The researchers have measured the temperature on Mars’ surface when its moon Phobos passes in front of the sun. The surface temperatures measured by the lander became slightly colder during these transits due to the lower amount of sunlight the surface received at this time. The transits only lasted for 20 to 35 seconds though, and therefore only the top layer of the ground a few millimetres thick has time to cool significantly. Based on this observation, the researchers conclude that the material in the top millimetres of the ground is different from that below. A possible explanation would be an increase of soil density with depth, a larger fraction of smaller particles such as dust at the top, or a less cemented layer of particles on top of a soil in which particles are slightly more cemented together.
– On Earth, the soil differs greatly between locations and the same is definitely true on Mars. In another location from where InSight landed, NASA’s Spirit rover got stuck in a sand pit. And in yet another location, near Mars’ north pole, the Phoenix lander found that the soil was somewhat sticky, as it tried to take a sample, Axel Hagermann says.
Cemented but porous
It was one of instruments on InSight, an infrared radiometer provided by the German Aerospace Agency DLR that measured the surface. This instrument can take the temperature of a surface in its field of view every two seconds. InSight also has a camera on board, so a solar eclipse can literally be seen. A drop in power coming from InSight’s solar panels can also measure the shadow of the moon that passes between InSight and the sun. Another of InSight’s instruments gave another clue to the soil puzzle: a drill consisting of a self-propelling mole, designed to dig into Mars’ soil to a depth of five meters.
– Strangely, this mole only got to a depth some 10s of centimetres, despite extensive tests on Earth. The thermal measurements of the surface fit into that picture because they show that Mars’ soil becomes more compact with depth. Possibly, there is also some cementation going on below the surface, Axel Hagermann explains.
The researchers still investigate exactly what the characteristics of Mars’ soil really are. One plausible explanation is that parts of Mars’ surface are covered by what is known as a “duricrust”, i.e. a cemented and yet rather porous type of soil.
– Rocks and soil on Mars behave in very peculiar ways; we clearly need to understand that before we think about colonising Mars, Axel Hagermann says.
Mars in Kiruna
In their lab at Space Campus in Kiruna, the research group in Atmospheric Science is aiming at creating analogues of this type of soil – material of low density that seems to be cemented but can disintegrate easily.
– We also need to investigate what kind of environment can help create this type of material and whether these conditions exist or could have existed on Mars, Axel Hagermann concludes.