NASA’s InSight lander has observed more than 500 Martian earthquakes since it landed on the red planet in 2018. But there is a glaring hole in his catalog: the lander has not yet detected any major rumble.
“We would have expected some strength 4 events, and maybe even a strength 5 at this point, given the number of smaller tremors,” Bruce Banerdt, InSight’s lead researcher, told Insider.
Instead, most of the shaking was so quiet that the average Californian would not even notice it. The four largest Mars tremors InSight’s seismometer felt to range in size from 3.1 to 3.6.
Mars seismologists are therefore beginning to worry. Either the InSight team just got unhappy, or Mars can’t produce big shakes at all. If it is the latter, Banerdt said: “we still do not really know what it means.”
The reason why scientists are so interested in Mars’ motion is that measuring earthquakes can show what the inside of the planet looks like. So far, Insight’s readings have indicated that Mars may have terrestrial layers deep beneath its crust, wrapped in a lunar outer shell orbited by asteroids.
The seismometer of the InSight lander, as captured by its camera on September 23, 2020.
NASA / JPL-Caltech
But large shakes will help scientists see deep into the Mars core, which could provide clues as to how the planet was born and how it evolved over time. A better understanding of the inside of Mars can be crucial in our efforts to find other worlds that can house life.
“By looking at Mars ‘core and looking at Mars’ crust and understanding that it hasn’t changed much in the last 4.5 billion years, we can get a glimpse into what the Earth could look like very early on,” said Banerdt. said. “Mars helps us understand how rocky planets form and how they evolve in general.”
Banerdt and his team hope to find out why they do not see large tremors on Mars – either so that they will know how to look for it better in a future mission, or so that they can determine which of the interior of Mars the large tremors so make. Rare.
Martian earthquakes indicate an earthy planet with a lunar crust
A 2019 concept on how seismic waves from a Martian earthquake can move through different layers of the Martian interior.
NASA / JPL-Caltech / ETH Zurich / Van Driel
Listening to earthquakes is like doing a CAT scan. When doctors do such a scan, a machine sends x-rays through your body and then analyzes how the waves return at different times and in different directions. This enables them to ‘put together the 3D geometry of what’s going on in your body’, Banerdt said.
With InSight, he continued, “we do the same with a planet and use Mars earthquakes as our ‘radiation waves’ and the seismometer is the detector.”
Scientists previously thought that Mars had to have a crust like Earth, which must have been ironed out from below by geological activity such as the movement of tectonic plates and the bubble of molten magma. But the InSight seismometer painted a more nuanced picture.
“It’s somewhere between the moon and the earth as it transmits seismic waves,” Banerdt said.
On the moon, the crust has broken up due to asteroid impact, giving seismic waves more cracks and surfaces to jump off. It’s like they’re doing a ‘drunk walk’, Banerdt said, and it makes lunar tremors last for hours.
Apollo astronauts have installed seismometers on the moon.
NASA / Eugene Cernan
On Earth, seismic waves do not resonate as much, so they weaken rapidly. Moisture in the planetary crust also allows it to absorb some of their energy. As a result, earthquakes usually last only a few seconds, although large large minutes can last.
Meanwhile, Martian tremors usually appear to last about 10 to 40 minutes.
The first few hundred tremors that InSight picked up carried the same as those on the moon. But because they were so small, they only enabled scientists to analyze the composition of the top layer of the crust. The handful of larger quakes – which gave the InSight team a look at deeper layers – acted more like earthquakes.
“I think maybe Mars has an outer layer that is quite lunar,” Banerdt said. “It’s pretty much broken up by impacts. But deeper in the planet, in the mantle, it looks like it could be more earthy.”
The mystery of the missing Big One
A dramatic, fresh impact crater dominates this image taken by NASA’s Mars Reconnaissance Orbiter on November 19, 2013.
NASA / JPL-Caltech / Univ. of Arizona
InSight’s Mars earthquake follows a similar pattern to earthquakes: the higher the magnitude, the rarer the earthquake.
“You get fewer and fewer tremors as you get bigger and bigger numbers, and that follows a kind of exponential law,” Banerdt said.
So far, the tail end of the exponential graph is missing. It can only be a quiet period on Mars – planets can have a lot of seismic activity and dry periods without major tremors. But Banerdt suspects that InSight’s data point to a bigger trend.
“There seems to be less large quakes on Mars, relative to the number of small quakes, than we would expect. It’s a bit puzzling,” he said. “We are still trying to figure out what the explanations may be.”
It is possible that NASA has just chosen a good place to hunt for large quakes. On earth, there are many areas that never see severe earthquakes. Or maybe Mars just never shakes that much.
“It could also be related to gravity, it could be related to the thickness of the brittle layer, it could be related to a lot of things. But at the moment we really do not have it right,” Banerdt said. . “This is an ongoing research area.”
InSight is about to fall asleep in ‘optimal’ earthquake time
On February 14, 2021, the InSight Lander’s camera captured an image of one of its solar panels covered with dust.
NASA / JPL-Caltech
The longer InSight waits and listens, the greater the chance of catching a major earthquake. Unfortunately, the lander is about to take a week-long break during the peak period of the earthquake.
This is because the Elysium Planitia, where InSight landed, surprised NASA with its lack of wind. There’s a little wind – enough to dispel the seismic noise of some distant tremors. But that’s not enough to keep red Mars dust from InSight’s solar panels.
The Martian winter is now approaching and a thick layer of dust is taxing the energy production of the robot. NASA has therefore decided to put InSight into hibernation. In February, the lander gradually began to turn off his scientific instruments and retain the power to keep himself warm.
In June, NASA expects to halt InSight’s scientific activities completely until Mars swings back to the sun in July.
The seismometer is still running, but Banerdt expects to close it in a month or so. This will be in the midst of the ‘optimal’ time to detect Martian earthquakes, as winds will lie in the depths of winter. The reduction in wind speed allows the seismometer to pick up distances with less interference.
“We hope to keep the seismometer running for as long as we can, and then get it running again – you know, after we pass this low power time, turn it on as quickly as possible,” Banerdt said. said. “But we’ll probably miss a few things in between.”
If InSight survives its power shortage, the seismometer could continue to listen to earthquakes in 2022.