Analysis of wind-induced vibrations from Mars sheds light on the underground properties of the planet – sciencedaily
Seismic data collected at Elysium Planitia, the second largest volcanic region on Mars, suggests the presence of a shallow sedimentary layer sandwiched between lava flows below the planet’s surface. These results were obtained as part of the NASA InSight mission (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport), in which several international research partners collaborate, including the University of Cologne. The article “Shallow Mars Structure at InSight Landing Site from Inversion of Ambient Vibrations” will appear in Nature Communication November 23.
Geophysicist Cédric Schmelzbach from ETH Zurich and his colleagues, whose earthquake specialists Dr Brigitte Knapmeyer-Endrun and doctoral researcher Sebastian Carrasco (MSc) from the Seismic Observatory at the University of Cologne in Bensberg, used seismic data to analyze the composition of the Elysium Planitia region. The authors examined the shallow subsoil to a depth of about 200 meters. Just below the surface, they discovered a layer of predominantly sandy regolith about three meters thick on top of a 15-meter layer of coarse boulder ejecta – boulders that were thrown out after an impact. meteorite and fell to the surface.
Beneath these upper layers, they identified around 150 meters of basalt rock, i.e., cooled and solidified lava flows, which was largely in line with the expected underground structure. However, between these lava flows, starting at a depth of around 30 meters, the authors identified an additional layer 30 to 40 meters thick with low seismic velocity, suggesting that it contains relatively weak sedimentary material. to stronger basalt layers.
To date the shallower lava flows, the authors used crater counts from existing literature. Established knowledge of the meteorite impact rate allows geologists to date rocks: surfaces with many meteor impact craters are older than those with fewer craters. Additionally, larger diameter craters extend into the lower layer, allowing scientists to date deep rock, while smaller ones allow them to date shallower rock layers.
They found that the shallower lava flows are around 1.7 billion years old and formed during the Amazon Period – a geological era on Mars characterized by low rates of meteorite and asteroid impacts and by cold, hyper-arid conditions, which began about 3 billion years ago. There are. In contrast, the deeper basalt layer beneath the sediment formed much earlier, about 3.6 billion years ago during the Hesperian Period, characterized by widespread volcanic activity.
The authors propose that the middle layer at low volcanic velocities could be composed of sedimentary deposits sandwiched between the Hesperian and Amazonian basalts, or within the Amazonian basalts themselves. These results offer the first opportunity to compare shallow subsoil ground truth seismic measurements with previous predictions based on orbital geological mapping. Prior to the landing, Dr Knapmeyer-Endrun had already developed models of the shallow subsoil velocity structure at the InSight landing site based on terrestrial analogues. The actual measurements now indicate additional stratification as well as more porous rocks in general.
“While the results help to better understand the geological processes of Elysium Planitia, the comparison with pre-landing models is also valuable for future ground missions, as it can help refine predictions,” noted Knapmeyer -Endrun. Knowledge of the properties of the shallow subsoil is necessary to assess, for example, its carrying capacity and ease of movement for rovers. In addition, details of the stratification in the shallow subsoil help to understand where it might still contain groundwater or ice. As part of his doctoral research at the University of Cologne, Sebastian Carrasco will continue to analyze the effect of the shallow structure of Elysium Planitia on earthquake records.
The InSight lander arrived on Mars on November 26, 2018, landing in the Elysium Planitia region. Mars has been the target of many planetary science missions, but the InSight mission is the first to specifically measure the subsurface using seismic methods.
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