Explanation found for encrusting of the Martian soil
October 30, 2024 | InSight mission: News from the HP³ Mars mole
Explanation found for encrusting of the Martian soil
InSight – Mission for geophysical measurements on Mars
This artist's impression shows the Mars lander InSight at its deployment site in the Elysium Planitia plain with unfolded solar panels, the platform with the two antennas, various technical and scientific components, the robotic arm with camera and the gripping mechanism used to place two of the main experiments from the platform onto the ground in front of the probe: On the left, the SEIS (Seismic Experiment for Interior Structures) seismometer provided by France for the mission, and on the right, the HP³ (Heatflow and Physical Properties Package) experiment developed by DLR. InSight landed on Mars on 26 November 2018. Due to a lack of power supply, the mission was declared terminated by NASA on 20 December 2022.
Image: 1/5, Credit:
NASA/Caltech
Structure of the Martian soil directly below the InSight landing site
Temperature measurements in the uppermost Martian soil were taken using DLR's HP³ experiment at the In-Sight landing site and collected over many Martian days and thus across seasonal changes. They provided new insights into the formation and structure (left) of Mars's 'duricrust soil'. The temperatures are high enough for brines (salty liquids) to form when the humidity of the atmosphere in the Martian winter and spring allows it. Thermal conductivity measurements using the HP³ probe have shown that the soil 'breathes' over the seasons. As humidity decreases, the brine crystallises and the salts encrust the Martian soil. This happens at the landing site down to a depth of around 20 centimetres, providing an explanation as to why the InSight 'mole' was unable to penetrate deeper into the Martian soil. When penetrating the duricrust, the mole created a hole as it hammered and therefore failed to generate enough friction in the upper 20 centimetres. The duricrust soil is covered by one to two centimetres of Martian dust and sand, with a layer of volcanic sand and debris below. On the right, the filled pit, traces from the lander's gripper arm can be seen, and the pressure point where the arm was used to compact the soil to enable deeper penetration through higher friction.
Image: 2/5, Credit:
GRL (T. Spohn et al.); NASA/JPL-Caltech
InSight's last selfie
On 24 April 2022, NASA's InSight Mars probe took its last selfie with its camera on the robotic arm - it was the 1211th Martian day of the mission. The solar panels were already heavily covered in Martian dust, which was quite expected after almost three and a half years of mission duration, so it was foreseeable that the power production would be sufficient for fewer and fewer functions. On 20 December 2022, after two unsuccessful communication attempts - what mission control at the Jet Propulsion Laboratory calls "dead bus mode" - NASA declared the InSight mission over. With 1446 sols (Martian days; 1 sol equals 24 hours and 39 minutes), InSight has a mission duration more than twice as long as planned. InSight launched on 5 May 2018 and landed on Mars on 26 November 2018.
Temperatures in the Martian soil and their fluctuations beneath the surface determine how quickly chemical and physical processes occur.
Although temperatures are below the freezing point of water, salts make it possible for brines to form, which crystallise and form a crust.
Crusts formed by crystallizing brines may explain the formation of cohesive sand, which prevented the HP3 thermal probe from penetrating to greater depths.
The temperature of the Martian soil is a critical factor in assessing the potential for microbial life.
Focus: Space, Mars, InSight mission, Solar System exploration
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.
First and last Insight selfies on Mars
These are the first and last complete selfies of NASA's InSight lander on Mars. They show the solar panels and the lander's deck with and without Martian dust. The first selfie, in which the solar panels are still completely dust-free, was taken on 6 December 2018 (Sol 10). The selfie consists of 11 images taken by the Instrument Deployment Camera at the elbow of the robot arm. These images were then stitched together into a mosaic. The deck is still covered by the scientific instruments that were deployed later on the ground, as well as the weather sensor booms and the UHF antenna. The last selfie was taken on 24 April 2022, the 1211th Mars day of the mission. The lander is now covered with much more dust than in its first selfie. As InSight's dust-covered solar cells produced less and less energy, the team moved the lander's robotic arm to its resting position in May 2022 and had to declare the end of the mission shortly after.
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.
