JuanCastillo29/space-mission-intelligence-data huggingface dataset:
Deriving the bulk composition of comet 67P

The bulk elemental abundance of 67P was not measured directly. Instead, two instruments measured two different components, which we call here the ice (anything measured as a gas in the coma) and refractory (measured on solid dust particles) components. The ice component was measured by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA; Balsiger et al., 2007). The refractory component was measured on dust particles collected by the COmetary Secondary Ion Mass Anaylzer (COSIMA; Kissel et al., 2007). To get the bulk elemental composition of 67P, we need to combine these two components. To do so and to properly track the errors and uncertainties, we model the data from both ROSINA and COSIMA, assuming that the respective measured values represent the median of a log-normal distribution and the respective errors are 1-σ errors. Taking one million random draws of each element with the respective distribution gives a good match to the data, as shown in the appendix (Fig. 6).
The refractory component from COSIMA

The COSIMA instrument collected dust particles in the coma of 67P. These particles were, amongst other things, analysed with a high-resolution time-of-flight secondary ion mass spectrometer. The elemental abundances of the cometary dust that we are using here were presented in Table 3 of Bardyn et al. (2017). The Nitrogen to carbon ratio, N/C, was measured by Fray et al. (2017).

Here, we use all the elemental abundances with respect to iron, Fe. But, because N/Fe was not directly measured the ’minus’-error became very large when deriving it from the measurements of the N/C, C/Si, and Si/Fe ratios. Because this large error leads to a negative lower bound for the N/Fe ratio, which is non-physical, we have chosen to use the logarithm of the ’positive’ error also for the ’negative’ error. This, in principle, can overestimate the abundance of Nitrogen, but given that Nitrogen is almost two orders of magnitude depleted with respect to the major elements of hydrogen, carbon, and oxygen, we believe this assumption is justified and will not affect our results in a significant way. Another assumption of the COSIMA measurements is that the hydrogen-to-carbon atomic ratio is 1.04 ± 0.16 (Isnard et al., 2019). This is justified by the fact that the macro-molecules that were detected in 67Ps particles contain more H than the Insoluble Organic Matter (IOM) measured in chondritic meteorites (Fray et al., 2016). The highest H/C in IOM that was measured is 0.8 (Alexander et al., 2007). We thus adopt this same assumption as Isnard et al. (2019) and Bardyn et al. (2017). Additionally, COSIMA found that the refractory component of 67P is made up by roughly one half in organics and the other half in minerals. All used measurements from Bardyn et al. (2017) and Fray et al. (2017), as well as the log-normal parameters used to model the measurements, are listed in Table 1. To zeroth order, the refractory component of 67P consists of equal parts of hydrogen, carbon, and oxygen (left panel of Fig. 6). The next most abundant element, silicon, is a factor of four less abundant than the three major elements. All other species are at least 1.5 orders of magnitude less abundant than the major elements. Potassium is the least abundant of the measured elements and is depleted by more than three orders of magnitude compared to hydrogen.
The ice component from ROSINA

The composition of the gases in the coma of 67P was measured by the ROSINA instrument. ROSINA consisted of three subsystems: the Double-Focusing Mass Spectrometer (DFMS), the Reflectron-type Time-Of-Flight mass spectrometer (RTOF), and the Comet Pressure Sensor (COPS). Rubin et al. (2019) determined the molecular abundances of 45 major and minor volatile species with respect to water. We use these values to derive the elemental abundances with respect to water (right panel of Fig. 6). Table 2 lists the ROSINA measurements and the log-normal parameters used to model the measurements. Compared to the refractory component, the volatiles are depleted in carbon. Hydrogen and oxygen are still the major elements, while sulphur and nitrogen are both less abundant by two orders of magnitude, similar to the refractory component. The three noble gases argon, krypton, and xenon are five to seven orders of magnitude less abundant than hydrogen.
Constraining the refractory-to-ice mass ratio

