Mars's Gale Crater experienced flooding on a truly immense scale, a study of its geologic structures has concluded. The finding adds to the evidence water was present on the Martian surface later than previously thought, although it is not clear if it lasted long enough to improve the prospects for life.
Gale was chosen as the Curiosity’s Rover's landing site because from orbit it appeared to have once been filled with water. Since the rover’s arrival in 2012, it has sent back a steady stream of supporting evidence, but we may have been misinterpreting the legacy of a gigantic flood 3.6-3.85 billion years ago.
As early as 1997 the Pathfinder Rover produced evidence its site at Ares Vallis had experienced a major flood, but Dr Alberto Fairén of Cornell University is proposing something on a different scale for Gale Crater.
Many of the rocks around Mount Sharp, Gale’s central peak, appear to be sedimentary, indicating this was once a shallow lake-bed. These sediments include wave-shaped gravel ridges known as "megaripples" 10 meters (33 feet) high and about 137 meters (450 feet) apart. Based on the ridges' slopes and the wavelength between them, the team has concluded they were made by walls of water at least 24 meters (80 feet) high traveling at a speed of 10 meters per second.
"We identified megafloods for the first time using detailed sedimentological data observed by the rover Curiosity," Fairén said in a statement. The shapes resemble those carved in floods of ice melt on Earth.
Features like this on Gale Crater's floor have previously been thought to have formed prior to Mount Sharp's emergence as surrounding areas eroded away. However, Fairén presents evidence most are younger than Mount Sharp, implying water returning to the system after a long absence.
On Earth such floods are usually a consequence of climatic shifts or volcanic heat under the ice. For Mars, particularly given the timing, the more likely scenario is that an asteroid or comet hit the planet, Fairén and colleagues conclude in Scientific Reports. The heat generated by impact turned vast quantities of water ice, frozen carbon dioxide, and methane stored beneath the surface to gas.
These gasses produced a short-lived but powerful greenhouse effect that warmed the planet sufficiently to melt even more ice, particularly at equatorial locations like Gale. Subsequent cooling created torrential rains, which added to water released as the ice melted, with deluges coming over the edge of Gale Crater and down Mount Sharp combining. “The flooding event may have only lasted a few days, but the warm and wet period persisted for much longer,” the paper concludes. The authors are confident of this based on layers overlying the flood deposits, but say the time could have been anything from weeks to centuries.
The authors have identified four craters formed at the right time large enough to have produced this effect.
Martian megafloods have previously been proposed based on Mars Express observations of Worcester Crater, which lies near the mouth of Kasei Valles, thought to be Mars's largest channel system. As impressive as that evidence was, we know much more about Gale Crater than Worcester and have the opportunity to study it further as long as Curiosity persists.