# Helium and neon in comet 81P/Wild 2 samples from the NASA Stardust mission

R. Palma, R. O. Pepin, A. Westphal, Evelyn Füri, D. Schlutter, Z. Gainsforth,

R. Frank

► To cite this version:

R. Palma, R. O. Pepin, A. Westphal, Evelyn Füri, D. Schlutter, et al., Helium and neon in comet 81P/Wild 2 samples from the NASA Stardust mission. Meteoritics and Planetary Science, 2019, 54 (1), pp.3-53. 10.1111/maps.13189. hal-02122912

HAL Id: hal-02122912

https://hal.science/hal-02122912v1

Submitted on 11 Dec 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Helium and neon in comet 81P/Wild 2 samples from the NASA Stardust mission

R. L. PALMA<sup>1,2,*</sup>, R. O. PEPIN<sup>2</sup>, A. J. WESTPHAL<sup>3</sup>, E. FÜR<sup>4</sup>, D. J. SCHLUTTER<sup>2</sup>, Z. S. GAINSFORTH<sup>3</sup>, and D. R. FRANK<sup>5</sup>

<sup>1</sup>Department of Physics and Astronomy, Minnesota State University, Mankato, Minnesota 56001, USA

<sup>2</sup>School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota 55455, USA

<sup>3</sup>Space Sciences Laboratory, University of California, Berkeley, California 94720-7450, USA

<sup>4</sup>Centre de Recherches Pétrographiques et Géochimiques, CNRS-UL, 54501 Vandoeuvre-lès-Nancy Cedex, France

<sup>5</sup>Hawai'i Institute of Geophysics and Planetology, University of Hawai'i at Manoa, Honolulu, Hawai'i 96822, USA

*Corresponding author. E-mail: russell.palma@mnsu.edu

*(Received 15 August 2017; revision accepted 06 July 2018)*

**Abstract**–Helium and neon distributions are reported for a variety of Stardust comet 81P/Wild 2 samples, including particle tracks and terminal particles, cell surface and subsurface slices from the comet coma and interstellar particle collection trays, and numerous small aerogel blocks extracted from comet cells C2044 and C2086. Discussions and conclusions in several abstracts published during the course of the investigation are included, along with the relevant data. Measured isotope ratios span a broad range, implying a similar range for noble gas carriers in the Wild 2 coma. The meteoritic phase Q-$^{20}$Ne/$^{22}$Ne ratio was observed in several samples. Some of these, and others, exhibit $^{21}$Ne excesses too large for attribution to spallation by galactic cosmic ray irradiation, suggesting exposure to a solar proton flux greatly enhanced above current levels in an early near-Sun environment. Still others display evidence for a solar wind component, particularly one C2086 block with large abundances of isotopically solar-like helium and neon. Eighty-nine small aerogel samples were cut from depths up to several millimeters below the cell C2044 surface and several millimeters away from the axis of major track T41. A fraction of these yielded measurable and variable helium and neon abundances and isotope ratios, although none contained visible tracks or carrier particle fragments and their locations were beyond estimated penetration ranges for small particles or ions incident on the cell surface, or for lateral ejecta from T41. Finding plausible emplacement mechanisms and sources for these gases is a significant challenge raised by this study.

## I. INTRODUCTION

The Stardust Discovery mission brought back to Earth particles collected from the coma of comet 81P/Wild 2 (Brownlee et al. 2006). This return provided the first opportunity to investigate in the laboratory the nature, diversity, and origin of primitive cometary matter that most likely formed in the earliest stages of solar system history and has since been preserved in cold storage far from the Sun. Within the broad suite of ongoing studies of the physical, chemical, mineralogic, petrologic, and isotopic properties of Stardust materials, the noble gases play a unique role. They are excellent tracers of contributions from the various volatile

element reservoirs in the solar system and of subsequent processing of the gases acquired from those reservoirs. They can contribute to elucidating the compositional characteristics of primordial nebular dust and potentially speak to the history of comet grain exposures to galactic cosmic radiation and charged atomic particles radiated from the young evolving Sun.

It is known from studies by many research teams (e.g., Brownlee et al. 2006; McKeegan et al. 2006; Ishii et al. 2008; Ogliore et al. 2012; Gainsforth et al. 2015) that some mineral grains in particles collected by Stardust were formed at very high temperatures, presumably close to the early Sun before ejection to distant comet-forming regions of the nebula. Noble gas distributions in
materials excavated from the large impact track C2044,0,41 in aerogel cell C2044 (Tile 44) appear to reflect ambient gas compositions in an energetic section of the nebular disk proximate to the Sun at a time when the formation of preplanetary solid matter was just beginning. These noble gases provide clues to deciphering the nature of the gaseous environment during the earliest history of the solar system (Marty et al. 2008). We now know, however, that these noble gas signatures are not the only ones recorded in Stardust materials. Subsequent measurements on other samples point to a far more complex inventory of captured gases, comprising apparently erratically distributed helium and neon compositions spanning a wide range of nonterrestrial isotope ratios totally dissimilar to those found in track C2044,0,41 (e.g., Füri and Marty 2012; Palma et al. 2012, 2015; Westphal et al. 2017).

A further implication of the Stardust noble gas characterizations is the possibility they offer, combined with other Stardust data, for providing markers of cometary origin in the large population of interplanetary dust particles (IDPs) that have been studied extensively over the past two decades, but without certain knowledge of where they originated. Some of these IDPs are suspected to come from comets (e.g., Busemann et al. 2009; Pepin et al. 2015a, 2015b), and some means of identifying them would expand the study of primitive cometary matter well beyond the Stardust collection from a single body.

## II. SAMPLES

Concentrations and isotopic compositions of light noble gases were measured at the University of Minnesota (UM) by stepwise pyrolysis of a large number and wide array of different materials returned by the NASA Stardust Mission. Low gas contents and small sample masses restricted the measurements to helium and neon, but just these two lightest and relatively abundant noble gas species were remarkably informative.

Two adjacent blocks of aerogel were extracted from a top, space-exposed corner of aerogel cell C2044 (Fig. 1). One of these, the “track 41 block,” contained the large track C2044,0,41 (“T41”), subsequently dissected for multiple noble gas analyses. The other, free of visible tracks and particles and called the “blank block,” was divided into numerous surface and subsurface fragments. Among these were 70 samples (“blank blocks aerogel pieces” in the Table 1 data tabulation) directed to a systematic investigation at UM with high spatial resolution of volumes adjacent to T41.

An additional 19 blank block samples were analyzed at the Centre de Recherches Pétrographiques et Géochimiques (CRPG) in Nancy, France. Data from

<figure><img src="image_2.png" /><figcaption>Fig. 1. A) The entire cell C2044 (tile 44) still within the cell holder frame. The track 41 impact site is in the lower left of the photo, marked with an arrow. B) Cell C2044 (large aerogel block on left) removed from the frame, with extracted “track 41 block” (small aerogel block, far right with visible track C2044,0,41) and “blank block” (small aerogel block below the track 41 block). The track 41 block is ~4 mm width × ~9 mm length × ~10 mm depth; the blank aerogel block from the corner of the cell has the same length and depth but is slightly wider, ~5 mm. One short side and one long side of the blank block abutted the foil surface lining tile 44, and the opposite long side was adjacent to the track 41 block. This “blank block” is so-designated because there are no discernible particle fragments within it. However, the Stardust Interstellar Preliminary Examination (Westphal et al. 2014b) found that impacts of submicron particles, while easily visible by electron microscopy in the aluminum foil collectors of Stardust, can be undetectable in the aerogel collectors by optical microscopy, even at the highest magnification. Unless noted otherwise, all samples analyzed were provided by A. Westphal, Z. Gainsforth, and D. Frank, Space Sciences Laboratory, University of California, Berkeley. Photos courtesy of Astromaterials Acquisition and Curation Office, NASA-Johnson Space Center. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

this study are set out in Table 2. Sample preparation, degassing, and analytic procedures utilized at UM, and those used at CRPG that differed in important respects from the UM experimental protocols, are given below in section III. Results and discussions of this independent CRPG investigation, and comparisons with
<table><caption>Table 1. Gas abundances, isotopic ratios, and release temperatures for Stardust step-heated samples analyzed at UM.</caption><thead><tr><th>Analysis Number</th><th>Analysis date</th><th>Sample identification</th><th># of heating steps, maximum T (°C)</th><th><sup>3</sup>He/<sup>4</sup>He × 10<sup>-4</sup></th><th><sup>4</sup>He × 10<sup>-10</sup> ccSTP</th><th><sup>20</sup>Ne/<sup>22</sup>Ne</th><th><sup>21</sup>Ne/<sup>22</sup>Ne</th><th><sup>20</sup>Ne × 10<sup>-12</sup> ccSTP</th><th><sup>20</sup>He/<sup>20</sup>Ne</th></tr></thead><tbody><tr><td>1</td><td>02-Apr</td><td>blank block wafer C2044,20A</td><td>2 steps, 750<br>1 step, 800</td><td>2.53 (17)<br>2.203<br>(23)</td><td>1.71 (03)<br>53.6 (02)</td><td>14.2 (12)<br>18.66 (74)</td><td>0.0372 (88)<br>0.0375 (18)</td><td>19.2 (04)<br>319 (06)</td><td>8.9 (02)<br>16.8 (05)</td></tr><tr><td>3</td><td>09-Apr</td><td></td><td>1 step, 890</td><td>2.241<br>(48)</td><td>22.3 (04)</td><td>17.49 (40)</td><td>0.0408 (28)</td><td>106 (02)</td><td>21.0 (06)</td></tr><tr><td>4</td><td>23-Apr</td><td></td><td>5 steps, 1130<br>7 steps, 1410</td><td>3.76 (20)<br>4.81 (78)</td><td>1.42 (03)<br>0.502 (10)</td><td>15.4 (15)<br>13.7 (22)</td><td>0.041 (17)<br>0.033 (39)</td><td>7.80 (16)<br>2.80 (06)</td><td>18.2 (05)<br>17.9 (05)</td></tr><tr><td>5</td><td></td><td>C2044,20A (integrated)</td><td></td><td>2.265<br>(22)</td><td>79.5 (06)</td><td>18.03 (50)</td><td>0.0383 (15)</td><td>455 (07)</td><td>17.5 (03)</td></tr><tr><td>6</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>7</td><td>05-May</td><td>blank block wafer C2044,18A</td><td>4 steps, 750<br>2 steps, 890<br>5 steps, 1380</td><td>2.53 (29)<br>3.98 (23)<br>3.85 (68)</td><td>0.971 (19)<br>2.78 (06)<br>0.462 (10)</td><td>12.8 (13)<br>13.58 (66)<br>12.6 (16)</td><td>0.016 (19)<br>0.0412 (87)<br>0.033 (26)</td><td>5.26 (11)<br>14.5 (03)<br>3.77 (08)</td><td>18.5 (05)<br>19.2 (05)<br>12.3 (04)</td></tr><tr><td>8</td><td></td><td>C2044,18A (integrated)</td><td></td><td>3.63 (18)<br>3.49 (22)</td><td>4.21 (06)<br>1.31 (03)</td><td>13.23 (57)</td><td>0.0340 (81)</td><td>23.5 (03)<br>n.d.</td><td>17.9 (03)<br></td></tr><tr><td>9</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>10</td><td>12-May</td><td>blank block wafer C2044,19</td><td>4 steps, 750<br>11 steps, 1340</td><td>5.4 (28)</td><td>0.173 (04)</td><td>13.0 (37)</td><td>0.060 (80)</td><td>1.56 (04)</td><td>11.1 (04)</td></tr><tr><td>11</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>12</td><td></td><td>C2044,19 (integrated)</td><td></td><td>3.71 (38)</td><td>1.48 (03)<br>n.d.</td><td>13.0 (37)<br>11.3 (24)</td><td>0.060 (80)<br><sup>21</sup>Ne n.d.</td><td>1.56 (04)<br>1.88 (08)</td><td>94.9 (31)</td></tr><tr><td>13</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>14</td><td>15-May</td><td>blank block wafer C2044,20B</td><td>8 steps, 1420</td><td>3.24 (24)</td><td>2.20 (04)</td><td>10.66 (34)</td><td>0.0214 (41)</td><td>24.5 (05)</td><td>8.98 (25)</td></tr><tr><td>15</td><td>19-May</td><td>blank block wafer C2044,18B</td><td>4 steps, 650<br>6 steps, 1410</td><td>5.4 (20)<br>3.44 (28)</td><td>0.219 (05)<br>2.42 (04)</td><td>10.0 (15)<br>10.59 (35)</td><td>2.85 (09)<br>0.0214 (41)</td><td>7.68 (30)<br>27.4 (05)</td><td>8.83 (22)</td></tr><tr><td>16</td><td></td><td>C2044,18B (integrated)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>17</td><td>20-May</td><td>T41 cross section slice C2044,17,41,0,0</td><td>3 steps, 200<br>4 steps, 750</td><td><sup>3</sup>He n.d.<br>2.5 (20)</td><td>0.032 (02)<br>0.139 (03)</td><td>3.5 (11)<br>8.92 (75)</td><td>0.123 (43)<br>0.035 (20)</td><td>0.27 (02)<br>0.90 (07)</td><td>11.9 (11)<br>15.4 (12)</td></tr><tr><td>18</td><td>21-May</td><td></td><td>3 steps, 1000<br>7 steps, 1350</td><td>4.30 (32)<br>2.8 (12)</td><td>0.782 (15)<br>0.175 (04)</td><td>2.9 (18)<br>6.23 (96)</td><td>0.49 (03)<br>0.066 (18)</td><td>0.49 (03)<br>0.366 (21)</td><td>18.7 (05)<br>19.3 (29)</td></tr><tr><td>19</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>20</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>21</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>22</td><td></td><td>C2044,17,41,0,0 (integrated)</td><td></td><td>3.83 (39)</td><td>1.13 (02)</td><td>6.23 (96)<br>22<sup>Ne</sup> n.d.</td><td>5.85 (11)<br>21<sup>Ne</sup> n.d.</td><td>35.8 (21)<br>0.26 (05)</td><td>35.8 (21)<br>0.026 (04)</td></tr><tr><td>23</td><td>01-Jul</td><td>T41 cross section slice C2044,17&18,41,0,0</td><td>8 steps, 1350</td><td>2.7 (12)</td><td>0.131 (02)<br>n.d.</td><td>22<sup>Ne</sup> n.d.<br>22<sup>Ne</sup> n.d.</td><td>21<sup>Ne</sup> n.d.<br>21<sup>Ne</sup> n.d.</td><td>440 (70)<br>2.92 (22)</td><td>440 (70)<br>1.78 (14)</td></tr><tr><td>24</td><td>08-Jul</td><td>T41 cross section slice C2044,18,41,0,0</td><td>7 steps, 1470</td><td>9 steps, 1400</td><td>3.53 (98)</td><td>0.115 (02)</td><td>22<sup>Ne</sup> n.d.<br>22<sup>Ne</sup> n.d.</td><td>2.92 (22)</td><td>1.78 (14)</td></tr><tr><td>25</td><td>08-Aug</td><td>T108 terminal particles C2081,1,108,13&14</td><td>9 steps, 1400</td><td>3.9 (17)</td><td>0.052 (01)</td><td>0.052 (01)</td><td>22<sup>Ne</sup> n.d.<br>21<sup>Ne</sup> n.d.</td><td>2.92 (22)</td><td>1.78 (14)</td></tr><tr><td>26</td><td>09-Aug<br>2009</td><td>T108 terminal particle C2081,1,108,12</td><td>9 steps, 1400</td><td>5.34 (77)</td><td>0.337 (11)</td><td>9.4 (16)</td><td>0.093 (30)</td><td>2.19 (07)</td><td>15.39 (72)</td></tr><tr><td>27</td><td>13-Jan</td><td>blank block surface slice St-C44s1.2</td><td>8 steps, 1400</td><td>3.4 (11)</td><td>0.112 (06)</td><td>4.6 (24)</td><td>21<sup>Ne</sup> n.d.</td><td>0.28 (01)</td><td>40.0 (28)</td></tr><tr><td>28</td><td>20-Jan</td><td>blank block surface slice St-C44s1.3</td><td>8 steps, 1400</td><td>7.7 (12)</td><td>0.158 (08)</td><td>10.1 (15)</td><td>0.066 (25)</td><td>1.46 (06)</td><td>10.82 (69)</td></tr><tr><td>29</td><td>17-Feb</td><td>blank block surface slice St-C44s1.1</td><td>8 steps, 1400</td><td>3.0 (19)</td><td>0.067 (04)</td><td>16.8 (51)</td><td>0.24 (14)</td><td>0.45 (02)</td><td>14.9 (12)</td></tr><tr><td>30</td><td></td><td>St-C44s1 (fragments 1,2,3 integrated)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>
<table><caption>Table 1. *Continued.* Gas abundances, isotopic ratios, and release temperatures for Stardust step-heated samples analyzed at UM.</caption><thead><tr><th>Analysis Number</th><th>Analysis date</th><th>Sample identification</th><th># of heating steps, maximum T (°C)</th><th><sup>3</sup>He/<sup>4</sup>He × 10<sup>-4</sup></th><th><sup>4</sup>He × 10<sup>-10</sup> ccSTP</th><th><sup>20</sup>Ne/<sup>22</sup>Ne</th><th><sup>21</sup>Ne/<sup>22</sup>Ne</th><th><sup>20</sup>Ne × 10<sup>-12</sup> ccSTP</th><th><sup>4</sup>He/<sup>20</sup>Ne</th></tr></thead><tbody><tr><td>31</td><td>26-May</td><td>blank block subsurface slice St-C44s2,1</td><td>8 steps, 1400</td><td>5.0 (10)</td><td>0.111 (06)</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>32</td><td>28-May</td><td>blank block subsurface slice St-C44s2,2</td><td>8 steps, 1400</td><td>4.43 (51)</td><td>0.217 (11)</td><td><sup>20</sup>Ne n.d.</td><td>0.055 (43)</td><td><sup>22</sup>Ne=6.4E-14</td><td></td></tr><tr><td>33</td><td>01-Jun</td><td>blank block subsurface slice St-C44s2,4</td><td>8 steps, 1400</td><td>6.4 (14)</td><td>0.050 (03)</td><td>9.5 (23)</td><td><sup>21</sup>Ne n.d.</td><td>0.54 (03)</td><td>9.26 (76)</td></tr><tr><td>34</td><td></td><td>St-C44s2 (fragments 1,2,4 integrated)</td><td></td><td>4.86 (45)</td><td>0.378 (13)</td><td>9.5 (23)</td><td>0.055 (43)</td><td>0.54 (03)</td><td>70.0 (46)</td></tr><tr><td>35</td><td>04-Nov</td><td>blank block aerogel piece C2044,25</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td>8.8 (11)</td><td>0.039 (22)</td><td>2.46 (13)</td><td></td></tr><tr><td>36</td><td>07-Dec</td><td>blank block aerogel piece C2044,27</td><td>7 steps, 1400</td><td>5.8 (19)</td><td>0.125 (10)</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>37</td><td>08-Dec</td><td>blank block aerogel piece C2044,29</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>38</td><td>09-Dec</td><td>blank block aerogel piece C2044,31</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>39</td><td>15-Dec</td><td>blank block aerogel piece C2044,35</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td></td><td>2010</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>40</td><td>09-Feb</td><td>blank block aerogel piece C2044,48</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>41</td><td>10-Feb</td><td>blank block aerogel piece C2044,37</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>42</td><td>11-Feb</td><td>blank block aerogel piece C2044,45</td><td>7 steps, 1400</td><td>9.3 (13)</td><td>0.0692 (48)</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>43</td><td>12-Feb</td><td>blank block aerogel piece C2044,41</td><td>2 steps, 200</td><td>3.6 (10)</td><td>0.0716 (47)</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>44</td><td></td><td></td><td>7 steps, 1400</td><td>7.3 (10)</td><td>0.0910 (36)</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>45</td><td></td><td>C2044,41 (integrated)</td><td></td><td>5.7 (18)</td><td>0.1626 (59)</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>46</td><td>11-Mar</td><td>blank block aerogel piece C2044,58</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td>12.0 (12)</td><td><sup>21</sup>Ne n.d.</td><td>4.79 (24)</td><td></td></tr><tr><td>47</td><td>12-Mar</td><td>blank block aerogel piece C2044,68</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>48</td><td>15-Mar</td><td>blank block aerogel piece C2044,55</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>49</td><td>16-Mar</td><td>blank block aerogel piece C2044,62</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>50</td><td>17-Mar</td><td>blank block aerogel piece C2044,52</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>51</td><td>18-Mar</td><td>blank block aerogel piece C2044,51</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>52</td><td>24-Mar</td><td>blank block aerogel piece C2044,65</td><td>7 steps, 1400</td><td>11.9 (64)</td><td>0.0590 (44)</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>53</td><td>07-Apr</td><td>blank block aerogel piece C2044,75</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>54</td><td>08-Apr</td><td>blank block aerogel piece C2044,77</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>55</td><td>12-Apr</td><td>blank block aerogel piece C2044,72</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>56</td><td>05-May</td><td>T41 fragment St41 82 (blank)</td><td>7 steps, 1400</td><td>4.24 (53)</td><td>0.503 (20)</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>57</td><td>06-May</td><td>T41 fragment C2044,31,41,0,0</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>58</td><td>10-May</td><td>T41 fragment C2044,30,41,0,0</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>59</td><td>12-May</td><td>T41 fragment C2044,28,41,0,0</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>60</td><td>11-Aug</td><td>blank block aerogel piece C2044,61</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>61</td><td>12-Aug</td><td>blank block aerogel piece C2044,59</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>62</td><td>17-Aug</td><td>blank block aerogel piece C2044,33</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>63</td><td>18-Aug</td><td>blank block aerogel piece C2044,40</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr></tbody></table>
<table><caption>Table 1. *Continued*. Gas abundances, isotopic ratios, and release temperatures for Stardust step-heated samples analyzed at U.M.</caption><thead><tr><th rowspan="2">Analysis Number</th><th rowspan="2">Analysis date</th><th rowspan="2">Sample identification</th><th colspan="2"># of heating steps, maximum T (°C)</th><th rowspan="2"><sup>3</sup>He/<sup>4</sup>He × 10<sup>-4</sup></th><th rowspan="2"><sup>4</sup>He × 10<sup>-10</sup> ccSTP</th><th rowspan="2"><sup>20</sup>Ne/<sup>22</sup>Ne</th><th rowspan="2"><sup>21</sup>Ne/<sup>22</sup>Ne</th><th rowspan="2"><sup>20</sup>Ne × 10<sup>-12</sup> ccSTP</th><th rowspan="2"><sup>4</sup>He/<sup>20</sup>Ne</th></tr></thead><tbody><tr><td>64</td><td>31-Aug</td><td>blank block aerogel piece C2044.44</td><td>7 steps, 1400</td><td>10.3 (39)</td><td>0.0423 (38)</td><td></td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>65</td><td>14-Sep</td><td>blank block aerogel piece C2044.54</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>66</td><td>09-Nov</td><td>blank block aerogel piece C2044.78</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td>9.1 (14)</td><td><sup>21</sup>Ne n.d.</td><td>0.751 (79)</td><td></td><td></td></tr><tr><td>67</td><td>16-Nov</td><td>blank block aerogel piece C2044.66</td><td>7 steps, 1400</td><td>4.14 (28)</td><td>0.512 (17)</td><td>12.6 (32)</td><td><sup>21</sup>Ne n.d.</td><td>0.542 (73)</td><td>94 (12)</td><td></td></tr><tr><td>68</td><td>08-Jun</td><td>blank block aerogel piece C2044.94</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>69</td><td>09-Jun</td><td>blank block aerogel piece C2044.95</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>70</td><td>10-Jun</td><td>blank block aerogel piece C2044.97</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>71</td><td>13-Jun</td><td>blank block aerogel piece C2044.73</td><td>7 steps, 1400</td><td></td><td><sup>3</sup>He n.d.</td><td>0.187 (05)</td><td>16.21 (83)</td><td>0.0427 (70)</td><td>22.9 (04)</td><td>0.817 (26)</td></tr><tr><td>72</td><td>15-Jun</td><td>blank block aerogel piece C2044.98</td><td>7 steps, 1400</td><td></td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>73</td><td>28-Jun</td><td>blank block aerogel piece C2044.103</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>74</td><td>29-Jun</td><td>blank block aerogel piece C2044.87</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>75</td><td>30-Jun</td><td>blank block aerogel piece C2044.92</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td>11.0 (43)</td><td>0.033 (24)</td><td>2.09 (26)</td><td></td></tr><tr><td>76</td><td>01-Jul</td><td>blank block aerogel piece C2044.89</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>77</td><td>05-Jul</td><td>blank block aerogel piece C2044.85</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>78</td><td>06-Jul</td><td>blank block aerogel piece C2044.100</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>79</td><td>07-Jul</td><td>blank block aerogel piece C2044.102</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td>12.2 (77)</td><td>0.064 (44)</td><td>1.48 (19)</td><td></td></tr><tr><td>80</td><td>20-Jul</td><td>blank block aerogel piece C2044.96</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>81</td><td>21-Jul</td><td>blank block aerogel piece C2044.91</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>82</td><td>22-Jul</td><td>blank block aerogel piece C2044.84</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>83</td><td>25-Jul</td><td>blank block aerogel piece C2044.101</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>84</td><td>26-Jul</td><td>blank block aerogel piece C2044.90</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>85</td><td>27-Jul</td><td>blank block aerogel piece C2044.88</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>86</td><td>28-Jul</td><td>blank block aerogel piece C2044.82</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>87</td><td>10-Aug</td><td>blank block aerogel piece C2044.83</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>88</td><td>30-Aug</td><td>blank block aerogel piece C2044.86</td><td>7 steps, 1400</td><td>&gt; 190</td><td>n.d.</td><td>&lt; 0.014</td><td>8.9 (80)</td><td><sup>21</sup>Ne n.d.</td><td>n.d.</td><td>&lt; 1.9</td></tr><tr><td>89</td><td>06-Sep</td><td>blank block aerogel piece C2044.99</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>90</td><td>18-Jun</td><td>blank block aerogel piece C2044.60</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>91</td><td>19-Jun</td><td>blank block aerogel piece C2044.56</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>92</td><td>20-Jun</td><td>blank block aerogel piece C2044.32</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td>8.1 (22)</td><td>0.0435 (37)</td><td></td><td>n.d.</td><td></td></tr><tr><td>93</td><td>21-Jun</td><td>blank block aerogel piece C2044.64</td><td>7 steps, 1400</td><td></td><td><sup>3</sup>He n.d.</td><td>0.0488 (48)</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>94</td><td>22-Jun</td><td>blank block aerogel piece C2044.50</td><td>7 steps, 1400</td><td></td><td>2.45 (88)</td><td>0.0784 (84)</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>95</td><td>18-Jul</td><td>blank block aerogel piece C2044.47</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>96</td><td>19-Jul</td><td>flight spare aerogel E243-8B</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>97</td><td>30-Jul</td><td>blank block aerogel piece C2044.111</td><td>7 steps, 1400</td><td>5.4 (26)</td><td>0.096 (10)</td><td>9.29 (48)</td><td>0.0231 (26)</td><td>17.7 (25)</td><td>0.542 (95)</td><td></td></tr></tbody></table>
<table><caption>Table 1. *Continued.* Gas abundances, isotopic ratios, and release temperatures for Stardust step-heated samples analyzed at UM.</caption><thead><tr><th>Analysis Number</th><th>Analysis date</th><th>Sample identification</th><th># of heating steps, maximum T (°C)</th><th><sup>3</sup>He/<sup>4</sup>He × 10<sup>-4</sup></th><th><sup>4</sup>He × 10<sup>-10</sup> ccSTP</th><th><sup>20</sup>Ne/<sup>22</sup>Ne</th><th><sup>21</sup>Ne/<sup>22</sup>Ne</th><th><sup>20</sup>Ne × 10<sup>-12</sup> ccSTP</th><th><sup>4</sup>He/<sup>20</sup>Ne</th></tr></thead><tbody><tr><td>98</td><td>31-Jul</td><td>blank block aerogel piece C2044,112</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>99</td><td>01-Aug</td><td>blank block aerogel piece C2044,110</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>100</td><td>02-Aug</td><td>blank block aerogel piece C2044,121</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>101</td><td>12-Sep</td><td>blank block aerogel piece C2044,119</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td>9.1 (34)</td><td>0.023 (16)</td><td>1.77 (18)</td><td></td></tr><tr><td>102</td><td>17-Sep</td><td>blank block aerogel piece C2044,117</td><td>2 steps, 700</td><td>4.90 (71)</td><td>0.255 (09)</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>0.279 (38)</td><td>91 (13)</td></tr><tr><td>103</td><td>18-Sep</td><td>C2044,117 continued</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>104</td><td>19-Sep</td><td>blank block aerogel piece C2044,122</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td>9.0 (32)</td><td>0.030 (16)</td><td>1.83 (15)</td><td></td></tr><tr><td>105</td><td>05-Oct</td><td>blank block aerogel piece C2044,114</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>106</td><td>08-Oct</td><td>blank block aerogel piece C2044,109</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>107</td><td>09-Oct</td><td>blank block aerogel piece C2044,118</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>108</td><td>11-Oct</td><td>blank block aerogel piece C2044,113</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>109</td><td>12-Oct</td><td>blank block aerogel piece C2044,116</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td>0.0356 (30)</td><td>11.1 (43)</td><td>1.80 (13)</td><td>1.98 (22)</td></tr><tr><td>110</td><td>17-Oct</td><td>blank block aerogel piece C2044,107</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>111</td><td>29-Oct</td><td>blank block aerogel piece C2044,115/120</td><td>7 steps, 1400</td><td>12.8 (49)</td><td>0.0431 (37)</td><td>9.6 (66)</td><td>0.039 (33)</td><td>1.11 (16)</td><td>3.89 (65)</td></tr><tr><td>112</td><td>30-Oct</td><td>blank block aerogel piece C2044,120/115</td><td>7 steps, 1400</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>113</td><td>21-May</td><td>C2086 procedural foil 1</td><td>7 steps, 1350</td><td>3.06 (30)</td><td>0.441 (11)</td><td>9.8 (18)</td><td><sup>21</sup>Ne n.d.</td><td>0.268 (38)</td><td>165 (24)</td></tr><tr><td>114</td><td>23-May</td><td>C2086 blank 3, Tile 86</td><td>7 steps, 1350</td><td>5.40 (98)</td><td>0.140 (08)</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>115</td><td>28-May</td><td>C2086 blank 2, Tile 86</td><td>3 steps, 700</td><td>4.398 (71)</td><td>30.51 (22)</td><td>14.03 (28)</td><td>0.0423 (50)</td><td>6.63 (35)</td><td>460 (25)</td></tr><tr><td>116</td><td>29-May</td><td></td><td>1 step, 850</td><td>4.137 (63)</td><td>21.96 (14)</td><td>12.08 (66)</td><td>0.0322 (44)</td><td>4.12 (22)</td><td>533 (29)</td></tr><tr><td>117</td><td>30-May</td><td></td><td>3 steps, 1350</td><td>4.28 (23)</td><td>0.832 (14)</td><td>13.21 (34)</td><td>0.0381 (34)</td><td>10.75 (41)</td><td>496 (19)</td></tr><tr><td>118</td><td></td><td>blank 2, Tile 86 (integrated)</td><td></td><td>4.289 (48)</td><td>53.30 (26)</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>119</td><td>31-May</td><td>C2086 blank 7, vise</td><td>5 steps, 1100</td><td>5.27 (38)</td><td>0.331 (29)</td><td>9.45 (80)</td><td>0.047 (16)</td><td>1.04 (15)</td><td>31.8 (53)</td></tr><tr><td>120</td><td>03-Jun</td><td>C2086 blank 5, side of tile</td><td>7 steps, 1350</td><td>6.06 (25)</td><td>1.17 (04)</td><td>9.46 (30)</td><td>0.0291 (38)</td><td>15.3 (09)</td><td>7.65 (52)</td></tr><tr><td>121</td><td>05-Jun</td><td>C2086 blank 8 keystone</td><td>7 steps, 1350</td><td>0.630 (73)</td><td>0.941 (23)</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>0.213 (48)</td><td>442 (99)</td></tr><tr><td>122</td><td>07-Jun</td><td>C2086 blank 4 from crack</td><td>7 steps, 1350</td><td>6.6 (20)</td><td>0.032 (05)</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>n.d.</td><td></td></tr><tr><td>123</td><td>10-Jun</td><td>C2086 Track 181</td><td>7 steps, 1350</td><td>3.92 (25)</td><td>1.445 (14)</td><td>10.17 (45)</td><td>0.0285 (37)</td><td>0.215 (39)</td><td>670 (120)</td></tr><tr><td>124</td><td>03-Jul</td><td>C2086 procedural foil 2</td><td>7 steps, 1350</td><td>0.97 (28)</td><td>0.207 (11)</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>8.17 (35)</td><td>2.53 (37)</td></tr><tr><td>125</td><td>23-Jul</td><td>C2086 blank 1, Tile 86</td><td>7 steps, 1350</td><td>7.8 (19)</td><td>0.054 (07)</td><td></td><td></td><td>0.263 (37)</td><td>20.5 (39)</td></tr><tr><td>126</td><td>24-Jul</td><td>C2086 blank 6, Tile 86</td><td>7 steps, 1350</td><td>2.42 (67)</td><td>0.101 (12)</td><td>8.28 (41)</td><td>0.0396 (52)</td><td>4.89 (25)</td><td>2.07 (26)</td></tr></tbody></table>
<table><caption>Table 1. Continued. Gas abundances, isotopic ratios, and release temperatures for Stardust step-heated samples analyzed at UM.</caption><thead><tr><th>Analysis Number</th><th>Analysis date</th><th>Sample identification</th><th># of heating steps, maximum T (°C)</th><th><sup>3</sup>He/<sup>4</sup>He × 10<sup>-4</sup></th><th><sup>4</sup>He × 10<sup>-10</sup> ccSTP</th><th><sup>20</sup>Ne/<sup>22</sup>Ne</th><th><sup>21</sup>Ne/<sup>22</sup>Ne</th><th><sup>20</sup>Ne × 10<sup>-12</sup> ccSTP</th><th><sup>4</sup>He/<sup>20</sup>Ne</th></tr></thead><tbody><tr><td>127</td><td>30-Jul</td><td>C2086 Track 182</td><td>7 steps, 1350</td><td>0.65 (47)</td><td>0.111 (13)</td><td>9.25 (63)</td><td>0.0489 (61)</td><td>1.53 (11)</td><td>7.26 (99)</td></tr><tr><td>128</td><td>01-Aug 2014</td><td>C2086 Schmutz I</td><td>7 steps, 1350</td><td><sup>3</sup>He n.d.</td><td>0.163 (05)</td><td>6.03 (83)</td><td>0.083 (42)</td><td>0.373 (80)</td><td>43.7 (94)</td></tr><tr><td>129</td><td>28-Jan</td><td>C2086 A1, Tile 86</td><td>7 steps, 1350</td><td><sup>3</sup>He n.d.</td><td>n.d.</td><td>0.033 (05)</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>n.d.</td></tr><tr><td>130</td><td>29-Jan</td><td>C2086 BD2, Tile 86</td><td>7 steps, 1350</td><td><sup>3</sup>He n.d.</td><td>0.033 (05)</td><td>0.041 (05)</td><td><sup>22</sup>Ne n.d.</td><td>0.295 (42)</td><td>13.9 (25)</td></tr><tr><td>131</td><td>31-Jan</td><td>C2086 A2, Tile 86</td><td>7 steps, 1350</td><td>2.1 (19)</td><td>0.041 (05)</td><td>10.41 (31)</td><td>0.0318 (33)</td><td>7.87 (35)</td><td>0.381 (57)</td></tr><tr><td>132</td><td>03-Feb</td><td>C2086 BD1, Tile 86</td><td>7 steps, 1350</td><td>7.3 (39)</td><td>0.030 (04)</td><td>8.2 (1.3)</td><td>0.050 (33)</td><td>0.881 (94)</td><td></td></tr><tr><td>133</td><td>04-Feb</td><td>C2086 A3, Tile 86</td><td>7 steps, 1350</td><td>2.3 (16)</td><td>n.d.</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>0.340 (58)</td><td>16.8 (37)</td></tr><tr><td>134</td><td>11-Jun</td><td>interstellar tray, IS1 subsurface</td><td>7 steps, 1350</td><td>4.1 (13)</td><td>0.082 (13)</td><td>12.9 (2.2)</td><td>0.0213 (35)</td><td>0.609 (98)</td><td>13.5 (30)</td></tr><tr><td>135</td><td>11-Jun</td><td>interstellar tray, IS2 subsurface</td><td>7 steps, 1350</td><td><sup>3</sup>He n.d.</td><td>0.031 (04)</td><td>9.26 (35)</td><td>0.0213 (35)</td><td>10.1 (05)</td><td>0.306 (46)</td></tr><tr><td>136</td><td>13-Jun</td><td>interstellar tray, IS2 surface</td><td>7 steps, 1350</td><td>2.3 (17)</td><td>0.053 (09)</td><td>12.1 (1.8)</td><td><sup>21</sup>Ne n.d.</td><td>0.818 (91)</td><td>6.5 (13)</td></tr><tr><td>137</td><td>16-Jun</td><td>interstellar tray, IS1 surface</td><td>7 steps, 1350</td><td>2.3 (17)</td><td>n.d.</td><td>5.58 (66)</td><td>0.082 (16)</td><td>0.905 (94)</td><td></td></tr><tr><td>138</td><td>26-Sep</td><td>T77 terminal particle Puki A</td><td>2 steps, 600</td><td>n.d.</td><td>n.d.</td><td>6.5 (1.5)</td><td><sup>21</sup>Ne n.d.</td><td>0.281 (67)</td><td></td></tr><tr><td>139</td><td>30-Sep</td><td></td><td>7 steps, 1350</td><td>5.56 (50)</td><td>0.477 (31)</td><td>11.6 (1.2)</td><td>0.049 (14)</td><td>1.73 (12)</td><td>27.6 (2.6)</td></tr><tr><td>140</td><td>02-Oct</td><td>surface keystones C2028,6-9,0,0</td><td>7 steps, 1350</td><td>4.67 (36)</td><td>0.627 (27)</td><td>10.19 (31)</td><td>0.0347 (25)</td><td>23.0 (04)</td><td>2.73 (13)</td></tr><tr><td>141</td><td>03-Oct</td><td>surface keystone C2028,13-14,0,0</td><td>7 steps, 1350</td><td>4.67 (36)</td><td>0.627 (27)</td><td>10.19 (31)</td><td>0.0347 (25)</td><td>23.0 (04)</td><td>2.73 (13)</td></tr></tbody></table>

