X-ray processing of a realistic ice mantle can explain the gas abundances in protoplanetary disks

The Atacama Large Millimeter Array has allowed a detailed observation of molecules in protoplanetary disks, which can evolve toward solar systems like our own. While CO, CO2, HCO, and H2CO are often abundant species in the cold zones of the disk, CH3OH or CH3CN are only found in a few regions, and more-complex organic molecules are not observed. We simulate, experimentally, ice processing in disks under realistic conditions, that is, layered ices irradiated by soft X-rays. X-ray emission from young solar-type stars is thousands of times brighter than that of today’s sun. The ice mantle is composed of a H2O:CH4:NH3 mixture, covered by a layer made of CH3OHand CO. The photoproducts found desorbing from both ice layers to the gas phase during the irradiation converge with those detected in higher abundances in the gas phase of protoplanetary disks, providing important insights on the nonthermal processes that drive the chemistry in these objects.

PNAS (published online, 2020)