The drilling probe of the NASA probe “Mars InSight” was actually supposed to penetrate up to five meters deep into the Martian subsurface, but it failed due to an unexpectedly porous and brittle surface layer. Now the temperature data collected by this probe despite these problems provides clues to the formation of this “duricrust”. But they also reveal how well the Martian crust insulates against temperature fluctuations on the surface, how high the density of the layers near the surface is and how they are structured. Together, these new findings provide valuable insights into the physical and chemical properties of the Martian subsurface – and thus the factors that are also important for past or even present-day microbial processes.
When NASA’s Mars InSight probe landed on the Red Planet in November 2018, expectations were high: With the help of its instruments, including thermal probes, seismometers and radio position detectors, Mars InSight was intended to provide more precise data on the internal composition of the Red Planet for the first time. With success: Shortly after landing, the seismometer registered an earthquake on Mars for the first time, and a good 1,300 more were to follow over the next four years. Planetary researchers used this data to determine, among other things, information about the size and structure of the core, mantle and crust of the Red Planet. Seismic data from the area around the Cerberus Fossae trench system also provided exciting evidence that there could still be a hot, partly molten zone in the subsurface there, possibly even a volcanic magma chamber.
HP3 Drill: Failed on a “Duricrust”.
But Mars InSight’s most important instrument, the HP3 (Heat Flow and Physical Properties Package) experiment developed by the German Aerospace Center (DLR), caused problems. The aim of this probe, also known as the “Mars Mole”, was to drill up to five meters deep into the Martian subsurface using a hammer mechanism. There, the rod-shaped probe was intended to measure the heat flow beneath the surface of Mars for the first time. But the mole did not manage to penetrate deeper than almost 40 centimeters into the Martian soil. This turned out to be unexpectedly encrusted, but at the same time highly porous, and therefore did not offer the “percussion drill” enough support and friction to be driven further into the subsoil. “To get an idea of the mechanical properties of the soil, I like to refer to the floral foam used in floristry, a light, highly porous material in which holes are created when plant stems are pressed into them,” explains first author Tilman Spohn, scientific director of the experiment HP3 from the DLR Institute of Planetary Research. The existence of such a “duric crust” was not expected based on orbiter data in the Mars InSight landing area.
Nevertheless, the Mars mole was not completely useless: After the drilling attempts were completed, the planetary researchers used the probe to measure the temperature changes at the depth reached. “We determined the thermal conductivity and temperature fluctuations at short intervals on seven Martian days,” reports Spohn. “In addition, we continuously measured the highest and lowest daily temperatures over the second Martian year. These recordings of the temperature profile across daily cycles and different seasons were the first of their kind on Mars.” The team has now published the analysis of this data. The results provide insight into the thermal conductivity, structure and composition of the uppermost 40 centimeters of Martian crust. At the same time, they also provided clues as to how and why the “duricrust soil” was formed.
Good insulation and liquid brine
The measurements showed that the temperature underground fluctuates by only around five to seven degrees around the average of around minus 56 degrees during a Martian day – and therefore significantly less than on the surface of the planet. The temperatures there change by up to around 100 degrees when the time changes between day and night. The Martian soil is therefore a good insulator and significantly dampens the large temperature differences even at shallow depths, as Spohn and his team report. Over the course of the seasons, the temperature fluctuated by 13 degrees at a depth of almost 40 centimeters. With this temperature data, the “Martian mole” also provided clues as to how the approximately 20 centimeter thick duric crust was formed, which ultimately caused it to fail. As Spohn and his colleagues determined, in winter and spring it is warm and humid enough during the day to allow a thin film of liquid, salty brines to form in the layer just below the surface of Mars. When the humidity on the surface of Mars drops again, the brine crystallizes and solidifies the grains of the Martian regolith. This forms the porous but hard structure that hindered the Martian drill’s advance.
By comparing the temperatures at depth and at the surface, Spohn and his team were able to determine thermal diffusivity, a measure of the rate of heat transport in a substance, as well as thermal conductivity. This in turn made it possible for the first time to estimate the density of the Martian crust near the surface, something that had never been achieved before with any lander. Others too
Physical properties such as the elasticity of the soil, the speed of seismic waves or the transport of material in the Martian soil can now be derived from the HP3 data. “The temperature also has a strong influence on chemical reactions that take place in the soil, on the exchange with the gas molecules in the atmosphere and thus also on potential biological processes with a view to possible microbial life on Mars,” explains Spohn. The findings about the properties and strength of the Martian soil are also of particular interest for future Mars exploration with humans.
Source: Tilman Spohn (German Aerospace Center DLR, Cologne) et al., Geophysical Research Letters, doi: 10.1029/2024GL108600