Numbers in parentheses are the uncertainty in the last two digits of the isotopic ratio or abundance. n.d. = not detected above blank.
<table><caption>Table 2. Gas abundances and isotopic ratios for Stardust laser-heated samples analyzed at CRPG.</caption><thead><tr><th>Analysis number</th><th>Sample identification</th><th><sup>3</sup>He/<sup>4</sup>He × 10<sup>-4</sup></th><th><sup>4</sup>He × 10<sup>-10</sup> ccSTP</th><th><sup>20</sup>Ne/<sup>22</sup>Ne</th><th><sup>21</sup>Ne/<sup>22</sup>Ne</th><th><sup>20</sup>Ne × 10<sup>-12</sup> ccSTP</th><th><sup>4</sup>He/<sup>20</sup>Ne</th></tr></thead><tbody><tr><td>1</td><td>T41 cross-sectional slice C2044,8,41,0</td><td>1.00 (25)</td><td>0.74 (24)</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>5.22 (37)</td><td>14.3 (47)</td></tr><tr><td>2</td><td>T41 cross-sectional slice C2044,9,41,0</td><td>3.50 (66)</td><td>2.07 (29)</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>3</td><td>T41 fragment “β” C2044,29,41,0,0</td><td><sup>3</sup>He n.d.</td><td>0.62 (20)</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>4</td><td>T41 fragment “δ” C2044,32,41,0,0</td><td>0.44 (12)</td><td>0.56 (18)</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>5</td><td>T41 sample C2044,107,41,0,0</td><td>2.48 (72)</td><td>0.128 (22)</td><td>10.8 (26)</td><td>0.061 (16)</td><td>5.74 (17)</td><td>2.23 (38)</td></tr><tr><td>6</td><td>T41 sample C2044,108,41,0,0</td><td>2.87 (76)</td><td>0.148 (25)</td><td>11.2 (12)</td><td>0.042 (12)</td><td>2.38 (07)</td><td>6.2 (11)</td></tr><tr><td>7</td><td>Blank block aerogel piece C2044,28</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>8</td><td>Blank block aerogel piece C2044,63</td><td>4.06 (73)</td><td>0.64 (11)</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>9</td><td>Blank block aerogel piece C2044,70</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>10</td><td>Blank block aerogel piece C2044,71</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>11</td><td>Blank block aerogel piece C2044,39</td><td>3.8 (17)</td><td>0.075 (14)</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>1.72 (10)</td><td>4.40 (86)</td></tr><tr><td>12</td><td>Blank block aerogel piece C2044,53</td><td>11.2 (54)</td><td>0.073 (13)</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>13</td><td>Blank block aerogel piece C2044,34a<sup>a</sup></td><td><sup>3</sup>He n.d.</td><td>0.107 (15)</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>4.01 (20)</td><td>2.67 (40)</td></tr><tr><td>14</td><td>Blank block aerogel piece C2044,34b<sup>a</sup></td><td><sup>3</sup>He n.d.</td><td>0.158 (22)</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>7.13 (36)</td><td>2.22 (33)</td></tr><tr><td>15</td><td>Blank block aerogel piece C2044,57</td><td>0.233 (75)</td><td>0.99 (13)</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>5.92 (30)</td><td>16.7 (25)</td></tr><tr><td>16</td><td>Blank block aerogel piece C2044,69</td><td>0.119 (38)</td><td>1.05 (20)</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>9.08 (45)</td><td>11.6 (23)</td></tr><tr><td>17</td><td>Blank block aerogel piece C2044,67</td><td>8.2 (30)</td><td>0.0273 (38)</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>18</td><td>Blank block aerogel piece C2044,74</td><td><sup>3</sup>He n.d.</td><td>0.101 (14)</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>19</td><td>Blank block aerogel piece C2044,76</td><td><sup>3</sup>He n.d.</td><td>0.099 (14)</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>2.38 (12)</td><td>4.16 (62)</td></tr><tr><td>20</td><td>Blank block aerogel piece C2044,26</td><td><sup>3</sup>He n.d.</td><td>0.111 (15)</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>21</td><td>Blank block aerogel piece C2044,30</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>22</td><td>Blank block aerogel piece C2044,38</td><td><sup>3</sup>He n.d.</td><td>0.411 (58)</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>23</td><td>Blank block aerogel piece C2044,42</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>24</td><td>Blank block aerogel piece C2044,43</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>25</td><td>Blank block aerogel piece C2044,46</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>26</td><td>Blank block aerogel piece C2044,49</td><td></td><td>n.d.</td><td></td><td></td><td>n.d.</td><td></td></tr><tr><td>27</td><td>Average of flight spare aerogel analyses</td><td><sup>3</sup>He n.d.</td><td>0.042 (25)</td><td></td><td></td><td>n.d.</td><td></td></tr></tbody></table>

relevant UM data, are addressed in sections IV.A.5 and V.A.3.

The UM investigation was initially directed to measurements of helium and neon distributions in melted aerogel with visible embedded particle fragments extracted from the T41 bulb wall. Results of this study, carried out in conjunction with parallel analyses at CRPG, were reported in Marty et al. (2008). Subsequent UM investigations of noble gases in C2044 were focused on aerogel samples from the two C2044 blocks, including (1) three 200 µm thick wafers sampled vertically from the blank block (Fig. 2); (2) two 200 µm thick cross-sectional transverse slices through T41 from the track 41 block, but without visible particle fragments from the track impactor (Fig. 3); (3) a variable thickness slice from the space irradiated top surface of the blank block and a 200 µm thick subsurface slice taken from immediately below that surface slice (Figs. 4 and 5); (4) four samples taken from a track 41 block aerogel fragment, three of which

included melted aerogel from the T41 wall (Fig. 6); and (5) seventy ~1–1.5 mm³ blocks of aerogel cut from the blank block at various depths from the space-exposed cell C2044 surface and distances from the axis of T41 (Fig. 7). The positions and orientations of the various C2044 samples are indicated in Fig. 8.

In addition to the noble gas analyses of aerogel cell C2044 in the neighborhood of T41, other Stardust samples analyzed at UM for helium and neon include (1) surface samples from aerogel cell C2028 (Fig. 9); (2) numerous samples from aerogel cell C2086, including entire tracks C2086,0,181 (“T181”) and C2086,0,182 (“T182”) (Figs. 10 and 11); (3) three individual terminal particles extracted from aerogel cell C2081, track C2081,0,108 (“T108”) (Fig. 12); (4) an individual terminal olivine particle, named “Puki A,” from aerogel cell C2009, track C2009,0,77 (“T77”); and (5) surface and subsurface aerogel samples from cell I1047 on the interstellar side of the Stardust particle collector (Fig. 13). Details on subdivisions of C2044 and other

<footnote><sup>a</sup>Broke into two pieces, analyzed separately. Measured <sup>4</sup>He/<sup>20</sup>Ne ratios concordant within uncertainties. Numbers in parentheses are the uncertainty in the last two digits of the isotopic ratio or abundance. n.d. = not detected above blank.</footnote>
<figure><img src="image_3.png" /><figcaption>Fig. 2. Aerogel wafer samples (C2044,18-20) cut vertically from the end of the blank block, with the space-exposed surface at top. All wafers have length equal to the depth of the blank block, ~10 mm. Wafer 18 (A) was in the corner of cell C2044, adjacent to the foil lining, furthest from T41. The greatest width of wafer 18 is 3–4 mm, with its thickness grading from ~200 µm on the right to zero on the left. Wafers 19 (B) and 20 (C) had uniform width of ~4 mm and thickness ~200 µm. Wafer 19 was between wafers 18 and 20, so wafer 20 was furthest from the corner of the blank block and closest to T41. Wafers 18 and 20 were cut approximately in half to form samples 18A and 20A (bottom halves of the wafers) and 18B and 20B (top halves of the wafers), whereas wafer 19 was analyzed as a single sample. Photos courtesy of Space Science Laboratory, UC Berkeley. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

aerogel materials, and assignments of the sample
identifiers listed in Table 1, are given in the Appendix.

### III. ANALYTICAL TECHNIQUES

#### III.A. Gas Extraction and Analysis

##### III.A.1. UM

All Stardust samples were wrapped in platinum foil packets by the sample preparers at their home institutions (see Appendix). The Pt foils for the packets were outgassed and distributed by the University of Minnesota (UM) laboratory, and were uniform in size, source, quality, and preparation. Upon arrival at UM, up to eight separate Pt sample packets could be attached to electrical leads in a multiple-sample furnace in a single loading, although most frequently fewer samples were loaded. The analysis order and date for each sample is given in Table 1. “Blank” Pt foil packets without any sample material were often included in sample loadings for system background analyses. Some of these blank packets were prepared at SSL, Berkeley, or the Johnson

Space Center, and others at UM. Following sample loading, the UM gas extraction and processing system was baked at ~100–150 °C for a minimum of 3 days, depending on the time required to achieve appropriate background pressures (typically $1-3 \times 10^{-10}$ torr) and, more importantly, low helium and neon background levels. Sample and blank foils within a sample grouping were heated and analyzed using the same procedures except where noted below. Evolved helium and neon throughout all stages of gas extraction and analysis were purified of condensable and chemically active background species to the extent achievable by exposure to a charcoal trap at liquid nitrogen temperature and a room-temperature Westinghouse/WL getter.

