Simulations suggest cold steam is the source of streaking in Martian soil

While nobody is about to accuse Mars of feeling humid any time soon, the red planet does seem to have a wet season. Or rather, some soil deposits have a wet season, as during the summer dark streaks of moist soil turn up on various slopes, sometimes hundreds of feet long and 16 feet wide. While scientists are still confident about the hydrated soil identified in 2015, this scenario still leaves a lot of questions to be answered on an otherwise dry planet with a strikingly thin atmosphere. While we’d obviously love to pin down the origin of this salty brine, scientists from the University of Nantes in France have also been trying to figure the exact mechanics involved in making the streaks themselves. But they’ve been working on that problem here on Earth.

Sandbox simulations

Since working with the actual Martian soil isn’t that practical, the research team set up a reasonable simulation in a couple of plastic containers at home. Soil was built into 30° slopes, with ice placed at the top of their simulated hillside. The temperature and air pressure where then adjusted to match the conditions on Mars as closely as possible. For comparison, a second terrarium was created, but that one simply matched the average air pressure and temperatures found on Earth. These ambient conditions were expected to be the key variables in the water’s behavior, mainly to see how much the water would freeze or boil on the surface of the red planet.

The reason atmospheric pressure was so important is because it alone can change when water boils or not. Our everyday experience with boiling water usually involves heating it on our stove, but that’s not the only way to turn liquid water into vapor. For water molecules to start flying around as vapor, they only need enough energy to overpower the air molecules sitting on top of them. If there’s less air, the water molecules need less energy to rip away from each other and behave as a gas (which is why water boils at less than 100° Celsius at higher elevations where there’s less air pressure.) Under dramatic conditions like a near vacuum, the boiling basically demands all the available energy, leaving the water molecules that still couldn’t get airborne with so little energy that they’ll freeze.

Brisk but still boiling

For the simulated Maritain slopes, that low-pressure boiling was active enough to more or less dig out the dark channels that had been previously observed. The ice blocks melted, and the liquid water dribbled a bit, but then boiled vigorously enough to blast grains of soil to the left and right. The ejected soil built up into ridges on either side of the water’s path, making shapes similar to what’s been seen on Mars itself.

Source: Boiling water may be cause of Martian streaks, study says by Mariëtte Le Roux, Phys.org