On November 26th, 2018, NASA's Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport (InSight) mission landed on Mars. This was a significant achievement in Mars exploration as it marked the first time a research station had been deployed to the Martian surface to probe the planet's interior. One of the critical instruments utilized by InSight was the Heat Flow and Physical Properties Package (HP3), developed by the German Aerospace Center (DLR). Commonly referred to as the Martian Mole, this instrument was designed to measure heat flow from deep within the planet's interior for four years.
The HP3 was engineered to drill up to five meters (~16.5 ft) into the Martian surface to capture heat signatures from lower depths. However, the Mole encountered unexpected challenges in its operation, ultimately digging only a short distance just beneath the surface. This outcome surprised scientists, but despite the limitations, the Mole managed to collect extensive data regarding daily and seasonal temperature variations beneath the surface. A recent analysis of the obtained data, conducted by a team from the German Aerospace Center, has provided new insights into the unique crusty composition of Martian soil. The findings suggest that the temperature dynamics of the upper 40 cm (~16 inches) of the Martian surface are conducive to the formation of salt films which solidify within the soil.
The research, carried out by a team from the Microgravity User Support Center (MUSC) at the DLR Space Operations and Astronaut Training Institution in Cologne, focuses on the heat data acquired from the interior, which could be fundamental in understanding the geological evolution of Mars and investigating theories surrounding its core region. Current hypotheses suggest that geological activity on Mars largely came to a halt by the late Hesperian period (around 3 billion years ago). Nevertheless, some evidence indicates that lava flows might still be present in certain areas today.
This phenomenon is likely attributable to the faster cooling of Mars' interior due to its lower mass and atmospheric pressure. Scientists hypothesize that this led to the solidification of Mars' outer core while its inner core remains in a liquid state—although this remains a subject of ongoing research. By juxtaposing the subsurface temperature data acquired by InSight with surface temperatures, the DLR team was able to compute the rate of heat transport within the Martian crust (known scientifically as thermal diffusivity) and its thermal conductivity. From these metrics, they could estimate the density of the Martian soil for the first time.
The research team observed that the density of the uppermost 30 cm (~12 inches) of the Martian soil is akin to that of basaltic sand—this was an unanticipated comparison based on previous orbiter data. This grainy material is prevalent on Earth and was created through the weathering of volcanic rock rich in iron and magnesium. Below this layer, the soil density becomes comparable to consolidated sand and coarser basalt fragments. Tilman Spohn, the principal investigator of the HP3 experiment at the DLR Institute of Planetary Research, elaborated in a press release:
“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. Over the course of seven Martian days, we measured thermal conductivity and temperature fluctuations at short intervals.
“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.
Due to the presence of a hard, encrusted layer in the Martian soil, known as "duricrust," extending to a depth of approximately 20 cm (~8 inches), the Mole's penetration was limited, only achieving a depth of about 40 cm (~16 inches)—well short of its intended depth of 5 m (~16.5 ft). Nevertheless, the data retrieved at this depth has been instrumental in enhancing our understanding of heat transport mechanisms present on Mars. According to the research findings, temperature fluctuations in the Martian ground were documented to be only 5 to 7 °C (9 to 12.5 °F) throughout a single Martian day, a significant contrast to the temperature fluctuations observed on the surface, which ranged from 110 to 130 °C (230 to 266 °F).
Seasonal observations indicated a fluctuation of approximately 13 °C (~23.5 °F) while remaining under the freezing point of water in surface-adjacent layers, demonstrating that Martian soil possesses excellent insulating qualities. This characteristic significantly reduces the substantial temperature differences experienced at shallow depths, in turn influencing various physical properties within the Martian soil, including elasticity, thermal conductivity, heat capacity, and the mobility of materials, as well as the velocity of seismic waves traversing through it.
According to Spohn, “Temperature also has a substantial effect on chemical reactions occurring in the soil, impacting gas exchanges with the atmosphere and potential biological processes related to microbial life on Mars. These insights into the properties and strength of the Martian soil are significantly relevant as we look ahead to future human explorations of the red planet.”
One particularly intriguing discovery regarding temperature fluctuations was the potential formation of salty brines during winter and spring, occurring for up to ten hours a day when moisture levels were sufficient in the atmosphere. The solidification of these brines could indeed be the most plausible explanation behind the formation of the duricrust layer beneath the surface, providing essential insights as future missions endeavor to uncover more about Mars' geological history.
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