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The presence of clathrates in comet 67P/Churyumov-Gerasimenko

In situ coma measurements of 67P suggest that volatiles in the nucleus are stored as clathrates and pure condensates.


INTRODUCTION
Although there is no doubt that cometary nuclei are, to a large extent, composed of H 2 O ice, it is the structure and phase of this H 2 O ice that provide insight into the place of origin, formation temperature, and evolution of icy agglomerates in the protosolar nebula (PSN). Whether cometary H 2 O ice originated directly from the interstellar medium (ISM) or was derived from the PSN has been a topic of active debate over the past three decades. An origin from the ISM implies formation at larger heliocentric distances, where pristine amorphous H 2 O ice could be maintained in the extremely low temperature, nonturbulent protoplanetary disc (1)(2)(3)(4)(5). An origin from the PSN implies formation at smaller heliocentric distances where crystalline ice could form at a temperature of~150 K in the cooling PSN (6,7). The phase in which other volatile species are stored in the nucleus strongly depends on the phase of the H 2 O ice.
Amorphous H 2 O ice very efficiently traps large amounts of volatiles in its highly porous structure [for example, (8,9)]. The trapped volatiles are then released simultaneously as a result of changes in the ice structure. The major release of trapped gases occurs during the exothermic transition from amorphous to crystalline ice (8).
On the other hand, free crystalline H 2 O ice could enable various volatile species to be encaged as guest species within clathrate hydrates. In a clathrate, volatile gases are locked inside cage-like structures of crystalline H 2 O ice. In the cooling PSN, H 2 O will be present in its crystalline structure, with no or very little amorphous H 2 O. The fraction of amorphous ice in the PSN would be negligible because condensation of the formerly vaporized H 2 O occurs at significantly higher temperatures in the crystalline phase than those needed for the formation of amorphous H 2 O ice. By the time the temperature of the PSN was low enough for amorphous H 2 O ice formation, most volatile gases themselves either had condensed or were trapped in clathrates. These processes leave no or very little H 2 O and other volatile gases available when conditions were right for amorphous ice to form (2,6,10). Lacking in situ measurements of the internal structure and ice phase of cometary nuclei, the composition of the coma and the outgassing pattern of volatile species of major and minor abundance provide the best clues about the ice structure and, as a result, the origin of cometary nuclei.
Recent measurements by the ROSINA/DFMS (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis/Double-Focusing Mass Spectrometer) (11) instrument onboard the Rosetta spacecraft showed a strongly heterogeneous coma in the major (12) and minor volatile species (13) of the Jupiter family comet (JFC) 67P/Churyumov-Gerasimenko (hereinafter 67P). In addition, a strong north-south asymmetry was present in the measured abundances during October 2014 (14).
Here, we use these recent coma measurements by ROSINA/DFMS over the September to October 2014 time period (12)(13)(14) to infer the structure of the icy agglomerates from which 67P was assembled. In particular, we restrict our analysis to data obtained when the poorly illuminated, winter southern hemisphere of the comet was in the view of Rosetta. The mid-to-high southern latitude scans revealed an interesting feature in the coma signal over two narrow sub-spacecraft longitude regions (12,13). Over these narrow southern hemisphere longitude regions, the signals of CO 2 , CO, and C 2 H 6 clearly deviated from the overall H 2 O signal, showing maxima at times of deep H 2 O minima (figs. S1 to S3). This telling feature was not present over the well-illuminated northern hemisphere, which was experiencing summer at the time. The higher temperatures experienced by the northern hemisphere make it difficult to reliably infer whether minor species are being released from different ice phases. The observed outgassing over the northern hemisphere with substantial H 2 O ice sublimation due to the higher temperatures would be consistent with gas release from either amorphous ice or clathrates, or both. Once the temperature is high enough for H 2 O sublimation, as was the case in the northern hemisphere during the period of observation, differences in outgassing due to gas release from amorphous ice, clathrate structures, and nucleus heterogeneity cannot be clearly distinguished. In contrast, temporal variations in the outgassing of volatiles from the southern (winter) hemisphere are well resolved and distinguishable from each other. At these lower temperatures, volatile outgassing is expected to be different based on the phase of H 2 O ice in the nucleus. Hence, the clearly distinguishable outgassing features in the southern hemisphere coma provide insight into the structure and history of 67P's nucleus.

