Interferometric Imaging of Titan's HC3N, H13CCCN, and HCCC15N

M. A. Cordiner, C. A. Nixon, S. B. Charnley, N. A. Teanby, E. M. Molter, Z.

Kisiel, V. Vuitton

► To cite this version:

M. A. Cordiner, C. A. Nixon, S. B. Charnley, N. A. Teanby, E. M. Molter, et al. Interferometric Imaging of Titan's HC3N, H13CCCN, and HCCC15N. The Astrophysical Journal Letters, 2018, 859, 10.3847/2041-8213/aac38d. insu-03693561

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# Interferometric Imaging of Titan's HC₃N, H¹³CCCN, and HCCC¹⁵N

M. A. Cordiner<fnref n="1" />,<fnref n="2" /> ID, C. A. Nixon<fnref n="1" /> ID, S. B. Charnley<fnref n="1" />, N. A. Teanby<fnref n="3" /> ID, E. M. Molter<fnref n="4" /> ID, Z. Kisiel<fnref n="5" /> ID, and V. Vuitton<fnref n="6" /> ID

Received 2018 March 27; revised 2018 May 8; accepted 2018 May 9; published 2018 May 23

## Abstract

We present the first maps of cyanoacetylene isotopologues in Titan's atmosphere, including H¹³CCCN and HCCC¹⁵N, detected in the 0.9 mm band using the Atacama Large Millimeter/submillimeter array (ALMA) around the time of Titan's (southern winter) solstice in 2017 May. The first high-resolution map of HC₃N in its $v_7 = 1$ vibrationally excited state is also presented, revealing a unique snapshot of the global HC₃N distribution, free from the strong optical depth effects that adversely impact the ground-state ($v = 0$) map. The HC₃N emission is found to be strongly enhanced over Titan's south pole (by a factor of 5.7 compared to the north pole), consistent with rapid photochemical loss of HC₃N from the summer hemisphere combined with production and transport to the winter pole since the 2015 April ALMA observations. The H¹³CCCN/HCCC¹⁵N flux ratio is derived at the southern HC₃N peak, and implies an HC₃N/HCCC¹⁵N ratio of $67 \pm 14$. This represents a significant enrichment in ¹⁵N compared with Titan's main molecular nitrogen reservoir, which has a ¹⁴N/¹⁵N ratio of 167, and confirms the importance of photochemistry in determining the nitrogen isotopic ratio in Titan's organic inventory.

**Key words:** planets and satellites: atmospheres – planets and satellites: individual (Titan) – submillimeter: planetary systems – techniques: imaging spectroscopy – techniques: interferometric

## 1. Introduction

Titan's nitrogen and methane-dominated atmosphere is by far the densest of any moon in the Solar System, and its origin has remained a mystery since its discovery by Kuiper (1944; see also Hörst 2017 and references therein). Titan's atmospheric nitrogen is theorized to have been outgassed as NH₃ or N₂, originally present as ice that was accreted in the Saturnian sub-nebula, perhaps with a contribution delivered by cometary impacts. Such theories can be tested by measurements of Titan's present-day atmospheric abundances.

Trace isotopic ratios can reveal crucial information about the origins of a wide variety of solar system materials (e.g., Mandt et al. 2009; Bockelée-Morvan et al. 2015; Alexander et al. 2018; Marboeuf et al. 2018), and provide unique insights into their thermal and chemical histories. The gases that were eventually incorporated into icy planetesimals may have become enriched (or depleted) in heavy isotopes during the formation of the solar system or prior interstellar cloud. The difference in zero-point energy between the reactants and products means that at low temperatures (below the activation energy for the reverse reaction), isotopic exchange reactions such as ¹⁵N⁺ + ¹⁴N₂ ⇌ ¹⁴N⁺ + ¹⁴N¹⁵N tend to proceed preferentially in the forward direction (Roueff et al. 2015). Charnley & Rodgers (2002) theorized that reactions of ¹⁵N-enriched N₂ with He⁺, followed by H₂, can lead to the production of ¹⁵N-enriched interstellar NH₃ ice. In the protosolar nebula, ¹⁵N-enriched NH₃ may have also arisen following the isotope-selective photodissociation of N₂ (Visser et al. 2018). Understanding Titan's atmospheric ¹⁴N/¹⁵N ratio may thus provide a crucial window into the thermal, chemical, and radiation history of its nitrogen-bearing ices.

