Three things NASA learned from Mars InSight

NASA’s InSight spacecraft landed on Mars on November 26, 2018 to study the planet’s deep interior. A little over a Martian year later, the hover lander detected more than 480 earthquakes and collected the most comprehensive weather data of any surface mission sent to Mars. The InSight probe, which struggled to dig underground to take the planet’s temperature, also made progress.

There was a time when the surfaces of Mars and Earth were very similar. Both were hot, humid, and shrouded in thick atmospheres. But 3 or 4 billion years ago, these two worlds took different paths. InSight’s mission (short for Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) was to help scientists compare Earth to its rusty brother. Studying the makeup of the depths of Mars, how this material is layered, and how quickly heat escapes from it could help scientists better understand how a planet’s starting materials make it more or less likely to support life.

While there’s more science to come from InSight, here are three discoveries about our red neighbor in the sky.

Low rumbles are the norm

The InSight seismometer, provided by the French space agency, the Center National d’Études Spatiales (CNES), is sensitive enough to detect slight rumblings at great distances. But it was not until April 2019 that seismologists from the Marsquake service, coordinated by ETH Zurich, detected their first marsquake. Since then, Mars has more than made up for the time lost by shaking frequently, albeit gently, without earthquakes above magnitude 3.7.

The absence of earthquakes with magnitude greater than 4 poses a mystery, considering how often the Red Planet shakes due to smaller earthquakes.

“It’s a little surprising that we didn’t see a bigger event,” said seismologist Mark Panning of NASA’s Jet Propulsion Laboratory in Southern California, who is leading the InSight mission. “It can tell us something about Mars, or it can tell us something about luck.”

In other words: Mars may simply be more static than expected – or InSight may have landed in a particularly quiet time.

Seismologists will have to continue to wait patiently for these larger earthquakes in order to study the deep layers beneath the crust. “Sometimes you get big, amazing flashes of information, but most of the time you tease what nature has to say to you,” said Bruce Banerdt, InSight Principal Investigator, JPL. “It’s more like trying to follow a tricky clue trail than having the answers presented to us in a well-wrapped package.”

The wind can hide earthquakes

Once InSight started detecting earthquakes, they became so regular that at some point, they happened every day. Then, at the end of June of this year, the detections essentially stopped. Only five earthquakes have been detected since then, all since September.

Scientists believe that the Mars wind is responsible for these seismically pristine periods: the planet entered the windiest season of the Martian year around June. The mission knew that winds could affect InSight’s sensitive seismometer, which is equipped with a windshield and dome-shaped heat shield. But the wind still shakes the ground itself and creates a literal noise that covers earthquakes. It could have also contributed to what appears to be the long seismic silence before InSight’s first earthquake, since the spacecraft landed as a regional dust storm set in.

“Before landing we had to guess how the wind would affect the surface vibrations,” Banerdt said. “Since we are working with events that are much smaller than what we would pay attention to on Earth, we find that we have to be much more aware of the wind.”

Surface waves are lacking

All earthquakes have two sets of body waves, which are waves that travel through the interior of the planet: primary waves (P waves) and secondary waves (S waves). They also wave along the top of the crust as part of a third category, called surface waves.

On Earth, seismologists use surface waves to learn more about the internal structure of the planet. Before arriving on Mars, InSight seismologists expected these waves to provide glimpses as deep as 250 miles (about 400 kilometers) below the surface, in a subcrustal layer called the mantle. But Mars continues to offer mysteries: Despite hundreds of earthquakes, none have included surface waves.

“It’s not totally unheard of to have earthquakes without surface waves, but it came as a surprise,” Panning said. “For example, you can’t see surface waves on the Moon. But that’s because the Moon diffuses a lot more than Mars.”

The dry lunar crust is more fractured than Earth and Mars, causing seismic waves to bounce in a more diffuse pattern that can last for over an hour. The lack of surface waves on Mars may be linked to extensive fracking in the first 10 kilometers under InSight. It could also mean that the earthquakes detected by InSight are coming from deep within the planet, as they would not produce strong surface waves.

Of course, unraveling such mysteries is the goal of science, and there is more to come with InSight.