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SYFY WIRE Bad Astronomy

Whoop whoop whoop! NASA InSight 'hears' an asteroid impact on Mars

Four separate impacts detected by the NASA lander.

By Phil Plait
A triple crater

On Sept. 5, 2021, NASA’s InSight lander “heard” the impact of an asteroid slamming into the surface of Mars — in fact the space rock broke into three pieces, each hitting the planet and creating a crater. Not only that, but it also detected the acoustic waves generated by the asteroid as it rammed through the thin air of Mars too!

It gets better: The HiRISE camera on Mars Reconnaissance Orbiter (or MRO) found the craters left behind by the impactor.

All four events were translated into sound, so you can actually hear it. Listen:

Spooky, and cool.

The three impacts were felt by the Seismic Experiment for Interior Structure, or SEIS, a seismometer lowered to the Martian surface shortly after InSight landed in 2018. It’s detected thousands of marsquakes since then, including some big ones. But acoustic (sound) waves travel through the air as well, and can be detected by SEIS as they pass.

The first sound is the asteroid passing through the air. The Martian atmosphere is exceedingly thin, less than 1% as thick as Earth’s, but sound can still travel through it. 

So why did the impact make that whoop-whoop-whoop sound?

That’s a little complicated. The speed of a wave through a medium, like air or rock, doesn’t depend on frequency. However, the speed of the wave does depend on temperature; a higher temperature means waves will travel through that medium more quickly. 

But the temperature and density of the air changes with height! When a wave passes through a medium with changing density, it refracts, or bends, and that does depend on the frequency. Lower frequencies are bent less, and take a more direct path. In the end, as a wave passes through the Martian air, the lower frequency reaches a distant observer first, then the higher frequencies. So you hear the lower pitch first, then the higher ones: Whoop!*

That’s amazing. InSight heard an asteroid impact on Mars!

Even better: A rough position of the impact was found by analyzing the SEIS data, and a year or so later MRO orbited over that spot. Combing through the images, scientists found the site of the new craters.

A triple crater

Note the colors aren’t what you’d see if you were there; the filters used in this image make the surface dust look gray, and basaltic rock exposed by the impact, which is dark by eye, looks blue in this shot.

It’s not uncommon for a small asteroid to break apart as it moves through air hypersonically, even the thin air of Mars. The extreme pressure felt by the rock compresses it, and if it’s enough it can break the rock apart. In this case it broke into three pieces, each leaving a crater.

And while this was the first impact identified, it wasn’t the last: One was found in SEIS data from May 2020, another in February 2021, and another in August 2021. All three left impacts seen by HiRISE as well.

Three new craters on Mars

This is more than just an extremely cool story. Seismic waves can be used to probe the interiors of planets — this is how we know about the solid and liquid cores of the Earth, and the mantle, and in fact why InSight was sent to Mars in the first place: To learn about the Red Planet’s insides. It’s detected marsquakes from various spots on Mars, and even determined that the core of Mars is larger than expected.

Having seismic and acoustic data, as well as images of craters, also yields a lot of insight (so to speak) on how cratering works on Mars; the thin atmosphere, different surface composition and temperature, and more all make things different there than here on Earth, so planetary scientists are eager to see data like this to help them understand what’s going on there.

It’s also something future astronauts will have to deal with. Impacts, even small ones, are still decently rare, but if we wind up with long-term scientific bases on Mars it’ll be something they’ll want to study both for science and for habitability. That’s another reason, besides solar and galactic radiation, that future bases may be built under the surface

*Note: The website for InSight says that the lower frequencies travel through the air more quickly, but as far as I can tell that’s not technically what’s going on, or at least not the complete story. I poked around a bit and didn’t see any more info on this, so I applied my own knowledge of physics here; I admit I might be wrong but I think my explanation is correct.