The first measurement of Titan's ¹⁴N/¹⁵N ratio was by Marten et al. (2002), who used mm-wave spectroscopy of HCN

to derive HC¹⁴N/HC¹⁵N = 60–70. A small ratio (compared to the Solar value of 440 and the terrestrial value of 272) was confirmed by Gurwell (2004), Vinatier et al. (2007), and Courtin et al. (2011), using a combination of ground- and space-based sub-mm and infrared observations. A refined disk-average measurement of 72.3 ± 2.2 was recently obtained by Molter et al. (2016) using ALMA archival flux-calibration observations of Titan. Meanwhile, the Cassini–Huygens mass spectrometer measured the ¹⁴N/¹⁵N ratio in tropospheric N₂ to be significantly higher at 167 ± 0.6 (Niemann et al. 2010). The difference in ¹⁵N fraction for these molecules is theorized to be a consequence of isotope-selective photodissociation of N₂ in the upper atmosphere. At altitudes $\gtrsim 800$ km, ¹⁴N₂ is more slowly dissociated than ¹⁵N¹⁴N due to self-shielding in the predissociating absorption lines (Liang et al. 2007). This gives rise to an enhanced abundance of atomic ¹⁵N that is theorized to carry through into other photochemically produced species. While this is likely sufficient to explain the observed HC¹⁴N/HC¹⁵N ratio, the theory remains to be tested for any molecules apart from HCN.

Additional complexity arises due to the many possible sources and sinks of atmospheric ¹⁴N and ¹⁵N, including outgassing from (or precipitation onto) the surface, and sputtering/escape from (delivery to) the top of the atmosphere, any of which may alter the overall nitrogen isotopic ratio over time (Mandt et al. 2009; Krasnopolsky 2016). Our present study is motivated by the need to accurately measure the ¹⁴N/¹⁵N ratio in Titan's photochemical products, to help elucidate the sources and sinks of ¹⁵N, which are crucial for a proper understanding of the primordial value of ¹⁴N/¹⁵N in Titan's ice at the time that it was accreted. Stratospheric HC₃N densities are theorized to be affected by many different reactions, so comparison between observed and predicted HCCC¹⁵N abundances provides a crucial check of the reaction

<footnote n="1">¹NASA Goddard Space Flight Center, 8800 Greenbelt Road, MD 20771, USA; martin.cordiner@nasa.gov</footnote>

<footnote n="2">²Institute for Astrophysics and Computational Sciences, The Catholic University of America, Washington, DC 20064, USA</footnote>

<footnote n="3">³School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol BS8 1RJ, UK</footnote>

<footnote n="4">⁴Astronomy Department, 501 Campbell Hall, University of California Berkeley, Berkeley, CA, USA</footnote>

<footnote n="5">⁵Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warszawa, Poland</footnote>

<footnote n="6">⁶Université Grenoble Alpes, CNRS, IPAG, F-38000 Grenoble, France</footnote>
<table><caption>**Table 1**

Detected HC<sub>3</sub>N Line Spectroscopic Parameters and Measured Fluxes</caption><thead><tr><th>Species</th><th>Transition</th><th>Rest Freq.<br>(MHz)</th><th>A<br>(10<sup>-3</sup> s)</th><th>g<sub>u</sub></th><th>E<sub>u</sub><br>(K)</th><th>Flux<br>(Jy kHz)</th></tr></thead><tbody><tr><td>HC<sub>3</sub>N</td><td>J = 39 – 38, v = 0</td><td>354697.463</td><td>3.571</td><td>79</td><td>340.5</td><td>475 ± 19</td></tr><tr><td>HC<sub>3</sub>N</td><td>J = 39 – 38, v<sub>7</sub> = 1e</td><td>355566.254</td><td>3.577</td><td>79</td><td>662.2</td><td>412 ± 17</td></tr><tr><td>HC<sub>3</sub>N</td><td>J = 39 – 38, v<sub>7</sub> = 1f</td><td>356072.445</td><td>3.592</td><td>79</td><td>662.7</td><td>402 ± 17</td></tr><tr><td>H<sup>13</sup>CCCN</td><td>J = 39 – 38</td><td>343737.400</td><td>3.250</td><td>79</td><td>330.0</td><td>35.5 ± 5.9</td></tr><tr><td>HCCC<sup>15</sup>N</td><td>J = 39 – 38</td><td>344385.348</td><td>3.254</td><td>79</td><td>330.6</td><td>47.3 ± 6.6</td></tr></tbody></table>

networks used in photochemical models (e.g., Loison et al. 2015; Krasnopolsky 2016; Vuitton et al. 2018), including photolysis cross sections, reaction rates, and branching ratios.