To combine the refractory and volatile/ice components and arrive at the bulk elemental composition of 67P, we are left with one unknown: the refractory-to-ice mass ratio, χ. There is considerable debate on the value of χ for 67P, or any comet for that matter (see review by Choukroun et al., 2020). Values from 0.1 to 10 and higher can be found in the literature. The large discrepancies in these values result primarily from a lack of knowledge about how much dust falls back to the surface and to what extent that dust has lost its ice component while on ballistic trajectories around the comet.
Rubin et al. (2019) derived bulk elemental abundances for hydrogen, carbon, oxygen, nitrogen, and silicon, assuming χ equal to one and three. Here, we take a different approach and start by being agnostic about the value of χ and thus initially leave it as a free parameter. We will see later that doing that will allow us to independently constrain χ using only arguments of the elemental composition. Additionally, we derive the bulk elemental abundances for 12 additional elements (Na, Mg, Al, S, K, Ar, Ca, Cr, Mn, Fe, Kr, Xe), which crucially also includes the noble gases argon, krypton, and xenon. Thus, for each value of χ, we draw a random composition for the refractory and volatile/ice components, assuming the log-normal distribution described above. Each component is mixed with the other at the respective χ, from which we can calculate the resulting bulk elemental composition. By performing one million such random draws, we can calculate the median elemental composition and the associated 1-σ errors.

[Figure: Figure 1: The top panel shows the bulk carbon-to-oxygen atomic ratio of comet 67P as a function of the refractory-to-ice mass ratio, χ. The dark blue shaded area encompasses 50% of the cases, and the light blue ±1σ. The solar value of 0.594 is taken from Asplund et al. (2021). The bottom panel shows the fraction of cases with a super-solar C/O ratio.]
The bulk composition of 67P

Given the arguments above, we have constrained the refractory-to-ice mass ratio to χ < 1.7. Figure 3 presents an overview of the bulk abundances for 67P relative to the solar values for three different values of χ. The full list of values presented in that figure can be found in Tables 3 and 4. Several things are of note. First, if χ < 0.3 xenon becomes super-solar in 84% of the cases. There is no particular reason that xenon should be super-solar, so we can consider this a mild lower limit on χ. Second, oxygen is super solar if χ < 0.5 and becomes consistent with the solar value at χ ∼0.5. Thus, a rather plausible range for χ is 0.5 < χ < 1.7. For this range of χ the bulk mass fraction of water is between 28% (χ = 1.7) to 52% (χ = 0.5). Interestingly, the prediction from Bitsch & Battistini (2020) for a solar-type star is a water mass fraction of 35%. This is well within our range with a water mass fraction of 35% at χ = 1.2. Oxygen is solar in the median case at χ = 1.1 and thus represents our preferred case, which also happens to lie in the middle of our constraint of χ (0.5 < χ < 1.7).

[Figure: Figure 2: The top panel shows the oxygen-to-silicon ratio for oxygen that is in water (subscript ’ex’) and that which is in anhydrous form (subscript ’an’). The values for CI chondrites are taken from Alexander (2019). The bottom panel shows the ratio of the two oxygen components. Note that the bottom panel is not simply the ratio of the lines in the top panel. Oex and Oan are not uncorrelated, and therefore, their ratio and respective errors need to be computed for each random draw. The darker shaded areas contain 50% of the cases while the lighter areas contain ±1σ]

[Figure: Figure 3: The top panels of each column show the bulk elemental composition of 67P with respect to silicon compared to the solar elemental composition (Asplund et al., 2021). Each column shows the results for a different refractory-to-ice mass ratio, χ. The bottom panels of each column show the abundances of each element normalised to the solar values. The green boxes are the values for 67P, while the purple dots show the composition for CI chondrites (Alexander, 2019). All bulk abundances and corresponding errors for 67P are also available in Tables 3 and 4.]
: The top panels of each column show the bulk elemental composition of 67P with respect to silicon compared to the solar elemental composition (Asplund et al., 2021). Each column shows the results for a different refractory-to-ice mass ratio, χ. The bottom panels of each column show the abundances of each element normalised to the solar values. The green boxes are the values for 67P, while the purple dots show the composition for CI chondrites (Alexander, 2019). All bulk abundances and corresponding errors for 67P are also available in Tables 3 and 4.]

