Mars shakes with earthquakes, but not all are triggered by phenomena occurring below the surface – many are the result of meteorite impacts.
Meteorites crash into Mars every day. After analyzing data from NASA’s InSight lander, an international team of researchers noticed that its seismometer, SEIS, detected six seismic events nearby. These were linked to the same atmospheric acoustic signal that meteors generate as they move through the Martian atmosphere. Further investigation identified all six as part of an entirely new class of earthquakes known as VF (very high frequency) events.
The collisions that generate VF earthquakes occur in fractions of a second, far less time than the several seconds that tectonic processes take to cause earthquakes of similar magnitude. These are some of the main seismological data that have helped us understand the occurrence of earthquakes caused by meteoric impacts on Mars. This is also the first time that seismic data has been used to determine how often impact craters form.
“Although a non-impact origin cannot be definitively ruled out for every VF event, we show that the VF class as a whole is plausibly caused by meteorite impacts,” the researchers said in a study recently published in Nature.
Seismic displacement
Scientists had typically determined the approximate rate of meteorite impact on Mars by comparing the frequency of craters on its surface with the expected rate of impacts calculated using counts of lunar craters left behind by meteorites. The lunar crater scale models were then adjusted to match Martian conditions.
Looking at the Moon as a basis for comparison was not ideal, as Mars is particularly prone to being hit by meteorites. The red planet is not only a more massive body that has greater gravitational pull, but it is located near the asteroid belt.
Another issue is that lunar craters are often better preserved than Martian craters because there is no place in the Solar System dustier than Mars. Craters in orbital images are often partially obscured by dust, making them difficult to identify. Sandstorms can complicate matters by covering craters with more dust and debris (something that can’t happen on the Moon due to the lack of wind).
InSight deployed its SEIS instrument after landing in the Elysium Planitia region of Mars. In addition to detecting tectonic activity, the seismometer can potentially determine the extent of the impact through seismic data. When meteorites hit Mars, they produce seismic waves much like tectonic earthquakes, and the waves can be detected by seismometers as they travel through the mantle and crust. A large earthquake picked up by SEIS was associated with a crater 150 meters (492 feet) wide. SEIS would later detect five more earthquakes that were all associated with an acoustic signal (detected by another sensor on InSight) that is a telltale sign of a falling meteor.
A big impact
Something else stood out about the six impact-driven earthquakes detected with seismic data. Because of the velocity of the meteorites (over 3,000 meters or 9,842 feet per second), these events occurred faster than any other type of earthquake, even faster than earthquakes in the high frequency (HF) class. That’s how they earned their classification: very high frequency earthquakes, or VF. When the InSight team used the context camera (CTX) of the Mars Reconnaissance Orbiter (MRO) to image the locations of the events obtained by SEIS, there were new craters present in the images.
There are other seismic events that have not yet been assigned to craters. They are thought to be small craters formed by meteorites about the size of basketballs, which are extremely difficult to see in orbital images from MRO.
The researchers were able to use the SEIS data to estimate the diameters of the craters based on the distance from InSight (by how long it took the seismic waves to reach the spacecraft) and the magnitude of the associated VF earthquakes. They were also able to extract the frequency of earthquakes obtained from SEIS. After a frequency estimate based on the data was applied to the entire Martian surface, they estimated that about 280 to 360 VF earthquakes occur each year.
“The case is strong that the unique marsquake VF class is consistent with impacts,” they said in the same study. “It is therefore worth considering the implications of attributing all VF events to meteoroid impacts.”
Their discovery has added to the estimated number of impact craters on Mars as many of them could not be seen from space before. What can VF impacts tell us? The rate of impact on a planet or moon is important in determining the age of that object’s surface. Using the impacts has helped us determine that the surface of Venus is constantly being renewed by volcanic activity, while most of the surface of Mars has not been covered by lava for billions of years.
Figuring out the scale of the meteorite impact could also help protect spacecraft and, one day, possibly Martian astronauts, from potential hazards. The study suggests that there are periods when impacts are more or less frequent, so it may be possible to predict when the sky is slightly more likely to be clear of falling space rocks — and when it isn’t. Meteorites are not very dangerous for Earth since most of them burn up in the atmosphere. Mars has a much thinner atmosphere that it can get through more, and there is no umbrella for a meteor shower.
Nature Astronomy, 2024. DOI: 10.1038/s41550-024-02301-z