In contrast to <sup>15</sup>N, a large body of remote and in situ observational (and laboratory) data shows that <sup>13</sup>C ratios are much less variable, with a value of ≈90 across a wide range of solar system materials (including all of Titan's hydrocarbons and nitriles for which measurements exist; Bézard et al. 2014). This implies either that no significant carbon fractioning processes are in operation, or that the isotopic production and loss mechanisms are closely balanced (Hörst 2017).

In this Letter, we present the first maps of Titan's <sup>13</sup>C and <sup>15</sup>N isotopologues of the cyanoacetylene molecule (HC<sub>3</sub>N), obtained using spatially resolved data from the Atacama Large Millimeter/submillimeter Array (ALMA). Pure mm/sub-mm rotational emission lines of the main H<sup>12</sup>C<sup>14</sup>N isotopologue (hereafter referred to as simply HC<sub>3</sub>N), are found to be unreliable as a tracer of the total HC<sub>3</sub>N abundance due to strong opacity effects, so we use the known atmospheric <sup>12</sup>C/<sup>13</sup>C ratio (assumed to apply to HC<sub>3</sub>N) combined with our ALMA measurements of H<sup>13</sup>CCCN and HCCC<sup>15</sup>N to derive the <sup>14</sup>N/<sup>15</sup>N ratio in HC<sub>3</sub>N for the first time.

## 2. Observations

Observations of Titan were obtained using ALMA on 2017 May 8 as part of the Director's Discretionary Time program 2016.A.00014.S. Following the initial bandpass and flux-calibration scans, our observations consisted of an interleaved sequence of three visits each to Titan and the phase calibrator J1751-1950. The phase-center was updated in real-time to track Titan's moving position on the sky.

The Band 7 receiver was used, and the ALMA correlator was configured to observe the frequency ranges 342.5–346.1 GHz and 354.2–356.1 GHz at moderate spectral resolution (976 kHz) to capture the CO 2–1 and HCN 4–3 lines (including their broad line wings), as well as the HC<sub>3</sub>N lines of interest to this study. The total on-source observing time for Titan was 18 minutes, with 46 antennas online, resulting in an rms noise of ≈4 mJy beam<sup>-1</sup>. Weather conditions were good, with a zenith precipitable water vapor of 0.76 mm.

Data were flagged and calibrated in CASA 5.1 (Jaeger 2008) using the automated pipeline scripts supplied by the Joint ALMA Observatory (Shinnaga et al. 2015). Flux calibration was performed with respect to the quasar J1733-1304, and is expected to be accurate to within about 5%. The spectral axis was transformed to Titan's rest frame and regridded to a 976 kHz channel width. Titan's continuum flux was subtracted using low-order polynomial fits to the spectral regions adjacent to our detected lines. Imaging and deconvolution were performed using the Clark clean algorithm with natural visibility weighting, a pixel size of 0.025, and a flux threshold

of 8 mJy. The resulting angular resolution was 0.23 × 0.17 from a Gaussian fit to the point-spread function.

The coordinate scales of the cleaned images were transformed to physical distances with respect to the center of Titan, which was 9.26 au from Earth at the time of observation. Titan's north pole was oriented 5.3° counter-clockwise from celestial north, and tilted toward the observer by 26°. This is close to the maximum polar tilt due to the proximity of our observations to Titan's southern winter solstice on 2017 May 24. Before plotting, our images were corrected for a small, 0.06 offset in declination (of unknown origin) that was identified in Titan's position with respect to the ALMA phase center.