RESULTS
Of the five species studied, CH 4 was the only volatile species whose signal showed no apparent correlation with either CO 2 or H 2 O (figs. S1 to S3) over the southern hemisphere scans (13). This outgassing behavior provides important clues about the nucleus of 67P. Coma heterogeneity was attributed to heterogeneity in the nucleus (13), including possible variations in surface properties (14,15). These previous results clearly imply the presence of some kind of heterogeneity related to the properties of the nucleus, though the observed time variation of all volatile species would be difficult to explain solely with such variation. A heterogeneous nucleus and/or surface properties would certainly affect the composition of the coma, though it is unclear what surface properties would be able to affect CH 4 differently from the other species. The distinct time variation displayed by CH 4 also strongly suggests that it does not sublimate from a segregated nonpolar ice phase, as proposed for CO, CO 2 , and C 2 H 6 (13).
We are also able to exclude gas release from amorphous H 2 O ice based on the available measurements. As has been shown in multiple laboratory experiments, large amounts of volatile gases are released in the phase transition from amorphous H 2 O ice to the cubic phase of crystalline H 2 O between 135 and 155 K [for example, (8)]. This transition is followed by the transition of cubic crystalline to hexagonal crystalline ice between 160 and 175 K [for example, (16)]. In these stages, the trapped volatile gases are released simultaneously and independently of their own volatility, with the exception of H 2 O. The simultaneous release that would occur with amorphous ice does not agree with the ROSINA/DFMS observations, in which not all minor volatile species were released together from the southern hemisphere nucleus (13). For instance, no HCN and CH 3 OH outgassing was observed at times of CO 2 , CO, and C 2 H 6 outgassing maxima over the southern hemisphere during these measurements. The outgassing pattern of CH 4 that is distinct from other major and minor volatile species is also inconsistent with gas release from amorphous ice as currently understood from laboratory studies.
The presence of clathrates would explain the observed time variation of CH 4 over the southern hemisphere (figs. S1 to S3). Stability curves of CH 4 and C 2 H 6 single-guest clathrates, as well as CH 4 , C 2 H 6 , and CO 2 condensation curves, are shown in Fig. 1 as a function of the total PSN gas pressure. In the cooling nebula, CH 4 clathrate forms at temperatures 20 to 30 K higher than the CH 4 condensate (Fig. 1). Figure 1 shows equilibrium curves with gas-phase mole fractions relative to H 2 in the PSN, derived specifically from the measured cometary gas/H 2 O ratios of 67P's southern hemisphere (17). Note that the abundances of volatiles measured by ROSINA/DFMS in the coma of 67P do not necessarily represent bulk abundances in the nucleus. Thus, several ratios were tested based on other available measurements for known comets. Indeed, CH 4 clathrates will preferentially form instead of condensed CH 4 ice in every case for gas-phase CH 4 mole fractions varied within the range of known comets. Thus, if CH 4 were present as clathrate in the nucleus of 67P, then CH 4 outgassing from the nucleus would occur when the ambient pressure drops below the equilibrium pressure of the CH 4 clathrate at a given temperature. In a cometary environment, the largest contribution to the total pressure within the nucleus comes from H 2 O, unlike in the PSN, where the total pressure is mainly given by H 2 . To represent clathrate decomposition in the nucleus of 67P, we calculated the equilibrium curves of volatile species as a function of total pressure (Fig. 2). For this calculation, the gas-phase mole fractions relative to H 2 O were taken directly from ROSINA/ DFMS measurements derived for the southern hemisphere (14). We see from these curves that the decomposition of CH 4 clathrate begins at temperatures significantly lower than those needed for the sublimation of the host H 2 O cages (Fig. 2). Thus, if CH 4 is present as a clathrate structure in the nucleus of 67P, it is not required for the outgassing pattern of CH 4 to follow that of H 2 O or CO 2 .
The curves in Fig. 2 are in agreement with the ROSINA/DFMS measurements that show poor correlations of CH 4 with CO 2 and H 2 O (R 2 of 0.18 and 0.57, respectively) [figs. S1 to S3 (13)]. However, the same ROSINA/DFMS observations show that C 2 H 6 follows closely and correlates very well with CO 2 in the southern hemisphere (R 2 of 0.77), unlike CH 4 (13). At the same time, Fig. 1 suggests that C 2 H 6 also preferentially forms clathrates instead of a pure condensate in the cooling PSN. We conclude that both CH 4 and C 2 H 6 are present as clathrates in the nucleus of 67P. In contrast with CH 4 , the strong correlation between C 2 H 6 and CO 2 is caused by the fact that the dissociation temperature of the C 2 H 6 clathrate is coincidentally close to the sublimation temperature of CO 2 ice. There is strong correlation between C 2 H 6 and CO 2 , as shown by their overlapping equilibrium curves in   . S4). In addition, the equilibrium pressure curves of the JFCs (67P and Hartley 2) have very similar temperature dependences, as do those of comets Halley and Hale-Bopp, which belong to the Oort cloud ( fig. S4). On the basis of our results, clathrate and pure condensate formation temperatures change as a function of the abundance ratio used. Thus, although the specific formation temperature of the nucleus of 67P cannot be constrained reliably, our results provide a strong argument for the presence of CH 4 and C 2 H 6 clathrates in the nucleus of 67P.

