Scientists are finding new mysteries since the geophysical mission landed two years ago.
NASA’s InSight spacecraft landed on Mars on November 26, 2018 to study the deep interior of the planet. Just over a Martian year later, the stationary lander has detected more than 480 earthquakes and collected the most comprehensive weather data from any surface mission sent to Mars. InSight’s probe, which has struggled to dig underground to measure the planet’s temperature, has also made progress.
There was a time when the surfaces of Mars and Earth were very similar. Both were warm, wet and enveloped in a thick atmosphere. 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 sibling. By studying what the depths of Mars are made of, how that material is layered, and how quickly heat seeps out, scientists can better understand how a planet’s source materials make it more or less likely to support life.
While more science is coming out of InSight, here are three findings about our red neighbor in the sky.
Vague rumble is the norm
The InSight seismometer, provided by the French space agency Center National d’Études Spatiales (CNES), is sensitive enough to detect light rumbling at great distances. But it wasn’t until April 2019 that seismologists from the Marsquake Service, coordinated by ETH Zurich, discovered their first marching quake. Since then, Mars has more than made up for lost time by shaking regularly, albeit gently, without earthquakes greater than magnitude 3.7.
The lack of earthquakes greater than magnitude 4 is somewhat of a mystery, given how often the Red Planet shakes due to smaller earthquakes.
“It’s a bit surprising that we haven’t seen a larger event,” said seismologist Mark Panning of NASA’s Jet Propulsion Laboratory in Southern California, which leads the InSight mission. “That might tell us something about Mars, or maybe it might tell us something about happiness.”
In other words, it could be that Mars is just more static than expected – or that InSight has landed in a particularly quiet period.
Seismologists will have to wait patiently for those larger earthquakes to study layers deep below the crust. “Sometimes you get big flashes of amazing information, but most of the time you tease what nature has to tell you,” said JPL’s InSight lead researcher Bruce Banerdt. “It’s more like trying to follow a trail of tricky clues than having the answers presented to us in a beautifully packaged package.”
The wind can hide earthquakes
Once InSight started to detect earthquakes, they became so regular that they happened every day at some point. Then, at the end of June this year, the detections essentially ceased. Only five earthquakes have been recorded since then, all since September.
Scientists believe that the winds from Mars are responsible for these seismically empty periods: the planet entered the windiest season of the Mars year around June. The mission knew that wind could affect InSight’s sensitive seismometer, which is equipped with a dome-shaped wind and heat shield. But the wind still shakes the ground itself, literally making noise that obscures earthquakes. This could also have contributed to what appears to be the long seismic lull before InSight’s first earthquake, since the spacecraft landed while a regional dust storm came down.
“Before landing, we had to guess how the wind would affect the vibrations on the surface,” Banerdt said. “Because we work with events that are much smaller than what we would pay attention to on Earth, we find that we have to pay a lot more attention to the wind.”
Surface waves are missing
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 ripple 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 planet’s internal structure. Before InSight’s seismologists arrived, InSight’s seismologists expected these waves to glimpse as much as 250 miles (about 400 kilometers) below the surface, in a lower crust layer called the mantle. But Mars still offers mysteries: Despite hundreds of earthquakes, there are no surface waves.
“It’s not completely unheard of to have earthquakes without surface waves, but it was a surprise,” said Panning. “For example, you can’t see surface waves on the moon. But that’s because the moon has much more scattering than Mars.”
The dry lunar crust is more fractured than Earth and Mars, causing seismic waves to bounce around in a more diffuse pattern that can last more than an hour. The lack of surface waves on Mars may be related to extensive rupture in the top 6 miles (10 kilometers) below InSight. It could also mean that the earthquakes detected InSight are coming from deep within the planet, as they would not produce strong surface waves.
Of course, unraveling such mysteries is what science is all about, and there’s more to come with InSight.
More about the mission
JPL manages InSight for NASA’s Science Missions Directorate. InSight is part of NASA’s Discovery Program, operated by the agency’s Marshall Space Flight Center in Huntsville, Alabama. Lockheed Martin Space in Denver has built the InSight spacecraft, including its cruise stage and lander, and is supporting spacecraft operations for the mission.
A number of European partners, including the Center National d’Études Spatiales (CNES) in France and the German Aerospace Center (DLR), support the InSight mission. CNES provided the Seismic Experiment for Interior Structure (SEIS) instrument to NASA, with the principal investigator at IPGP (Institut de Physique du Globe de Paris). Significant contributions to SEIS came from IPGP; the Max Planck Institute for Solar System Research (MPS) in Germany; the Swiss Federal Institute of Technology (ETH Zurich) in Switzerland; Imperial College London and Oxford University in the United Kingdom; and JPL. DLR has the Heat Flow and Physical Properties (HP3) instrument, with significant contributions from the Space Research Center (CBK) of the Polish Academy of Sciences and Astronomy in Poland. The Spanish Centro de Astrobiología (CAB) supplied the temperature and wind sensors.
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Jet Propulsion Laboratory, Pasadena, California.