Martian Rain

And what it tells us about the atmospheric pressure on early Mars.

Evidence for flowing water on Mars, as seen by the HiRISE camera on the Mars Reconnaissance Orbiter. How much was contributed by rainfall?

In a new paper in the journal Icarus, Robert Craddock of the Smithsonian National Air and Space Museum and Ralph Lorenz of Johns Hopkins University look at the effects the Martian atmosphere would have had on rainfall early in the planet’s history. Based on observed features like drainage patterns, including results from the Mars Pathfinder mission of the 1990s and the more recent Curiosity rover, evidence that it rained on early Mars is very strong. So Craddock and Lorenz asked themselves: What can the size of raindrops tell us about the early atmosphere on Mars?

Their results are intriguing. First, the scientists found that Mars could not have had an atmosphere thicker than 4 bars, or 4 times Earth’s atmospheric pressure, because any more than that would have produced only fog, not raindrops large enough to modify the Martian surface. This would run counter to the evidence we see for a water-shaped landscape.

As Mars lost more and more of its initial atmosphere over time, the raindrops would have grown in size. At about 3 bars of atmospheric pressure, Martian rain would have become a drizzle or light rain. At 1.5 bars, raindrop sizes would have been equivalent to what a storm produces on Earth. The intensity of these storm events would increase at 0.5 bar, at which point raindrops would have grown to a maximum size of 7.3 millimeters (almost a third of an inch), compared to a maximum size of 6.5 mm on Earth. However, even though the raindrops on Mars would have been bigger, the storm event would be less severe due to the lower gravity. And below 0.5 bar, the researchers say there would have been no more Martian rain.  

Rainfall and the modification of the landscape would have been critical, of course, to any life that may have existed on early Mars. Unfortunately, the study doesn’t go as far as pinning down a specific time frame when each type of rainfall regime was prevalent. But a thorough study of surface features should be able to narrow that down.

The main problem is that we lack the absolute dating of features on Mars that we have for Earth, so uncertainties will remain large. Nevertheless, the study contributes to our understanding of a planet that was once habitable, and became less so over time. Most intriguingly, the highest degree of habitability, at least regarding rainfall, was not right at the beginning, but when the Martian atmosphere got down to about 1.5 or 1 bar of atmosphere pressure: the Goldilocks zone of rainfall, I suppose.

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