Gas Phase Ion Species Released During Grain Collisions: Implications For Protoplanetary Disks

Grain charging and gas ionization are important processes in protoplanetary disks. Both occur in mutual collisions between grains, as charge is exchanged between grain surfaces but also released into the surrounding gas as ions. The charge carrier for tribocharging, the origin of the gaseous ions, and their composition are currently unknown. However, it is important to know to validate the significance of these processes under disk conditions. In this work, we approach these questions by detecting molecules ejected during grain collisions by mass spectrometry. As tribocharging works well under normal atmospheric conditions, we use untreated "dirty" particles here. Without collisions, our measurements show a background mix of molecules. Among these are organics, but especially water-related molecules. During collisions, the abundances of not all but quite a few molecules change. Water-related molecules account for one of the largest changing fractions. These results suggest that particle collisions release adsorbates even at very low pressure, which is relevant for protoplanetary disks. As monolayers of water and organics are present on all surfaces in cool to moderately tempered parts of protoplanetary disks, this supports the importance of triboionization in disks.