The quakes suggest the planet has an Earth-like interior wrapped in a dry, broken, moon-like crust.
But big Mars quakes are mysteriously missing, and scientists aren't sure why.
NASA's InSight lander has observed more than 500 Mars quakes since it landed on the red planet in 2018. But there's a glaring hole in its catalogue: The lander has yet to detect any big rumblings.
"We would have expected a few magnitude 4 events, and maybe even a magnitude 5 at this point, given the number of smaller quakes," Bruce Banerdt, the principal investigator for InSight, told Insider.
Instead, most of the quakes have been so quiet that the average Californian wouldn't even notice them. The four biggest Mars quakes InSight's seismometer has felt ranged in magnitude from 3.1 to 3.6.
So Mars seismologists are beginning to scratch their heads. Either the InSight team has just gotten unlucky, or Mars can't produce big quakes at all. If it's the latter, Banerdt said, "we don't really know what that means yet."
The reason scientists are so interested in Mars' movement is that measuring quakes can reveal what the interior of the planet looks like. So far, Insight's readings have indicated that Mars may have Earth-like layers deep below its crust, which are wrapped in a moon-like outer shell that's been battered by asteroids.
But really big quakes would help scientists see deep into the Martian core, which could yield clues about how the planet was born and how it has evolved over time. A better understanding of Mars' insides could be crucial in our efforts to find other worlds that might host life.
"By looking at Mars' core and looking at Mars' crust, and understanding that these haven't changed very much in the last 4.5 billion years, we can get a glimpse into what the Earth might have looked like very early on," Banerdt said. "Mars is helping us to understand just how rocky planets form and how they evolve in general."
Banerdt and his team hope to figure out why they're not seeing big quakes on Mars - either so they'll know how to better look for them in a future mission, or so they can pinpoint what about the Martian interior makes major quakes so scarce.
Mars quakes hint at an Earth-like planet with a moon-like crust
Listening for a planet's quakes is like doing a CAT scan. When doctors do that kind of scan, a machine sends X-rays through your body, then analyzes how the waves come back at different times and in different directions. That enables them to "piece together the 3D geometry of what's going on inside your body," Banerdt said.
With InSight, he continued, "we're doing the same thing with a planet, using Mars quakes as our 'radiation waves,' and the seismometer is the detector."
Scientists used to think that Mars must have a crust like Earth's, which has been smoothed out by geologic activity like the movement of tectonic plates and the bubbling of molten magma from below. But InSight's seismometer has painted a more nuanced picture.
"It's somewhere between the moon and the Earth in the way it transmits seismic waves," Banerdt said.
On the moon, the crust has been broken up because of asteroid impacts, which gives seismic waves more cracks and surfaces to bounce off. It's as if they're doing a "drunken walk," Banerdt said, and that leads moon quakes to last for hours.
On Earth, seismic waves don't reverberate that much, so they weaken quickly. Moisture in our planet's crust also allows it to absorb some of their energy. As a result, earthquakes usually last just a few seconds, though really big ones can last minutes.
Mars quakes, meanwhile, generally seem to last about 10 to 40 minutes.
The first few hundred tremblings InSight picked up on Mars behaved similarly to those on the moon. But because they were so small, they only enabled scientists to analyze the makeup of the upper layer of the crust. The handful of larger quakes - which gave the InSight team a peek at deeper layers - have acted more like earthquakes.
"I think maybe Mars has an outer layer which is rather lunar-like," Banerdt said. "It's quite broken up by impacts. But deeper into the planet, into the mantle, it appears like it might be more Earth-like."
The mystery of the missing Big One
InSight's Mars quakes follow a similar pattern to earthquakes: The higher the magnitude, the rarer the quake.
"You get fewer and fewer quakes as as you get larger and larger numbers, and it follows a sort of an exponential law," Banerdt said.
So far, the tail end of that exponential graph is missing. It could just be a quiet period on Mars - planets can have spells with lots of seismic activity and dry periods with no big quakes. But Banerdt suspects that InSight's data points to a larger trend.
"It looks like there are fewer large quakes on Mars, relative to the number of small quakes, than we would expect. It's a little bit puzzling," he said. "We're still trying to figure out what explanations for that could be."
It's possible that NASA just didn't pick a good spot to hunt for big quakes. On Earth, there are plenty of areas that never see major earthquakes. Or maybe Mars just never shakes that much.
"It could be also related to the gravity, it could be related to the thickness of the brittle layer, it could be related to a lot of things. But right now, we really don't have a handle on that," Banerdt said. "It's an ongoing area of research."
InSight is about to hibernate through 'optimum' quake time
The longer InSight waits and listens, the more likely it is to catch a big quake. Unfortunately, the lander is about to take a weeks-long break during peak quake-listening time.
That's because the Elysium Planitia, where InSight landed, has surprised NASA with its lack of wind. There is some wind - enough to drown out the seismic noise of some faraway quakes. But it's not enough to keep red Martian dust off of InSight's solar panels.
Now, the Martian winter is setting in and a thick layer of dust is taxing the robot's energy production. So NASA has decided to put InSight into hibernation. In February, the lander began incrementally shutting off its scientific instruments, conserving power to keep itself warm.
In June, NASA expects to shut down InSight's scientific operations entirely until Mars swings back toward the sun in July.
The seismometer is still running, but Banerdt expects to shut it down in a month or so. That will be in the middle of the "optimum" time for detecting Mars quakes, he said, since winds die down in the depths of winter. Reduced windiness allows the seismometer to pick up distant quakes with less interference.
"We're hoping to keep the seismometer going as long as we can, then start it up again - you know, after we pass this low-power time - turn it on as quickly as we can," Banerdt said. "But we will probably be missing some things in between."
If InSight survives its power shortage, the seismometer could keep listening for quakes into 2022.
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