The second-largest volcanic zone on Mars, Elysium Planitia, recorded seismic data that point to the existence of a shallow sedimentary layer sandwiched between lava flows beneath the planet’s surface.
These results were obtained as part of NASA’s InSight project, which stands for Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport. The University of Cologne is one of the mission’s international scientific partners.
The paper ‘The shallow structure of Mars at the InSight landing site from inversion of ambient vibrations’ will appeared in Nature Communications on 23rd November.
Geophysicist Dr. Cédric Schmelzbach from ETH Zurich and colleagues, including the earthquake specialists Dr. Brigitte Knapmeyer-Endrun and doctoral researcher Sebastian Carrasco (MSc) from the University of Cologne’s Seismic Observatory in Bensberg, used seismic data to analyse the composition of the Elysium Planitia region.
The shallow subsurface down to a depth of around 200 meters was studied by the authors. They found a regolith layer just beneath the surface, which was about three meters deep and dominated by sand, above a 15-meter layer of coarse, blocky ejecta stone blocks that were expelled following a meteorite impact and returned to the surface.
They discovered 150 meters of basaltic rocks, or cooled and hardened lava flows, beneath these top layers, which was broadly consistent with the anticipated subsurface structure.
While the results help to understand the geological processes in Elysium Planitia better, comparison with pre-landing models is also valuable for future landed missions, since it can help to refine predictions.
Dr. Knapmeyer-Endrun
The scientists discovered an extra layer between these lava flows, commencing at a depth of around 30 meters, that was 30 to 40 meters thick and had a low seismic velocity, indicating it contained weak sedimentary elements in contrast to the stronger basalt layers.
To date the shallower lava flows, the authors used crater counts from existing literature. Established knowledge about the impact rate of meteorites allows geologists to date rocks: surfaces with many impact craters from meteorites are older than ones with fewer craters.
Also, craters with larger diameters extend into the lower layer, allowing the scientists to date the deep rock, while smaller ones allow them to date the shallower rock layers.
They discovered that the shallower lava flows are roughly 1.7 billion years old and formed during the Amazonian period, a geological age on Mars that started around 3 billion years ago and was defined by low rates of meteorite and asteroid impacts as well as by cold, hyper-arid conditions.
The deeper basalt layer beneath the sediments, on the other hand, originated considerably more recently, roughly 3.6 billion years ago, during the Hesperian epoch, which was characterized by intense volcanic activity.
The authors suggest that the intermediate layer with low volcanic velocities may be made up of sedimentary deposits that are either embedded inside or sandwiched between the Amazonian and Hesperian basalts. These findings offer the first chance to compare orbital geological mapping-based forecasts to seismic ground-truth measurements of the shallow subsurface.
Prior to the landing, Dr. Knapmeyer-Endrun had already developed models of the velocity structure of the shallow subsurface at the InSight landing site based on terrestrial analogues. The actual measurements now indicate additional layering as well as more porous rocks in general.
“While the results help to understand the geological processes in Elysium Planitia better, comparison with pre-landing models is also valuable for future landed missions, since it can help to refine predictions,” Knapmeyer-Endrun remarked.
To evaluate, for instance, the shallow subsurface’s load-bearing capacity and rover trafficability, one needs to be aware of its qualities. A deeper understanding of the shallow subsurface’s layering also aids in determining whether or not it still contains ice or groundwater.
Sebastian Carrasco will continue to examine the impact of the shallow structure of Elysium Planitia on marsquake records as part of his dissertation thesis at the University of Cologne.
The InSight lander arrived on Mars on 26 November 2018, touching down in the Elysium Planitia region. Mars has been the target of numerous planetary science missions, but the InSight mission is the first to specifically measure the subsurface using seismic methods.
