Hunting for Mars’ missing carbon(ate)
Mars wasn’t always so dry and dusty. Many geographical features point to valleys that were carved long ago by flowing water eroding channels between the mountains. However to have liquid water on the surface, there needed to be a thick enough atmosphere on the planet to keep the water from evaporating and retain heat to keep the water from freezing. Recent analysis suggests that that atmosphere may not have been available at the time these valleys were being carved, narrowing the options for Mars’ wetter history.
When looking for missing atmosphere, one place to start is the rocks. Carbon from the atmosphere can trapped or sequestered in rock formations as carbonate minerals. If the missing carbon was indeed trapped in this way, shrinking the Martian atmosphere from the ground, large deposits of carbonate minerals should be detectable. But while surveys from a variety of instruments and missions have found some carbonate deposits, even the largest collection, at the size of Delaware, isn’t enough to account for what would have been necessary for a robust atmosphere. There might be more carbonate deeper underground, but it would have been deposited there before the valleys were eroded, leaving the presumed liquid source of erosion high and dry.
The answer to the riddle may be that the valleys were instead formed by tiny, icy trickles. If the atmosphere was instead dissipating into space, instead of being absorbed into the ground, there wouldn’t be much heat to maintain temperatures for flowing water. That water may have been on the surface as ice and snow however, where tiny shifts in temperature could have occasionally melted tiny trickles of water. Those small drops could carve and then refreeze, expanding and widening grooves in the Martian rock, eventually forming the valleys we see today.
Source: What Happened to Early Mars' Atmosphere? New Study Eliminates One Theory by Tony Greicius, NASA