Scientists believe that ancient Mars had a relatively warm, wet climate. Running water carved deep and complex networks of valleys and collected in lake beds, the remains of which are still visible in satellite images today. About 3.5 billion years ago, the Red Planet became colder and drier, and valley formation slowed as precipitation dwindled—or so scientists thought.
New research from Wilson et al. suggests that flowing water, perhaps from snowmelt, formed midlatitude valleys between 2 billion and 3 billion years ago, near the transition between Mars’s Hesperian and Amazonian periods. This could be evidence that Mars had a cold, wet climate during the transition and possibly well into the Amazonian.
These findings arose from efforts to uncover the history of distinctive, water-carved features known as fresh shallow valleys, which are found in midlatitude regions across Mars. Fresh shallow valleys are narrower, shallower, and less degraded than other ancient valleys, suggesting that they might also be younger.
Wilson’s team used high-resolution images and topographic data collected by satellites to examine the size, shape, age, and distribution of fresh shallow valleys in Arabia Terra, a large region just north of the Martian equator. They also used a computer model to simulate water flow and recreate rivers and lakes that may have existed in the Red Planet’s past.
The results suggest that fresh shallow valleys in Arabia Terra formed between the late Hesperian period, which ended 3 billion years ago, and the mid-Amazonian, about 2 billion years ago. The shape and shallow depths of the valleys indicate that they were likely active for a geologically short period of time, probably carved by seasonal melting of snow that flowed across the surface.
Other researchers have speculated that volcanic activity, asteroid impacts, or other local heat sources could have melted enough water to form relatively young valleys on Mars. However, these hypotheses are inconsistent with the authors’ findings. Instead, the researchers say the appearance and wide distribution of the valleys in the midlatitudes point to a global Martian climate that was wet—and potentially habitable—much later than has been assumed. (Journal of Geophysical Research: Planets, doi:10.1002/2016JE005052, 2016)
—Sarah Stanley, Freelance Writer