Analysis of Martian vibrations caused by wind sh

image: Artist’s impression: The InSight probe is located in Homestead Hollow, a small crater. The SEIS seismometer used in this study is the light-colored hemisphere on the ground in front of the probe. The ground beneath it consists of a rich, sandy layer on top of alternate layers of sediments (yellow-orange colours) and basaltic rocks, i.e. past lava flows (brown colours). Show more

Credit: Geraldine Zenhäusern / ETH Zürich

Seismic data collected at Elysium Planitia, the second largest volcanic region on Mars, indicates a shallow sedimentary layer sandwiched between lava flows beneath the planet’s surface. These results were obtained under the NASA InSight (Internal Exploration Using Seismic Investigations, Geodesy and Thermal Transport) mission, where several international research partners collaborate, including the University of Cologne. The research paper, “Shallow structure of Mars at the InSight landing site from reflection of surrounding vibrations,” will appear in Nature Communications on November 23.

Geophysicist Dr. Cedric Schmelsbach of the Swiss Federal Institute of Technology in Zurich and colleagues, including seismologists Dr. Brigitte Knappmayer Endron and PhD researcher Sebastian Carrasco (MSc) from the Seismological Observatory of the University of Cologne in Pennsburg, used seismic data to analyze the composition of the Elysium-Planetia region. The authors examined the shallow subsurface of about 200 m in depth. Directly below the surface, they discovered a rich layer of predominant sandy material about three meters thick on top of a 15-meter layer of jagged block projectiles — blocks of rock that were ejected after a meteorite impact and fell back to the surface.

Beneath these upper layers, they identified about 150 meters of basaltic rock, that is, cooled and solidified lava flows, which were largely consistent with the projected subsurface structure. However, between these lava flows, starting at about 30 m depth, the authors identified an additional 30–40 m thick layer with low seismic velocity, indicating that they contain weaker sedimentary material compared to the stronger basalt layers.

To date the shallow pyroclastic flows, the authors used crater counts from the existing literature. Well-established knowledge about the rate of impact of meteorites allows geologists to date the rocks: surfaces with many impact craters from meteorites are older than those with fewer craters. Also, craters with larger diameters extend into the lower stratum, allowing scientists to date deep rocks, while smaller craters allow them to date shallower strata.

They found that shallow lava flows are about 1.7 billion years old, and formed during the Amazonian period – a geological epoch on Mars characterized by low rates of meteorite impacts and asteroids and cold, extremely dry conditions, which began nearly 3 billion years ago. In contrast, the deeper basalt layer beneath the sediments formed much earlier, roughly 3.6 billion years ago during the Hesperian period, which was characterized by widespread volcanic activity.

The authors suggest that the intermediate layer with lower volcanic velocities could consist of sedimentary deposits sandwiched between the Hesperian and Amazonian basalts, or within the Amazonian basalts themselves. These results provide the first opportunity to compare the seismic ground truth measurements of the shallow surface layer with previous predictions based on orbital geological mapping. Prior to the landing, Dr. Knapmeyer-Endrun had already developed models of the shallow subsurface velocity structure at the InSight landing site based on their ground-based analogues. Actual measurements now indicate additional layers as well as generally more porous rocks.

“While the results help to better understand geological processes at Elysium Planitia, comparison with pre-landing models is also valuable for future landing missions, as they can help improve predictions,” Knapmeyer-Endrun noted. Knowledge of the characteristics of the shallow surface layer is required to assess, for example, its bearing capacity and passability for roving vehicles. Besides, details of the layers below the shallow surface helps to understand where it may still contain groundwater or ice. In the framework 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 seismic recordings.

The InSight lander arrived at Mars on November 26, 2018, and landed at Elysium Planitia. Mars has been the target of several planetary science missions, but the InSight mission is the first to specifically measure Earth’s interior using seismic methods.

The Jet Propulsion Laboratory (JPL) manages the InSight program of NASA’s Science Mission Directorate. InSight is part of NASA’s Discovery Program, which is managed by the agency’s Marshall Space Flight Center in Huntsville, Alabama. Lockheed Martin Space Corporation of Denver built the Insight spacecraft, including a cruise stage and lander, and supports the mission’s spacecraft operations. A number of European partners, including the French National Center for Space Studies (CNES) and the German Aerospace Center (DLR), are supporting the InSight mission. The National Center for Space Studies has provided the Inner Structure Seismic Experiment (SEIS) instrument to NASA, with the principal investigator of the IPGP (Institut de Physiques de la Physique in the World in Paris). Important contributions to the Common Environmental Information System came from the IPGP; Max Planck Institute for Solar System Research (MPS) in Germany; Swiss Federal Institute of Technology (ETH Zurich) in Switzerland; Imperial College London and Oxford University in the UK; and JPL.

magazine

Nature Communications

Research topic

do not apply

Article title

Shallow structure of Mars at the InSight landing site from reflection of surrounding vibrations

The date the article was published

November 23, 2021

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