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My first (and so far, only) total solar eclipse was in August 2017. I was in Wyoming, and the minutes leading up to totality were odd. The lighting around us changed, the temperature dropped, and we could all feel conditions feeling weirder and weirder right up until the Moon completely blocked the Sun.
One thing, we didn't feel, though, were any seismic changes.
That, however, is not the case for a seismometer on NASA's InSight Mars lander. During several eclipse-like events on the Red Planet, it has detected extremely slight but real changes in the surface of Mars! That's really strange, but it turns out to be literally cool.
Mars has two moons, tiny space potatoes called Deimos and Phobos. They're irregular in shape, with Deimos being roughly 12 km across and Phobos twice that. They orbit pretty close in, about 24,000 and 6,000 km above the surface, respectively, in orbits that are fairly well aligned with the equator of Mars.
That means that if you are standing on Mars in an equatorial region, you can sometimes see these moons pass directly in front of the Sun. A Martian eclipse! Although the way the sizes all work out, the moons appear smaller than the Sun so they can't completely block it out. Technically we call events like these transits.
They only happen about twice every Martian year (or once per Earth year, given the longer year of Mars) due to the way the orbits have to align, but when they do there are usually a handful of transits seen over the course of a few days. Deimos is smaller and farther away, so the transits last longer (Deimos orbits more slowly), about 2 minutes, and don't dim the Sun much. Phobos is bigger and closer, so the transits dim the Sun more but only last for about 30 seconds.
The first transits seen from the surface were by the rovers Spirit and Opportunity, but many have been seen since then by Curiosity as well.
InSight is not a rover, but a stationary lander that touched down in November 2018. It has a series of scientific instruments on board mostly focused on learning more about the interior of Mars. This includes SEIS, the Seismic Experiment for Interior Structure, which is inside a small dome to protect it from the wind. It sits directly on the Martian surface.
During Phobos transits, the solar panels see a drop in power generation, which isn't surprising (I see the same thing in my own house's panels when clouds pass in front of the Sun). What is very interesting, though, is that the magnetometer also saw a change in the magnetic field measurements, and, even more bizarrely, the seismometer detected a shift in the tilt of the planet's surface!
The magnetometer change is likely related to the solar panel current drop. A change in current flow creates a magnetic field — this is how we use turbines, for example, to make electricity — so seeing that makes some sense.
But what of the seismometer? Mind you, it didn't detect a Marsquake (though it has felt quite a few of those since it was switched on), but essentially a tilt in the land around it. That tilt was extremely small, just one part in a hundred million! But it happened three times out of eight Phobos transits seen, and it only happened in transits when the Sun was almost overhead, near local noon. When the Sun was lower, SEIS saw no changes.
The scientists' first thought (and mine as well) was that it may have been due to the gravity of Phobos tugging on the seismometer. When it's overhead it's closer, so the gravity stronger, which would explain why it was seen when at noon but not at other transit times. Alas, though, the gravity of the tiny moon isn't enough to cause this change.
They looked at other causes, and while they can't be 100% sure, they conclude that the tilt is caused by the transit cooling the surface of Mars! The moon blocks a decent percent of sunlight (up to 40%), which is certainly enough to cause a slight change in temperature. The ground reacts quickly, cooling a little bit and contracting. This happens in the shadow but not outside of it, so the ground tilts a teeny tiny bit. The effect is incredibly small, but SEIS is sensitive enough to detect it. And when the Sun is lower, the effect is smaller, too, so it's harder to detect. It all fits.
That's amazing. These transits on Mars are actually useful, too. InSight's location on Mars is the best established of anything on the planet (due to images of it from orbit as well as radio measurements from another of its instruments), so when it sees a transit a very accurate position for the moon can be determined, which allows astronomers to get a better orbit determination for the moons as well. The orbits change slightly due to the effects of the tides of Mars on the moons, too, so these measurements allow a way to measure that, too.
And now they can be used to look at the way the Martian surface reacts to changes in temperature. I know it's on the nose to say this, but that is so cool.
Not too long ago our knowledge of the conditions on Mars was so hazy we weren't even sure if there could be extensive life there or not, but now we measure some things so accurately we can detect a shift in a few parts per billion. That's phenomenal.