In principle, at least, the use of seismic – earthquake – waves to understand the interior of a planet is less complicated than it first seems. You’re probably familiar with bats’ use of echolocation to navigate the night skies: they emit high-pitched squeaks, and then use the reflections, the echoes, to navigate and find prey. Seismic reflection is much the same, except the equivalent of the bat “squeak” is a remote earthquake, and the reflections come for the differentiated layers of the planet’s structure.
Geologists and geophysicists have been using seismology to analyze the Earth’s structure for years. The same technique has been applied to Mars.
In February 2019, Mars Insight went on-line, a sophisticated seismometer placed on Elysium Planitia near the Martian equator. Since, Mars Insight has tracked Marsquakes and the pressure and shear waves those quakes generate as they reflect and refract inside the the planet.
The whole project was a bit of scientific draw for an inside straight: were there enough Marsquakes and were they strong enough to be useful? Earthquakes, after all, are a product of plate tectonics and Mars has no plate tectonics activity. Was Mars’ crust, mantle and core sufficiently differentiated to create the kind of seismic reflections that are useful? Did Mars even have a molten core?
Mars Insight has returned more than 1,200 Martian days of data now. Mars has quakes – earlier this month Mars Insight detected a ML 5.0 temblor, a record. The quakes are more than sufficient to permit seismography. And Mars has differentiation – separation into layers of crust, mantle and core – like and yet unlike Earth’s. Mars’ core, for example, is disproportionately large, which implies a lower density than Earth’s.
Mars’ crust turns out to be far thinner than expected and may have two or even three sub-layers. It goes as deep as 12 miles (20 kilometers) if there are two sub-layers, or 23 miles (37 kilometers) if there are three. Parts of the Earth’s crust are more than 200 miles thick. Beneath that Martian crust is the mantle, which extends 969 miles (1,560 kilometers) below the surface. The Earth’s mantle clocks in at 1,800 miles thick. And a the heart of Mars is the semi-liquid core, which has a radius of 1,137 miles.
But like all good scientific inquiries, Mars Insight answered questions but also asks a whole mess of new ones. For example, all of InSight’s most significant quakes appear to have come from one area, Cerberus Fossae, a region volcanically active enough that lava may have flowed there within the last few million years. In terms of geologic time, that’s pretty much yesterday.
Curiously, Insight has detected no quakes have been from the much more prominent volcanic regions, like Tharsis, home to three of the biggest volcanoes on Mars. Perhaps the largest in the solar system. You’d expect that a 21 mile high volcano like Olympus Mons would show some tectonic action. Not that Mars Insight has detected. It may be that Insight lies in a seismic shadow of the martian planetary core, and cannot detect Marsquakes there. The solution to the puzzle would be to plant another seismometer nearer to Olympus Mons. Teasing out the structural detail of that monster pile of rock would be fascinating. But NASA doesn’t seem to have that budgeted. Too bad.
And what generates Marsquakes? What is the physical mechanism? Mars insight wasn’t designed to answer that question, but rather to take advantage of the existence of Marsquakes. And Insight may be nearing the end of its functional life; Mars dust is accumulating on its solar panels, greatly diminishing their efficiency. It’s a long ways to someone or something that might clean them. But the ongoing analysis of the data Mars Insight has transmitted to Earth is just beginning.
Perseverance may get all the press, but Insight has done a fine job. We know a remarkable amount already about the Red Planet, even without setting foot on it.