Similarly, Bitsch & Battistini (2020) predict an oxygen mass fraction of 55% whereas we get a range of 58% (χ = 1.7) to 69% (χ = 0.5). An updated overview of literature values of χ for comet 67P can be found in the appendix in Fig. 7. Third, the noble gases argon, krypton, and xenon are all enriched by orders of magnitude compared to CI chondrites. Fourth, and maybe most importantly, xenon (the least volatile of the three mentioned noble gases) is present in quasi-solar abundance for all the plausible cases of χ (within 1σ xenon is solar for χ < 1.2 and slightly sub-solar for larger χ). We can also look at Xe/H2O for a sanity check, which comes only from the ROSINA measurements and does not rely on our modelling of the bulk abundances. For 67P, Xe/H2O = (2.4 ± 1.1) · 10−7 (Rubin et al., 2019) while for the Sun using the values by Lodders (2003) it is (3.89 ± 0.48) · 10−7 [assuming all oxygen is in water], (6.95 ± 0.86) · 10−7 [assuming 56% of the oxygen atoms are in water as in the case of our χ = 1.1], and (7.78 ± 0.96) · 10−7 [assuming half of the oxygen is in water]. The 67P xenon to water ratio is sub-solar by factors of 1.6 ± 0.8, 2.9 ± 1.4, and 3.2 ± 1.5 for the three cases assuming different amounts of oxygen trapped in water. Thus, even for the measurements only derived from ROSINA, xenon is quasi-solar. Fifth, Krypton, which is slightly more volatile than xenon, is slightly depleted (factor of 3-10) with respect to the solar value. Argon, on the other hand, the most volatile of the three noble gases, is strongly depleted (∼3 orders of magnitude) with respect to the solar value.
on short timescales (e.g., Takeuchi & Lin, 2002, 2005), and the dust disk begins to contract (c). Eventually, structures in the disk form (Andrews, 2020) and trap dust, thus preventing the complete loss of the dust disk (d). Dust pebbles trapped within these pressure bumps remain there until they are accreted into planetesimals. Depending on the location in the disk and the accretion time, pebbles lose volatiles/retain the respective volatile species. The xenon, krypton, and argon lines are illustrated as examples.]

[Figure: Figure 5: The temperature as a function of heliocentric distance is shown for a large set of models of evolved protoplanetary disks (light grey lines). The black dashed line shows the temperature profile of a passive disk (Chiang & Youdin, 2010). The orange lines show the sublimation lines for xenon (solid), krypton (dashed), and argon (dotted) for the values presented in Öberg & Wordsworth (2019).]

Additionally, CI chondrites are depleted relative to comets by at least two orders of magnitude even when we assume a χ = 10 (the order of magnitude for the amount of water in CIs; Fig. 3). In our scenario, this implies that CIs formed inside the xenon sublimation line, i.e., inside 12 au, namely the giant planet region. This distance is also consistent with dynamical models of CI formation beyond Jupiter and implantation into the asteroid belt (Raymond & Izidoro, 2017). In our model, because the comet forms well inside the argon condensation line, the only argon present would be the one adsorbed from the protoplanetary disk. This is consistent with the results of Yokochi (2022), who showed experimentally that the adsorption of argon at 36-45 K (i.e. between the Kr and Xe lines) would lead to the observed Ar/H2O ratio of 67P. Additionally, argon, adsorbed from the gas of the disk, would have solar isotopic composition, as observed (Balsiger et al., 2015). Both heating from solar radiation (e.g., Guilbert-Lepoutre et al., 2016; Gkotsinas et al., 2024) and collisions (e.g., Bottke et al., 2023; Jutzi & Michel, 2020) could further deplete argon.
Conclusions