## 3. Results

Emission from HC<sub>3</sub>N, including lines from the ground (v = 0) vibrational state, (v<sub>7</sub> = 1) vibrationally excited state, and the isotopologue lines were identified using the Cologne Database for Molecular Spectroscopy (CDMS) catalog (Müller et al. 2001), based on the laboratory frequencies of Thorwirth et al. (2000, 2001). Relevant spectroscopic parameters and integrated line fluxes are given in Table 1. Note that although the *J* = 38 – 37 lines of HC<sup>13</sup>CCN and HCC<sup>13</sup>CN were detectable in addition to the *J* = 39 – 38 line of H<sup>13</sup>CCCN, the other <sup>13</sup>C isotopologues were excluded from the present study as they cannot be properly disentangled from the steeply rising wing of the overlapping HC<sup>15</sup>N *J* = 4 – 3 line (at 344,200 MHz).

Figure 1 shows the detected spectral lines and Figure 2 shows the emission maps, integrated over the full extent of the detected flux for H<sup>13</sup>CCCN and HCCC<sup>15</sup>N. Only the central five spectral channels were included for HC<sub>3</sub>N v = 0 and v<sub>7</sub> = 1 to facilitate intercomparison of their maps—avoiding the introduction of undue noise from the weak, pressure-broadened wings that are present at the north pole for v = 0 (but not detected for v<sub>7</sub> = 1). HC<sub>3</sub>N (v = 0) shows a limb-brightened flux distribution, characteristic of high-altitude atmospheric emission. There is a strong intensity peak over the south pole, with a weaker, secondary peak in the north. By contrast, emission from the <sup>13</sup>C and <sup>15</sup>N isotopologues was only detected within a compact region over the south pole, with a peak flux 0.03 away from Titan's disk. This corresponds to a sky-projected altitude of 200 km above the southern limb.

The Einstein A coefficients and degeneracies ($g_u$) for the v = 0 and v<sub>7</sub> = 1 lines are practically identical, but the upper-state energy ($E_u$) for v<sub>7</sub> = 1 is 322 K above v = 0. At the ~160 K temperature in the middle atmosphere where the majority of the HC<sub>3</sub>N emission originates, the difference in Boltzmann factors results in a factor of 7.4 reduction in the population of the vibrationally excited state (assuming local thermodynamic equilibrium; LTE), with a corresponding drop
<figure><img src="image_3.png" /><figcaption>**Figure 1.** ALMA spectra of HC₃N extracted from a beam centered 200 km above the south polar limb, including the (*J* = 39 – 38, *v* = 0) rotational transition of the vibrational ground state, the (*J* = 39 – 38, *v* = 1f) transition of the vibrationally excited state, and the ground-state rotational transitions of the detected ¹³C and ¹⁵N isotopologues (see Table 1).</figcaption></figure>

in emitted line flux. In fact, the measured *v* = 0 line flux is only 1.2 times stronger than *v*<sub>7</sub> = 1, and this discrepancy is probably due to a very high optical depth of the *v* = 0 line in a compact region near the south pole (see Section 4). Expressed as a Rayleigh–Jeans antenna temperature, the HC<sub>3</sub>N *v* = 0 line peak of 78 K does not expose the full extent of the line saturation, which is more readily revealed by comparing the emission from the north (N) and south (S) poles. The ratio of S to N polar peaks for *v*<sub>7</sub> = 1 is 5.7 ± 0.3, and for *v* = 0 the ratio is only 1.6 ± 0.1. This difference cannot be attributed to differences in temperatures between the poles, which are typically <~20 K— such a temperature variation only amounts to ~20% difference in the LTE *v*<sub>7</sub> = 1 and *v* = 0 level populations. Thus, we deduce the presence of a south polar region smaller than the telescope beam, in which the HC<sub>3</sub>N *J* = 39 – 38, *v* = 0 line is completely optically thick, significantly depressing the measured flux from that region.

The high opacity of the HC₃N *J* = 39 – 38, *v* = 0 line rules out its use for deriving an accurate abundance over the south pole where the isotopologues were detected. We have also conducted tests to retrieve the HC₃N abundance using the *v*<sub>7</sub> = 1 transition, but this line is found to be too sensitive to errors in the adopted temperature profile. Moreover, it has

recently been shown that, due to the rapidly decreasing density, non-LTE effects start to become important for vibrationally excited lines in Titan’s atmosphere above about 350 km (Kutepov et al. 2013), resulting in unreliable abundance retrievals. Given that a significant proportion of the HC₃N flux detected through high-resolution mm/sub-mm spectroscopy originates from altitudes above 350 km (Marten et al. 2002; Cordiner et al. 2014), we find ourselves in the unfortunate situation of being unable to derive accurate enough abundances for the main H¹²C₁⁴N isotopologue to allow a direct calculation of the ¹²C/¹³C and ¹⁴N/¹⁵N ratios in this molecule.