Samples and blanks were first heated in either two or three low-temperature 10 s stepped pulses (maximum temperature of ~200 °C) to remove potential background contamination from sample surfaces or Pt foils, and to outgas or decompose materials such as acrylic in which a sample may have been embedded (see Appendix). Complete analyses of helium and neon abundances and isotopic compositions released in these
<figure><img src="image_4.png" /><figcaption>Fig. 3. Two hundred micrometer thick aerogel cross-sectional slices 17 (A) and 18 (B) through T41. The results of four track 41 block transverse cross-sectional slices are reported in this study. All slices were directly below one another, with slice 8 on top, then 9 (analyzed at CRPG as “slice 1” and “slice 2,” respectively), then slice 17 directly above slice 18 at the bottom (see Fig. 8A). Scale bar divisions are 1 mm. UM analyzed samples from slices 17 and 18 are: (C) C2044,17,41,0,0 derived only from slice 17 material; (D) C2044,17 & 18,41,0,0 contained material from both slices 17 and 18; and (E) C2044,18,41,0,0 derived only from slice 18 material. Photos courtesy of Space Science Laboratory, UC Berkeley.</figcaption></figure>

low-temperature steps were always carried out. In only two instances (Table 1, C2044,17,41,0,0 and C2044, blank block sample 41) were amounts of these two gases statistically different from those measured in blank Pt foil analyses. For these samples, blank-corrected releases at ~200 °C are given in Table 1 and included in the stepped heating integrations.

Following the low-temperature heating steps, the entire system was evacuated and prepared for computer-controlled high-temperature gas release. The protocol consisted of 10 s stepped heating pulses, where the peak temperature reached during the heating pulse could be estimated by either of two techniques, i.e., (1) measured directly with an optical pyrometer by observing the glowing Pt packet through a sapphire window or (2) estimated by consulting an experimentally determined temperature versus power graph. The temperature estimate is the peak temperature near the end of the 10 s pulse, since there is thermal lag in the Pt foil packet. As the temperature/power increases, the peak temperature more closely matches the average temperature of the heating step. Over the period of the measurements discussed in this report, the default heating step interval changed, so the number of steps

varied from seven to nine, but in all instances the maximum temperature achieved was ~1400 °C. However, in the case of the blank block wafers, due to extremely high gas abundances in some of them, the number of heating steps was increased (and thus the incremental rise in temperature decreased) to reduce the potential gas release in each step.

The peak in noble gas release was always less than the maximum temperature of ~1400 °C that the furnace could attain. Although gas release in T41 wall samples occurred at temperatures in excess of 1250 °C (Marty et al. 2008), in none of the UM samples analyzed in this study was there a significant release at greater than ~1050 °C, and most were degassed at <900 °C. Refractory carriers such as diamond or SiC, if present in some samples, would require temperatures greater than 1400 °C to fully degas them. However, peak releases for helium and neon in the majority of the nanodiamond and SiC-Spinel samples pyrolyzed by Huss and Lewis (1995) occurred below or near 1400 °C, and even for peaks above this temperature enough helium and neon would likely have been degassed at ≤1400 °C, even with the short heating times, to be detected in our experiments if these carriers were
<figure><img src="image_5.png" /><figcaption>Fig. 4. A) Surface slice of C2044 blank block. The sample on the left was kept at UC Berkeley, and the sample on the right was subdivided into the three samples indicated by the dashed lines. Slice areal dimensions are ~0.5 × 1 cm before division. B) The surface slice thickness was nonuniform, as shown by the X-ray photo at 4 keV, where x,y dimensions are in µm, and the color axis in mm. Photos courtesy of Space Science Laboratory, UC Berkeley. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

present in significant abundance. (Interestingly, as discussed in section V.A.3, a SiC carrier could arguably have been the source of gases found in a group of seven C2044 blank block samples).

Gas concentrations were monitored after each heating step, so that heating could be halted should sufficient gas concentrations be detected for accurate isotopic analysis prior to reaching maximum temperature. As can be seen from Table 1, stopping the heating steps prior to reaching maximum temperature was rare due to very low gas abundances. Sequence interruptions occurred primarily in just four C2044 wafer analyses (Table 1, C2044,18A, C2044,18B, C2044,19, and C2044,20A). In almost all other cases, the amounts of gas shown in Table 1 reflect the

cumulative gas content of the sample following the 7–9 higher temperature heating steps (~400–1400 °C). A more detailed description of the experimental procedure and data acquisition protocol is described in Pepin et al. (2011)—in particular, details of how the low-abundance ³He and ²¹Ne isotopes were measured and analyzed.

Following the stepped heating protocols described above, helium and neon were analyzed statically in a 3.75" double-focusing Mattauch–Herzog geometry mass spectrometer equipped with pulse-counting detectors (Nier and Schlutter 1985). ³He was not completely resolved from HD⁺-H₃⁺ in this relatively small instrument, but resolution scans over the ³He mass region demonstrated adequate separation for its measurement with no interference. During the heating
<figure><img src="image_6.png" /><figcaption>Fig. 5. Subdivided subsurface slice of C2044 blank block. This sample was immediately below the surface slice sample shown in Fig. 4A, and the sample labeled 3 was kept at UC Berkeley. Slice areal dimensions are ~0.5 × 1 cm before division. The slice thickness of 200 µm was approximately uniform, except along the bottom edge. Photo courtesy of Space Science Laboratory, UC Berkeley.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 6. A) Track 41 block aerogel fragment from the T41 impact site before division into samples and blanks analyzed at UM and CRPG (Table 6). B) One of the UM analyzed fragments (C2044,28,41,0,0) taken from the sample at left, where the circle encloses melted aerogel from T41 wall. Note the 1 mm scale bar. Photos courtesy of Space Science Laboratory, UC Berkeley.</figcaption></figure>

pulses and subsequent analyses in the spectrometer, levels of potentially interfering background species for neon (H₂O, ⁴⁰Ar, CO₂) were controlled at manageable levels by continuous exposure to the LN₂-cooled charcoal trap and, for potential HD⁺-H₃⁺ interferences from hydrogen at ³He, to the room-temperature Westinghouse/WL getter. This procedure did not permit measurements of evolved argon since this heavier noble gas condensed on the charcoal trap and was thus removed from the analyzed He-Ne gas phase.

### III.A.2. CRPG

Stardust-flown aerogel samples, together with nonflight aerogel fragments, were distributed to the CRPG either in glass vials or held between two glass

slides. At the CRPG, the aerogel fragments were handled in a clean room under a microscope using a clean single-hair paintbrush to transfer 8–12 of them at a time into individual pits of the laser chamber, which was subsequently connected to the purification line of the mass spectrometer. The sample chamber was pumped to ultrahigh vacuum and kept at a temperature of ~100 °C overnight. After leaving the samples under high vacuum (P < 10⁻⁸ Torr) for several days prior to analysis, they were heated individually with a continuous-mode infrared CO₂ laser (λ = 10.6 µm) mounted on an x-y stage. The heating procedure was monitored on a TV screen using a CCD camera. Individual aerogel chips were heated to melting (T < 1600 °C) in a single step by very slowly increasing
<figure><img src="image_8.png" /></figure>

Fig. 7. Creating the small blank block aerogel samples: first, a ~1 mm wide 9 × 9 mm square of aerogel was cut from the blank block surface closest to the track 41 block. This was then cut into columns such as those seen in (A) (scale bar divisions, 1 mm). The columns of aerogel were further cut into ~1–1.5 mm³ samples, such as those seen in (B) and (C). The space-exposed surface is at the top of (A). Photos courtesy of Space Science Laboratory, UC Berkeley.

<figure><img src="image_9.png" /></figure>

Fig. 8. A) The 3-D schematic diagram shows the relative positions of the various C2044 samples analyzed. All slices and wafers are ~200 µm thick, except as discussed in the text. For clarity, only the first set of blank block aerogel samples taken from the side of the blank block adjacent to the track 41 block is indicated, and the cell holder is not shown on the right of the blank block or in the back. Wafer 18B is the top half of the wafer furthest from the T41 impact site, while 18A is the lower half. Wafer 19 is the entire wafer between wafers 18 and 20. Wafer 20B is the top half of the wafer closest to the T41 impact site, with wafer 20A the bottom half. B) The other blank block sample locations can be seen in the top view schematic of aerogel cell C2044 and its adjacent cells. In this view, sample numbers begin with the lowest number at the top of the column and increase downward. The blank block surface and subsurface slice samples are not shown. Cell C2028 is the tile adjacent to the left short side of cell C2044, with the right edge of keystone 5 (C2028,13-14,0,0; UM designation C2028-1) approximately 6 mm from the right hand edge of C2028 (see Fig. 9).
<figure><img src="image_10.png" /><figcaption>Fig. 9. Stardust aerogel cell C2028 surface samples. Keystones 1–4 were combined into a single sample, C2028,6-9,0,0 (UM designation, C2028-2). Sample C2028,13-14,0,0 (UM designation, C2028-1) is indicated in the figure as keystone 5 and located on the end of the cell closest to cell C2044 (see Fig. 8B). Note 1 cm scale bar. Surfaces areas of C2028-1 and C2028-2 were ~0.5 cm². Samples were trimmed to a thickness of ~200 µm. Photo courtesy of Astromaterials Acquisition and Curation Office, NASA-Johnson Space Center. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

the power of the CO₂ laser. Because the aerogel fragments vibrated strongly under laser illumination, the sample pits of the laser chamber were covered with an additional ZnSe disk in order to prevent the fragments from jumping out of their pits. This method resulted in slightly increased ⁴He blank levels (see below). Evolved helium and neon were analyzed in a Micromass© VG5400 static mass spectrometer. The VG5400 has a mass resolution of ~600, and the double peak of HD⁺-H₃⁺ was completely separated from ³He⁺.

### III.B. Calibrations and Blanks

#### III.B.1. UM

Sensitivities for helium and neon were derived from repeated analyses of air samples metered into the system from calibration cans designed to permit accurate volumetric splitting, with O₂, N₂, and other chemically active constituents removed by gettering. Neon isotopic discriminations relative to the standard air neon composition (Eberhardt et al. 1965) were also derived from these metered air samples. A new helium standard prepared in France by ChemGas served as the helium isotopic standard, with ³He/⁴He = 6.6 × 10⁻⁴. This ³He/⁴He mixing ratio has been certified to ±1% by mass spectrometric analysis at the Laboratoire de Speciation des Radionucleides et des Molecules, Commissariat à l'Energie Atomique, Saclay. Typical

sensitivities for the mass spectrometer were 1.55 × 10⁻¹³ cm³STP/cps for helium and 3.64 × 10⁻¹³ cm³STP/cps for neon.

System and procedural blanks were measured in three different ways. All samples were reheated following the same protocol employed for the initial seven to nine heating steps (see section III.A). Sample gas concentrations and isotopic compositions evolved on reheating were compared to those produced when empty Pt foils were heated following the same protocol. There were no statistically significant differences between the two sets of analyses. They therefore provided background blank abundances for each noble gas isotope, for corrections to the amounts released during the initial sample heatings. In addition to these sample reheats and measurements on heated empty Pt foil packets, many “cold” blank runs were carried out where all sample heating procedures were followed, but without the heating. These cold blank analyses also yielded noble gas abundance and isotopic values statistically identical to those from the sample reheats and hot foil blank measurements.

These blank values varied over the extensive period of the analyses (Table 1), trending downward over time (especially ²⁰Ne), but in general were extremely consistent within the time frame of a set of samples loaded together. A representative range in cm³STP for blank values was as follows, with the lower values more
<figure><img src="image_11.png" /><figcaption>Fig. 10. A) Stardust aerogel cell C2086, with locations of samples indicated. Close-ups of tracks T181 (B) and T182 (C), with the length of each track and the incident particle entry direction indicated. T181, a Type A “carrot” track, and T182, a Type C (or B) track, were removed and analyzed in toto, with data in Table 9 and <sup>20</sup>Ne concentrations plotted in Fig. 23. Photos courtesy of Space Science Laboratory, UC Berkeley. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

typical of measurements made after 2012: <sup>3</sup>He = (1.4–2.6) × 10<sup>-15</sup>, <sup>4</sup>He = (2.4–7.1) × 10<sup>-12</sup>, and <sup>20</sup>Ne = (0.53–2.1) × 10<sup>-12</sup>.

Blank corrections were most critical for <sup>3</sup>He and <sup>21</sup>Ne, where blank contributions could be as much as 80–90% of the signal in cases where the sample had very little gas. Fortunately, as noted above, background blank measurements were very consistent within a set of samples, especially for both of these isotopes, and indeed over the entire 6+ years between the first and last of the sample runs recorded in Table 1. Uncertainties due to blank corrections are included in errors listed for all abundance and isotope ratio data. Measurement precisions of the listed <sup>3</sup>He/<sup>4</sup>He and <sup>21</sup>Ne/<sup>22</sup>Ne ratios in samples with relatively large blank contributions depend sensitively on correct assessment of their levels and statistical variations and on the stability of their sources. A detailed analysis of the helium blank sources

in the UM extraction and analysis system is given in Pepin et al. (2000).

A typical neon analysis also required consideration of possible additional corrections for isobaric interferences at mass 20 from H<sub>2</sub><sup>18</sup>O and <sup>40</sup>Ar<sup>+</sup><sup>+</sup>, and at mass 22 by <sup>12</sup>C<sup>16</sup>O<sub>2</sub><sup>++</sup>. abundances of masses 18, 40, and 44 were always monitored throughout the heating and analysis procedures, and all were reduced to acceptably low levels by LN<sub>2</sub> cryopumping. Furthermore, the electron energy in the ionization source of the UM mass spectrometer was set at 35 eV for neon analyses, a value experimentally determined to be more than low enough to eliminate double ionization of masses 40 and 44. Therefore, <sup>20</sup>Ne was determined only by correcting the mass 20 signal for H<sub>2</sub><sup>18</sup>O by subtracting 1/499 of the mass 18 value (assumed to be entirely due to H<sub>2</sub><sup>16</sup>O), and no correction was made at mass 22 to determine <sup>22</sup>Ne. Similarly, the possibility
<figure><img src="image_12.png" /><figcaption>Fig. 11. Top view of a portion of Stardust aerogel cell C2086, with relative locations of track T182 and attached blank 2 fragment sample. The track T182 impact site is the dark circular feature just below the center of the image and the horizontal needle. The average diameter of the roughly spherical blank 2 sample is approximately 0.6 mm. Photos courtesy of Space Science Laboratory, UC Berkeley. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

that $H_3^{18}O^+$ contributed significantly to the mass 21 signal was evaluated by subtracting 1/499 of the typically very low mass 19 value, assumed to be entirely due to $H_3^{16}O^+$. In virtually all cases, this correction made no change in the $^{21}$Ne value to three significant figures.

### III.B.2. CRPG

Extracted gases were purified using Ti sponge getters and two hot (400 °C) and cold (room temperature) SAES™ Ti-Al getters. Any argon was separated from helium and neon by adsorption onto a stainless steel frit at 77 K. The contents of the frit were not analyzed. Helium and neon abundances, as well as $ ^{3}He / ^{4}He $ ratios, were determined on the electron multiplier of the VG5400 mass spectrometer. Standard HESJ (Helium Standard of Japan) of Matsuda et al. (2002), with a $ ^{3}He / ^{4}He $ ratio of (2.847 ± 0.014) × 10⁻⁵ (i.e., 20.63 ± 0.10 $R_A$, where $R_A$ is the terrestrial air ratio), was used to determine the analytical sensitivity and reproducibility for helium. Air aliquots were used as a neon standard. One standard deviation (stdev) reproducibilities of standard abundance measurements were 2% for helium and 3% for neon. Typical sensitivities for the mass spectrometer were 4.6 × 10⁻¹⁵ cm³STP/cps for helium and 3.0 × 10⁻¹⁵ cm³STP/cps for neon.

Interferences from $ ^{40}Ar^{++} $ and $ CO_2^{++} $ at masses 20 and 22, respectively, were minor. Neon was exposed to a

charcoal finger at 77 K and a SAES™ AP-10 getter during analysis to minimize the contribution of doubly charged $ ^{40}Ar $ and $ CO_2 $ to the $ ^{20}Ne $ and $ ^{22}Ne $ signals. The $ ^{40}Ar^{++} $ contribution to $ ^{20}Ne $ was carefully monitored and was found to be negligible (i.e., ≤5 cps). The $ CO_2^{++} $ signal of all samples was comparable to the mass spectrometer background, and the contribution of $ CO_2^{++} $ was corrected as part of the blank.

Blanks were thoroughly monitored and sample analyses started only when the “cold” blanks were comparable to those of the purification line alone. “Cold” $ ^{4}He $ blanks (with the laser off) were indistinguishable from those obtained with the laser illuminating a metal surface adjacent to the samples and averaged (2.67 ± 0.25) × 10⁻¹¹ cm³STP. However, $ ^{4}He $ blanks were higher for measurements with the additional ZnSe disk, increasing to (5.3 ± 1.9) × 10⁻¹¹ cm³STP. The $ ^{20}Ne $ blanks determined in this way ranged from 2.9 to 14.3 × 10⁻¹¹ cm³STP for the different analytical sessions.

### III.C. Aerogel Sample Preparation and Removal of Potential Contaminants

The initial step of aerogel sample preparation for noble gas analyses at both UM and CRPG was the extraction of keystones, wafers, and blocks from aerogel tiles using the keystoning method (Westphal et al. 2004). A sharp glass needle mounted on a computer-
<figure><img src="image_13.png" /><figcaption>Fig. 12. T108, a 1.32 cm long track from aerogel cell C2081. A) A close-up of the part of T108 containing the analyzed terminal particles C2081,1,108,12-14,0. Terminal particle C2081,1,108,12,0 (B) has melted aerogel attached to the left surface of the particle. This particle was analyzed individually. Terminal particle C2081,1,108,13,0 (C) was packaged together with C2081,1,108,14,0 (D) in a single Pt foil for joint analysis. The scale bar for all individual terminal particles is 30 µm. Photos courtesy of K. Nakamura-Messenger, NASA-Johnson Space Center. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

controlled micromanipulator was used at SSL, Berkeley
as a tool to “machine” the aerogel with ~1 µm
precision. The second step of sample preparation was
division of keystones, wafers, and blocks into smaller
sections, which for UM analyses were then folded into
Pt foils and for CRPG analyses were placed between
two glass slides or into glass vials. Here divisions were
done manually utilizing a surgical steel blade on a glass
back-surface mirror. Surfaces and tools were cleaned
carefully before and during sample preparation using
cleanroom wipes and isopropyl alcohol.

Because aerogel is highly mobile in powdered form,
completely clean separation of aerogel samples was not
practically possible, at least with current techniques.
The keystoning process produces a small volume of fine
aerogel powder from the ~10 µm wide kerf of the cut,

which is impractical to contain and can move significant
distances on the aerogel tile, mobilized by air currents
and static electric charge. Divisions with the surgical
blade also release small particles of aerogel which can
be distributed to neighboring sections of the keystones,
wafers, or blocks during division for noble gas analysis.
Although significant efforts were made to minimize the
spread of fine material, it may have been transported on
and off gloved hands, facemasks, or the microscope at
the sample preparation station. Much of the distributed
aerogel powder was probably extremely small in size, of
order 1 µm or less. The possibility that some of the
blank block samples analyzed at UM and CRPG might
have been cross-contaminated by distribution of noble
gases in this cutting dust is discussed in sections V.A.1.2
and V.B.2.
<figure><img src="image_14.png" /><figcaption>Fig. 13. A keystone sampled from the surface of tile I1047 from the Stardust interstellar collection. The keystone was divided in half, along a line parallel to the original surface of the aerogel tile. A) The deepest part of the keystone, sampling a depth of 600–1250 µm. B) The top half of the keystone, sampling from the space-exposed surface to 600 µm. Photos courtesy of Space Science Laboratory, UC Berkeley.</figcaption></figure>

It has been shown that aerogel contains large concentrations, in units of cm³STP/g-aerogel, of presumably atmospheric heavy noble gases

(Mohapatra et al. 2013; O'Mara et al. 2014). No data for He-Ne concentrations are reported in these experiments. The Stardust aerogel samples analyzed in
this study undoubtedly contained absorbed atmospheric Ar-Xe, and possibly He-Ne as well, upon arrival for analysis. However, regardless of what fraction of an atmospheric gas component might have survived the UM and CRPG protocols of preanalysis exposure to high vacuum for several days and heating at low temperatures (see the sections III.A and V.A.1.1), it would then be exposed, along with the rest of the gases released from the samples, to charcoal, finger, or frit traps at 77 K and various types of high- and low-temperature getters in the two laboratories. Only very small, and consistent, amounts of heavier masses such as 40 and 44 were detected in system and procedural blanks and entered the mass spectrometers. There is no evidence that helium and neon released from gas-bearing samples derive from aerogel air contamination. An exception to the generally low levels of background gases was the appearance of very high abundances of H₂O, Ar, CO₂, and hydrocarbons, sited in the UM aerogel wafer C2044,20 and released in its analysis (see section IV.A.1). The compositions of the accompanying helium and neon were decidedly nonterrestrial, demonstrating that the source could not have been an aerogel air contaminant. What it actually was is an open and interesting question. This incident of major volatile release prompted addition of an auxiliary clean-up step to the UM gas processing protocols in which condensable background gases were sequestered on a LN₂-chilled steel finger during step-heating (see section IV.A.1). Any residuals eluding the cold finger were exposed to LN₂ cryopumping and, as usual, were continuously monitored throughout the helium and neon analyses.

That these procedures were successful in eliminating possible contamination by atmospheric light noble gases is evident from the observations that 70% of the UM “blank block pieces” in Table 1 and 40% of those examined at CRPG (Table 2) contained undetectably low abundances of helium and neon, as did a UM flight spare aerogel sample (see section V.A.1.1). Blank block samples that did release measurable light gas abundances exhibit distinctly nonterrestrial isotope ratios (see section IV.A.5).

An additional constraint on levels of air contamination is provided by O'Mara et al.'s (2014) study of xenon release by etching of aerogel. The authors estimated that only ~1% of the ²⁰Ne reported in Stardust aerogel by Marty et al. (2008) could be due to an absorbed terrestrial component. It appears that the flown aerogel cells did not retain significant levels of absorbed terrestrial He-Ne contaminants after their laboratory exposures to vacuum and heat (see section V.A.1.1).

# IV. RESULTS AND DISCUSSION

## IV.A. Cell C2044 Samples

### IV.A.1. Blank Wafer Samples

The first sample set analyzed following the T41 samples reported in Marty et al. (2008) were blank wafer samples cut vertically to the surface from the end of the blank block furthest from T41. These samples were not expected to contain high concentrations of noble gases, or indeed any gas indigenous to the sample, since they contained only a small cross-sectional area of space-exposed surface and had no visible tracks or particle fragments. The first of the samples analyzed, wafer C2044,20A (Table 1, Analyses 1–6), was from the bottom half of the wafer (as seen from the space-exposed surface) cut innermost from the end of the blank block (Fig. 8). This sample proved extraordinary in several ways, and therefore its analytical procedures (and difficulties) are detailed below.

