Mars Quakes Expose Daily Meteor Strikes on the Red Planet

An international research team has made a groundbreaking discovery: Mars is hit by 280 to 360 large meteorites annually. This estimate, based on seismic data from NASA’s InSight Mission, is five times higher than previous estimates derived from orbital images, demonstrating the effectiveness of seismology in impact studies.

For the first time, researchers have used seismic data to estimate a global meteorite impact rate, revealing that meteoroids the size of a basketball strike Mars almost daily. Impact-generated seismic signals indicate that meteorite impacts are significantly more abundant than previously thought. This seismic data offers a new tool, complementing observational data, for calculating meteorite impact rates and planning future Mars missions.

The international research team, co-led by ETH Zurich and Imperial College London, published their findings on June 28 in the journal *Nature Astronomy*. The study provides the first estimate of global meteorite impacts on Mars using seismic data. The findings suggest that between 280 to 360 meteorites strike the planet each year, forming impact craters greater than 8 meters (about 26 feet) across.

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Geraldine Zenhaeusern, who co-led the study, commented, “This rate is about five times higher than the number estimated from orbital imagery alone. Our findings, aligned with orbital imagery, demonstrate that seismology is an excellent tool for measuring impact rates.”

**Seismic “Chirp” Signals New Class of Quakes**

Using data from the seismometer deployed during NASA's InSight Mission to Mars, researchers found that six seismic events recorded near the station had previously been identified as meteoric impacts. This identification was made possible by the recording of a specific acoustic atmospheric signal generated when meteorites enter the Martian atmosphere.

Now, Zenhäusern of ETH Zurich, co-lead Natalia Wójcicka of Imperial College London, and their research team have discovered that these six seismic events belong to a much larger group of marsquakes, referred to as very high frequency (VF) events. The source process of these quakes occurs much faster than that of a tectonic marsquake of similar size. While a normal magnitude 3-quake on Mars takes several seconds, an impact-generated event of the same size takes only 0.2 seconds or less due to the hypervelocity of the collision. By analyzing marsquake spectra, researchers identified an additional 80 marsquakes now thought to be caused by meteoroid strikes.

**Meteorite Impact Detection Methodology**

The research quest began in December 2021, a year before accumulated dust on the solar panels ended the InSight mission. A large distant quake recorded by the seismometer reverberated a broadband seismic signal throughout the planet. Remote sensing associated the quake with a 150-meter-wide crater. To confirm this, the InSight team partnered with the Mars Reconnaissance Orbiter (MRO) Context Camera (CTX) to search for other fresh craters that matched the timing and location of the seismic events detected by InSight. The team's detective work paid off, leading to the discovery of a second fresh crater over 100 meters (320 feet) in diameter. However, smaller craters formed by basketball-sized meteoroids, which should be far more common, remained elusive. Now, the number of meteorite strikes is newly estimated by the occurrence of these special high-frequency quakes.

**First Meteorite Impact Rate From Seismic Data**

Approximately 17,000 meteorites fall to Earth each year, but unless they streak across the night sky, they are rarely noticed. Most meteors disintegrate as they enter Earth’s atmosphere, but on Mars, the atmosphere is 100 times thinner, leaving its surface exposed to larger and more frequent meteorite strikes.

Until now, planetary scientists have relied on orbital images and models inferred from well-preserved meteorite impacts on the Moon. However, extrapolating these estimates to Mars proved challenging. Scientists had to account for Mars' stronger gravitational pull and proximity to the asteroid belt, which both increase the number of meteorites hitting the red planet. On the other hand, regular sandstorms result in craters that are much less well-preserved than those on the Moon, making them not as easily detected with orbital imagery. When a meteorite strikes the planet, the seismic waves of the impact travel through the crust and mantle and can be picked up by seismometers.

**Insights From Mars’ Seismological Data**

Wójcicka explained, “We estimated crater diameters from the magnitude of all the VF-marsquakes and their distances, then used this data to calculate how many craters formed around the InSight lander over the course of a year. We then extrapolated this data to estimate the number of impacts that happen annually on the whole surface of Mars.”

Zenhäusern added, “While new craters can best be seen on flat and dusty terrain where they really stand out, this type of terrain covers less than half of the surface of Mars. However, the sensitive InSight seismometer could detect every single impact within the lander’s range.”

**Implications for Understanding Mars’ Geological Age**

Much like the lines and wrinkles on our faces, the size and density of craters from meteorite strikes reveal clues about the age of different regions of a planetary body. The fewer the craters, the younger the region of the planet. For example, Venus has almost no visible craters because its surface is continually reworked by volcanism, while Mercury and the Moon, with their ancient surfaces, are heavily cratered. Mars falls in between these examples, with some old and some young regions distinguishable by the number of craters.

New data shows that an 8-meter (26-foot) crater forms somewhere on Mars nearly every day, and a 30-meter (98-foot) crater occurs about once a month. Since hypervelocity impacts create blast zones that are easily 100 times larger in diameter than the crater, knowing the exact number of impacts is crucial for the safety of robotic and future human missions to the red planet.

“This is the first paper of its kind to determine how often meteorites impact the surface of Mars from seismological data – a primary mission goal of the Mars InSight Mission,” said Domenico Giardini, Professor of Seismology and Geodynamics at ETH Zurich and co-Principal Investigator for the NASA Mars InSight Mission. “Such data is essential for planning future missions to Mars.”

According to Zenhäusern and Wójcicka, the next steps in advancing this research involve using machine learning technologies to help identify further craters in satellite images and detect seismic events in the data.

July 4, 2024 10:52 a.m. 422

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