DISCUSSION
Circumstantial evidence for the existence of amorphous ice in comets was previously given based on the onset of cometary activity at large heliocentric distances [for example, (18)], as well as distant and nearperihelion outbursts (19). These phenomena could be explained by the crystallization of amorphous ice, as was proposed for the distant out-bursts of 1P/Halley (20) and Hale-Bopp (21)(22)(23), the perihelion activity of 2060 Chiron (24), and the erratic activity of comet Schwassmann-Wachmann (25). In the case of comet 17P/Holmes, however, the crystallization of H 2 O ice was likely not responsible for its 2007 post-perihelion megaburst (26). Recently, the exothermic phase transition of amorphous to crystalline H 2 O ice has been proposed as a potential cause for the observed active pits in the nucleus of 67P (27). At the same time, crystallization of amorphous H 2 O ice is not the only process capable of producing outbursts and pits. Pressure pulses resulting in outbursts could also be caused by clathrate decomposition at depths comparable to the depths of observed pits on time scales shorter than the lifetime of 67P, as has been shown recently (28). The kinetics of clathrate formation at low temperatures are not well known, but the nature of the differences in prevailing thermodynamic conditions (P, T) in the PSN versus the ISM makes it more likely that clathrate will form in the former as opposed to the latter (5). Clathrates could also form later in the nucleus in the presence of free crystalline H 2 O ice, even if the original ice phase of 67P was amorphous H 2 O. If clathrates formed in the nucleus of 67P at a later period after the comet's formation, then clathration would have had to occur on significantly shorter time scales than the formation of crystalline H 2 O and clathrate ice grains in the PSN. Unfortunately, the currently loosely constrained kinetics of clathrate formation (29) do not allow us to distinguish between a nebular and a postnebular formation of clathrate in 67P.
Our results do not exclude the existence of amorphous ice in the solar nebula, which may have been quite abundant in the low-temperature, outer regions of the disc [for example, (4,30)]. In addition, the presence of clathrates in the nucleus of 67P does not prove that comets, including 67P, formed out of clathrates. Yet, our results, along with other recent efforts supporting the presence of N 2 , Ar, and CO clathrates in the nucleus of 67P (31,32), suggest a picture of the origin of 67P that is different from what was envisaged before, where crystalline H 2 O ice, pure condensates, and clathrates, rather than amorphous ice, may play a leading role. If clathrates did form in the PSN, then 67P would have agglomerated from ices condensed and altered in the PSN instead of pristine ices from the ISM or the outskirts of the disc, where amorphous ice may dominate (4,5). This idea is consistent with scenarios arguing that the building blocks of giant planets and satellites were formed in a similar manner in the nebula (33,34) and that Titan accreted from clathrate-rich planetesimals originating from Saturn's feeding zone (3). Dynamical model results suggest that both JFCs and Oort cloud comets may have formed in the same environment extending over heliocentric distances of tens of astronomical units (35). If the nucleus of 67P agglomerated from crystalline ices and clathrates, then it likely formed closer to the Sun than previously considered for JFCs (35). This also implies that other comets with measured D/H ratios lower than that of 67P (36) should have been formed from crystalline ices and clathrates, because they probably formed closer to the Sun than 67P (37). In any case, the presence of clathrates in 67P would indicate that the snow line, whose position is still loosely constrained (38), was located beyond the present asteroid belt in the PSN. This would suggest that comets formed from at least two distinct reservoirs: a crystalline H 2 O reservoir located inside the disc and an amorphous H 2 O ice reservoir located outside the disc (5). Future direct sampling of ices from other comets will be crucial in locating the boundary between these reservoirs and will better constrain the phase of ice in various comets at the time of their formation.

MATERIALS AND METHODS
Calculation of pure condensate and clathrate equilibrium curves Condensation curves of CH 4 , C 2 H 6 , and CO 2 were calculated based on Fray and Schmitt (39). The fitting expression used is lnP eq ¼ A 0 þ ∑ n i¼1 A i =T i . Fitting parameters A i for CH 4 , C 2 H 6 , and CO 2 are summarized in table S2. The condensation curve of H 2 O vapor was calculated via the following equations (39) ln To test the sensitivity of the clathrate and condensate equilibrium curves, the gas-phase X/H 2 O was varied within the range of known comets (14) and taken relative to H 2 as described above. Varying the gas-phase X/H 2 O (hence, the X/H 2 ) does not change the ordering of the curves as a function of temperature, and the CO 2 condensate and C 2 H 6 clathrate curves match each other well for the variety of X/H 2 O ranges of known comets. To demonstrate the latter point, equilibrium curves of four selected comets are shown in fig. S4.

SUPPLEMENTARY MATERIALS
Supplementary material for this article is available at http://advances.sciencemag.org/cgi/ content/full/2/4/e1501781/DC1  Table S1. Fitting parameters used in calculating clathrate equilibrium curves of CH 4 and C 2 H 6 . Table S2. Fitting coefficients used to calculate condensation curves. Table S3. Fitting coefficients for the condensation pressure of crystalline H 2 O ice. References (42,43)