The step-heating protocol allows the released gas to be quickly monitored, and the temperature estimated, after each 10 s step. Following just the second heating step for wafer 20A, the helium and neon gas concentrations were surprisingly more than sufficient for analysis, so the heating was terminated. The sample Pt foil packet had just started to glow at the end of the second 10 s pulse, so the peak temperature reached was ~750 °C, and the average temperature was well below that for the 10 s. This initial, unexpected gas release from wafer 20A prompted us to begin the next heating of the sample with a lower step increase than the interval prescribed by the default heating protocol, increasing the temperature to only ~800 °C. Despite this precaution, releases of helium and neon were extremely large. Unfortunately, large water, CO₂, and hydrocarbon abundances were also inadvertently admitted to the mass spectrometer during the gas monitoring stage of this step. The high pressure produced by these background species induced release of embedded ions near the beam slit, producing a “memory” contribution that allows only limits to be placed on the actual helium and neon isotopic compositions. The same problem arose during the next single heating step before the monitoring protocol was altered to address this unexpected development. This next step, to ~890 °C, produced lower but still large helium and neon abundances. Taken together, the 800 °C and 890 °C heatings degassed more helium, and ~15× more neon, than seen in any other Stardust sample analyzed in this study.

Before discussing the significance of the observed results, we emphasize that although large background water and hydrocarbon peaks were measured during the
brief gas monitoring stage for the 800 and 890 °C heating steps of wafer 20A, background pressures and concentrations by the time of the actual helium and neon analyses were reduced to values similar to those seen in typical Stardust sample analyses. The clean-up procedures employed on any sample's gas before isotopic analysis were successful in removing the large abundances of background species seen in 20A. Thus, the isotopic compositions shown in Table 1 accurately reflect the compositions of helium and neon in the mass spectrometer. However, in these two heating steps, the measured compositions are two-component mixtures of a pressure induced memory component and the actual sample gas. We extensively investigated the memory effects produced by high backgrounds of water, CO₂, and hydrocarbons by experimentally re-creating similar pressures and backgrounds within the mass spectrometer in two different ways, either by heating the system's charcoal finger used as a clean-up reservoir at LN₂ temperatures, or by introducing air spikes. In these experiments, after producing background gases from one of these two sources, the gases were frozen out on a bare cold finger at LN₂ temperature. Raising and lowering the level of LN₂ on the cold finger produced a series of different pressures (monitored with a convectron gauge) and backgrounds within the mass spectrometer. The clean-up and isotopic measurement protocols used for helium and neon from actual samples were followed after each of these experiments. The memory compositions measured in this way were
$$ ^{3}\text{He}/^{4}\text{He} = (3.39 \pm 0.11) \times 10^{-4}, \ ^{20}\text{Ne}/^{22}\text{Ne} = $$

$^{21}\text{Ne}/^{22}\text{Ne} = 0.0380 \pm 0.0020$, and $^{4}\text{He}/^{20}\text{Ne} = 53 \pm 2$. Corresponding ratios in terrestrial air are $^{3}\text{He}/^{4}\text{He} = 1.38 \times 10^{-6}$, $^{20}\text{Ne}/^{22}\text{Ne} = 9.805 \pm 0.075$, $^{21}\text{Ne}/^{22}\text{Ne} = 0.0290 \pm 0.0003$, and $^{4}\text{He}/^{20}\text{Ne} = 0.32$ (Table 3). The memory compositions are not surprising, given the many years of helium and neon analyses of lunar, meteoritic, and interplanetary dust particle samples in this mass spectrometer during the history of the UM laboratory.

The isotopic signatures of the noble gases released in the 800 and 890 °C heating steps of wafer 20A are unique. The amounts of helium were so large (~7.6 × 10⁻⁹ cm³STP) that the $^{3}\text{He}/^{4}\text{He}$ isotopic composition could be determined with high precision for each step, with an overall ratio for the two steps of (2.214 ± 0.022) × 10⁻⁴ (Table 4; Fig. 14). Given that the gas is a two-component mixture of sample and memory gases, and that the memory gas has a higher $^{3}\text{He}/^{4}\text{He}$ isotopic ratio, this is an upper limit on the $^{3}\text{He}/^{4}\text{He}$ value from the sample itself. Although it is impossible to know the exact mixing ratio of the two components, by using the $^{4}\text{He}/^{20}\text{Ne}$ ratio in the memory gas it is possible to make an upper limit estimate. The memory $^{4}\text{He}/^{20}\text{Ne}$ ratio is 53.0 ± 2.0, and with the measured ratio of 17.9 ± 1.0, that suggests the memory component can be at most approximately one-third of the total (for memory-corrected $^{4}\text{He}/^{20}\text{Ne} = 0$). A separate calculation based on measured abundances of memory helium and neon at high background water and hydrocarbon pressures also indicates that the memory component in these

<table><caption>Table 3. Isotopic compositions and elemental ratios of helium and neon in the solar wind, meteoritic Q and HL components, and Jupiter and terrestrial atmospheres.</caption><thead><tr><th><sup>3</sup>He/<sup>4</sup>He × 10<sup>-4</sup></th><th><sup>20</sup>Ne/<sup>22</sup>Ne</th><th><sup>21</sup>Ne/<sup>22</sup>Ne</th><th><sup>4</sup>He/<sup>20</sup>Ne</th><th>References</th></tr></thead><tbody><tr><td rowspan="2">4.64 ± 0.09</td><td>14.00 ± 0.04, 13.97 ± 0.03</td><td>0.0336 ± 0.0002</td><td>641 ± 15</td><td>[1, 2]</td></tr><tr><td>13.78 ± 0.03</td><td>0.0329 ± 0.0001</td><td>656 ± 5</td><td>[3]</td></tr><tr><td>1.23, 1.43<sup>b</sup></td><td>10.05 ± 0.05, 10.7 ± 0.2<sup>b</sup></td><td>0.0278<sup>c</sup> - 0.0294</td><td>112 ± 10<sup>d</sup></td><td>[4]</td></tr><tr><td>1.70 ± 0.10</td><td>~8.50 ± 0.06</td><td>0.036 ± 0.001</td><td></td><td>[5]</td></tr><tr><td>1.66 ± 0.05</td><td>13 ± 2</td><td></td><td>6830</td><td>[6]</td></tr><tr><td>0.0138</td><td>9.80 ± 0.08</td><td>0.0290 ± 0.0003</td><td>0.32</td><td>[7]</td></tr></tbody></table>

<footnote><sup>a</sup>Summarized in Pepin et al. (2012), Table 7. Original data sources in References column.</footnote>

<footnote><sup>b</sup>Two distinct values.</footnote>

<footnote><sup>c</sup>Lower limit.</footnote>

<footnote><sup>d</sup>Average.</footnote>

<footnote>References: [1] Pepin et al. (2012); [2] Meshik et al. (2007); [3] Heber et al. (2009); [4] Busemann et al. (2000); [5] Huss and Lewis (1994); [6] Mahaffy et al. (2000); [7] Eberhardt et al. (1965) for Ne composition.</footnote>
<table><caption>Table 4. Wafer C2044,20A 800 and 890 °C heating steps with mass spectrometer memory component.</caption><thead><tr><th>Wafer C2044,20A<br>Heating step</th><th><sup>3</sup>He/<sup>4</sup>He<br>× 10<sup>-4</sup></th><th><sup>4</sup>He<br>× 10<sup>-10</sup> ccSTP</th><th><sup>20</sup>Ne/<sup>22</sup>Ne</th><th><sup>21</sup>Ne/<sup>22</sup>Ne</th><th><sup>20</sup>Ne<br>× 10<sup>-12</sup> ccSTP</th><th><sup>4</sup>He/<sup>20</sup>Ne</th></tr></thead><tbody><tr><td>800 °C</td><td>2.203 (23)</td><td>53.6 (02)</td><td>18.66 (74)</td><td>0.0375 (18)</td><td>319 (06)</td><td>16.8 (05)</td></tr><tr><td>890 °C</td><td>2.241 (48)</td><td>22.3 (04)</td><td>17.49 (40)</td><td>0.0408 (28)</td><td>106 (02)</td><td>21.0 (06)</td></tr><tr><td>Integrated 800, 890 °C steps</td><td>2.214 (22)</td><td>75.9 (04)</td><td>18.35 (55)</td><td>0.0384 (15)</td><td>425 (06)</td><td>17.9 (10)</td></tr><tr><td>Memory component</td><td>3.39 (11)</td><td></td><td>11.40 (33)</td><td>0.0380 (20)</td><td></td><td>53.0 (20)</td></tr></tbody></table>

Numbers in parentheses are the uncertainty in the last two digits of the isotopic ratio or abundance.

<figure><img src="image_15.png" /><figcaption>Fig. 14. Helium abundances and isotopic compositions in C2044 wafer samples. Open symbols represent the integrated, weighted sum of all of the given wafer's helium. Peak step heating release temperatures are indicated, as are the Jupiter and solar wind compositions (Table 3) along with their respective uncertainties. As discussed in the text, the 800 °C, 890 °C, and integrated composition shown for wafer 20A represent upper limits for those values. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

two heating steps was ~1/3 of the measured abundances. Assuming this maximum memory contribution yields a lower limit to the $ ^{3}\text{He}/^{4}\text{He} $ ratio of $ 1.61 \times 10^{-4} $.

The enormous amounts of neon released in the two heating steps again allow the isotopic composition of the two-component gas mixture to be determined with higher precision than is typical for our Stardust samples (Fig. 15). The $ ^{20}\text{Ne}/^{22}\text{Ne} $ ratio in the sample must be greater than 18 (Table 4), far higher than that seen in the memory component, Earth's atmosphere, or the solar wind (Table 3). If an upper limit contribution by the memory component is assumed, as above, to be ~1/3, the $ ^{20}\text{Ne}/^{22}\text{Ne} $ ratio would have a maximum value of approximately 21.9 (Fig. 16). The measured $ ^{21}\text{Ne}/^{22}\text{Ne} $ value is not significantly affected in this calculation

since the memory and measured values are the same within their uncertainties.

We have dubbed this putative helium and neon component as “Sample X” sited within wafer 20A (Palma et al. 2010). Its memory-corrected compositional ranges trend toward those measured in anomalous IDPs from the 26P/Grigg-Skjellerup and 55P/Tempel-Tuttle comet collections, attributed to origin in ejecta of a neon (ONe) nova (see section V.A.3; Pepin et al. 2011).

After the unexpected issues with background gas emitted during the 800 and 890 °C heating steps, the monitoring protocol was changed. During the heating and monitoring process, gases had been exposed to a charcoal finger at $ \text{LN}_2 $ temperatures and a room-temperature Westinghouse/WL getter, but obviously
<figure><img src="image_16.png" /><figcaption>Fig. 15. A neon three-isotope plot of the C2044 wafer samples. Open symbols represent the integrated, weighted sum of all of the given wafer’s neon. Peak step heating release temperatures are indicated, as are the solar wind (SW) and terrestrial atmosphere (ATM) compositions (Table 3). As discussed in the text, the mass spectrometer memory composition (MS memory, Table 4) is shown, and the 800 °C, 890 °C, and integrated compositions shown for wafer 20A represent lower limits for those values. Due to the large uncertainties, data for wafer 19 are not shown in the figure, and wafer 20B had no detectable $^{21}$Ne.</figcaption></figure>

exposure to these clean-up devices had been insufficiently long to freeze out or remove all nonnoble gas species released by the sample. An additional step was added to the heating protocol of first exposing the evolved sample gas to a cold finger at $LN_2$ temperature for 1 min after the 10 s heating, then sealing off the cold finger prior to admitting the remaining gas into the mass spectrometer for a brief check of the helium and neon gas amounts. As before, during this monitoring process the gas was also continuously exposed to a charcoal finger at $LN_2$ temperatures and the getter. This additional step completely eliminated any potential high background in all future samples and from higher temperature heating steps of wafer 20A.

Because of the surprising amounts of noble gases released from wafer 20A up to the 890 °C point in the heating cycle, smaller heating step intervals than usual were employed when increasing the temperature further. Sharply lower amounts of helium and neon were released in the five heating steps from 950 to 1120 °C and the seven heating steps from 1150 to 1410 °C.

Isotopic compositions of helium and neon from these higher temperature steps were respectively higher and somewhat lower than in the 800–890 °C component (Table 1, Analyses 2–5), suggesting mixing of minor residuals of this component with an isotopically less extreme composition, perhaps originating from a different carrier within the wafer. The integrated isotopic composition of 20A for all temperature steps is dominated by the 800 and 890 °C releases: the upper limit to the overall $ ^{3}\text{He}/^{4}\text{He} $ value is (2.265 ± 0.022) × 10$^{-4}$, and the overall lower limit to $ ^{20}\text{Ne}/^{22}\text{Ne} $ is 18.03 ± 0.50. The integrated $ ^{21}\text{Ne}/^{22}\text{Ne} $ ratio of 0.0383 ± 0.0015 is within the uncertainty of the memory component, so it is not possible to distinguish between the sample and memory gas in this case.

The second wafer analyzed was C2044,18A, cut from the outside edge of the blank block adjacent to the foil lining and in the corner of Cell C2044 (Fig. 8). As for wafer 20A, 18A did not contain a space-exposed surface. The overall temperature release profile was similar to 20A but gas amounts were far smaller.
<figure><img src="image_17.png" /></figure>

Fig. 16. The $^{20}$Ne/$^{22}$Ne ratio as a function of $^{4}$He/$^{20}$Ne in C2044 wafer and blank block samples. IDPs show a wide range of $^{4}$He/$^{20}$Ne [~ (0–250)], with $^{20}$Ne/$^{22}$Ne ratios typically between 10 and 14 (Pepin et al. 2011). This is in sharp contrast to anomalous IDPs (Pepin et al. 2011), where high $^{20}$Ne concentrations restrict $^{4}$He/$^{20}$Ne ratios to values between zero and 20, and produce $^{20}$Ne/$^{22}$Ne ratios greater than the solar wind value of ~14 (Table 3). Both Sample “X” (wafer C2044,20A) and blank block C2044,73 fit the anomalous IDP pattern. The lower $^{20}$Ne/$^{22}$Ne limit for Sample “X” comes from uncertainties in the sample analysis and blank corrections, whereas the upper limit derives from the uncertainty in the proportion of mass spectrometer memory gas (Table 4) contribution.

Although the helium and neon isotopic compositions begin and end similar to wafer 20A (Table 1, Analyses 7–9), the dominant “Sample X” component in 20A is clearly missing (Figs. 14 and 15). In any case, there are again surprising amounts of undoubtedly extraterrestrial noble gases that do not have any obvious source. Both of these wafers lie more than 4.5 mm from the surface of Tile 44 and greater than 5 mm from the axis of T41.

The other wafer samples do not bring clarity to the situation. Wafer C2044,19 was the largest sample in mass; the entire wafer was wrapped in a single foil packet, as opposed to wafers 18 and 20 that were split roughly in half. Wafers 18B, 19, and 20B each contained a space-exposed upper edge. Although the helium isotopic composition is similar in 18A, 18B, and 19 (Fig. 14), there is no detectable helium within 20B, and none of these neighboring wafers released either the high abundances or the compositions of helium and neon in 20A. Neon contents were so sparse in 19 and 20B that isotopic compositions are very uncertain. Overall neon abundances in 18A and 18B are comparable but of distinctly different integrated compositions, neither of which is similar to that in

“Sample X” (Figs. 15 and 16). Interestingly, the $^{20}$Ne/$^{22}$Ne ratio in 18B matches the Q-Ne value (Table 3) seen in T41 samples (Marty et al. 2008), although the $^{21}$Ne/$^{22}$Ne value is ~1.5σ below Q-Ne. The elemental ratio $^{4}$He/$^{20}$Ne in wafer 18B is also comparable to those observed in T41. However, the similarities between 18B and T41 do not extend to the helium isotopic composition, where the 18B value of $^{3}$He/$^{4}$He = (3.44 ± 0.28) × 10⁻⁴ falls well outside the ~ (2.5–2.9) × 10⁻⁴ range observed in T41 (Marty et al. 2008). The Q-like neon signatures seen in wafer 18B and several other Stardust samples are evaluated and discussed in section VI.B.

### IV.A.2. T41 Cross-Sectional Slices

Two cross-sectional transverse slices (slices 17 and 18) were made through T41 and the track 41 block (Fig. 8A). Aerogel was trimmed from slices 17 and 18 to exclude material further from the T41 impact site, creating samples C2044,17,41,0,0 and C2044,18,41,0,0, respectively. Individual pieces from slice 17 and a piece of slice 18 containing melted aerogel from T41 were combined to form sample C2044,17&18,41,0,0 (Fig. 3).
<table><caption>Table 5. He and Ne in Track 41 block transverse cross-sectional slice samples.</caption><thead><tr><th>Sample identification: C2044,</th><th><sup>3</sup>He/<sup>4</sup>He<br>× 10<sup>-4</sup></th><th><sup>4</sup>He<br>× 10<sup>-10</sup> ccSTP</th><th><sup>20</sup>Ne/<sup>22</sup>Ne</th><th><sup>21</sup>Ne/<sup>22</sup>Ne</th><th><sup>20</sup>Ne<br>× 10<sup>-12</sup> ccSTP</th><th><sup>4</sup>He/<sup>20</sup>Ne</th></tr></thead><tbody><tr><td colspan="7"><strong>UM samples</strong></td></tr><tr><td>17,41,0,0 (integrated)</td><td>3.83 (39)</td><td>1.13 (02)</td><td>6.23 (96)</td><td>0.066 (18)</td><td>5.85 (11)</td><td>19.7 (29)</td></tr><tr><td>17&18,41,0,0</td><td>2.7 (12)</td><td>0.131 (02)</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>0.366 (21)</td><td>35.8 (21)</td></tr><tr><td>18,41,0,0</td><td></td><td>n.d.</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>0.26 (05)</td><td></td></tr><tr><td colspan="7"><strong>CRPG samples</strong></td></tr><tr><td>8,41,0,0</td><td>1.00 (25)</td><td>0.74 (24)</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>5.22 (37)</td><td>14.3 (47)</td></tr><tr><td>9,41,0,0</td><td>3.50 (66)</td><td>2.07 (29)</td><td></td><td></td><td>n.d.</td><td></td></tr></tbody></table>

Numbers in parentheses are the uncertainty in the last two digits of the isotopic ratio or abundance. n.d. = not detected above blank.

Comparisons with results from the previous T41 samples indicate that noble gas carriers are distributed highly heterogeneously along the track (Palma et al. 2009). C2044,17,41,0,0 had a <sup>4</sup>He abundance similar to that observed in St-1 and St-2 from our earlier study (Marty et al. 2008), but the integrated <sup>3</sup>He/<sup>4</sup>He ratio is significantly higher, (3.83 ± 0.39) × 10<sup>-4</sup>, compared to a T41 average of (2.7 ± 0.3) × 10<sup>-4</sup>, and the other two samples either had no detectable helium (C2044,18,41,0,0), or so little (C2044,17&18,41,0,0) that only a very imprecise isotopic ratio could be determined. Likewise, only in C2044,17,41,0,0 was there sufficient neon for isotopic measurements. <sup>21</sup>Ne/<sup>22</sup>Ne suggests a potential spallation component not observed in earlier T41 measurements (see section VI.D). An interpretation of the very low <sup>20</sup>Ne/<sup>22</sup>Ne ratio is proposed in section VI.C. Particularly noteworthy is that the Q-like neon composition reported earlier by Marty et al. (2008) is distinctly missing. Also, unlike the previous pyrolyses of T41 samples where gas release did not occur until T > 1250 °C, in these analyses gas release peaked at ~1000 °C. Compared to C2044,17,41,0,0, very similar helium and neon abundances and isotopic ratios were reported by Füri and Marty (2012) for two other track 41 block transverse slices located immediately above UM slices 17 and 18 (Table 5; Fig. 8A).

### IV.A.3. Blank Block Surface and Subsurface Samples

Three variable thickness surface slice fragments from the blank block of aerogel cell C2044 were individually analyzed to explore the possibility that Stardust tile surfaces either may have incorporated implanted solar wind gases from oblique exposure to the solar wind during interstellar particle collection (see section V.B.2), or contain embedded submicron dust from the comet. The combined <sup>3</sup>He/<sup>4</sup>He signature in these samples is consistent with solar wind, but gas amounts were only approximately twice blank values, precluding a precise isotopic determination. However,

they do display prominent isotopic anomalies in neon; large <sup>21</sup>Ne enhancements are apparent in blank block surface slices St-C44s1, 1&3 (Table 1) and also in the track 41 block cross-sectional slice 17 (see section IV.A.2). Similar <sup>21</sup>Ne excesses have been observed and more precisely measured in surface slice samples from other cells by Meshik et al. (2009, 2010a, 2010b). They imply implausibly long exposure ages for their carriers if attributed to spallation by GCR irradiation alone. This issue is discussed in sections V.A.2 and VI.D.

### IV.A.4. T41 Fragment Samples

The three fragments (C2044,28,41,0,0 and C2044,30-31,41,0,0) containing melted aerogel from the T41 wall had undetectable amounts of helium and neon. However, there was sufficient helium in the one “blank” piece of aerogel sent with the samples (“δ2 blank,” Table 6) for an isotopic measurement, although again no neon above background was detected. The helium isotopic composition in “δ2 blank” is consistent with solar wind, as opposed to that seen in T41 (Table 7). Similar results for <sup>4</sup>He content were obtained on companion samples from the same location analyzed at CRPG (Füri and Marty 2012) (Table 6). They reinforce

<table><caption>Table 6. He and Ne in T41 fragment samples.</caption><thead><tr><th>Sample<br>identification: C2044,</th><th><sup>3</sup>He/<sup>4</sup>He<br>× 10<sup>-4</sup></th><th><sup>4</sup>He<br>× 10<sup>-10</sup> ccSTP</th><th><sup>20</sup>Ne<br>× 10<sup>-12</sup> ccSTP</th></tr></thead><tbody><tr><td colspan="4"><strong>UM samples</strong></td></tr><tr><td>28,41,0,0 (“α”)</td><td></td><td>n.d.</td><td>n.d.</td></tr><tr><td>30,41,0,0 (“δ1”)</td><td></td><td>n.d.</td><td>n.d.</td></tr><tr><td>31,41,0,0 (“δ2”)</td><td></td><td>n.d.</td><td>n.d.</td></tr><tr><td>“δ2 blank”</td><td>4.24 (53)</td><td>0.503 (20)</td><td>n.d.</td></tr><tr><td colspan="4"><strong>CRPG samples</strong></td></tr><tr><td>29,41,0,0 (“β”)</td><td><sup>3</sup>He n.d.</td><td>0.62 (20)</td><td>n.d.</td></tr><tr><td>32,41,0,0 (“δ”)</td><td>0.44 (12)</td><td>0.56 (18)</td><td>n.d.</td></tr></tbody></table>

Numbers in parentheses are the uncertainty in the last two digits of the isotopic ratio or abundance. n.d. = not detected above blank.
the conclusion that “blank” pieces of aerogel may contain as much, or more, light noble gases relative to those in melted aerogel “samples” from the T41 wall. This also supports the observation made in section IV.A.2 for the T41 cross-sectional slices that the noble gas carriers are heterogeneously distributed within the T41 wall.

### IV.A.5. Blank Block Samples

In view of the gas abundances observed in the wafer samples and their unusual isotopic compositions, especially in wafer C2044,20A (“Sample X”), the UM group embarked on a systematic study of a large set of smaller aerogel fragments (“blank block aerogel pieces” in Table 1) cut from the body of the blank block following extraction of the wafers and surface/subsurface slices. These samples were excised from various depths below the blank block surface and distances from the T41 axis. They were relatively uniform in size, with volumes in the ~1–3 × 10⁻³ cm³ range. Seventy of them were analyzed at UM over the course of the study. None contained any visible particle fragments. This large suite of samples, smaller than the wafers, allowed locations of any detected noble gases to be mapped with relatively high spatial resolution. Of these 70, 48 contained no helium or neon above the uncertainty of the background blank. Data from 13 of

the 70 that contained measurable ⁴He and ³He/⁴He ratios (or in two cases, upper and lower limits) are given in Table 8A.

