Mars impacts seen and heard
&ball; Physics 15, 164
Linking acoustic and seismic signals from meteor impacts to images from the orbiter is a step towards mapping the planet’s interior.
NASA’s InSight lander team recently discovered that meteorites crashing into Mars can produce distinctive sounds in the form of low-frequency “bloops.” By combining these bloops with seismic wave data, researchers have now identified matching craters in satellite images. . Further analysis revealed 1,200 recently created craters, providing a toolkit for interpreting the history and internal structure of Mars and suggesting that such impacts are more frequent than previously thought. . Ingrid Daubar of Brown University in Rhode Island presented the results at the 54th Meeting of the Planetary Science Division of the American Astronomical Society in London, Canada earlier this month.
The Insight The lander arrived on Mars in 2018, equipped with a seismometer, pressure sensors, a thermal probe and other geophysical instruments. One of its main objectives is to study the interior of Mars by measuring the seismic activity of the planet. Much of what we know about the Earth’s interior comes from measuring the propagation of seismic waves from an earthquake in one location to detectors in distant locations. InSight has listed over 1000 march tremors, but the amount of information provided by these data is limited due to uncertainty about the origin of the waves. Meteor impacts could offer an alternative source of seismic waves, but they are only recognizable in retrospect by identifying the crater site. One of InSight’s seismic tasks is to record these events and verify their location through images of new craters.
In September 2021, InSight pressure sensors picked up a bloop sound, characterized by lower frequencies arriving before higher pitches. InSight scientists recognized the bloop as an acoustic wave passing through a heated atmosphere after sunset – a similar sound effect is seen in deserts on Earth. “It was really exciting,” says Daubar, as a meteorite impact was the most likely source of these sound waves.
Screening of seismometer data revealed matching seismic signals that passed through the ground from impact, and calculations showed that the time it took for the two to reach the lander matched patterns of a Martian impact. To verify that the signals came from a meteoroid (as a meteorite is called before it hits the ground), the team had new images of the predicted crash site taken by NASA’s Mars Reconnaissance Orbiter (MRO), a satellite that has been orbiting the red planet since 2006. “We were happy to find fresh-looking craters that weren’t there in the older images,” says Daubar.
The impact had actually resulted in a cluster of craters, formed when the meteoroid broke through the atmosphere and landed about 85 kilometers from the lander. “There were three distinct ‘bloops’, which we were able to relate to the explosion in the atmosphere and the impact on the ground,” explains Daubar. The team also identified in archived seismic data three other impacts that had occurred in 2021 and had been recorded by InSight’s seismometers and MRO.
With a clear picture in mind of what a newly formed crater looks like, researchers continued to sift through old MRO records for “before” and “after” images of the surface of Mars near InSight. They sourced a catalog of 1,200 newly created impact craters, recording their sizes, ages, and visible features such as radial striations and reflectivity. “We can use this information to learn more about the types of materials hitting Mars, their strength, and their breakdown in the atmosphere,” she explains.
The crater datasets revealed that the impact rate in the surveyed regions appears higher than previously measured, and slightly higher than expected. Such observations could provide a clearer picture of near-Earth objects raining down on Mars and could pose a danger to future space missions or to Earth.
Next, the researchers plan to correlate the observed impacts with four years of archived seismic wave data from InSight. Daubar imagines that there should be other meteorite impacts producing observable seismic signals, in addition to the four they recently identified. Connecting these datasets could provide a map of impacts with known magnitudes and locations, allowing detailed measurements of the time it takes for seismic energy to travel from each to the seismometer. This is the key to deciphering the shallow structure of Mars’ crust and, by modeling the associated acoustic waves, deciphering the planet’s atmospheric layers.
The work demonstrates the interest of combining seismic, acoustic and imaging tools on other objects in the solar system. NASA’s Dragonfly mission to Saturn’s icy moon Titan is scheduled to launch in 2026 and arrive in 2034 with seismic and acoustic wave detectors. Daubar wonders if bloop-like sounds will help researchers study this celestial body. “Does Titan’s thick atmosphere produce different acoustic signals?” she asks. Ever since the first modern seismograph recorded tremors on Earth two centuries ago, researchers have continued to learn more about the layers of matter that make up our planet. The possibilities for other planets are equally enticing.
Rachel Berkowitz is the corresponding editor for physics journal based in Vancouver, Canada.