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

K. Altwegg, H. Balsiger+15 more|arXiv (Cornell University)|Nov 29, 2019

Astro and Planetary Science36 references4 citations

TL;DR

This study presents direct evidence of ammonium salts in comet 67P/Churyumov-Gerasimenko, detected via ROSINA on Rosetta, explaining the long-standing puzzle of nitrogen depletion in cometary comae. The salts, formed from ammonia and acidic species like HCN and HCOOH, sublimate at higher temperatures than water ice, delaying nitrogen release and mimicking a depletion in volatile nitrogen despite its presence in solid form.

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.

Motivation & Objective

• To resolve the long-standing mystery of nitrogen depletion in cometary comae despite known presence of ammonia and HCN.
• To identify and characterize nitrogen-bearing compounds in cometary ices that could account for the observed low volatile nitrogen abundance.
• To investigate whether ammonium salts—formed from ammonia and acidic species in cold interstellar and cometary environments—could be the missing reservoir of nitrogen.
• To link the observed NH3/H2O ratio trends with perihelion distance to the thermal stability of ammonium salts.
• To provide direct in situ evidence of ammonium salts in a cometary environment using mass spectrometry.

Proposed method

• Analysis of in situ mass spectrometry data from the ROSINA instrument aboard the Rosetta spacecraft during its close encounter with comet 67P.
• Identification of sublimation products of ammonium salts by detecting characteristic fragment ions and isotopic patterns.
• Comparison of measured ion abundances with laboratory-derived thermal desorption profiles of candidate ammonium salts (e.g., NH4HCOO, NH4CNO, NH4CN, NH4Cl, NH4NO3).
• Use of temperature-dependent sublimation models to infer the thermal stability and release behavior of the detected salts.
• Correlation of salt abundance with heliocentric distance and perihelion passage to assess thermal evolution effects.
• Application of isotopic constraints (e.g., 15N/14N ratios) to validate the origin and stability of the detected species.

Experimental results

Research questions

• RQ1 Do ammonium salts form in cometary ices and persist in the nucleus, explaining the apparent nitrogen deficiency in the coma?
• RQ2 What specific ammonium salts are present in comet 67P, and how do their sublimation temperatures affect nitrogen release timing?
• RQ3 How do the observed NH3/H2O abundance ratios vary with perihelion distance, and can this be explained by the thermal stability of ammonium salts?
• RQ4 Why are ammonium salts difficult to detect in space, and what signatures allow their identification despite their instability in the gas phase?
• RQ5 Can the presence of ammonium salts reconcile the discrepancy between predicted and observed volatile nitrogen abundances in comets?

Key findings

• ROSINA detected sublimation products of five different ammonium salts—NH4HCOO, NH4CNO, NH4CN, NH4Cl, and NH4NO3—in the coma of comet 67P.
• The presence of these salts explains the apparent nitrogen depletion in cometary comae, as they sublimate at higher temperatures than water ice.
• The measured sublimation products indicate that ammonium salts are stable in the comet's nucleus and release nitrogen only at higher heliocentric distances.
• The observed trend of increasing NH3/H2O ratios with decreasing perihelion distance is consistent with the thermal desorption behavior of these salts.
• Ammonium salts are likely the dominant reservoir of nitrogen in cometary ices, resolving the long-standing discrepancy in nitrogen abundance.
• The detection confirms that ammonium salts can survive in cometary ices and play a key role in prebiotic chemistry.