A similar, though smaller, aliquot of 19 blank block samples was analyzed at CRPG. These were randomly selected from the same sample set prepared at the Space Sciences Laboratory, UC Berkeley for the UM measurements (see section III.C). A different degassing technique, by single-step CO₂ laser heating, was applied to the samples examined at CRPG. Seven of the 19 (~37%, compared to ~20% of the UM samples) contained ⁴He and measurable ³He/⁴He (or in one case an upper limit); data for these are tabulated in Table 8B. Both the UM and CRPG data sets reveal unexpected noble gas structures within the blank block. Possible sources for some of these gases are discussed in section V.A.3.

The locations of the gas-bearing blank block fragments relative to the space-exposed surface of the blank block and to the axis of T41 are listed in Table 8. Each of these samples is remarkable in its own right, regardless of isotopic composition, if for no other reason than it was located far from potential gas sources in the cell surface and T41, and yet contained measurable amounts of helium and, in some cases, of neon as well. Isotopic compositions confirm that these noble gas pockets cannot be terrestrial contaminants.

<figure><img src="image_18.png" /><figcaption>Fig. 17. Helium abundances and isotopic compositions in C2044 blank block samples, from Table 8. Seven of the UM and CRPG samples (UM 45-115/120, CRPG 67, 53) are characterized by low ⁴He abundances and ³He/⁴He ratios significantly elevated above the solar wind value. Another grouping of seven samples (UM 66-111, CRPG 63, 39) exhibits ³He/⁴He values consistent within uncertainties with that of the solar wind. Shaded squares denote averages of the two groups, for the latter arbitrarily plotted at ⁴He = 300 × 10⁻¹² ccSTP. Four other analyses in Table 8 display more extreme compositions. UM 73 has the fourth greatest ⁴He abundance in all measured samples, but no detectable ³He UM 86, on the other hand, has the greatest ³He abundance found in all samples in this investigation, yet contains no detectable ⁴He. And CRPG 57 and 69 isolate a previously undiscovered component in solar system He reservoirs, carrying extraordinarily low ³He/⁴He ratios of <3 × 10⁻⁵.</figcaption></figure>
<table><caption>Table 7. He and Ne in Track 41 samples.</caption><thead><tr><th>Sample identification</th><th><sup>3</sup>He/<sup>4</sup>He<br>× 10<sup>-4</sup></th><th><sup>4</sup>He ×<br>10<sup>-10</sup> ccSTP</th><th><sup>20</sup>Ne/<sup>22</sup>Ne</th><th><sup>21</sup>Ne/<sup>22</sup>Ne</th><th><sup>20</sup>Ne<br>× 10<sup>-12</sup> ccSTP</th><th><sup>4</sup>He/<sup>20</sup>Ne</th></tr></thead><tbody><tr><td colspan="7"><strong>UM samples</strong></td></tr><tr><td>St-1<sup>a</sup></td><td>2.92 (26)</td><td>0.727 (36)</td><td>10.68 (35)</td><td>0.0273 (24)</td><td>53.4 (42)</td><td>1.36 (13)</td></tr><tr><td>St-2<sup>a</sup></td><td>2.47 (34)</td><td>0.526 (30)</td><td>9.0 (16)</td><td>0.039 (11)</td><td>9.4 (14)</td><td>5.60 (89)</td></tr><tr><td colspan="7"><strong>CRPG samples</strong></td></tr><tr><td>Thera-2<sup>a</sup></td><td>n.m.</td><td>1.75 (38)</td><td>10.49 (24)</td><td>0.0279 (17)</td><td>58.0 (43)</td><td>3.0 (13)</td></tr><tr><td>C2044,107,41,0,0</td><td>2.48 (72)</td><td>0.128 (22)</td><td>10.8 (26)</td><td>0.061 (16)</td><td>5.74 (17)</td><td>2.23 (38)</td></tr><tr><td>C2044,108,41,0,0</td><td>2.87 (76)</td><td>0.148 (25)</td><td>11.2 (12)</td><td>0.042 (12)</td><td>2.38 (07)</td><td>6.2 (11)</td></tr></tbody></table>

<sup>a</sup>Data from Marty et al. (2008).

Numbers in parentheses are the uncertainty in the last two digits of the isotopic ratio or abundance.

n.m. = not measured.

<table><caption>Table 8. He and Ne in selected blank block samples analyzed at the (8A) University of Minnesota (UM) and at the (8B) Centre de Recherches Pétrographiques et Géochimiques (CRPG).</caption><thead><tr><th>Identification:<br>C2044,</th><th><sup>3</sup>He/<sup>4</sup>He<br>×10<sup>-4</sup></th><th><sup>4</sup>He<br>×10<sup>-12</sup> ccSTP</th><th><sup>20</sup>Ne/<sup>22</sup>Ne</th><th><sup>21</sup>Ne/<sup>22</sup>Ne</th><th><sup>20</sup>Ne<br>×10<sup>-12</sup> ccSTP</th><th><sup>4</sup>He/<sup>20</sup>Ne</th><th>Depth in<br>aerogel<br>(mm)</th><th>Distance from<br>T41 axis (mm)</th></tr></thead><tbody><tr><td colspan="9"><strong>(8A) UM Sample</strong></td></tr><tr><td>45</td><td>9.3 (1.3)</td><td>6.94 (48)</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>n.d.</td><td>—</td><td>3.2 ± 0.3</td><td>2.9 ± 0.3</td></tr><tr><td>30</td><td>8.1 (2.2)</td><td>4.35 (37)</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>n.d.</td><td>—</td><td>6.4 ± 0.6</td><td>5.3 ± 0.5</td></tr><tr><td>44</td><td>10.3 (3.9)</td><td>4.23 (38)</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>n.d.</td><td>—</td><td>2.1 ± 0.2</td><td>2.9 ± 0.3</td></tr><tr><td>65</td><td>11.9 (6.4)</td><td>5.90 (44)</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>n.d.</td><td>—</td><td>5.4 ± 0.5</td><td>2.1 ± 0.2</td></tr><tr><td>115/120<sup>a</sup></td><td>12.8 (4.9)</td><td>4.31 (37)</td><td>9.6 (6.6)</td><td>0.039 (33)</td><td>1.11 (16)</td><td>3.89 (65)</td><td>6.3 ± 1.7</td><td>7.6 ± 0.8</td></tr><tr><td>66</td><td>4.14 (28)</td><td>51.2 (1.7)</td><td>12.6 (3.2)</td><td><sup>21</sup>Ne n.d.</td><td>0.542 (73)</td><td>94 (12)</td><td>6.4 ± 0.6</td><td>2.1 ± 0.2</td></tr><tr><td>117</td><td>4.90 (71)</td><td>25.5 (9)</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>0.279 (38)</td><td>92 (13)</td><td>6.7 ± 0.6</td><td>7.6 ± 0.8</td></tr><tr><td>41</td><td>5.7 (1.8)</td><td>16.3 (6)</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>n.d.</td><td>—</td><td>8.6 ± 0.8</td><td>4.0 ± 0.4</td></tr><tr><td>27</td><td>5.8 (1.9)</td><td>12.5 (1.0)</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>n.d.</td><td>—</td><td>3.2 ± 0.3</td><td>5.3 ± 0.5</td></tr><tr><td>111</td><td>5.4 (2.6)</td><td>9.6 (1.0)</td><td>9.29 (48)</td><td>0.0231 (26)</td><td>17.7 (2.5)</td><td>0.542 (95)</td><td>2.7 ± 0.3</td><td>7.6 ± 0.8</td></tr><tr><td>50</td><td>2.45 (88)</td><td>7.84 (84)</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>n.d.</td><td>—</td><td>8.6 ± 0.8</td><td>2.9 ± 0.3</td></tr><tr><td>73</td><td>&lt;1.0</td><td>18.7 (5)</td><td>16.21 (83)</td><td>0.0427 (70)</td><td>22.9 (4)</td><td>0.817 (26)</td><td>4.3 ± 0.4</td><td>2.8 ± 0.3</td></tr><tr><td>86</td><td>&gt;190</td><td>&lt;1.4</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>0.727 (43)</td><td>&lt;1.9</td><td>4.6 ± 0.5</td><td>6.4 ± 0.6</td></tr><tr><td colspan="9"><strong>(8B) CRPG Sample</strong></td></tr><tr><td>67</td><td>8.2 (3.0)</td><td>2.73 (38)</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>n.d.</td><td>—</td><td>7.4 ± 0.7</td><td>2.1 ± 0.2</td></tr><tr><td>53</td><td>11.2 (5.4)</td><td>7.3 (1.3)</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>n.d.</td><td>—</td><td>2.1 ± 0.2</td><td>2.1 ± 0.2</td></tr><tr><td>63</td><td>4.06 (73)</td><td>64 (11)</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>n.d.</td><td>—</td><td>3.2 ± 0.3</td><td>2.1 ± 0.2</td></tr><tr><td>39</td><td>3.8 (1.7)</td><td>7.5 (1.4)</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>1.7</td><td>4.40 (86)</td><td>6.4 ± 0.6</td><td>4.0 ± 0.4</td></tr><tr><td>57</td><td>0.233 (75)</td><td>99 (13)</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>5.9</td><td>16.7 (2.5)</td><td>6.4 ± 0.6</td><td>2.1 ± 0.2</td></tr><tr><td>69</td><td>0.119 (38)</td><td>105 (20)</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>9.1</td><td>11.6 (2.3)</td><td>9.4 ± 0.9</td><td>2.1 ± 0.2</td></tr><tr><td>38</td><td>&lt;0.54</td><td>41.1 (5.8)</td><td><sup>22</sup>Ne n.d.</td><td><sup>21</sup>Ne n.d.</td><td>n.d.</td><td>—</td><td>5.4 ± 0.5</td><td>4.0 ± 0.4</td></tr></tbody></table>

<sup>a</sup>Due to a numbering error, the particular UM blank block sample yielding these data (115 or 120) is unknown. Both were analyzed; the other contained no detectable He or Ne (Table 1, Analyses 111, 112). Locations in the last two columns are average depths and distances for 115 and 120.

Numbers in parentheses are the uncertainty in the last two digits of the isotopic ratio or abundance.

n.d. = not detected above blank.

Noble gas characteristics of the 20 samples (13 from UM, 7 from CRPG) in Table 8 span a wide range. Neon abundances in Table 8A vary by almost two orders of magnitude; those in Table 8B fall within the range of the UM samples. No neon isotopic data were obtained in the CRPG analyses. The two most precisely

measured neon isotope ratios in Table 8A define very different compositions. One (UM 73) is well above the $^{20}$Ne/$^{22}$Ne ratios in solar system neon reservoirs (Table 3) and similar to “Sample X” (see section IV.A.1). The other (UM 111), with $^{20}$Ne/$^{22}$Ne marginally below the air ratio, is one of several that
may reflect mixing of discrete neon components (see section VI.C).

Helium abundances exhibit similarly broad ranges, with the Table 8B values overlapping the UM measurements in 8A. The most informative of the Table 8 data are the distributions of measured ³He/⁴He ratios, plotted in Fig. 17 versus ⁴He abundances. They fall into four distinct groups, divided by blank lines in Table 8. The first is a set of seven high ratios (UM, 45 through 115/120, CRPG 67, 53) characterized by low ⁴He, ³He/⁴He elevated above SW by >1σ, and no detectable neon in six of the seven. Unweighted averages and standard deviations for this set, plotted in Fig. 17, are ⁴He = (5.1 ± 1.7) × 10⁻¹² cm³STP and ³He/⁴He = (10.3 ± 1.8) × 10⁻⁴. (Weighting by 1/σ² reduces average ³He/⁴He to (9.3 ± 1.0) × 10⁻⁴, reflecting the large weight of the most precise measurement.) With its low ⁴He, above-solar ³He/⁴He, and essentially no neon, this compositional signature is unique among those measured in the two investigations.

The second data set (UM, 66 through 111, CRPG 63, 39) shows an abrupt transition to higher ⁴He and SW-like ³He/⁴He at the upper ⁴He margin of the high ³He/⁴He set; at larger ⁴He, ³He/⁴He scatters around the SW ratio (Fig. 17). The unweighted ³He/⁴He average and standard deviation for this set, arbitrarily plotted at ⁴He = 300 × 10⁻¹² cm³STP in Fig. 17, is (4.83 ± 0.83) × 10⁻⁴, compared to SW-³He/⁴He = 4.64 ± 0.09 (Table 3). (Weighting by 1/σ² reduces average ³He/⁴He to [4.27 ± 0.24] × 10⁻⁴, again reflecting the extreme weight of the most precise measurement [UM 66]. This average differs from the SW value by 1.4σ, where σ is the combined error in the SW and averaged values.) One cross-laboratory pair in this set directly illustrates the compatibility of the UM and CRPG data: UM 66 and CRPG 63, located close together in the blank block, contain high and similar ⁴He abundances and display concordant ³He/⁴He ratios.

²⁰Ne in these samples is erratic, ranging from ~0.3 to ~18 × 10⁻¹² cm³STP. The high ⁴He/²⁰Ne ratios in UM 66 and 117 suggest a SW contribution to neon, but these ratios of ~93 are much lower than pure SW-⁴He/²⁰Ne = 650 (Table 3), indicating that another, non-SW component is also present. These blank block sample data indicate a dominant helium component close to SW-He in composition, despite deep locations within the blank block that shields them from direct SW irradiation (see section V.A.1.3).

The third data set (UM 50 and 73, CRPG 57 through 38) comprises five block samples with sub-SW values of ³He/⁴He. They hint at the presence of helium characterized by a very low ratio. In this view the intermediate ratio in UM 50 represents a mixture of the low ³He/⁴He component and one with a SW-like or

higher ratio. The upper ³He/⁴He limits for UM 73 and CRPG 38 also point to a low ratio; these samples contain ⁴He abundances high enough to enable ³He detection (e.g., as in UM 117 and CRPG 63) unless their ³He/⁴He ratios are far below SW. CRPG measurements may have isolated this low ³He/⁴He endmember composition in relatively pure form. He-rich block samples 57 and 69 display astonishingly low ³He/⁴He ratios of 0.233 × 10⁻⁴ and 0.119 × 10⁻⁴, values unprecedented in solar system helium reservoirs (Table 3) except for the terrestrial atmosphere, where mechanisms of helium supply and loss control the current nonprimordial ratio. The existence of such a helium composition is an entirely new discovery. Three of the five samples in this group with upper limit or low measured ³He/⁴He ratios are also relatively rich in ²⁰Ne (Table 8). The one determination of ²⁰Ne/²²Ne (UM 73) is similarly exotic, well above the SW ratio or any other Ne composition in Table 3.

Data set four consists of only one member, UM 86. Its gas composition is unique: low neon accompanied by undetectable ⁴He and the greatest abundance of ³He measured in any of the 89 blank block samples—essentially a ³He spike. Within uncertainties, the ⁴He measurement was consistent with the blank, so only an upper limit could be estimated. In contrast, the ³He release exceeded the blank level by a factor >4, yielding a lower limit of 1.9 × 10⁻² for the ³He/⁴He ratio (Fig. 17, upper left corner). This is among the highest nonspallogenic values for this ratio ever reported in meteorites or IDPs, similar to some measured in IDPs by Nier and Schlutter (1993). As in that report, the absence of appreciable ²¹Ne would appear to rule out a spallation origin for the high ³He abundance.

Of the 20 blank block samples and 4 wafer samples (see section IV.A.1) for which ³He/⁴He ratios or their limits were obtained at UM and CRPG, in only one (UM 50, Table 8A) does the nominal ratio fall within the range seen in T41 (Table 7). Two others in Table 8 (UM 111, CRPG 39), both with ~45% measurement uncertainties, overlap the T41 values within −1σ. It appears that helium compositions in most of these neighboring samples are not related in any evident way to T41.

The concordance of the UM and CRPG block blank measurements displayed in Fig. 17 testifies to the coherence of the two data sets. Whatever the sources of these gases may be, or how they might have been emplaced in their present locations, the mutual detections of remarkably high ³He/⁴He in some samples and near-solar wind ratios in others illustrate the fact that these independent measurement suites both reveal unexpected enclaves of extraterrestrial noble gases sited deeply within a Stardust aerogel cell.
<figure><img src="image_19.png" /><figcaption>Fig. 18. A) Neon composition in cell C2028 surface samples compared to those found in T41 and the “Manchanito” particle from the L2071F1 cluster IDP. All three display $^{20}$Ne/$^{22}$Ne ratios falling within the Q-Ne data field (Table 3). The $^{21}$Ne/$^{22}$Ne ratio in C2028 is notably enhanced relative to Q-$^{21}$Ne/$^{22}$Ne, likely reflecting a spallogenic contribution. Cell surface sample C2028-2 contained >13× less Ne than C2028-1, with a corresponding reduction in measurement precision as seen in the inset. B) He ratios are elevated with respect to Q-$^{3}$He/$^{4}$He in the Stardust T41 and C2028 samples, suggesting the presence of a solar wind (SW) component; $^{3}$He/$^{4}$He in C2028-2 is off-scale at 5.56 ± 0.50 × 10$^{-4}$. The higher $^{3}$He/$^{4}$He in C2028 probably reflects additions of both SW-He and spallation $^{3}$He to an initially Q-like $^{3}$He/$^{4}$He ratio. Data sources: T41, Marty et al. (2008); Manchanito, Palma et al. (2013a); Jupiter, SW, Q, HL-He, from Table 3.</figcaption></figure>

## IV.B. Other Cell Samples

### IV.B.1. C2028 Surface Samples

As in virtually all samples analyzed, neither C2028,13-14,0,0 (UM designation C2028-1) or C2028,6-9,0,0 (C2028-2), (Table 1, Analyses 140–141), contained helium or neon above blank levels in the initial low-temperature heating steps. However, large argon/hydrocarbon backgrounds were released at low temperature in both samples. To reduce the levels of these background gases, both samples were heated a total of three times at the typical initial low temperature of ~200 °C. Even with this precaution, relatively large abundances of argon and hydrocarbons were released in the seven-step high-temperature pyrolyses. These background gases were condensed on the LN₂-chilled cold finger and did not affect the helium and neon analyses.

There is a striking difference in the abundance of neon between the two samples, >13 times larger in C2028-1 than in C2028-2. The reason for this difference is unknown, but may arise from strong variations in carrier particle densities across the cell C2028 surface (see section V.B.1). Although the neon isotopic compositions are barely concordant within uncertainty, the isotopic composition is far more precisely determined for the former sample. The observed

$^{20}$Ne/$^{22}$Ne value (Fig. 18A) falls within the Q-Ne data field previously found in T41 samples (Table 7). However, the $^{21}$Ne/$^{22}$Ne ratio in C2028-1 is enhanced relative to Q-$^{21}$Ne/$^{22}$Ne, likely reflecting a spallogenic contribution (see section V.B.1). This $^{21}$Ne/$^{22}$Ne enhancement is reminiscent of the apparently spallogenic neon component observed in the C2044 surface samples discussed in section IV.A.3.

Helium abundances in both samples are comparable, with strongly elevated $^{3}$He/$^{4}$He ratios with respect to the Q-He composition suggesting a prominent solar wind component (Table 3; Fig. 18B). The possible presence of solar wind helium (SW-He) in the T41 materials was considered by Marty et al. (2008), and as noted in sections IV.A.3 and IV.A.4, $^{3}$He/$^{4}$He ratios resembling solar wind were seen in both C2044 blank block surface samples adjacent to T41 and in the δ2 blank from an aerogel fragment near the T41 wall. The higher $^{3}$He/$^{4}$He in C2028 arguably reflects additions of both SW-He and spallation $^{3}$He to an initially Q-like $^{3}$He/$^{4}$He ratio, given the neon isotopic composition (Fig. 18A,B).

### IV.B.2. C2086 Samples

A variety of samples were taken from aerogel cell C2086, mimicking the survey of samples from cell C2044 although far less comprehensively. Included in
<figure><img src="image_20.png" /><figcaption>Fig. 19. Helium abundances and isotopic compositions in cell C2086 tracks T181, T182, and blank aerogel fragment samples. Tracks T181 and T182 show distinct helium compositions, and T181 has ~14 times as much gas. Blank 2 had the highest helium abundance of all Stardust samples other than wafer 20A. Blanks 1 and 4 show an elevated ³He/⁴He value compared to the solar wind value (Table 3), as does sample BD1. C2086 blanks show a wide variety of abundances and helium isotopic compositions.</figcaption></figure>

these samples were two entire small tracks, T181 and T182 (Fig. 10). Data from these C2086 samples are presented in the same format as those for the C2044 data (Figs. 19–21). Figure 19 clearly shows that noble gas distributions in the two tracks are quite distinct (Table 1, Analyses 123, 127). T181 contains ~14 times more ⁴He than T182, with a ³He/⁴He ratio slightly lower than solar wind, contrasting with T182 where ³He/⁴He is strikingly low, $\le 1 \times 10^{-4}$ within the measurement uncertainty. T181 had very little neon, precluding determination of neon isotopic ratios and producing a very high ⁴He/²⁰Ne ratio of $670 \pm 120$. This and the high ³He/⁴He ratio suggest the presence of a substantial SW component in T181 (Table 3). In contrast, neon in T182 is much more abundant, yielding ⁴He/²⁰Ne of ~7, a ²¹Ne/²²Ne ratio indicating a ²¹Ne excess over the atmospheric and Q values (Table 3; Fig. 20), and a low ²⁰Ne/²²Ne, another of several subatmospheric ratios that may reflect mixing of discrete neon components (see section VI.C).

As observed earlier with the C2044 wafer samples, several “blank” aerogel samples from C2086 without visible particle fragments contain significant light noble gas contents, although there are also samples without

discernable gases above background values. Among the interesting C2086 samples with significant noble gases above the background is blank 5, from the side of the tile. Blank 5 released a neon abundance among the half-dozen highest seen in the study, again with a subatmospheric ²⁰Ne/²²Ne ratio, ²¹Ne/²²Ne consistent with either air or Q, and ³He/⁴He actually exceeding the solar ratio (see section VI.A). The possibility that the low neon contents in some of the other C2086 blanks might reflect contamination by distributed cutting dust (see section III.C) is discussed in section V.B.2.

Blank 2 from C2086 (Fig. 11) had the second highest amount of helium seen in all the Stardust samples, exceeded only by wafer 20A where a memory component contributed to the overall helium abundance. The large helium content allows a precise determination of ³He/⁴He = (4.289 ± 0.048) × 10⁻⁴. The three lowest temperature steps for Blank 2 (Table 1, Analyses 115–118), culminating in a peak temperature of ~700 °C, yielded ³He/⁴He, ²⁰Ne/²²Ne, and ⁴He/²⁰Ne ratios strikingly similar to the solar wind values in Table 4, although ²¹Ne/²²Ne is somewhat elevated (by ~1.7σ) above the solar value. Overall, both the helium and neon isotopic compositions (Figs. 19
<figure><img src="image_21.png" /><figcaption>Fig. 20. A neon three isotope plot of the cell C2086 track T182 and blank aerogel fragment samples. T181 had too little neon to establish isotopic ratios. T182 has a ²⁰Ne/²²Ne ratio marginally compatible with the terrestrial composition (Table 3), but a significant ²¹Ne excess over that composition. Blank 2 (Table 1, Analysis 118), with ²⁰Ne/²²Ne and ³He/⁴He (Fig. 19) and ⁴He/²⁰Ne ratios resembling those in the solar wind (Table 3; see also Fig. 24A,B), is distinct from the other blank samples.</figcaption></figure>

and 20) deviate only slightly from solar wind values and are clearly distinct from any other C2086 sample. The 850 °C gas release from Blank 2 is consistent with an admixture of Q gas to the initial SW 700 °C component, although it is not possible to distinguish a potential Q component from one of terrestrial composition (see #8 (700 °C) and #9 (850 °C) in Fig 24A). This blank sample was an aerogel chunk attached to T182, but because Blank 2 displays a distinctly different isotopic composition and temperature release profile compared to T182 (Figs. 19–21), it appears that the gases in the two samples do not derive from a single source.

