Mars shakes with marsquakes, but not all quakes are caused by phenomena that occur beneath the surface. Many quakes 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 their seismometer, SEIS, had detected six nearby seismic events. These were linked to the same atmospheric acoustic signal that meteorites generate as they hurtle through the Martian atmosphere. Further investigation identified all six as part of an entirely new class of earthquakes known as very high frequency (VF) events.
The collisions that generate VF quakes occur in fractions of a second, much less time than the few seconds it takes for tectonic processes to produce quakes of similar magnitude. These are some of the key seismological data that have helped us understand the occurrence of earthquakes caused by meteor impacts on Mars. This is also the first time that seismic data has been used to determine how often impact craters form.
“While 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 shift
Scientists had typically estimated the meteorite impact rate on Mars by comparing the frequency of craters on the surface to the expected impact rate, calculated using counts of lunar craters left by meteorites. Models of lunar crater rate were then adjusted to fit conditions on Mars.
Using the Moon as a comparison point was not ideal, as Mars is particularly susceptible to meteors. The Red Planet is not only a more massive body with a stronger gravity, but it is also located near the asteroid belt.
Another problem is that lunar craters are often better preserved than Martian craters, because there is no place in the solar system that is dustier than Mars. Craters in orbital images are often partially covered 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 because of 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 could potentially determine impact velocity from seismic data. When meteorites strike Mars, they produce seismic waves, much like tectonic marsquakes, and the waves can be detected by seismometers as they travel through the mantle and crust. One massive quake picked up by SEIS was linked to a 150-meter (492-foot) wide crater. SEIS would later detect five more marsquakes, all of which were associated with an acoustic signal (detected by another sensor on InSight) that is a telltale sign of a falling meteorite.
A huge impact
There was something else that stood out about the six impact-driven Marsquakes detected by seismic data. Because of the speed of meteorites (more than 3,000 meters or 9,842 feet per second), these events occurred faster than any other type of Marsquake, even faster than quakes in the high frequency (HF) class. Thus, they earned their own classification: very high frequency, or VF, quakes. When the InSight team used the Context Camera (CTX) on the Mars Reconnaissance Orbiter (MRO) to image the locations of the events picked up by SEIS, new craters were 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 basketball-sized meteorites, which are extremely difficult to see in MRO orbital images.
The researchers were able to use SEIS data to estimate crater diameters based on their distance from InSight (depending on how long it took for seismic waves to reach the spacecraft) and the size of the associated VF quakes. They were also able to infer the frequency of the quakes picked up by SEIS. After applying a frequency estimate based on the data to the entire Martian surface, they estimated that about 280 to 360 VF quakes occur each year.
“There is strong evidence that the unique VF quake 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 detection has contributed to the estimate of the number of impact craters on Mars, since many of them were previously invisible from space. What can VF impacts tell us? The velocity of impacts on a planet or moon is important for determining the age of the surface of that object. Impacts have shown 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.
Knowing the rate of meteorite impacts could also help protect spacecraft and, one day, perhaps Mars astronauts, from potential hazards. The study suggests that there are periods when impacts are more or less frequent, so it might be possible to predict when the sky is likely to be clear of falling space rocks—and when it isn’t. Meteorites don’t pose a major threat to Earth, since most of them burn up in the atmosphere. Mars has a much thinner atmosphere that more can pass through, and there’s no umbrella for a meteor shower.
Natural Astronomy, 2024. DOI: 10.1038/s41550-024-02301-z