Using seismic data collected by the SEIS (Seismic Experiment for Interior Structure) instrument aboard NASA’s InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) lander, planetary researchers have resolved, for the first time on Mars, the shallow subsurface to around 200 m (656 feet) depth. Their results, published in the journal Nature Communications, suggest the presence of a shallow sedimentary layer sandwiched between lava flows.
This artist’s impression shows NASA’s InSight lander in Homestead Hollow, a small impact crater; the r SEIS seismometer is the light-colored hemisphere on the ground in front of the lander; the ground beneath it consists of a sandy regolith layer on top of alternate layers of sediments (yellow-orange colors) and basaltic rocks, i.e. former lava flows (brown colors). Image credit: Géraldine Zenhäusern / ETH Zürich.
“Mars has been the target of a large number of planetary science missions involving flybys, orbiters, landers, and rovers that have focused on surface and atmospheric remote sensing as well as surface geochemistry and mineralogy,” said Dr. Cédric Schmelzbach from ETH Zurich and his colleagues.
“NASA’s InSight mission is the first to specifically target the subsurface using seismic methods, deploying a very broad-band seismometer: SEIS.”
“SEIS operates continuously with the primary goal to detect marsquakes in order to quantify Martian seismicity and to infer the interior structure of Mars at all scales.”
In the study, Dr. Schmelzbach and co-authors used SEIS data to examine the shallow subsurface of Elysium Planitia, a flat-smooth plain just north of the Martian equator.
Right beneath the surface, they discovered a regolith layer of dominantly sandy material approximately 3 m (9.8 feet) thick above a 15-m (49-foot) layer of coarse blocky ejecta — rocky blocks that were ejected after a meteorite impact and fell back to the surface.
Below these top layers, they identified around 150 m (492 feet) of basaltic rocks, i.e., cooled and solidified lava flows, which was largely consistent with the expected subsurface structure.
However, between these lava flows, starting at a depth of about 30 m (98 feet), the researchers identified an additional layer 30 to 40 m (98-131 feet) thick with low seismic velocity, suggesting it contains weak sedimentary materials relative to the stronger basalt layers.
To date the shallower lava flows, the scientists used crater counts from existing literature.
They found that the shallower lava flows are approximately 1.7 billion years old, forming during the Amazonian period, a geological era on Mars characterized by low rates of meteorite and asteroid impacts and by cold, hyper-arid conditions, which began approximately 3 billion years ago.
In contrast, the deeper basalt layer below the sediments formed much earlier, approximately 3.6 billion years ago during the Hesperian period, which was characterized by widespread volcanic activity.
The team proposes that the intermediate layer with low volcanic velocities could be composed of sedimentary deposits sandwiched between the Hesperian and Amazonian basalts, or within the Amazonian basalts themselves.
“While the results help to better understand the geological processes in Elysium Planitia, comparison with pre-landing models is also valuable for future landed missions, since it can help to refine predictions,” said Dr. Brigitte Knapmeyer-Endrun, a researcher at the University of Cologne’s Bensberg Observatory.
“Knowledge of the properties of the shallow subsurface is required to assess, for example, its load-bearing capacity and trafficability for rovers.”
“Besides, details on the layering in the shallow subsurface help to understand where it might still contain ground water or ice.”
M. Hobiger et al. 2021. The shallow structure of Mars at the InSight landing site from inversion of ambient vibrations. Nat Commun 12, 6756; doi: 10.1038/s41467-021-26957-7