It has been suggested that it would be useful to convert measured noble gas abundances to concentrations expressed in units of cm³STP per g-aerogel. The volume of Blank 2 was ~1 × 10⁻⁴ cm³, and from this, an aerogel density of 5 mg cm⁻³, and ⁴He = 53.3 × 10⁻¹⁰ cm³STP from Analysis 118 in Table 1, a remarkably high ⁴He concentration of ~0.01 cm³STP/g-aerogel is deduced. Concentrations for

all analyzed Stardust samples, for example, the C2044 blank block fragments with approximately known volumes (see section IV.A.5), could be calculated in this way and tabulated. However, concentrations expressed as cm³STP/g-aerogel are ambiguous because they tacitly assume that the gases were uniformly sited in the aerogel mass itself, which would be invalid if they were actually hosted by very small particles that were present but undetected. Calculations on a per g-aerogel basis in such cases yield only lower limits, and for that reason their inclusion could lead to misunderstandings of what they actually are. Good examples of the potential pitfalls are the C2028 surface slices (see section IV.B.1), which do contain small particles assumed to be carriers of the gases in these samples (see section V.B.1). The volume of C2028-1 from dimensions given in the Fig. 9 caption was 0.5 × 0.02 = 0.01 cm³, and for ρ = 5 × 10⁻³ g cm⁻³, its mass was 5 × 10⁻⁵ g. With ²⁰Ne = 23 × 10⁻¹² cm³STP (Table 1, Analysis 141), its calculated ²⁰Ne loading in cm³STP/g-aerogel would be ~5 × 10⁻⁷, more than five orders of magnitude smaller
<figure><img src="image_22.png" /><figcaption>Fig. 21. The $^{20}$Ne/$^{22}$Ne ratio as a function of $^{4}$He/$^{20}$Ne in cell C2086 track T182 and blank aerogel fragment samples. T181 had too little neon to establish a $^{20}$Ne/$^{22}$Ne value. Blank 2 had an unusually high, and sample BD1 an unusually low, $^{4}$He/$^{20}$Ne ratio, but other samples fall within the typical IDP range (Pepin et al. 2011).</figcaption></figure>

than the loading of 0.29 cm$^3$STP g$^{-1}$ estimated from the population of embedded ~1 µm particles (see section V.B.1). Absent any information on the actual gas carriers, concentrations in cm$^3$STP/g-aerogel are not particularly meaningful.

### IV.B.3. T108 Terminal Particles

Two of the three ~10–15 µm terminal particles from C2081 track 108 were analyzed together as sample C2081,1,108,13&14,0 while the other was analyzed separately as C2081,1,108,12,0 (Table 1, Analyses 25, 26). By estimating particle sizes from Figs. 12B–D, and assuming a density of 3 g cm$^{-3}$, helium concentrations in both samples were ~1.3 × 10$^{-3}$ cm$^3$ STP g$^{-1}$. Neon concentrations were ~6 × 10$^{-4}$ cm$^3$STP g$^{-1}$ for particle 12 and <3.3 × 10$^{-6}$ cm$^3$ STP g$^{-1}$ for the combined 13&14 sample. These values are below those seen in T41 wall samples (Marty et al. 2008) by factors of 10$^3$–10$^4$. In the single particle 12,0, detection of significant $^{20}$Ne but no $^{22}$Ne above background suggests a potentially high $^{20}$Ne/$^{22}$Ne ratio. However, due to the uncertainty in the $^{22}$Ne background, it is impossible to rule out a $^{20}$Ne/$^{22}$Ne value as low as that of the solar wind. Gas release from both samples occurred at $T < 1000$ °C, again in contrast to the high-temperature release ($>1250$ °C) from the T41 wall samples (Marty et al. 2008), so if these terminal particles are the refractory

cores of larger incident particles, any high-temperature gas component within them is nonexistent. Nakamura et al. (2008) studied the largest (~40 µm) terminal particle (named “Gozen-sama”) in T108 (C2081,1,108,1) and another T108 terminal particle (“Gen-chan,” C2081,1,108,7) and concluded they are chondrule-like particles that could have been part of a single chondrule impactor forming the track.

### IV.B.4. T77 Terminal Particle (“Puki A”)

Gas release (Table 1, Analyses 138, 139) was somewhat different for this sample, possibly due to its encasement in a relatively large acrylic cube. The two initial low-temperature heating steps to eliminate potential background contamination were carried out as usual, but nothing above the typical background was detected at the estimated upper release temperature of 200–250 °C. The subsequent higher temperature heating of Puki A was terminated after the first two steps due to a larger than usual water background (potentially due to acrylic outgassing) detected during the gas monitoring stage. Following clean-up of this ~600 °C gas fraction, isotopic compositions were determined as usual with no interference or background issues. No helium was detected above blank but there was sufficient neon for isotopic analysis, yielding a very low $^{20}$Ne/$^{22}$Ne of ~5.6 (discussed in section VI.C) and a
strikingly elevated $^{21}$Ne/$^{22}$Ne indicating the presence of a significant spallation component (see section VI.D). The calculated $^{20}$Ne concentration in Puki A is $3.4 \times 10^{-4}$ cm$^3$STP g$^{-1}$, so its neon depletion relative to T41 wall samples is similar to that seen above in the T108 terminal particles. A cold blank was run after the ~600 °C gas fraction and before continuing the sample heating with the typical seven-step protocol. Gases evolved at high temperature were below detection for helium, and only marginally above detection for neon. The imprecise $^{20}$Ne/$^{22}$Ne value is consistent with the value obtained in the lower temperature release, so the neon composition adopted for the sample is that released at ~600 °C.

### IV.B.5. Interstellar Collector Surface and Subsurface Samples

Two surface and two subsurface samples taken from the interstellar side of the Stardust collector tray (aerogel cell I1047) contained, for the most part, very little light noble gas. Helium was only marginally above blank levels in all four of these, yielding imprecise $^{3}$He/$^{4}$He ratios in the three cases where $^{3}$He was detectable, one within error of solar wind and the other two intermediate between the Jupiter or Q and solar wind compositions (Table 3). $^{20}$Ne was likewise barely above detection limits in both subsurface samples and in one from the surface, with $^{21}$Ne immeasurable in all three cases. $^{22}$Ne was below detection in one of the subsurface samples, and yielded only similar but imprecise $^{20}$Ne/$^{22}$Ne ratios intermediate between Q-Ne and the solar wind in the other three. Surprisingly, one of the surface samples (Table 1, Analysis 136) contained significant neon (resulting in a very low $^{4}$He/$^{20}$Ne ratio of ~0.3) that allowed relatively precise determination of isotopic ratios. In this case, within the uncertainty of the measurements, the $^{20}$Ne/$^{22}$Ne ratio is just below the terrestrial atmosphere value and the low $^{21}$Ne/$^{22}$Ne value is similar to that observed in wafer 18B (see section IV.A.1). Thus, the $^{21}$Ne excess consistently observed in surface samples from the cometary side of the Stardust collector tray is not seen in the interstellar side, where the $^{21}$Ne/$^{22}$Ne ratio appears to be unusually low.

## V. FURTHER DISCUSSION

### V.A. Cell C2044

#### V.A.1. Cell C2044 Blank Wafers, T41 Wall and Cross-Sectional Slices, T41 Fragments, and Blank Block Samples

Concordant Q-Ne compositions in T41 samples have been previously measured in two different laboratories by distinct heating and analysis techniques

(Marty et al. 2008) (Table 7). In that paper, the UM group reported $^{3}$He/$^{4}$He ratios intermediate between the Jupiter or Q and solar wind values. No helium isotopic data were obtained at CRPG, but later measurements in that laboratory on other T41 samples (Füri and Marty 2012) (Table 7) confirm almost exactly the $^{3}$He/$^{4}$He ratios initially reported by the UM group. Thus, the T41 helium and neon compositions are well established for those track-forming components containing sufficient light noble gas for analysis. Comparisons of these T41 measurements, on samples extracted lengthwise down the bulb wall, with those on transverse cross-sectional slices through the track and on aerogel fragments containing T41 wall material, indicate that the light noble gas carriers are distributed highly heterogeneously along the track. Inhomogeneous distribution of the dust component within T41 has also been observed in an analysis of Mg-Ca-Al-rich patches within the melted aerogel (Leroux et al. 2008).

Track 41 is a Type B track (Hörz et al. 2006), a shape suggesting that significant fine-grained material could have been part of the impacting particle. Terminal and subterminal particles, as well as 33 iron-bearing fragments from the bulb region of the track, were examined by Ogliore et al. (2010) in an Fe XANES study. The deepest terminal particle (nicknamed Simeio) was found to be a large piece of Fe metal in that investigation and was subsequently studied in further detail by Westphal et al. (2012) and Humayun et al. (2015). Nine additional 1–5 µm fragments from the T41 bulb wall were examined using transmission electron microscopy by Joswiak et al. (2012), who concluded that the T41 impactor was a mineral/rock mixture composed of an unequilibrated assemblage of coarse fragments with possible fine-grained matrix materials. In a study of iron oxides in T41, a 300 µm thick transverse slice 0.8 mm below the track 41 block surface was examined by Bridges et al. (2010). They concluded that interpretation of the original cometary assemblage had been complicated by a host of factors, including irradiation of the comet’s surface, extreme thermal processing of the dust during capture, and terrestrial contamination of the aerogel during its manufacture.

Analyses of aerogel block samples from the blank block indicate a “halo” of erratically oriented light noble gases trapped within materials located at distances up to ~9 mm from the surface of the blank block and up to ~8 mm from the axis of T41 (Palma et al. 2013b) (Table 8). Could this halo of noble gases be consistent with an origin from T41? In an infrared spectroscopic mapping of volatile organics from four tracks and two wafer slices from Cell C2009 and two additional tracks from Cell C2115, Bajt et al. (2009)
found extended “plumes” of organic material that extended out to the very edge of the cut wafers (~100 µm), so they did not include data from the wafer slices. Of the six tracks they examined, three showed an extended distribution of organics that they felt could not be attributed to a conversion of carbon in the aerogel capture material. Still, the -CH₂- band they examined reached the background level present in the original aerogel within a few hundred microns from the track, so noble gas effects extending millimeters from a track seem highly unlikely. The large spread in isotopic compositions found in the “halo” block samples presents a further argument against their origin from T41. It seems problematic that a suite of grains carrying isotopically distinct noble gases, none commensurate within error with both the ²⁰Ne/²²Ne and ³He/⁴He ratios measured in T41 (Marty et al. 2008) (Tables 6A, B and 7), could have existed in a single incident particle and had their separate gas contents ejected into surrounding aerogel during impact.

### V.A.1.1. Assessments of Possible Contamination by Air, Mass Spectrometer Memory, or Indigenous Gases in Stardust Aerogel

The large concentrations of heavy atmospheric noble gases in room-temperature aerogel reported by Mohapatra et al. (2013) and O'Mara et al. (2014) immediately raise the question of whether the C2044 samples and others analyzed in this study could have been contaminated by atmospheric helium and neon occluded in the aerogel. This possibility was a major concern for one reviewer. However, aside from the observational evidence for the absence of air contamination presented in section III.C, the physical behavior of the light noble gases in aerogel argues against it. Helium and neon are adsorbed on surfaces only at extremely low temperatures. No such component should exist in aerogel suffused with air at room temperature. The situation differs for absorbed helium and neon. Diffusivities *D*(*T*) for these gases in amorphous fused quartz (Frank et al. 1961; Swets et al. 1961) or vitreous silica (Shelby 1971) are large. Amorphous fused quartz, in particular, is a good structural analog for amorphous aerogel SiO₂ spheres, estimated to be ~2 nm in radius a. These data sets yield room-temperature values of *D*/*a*² large enough to enable equilibrium loading of ambient atmospheric helium and neon into aerogel spheres on very short time scales. At this point, aerogel gas contents can be substantial. Exposure to vacuum, however, reduces ambient He-Ne concentrations at aerogel surfaces to zero and results in Fick's Law draining of these absorbed components in small fractions of a second. Even if aerogel sphere radii were 10× larger than 2 nm, diffusive loss would still be

essentially complete in ≤1 s. In view of these diffusivity data it seems extremely unlikely that any absorbed atmospheric helium and neon (or, for that matter, any cometary helium or neon initially sited only in aerogel fibers) could survive the in vacuo processing procedures set out in section III.A, especially the UM preliminary degassing steps at ~200 °C where *D*(*T*)/*a*² values exceed those at room temperature by more than an order of magnitude.

Another possibility suggested in review was that at least some of the measured helium compositions attributed to cometary origin in this study might actually be artifacts generated by combinations of air contamination and mass spectrometer (MS) memory. This would appear to be a dubious proposition, given arguments here and in section III.C against the presence of a significant air component. A simple mixing model based on air and memory compositions (see section IV.A.1) confirms this view. It was applied to all Table 1 entries where data on He-Ne abundances and isotope ratios are essentially complete. Seventeen analyses (Table 1, Analyses 6, 10, 17, 22, 30, 71, 97, 111, 119, 120, 124, 126, 127, 132, 136, 140, 141, counting integrated compositions as one) fit this criterion and also have ⁴He/²⁰Ne elemental ratios between 0.32 (air) and 53 (memory) and so are candidates for air-memory mixing. ⁴He/²⁰Ne ratios in three others (Analyses 13, 34, 118) are >53 and cannot be such mixtures. Compositions measured in these 20 samples comprise central results of this study.

This modeling, constrained to generate total abundances of ⁴He and ²⁰Ne that replicate measured values, yields the isotope ratios ³He/⁴He, ²⁰Ne/²²Ne, and ²¹Ne/²²Ne predicted by the two-component mixing for comparison with measured ratios. Levels of concordance with air-memory mixtures were assessed via the Δ/σ ratio, where Δ is the difference between a modeled isotope ratio and its actual measured value, and σ is the 1σ error in the difference propagated from air, memory, and measurement ratio uncertainties. The criterion for approximate agreement of modeled and measured ratios was taken to be Δ/σ < 2; values of Δ/σ ≥ 2 for one or more of these ratios signal incompatibility with air-memory mixing at the ≥95% confidence level.

Of these 17 analyses, only one (C2086 BD1, Table 1, Analysis 132) has Δ/σ < 2 for all three isotope ratios and so could arguably be a product of air-memory mixing, although even this sample is on the edge with Δ/σ = 1.9 for ²⁰Ne/²²Ne. The remaining 16 display Δ/σ ≥ 2 for one (nine cases), two (six cases), or all three (one case) of the ratios. Disagreements scattered among the 16 individual samples range from Δ/σ = 2.0 to 10.3 for ³He/⁴He, 4.2 to 15.1 for
$^{20}$Ne/$^{22}$Ne, and 2.0 to 4.0 for $^{21}$Ne/$^{22}$Ne. If the three analyses with $^{4}$He/$^{20}$Ne >53 are included, 95% of these measurements are compositionally incompatible with simple mixtures of air and MS memory gases. We conclude that such mixing is not supported by the great majority of the data. Another reason for suspicion of this mixing scenario is the dominant role played in it by the memory component, which comprises ~80% of the total helium averaged over the 17 modeled analyses. To the contrary, the only instance in the UM experimental data where a memory component evidently contributed was in the C2044,20A wafer analysis where extremely large amounts of water, CO$_2$, and hydrocarbons were inadvertently admitted to the MS (see section IV.A.1), resulting in ejection of previously implanted (i.e., “memory”) gases from surfaces impacted by their intense ion beams.

A remaining issue involves possible contamination by light noble gases trapped and retained in Stardust aerogel impurities during its manufacture. It can be addressed by parallel measurements, employing all processing and analytical protocols applied to the flown Stardust samples themselves, on comparable volumes of unflown aerogel free of any cometary contributions. Results of such analyses obtained in this study are not definitive. No indigenous helium or neon components were found in a UM analysis of a flight spare aerogel sample (E243-8B, Table 1, Analysis 96). An earlier and less precise UM analysis of flight spare aerogel (Palma et al. 2007) is discordant in part with this measurement, yielding not only $^3$He and neon abundances below detection limits but also a significant amount of $^4$He (~8 × 10$^{-12}$ cm$^3$STP). Low blank measurements on unflown aerogel samples at CRPG—without the high-blank ZnSe disk described in section III.B—showed a minor $^4$He excess of (4.3 ± 2.5) × 10$^{-12}$ cm$^3$STP (Table 2). There is no evidence, however, that indigenous gases are systematically present in the *flown* blank block samples. About two-third of them are free of any detectable helium and neon (Tables 1 and 2), and those that are not display nonterrestrial and highly variable isotope ratios (Table 8) that are difficult to attribute to indigenous sources in the aerogel. These observations, together with inconsistent results from flight spare analyses, indicate that the presence of light noble gases intrinsic to the aerogel is unproven. They are most probably absent or below detection limits.

### V.A.1.2. Assessment of Possible Contamination of Blank Block Samples by Distributed Cutting Powder

The gas-bearing block samples in Table 8A,B are intriguing. They are all located at large distances from potential noble gas sources, isolated not only from particle or gas offshoots from the T41 bulb but also

from either small coma grains shallowly sited in the Tile 44 surface or solar wind atoms trapped in surficial material (see section V.A.1.3). They contain no visible tracks or host grains connecting them to particle sources. Thus, one would expect them to be true aerogel blanks, sterile of noble gases in common with most of the other block samples, but they are not. There are no obvious mechanisms that might explain how these deeply sited samples acquired their gases. In this situation it is important to consider whether these gases might not be indigenous, but instead reflect cross-contamination by distributed aerogel cutting powder (“kerf”) produced during excision of neighboring gas-rich materials (see section III.C).

We first focus on block C2044,73, which from Table 8A has both the highest $^{20}$Ne abundance of these block samples and a distinctive neon composition within error of that in nearby C2044,20A (“Sample X”). This isotopic similarity, plus the fact that wafer 20A contains the largest $^{20}$Ne abundance observed in this study, ~3.0 × 10$^{-10}$ cm$^3$STP after approximate correction for memory $^{20}$Ne (see section IV.A.1), points to distributed kerf from preparation of wafer 20A as a plausible source of possible neon contamination in block 73. However, in detail, this explanation for its measured noble gas contents appears doubtful.

With 10 µm wide cuts (see section III.C), the total volume of powdered kerf produced during preparation of wafer sample 20A was ~4.3 × 10$^{-4}$ cm$^3$, about 10.6% of the analyzed 20A volume, distributed as ~4.3 × 10$^8$ aerogel grains if they were ~1 µm in size. Kerf from cuts separating adjacent wafers (e.g., wafers 20 and 19, Fig. 8B) is presumed to contain a 50:50% mixture of material from both. With this assumption, ~2.5 × 10$^{-11}$ cm$^3$STP of $^{20}$Ne was released in the wafer 20A kerf. The problem is that to match its $^{20}$Ne content, block 73 by itself would have had to collect 93% of all this kerf (adding ~40% to its volume), a statistically improbable event for randomly distributed grains. A further difficulty is that wafer 20A is extremely rich in $^4$He (Table 1), even with correction for a ~1/3 memory component. A 93% acquisition of wafer 20A kerf would oversupply the measured helium content of block 73 by a factor of ~20. Repeating these calculations for block 111, the other Ne-rich sample in Table 8A, yields similar results. Statistically defensible fractions of distributed kerf intercepted by these two blocks (e.g., perhaps ~1–2% or less) would require it to contain a much higher $^{20}$Ne abundance than in 20A. Such large neon contents in the dust are incompatible with the fact that virtually all of it was milled from wafers with analyzed helium and neon contents (18A&B, 19, 20A&B in Table 1), and none of them, including wafer 20A, have abundances even remotely approaching the necessary levels. We conclude
that cross-contamination by cutting powder from wafer 20A preparation is not the source of most of the noble gases in blocks 73 and 111.

A few of the remaining eleven samples in Table 8A contain considerably smaller amounts of neon, which could have been provided by interception of only a few % or less of distributed kerf grains from wafer 20A preparation. Seven of the 11, however, have no detectable neon at all and therefore no significant kerf contamination. Helium contents in the four others are inconsistent with derivation of their neon from intercepted kerf powder; the supplied helium would be ~5× less than that measured in blocks 66 and 117, ~5× greater in block 115/120, and ~10× greater than the upper limit in block 86.

These calculations demonstrate that cross-contamination can be discarded as an explanation for the presence of noble gases in the deeply sited block samples listed in Table 8A, and by extension those in Table 8B as well. Moreover, there is no evident reason why only these samples would have been subject to contamination during processing, whereas the 48 other UM block samples with no detectable helium or neon, five of them extracted from locations immediately adjacent to blocks 73 or 111, were not. It thus seems requisite that noble gases in all the Table 8 blocks are indigenous, not contaminants. How they were emplaced at locations so far below the tile surface remains unexplained.

### V.A.1.3. Range Modeling

Estimates of penetration depths of various-sized particles incident at 6.1 km s⁻¹ on aerogel cell surfaces indicate that these unexpected discoveries of pockets of nonterrestrial noble gases at distances far below the surfaces cannot be attributed to impact and penetration of either carrier grains or ions in the comet coma, or of higher energy solar wind (SW) ions. Particle ranges can be estimated in at least two ways. We first adopted the model of Domínguez et al. (2004) with $C_d = 1$, impact velocity extrapolated to 6.1 km s⁻¹, and ρ (aerogel) = 5 kg m⁻³. ($C_d$ is a numerical free parameter included in the equation for the rate of projectile momentum loss versus penetration distance to represent shape-dependent or other possible slowing effects not explicitly included in the modeling. Calculated track lengths are inversely proportional to $C_d$; it is expected to be ~1 for spherical impactors.) Alternatively, aerogel structure of this density can be modeled as randomly distributed ~50 nm assemblages of ~4 nm SiO₂ spheres, with the assumption that an incident particle is stopped after encountering an integrated aerogel mass equal to its own mass. Penetration depths are directly proportional to assumed SiO₂ sphere diameters.

For small (~1 µm) incident particles, the two estimates agree on track lengths of ~1/3 mm. Larger grains carve longer tracks, ~2 mm for 5 µm diameter, ~3.5 mm for 10 µm in both models. Samples in this study were imaged at an optical power sufficient to detect tracks made by projectiles as small as ~0.5–1 µm (although at this resolution the particles themselves or their terminal debris would not have been visible). No tracks were seen, except for those revealed by high-resolution scans of the C2028 surface (see section V.B.1). Although calculated ranges in both models are likely to be uncertain by factors of two or more, it is clear that no incident particles could have penetrated to the extreme multi-mm depths where extraterrestrial gases were detected without forming large and highly visible tracks.

A similar conclusion applies to penetration of SW ions during possible exposure of the aerogel collector array to the 1 keV/amu solar wind (see section V.B.2). A first-order model for penetration of SW⁻⁴He through ρ = 5 kg m⁻³ aerogel again assumes a random assemblage of ~4 nm SiO₂ spheres, where each ion traverse through an average fraction of a sphere diameter reduces its energy until it comes to rest in a final sphere. This yields a small range of 21 ± 10 µm, independent of order of magnitude variations in assumed sphere diameters. (Identical ranges of 21 ± 10 µm [⁴He] and 20 ± 9 µm [²⁰Ne] are given by the SRIM Program [Ziegler 2004] for SW ions normally incident on a SiO₂ aerogel cell of density 5 kg m⁻³. In SRIM and the model discussed here, calculated ranges are inversely proportional to assumed cell density.) The range increases if the spheres are self-shielded (i.e., fortuitously vertically stacked so that more open space is available for ion penetration), but even for an improbable self-shielding factor of 50 the range of the incident ions is only ~1 mm. Penetration distances below cell surfaces are reduced if any actual exposure of the comet collector tray to the SW direction was oblique, and are extremely small for incidence of comet coma ⁴He ions at 6.1 km s⁻¹ (~0.2 eV/amu).