A similar situation is encountered for HCN in the interstellar medium, and can plausibly be resolved using the “double isotope” method: the more easily measured abundance of an (optically thin) ¹³C-substituted isotopologue is combined with the known ¹²C/¹³C ratio to infer the abundance of the (optically thick) ¹²C isotopologue. The use of this method for Titan’s HC₃N rests on the assumption that the HC₃N/H¹³CCCN ratio is the same as the bulk ¹²C/¹³C ratio measured from other gases.

The ¹²C/¹³C ratios across the planets, moons, and minor bodies of the solar system tend to cluster around 90
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Integrated emission maps of HC₃N, including the ground-state (J = 39 – 38, v = 0) and vibrationally excited (J = 39 – 38, v₇ = 1e and 1f) lines (top two panels), and the detected ¹³C and ¹⁵N isotopologues (bottom two panels). The contour interval is 5σ for the HC₃N v = 0 and v₇ = 1 lines, and 3σ for the isotopologues, where σ is the rms noise level. Wire frame shows Titan's solid body, with 22°5 increments in latitude and 30° in longitude. Ellipses (lower left) indicate the spatial resolution.</figcaption></figure>

(Woods 2009). Similarity between the ¹²C/¹³C ratios for Jupiter, Saturn, the Earth, and Sun led Sada et al. (1996) to conclude that there is little or no ¹³C fractionation occurring in the atmospheres of the Giant Planets. The range of individual ¹²C/¹³C measurements for Titan's gases tabulated by Bézard et al. (2014) are all consistent with the (error-weighted) average value of 88.6 ± 0.8. This includes the prior measurement of 79 ± 17 in HC₃N by Jennings et al. (2008), and we take this as good evidence for a lack of strong ¹³C fractionation processes operating in Titan's atmosphere.

As the molecular partition functions, Einstein A values, and upper-state energies for the observed H¹³CCCN and HCCC¹⁵N transitions are identical to within 0.2%, the H¹³CCCN/HCCC¹⁵N abundance ratio can be accurately derived, independent of assumptions regarding the temperature and excitation of the gas. Adopting a value H¹²C₃·¹⁴N/H¹³C¹²C₂·¹⁴N = 89, the ratio of our observed H¹³CCCN and HCCC¹⁵N line fluxes (0.75 ± 0.16) implies a ¹⁴N/¹⁵N ratio of 67 ± 14 in cyanoace-tylene. The measured fluxes are dominated by emission from the (narrow) spectral line cores, which originates primarily from altitudes ~200–400 km (e.g., Marten et al. 2002), so the

measured ¹⁴N/¹⁵N ratio should be considered an average over this range.

## 4. Discussion

Our value for ¹⁴N/¹⁵N in HC₃N is consistent with the error-weighted average of the prior HCN measurements: 71 ± 2 (Bézard et al. 2014; Molter et al. 2016). This may be understood within the framework of recent models for Titan's nitrile photochemistry. A source of ¹⁵N-enriched atomic nitrogen is produced in the upper atmosphere as a result of preferential photodissociation of ¹⁵N¹⁴N compared with the dominant ¹⁴N₂ isotopologue. The line radiation required to predissociate ¹⁴N₂ becomes attenuated with distance into the atmosphere, and this effect is weaker for ¹⁵N¹⁴N due to its lower abundance and differing predissociation wavelengths compared with the main isotopologue (Liang et al. 2007). The resulting ¹⁵N-enriched atomic nitrogen becomes incorporated into HCN, and then into HC₃N (through the CN radical intermediary), as highlighted by the following reaction sequence (cf. Wilson & Atreya 2004):

$$ ^{15}\text{N}^{14}\text{N} + h\nu \rightarrow ^{15}\text{N} + ^{14}\text{N}, \quad (1) $$
$$^{15}\text{N} + \text{CH}_3 \rightarrow \text{H}_2\text{C}^{15}\text{N} + \text{H}, \qquad (2)$$

$$\text{H}_2\text{C}^{15}\text{N} + \text{H} \rightarrow \text{HC}^{15}\text{N} + \text{H}_2, \qquad (3)$$

$$\text{HC}^{15}\text{N} + h\nu \rightarrow \text{C}^{15}\text{N} + \text{H}, \qquad (4)$$

$$\text{C}^{15}\text{N} + \text{C}_2\text{H}_2 \rightarrow \text{HC}_3^{15}\text{N} + \text{H}. \qquad (5)$$

