Mark studies aerosols in planetary atmospheres via remote sensing and modeling. His long-time focus has been Mars: for Opportunity and Curiosity, he monitors atmospheric dust load, studies dust properties, measures dust lifting in dust devils, and moonlights in astronomical imaging. He is currently preparing for a similar role on Mars 2020 with Mastcam-Z and Skycam. Previously, he was the imaging team lead for the Phoenix mission, as well as a participant in Spirit's mission and a member of the imaging teams for Mars Pathfinder and (very briefly) Mars Polar Lander. In non-Martian studies, he studied Jupiter's clouds with the Net Flux Radiometer on the Galileo Probe, and Titan's aerosols with the Hubble Space Telescope and the Huygens Probe's Descent Imager/Spectral Radiometer.