Laboratory studies for understanding the findings of the ROSETTA spacecraft on Comet 67P/ Churyumov–Gerasimenko.
The era of space exploration has brought with it a unique opportunity to explore small bodies in the solar system, their composition and structure and even their link to the early epochs of the solar system and the primordial Earth. Comets are small icy bodies, among the most intriguing objects in our solar system and as such, targeted by space missions. A few successful missions to comets, essentially fly-bys, preceded the ambitious Rosetta mission, which monitored comet 67P/Churyumov-Gerasimenko (67P/C-G) by means of different instruments, for over two years in orbit around the comet.
The Rosetta mission made many new discoveries regarding the comet nucleus structure and evolution along the orbit. In the present work, we aim at explaining some of the Rosetta mission findings by laboratory experiments that simulate comet formation and evolution conditions. The laboratory results are then combined with theoretical models and confronted with the Rosetta findings.
The ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) instrument on-board the spacecraft measured the production rates and coma abundances of H2O and other volatiles: CO2, CO, O2, N2, Ar, Kr, Xe and many more. Our laboratory experiments followed the behavior of amorphous H2O ice in which various volatiles are trapped, from formation at low temperatures and pressures, through heating and release of the trapped volatiles, until the temperature is high enough for the ice to sublimate. We focused on CO2 as a major volatile, and carried out a series of experiments examining the effect of CO2 on the trapping and release of additional volatiles in its presence. The results shed light on the observed activity of comet 67P/C-G, and explain in particular the anti-correlation between the measured H2O and CO2 production rates.
In light of the first O2 measurement in comets, we performed experiments with O2 trapped in amorphous, measuring its release from the ice upon crystallization and H2O sublimation. Our results agree with the in-situ measurements and confirm the observed correlation between O2 and H2O.
Finally, we performed experiments with Ar, Kr and Xe, which were measured for the first time in comets. We found that the isotopic fractionation of Kr and Xe in the laboratory experiments agreed with the in-situ measurements, both differing from the solar isotopic ratios.
To complement the experimental study in order to justify the direct comparison of the laboratory results, which simulate the behavior of ice in the interior of the nucleus, with observations related to surface activity that determines the coma composition, we applied a thermo-physical model to comet 67P/C-G. We found that, globally, the abundance ratios in the interior are preserved in the ejected material, although during outbursts they may vary.
We conclude that our laboratory results for trapping and release of volatiles support the hypothesis that cometary ice is originally amorphous, and this not only explains some of the typical features of cometary activity, but also provides insight on the conditions under which comets formed.