Critically, these S-waves are slower than P-waves, so when a quake pops off, they arrive at InSight’s seismometer a bit later. “This difference between the arrival of the P and S waves can give you an idea about what’s the location of the quake; how far it was away from your station,” says Knapmeyer-Endrun. The waves also differ in what mediums they can travel through, versus which ones they bounce off of. P-waves move through solids, liquids, and gases, while S-waves only travel through solids.
By analyzing the waves that reach InSight’s seismometer, scientists can get an idea of the composition of Mars’ insides. Since S-waves can’t travel through the liquid core, all of their energy bounces off the boundary between core and mantle. Think of it like binary code for computers: Just as two elements—ones and zeros—can combine to produce extremely complex programming, so too can two kinds of waves combine to produce a sophisticated picture of the Red Planet’s guts. “We also look at differences in arrival times, and then we can say, ‘OK, this tells us something about the thickness of the layer,’” says Knapmeyer-Endrun.
Using this technique, she and her colleagues were able to estimate the thickness of the crust. Previously, scientists had used satellites flying overhead to measure the differences in gravity and topography across the planet, and they had taken a stab at the crust’s thickness that way, landing on an estimate of a global average of 110 kilometers. “Now, with our measurements from inside, we can say that that’s definitely too much,” says Knapmeyer-Endrun. They now think the maximum figure for average thickness is 72 kilometers.
The researchers reckon that the crust is made of two or three layers. There’s a topmost layer that’s 10 kilometers thick, which InSight’s measurements revealed to be unexpectedly light, perhaps because it’s made of fractured rock left over from meteorite impacts. The layer below that goes down to about 20 kilometers. “Unfortunately, we are not sure what follows next, if it’s already the mantle or if we have a third layer in the crust. There’s some ambiguities that we haven’t resolved,” says Knapmeyer-Endrun. “We can definitely say that the crust is not as thick as has been predicted previously, and it has a lower density.”
Planetary seismologist Simon Stähler of ETH Zürich led the effort to characterize the hottest and innermost chunk of Mars’ interior—its core. Though they lack the ability to actually see inside the planetary center, Stähler’s team was able to extract some information just by analyzing the S-waves that bounce off the core-mantle boundary. These rumblings, unable to penetrate the liquid core, find their way back up to the Martian surface, where they are picked up by InSight’s receivers. “It takes a good 10 minutes,” Stähler says, from the time of the quake to the detection of the signal reflected by the core. By measuring this interval, his team was able to deduce how deep into the planet the waves are traveling, thus measuring the depth of the core itself: around 1,550 kilometers from the surface.