Given the expected altitude dependence of the atomic $^{14}\text{N}/^{15}\text{N}$ ratio (as $^{15}\text{N}^{14}\text{N}$ is photodissociated to greater depths than $^{14}\text{N}_2$), and the possibility of multiple formation and destruction pathways for HC$_3$N (e.g., Loison et al. 2015), which become important in different parts of the atmosphere, the $^{14}\text{N}/^{15}\text{N}$ ratio in HC$_3$N cannot be easily interpreted without the aid of a detailed chemical model. The latest models by Vuitton et al. (2018) and Dobrijevic & Loison (2018) incorporate a comprehensive nitrile chemistry, including $^{15}\text{N}$-bearing species as well as ion, cosmic-ray, and photolytic processes. Vuitton et al. (2018) were able to reproduce the previously obtained HCN/HC$^{15}\text{N}$ ratio from Cassini and predict HC$_3$N/HCCC$^{15}\text{N}$ = 52 at 200 km, which is in reasonably good agreement with our ALMA observations. Dobrijevic & Loison (2018) predicted a higher HC$_3$N/HCCC$^{15}\text{N}$ ratio of $80 \pm 7$ by including dissociation by energetic electrons from Saturn's magnetosphere. Unfortunately, our measured HC$_3$N/HCCC$^{15}\text{N}$ ratio is not accurate enough to conclusively distinguish between these models, so additional observations at higher sensitivity are needed. Another source of uncertainty stems from our assumption of $\text{H}^{12}\text{C}_3\text{N}/\text{H}^{13}\text{CCCN} = 89$; limited accuracy of the prior $^{12}\text{C}/^{13}\text{C}$ measurement in HC$_3$N (Jennings et al. 2008) means that the true HC$_3$N/HCCC$^{15}\text{N}$ ratio could be as low as 43, so more accurate measurements of HC$_3$N/H$^{13}\text{CCCN}$ are also warranted. Combined with information on the $^{12}\text{C}/^{13}\text{C}$ ratios in C$_2$H$_2$, HCN and other molecules, ALMA observations of the remaining $^{13}\text{C}$ isotopologues HC$^{13}\text{CCN}$ and HCC$^{13}\text{CN}$ would allow the individual $^{13}\text{C}$ atoms to be tracked through the chemical network, providing a unique test for our understanding of the HC$_3$N formation mechanism(s) (e.g., Taniguchi et al. 2017).

Through a combination of laboratory, modeling, and observational studies (e.g., Clarke & Ferris 1997; Wilson & Atreya 2003; Teanby et al. 2008), it has been shown that Titan's nitriles (including HCN and HC$_3$N) are likely to become incorporated into more complex polymers and aerosol particles. As a result, the preferential removal of $^{15}\text{N}$ from the atmosphere, through its incorporation into photochemical products and subsequent precipitation onto the surface, should be considered as an important $^{15}\text{N}$ loss process, and thus a possible factor in the time evolution of Titan's bulk atmospheric $^{14}\text{N}/^{15}\text{N}$ ratio. The detection of a strong $^{15}\text{N}$-enrichment in a second molecule (after HCN) confirms the likely importance of this fractionation process. The consequent increase in precipitation rate for atmospheric $^{15}\text{N}$ (relative to $^{14}\text{N}$) means that the $^{14}\text{N}/^{15}\text{N}$ ratio in N$_2$ may have been lower in the past (e.g., Krasnopolsky 2016), perhaps close to the value of 136 found in cometary NH$_3$ ice (Shinnaka et al. 2016).

Our high-resolution HC$_3$N maps also reveal new details on Titan's atmospheric dynamics. By virtue of its large abundance at high altitude, its strong rotational emission spectrum and short lifetime (~1 years; Wilson & Atreya 2004; Krasnopolsky 2009), HC$_3$N is an excellent tracer of Titan's seasonally variable atmospheric circulation. Over the 24-month period since the 2015 April ALMA observations of HC$_3$N (Lai et al. 2017), the ratio of S to N polar emission peak intensities

has increased by almost a factor of two (from 3.1 to 5.7). This is explained by the combination of (1) rapid photolytic breakdown of HC$_3$N in the northern hemisphere due to the increase in solar insolation approaching the 2017 solstice, and (2) the transport of freshly synthesized HC$_3$N from mid latitudes toward the south pole by the strengthening winter polar circulation system (Teanby et al. 2012; Lora et al. 2015). Details regarding these photolytic and transport mechanisms may be elucidated through future monitoring of the HC$_3$N distribution at high resolution.

