When NASA’s InSight spacecraft landed on Mars and began exploring its depths, a special underground probe was on board and found that the planet’s surface was a solid but tough crust. Now they’ve identified the reason for this.
InSight mission research
On November 26, 2018, NASA’s Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport (InSight) mission landed on Mars. This was a milestone in the exploration of Mars, as it was the first time a research station was brought to the surface to study the planet’s interior.
One of the most important tools InSight used for this was the Heat Flow and Physical Properties Package developed by the German Aerospace Center DLR. The instrument, also known as the Martian Mole, has measured heat flows from deep inside the planet for four years.
The instrument was designed to go five meters (~16.5 feet) below the surface to detect heat in the depths of Mars. Unfortunately, the mole failed to burrow and eventually ended up under the surface, which came as a surprise to the scientists. Nevertheless, he gathered a significant amount of data on daily and seasonal fluctuations beneath the surface.
Analysis of this data by a team from DLR has provided new insights into why Martian soil has such a crust. According to their findings, temperatures in the upper 40 centimeters of the Martian surface lead to the formation of salt films that make the soil harder.
Results of Martian subsurface exploration
The analysis, published in the journal Geophysical Research Letters, was conducted by a team from the Microgravity User Support Center (MUSC) of the DLR Space Operations and Astronaut Training Institution in Cologne, which is responsible for overseeing the HP3 experiment.
Heat data from the subsurface could be integral to understanding the geologic evolution of Mars and testing theories about its core. Scientists now suspect that geologic activity on Mars largely ceased at the end of the Hesperian period (about 3 billion years ago), although there is evidence that lava continues to flow there today.
This was probably because the interior of Mars cooled faster due to its smaller mass and lower pressure. Scientists speculate that this caused Mars’ outer core to solidify and its inner core to become liquid. Although this question remains open.
Comparing subsurface temperatures obtained by InSight to surface temperatures, the DLR team was able to measure the rate of heat transport in the crust (thermal diffusion) and thermal conductivity. Based on this, the density of the Martian soil could be estimated for the first time.
The team determined that the density of the upper 30 cm (~12 inches) of soil is comparable to basaltic sand, something that would not be expected based on the orbiting satellite data. This material is widespread on Earth and is formed by the weathering of volcanic rock rich in iron and magnesium.
Under this layer, the density of the ground is comparable to that of compacted sand and coarse basalt debris.
Martian soil temperature and heat transport
Since the Martian soil crust (the so-called “duricrust”) extends to a depth of 20 centimeters, the mole managed to penetrate just over 40 centimeters — much less than 5 meters from its target. However, the data obtained at this depth has provided valuable information about heat transport on Mars.
Accordingly, the team found that soil temperatures only fluctuate between 5°C and 7°C during a Martian day, a tiny fraction of the fluctuations observed on the surface — between 110°C and 130°C.
They noted seasonal temperature fluctuations of 13°C, while remaining below the surface freezing point of water on Mars. This indicates that the Martian soil is an excellent insulator, greatly reducing large temperature variations at low depths.
This affects various physical properties of the Martian soil, including elasticity, thermal conductivity, heat capacity, the movement of material within it, and the speed at which seismic waves can travel through it.
Temperature fluctuations and crust formation in Martian soil
Temperature also has a strong impact on the chemical reactions occurring in the soil, on the exchange with gas molecules in the atmosphere, and hence on potential biological processes concerning possible microbial life on Mars. This knowledge of the properties and strength of the Martian soil is also of particular interest for future human exploration of the Red Planet.
But especially interesting was the way temperature fluctuations make it possible for salty brines to form for 10 hours a day (when there is enough moisture in the atmosphere) in winter and spring. Therefore, the solidification of this brine is the most likely explanation for the duricrust layer below the surface. This information could be very useful for future missions that will explore Mars and try to get beneath the surface to learn more about its history.
Provided by hys.org
