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SYFY WIRE Bad Astronomy

Mars sounds weird

By Phil Plait

A few days ago I posted a spectacular image of Mars, showcasing its jaw-dropping beauty.

Today, let me show you something quite different: Just how weird Mars is. You can hear it.

NASA's Mars InSight landed on the Red Planet on 26 November, 2018, designed to investigate the interior of Mars as well as conditions on its surface (including temperature, wind speed, and the like). It has a seismograph, called SEIS (for Seismic Experiment for Interior Structure), that it placed on the surface next to the lander on 19 December, which was built to detect marsquakes.

Mars doesn't have active tectonics the way the Earth does (at least not now, but a billion or three years ago things may have been different). Still, there's still some seismic activity. Small asteroid impacts create seismic waves, for example, and other sources include landslides and strong atmospheric phenomena like dust devils. Perhaps there might even be activity due to crustal shifts, though it's not clear if there's any activity (like volcanoes) that could produce such movement today.

But that's why SEIS is there. By measuring waves that travel through the ground a lot of insight can be gathered on what happens beneath it. It's extremely sensitive; out of about 100 events detected so far, scientists consider 21 of them strong enough to be considered due to marsquakes. For the remaining 80 or so the jury is still out, but analysis is ongoing.

NASA just released recordings of a few of the events SEIS has detected. Because it's detecting waves passing through the surface, these can be converted to sound waves so that you can hear them. It's a little tricky doing this because the wavelengths made are very long, well below what the ear can detect, so they have to be "sped up" to make them audible, and adjusted a bit so they make sense. This process is called sonification.

But a sound file is worth a hundred words (extrapolating downward from a picture's worth, at least), so here, have a listen. First up: a small (magnitude 3.7) quake from 22 May, 2019 (or Sol 173, the 173rd Martian day after InSight landed).

It's mostly just a low rumbling, with a few pops or clicks (hang tight, we'll get to those). Here's another quake that was on 25 July, 2019 (Sol 235). It's much shorter and slightly less powerful (magnitude 3.3), but gets pretty bassy toward the end.

Cooooool. This is more than just an interesting and fun way to listen to an earthquake; when you use a different sense, like hearing versus seeing, it's possible that nuances you might miss one way can be detected using the other. Just examining a plot of the wave strength over time is one thing, but hearing it is another. It's similar to adding color to images from telescopes by assembling multiple images; different features are suddenly obvious that may have been hidden before (like this particularly eerie sonification of radio waves emitted by Saturn's moon Enceladus).

So what about those other noises SEIS "heard"? Those are a bit more local. They're caused by the movement of InSight's robot arm, friction inside SEIS (probably thermal expansion and contraction, similar to the pops and groans you can hear in a house at night as the structure cools down), and even wind gusts. JPL put together a short video with those, from 6 March, 2019 (Sol 98):

The first 20 seconds or so are intro, and for some reason they used weird musical tones for that; I was confused for a moment thinking that was what SEIS was hearing! But the actual sounds start at 0:25. When that starts, you can see the indicator moving across the graph showing the waveforms of the sounds, and when something weird happens it's explained in a popup to the right of the graph. Remember, this audio is sped up (the text for the video says it's a factor of 10, but I think it's actually 20, gauging from the graph itself).

The pops they detect are nicknamed dinks and donks, and it becomes clear why in this collection taken on 16 July, 2019 (Sol 226), just after sundown as SEIS cooled:

It's funny; the air on Mars is so thin — the average surface pressure is only 0.6% of Earth's at sea level — that even if they were in our audio range you'd probably still not hear any of this. The sound wouldn't carry at all. I wonder about the quakes, though. I lived in California for several years and felt a few small earthquakes. A mag 3 might be strong enough to actually feel; I certainly felt a 4.5 (very, very certainly), and while I didn't feel a 2.2 we had, it made the closet doors in my bedroom rattle enough to wake me up one night.

My hope is that a decent-sized meteorite (like a meter or so across) smacks into the surface of Mars while InSight is still operating, and the HiRISE camera on the Mars Reconnaissance Orbiter can see the crater. That would be very cool, and a scientific bonanza. We've seen new craters since HiRISE started imaging the surface (in before-and-after shots), so knowing how big the crater is and how far from InSight it was compared to the signal strength should provide scientists with lots of fun data to examine.

I love that we now have enough instruments above and on Mars that we can start to look at more unusual phenomena in this way. Mars is pretty similar to Earth in many ways, but it's still an alien world. We're just now starting to get a good look at the weirdnesses over there.