Comparison of the HC$_3$N $v=0$ and $v_7=1$ line strengths reveals a high opacity in the $v=0$ line, approaching complete saturation at the winter pole. From a fitted 2D Gaussian, the S-polar peak is $0.21 \times 0.38$ in size (unresolved on the short axis), and the relatively low peak line brightness temperature of 78 K (compared to the $\approx 150-180$ K gas temperature) indicates that saturated emission fills less than half the beam. This demonstrates the presence of one or more extremely compact regions of enhanced HC$_3$N column density at the winter pole, similar to the chemically enriched gas recently observed at $\sim -80^\circ$ latitude by Cassini CIRS (Teanby et al. 2017; Vinatier et al. 2017).

## 5. Conclusions

We have detected and mapped for the first time $^{13}\text{C}$ and $^{15}\text{N}$ isotopologues of HC$_3$N in Titan's atmosphere, revealing a high-resolution snapshot of the global distributions for these trace gases. Similar to the main isotopologue, the $\text{H}^{13}\text{CCCN}$ and HCCC$^{15}\text{N}$ show compact emission peaks in the vicinity of the south pole, consistent with a short photochemical lifetime and advective transport by Titan's seasonally variable atmospheric circulation cell.

Our derived $^{14}\text{N}/^{15}\text{N}$ ratio in HC$_3$N of $67 \pm 14$ represents a significant enrichment in $^{15}\text{N}$ compared with the bulk (precursor) N$_2$ reservoir, and is the second such molecule (after HCN) for which this effect has been observed. Good agreement between our HC$_3$N observations and the latest chemical modeling work demonstrates a reasonable understanding regarding the synthesis of this molecule in Titan's atmosphere, and confirms the importance of isotope-selective N$_2$ photochemistry. Additional chemical/dynamical modeling is needed to investigate the full extent of Titan's $^{15}\text{N}$ and $^{14}\text{N}$ sources and sinks to help further constrain the detailed time evolution of the bulk $^{14}\text{N}/^{15}\text{N}$ ratio, which may provide new insights into the origin of Titan's nitrogen atmosphere. Such models may be tested through comparison of predicted gas abundance distributions with future ALMA observations at higher sensitivity.

This work was supported by the NSF under grant No. AST-1616306. It makes use of ALMA data ADS/JAO. ALMA#2016.A.00014.S. ALMA is a partnership of ESO, NSF (USA), NINS (Japan), NRC (Canada), NSC and ASIAA (Taiwan), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. NRAO is a facility of the NSF operated under cooperative agreement by AUI. C.A.N. and S.B.C. were supported by the NASA HQ Science Innovation Fund. N.A.T. was funded by the UK Science and Technology Facilities Council.
ORCID iDs

M. A. Cordiner https://orcid.org/0000-0001-8233-2436

C. A. Nixon https://orcid.org/0000-0001-9540-9121

N. A. Teanby https://orcid.org/0000-0003-3108-5775

E. M. Molter https://orcid.org/0000-0003-3799-9033

Z. Kisiel https://orcid.org/0000-0002-2570-3154

V. Vuitton https://orcid.org/0000-0001-7273-1898

References

Alexander, C. M. O., McKeegan, K. D., & Altwegg, K. 2018, SSRv, 214, 36
Bézard, B., Yelle, R., & Nixon, C. 2014, in Titan: Surface, Atmosphere and
Magnetosphere, ed. I. Muller-Wodarg et al. (Cambridge: Cambridge Univ.
Press), 158, Ch. 6

