On 24 December 2021, NASA's InSight lander felt the ground shake. Its SEIS seismometer registered a marsquake of magnitude 4. Independently, a 150-metre diameter crater was photographed from orbit by the cameras of the Mars Reconnaissance Orbiter. The camera team was able to determine that it was formed on 24 December, and realised that this was the same day as the reported quake. When the two teams compared notes, they found that the location derived from the seismic data was the same as the crater location and concluded that the seismic signal originated from the impact. This is the first time that a meteorite impact on another planet has been recorded both photographically and seismically. To the surprise of scientists, large amounts of water ice were thrown on to the surface by the impact, officially named S1094b. Two articles published today in the journal Science describe the event and its effects in detail. Researchers from the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) are involved in the analyses. In addition, an article on tectonics on Mars was published in Nature Astronomy at the same time, which explains that the marsquakes observed with InSight in recent years are the result of earlier volcanic activity in the Cerberus Fossae area.
From a seismological perspective, the photographic evidence of the impact makes it possible to determine the exact distance to the epicentre, which could otherwise only be estimated much less precisely with a single seismometer. This makes it possible to more accurately measure the path of the seismic waves through Mars and the properties of the rocks along this path. Observing meteorite impacts thus helps to understand the interior of Mars even better.
When the meteoroid struck, in a region called Amazonis Planitia, it blasted a crater roughly 150 metres across and 21 metres deep into the Martian soil. The dark halo of churned-up material surrounding it measures at least 37 kilometres across. With the seismic detection by InSight and the subsequent images from the Mars Reconnaissance Orbiter, the researchers had the extremely rare good fortune to 'witness' the formation of a crater of this size. Many larger craters exist on the planet but they are several million or billion years old.
Ice beneath the surface near the Martian equator
Brand new craters offer insights into the processes of crater formation, exposing fresh subsurface material that has yet to be modified by wind, weather and sunlight. In this case, large chunks of ice scattered by the impact were captured by NASA's Mars Reconnaissance Orbiter HiRISE (High-Resolution Imaging Science Experiment) colour camera, suggesting that the impact exposed a layer of ice from 10 to 20 metres below the surface. For future crewed Mars missions, it would be particularly beneficial to see where subsurface ice for human use can be found on Mars: subsurface water ice has been spotted several times in the northern lowlands, but never so close to the Martian equator, where Mars is warmest.
Second impact discovered in seismic data
After examining the seismic signal of the impact, the research team also revisited older data to look for similar seismograms. The data revealed that the epicentre of a quake that had occurred on 18 September 2021 also corresponded to a fresh crater of more than 100 metres in diameter. This impact is also described in the study. "It's unprecedented to find a fresh impact of this size," said Ingrid Daubar of Brown University, who leads InSight’s Impact Science Working Group. "It's an exciting moment in geologic history, and we got to witness it."
Surface waves provide conclusions about Martian crust
The quake triggered by this massive impact in December 2021 was the first observed by the mission to involve surface waves – a type of seismic wave that propagates along the top of the planet's crust. The second of two papers published today in the journal Science describes how scientists used these waves to study the structure of Mars' crust. "The analyses of the two impact events show that the crustal structure on the respective route from the impact to the InSight platform differs from the crustal structure at the landing site itself," explains Ana-Catalina Plesa. "The higher average propagation velocities of the seismic waves indicate a different composition of the crust in these areas. A lower porosity of the crust there could also be a cause. Both, in turn, would indicate a higher crustal density and local variations in the density of the Martian crust that we have not seen before." Preliminary analyses suggest that the crustal structure of the northern and southern Martian hemispheres may be similar at depths ranging between five and 30 kilometres. "Further analyses and direct comparison of seismic waves from both impact events will give us important clues about the formation and expression of the 'Martian dichotomy', which describes the division between the northern lowlands and southern highlands on Mars," Plesa added.
Many marsquakes provide clues to Mars tectonics
In another recent publication in the journal Nature Astronomy, the marsquake recorded over the past three years have been put into a geological context: Most of the quakes for which an epicentre could be calculated occurred in the Cerberus Fossae area, about 1500 kilometres east of InSight’s location. This is a relatively young volcanic centre, with the last eruptions occurring around 50,000 to 200,000 years ago. "A striking feature of Cerberus Fossae are grabens hundreds of kilometres long but very narrow and deep, running through the landscape like cracks in a rising dough," explains seismologist Martin Knapmeyer from the DLR Institute of Planetary Research, who is involved in this study. "Such graben are created when volcanic 'veins' form; that is, when magma from greater depths penetrates cracks in the upper crust, causing the entire region to bulge and rise. Over time, the magma solidifies and contracts somewhat in the process. Some of the recorded marsquakes took place at locations close to the last ejected lava and some just below the visible graben. This shows seismograms that fit well with cooling gangue rocks," added Knapmeyer.
Another 'family' of marsquake, on the other hand, shows an unusually slow fracture propagation, as is known from volcanic areas on Earth, such as the Eifel. This slow fracture propagation is related to the heating of the rock by the intruded magma. "Thus, the SEIS data show that Cerberus Fossae also resembles known volcanic areas in Earth's subsurface, and that volcanism there, like in the Eifel, is perhaps not yet completely extinct, but is currently only dormant," Knapmeyer emphasises.
InSight provides a glimpse into the Martian interior
InSight was sent to the Red Planet to study Mars' deep interior – its crust, mantle and core – which can give scientists an insight into the formation of all terrestrial bodies, including Earth and the Moon. Seismic waves are key to this improved understanding. Since InSight's touchdown on November 2018, the SEIS experiment has recorded 1318 marsquakes, including several caused by much smaller meteoroid impacts. Most of the quakes, however, were caused by tectonic tremors, that is, displacements of rock masses like in earthquakes.
The frequency of large meteorite impacts is of interest not only because of the possible danger to future astronauts; the number and size of craters on other planets is used to estimate the age of their surfaces. This statistical evaluation includes the impact frequency, which can be determined more precisely the more impacts can be directly observed. Finally, the recent experiment of the DART mission was aimed at preventing impacts like S1094b on Earth.
The InSight mission is being carried out by NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, on behalf of the agency's Science Mission Directorate. InSight is part of NASA's Discovery Program. The German Space Agency at DLR, with funding from the German Federal Ministry for Economic Affairs and Climate Action, supported a contribution by the Max Planck Institute for Solar System Research to the French main instrument SEIS (Seismic Experiment for Interior Structure). DLR researchers are involved in the evaluation of the SEIS data. In addition, DLR contributed the Heat Flow and Physical Properties Package (HP3) experiment to the mission.
Detailed information on the InSight mission and the HP3 experiment is available on DLR's dedicated mission site.