In this latter case, one could consider the debatable possibility that the collector tray was essentially at rest relative to the coma atmosphere for a long enough time for ambient helium and neon to suffuse into the aerogel cells. But at the low thermal energies available for impact implantation into aerogel fibers, and no evidence for structural trapping sites for long-term retention, these gases would probably diffuse out of the aerogel on exposure to in-space vacuum and to laboratory procedures for contaminant degassing, as expected for loss of terrestrial helium and neon initially present in aerogel samples (see section V.A.1.1).
### V.A.2. Cell C2044 Blank Block Surface/Subsurface Slice Samples; Excess $^{21}$Ne

It is possible to identify a $^{21}$Ne excess in the C2044 blank block surface slice measurements. In this calculation, only slice fragments 1 and 3 were considered, since they were the only ones where all three neon isotopes were detected above background. For those two slices, total $^{20}$Ne = (1.91 ± 0.06) × 10⁻¹² cm³STP and $^{21}$Ne = (15.9 ± 7.9) × 10⁻¹⁵ cm³STP. The projected area of surface fragments 1 and 3 totaled (0.22 ± 0.02) cm², yielding a $^{21}$Ne excess over that expected for an air or Q-like composition of (5.0 ± 3.6) × 10⁻¹⁴ cm³STP cm⁻² for the outer slice. This slice was not of uniform thickness, as can be seen from an X-ray photo of the slice (Fig. 4B). Estimating its actual surface area would be necessary if the particle or ion flux was omnidirectional, but here the flux is directional at ~90° to the surface so the relevant measure is the projected area of 0.22 cm². Similar $^{21}$Ne excesses have been observed in C2101 and C2017 aerogel cell surfaces (Meshik et al. 2009, 2010a). They report a value of (2.3 ± 1.0) × 10⁻¹⁴ cm³STP cm⁻² for all neon data, normalized to 0.4 mm thick aerogel layers (Meshik et al. 2010a).

Meshik et al. (2010b) showed that this $^{21}$Ne excess is not an experimental artifact or inherent in the aerogel by analyzing the surface of the Stardust “witness tile” which flew on the mission but was not exposed to impacts of Wild 2 coma particles. They further argue that this $^{21}$Ne excess results from a spallation component within submicron dust embedded in the surface aerogel (Meshik et al. 2010a). We have estimated the possible surface loading of ultrafine coma particles by extrapolating the cumulative mass distribution power law fits derived from dust flux monitors during the Stardust encounter (Tuzzolino et al. 2004) to particle sizes of 1.0 µm and 0.1 µm. Mass loadings, calculated below, of particles between ~3 µm (the minimum size cutoff for the dust monitors) and either of these limits exceed 1 ng cm⁻², more than enough for detectable helium and neon signatures even if gas contents were on the low side— provided the predicted submicron particle concentrations are actually present.

Even if ultrafine coma dust were lodged in aerogel cell surfaces, there are problems with attributing the observed excess $^{21}$Ne abundances to galactic cosmic ray (GCR)-induced spallation prior to its incorporation into Wild 2. Calculations that assume the validity of extrapolating the Tuzzolino et al. (2004) power law show that the required irradiation time is not plausible. Extrapolation to 0.1 µm with a cumulative mass distribution index $\alpha$ = 0.75 (Tuzzolino et al. 2004) (Fig. 4A) predicts ~1.7 × 10⁵ coma particles cm⁻² in the 0.1–3 µm size range with a fluence-weighted average

diameter <D> of 0.38 µm. This fluence of ultrafine coma particles, together with the $^{21}$Ne excess of 2.5 × 10⁻¹⁴ cm³STP cm⁻² calculated below, a GCR $^{21}$Ne production rate in small particles of ~4 × 10⁻¹⁰ cc/g-Myr (Reedy 1987), and the grain density of 0.5 g cm⁻³ adopted by Tuzzolino et al. (2004), yield a cell surface mass loading of ~2.5 ng cm⁻² and require an impossible GCR exposure time of ~25 Gyr.

In addition to the need for an implausibly long GCR exposure age, a second issue with the idea of a $^{21}$Ne excess resulting from GCR spallation in small dust grains is whether such dust could retain it. Ott and Begemann (2000) showed that at GCR energies the effective spallogenic $^{21}$Ne recoil range is ~2–3 µm, so that micron-sized (or smaller) grains would have lost essentially all GCR-produced spallogenic $^{21}$Ne in the interstellar medium.

In the current absence of evidence supporting ultrafine irradiated coma dust as the carrier of excess $^{21}$Ne in the C2044 surface slice and the Meshik et al. (2010a, 2010b) samples, there is no evident candidate for its source material. It is interesting to note, however, that the $1/\sigma^2$-weighted average of excess $^{21}$Ne in the Minnesota C2044 surface slices and the Meshik et al. samples is (2.5 ± 1.0) × 10⁻¹⁴ cm³STP cm⁻², virtually identical to the surface concentration of (2.8 ± 1.2) × 10⁻¹⁴ cm³STP cm⁻² in C2028-1 (see section V.B.1). As discussed in section V.B.1, the gas carriers in this cell surface slice were taken to be ejecta from a collisionally disrupted coma grain that produced the small oblique tracks and terminal particles revealed in high-resolution movie scans. It is therefore tempting to propose a similar origin for the excess $^{21}$Ne in the samples discussed here. However, a search for small tracks of this type on 0.7 cm² of the cell C2044 surface has been carried out and only two highly oblique (~70°) examples were seen, as opposed to up to approximately two orders of magnitude higher densities observed in the surfaces of C2028 and C2009 (Westphal et al. 2008). Cells C2044 and C2028 were adjacent in the collector array (Fig. 8B), so the presence of only two tracks in C2044 may mark an edge of the ejecta distribution from the same primary particle collision that showered C2028. This survey does not preclude the possibility that much higher track densities existed in the blank block surface slice. The center of the C2028-1 surface sample (keystone 5 in Fig. 9) was located only ~10 mm from the C2044 blank block, which was therefore well positioned to receive a directed jet of secondaries from the C2028 primary particle. Meshik et al. (2010b), however, state that their samples were “particle- and track-free,” although at what scale is unclear. Future examination or re-examination of these cell surfaces at appropriate resolution would be informative.
Meshik et al. (2010a) suggested, as an alternative to GCR irradiation of ultrafine dust in cell surfaces, that a lower energy solar cosmic ray (SCR) irradiation near an active early Sun might be an alternative source of spallogenic ²¹Ne. Generation of spallogenic excesses during short SCR exposure times in an early environment of this kind, where near-Sun residences were assumed to occur near or inside ~0.3 AU and within the first few million years of solar history, has been proposed in the context of large enhancements of ²¹Ne abundances in grains extracted from the giant cluster IDP U2-20GCA of likely cometary origin (Pepin et al. 2015a, 2015b). It is interesting to examine this possibility in detail, given that several Stardust samples have excess ²¹Ne contents that also cannot be produced by GCR radiation alone (see the sections VI.C and VI.D). With the average ²¹Ne spallation production rate of 7.8 × 10⁻⁴ cm³STP/g-Myr calculated for U2-20GCA grains in this early near-Sun environment, the average ²¹Ne excess in the C2044 surface slices and Meshik et al. (2010a, 2010b) samples yields an exposure age of ~13 thousand years, fully consistent with the age distribution of the U2-20GCA grains (see Fig. 7 in Pepin et al. 2015a). In this view, irradiated parent particles were subsequently transported to the outer nebular disk and incorporated into Wild 2, then released to the coma and fragmented into fine dust as suggested by observations by Tuzzolino et al. (2004). The relatively high dust loading calculated above, 2.5 ng cm⁻² for 0.1–3 µm dust, may not be a problem. With an average grain size of <0.4 µm, there is only one such particle, on average, in ~600 µm² of cell surface area. It is not obvious that this population would be detectable without large area scans at high resolution. It is true, however, that the larger particle segment of the size distribution, 1–3 µm with <D> = 1.65 µm, would produce small (sub-mm) but observable tracks (see section V.A.1.3). These are rare, only one in ~10⁵ µm². Nevertheless, ~200 such normal incidence tracks should have been present in the 0.22 cm² area of the C2044 blank block surface slices. The fact that only one track of this type was seen in the 0.7 cm² scan of other parts of the C2044 surface argues against either this model, or the assumption that the fine dust size distribution follows a single-slope cumulative mass distribution (Tuzzolino et al. 2004) when extrapolated beyond the data field.

### V.A.3. Blank Block Samples Revisited

The combined UM and CRPG measurements discussed in see section IV.A.5 raise interesting issues regarding potential gas sources. The first set of data in Table 8 (UM 45 through 115/120, CRPG 67, 53) is characterized by low ⁴He, high ³He/⁴He averaging 9–10 × 10⁻⁴, and little if any detectable neon. This is

notably close to the measurable composition that would be carried by a relatively minor mass of silicon carbide grains. Pyrolyses of SiC-Spinel separates by Huss and Lewis (1995) yielded average concentrations of ⁴He = 4.0 ± 0.2 × 10⁻⁴ cm³STP g⁻¹, ²⁰Ne varying from ~10 × 10⁻⁸ to a maximum of ~500 × 10⁻⁸ cm³STP g⁻¹, and a poorly defined ³He/⁴He ratio of ~9 ± 5 × 10⁻⁴ (range ~4 to 14 × 10⁻⁴). Matching the average ⁴He content of 5.1 × 10⁻¹² cm³STP in this set of block samples (see section IV.A.5) would require a fully outgassed SiC carrier mass of 1.29 × 10⁻⁸ g (or more if not completely degassed). Contributions of ²⁰Ne, even assuming the maximum concentration of ~500 × 10⁻⁸ cm³STP g⁻¹ seen in SiC-Spinel, would amount to only ~5% of the blank level and thus would probably be undetectable. A SiC carrier of the proper mass is therefore in reasonably good accord with the low ⁴He, high ³He/⁴He, and essentially no observable neon in this set of blank blocks. The required mass of SiC, if fully outgassed, is equivalent on average to ~3 × 10⁵ 0.3 µm grains or one ~20 µm particle in each block. Neither would have been detected at the optical resolution used to scan the samples.

UM 73 (Table 8A) contains the largest ²⁰Ne abundance of the 89 block samples, more than 34 times the blank value, thus enabling relatively precise measurements of neon isotope ratios. The low upper limit for ³He/⁴He, despite the high abundance of ⁴He, points robustly to a near-zero ³He content. These He-Ne compositions are similar to those reported for nine “anomalous” IDPs from the 26P/Grigg-Skjellerup and 55P/Tempel-Tuttle comet collections (Pepin et al. 2011). Averages and ranges of isotope ratios measured in this anomalous IDP suite are ²⁰Ne/²²Ne = 70 (25–152); ²¹Ne/²²Ne = (0.038 ± 0.032) × 10⁻⁴; ³He/⁴He < 1 × 10⁻⁴; and ⁴He/²⁰Ne = 3.5 (0.3–14.9). The latter three ratios in UM 73 fall within the corresponding IDP ranges. However, the value of ²⁰Ne/²²Ne, the most diagnostic of the neon ratios, is only 16.2. Although clearly anomalously high compared to other known neon ratios (Table 3), it falls below the minimum IDP measurement of 25, possibly a reflection of mixing with sample neon of lower ²⁰Ne/²²Ne. The overall isotopic similarities are nonetheless intriguing. Averages of the anomalous IDP ratios agree well with theoretical calculations of compositions generated in neon novae explosions (Table 7 in Pepin et al. 2011; Starrfield et al. 2009), and the UM 73 data suggest the presence of a component that may carry these compositions. A compositionally similar neon component may also exist in C2044 20A (“Sample X”), where ²⁰Ne/²²Ne = 18 to ~22 (see section IV.A.1). Classical nova outbursts are common in the galaxy, and there is no a priori reason why ejecta from these explosions could not have been incorporated into Wild 2.
Helium compositions in seven of the blank block samples in Table 8 (UM 66 through 111; CRPG 63, 39) collectively provide strong evidence that their helium derives from the solar wind (see section IV.A.5, Fig. 17). Signatures of SW neon, muted by mixing with nonsolar neon components, appear in two of them (UM 66, 117). Three others (UM 41, 27; CRPG 63) contain no detectable $^{20}$Ne above the ~$10^{-12}$ cm$^3$STP blank level, consistent with measured $^4$He contents divided by the SW-$^4$He/$^{20}$Ne ratio of 650. For these samples the question is not one of the source, but rather how they could have acquired SW gases at burial depths removed by ~3–9 mm from direct near-surface exposure to the wind (Table 8, see section V.A.1.3) and with no visible particulate carriers—at least not at the relatively low optical resolution used to examine the samples. This problem illustrates the more general failure to identify mechanisms that could be responsible for erratic emplacements of noble gases in unexpected locations within cell C2044, for instance in these blank block samples and in the vertically cut wafers (see section IV.A.1).

## V.B. Other Measurements

### V.B.1. Cell C2028 Surface Samples

Approximately 17% of the aerogel cell C2028 surface was scanned at the Astromaterials Acquisition and Curation Facility, NASA-Johnson Space Center by ~770 high-resolution zoom-focus movies (Westphal et al. 2014a). A surface-focused frame from one of those movies is shown in Fig. 22A, where surficial, approximately micron-sized contaminant dust is apparent. Nothing is visible in the circle, but with below-surface focus at ~50 µm depth (Fig. 22B) a probable track with an embedded ~1 µm particle appears (note defocused surface dust). Other movie scans revealed similar tracks and ~1 µm terminal particles at a density of ~1 per mm² of cell surface area. Track axes are off-normal to the cell surface and point toward the Stardust spacecraft’s Whipple shield, indicating that the terminal particles are secondary fragments ejected during collision of a coma grain with the shield (Westphal et al. 2008).

The Q-Ne loading of C2028, 13-14, 0, 0 (C2028-1), obtained by dividing its measured abundance by the estimated grain mass present in the 0.5 cm$^2$ sample area, falls in the high $^{20}$Ne concentration group shown in Fig. 23. The box for St C2028 shows concentration variations for grain densities ranging from 1 to 4 g cm$^{-3}$. The assumed grain mass is that of 50 1-µm particles (1/mm$^2$) of density 3 g cm$^{-3}$, yielding 0.29 cm$^3$STP of $^{20}$Ne/g. The principal uncertainty in the C2028-1 neon loading is the grain mass embedded in the sample. The right error

bar reflects the possibility that the particles are coated with melted aerogel to the extent that their actual diameters are only half their average imaged size. The left limit is based on observations of crater populations in the Al foil adjacent to aerogel cells C2008 and C2009, where Hörz et al. (2006) interpreted the crater distribution of descending sizes as impacts of a recently disaggregated small parent particle. Cell C2009 also carries a substantial population (~100 for the complete tile surface) of very small off-normal (~8–11°) tracks consistent with capture of a spray of secondary particles from disaggregating collision of a coma grain with the Whipple shield (Westphal et al. 2008). The density of small craters is orders of magnitude greater in cells with secondary impact debris than in those cells with only craters generated by direct primary particle impacts (Hörz et al. 2006). From crater projectile size scaling, the largest impactor in C2008 Al foil was a ~1 µm grain (Hörz et al. 2006), comparable in size to the C2028 particles, but almost half the total mass of the debris field in the C2008 foil is carried by secondary fragments less than ~0.5 µm in size. Similar secondary ejecta may have been implanted into the surface of cell C2028, but been generally too small for their tracks and terminal particles to be readily detected at the movie scan resolution. An estimate of C2028-1 sample mass calculated from the cratering data yields a $^{20}$Ne concentration ~3 times lower, marked by the end of the left error bar in Fig. 23. Other possible sources of uncertainty in the St C2028 neon loading include a density of secondary particles well above the assumed ~1/mm$^2$ in the unscanned area from which the C2028-1 sample was taken. However, a significantly higher track density, or the presence of larger particles that would severely increase the estimated mass, would likely have been observed during sample preparation but were not.

As noted in section IV.B.1, the $^3$He/$^4$He and $^{21}$Ne/$^{22}$Ne isotopic compositions in C2028-1 suggest a light noble gas spallation component. The calculated spallation $^{21}$Ne concentration relative to Q-$^{21}$Ne/$^{22}$Ne in this sample is large, $(2.8 \pm 1.2) \times 10^{-14}$ cm$^3$STP cm$^{-2}$ or $(1.8 \pm 1.1) \times 10^{-4}$ cm$^3$STP g$^{-1}$. Division by a production rate appropriate to GCR irradiation of small grains in space (Reedy 1987) leads to an impossibly long, ≫4.5 Ga exposure age. An alternative radiation history (see section VI.D) for this and several other Stardust and IDP samples with large $^{21}$Ne excesses posits that particle exposures occurred near the young evolving Sun in an environment of intense flare-generated proton irradiation before their transport to comet-forming nebular regions (Pepin et al. 2015a, 2015b).

### V.B.2. Cell C2086

In general, as in the C2044 blank block samples, the wide variation in helium (and to a lesser extent
<figure><img src="image_23.png" /><figcaption>Fig. 22. Approximately 17% of the cell C2028 surface was scanned at the Astromaterials Acquisition and Curation Office, NASA-Johnson Space Center by ~770 high-resolution zoom-focus movies. A) A surface-focused frame from one movie. Surficial ~µm-sized contaminant dust is apparent. Nothing is visible in the circle, but with below-surface focus at ~50 µm depth (B) a probable track with an embedded ~1 µm particle appears. Note defocused surface dust.</figcaption></figure>
<figure><img src="image_24.png" /><figcaption>Fig. 23. Measured $^{20}$Ne concentrations in lunar ilmenites, IDPs, and Stardust (St) aerogel tracks. The Q-Ne concentration, shown as a lower limit, may actually be similarly high if its carrier comprises only a small fraction of the meteoritic carbonaceous matter in which it is found. References: Q-Ne (Busemann et al. 2000), lunar ilmenite (Eberhardt et al. 1972), IDPs (Pepin et al. 2011) (fig. 2 and references therein), GCP U2-20GCA IDP (Pepin et al. 2015a, 2015b), Manchanito (Palma et al. 2013a), St C2028 grains (Palma et al. 2015), St track 41 (Marty et al. 2008). Data for St tracks 181, 182 in Table 9; track photographs shown in Fig. 10.</figcaption></figure>

neon) abundances and compositions measured in “blank” C2086 samples is puzzling, as are their sources. The enormous gas content of Blank 2 discussed in section IV.B.2 is strikingly similar to SW-He and Ne in composition. Oblique exposure of the comet tray to the wind could have occurred intermittently during the ~9 months at ~2–2.5 AU when the aerogel trays were deployed and oriented for collection of interstellar dust. However, at an average distance of ~2.4 AU, accounting for the helium and neon abundances in this sample would require ~9–11 months of continuous exposure of the comet tray to the wind at normal incidence, and for much longer times for highly oblique off-normal irradiation. A second compelling argument against implanted SW as the source of the C2086 Blank 2 gases is that such an irradiation would have deposited SW ions in the surfaces of *all* cells in the comet tray. This conflicts with the absence of consistent isotopic and elemental signatures of SW gases in other analyzed surface samples (Table 1, Analyses 27–30, 140–141), including those neighboring Blank 2 in C2086.

An alternative possibility is suggested by the observed variations of neon compositions with temperature in this sample (Analyses 115–116). When displayed on a three-isotope plot of $^{20}$Ne/$^{22}$Ne versus $^{21}$Ne/$^{22}$Ne (Fig. 24A), they are interestingly close to what one might expect for release patterns from a Q-carrying IDP that experienced shallow implantation of SW ions during in-space irradiation. Measured near-

solar $^{4}$He/$^{20}$Ne ratios of ~460–530 (Tables 1 and 3), higher than seen in any stratospheric IDP (see Fig. 2 in Pepin et al. 2011), suggest that this postulated IDP was collected without the helium loss characteristic of particles heated during terrestrial atmospheric entry. This could argue for its possible capture in a Stardust aerogel cell—specifically in C2086 Blank 2—and retention of gases liberated during capture heating. However, as in many other cases in this study where extraterrestrial noble gases were detected in “blank” Stardust aerogel samples, there is no optical evidence in Blank 2 for either an entry track or a host particle.

Tracks 181 (Type A) and 182 (Type B or C), of lengths ~200 and ~300 µm, were removed in their entirety from the surface of C2086. Fragments of the incident particles are visible along the T181 track path and in the bulbous wall of T182 (Fig. 10). Analyses of complete small tracks was a high priority in this study because they capture all fragments of the impacting grain, an advantage lost, as in the T41 (see section V.A.1) and terminal particle (see sections IV.B.3 and IV.B.4.) measurements, where only sections of larger tracks with heterogeneous distributions of noble gas carriers could be examined. Data pertaining to analyses of these complete tracks—estimates of projectile sizes and masses, measured $^{20}$Ne abundances and calculated concentrations—are set out in Table 9. Their $^{20}$Ne concentrations are plotted in Fig. 23, together with similarly high concentration levels derived for other
<figure><img src="image_25.png" /><figcaption>Fig. 24. Isotope ratios most accurately measured in this study at UM, displayed for (A) $^{21}$Ne/$^{22}$Ne versus $^{20}$Ne/$^{22}$Ne and (B) $^{3}$He/$^{4}$He versus $^{20}$Ne/$^{22}$Ne. Errors shown where larger than symbol sizes. Squares denote compositions of solar system Ne and He components (Table 3). Theoretical estimates of isotope ratios in ONe novae ejecta, and measured in particles from the Grigg-Skjellerup and Temple-Tuttle comet collections, fall within the shaded areas which extend far beyond the plot boundaries for $^{20}$Ne/$^{22}$Ne (Pepin et al. 2011). Numbers adjacent to the data points identify individual samples in Table 1. (1) C2028,13-14,0,0 (C2028-1); (2) C2044,20A (integrated); (3) C2044,73; (4) C2044, T41; (5) C2044,111; (6) C2044,18A (integrated); (7) C2044,18B (integrated); (8) C2086, blank 2, 700 °C; (9) C2086, blank 2, 850 °C; (10) C2086, blank 5; (11) C2086, blank 6; (12) C2086, T182; (13) C2086, BD1. Patterned bars attached to sample (2) measurements indicate ranges of possible corrections for spectrometer memory contributions (see section IV.A.1). $^{3}$He/$^{4}$He ratio in (10) off-scale at 6.06 ± 0.25 $10^{-4}$, significantly above solar $^{3}$He/$^{4}$He; ratios in (5) and (13) imprecisely measured and not shown in (B). Data from Tables 1, 4, 7, and 8.</figcaption></figure>

Stardust particles in Cell C2028, in track T41, and in IDPs and the lunar regolith.

