27 August 2013

Mars once had falling snow, but lost it with the rest of its atmosphere

When dreamers looked at grainy, pre-Viking shots of the Martian surface, they interpreted the long, seemingly even furrows as farmland. They made up elaborate stories of doomed canal-builders working frantically to hold off the advance of the frozen tundra of Mars. They were dramatic stories that arrested people’s imaginations for decades, but despite evidence disproving the canal hypothesis we still struggle to fully explain the origins of many features of the Martian surface. Some, astronomers now believe, are ancient lake and river beds, valleys carved out by the flow of ancient liquid water. But others geological features don’t fit that model — so what caused them?

A new report from Brown University presents evidence that snowfall may be one answer. We find ice at the polar caps, of course, but actual snowfall is a remarkably Earth-like meteorological feature that has implications for early Martian conditions, and could even change to rain at times. In 2008, NASA announced having detected snow falling from Martian clouds, but it was entirely vaporized before reaching the ground. The researchers claim that actual snowfall, and buildup on the surface leading to melting and runoff, could have created many of the tributary networks observed near tall mountain-ranges.

They even have some evidence to back this up. The paper makes use of a computer simulation from the Laboratoire de Météorologie Dynamique called the Mars global circulation model (GCM). This compiles evidence about the early composition of the red planet’s atmosphere to predict global circulation patterns. Since other models predict that Mars would have been quite cold, they looked at what the model had to say about the likelihood of snow, and sure enough the program put the highest probabilities of snowfall over the densest valley systems. During periodic warming, these could even have been short-lived bouts of rainfall.

An artist’s concept of Mars’ west hemisphere, billions of years ago, when it still had its oceans and atmosphere

It’s not surprising that the team looked to snowfall as an explanation, since lead researcher Kat Scanlon’s graduate studies were based in Hawaii, which displays the same orographic (science for “studying mountains”) tendencies as are now posited on Mars. When winds approach from the East, they lack the kinetic energy to get all the way over the tallest mountains. This causes their moisture to condense and fall back down, making the Eastern side of the Island temperate and jungle-like, and the West dry and desert-like. In a cold enough climate, this condensed moisture would fall as snow, rather than rain.
Curiosity has everything from drills to lasers for taking and analyzing samples of the red planet.

NASA’s Curiosity rover recently returned information that might explain why Mars no longer displays this kind of behavior: it’s losing its atmosphere. It has taken detailed assays of the current atmosphere, which is almost entirely carbon dioxide and about 0.6% the pressure of Earth’s at sea-level. More notably, it has used its ability to laser-blast solid samples and analyze the resulting vapor to determine that Mars has an unusually high ratio of heavy to light isotopes — most importantly of deuterium to hydrogen.

The main hypothesis to explain this is atmospheric loss, since light isotopes will escape slightly more quickly than heavy. Over billions of years, this can lead to non-standard isotope levels the show a loss of atmosphere. One major theory that might explain this loss say that about 4.2 million years ago Mars collided with an object about the size of Pluto. An impact from this body, a bit under a fifth the mass of Earth’s moon, would have caused a huge expulsion of atmosphere, followed by a slow, continued loss from then on. Their data don’t prove the collision hypothesis, but do fall in line with it.

All this seems most relevant to that inescapable question with regard to Mars: is there, or was there ever, life? While we may still stumble on some Lake Vostok-like reserve of microbial life under the surface, it seems more likely that life may have once existed there. That seems much more likely if we can prove a thicker atmosphere containing with a wider array of elements and compounds, and featuring the sorts of weather patterns we know can allow life to flourish.

Now read: The giant black hole at the center of the Milky Way is murdering a huge gas cloud.


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