Explanation found for encrusting of the Martian soil

Temperature measurements of the HP³ Mars mole have provided new insights

Mole, the Martian mole: for four years it had a small but highly regarded 'career' in planetary research. The experiment, unprecedented in the field, was named after the recognisable tunnel-digging mammal, but its official name is the Heat Flow and Physical Properties Package (HP³). In January 2019, at the landing site of NASA's InSight mission, the instrument – developed by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) in collaboration with European institutions – was placed onto the Martian surface. It was designed to dig up to five metres into the ground and measure the heat flow from deep inside the planet. Scientists were surprised when the mole struggled to bury itself, eventually getting just under the surface. Despite this, analysing the instrument's measurements of the mole's daily and seasonal temperature fluctuations at and just below the surface has yielded new and surprising results: temperatures in the top 40 centimetres of Martian soil promote the formation of crusty salt films, known as 'duricrust', which harden the soil.

Temperature measurements in the uppermost Martian soil at the InSight landing site, collected over many Martian days and across seasonal changes, have provided valuable insights into the formation of the 'duricrust' soil. During the nearly four years (that is, two Martian years) that InSight conducted experiments on the Martian surface, the mole, developed at the DLR Institute for Planetary Research, struggled to dig into Martian sub surface. The Martian soil proved unexpectedly difficult to penetrate – encrusted down to an approxi-mately 20 centimetre depth, yet also highly porous. "To get an idea of the mechanical properties of the soil, I like to compare it to floral foam, widely used in floristry for flower arrangements. It is a lightweight, highly porous material in which holes are created when plant stems are pressed into it," explains Tilman Spohn, principal investigator of the HP³ experiment at the DLR Institute of Planetary Research.

As a result, the mole failed to gain sufficient friction at the interface between the metal and the soil to absorb the remaining recoil of the hammer mechanism, preventing deeper penetration. The HP³ DLR experiment, designed to measure heat flow from Mars' interior, was therefore only partially successful. Attempts to hammer into the ground were discontinued at the start of 2021. The results from subsequent temperature measurements have now been published in the journal Geophysical Research Letters.

Insight into the interior of Mars

Because of the caked Martian soil down to a depth of 20 centimetres – which was not anticipated based on orbiter data – the mole managed to penetrate just a little more than 40 centimetres. After hammering tests were completed, the device was repurposed as a thermal probe. "Over the course of seven Martian days, we measured thermal conductivity and temperature fluctuations at short intervals," reports Tilman Spohn. "Additionally, we continuously measured the highest and lowest daily temperatures over the second Martian year. The average temperature over the depth of the 40-centimetre-long thermal probe was minus 56 degrees Celsius (217.5 Kelvin). These records, documenting the temperature curve over daily cycles and seasonal variations, were the first of their kind on Mars."

The temperatures in the near-surface Martian ground influence various physical properties, including the elasticity of the soil, the speed of seismic waves, thermal conductivity and heat capacity, as well as the movement of material within the soil. "Temperature also has a strong influence on chemical reactions occurring in the soil, on the exchange with gas molecules in the atmosphere and therefore also on potential biological processes regarding possible microbial life on Mars," Spohn continues. "These insights into the properties and strength of the Martian soil are also of particular interest for future human exploration of Mars"

Crystallised salty solutions harden the Martian soil

The ground temperature was measured to fluctuate by only five to seven degrees during a Martian day, which is a mere fraction of the 110- to 130-degree variations seen on the surface. This demonstrates that the Martian soil acts as an excellent insulator, significantly reducing the large temperature differences at shallow depths – by 10 to 20 times more than Earth's near-surface soil. Seasonally, the temperature fluctuated by 13 degrees and, in the layers near the surface, it remained below the freezing point of water on Mars.

Of particular interest, the temperature enables the formation of thin films of liquid, salty brines, for ten hours or more during a Martian day in winter and spring, when there is sufficient moisture in the atmosphere. Solidification of this brine is therefore the most likely explanation for the observed approximately 20-centimetre-thick duricrust layer of solidified, cohesive sand. This hardened layer is thought to have been the primary factor preventing the mission's thermal probe from penetrating to greater depths.

Determining the density of Martian soil for the first time

By comparing soil temperatures with surface temperatures, scientists were able to calculate thermal diffusivi-ty – a temperature-dependent measure of the rate of heat transport in a material – and thermal conductivity. From the ratio of thermal conductivity and diffusivity, the density of the Martian soil could be estimated for the first time – something not possible with previous landers. The density of the uppermost 30 centimetres of soil (including the duricrust) is comparable to that of basaltic sand, a weathering product of volcanic rock rich in iron and magnesium, and common on Earth. Below this layer, the soil corresponds to consolidated sand and coarser basalt fragments.

A Mars exploration premiere

When NASA's InSight mission gently touched down near the equator on the Elysium Planitia plain on 26 November 2018, it marked a milestone in Mars exploration. For the first time, a research station was placed on the surface of the 'Red Planet' with the aim of probing its interior by taking geophysical measurements. Two of InSight's most important instruments came from Europe: the French SEIS experiment, designed to measure ground vibrations caused by Mars quakes and asteroid impacts, and the thermal HP³ probe, provided by DLR to measure heat flow at and below the surface. This heat data would be crucial for understanding Mars' thermal evolution and the existence of a solid or perhaps still liquid metal core.

The HP³ experiment was controlled by the Microgravity User Support Center (MUSC) of the DLR Space Operations and Astronaut Training Institution in Cologne. As dust increasingly accumulated on the lander's solar panels, the power supply for the platform, experiments and communication became critical by the second half of 2022. Consequently, NASA decided to shut down the InSight mission on 15 December 2022, officially closing an extraordinary chapter in planetary exploration.

Related links

• Publication in the journal Geophysical Research Letters
• NASA – InSight mission
• DLR – InSight mission
• DLR – HP³ heat flow proble
• DLR blog – Insight logbook
• DLR Institute of Planetary Research
• DLR Space Operations and Astronaut Training institution

Contact

Falk Dambowsky
Head of Media Relations, Editor
German Aerospace Center (DLR)
Corporate Communications
+49 2203 601-3959

Prof. Dr. Tilman Spohn
HP³ Principal Investigator
German Aerospace Center (DLR)
DLR Institute of Planetary Research
www.dlr.de/pf

Press release DLR, 30 October 2024