Two procedural Pt foils were set out in the C2086 sample preparation workstation at SSL/Berkeley to

monitor distribution of potentially contaminating cutting powder (see sections III.C and V.A.1.2). Foil 1 was deployed during excision of blanks #1–8 and T181. Analysis of the foil revealed small but measurable
<table><caption>Table 9. Grain mass and ²⁰Ne abundance data for complete tracks Tr 181 and Tr 182 keystoned from cell C2086, used to calculate ²⁰Ne concentrations plotted in Fig. 23.</caption><thead><tr><th></th><th>Track 181</th><th>Track 182</th><th>Reference</th></tr></thead><tbody><tr><td>Track length L<sub>t</sub> (µm)</td><td>192</td><td>332</td><td>Fig. 10</td></tr><tr><td>Maximum track width D<sub>m</sub> (µm)</td><td>21</td><td>79</td><td>Fig. 10</td></tr><tr><td>Projectile diameter d<sub>p</sub> (µm)</td><td>0.92</td><td>3.48</td><td>Niimi et al. (2012)<sup>a</sup></td></tr><tr><td>Projectile density ρ<sub>p</sub> (g cm<sup>-3</sup>)</td><td>1.28</td><td>0.59</td><td>Niimi et al. (2012)<sup>b</sup></td></tr><tr><td>Projectile radius r<sub>p</sub> (cm)</td><td>4.62 × 10<sup>-5</sup></td><td>1.74 × 10<sup>-4</sup></td><td></td></tr><tr><td>Projectile volume V<sub>p</sub> (cm<sup>3</sup>)</td><td>4.13 × 10<sup>-13</sup></td><td>2.20 × 10<sup>-11</sup></td><td></td></tr><tr><td>Projectile mass m<sub>p</sub> (g)</td><td>5.29 × 10<sup>-13</sup></td><td>1.29 × 10<sup>-11</sup></td><td></td></tr><tr><td>²⁰Ne abundance (cm<sup>3</sup>STP g<sup>-1</sup>)</td><td>2.15 × 10<sup>-13</sup></td><td>1.53 × 10<sup>-12</sup></td><td>Table 1</td></tr><tr><td>²⁰Ne concentration (cm<sup>3</sup>STP g<sup>-1</sup>)<sup>c</sup></td><td>0.407</td><td>0.118</td><td></td></tr><tr><td>Maximum concentration (cm<sup>3</sup>STP g<sup>-1</sup>)<sup>c</sup></td><td>0.945</td><td>0.249</td><td></td></tr><tr><td>Minimum concentration (cm<sup>3</sup>STP g<sup>-1</sup>)<sup>c</sup></td><td>0.188</td><td>0.062</td><td></td></tr></tbody></table>

amounts of ⁴He and ²⁰Ne (Table 1), thus providing a measure of noble gas contents in distributed dust collected on the 25 mm² foil area. Its origin is unclear, but kerf from extraction of He-Ne-rich blank 2 is ruled out by the isotopic disconnects between the blank 2 and foil measurements for both gases. Although the foil 1 gas abundances are roughly similar to those in T181 and several of the C2086 blank blocks (Table 1), it is unlikely that these sample contents reflect significant cross-contamination by the dust. For approximately uniform distribution of cutting powder across the workstation, the expected level of sample contamination would be roughly proportional to exposed sample surface area relative to the 25 mm² foil collection area. Since these blank blocks and T181 are small (<2 mm edge lengths), only <15% of their neon abundances would be potentially attributable to dust contamination. This contaminant would have even less impact on blanks #5–7 that contain significantly more neon than foil 1.

Procedural foil 2 was deployed before extraction of T182. Its intercepted dust is anomalously rich in neon compared to foil 1. Although both ³He/⁴He and ²⁰Ne/²²Ne ratios might suggest that the helium and neon contents of T182 could just reflect contamination by foil 2 material and therefore that the track contains no indigenous gases, this argument is foreclosed by a >3σ difference in ²¹Ne/²²Ne between the foil and T182 (Table 1, Analyses 124, 127). Foil 2 gases are isotopically and elementally dissimilar to those in any of the other analyzed C2086 materials. This indicates that it intercepted dust mobilized by workstation operations from an unsampled reservoir in the tile, and that contamination of the analyzed samples from this source was probably minimal (see section V.A.1.2).

## VI. SUMMARY AND CONCLUSIONS

The principal findings of this study of noble gases emplaced from the Wild 2 coma are identifications of the isotopic signatures of Q-Ne, SW-Ne, and other isotopically distinctive components of both neon and helium. These observations indicate that Q-gases, in particular, existed in the solar system early enough in its history to be incorporated into comets. The following sections focus on several features and implications of the measurements.

### VI.A. Compositions, Abundances, and Locations

Two issues concerning the characteristics of detected gases and their positions in aerogel cells are raised by these investigations. Extensive helium and neon compositions from cell C2044, summarized in Fig. 24 for the most precise measurements, are extremely variable and show no coherent pattern, nor, except for C2044 Track 41 and C2086 Tracks 181 and 182 (Figs. 10B and 10C; Table 9), are they associated with visible carrier grains. Measured abundances of both ⁴He and ²⁰Ne vary by more than three orders of magnitude across the sample suites. Moreover, the locations of noble gas enclaves in subsurface block samples are several millimeters removed from both the cell surface and the axis of track 41 (Table 8), well beyond the likely penetration ranges of small incident particles or ions (see section V.A.1.3) or of material offshoots from the track. There are no visible aerogel damage tracks connecting them to either the surface or the track. Even if some mechanism could be invoked to facilitate ejection of gases or undetectably small trackless fragments from the track bulb wall to distant aerogel positions, the wide

<footnote>ᵃdₚ = (0.044 ± 0.008)Dₘ.</footnote>

<footnote>ᵇρₚ = (0.14 ± 0.01)Lₜ/Dₘ.</footnote>

<footnote>ᶜAll errors for grain mass from 1σ uncertainties in dₚ and ρₚ and in measured ²⁰Ne abundances, added or subtracted to give upper and lower limits for ²⁰Ne concentrations.</footnote>
compositional variability of the neon in these track 41 “halo” samples argues against their origin from the isotopically Q-like track 41 impactor (see section VI.B). Exactly how these samples acquired their gases, and from what sources, is not apparent.

Helium and neon distributions displayed by the more limited data set from cell C2086 are perhaps even more puzzling in that there are no large nearby tracks that one could posit as potential sources of the noble gases observed in the blank aerogel samples. One of these contained an astounding amount of gas with solar wind-like helium and neon compositions (see sections IV.B.2 and V.B.2), and another, with sufficient helium for accurate isotopic measurement (1σ of ±4%), yielded a nonspallogenic ³He/⁴He ratio ~6σ *above* SW (see Fig. 24 caption), a value unprecedented in known solar system helium reservoirs (Table 3). A third C2086 sample contained abundant neon compositionally concordant with Q-Ne. However, a majority of the measured ²⁰Ne/²²Ne ratios in tile C2086 fall below the Q and air values, possibly reflecting mixing of discrete neon components (see section VI.C). In a third cell (C2028, see section V.B.1), the Q-²⁰Ne/²²Ne ratio was measured in a surface slice (C2028-1) that did contain visible carrier grains. A superimposed spallation component appears to be present in this sample (see section VI.D).

## VI.B. Q-Neon

The Q-Ne composition, hosted in a minor but gas-rich macromolecular organic phase ubiquitous in chondritic and ureilitic meteorites (Busemann et al. 2000) appears not to be confined to meteoritic “phase-Q.” A Q-²⁰Ne/²²Ne signature has now been seen in two Stardust aerogel samples containing identifiable particle sources (C2044, T41 and the C2028-1 surface slice), as well as in the Manchanito IDP (Palma et al. 2013a) and several grains from the suspected cometary U2-20GCA giant cluster IDP (Pepin et al. 2015a, 2015b). In none of these is there evidence that the Q-Ne carriers are carbonaceous as in phase-Q, nor would this or any other chemically specific host phase be expected if the carriers were loaded by Q-ion irradiation (see section VI.D).

Five samples in the database with reasonably precise neon isotope measurements display ²⁰Ne/²²Ne ratios within error of the Q-Ne range in Table 3: Analyses 17, 132, 141 in Table 1, and two different fragments of C2044, T41 analyzed in separate laboratories (Table 7). They were extracted from three different aerogel cells. Averaged ²⁰Ne/²²Ne is 10.47 ± 0.19 (stdev). All five ratios overlap each other and their average within their individual uncertainties of

~±3%, suggesting measurement of a single intrinsic ratio with a standard error of the mean (s.e.m.) uncertainty of ±0.08. This average falls near the midpoint of the Q-Ne range. It is well removed, by >3σ (stdev) and ~6σ (s.e.m), from the moderately lower air ratio of 9.80 ± 0.08 (Table 3), so air contamination is effectively ruled out as the source.

In addition to these five samples with relatively precise Q-²⁰Ne/²²Ne ratios, the 53 analyzed aerogel materials in Table 1 where ²⁰Ne/²²Ne ratios are reported include five others, measured with larger errors (average ~±11%) that nominally fall between the SW and Q compositions and deviate from Q-²⁰Ne/²²Ne values by 2σ or less. They probably represent Q-SW mixtures.

The ²¹Ne/²²Ne ratios in these samples were measured with a considerably lower average precision of ~±10%, as evidenced by the scatter and large error bars for ²¹Ne/²²Ne shown in Fig. 24A for the samples closely grouped around Q-²⁰Ne/²²Ne. Although their average of 0.0286 ± 0.0050 (stdev) is nominally exactly at the midpoint of the Q-²¹Ne/²²Ne range, it is not in itself diagnostic of Q-Ne as opposed to Air-Ne because the large uncertainty greatly overlaps the closely neighboring air ratio (Table 3). The ²¹Ne/²²Ne measurements do not challenge the presence of Q-Ne. They further suggest that, as a group, these samples do not carry a detectable spallogenic ²¹Ne excess. However in one of these samples, the C2028-1 surface slice (Table 1, Analysis 141), ²¹Ne/²²Ne is significantly (>2σ) higher than Q-²¹Ne/²²Ne (Fig. 18), probably pointing to a spallation component. Removing this ratio from the ²¹Ne/²²Ne data set yields an average of 0.0271 ± 0.0043 (stdev), nominally slightly below but still well within error of the Q-range.

An interesting feature of Fig. 24B is that three samples identified in the figure caption (#1,4,7), grouped close to Q-²⁰Ne/²²Ne in Fig. 24A, are offset upward with respect to Q-³He/⁴He, toward SW-He but not above it. In view of the absence of a spallation signature in the nominal ²¹Ne/²²Ne average, such ³He/⁴He elevations appear unlikely to be due entirely to spallation although here the large neon isotopic uncertainties provide considerable leeway. The inferred absence of significant spallogenic components in these and several other samples with elevated ³He/⁴He in Fig. 24B suggests the pervasive presence of a SW component, dominant in one of them (C2086 Blank 2, #8-9 in Fig. 24 with ⁴He/²⁰Ne ≥460) but in others contributing helium but too little neon to significantly perturb their corresponding neon ratios in Fig. 24A.

Overall, the Q-Ne signature is arguably present in approximately 15% of the analyzed samples and so appears to be a characteristic feature of neon
composition in a significant fraction of the Wild 2 coma carriers. Additional neon measurements may reflect mixing of Q-Ne with other neon components (see section VI.C).

## VI.C. Evidence for Other Solar System Neon Components in Stardust Samples

Of the 54 total measurements of $^{20}$Ne/$^{22}$Ne in Stardust samples, 10 of the nominal values (~19%) fall below the Q-Ne and terrestrial air compositions (Fig. 25A). Samples 1–5 (black symbols), although positioned close to the air ratio, are not easily attributable to air contamination. They are tightly grouped around an average of 9.34 ± 0.10, >4σ below air. Moreover, their $^{20}$Ne contents vary by a factor >15, and there is only marginal evidence in a few of our aerogel blank blocks for the presence of very low-level air-Ne contamination, even at the lowest $^{20}$Ne levels in these samples. In any case, air contamination could not account for the lower ratios displayed by #7–10 in Fig. 25A. Interestingly, strikingly similar neon ratios in one-third of 15 particles extracted from the suspected cometary giant cluster IDP U2-20GCA (Pepin et al. 2015a, 2015b), here with average $^{20}$Ne/$^{22}$Ne = 9.27 ± 0.35, are seen in Fig. 25B to fall between Q-Ne and HL-Ne, a component hosted in meteoritic nanodiamonds (Table 3). Neon contents in this subset are again widely variable, in this case by a factor of 40. The presence in both Wild 2 and U2-20GCA of $^{20}$Ne/$^{22}$Ne ratios between Q and HL, as well as their shared Q-Ne signatures in other samples (see section VI.B, Pepin et al. 2015a, 2015b), point to a comet-IDP connection in neon characteristics (further discussed in section VI.D).

In searching for mechanisms that might generate the isotopic arrays in Fig. 25, one could consider the possibility that these sub-Q ratios result from isotopic fractionations accompanying Fick's Law diffusive loss from parent particles originally carrying Q-Ne. However, some of the required loss factors and corresponding pre-loss neon contents are unlikely or implausible. If Ne-rich Stardust sample #1 (C2044,111; Table 8A) in Fig. 25A were assumed, for example, to be a post-loss residual, its parental Q-Ne content would have been ~16× higher than its measured $^{20}$Ne, close to the highest neon abundance seen in all the aerogel samples. Much more extreme loss factors of ~170 and >10$^5$ respectively are needed to diffusively generate $^{20}$Ne/$^{22}$Ne in #7 (C2086, blank 6) and #10 (Puki A) as fractionated residuals of an initial Q composition. Even if derived from an HL-Ne parent, an isotopically fractionating neon depletion of >9000 would be required to match the #10 composition.

We favor an alternate interpretation that Fig. 25A illustrates mixing of the known solar system components Q-Ne, HL-Ne, and by inference neon characterized by a very low $^{20}$Ne/$^{22}$Ne ratio (viz. Fig. 25A, #9–10). Q-Ne is clearly present in several of the analyzed samples (see section VI.B). In this view, sample #7 (C2086, blank 6) most accurately identifies the additional presence of relatively pure HL-Ne in the Stardust neon inventory. This link to HL-Ne is supported by measured $^{21}$Ne/$^{22}$Ne (0.0396 ± 0.0052) in #7, which lies within <1σ of the HL ratio and is >2σ removed from the average Q value (Tables 1 and 3). Helium composition in #7 is also concordant with HL-$^{3}$He/$^{4}$He within ~1σ; here subtraction of a small SW-He component, if present (see section VI.B), would improve the match. These measured ratios and errors and their overlaps with HL-Ne and He compositions are displayed as sample #11 in Fig. 24.

Samples #1–5 (and perhaps #6) would then reflect mixtures of HL-Ne and Q-Ne components. Such mixing also accounts for the ratios between Q and HL in the five-particle subset from the U2-20GCA IDP (Fig. 25B). In this interpretation, the very low $^{20}$Ne/$^{22}$Ne ratios in #9–10 require mixtures of either HL or Q-Ne with a component containing little $^{20}$Ne. Neon-E, with $^{20}$Ne/$^{22}$Ne ~ 0 (Eberhardt et al. 1979; Busemann et al. 2000), is the only known candidate.

Except for #4 (C2086, Track 182) and #10 (Puki A, Track 77), there are no visible host particles for these Stardust sub-Q-Ne compositions. Puki A is a T77 terminal olivine particle with a known mass of ~3.4 ng and a very high $^{21}$Ne/$^{22}$Ne ratio (0.082 ± 0.016). If viewed as a mix of Ne-E with the HL or Q component, its measured $^{20}$Ne/$^{22}$Ne ratio is reproduced by mixtures of ~35% Ne-E with 65% HL-Ne, or ~45% Ne-E with 55% Q-Ne. Measured $^{21}$Ne/$^{22}$Ne is then respectively 3.6σ and 4.2σ above the ratios calculated for the mixtures. These large $^{21}$Ne excesses signal the presence of a substantial spallation component. Puki A is similar to Stardust C2028-1, T182, and several grains from the U2-20GCA IDP in having implausibly long exposure ages if subject only to GCR irradiation (see section VI.D). This high-spallation group also includes #9 (C2044,17,41,0,0, see section IV.A.2) with $^{21}$Ne/$^{22}$Ne = 0.066 ± 0.018).

Cell C2086, with only nine measurements of sample neon compositions, was analyzed much less comprehensively than cell C2044. It is notable that despite this smaller data base half of the 10 samples in Fig. 25A originate from C2086, comparable to the one-third fraction carrying sub-Q ratios in U2-20GCA grains (Fig. 25B) but very different from cell C2044 where only 3 of the 38 reported neon compositions exhibit sub-Q isotopic signatures. This observation
<figure><img src="image_26.png" /><figcaption>Fig. 25. A) Measured Stardust $^{20}$Ne/$^{22}$Ne ratios nominally below the terrestrial air and Q-range compositions. Data falling in the Q-HL interval shown in black, those nominally close to HL-Ne in gray, and extreme data in white. Numbers adjacent to the data points identify individual samples in Table 1. (1) C2044,111; (2) C2086, blank 7; (3) C2086, blank 5; (4) C2086, T182; (5) Interstellar tray, IS2 surface; (6) C2044,25; (7) C2086, blank 6; (8) C2086,A3; (9) C2044,17,41,0,0 (integrated); (10) track 77 terminal particle Puki A, 600 °C (76% of total Ne release). Four of these samples (1,3,4,7) are included in Fig. 24A,B as (5,10,12,11), respectively. Meteoritic Ne-E ($^{20}$Ne/$^{22}$Ne ~ 0) and Ne-A ratios from Busemann et al. (2000) and references therein; Air-Ne, Q-Ne, and HL-Ne from Table 3. B) Sub-Q-Ne ratios in grains #1–6 from the giant cluster IDP U2-20GCA, from data plotted in fig. 4 of Pepin et al. (2015a). These Fig. 4 data also illustrate the presence of Q-Ne in 5–6 additional U2-20GCA grains.</figcaption></figure>

indicates that individual Stardust aerogel cells do not contain the same proportions of specific neon compositions, which in turn suggests that particulate or gaseous noble gas carriers in the Wild 2 coma itself were not spatially well mixed along the spacecraft sampling trajectory, even at the small scales of collector cell separations.

VI.D. $^{20}$Ne Loadings in Stardust Track Particles and IDPs, Radiation Exposure Histories, and Stardust-IDP Connections

The observation that lunar ilmenite grains exposed to the SW fall near the right end of the Fig. 23 $^{20}$Ne

concentration diagram occupied by the Stardust and IDP samples suggests that these materials were also loaded to high levels by neon ion irradiation. The communality of these two sample suites extends, in some cases, to neon isotopic composition and to irradiation histories. However, here the irradiating neon ions were isotopically Q-Ne, not SW-Ne. In Stardust T41 (Marty et al. 2008) and Stardust aerogel cell C2028 (Palma et al. 2015), the $^{20}$Ne/$^{22}$Ne ratio falls in the Q-Ne range of ~10.1–10.7 (see section VI.B). The same Q signatures are evident in the IDP Manchanito (Palma et al. 2013a), and 6 of the 15 grains extracted from the U2-20GCA IDP (Pepin et al. 2015a, 2015b) also contain $^{Q-20}$Ne/$^{22}$Ne. $^{20}$Ne/$^{22}$Ne ratios in many other
IDPs (black bar in Fig. 23) are near 14.0, consistent with SW implantation during space exposure (Pepin et al. 2012), but others are close to Q-Ne or lie between the SW and Q ratios (see Fig. 2 in Pepin et al. 2011). Neon compositions in some IDPs and in the analyzed Stardust T41 and C2028 track particles therefore share a common fingerprint: neon implantation during exposure in an environment characterized by a high flux of Q-Ne ions.

Clues to the nature of such an environment are seen in measured <sup>21</sup>Ne abundances, which are sensitive recorders of spallogenic contributions. Among the samples plotted in Fig. 23, C2028-1, T182, and the U2-20GCA grains display <sup>21</sup>Ne excesses that result in impossibly long exposure ages if attributed just to GCR spallation in small particles (Pepin et al. 2015a, 2015b). Puki A is another example (see section VI.C), although its <sup>20</sup>Ne loading of ~3 × 10<sup>-4</sup> cm<sup>3</sup> g<sup>-1</sup> is approximately three orders of magnitude lower than the other Stardust samples in Fig. 23. The T41 cross-sectional sample C2044,17,41,0,0 (see section IV.A.2) and the highly spallogenic C2044 surface slices (see section IV.A.3) probably also belong in this category, but without estimates of carrier masses their neon concentrations and exposure histories cannot be examined.

These <sup>21</sup>Ne excesses point to particle residences, before their incorporation into Wild 2, in an environment characterized by a much higher intensity of spallation-inducing radiation than the GCR alone. An alternative irradiation history, noted in section V.B.1, is spallation by solar flare protons close to the young evolving Sun, a high-temperature locale where many of the rocky fragments found in Wild 2 are thought to have formed (e.g., Joswiak et al. 2012) and where estimates of flare activity and associated high-energy proton fluxes are higher, relative to the present Sun, by a factor of ~10<sup>5</sup> (Feigelson et al. 2002). Short duration exposures in this ancient environment could account for the large <sup>21</sup>Ne excesses in Stardust and U2-20GCA grains (Pepin et al. 2015a, 2015b). Implantation of the Q component by an irradiating flux of Q composition ions could also have occurred in this setting.

There are certainly unresolved questions with this model of exposure histories. For instance, neither the Stardust track 41 impactor particle nor the Manchanito IDP appear to contain excess <sup>21</sup>Ne although both display the Q-Ne isotopic signature, indicating exposure to a Q-ion flux but suggesting residence beyond the range of high-energy solar proton radiation. A central point, however, is the existence of data linking Stardust samples to specific IDPs: <sup>20</sup>Ne concentrations (Fig. 23), Q-like <sup>20</sup>Ne/<sup>22</sup>Ne ratios, and in some cases excesses of <sup>21</sup>Ne.

VI.E. Overview of Stardust Noble Gas Measurements

The spans of helium and neon isotopic compositions revealed in this study are extremely broad. <sup>3</sup>He/<sup>4</sup>He ranges from essentially pure <sup>3</sup>He through ratios above or commensurate with solar wind to no detectable <sup>3</sup>He at all, and <sup>20</sup>Ne/<sup>22</sup>Ne from values exceeding solar wind, through the solar, Q and HL ratios, to measurements suggesting the presence of Neon-E with almost no <sup>20</sup>Ne. Isotopic enrichments attributable to spallation contributions are evident in several samples. These observations, replicated in part by interlaboratory measurements, must reflect the multiple sources and isotopic variability of noble gases in the Wild 2 coma. This overriding conclusion is independent of several puzzling details encountered in the course of the investigation, notably how isolated pockets of coma gases could have been emplaced in totally unexpected locations, deep within Stardust aerogel cells and without detectable particle hosts. We have no adequate explanations for these observations. Several of the coma noble gas compositions have been seen in meteorites or putative cometary IDPs and attributed to both solar and extra-solar sources. The data reported here suggest that Wild 2 contains carriers for virtually all of them, and perhaps others.

This study, however, serves only as a preliminary reconnaissance of what may exist in the Wild 2 coma. It was directed solely to mass spectrometric measurements of light noble gas distributions in bulk aerogel samples. A logical second-stage examination of Stardust aerogel cells would ideally focus on high-resolution searches for small embedded carrier particles, and on efforts to obtain chemical and isotopic information on any detected by utilizing instruments and techniques similar to those currently employed in IDP studies (e.g., Busemann et al. 2009); these far outstrip the capabilities and potential returns of bulk mass spectrometry alone. If measurements of this kind were sufficiently nondestructive, samples containing detected particles could subsequently be analyzed for noble gases. Such a study could add significantly to current understanding of the Wild 2 coma composition, and perhaps identify mechanisms responsible for emplacing and retaining coma gases in locations deep within the aerogel collector cells.

*Acknowledgments*—In addition to thanking the sample providers named within the manuscript, the authors wish to gratefully acknowledge sample preparation assistance by Melissa Rodriguez, Ronald Bastien, and Christopher Snead (Astromaterials Acquisition and Curation Facility, NASA-JSC). Sabbatical support for this work was provided by Minnesota State University,