Expected constraints on the outer solar system formation conditions from the Rosetta-ROSINA measurements

Formation scenarios of the protosolar nebula invoke two main reservoirs of ices that took part in the production of icy planetesimals. The first reservoir, located within the inner region of the protosolar nebula, contains ices (dominated by H2O, CO, CO2, CH4, N2 and NH3) originating from the ISM, which, due to their near solar vicinity, were initially vaporized. With time, the decrease of temperature and pressure allowed the water in this reservoir to condense at ~150 K in the form of crystalline ice. It is postulated that a substantial fraction of the volatile species were trapped as clathrates during this condensation phase as long as free water ice was available and there was enough time to overcome the slow kinetics of clathration. On the other hand, the remaining volatiles that were not enclathrated (due to the lack of available water ice or a low kinetics of clathration) probably formed pure condensates at lower temperatures in this part of the nebula. The second reservoir, located at larger heliocentric distances, is composed of ices originating from the ISM that did not vaporize when entering into the disk. In this reservoir, water ice was essentially in the amorphous form and the other volatiles remained trapped in the amorphous matrix. The location of the boundary between these two reservoirs is loosely constrained and may vary between 5 and 30 AU from the Sun, depending on the postulated nebula’s thermodynamic conditions. The uncertainty in the distance of the boundary implies that comets may have formed from amorphous ice as well as from crystalline ices and/or clathrates. Here we review the key in situ measurements that are within the capabilities of the ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) instrument aboard the Rosetta spacecraft during its approach of comet 67P/Churyumov-Gerasimenko. These key measurements may allow disentangling between the different formation scenarios.