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Researchers use acoustic waves to locate meteoroids on Mars

The eyes and ears of a NASA mission located four meteoroids that hit Mars.

(CN) — In a study published Monday in Nature Geoscience, researchers including University of Toulouse professor Raphael Garcia and Dr. Ingrid Daubar of Brown University analyzed seismometer data from NASA’s InSight lander and calculated the location of four meteor impact events by having their instruments listen for seismic waves.

Then, they requested imaging from NASA’s Mars Reconnaissance Orbiter to confirm three of the impact sites, a technique Daubar described as the study’s “ears” and “eyes." The study says that the researchers did not need to do the same with the fourth site because it was a seismic event associated with a previously detected impact.

Of note are two polarized arrivals.

Per the study, the SEIS record observed each came from two different points along the impactor trajectory. During each event, the study says the meteoroid hitting the planet generated acoustic waves that signaled an acoustic arrival.

During the impact process that created the first signal, “the meteoroid compressed the atmosphere along its track and that created a shock wave that is transformed into an acoustic wave,” wrote Garcia via email.

For the second signal, Garcia wrote that the meteoroid fragmented into smaller pieces due to strong compression in Mars’ air. Comparing it to an “atmospheric explosion,” Garcia added that it bore similarities to the 2013 Chelyabinsk meteor. The meteor exploded over the southern Ural region in Russia, generating a bright flash producing a hot cloud of dust and gas that penetrated to 26.2 km (16.3 mi).

Something Garcia noted was the oddness that their research equipment detected four impacts during the mission’s three-year period, considering the factors that could confuse their readings.

“We would expect the meteoroid flux to be constant, so should be the detection rate,” explained Garcia. “However, the signal/noise and the capability of acoustic waves to propagate over long distances varies a lot with time of day and season. In particular, if an impact occurs during daytime, you have both much higher background noise and worse propagation conditions for acoustic waves. It may be that all previous events occurred during daytime.”

While they hope that future studies will reveal why this happened, the researchers also believe their findings show planetary seismology’s importance for studying impact processes on other worlds.

“Detecting these impacts with InSight's seismic sensors and confirming them with orbital images from the Mars Reconnaissance Orbiter means that for the first time, we know exactly where and how the source of a seismic signal occurred on Mars,” wrote Daubar via email. “This allows us to calibrate all of our other estimates of InSight event locations, and confirms scaling relationships between the size of a crater and the seismic waves it will produce. These relationships can be applied to other planets, so when we send seismometers to other places in our Solar System, we'll be able to interpret those data better.”

Garcia concurred about the potential of their research with three reasons of his own.

“For science, it is the confirmation that our current models of Martian crust are valid, and that we can scale the seismic and acoustic waves of impacts to the vertical impactor momentum (or crater size),” wrote Garcia. From a historical perspective, as Garcia put it, it is proof that the observations by landers and orbiters provide more information than either could do alone.

As for the third reason, Garcia ackowledged it was fun “hearing sound (infrasound) with the ground rotations recorded by a seismometer.”

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