A crust of brine prevented the Martian mole from drilling

Oct. 30 () –

A hard brine crust is the most likely explanation why NASA’s Insight mission’s HP3 drill on Mars could not sink to 5 meters as planned.

The Martian mole, which participated in planetary research for four years, was the first of its kind in the world. The experiment, unprecedented in its field, was named after the well-known tunnel-digging mammal, but its official name was 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 (DLR) in collaboration with European institutions, was placed on the Martian surface. It was designed to dig up to five meters into the ground and measure the flow of heat from the depths of the planet.

Scientists were surprised when the mole had difficulty burying itself, eventually reaching just below the surface. Despite this, analysis of the instrument’s measurements of the mole’s daily and seasonal temperature fluctuations at the surface and just below it has produced surprising new results: Temperatures in the upper 40 centimeters of Martian soil promote the formation of crusty salt films, known as “duricrust”, which harden the soil.

Temperature measurements on the top of the Martian soil at the InSight landing site, collected over many Martian days and throughout seasonal changes, have provided valuable information about the formation of “duricrust” soil. During the nearly four years (that is, two Martian years) that InSight conducted experiments on the Martian surface, the mole struggled to burrow into the Martian subsurface. The Martian soil proved unexpectedly difficult to penetrate: it was embedded to a depth of about 20 centimeters, but it was also very 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 floral arrangements. It is a light and very porous material in which holes are created when the stems of the plants are pressed,” he explains. in a statement Tilman Spohn, principal investigator of the HP3 experiment at the DLR Planetary Research Institute.

As a result, the mole failed to generate enough friction at the metal-ground interface to absorb the remaining recoil of the hammer mechanism, preventing deeper penetration. Therefore, the DLR’s HP3 experiment, designed to measure heat flow from the interior of Mars, was only partially successful. Attempts to drive the hammer into the ground were discontinued in early 2021.

The results of subsequent temperature measurements have now been published in the journal Geophysical Research Letters.

Because the Martian soil was packed to a depth of 20 centimeters (something that was not expected according to the orbiter data), the mole managed to penetrate just over 40 centimeters. After completing hammering tests, the device was reused as a thermal probe.

AT 56 DEGREES BELOW ZERO

“Over the course of seven Martian days, we measured thermal conductivity and temperature fluctuations at short intervals,” reports Tilman Spohn. “In addition, we continuously measured the highest and lowest daily temperatures during the second Martian year. The average temperature at the depth of the 40-centimeter-long thermal probe was minus 56 degrees Celsius (217.5 Kelvin). These records, that document the temperature curve throughout daily cycles and seasonal variations, They were the first of their kind on Mars“.

Temperatures in the near-surface Martian soil influence several physical properties, including soil elasticity, seismic wave speed, thermal conductivity, and thermal capacity, as well as the movement of material within the soil.

“Temperature also has a strong influence on the chemical reactions that occur in the soil, on the exchange with gas molecules in the atmosphere and, therefore, also on possible biological processes related to possible microbial life on Mars. ” continues Spohn. “These insights into the properties and strength of Martian soil “They are also of special interest for future human exploration of Mars.”

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

A HARD 20 CENTIMETRE SCAB

Of particular interest, the temperature allows 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. Therefore, solidification of this brine is the most likely explanation for the observed approximately 20 centimeters thick hard crust layer of solidified, cohesive sand. This hardened layer is believed to have been the main factor preventing the mission’s thermal probe from penetrating to greater depths.

By comparing soil temperatures to surface temperatures, scientists were able to calculate thermal diffusivity (a temperature-dependent measure of the rate of heat transport in a material) and thermal conductivity. From the relationship between thermal conductivity and diffusivity, the density of the Martian soil could be estimated for the first time, something that was not possible with previous landers. The density of the top 30 centimeters of soil (including the hard crust) is comparable to that of basaltic sand, a product of the erosion of volcanic rock rich in iron and magnesium, and common on Earth. Below this layer, the soil corresponds to consolidated sand and thicker basalt fragments.