The evidence for liquid water on the surface of Mars in the distant past is strong, but a discovery a few years ago provided a glimmer of hope that the wet stuff might still be making occasional appearances on the Red Planet. Fresh, dark streaks show up on steep slopes during the “warm” season, almost as if something wet is trickling downhill. To some researchers, however, these “recurring slope lineae,” which are a few meters wide and a few hundred meters long, look more like downward slides of destabilized sediment.

The question is, what could destabilize the sediment? The presence of briny water? (Water has been detected as a component of some of the minerals present, at least.) Could the thawing of carbon dioxide ice play a role? There is debate about which of these explanations can work and where water could possibly be coming from.

A new study led by Frédéric Schmidt of the University of Paris-Sud throws out a possible alternative that doesn’t involve thawing anything. If you’re holding out for water, you might consider that bad news, but it is at least a satisfyingly weird process.

The explanation relies on a phenomenon that is hard to find on Earth called a “Knudsen pump." At very low gas pressures, where collisions between molecules are relatively rare, gases in a very thin tube will do something strange: creep from a cold end of the tube to a warm end of the tube. That helps raise the gas pressure at the warm end of the tube.

Mars’ atmosphere is thin enough to satisfy the pressure requirement, and the small spaces between the grains of dust on the surface act as suitably tiny “tubes.” So when the Sun warms the surface of the Martian regolith, the temperature at the very surface of the dust rises more than it does a centimeter or two below. That creates a Knudsen pump that forces subsurface gas to move upward.

Why does this weird phenomenon matter? Because it can actually be a significant part of processes that move dust around the Red Planet. The researchers thought it might also help explain those dark streaks showing up on slopes. Any pile of sediment has maximum stable steepness, depending on the characteristics of the sediment and the local conditions. Landslides on Earth often follow heavy rains, when the soaking reduces that maximum tolerable steepness and destabilizes the slope.

A shadow pump

To investigate what might be happening on Mars, the team ran model simulations for slopes facing different directions and throughout the Martian year. At first, they actually found that the “Knudsen pump” didn’t have much effect at all on the maximum stable steepness, making it an unlikely trigger for the seasonal dark streaks.

But if you throw in a boulder to cast a little shade, things get interesting. In the first few minutes that a shadow extends over the surface, the temperature profile changes. The temperature just below the surface doesn’t change much, but the surface itself cools very quickly. Now, the highest temperature isn’t found right at the surface, but a little bit below the surface—with cooler temperatures both above and below that warmest point. Now we have two “Knudsen pumps” directing gas from both directions toward a single point below the surface. That slightly increases the gas pressure below the surface, which provides a little lifting force on the dust grains.

That lifting force can be enough to destabilize a slope (by reducing the maximum stable steepness several degrees) and cause a little slide. The simulated instabilities occurred around the same time of year that the dark streaks have been observed to appear. And those streaks start from the upper part of slopes where weathered bedrock is exposed, so shadows would be common.

That’s not to say this is the right answer and liquid water is off the table, but it seems like a plausible hypothesis that could be tested further. Mars might not even be the only place this process could play out, as the researchers note: “As a perspective, this exotic dry flow mechanism could also occur in other planetary environments, in extremely rarefied gas such as those of [Pluto], Triton, or even the Moon.”

Nature Geoscience, 2016. DOI: 10.1038/NGEO2917 (About DOIs).