How Rain Helped Shaped The Surface Of Mars

Valleys on Mars. NASA/JPL-Caltech/Arizona State University.

American researchers have discovered several intriguing facts about the past of the Red Planet by looking at the erosion on its surface due to precipitation.

The research, published in Icarus, focuses on the Noachian time period, between 3.7 and 4.1 billion years ago, when the surface of Mars was covered in water and under heavy bombardment from asteroids and meteorites. Specifically, the team looked at the valleys on Mars likely caused by heavy rainfall.

“By using basic physical principles to understand the relationship between the atmosphere, raindrop size, and rainfall intensity, we have shown that Mars would have seen some pretty big raindrops that would have been able to make more drastic changes to the surface than the earlier fog-like droplets,” co-author Dr. Ralph Lorenz, from the Johns Hopkins University Applied Physics Laboratory, said in a statement.

Based on the erosion, the researchers suggest that the pressure on Mars never exceeded four atmospheres. At that pressure, the raindrops would be small and sediment transport would be limited, so it wouldn’t have formed the valleys we see. That happens when the pressure falls below 1.5 atmospheres.

Less than that threshold, the droplets can grow big enough – up to 7.3 millimeters bigger, which is about a millimeter bigger than on Earth. Only then could heavy rains begin to erode the landscape. The intensity of the rain would be less than on our planet due to the lower gravity, but that would be enough to penetrate the soil and begin the formation of the valley networks, which still remain visible to this day.

“There will always be some unknowns, of course, such as how high a storm cloud may have risen into the Martian atmosphere, but we made efforts to apply the range of published variables for rainfall on Earth,” co-author Dr. Robert Craddock explained. “It’s unlikely that rainfall on early Mars would have been dramatically different than what's described in our paper. Our findings provide new, more definitive, constraints about the history of water and the climate on Mars.”

The dense atmosphere of the Red Planet in its early days is consistent with recent research that looked at how quickly Mars lost its atmosphere, which suggested that back then it was at least as dense as our own planet.

Today, Mars is a barren, freezing desert with an atmospheric pressure that is about 0.6 percent of what we have at sea level on Earth. In its hey-days full of water, life might have developed, and it's possible that some were able to cling on as the Red Planet slid into its current frigid climate.   

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