We have combined COSIMA and ROSINA data on the elemental abundances of comet 67P’s refractory and volatile/ice components. To calculate the bulk elemental abundance of 67P, we started out being agnostic about the refractory-to-ice mass ratio, χ, and left it as a free parameter. The resulting elemental composition showed, though, that only 0.5 < χ < 1.7 leads to reasonable results. These constraints on χ come from the amount of oxygen in water vs anhydrous material being not smaller than in CI chondrites as well as other elements (such as xenon and oxygen) not being in super-solar abundance relative to refractory elements. These values are consistent with other estimates from the coma of 67P (e.g. Biver et al., 2019; Marschall et al., 2020b; Läuter et al., 2020; Combi et al., 2020; Choukroun et al., 2020) and suggest little volatile loss from material falling back to the nucleus, which is also consistent with thermophysical studies of the fallback material (Davidsson et al., 2021). For these values of χ comet 67P would have a 0.2 < C/O < 0.4. Furthermore, for the mentioned range in χ, xenon is present in quasi-solar abundance. At the same time, krypton is slightly depleted relative to the solar value, and argon is heavily depleted. This pattern naturally occurs if 67P formed from pebbles that were trapped and retained in the 25-35 au region for millions of years. Over this retention time, diffusion causes the volatiles to reach the surface and sublimate if the temperature is high enough. Xenon was thus accreted in the solid form and, therefore, in solar abundance. In contrast, krypton was partially lost, and argon only accreted through adsorption from the gas.
Acknowledgements
We acknowledge the funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant Agreement No. 101019380). Additionally, we acknowledge support from programme ANR-20-CE49-0006 (ANR DISKBUILD). We thank Martin Rubin for clarifying discussions about their work (Rubin et al., 2019) from which we used the elemental abundances derived from ROSINA data. We thank Nicolas Fray, Anaïs Bardyn, Hervé Cottin, and Martin Hilchenbach for similar clarifying discussions about the most up-to-date COSIMA data to use for the elemental composition of the refractories (Bardyn et al., 2017; Fray et al., 2017). Furthermore, we thank Niels Ligterink for insightful discussions on ad-/desoption and diffusion of noble gases.
Elemental abundances of COSIMA and ROSINA

Each element is assumed to follow a log-normal distribution
� .
1 xσ √
2π exp � −(ln x −µ)2
2σ2 The values of µ and σ are shown in Tables 1 and 2, and the comparison of the ’model’ to the measurements is shown in Fig. 6.

[Figure: Figure 6: The left panel shows the elemental abundances (number of atoms) of the refractory component with respect to iron, Fe, as measured by COSIMA, while the right panel shows the elemental abundances (number of atoms) of the volatile/ice component with respect to molecular water, H2O, as measured by ROSINA. The green points represent the measurements of the respective instruments, and the purple box plots the results from the one million random draws from the respective log-normal distributions. The upper and lower bounds of the box contain 50% around the median while the whiskers contain 68%, i.e. ±1σ.]

[Table: Table 1: The COSIMA elemental abundances (Bardyn et al., 2017) with respect to Iron as well as the derived µ and σ of the log-normal distribution used to model the data.] µ σ 1 H Hydrogen 19.76 15.36 33.04 2.98 0.96 6 C Carbon 19 14 25 2.94 0.85 7 N Nitrogen 0.66 0.69 1.1 -0.42 1.02 8 O Oxygen 19 11 22 2.94 0.75 11 Na Sodium 0.28 0.18 0.56 -1.27 1.01 12 Mg Magnesium 0.4 0.2 0.4 -0.92 0.67 13 Al Aluminum 0.061 0.037 0.095 -2.80 0.88 14 Si Silicon 3.5 1.7 3.7 1.25 0.68 16 S Sulfur 0.5 0.094 0.116 -0.69 0.21 19 K Potassium 0.0074 0.0035 0.0069 -4.91 0.63 20 Ca Calcium 0.019 0.008 0.015 -3.96 0.56 24 Cr Chromium 0.0093 0.0035 0.0062 -4.68 0.49 25 Mn Manganese 0.015 0.003 0.003 -4.20 0.20 26 Fe Iron 1 0 0 0.00 0.00
Element
Name
X/Fe
-
+