Bockelée-Morvan, D., Calmonte, U., Charnley, S., et al. 2015, SSRv, 197, 47

Charnley, S. B., & Rodgers, S. D. 2002, ApJL, 569, L133

Clarke, D. W., & Ferris, J. P. 1997, Icar, 127, 158

Cordiner, M. A., Nixon, C. A., Teanby, N. A., et al. 2014, ApJL, 795, L30

Courtin, R., Swinyard, B. M., Moreno, R., et al. 2011, A&A, 536, L2

Dobrijevic, M., & Loison, J. C. 2018, Icar, 307, 371

Gurwell, M. A. 2004, ApJL, 616, L7

Hörst, S. M. 2017, JGRE, 122, 432

Jaeger, S. 2008, in ASP Conf. Ser. 394, Astronomical Data Analysis Software and Systems XVII, ed. R. W. Argyle, P. S. Bunclark, & J. R. Lewis (San Francisco, CA: ASP), 623

Jennings, D. E., Nixon, C. A., Jolly, A., et al. 2008, ApJL, 681, L109

Krasnopolsky, V. A. 2009, Icar, 201, 226

Krasnopolsky, V. A. 2016, P&SS, 134, 61

Kuiper, G. P. 1944, ApJ, 100, 378

Kutepov, A., Vinatier, S., Feofilov, A., Nixon, C., & Boursier, C. 2013, in AAS/Division for Planetary Sciences Meeting 45, abstract #, 207.05

Lai, J. C.-Y., Cordiner, M. A., Nixon, C. A., et al. 2017, AJ, 154, 206

Liang, M.-C., Heays, A. N., Lewis, B. R., Gibson, S. T., & Yung, Y. L. 2007, ApJL, 664, L115

Loison, J. C., Hébrard, E., Dobrijevic, M., et al. 2015, Icar, 247, 218

Lora, J. M., Lunine, J. I., & Russell, J. L. 2015, Icar, 250, 516

Mandt, K. E., Waite, J. H., Lewis, W., et al. 2009, P&SS, 57, 1917

Marboeuf, U., Thiabaud, A., Alibert, Y., & Benz, W. 2018, MNRAS, 475, 2355

Marten, A., Hidayat, T., Biraud, Y., & Moreno, R. 2002, Icar, 158, 532

Molter, E. M., Nixon, C. A., Cordiner, M. A., et al. 2016, AJ, 152, 42

Müller, H. S. P., Thorwirth, S., Roth, D. A., & Winnewisser, G. 2001, A&A, 370, L49

Niemann, H. B., Atreya, S. K., Demick, J. E., et al. 2010, JGRE, 115, E12006

Roueff, E., Loison, J. C., & Hickson, K. M. 2015, A&A, 576, A99

Sada, P. V., McCabe, G. H., Bjoraker, G. L., Jennings, D. E., & Reuter, D. C. 1996, ApJ, 472, 903

Shinnaga, H., Humphreys, E., Indebetouw, R., et al. 2015, in ASP Conf. Ser. 499, Revolution in Astronomy with ALMA: The Third Year, ed. D. Iono et al. (San Francisco, CA: ASP), 355

Shinnaka, Y., Kawakita, H., Jehin, E., et al. 2016, MNRAS, 462, S195

Taniguchi, K., Ozeki, H., & Saito, M. 2017, ApJ, 846, 46

Teanby, N. A., Bézard, B., Vinatier, S., et al. 2017, NatCo, 8, 1586

Teanby, N. A., Irwin, P. G. J., de Kok, R., et al. 2008, Icar, 193, 595

Teanby, N. A., Irwin, P. G. J., Nixon, C. A., et al. 2012, Natur, 491, 732

Thorwirth, S., Mueller, H. S. P., & Winnewisser, G. 2001, PCCP, 3, 1236

Thorwirth, S., Müller, H. S. P., & Winnewisser, G. 2000, JMoSp, 204, 133

Vinatier, S., Bézard, B., & Nixon, C. A. 2007, Icar, 191, 712

Vinatier, S., Bézard, B., Teanby, N., et al. 2017, in AAS/Division for Planetary Sciences Meeting 49, abstract #, 304.03

Visser, R., Bruderer, S., Cazzoletti, P., et al. 2018, A&A, in press (arXiv:1802.02841)

Vuitton, V., Yelle, R. V., Klippenstein, S. J., Horst, S. M., & Lavvas, P. 2018, Icar, submitted

Wilson, E. H., & Atreya, S. K. 2003, P&SS, 51, 1017

Wilson, E. H., & Atreya, S. K. 2004, JGRE, 109, 6002

Woods, P. M. 2009, arXiv:0901.4513