First direct observations of atmospheric sputtering at Mars.

Billions of years ago, Mars' ability to sustain liquid water waned as the solar wind and radiation began to erode the atmosphere. Sputtering is an atmospheric escape process that may have been dominant during earlier epochs of our Sun according to isotopic evidence, but is difficult to detect under current solar conditions. Using over 9 years of data from the Mars Atmosphere and Volatile Evolution mission, we present the first observations of present-day sputtering in the martian upper atmosphere. By correlating argon densities with solar electric fields, we find that sputtered rates of argon are over four times higher than model predictions. We also present evidence of enhanced sputtering during a solar storm, offering a glimpse at more intense conditions in the early solar system. Observationally establishing the role of sputtering in the loss of Mars' atmosphere is critical to understanding the conditions that allowed liquid water to exist on the martian surface and the implications for habitability.