[Table: Table 2: The ROSINA elemental abundances (Rubin et al., 2019) with respect to water as well as the derived µ and σ of the log-normal distribution used to model the data.] µ σ 1 H Hydrogen 2.1002E+00 3.1708E-02 3.4491E-02 0.74 0.02 6 C Carbon 9.8983E-02 2.9185E-02 3.0450E-02 -2.31 0.29 7 N Nitrogen 9.3630E-03 2.8593E-03 2.8593E-03 -4.67 0.30 8 O Oxygen 1.1980E+00 6.3501E-02 6.9131E-02 0.18 0.06 16 S Sulfur 1.8557E-02 9.7949E-03 1.4148E-02 -3.99 0.59 18 Ar Argon 5.8000E-06 2.2000E-06 2.2000E-06 -12.06 0.37 36 Kr Krypton 4.9000E-07 2.2000E-07 2.2000E-07 -14.53 0.43 54 Xe Xenon 2.4000E-07 1.1000E-07 1.1000E-07 -15.24 0.44
Element
Name
-
+

[Figure: Figure 7: Summary of literature values for the refractory-to-ice mass ratio, χ: Measurements in the coma are shown in blue and include the ones derived early in the mission from the GIADA instrument (Rotundi et al., 2015), integrated values combining coma gas/dust measurements and the total mass loss (Biver et al., 2019; Marschall et al., 2020b). Estimates from nucleus measurements are shown in purple (Herique et al., 2016; Pätzold et al., 2016; Fulle et al., 2017, 2019). The value we constrain here purely from the elemental composition is shown in green. Additionally, we show the values of χ for which the C/O ratio becomes super solar and the value of χ when at least 50% of cases have less oxygen in water than anything else.]
element
name
X/Si
+
-
X/Si
+
-

1 H Hydrogen 8.2689E+01 9.5216E+01 4.1470E+01 5.2849E+01 6.1041E+01 2.6713E+01 6 C Carbon 9.2908E+00 1.4429E+01 5.3951E+00 7.7621E+00 1.2871E+01 4.6455E+00 7 N Nitrogen 5.8488E-01 7.9705E-01 3.1997E-01 4.3520E-01 6.4395E-01 2.4500E-01 8 O Oxygen 4.8396E+01 5.8151E+01 2.4576E+01 3.1377E+01 3.8295E+01 1.6105E+01 11 Na Sodium 7.9736E-02 1.8869E-01 5.6142E-02 7.9736E-02 1.8869E-01 5.6142E-02 12 Mg Magnesium 1.1364E-01 1.8106E-01 6.9558E-02 1.1364E-01 1.8106E-01 6.9558E-02 13 Al Aluminum 1.7209E-02 3.4748E-02 1.1480E-02 1.7209E-02 3.4748E-02 1.1480E-02 14 Si Silicon 1.0000E+00 0.0000E+00 0.0000E+00 1.0000E+00 0.0000E+00 0.0000E+00 16 S Sulfur 8.2997E-01 1.1444E+00 4.5791E-01 5.6403E-01 7.3344E-01 3.0396E-01 19 K Potassium 2.1090E-03 3.1999E-03 1.2671E-03 2.1090E-03 3.1999E-03 1.2671E-03 18 Ar Argon 2.0555E-04 2.7418E-04 1.1058E-04 1.2333E-04 1.6451E-04 6.6349E-05 20 Ca Calcium 5.4035E-03 7.6795E-03 3.1547E-03 5.4035E-03 7.6795E-03 3.1547E-03 24 Cr Chromium 2.6481E-03 3.4905E-03 1.5036E-03 2.6481E-03 3.4905E-03 1.5036E-03 25 Mn Manganese 4.2759E-03 4.3790E-03 2.1680E-03 4.2759E-03 4.3790E-03 2.1680E-03 26 Fe Iron 2.8590E-01 2.7567E-01 1.4117E-01 2.8590E-01 2.7567E-01 1.4117E-01 36 Kr Krypton 1.7435E-05 2.4472E-05 9.6898E-06 1.0461E-05 1.4683E-05 5.8139E-06 54 Xe Xenon 8.5395E-06 1.1927E-05 4.7560E-06 5.1237E-06 7.1563E-06 2.8536E-06 element name X/Si +

[Table: Table 4: Continuation of Table 3 with the bulk elemental abundances of 67P with respect to silicon for the refractory-to-ice mass ratio, χ, of 1.1 and 1.7. The errors are ±1σ.
χ = 1.1 χ = 1.7]
-
X/Si
+
-

