On average, there is a crash once a day: Researchers have used the seismic sensitivity of the InSight probe to redetermine the impact rate of meteorites on Mars. According to the results, this rate has been significantly underestimated so far. The new data shows that over 300 meteorites hit the planet every year, forming craters over eight meters wide. This information is important for determining the age of Martian surfaces and for assessing the risks of planned Mars missions, say the researchers.
In the case of the Earth, they are often only noticeable by a brief flash in the night sky: most smaller celestial bodies do not reach the surface because they disintegrate in the atmosphere and burn up. But significantly more meteorites make it through to our neighboring planet because Mars’ gas envelope is a hundred times thinner than Earth’s. But how many actually hit the planet? So far, this has been estimated based on models and fresh impact craters that can be seen in space images of the Martian surface. However, there was a considerable uncertainty factor. Image resolutions are limited and material from sandstorms can also obscure the craters on Mars.
Now an international research team has shown a better way to estimate impact rates. The results are based on data from NASA’s InSight space probe, which landed on the surface of Mars in November 2018. Since then, it has been continuously monitoring the planet. The sensitive seismometer has already detected many Marsquakes that can provide information about the structures deep within the planet. But in addition to tectonic quakes, InSight can also record the tremors of the ground when a meteorite hits nearby.
Seismic evidence
As part of the study, the team was able to show that certain high-frequency seismic waves are the result of meteorite impacts. Meteorite impacts that were initially detected by the InSight probe’s seismometer were subsequently confirmed by images from the Mars Reconnaissance Orbiter. “New craters are best seen on flat, dusty terrain, because that’s where they stand out most. However, this type of terrain is only found on less than half of the surface of Mars. However, the sensitive seismometer of the InSight mission was able to record every single impact within the probe’s range,” says co-first author Géraldine Zenhäusern from ETH Zurich.
The researchers then obtained further information from the collected seismic data of many impacts: “Based on the strength and distance of the high-frequency Marsquakes, we were able to estimate the diameters of the craters. Using this information, we then calculated how many craters had formed around the InSight probe over the course of a year. We extrapolated this data to estimate the number of annual impacts on the entire surface of Mars,” explains co-first author Natalia Wójcicka from Imperial College London.
The new data now show that a new crater of at least eight meters in diameter forms on the surface of Mars almost every day, and a crater of 30 meters in diameter about once a month. “The rate determined is about five times higher than the number of impacts estimated from imaging alone. Our results show that seismology combined with the analysis of space images is an excellent tool for measuring impact rates,” says Zenhäusern. Co-author Gareth Collins from Imperial College London adds: “Listening for impacts seems to be more effective than looking for them if we want to understand how often they occur.”
Significance for dating and space travel
The information is important for estimating the age of surface structures on Mars, the researchers explain. The fewer craters a region has, the younger it is. This makes it possible to draw conclusions about when changes to the surface occurred due to volcanism. The average crater formation rate therefore plays an important role in estimating the age. “By using seismic data to better understand how often meteorites hit Mars and how these impacts change the surface, we can begin to create a timeline of the geological history and development of the red planet,” says Wojcicka.
The study results also have significance for space travel, say the researchers. This is because meteorite impacts can, due to scattering effects, affect areas whose diameter is a hundred times larger than the actual crater. More precise information about the impact rate is therefore particularly important for assessing the risks of manned missions to Mars. “The new data can now be incorporated into the planning of future missions to Mars,” says senior author Domenico Giardini from ETH Zurich.
Sources: Imperial College London, ETH Zurich. Specialist article: Nature Astronomy, doi: 10.1038/s41550-024-02301-z