Evidence of ammonium salts in comet 67P as explanation for the nitrogen depletion in cometary comae

Abstract

Cometary comae are generally depleted in nitrogen. The main carriers for volatile nitrogen in comets are NH3 and HCN. It is known that ammonia readily combines with many acids like e.g. HCN, HNCO, HCOOH, etc. encountered in the interstellar medium as well as in cometary ice to form ammonium salts (NH4 + X -) at low temperatures. Ammonium salts, which can play a significant role in prebiotic chemistry, are hard to detect in space as they are unstable in the gas phase and their infrared signature is often hidden by thermal radiation or by e.g. OH in minerals. Here we report the presence of all possible sublimation products of five different ammonium salts at comet 67P/Churyumov-Gerasimenko measured by the ROSINA instrument on Rosetta. The relatively high sublimation temperatures of the salts leads to an apparent lack of volatile nitrogen in the coma. This then also explains the observed trend of higher NH3/H2O ratios with decreasing perihelion distances in comets. MainNitrogen in the volatile part of a comet nucleus is predominantly in the form of NH3 and HCN, which are on average (0.80 ± 0.20) % and (0.21 ± 0.02) %, respectively relative to water, e.g. (1). The numbers for HCN are somewhat uncertain as IR observations generally differ from radio observations (2). Apart from these two molecules, nitrogen bearing species have rather low abundances in comets (2). Especially, neutral N2 escaped detection before the Rosetta mission. Already in 1988, after the Giotto flyby at comet 1P/Halley, Geiss (3) recognized that, while carbon and oxygen relative to silicon are close to solar abundance, comet Halley was clearly lacking nitrogen. One explanation for this depletion at that time was the high volatility of N2, which may not have been condensed in the cometary ice or may have been lost in the last 4.6 Gy. For comet 67P / Churyumov-Gerasimenko (67P hereafter), neutral N2 has now been found on the level of (8.9 ± 2.4) × 10 -4 relative to water (~3 % relative to CO) (4). Recently, a high N2/CO ratio of 6% has been reported for comet C/2016 R2 (Pan-STARRS) (5). This shows that N2 is condensed and stored in cometary ice, but the reported abundances are by far not enough to explain the deficiency in nitrogen. N/C atomic ratio in the solar photosphere is about 0.3 ± 0.1 (6). In the refractory phase, comets are also depleted in nitrogen with N/C = 0.05 ± 0.03 in comet 1P (7) and N/C= 0.035 ± 0.011 in comet 67P (8).While the spread in relative abundances of HCN is quite small among comets, the variation for NH3 seems to be much larger (1). What is quite remarkable is the fact that comets with small perihelion distances seem to have much higher NH3/H2O values (1). This suggests that ammonia has a higher sublimation temperature in comets than water, although for pure ice sublimation temperatures are 90 K and 140 K, respectively. In comet D/2012 S1 (ISON) between 1.2 and 0.34 AU, Di Santi et al. (9) found an increase in NH3/H2O from < 0.78 % up to (3.5 ± 0.3) % and in addition a distribution of NH3, inconsistent with release from ...

On the Origin and Evolution of the Material in 67P/Churyumov-Gerasimenko

“…-Comets may contain significant amounts of ammonium salts (Mumma et al 2019) that are likely to account for the to-date observed deficiency of nitrogen in comets (Altwegg et al 2020;Geiss 1987;Poch et al 2020;Quirico et al 2016). -The relative abundances of the volatiles in comets exhibit similarities to the ISM and comet 67P/C-G is no exception (Drozdovskaya et al 2019).…”

“…The jury is still out on whether highly volatile species are predominantly trapped in CO 2 as opposed to H 2 O in the case of amorphous ices. However, this discussion must be revisited in the context of the recent discovery of abundant ammonium salts in 67P, that suggest the 'missing' N in comets may be hidden in them (Altwegg et al 2020;Poch et al 2020). These semi-volatile salts can form in the interstellar medium at temperatures below 15 K and then remain stable until warmed to much higher temperatures (see later discussion).…”

“…An emerging view has some volatiles produced at least in part from semi-volatile ammonium salts (e.g., NH + 4 CN − ) (cf. Altwegg et al 2020;Hänni et al 2019;Mumma et al 2019;Poch et al 2020;Quirico et al 2016).…”

“…Abundances equal to CH 3 OH and HCN are denoted by the vertical and horizontal dotted lines presence of ammonium salts in 67P/C-G (Poch et al 2020;Quirico et al 2016) and other comets (Mumma et al , 2018(Mumma et al , 2019. Indeed, the mass spectra obtained by ROSINA DFMS (Altwegg et al 2020;Hänni et al 2019) showed all possible sublimation products of five different ammonium salts. Significant amounts of nitrogen may be bound in ammonium salts and hence hidden from observations due to high sublimation temperatures, which lead to the well-known deficiency of nitrogen in comets (Geiss 1987).…”

“…However, the cometary volatile abundances observed in comet C/1995 O1 (Hale-Bopp) showed striking similarities to the volatile molecules from the cold interstellar medium . Moreover, comparisons of HCN and NH 3 with C 2 H 6 and H 2 O amongst 30 comets suggested that both HCN and NH 3 were better associated with C 2 H 6 production rather than H 2 O, and both showed enrichments in comets within 1 au of the Sun consistent with dissociation of the semi-volatile salt, ammonium cyanide (Altwegg et al 2020;Hänni et al 2019;Mumma et al , 2018Mumma et al , 2019. Evidently, multiple processes are at work, although their relative importance may differ from not only one comet to another but also among the various molecules.…”