1 H Hydrogen 2.7996E+01 3.3598E+01 1.4465E+01 2.0543E+01 2.5603E+01 1.0819E+01 6 C Carbon 6.4843E+00 1.1635E+01 4.0489E+00 6.1019E+00 1.1317E+01 3.8755E+00 7 N Nitrogen 3.0525E-01 5.2457E-01 1.8182E-01 2.6449E-01 4.9492E-01 1.6259E-01 8 O Oxygen 1.7401E+01 2.2356E+01 9.1901E+00 1.3257E+01 1.7670E+01 7.1568E+00 11 Na Sodium 7.9736E-02 1.8869E-01 5.6142E-02 7.9736E-02 1.8869E-01 5.6142E-02 12 Mg Magnesium 1.1364E-01 1.8106E-01 6.9558E-02 1.1364E-01 1.8106E-01 6.9558E-02 13 Al Aluminum 1.7209E-02 3.4748E-02 1.1480E-02 1.7209E-02 3.4748E-02 1.1480E-02 14 Si Silicon 1.0000E+00 0.0000E+00 0.0000E+00 1.0000E+00 0.0000E+00 0.0000E+00 16 S Sulfur 3.4247E-01 4.0175E-01 1.7770E-01 2.7567E-01 3.0737E-01 1.4067E-01 19 K Potassium 2.1090E-03 3.1999E-03 1.2671E-03 2.1090E-03 3.1999E-03 1.2671E-03 18 Ar Argon 5.6059E-05 7.4775E-05 3.0159E-05 3.6273E-05 4.8384E-05 1.9514E-05 20 Ca Calcium 5.4035E-03 7.6795E-03 3.1547E-03 5.4035E-03 7.6795E-03 3.1547E-03 24 Cr Chromium 2.6481E-03 3.4905E-03 1.5036E-03 2.6481E-03 3.4905E-03 1.5036E-03 25 Mn Manganese 4.2759E-03 4.3790E-03 2.1680E-03 4.2759E-03 4.3790E-03 2.1680E-03 26 Fe Iron 2.8590E-01 2.7567E-01 1.4117E-01 2.8590E-01 2.7567E-01 1.4117E-01 36 Kr Krypton 4.7550E-06 6.6742E-06 2.6427E-06 3.0767E-06 4.3186E-06 1.7100E-06 54 Xe Xenon 2.3289E-06 3.2529E-06 1.2971E-06 1.5070E-06 2.1048E-06 8.3929E-07
Using CI composition for the refractory component

One might imagine that instead of the COSIMA measurements, one could use the composition of CI chondrites for the refractory component. If one were to do that, how would our results change? If we replace the values used from COSIMA (Table 1) with CI composition, the constraint on the refractory-to-ice ratio becomes 0.35 < χ < 1.35, which is very similar to what we find using the COSIMA composition (0.5 < χ < 1.7). Figure 8 is equivalent to Fig. 2 that we used to constrain the upper limit for χ. For this range (0.35 < χ < 1.35), xenon is solar when 0.35 < χ < 0.9. At the upper bound, when χ = 1.35, xenon is slightly sub-solar by 30%. Therefore, even if comet 67P were composed of refractories with CI composition, all our conclusions hold.

[Figure: Figure 8: The top panel shows the oxygen-to-silicon ratio for oxygen that is in water (subscript ’ex’) and that which is in anhydrous form (subscript ’an’). Compared to Fig 2 we assume CI composition for the refractory component of 67P. The values for CI chondrites are taken from Alexander (2019). The bottom panel shows the ratio of the two oxygen components. Note that the bottom panel is not simply the ratio of the lines in the top panel. Oex and Oan are not uncorrelated, and therefore, their ratio and respective errors need to be computed for each random draw. The darker shaded areas contain 50% of the cases while the lighter areas contain ±1σ]
Matthias Läuter1 and Tobias Kramer2
1 Zuse Institute Berlin, 14195 Berlin, Germany, e-mail: laeuter@zib.de 2 Institute for Theoretical Physics, Johannes Kepler Universität Linz, Austria Received: 21 January 2025 / Accepted: 30 May 2025 / Published: 04 July 2025 Published in Astronomy & Astrophysics, Volume 699, A75, doi:10.1051/0004-6361/202553845