3 Things We've Learned From NASA's Mars InSight
Scientists are finding new mysteries since the geophysics mission landed two years ago.
NASA's InSight spacecraft touched down Nov. 26, 2018, on
Mars to study the planet's deep interior. A little more than one Martian year
later, the stationary lander has detected more than 480 quakes and collected the
most comprehensive weather data of any surface mission sent to Mars. InSight's probe,
which has struggled to dig underground to take the planet's temperature, has made
progress, too.
There was a time when the surfaces of Mars and Earth were
very similar. Both were warm, wet, and shrouded in thick atmospheres. But 3 or
4 billion years ago, these two worlds took different paths. The mission of
InSight (short for Interior Exploration using Seismic Investigations, Geodesy
and Heat Transport) has been to help scientists to compare Earth to its rusty
sibling. Studying what the depths of Mars is made of, how that material is
layered, and how quickly heat seeps out of 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
findings about our red neighbor in the sky.
Faint Rumblings Are the Norm
InSight's seismometer, which was provided by the French
space agency, Centre National d'Études Spatiales (CNES), is sensitive enough to
detect slight rumblings from great distances. But it wasn't until April 2019 that
seismologists with the Marsquake Service, coordinated by ETH Zurich, detected
their first marsquake. Since then, Mars has more than made up for lost time
by shaking frequently, albeit gently, with no quakes larger than magnitude 3.7.
The lack of quakes larger than magnitude 4 poses something
of a mystery, considering how frequently the Red Planet shakes due to smaller
quakes.
"It's a little surprising we haven't seen a bigger
event," said seismologist Mark Panning of NASA's Jet Propulsion Laboratory
in Southern California, which leads the InSight mission. "That may be
telling us something about Mars, or it may be telling us something about luck."
Put another way: It could be that Mars is just more static
than anticipated - or that InSight landed in an especially quiet period.
Seismologists will have to keep waiting patiently for those
larger quakes in order to study layers deep below the crust. "Sometimes
you get big flashes of amazing information, but most of the time you're teasing
out what nature has to tell you," said InSight Principal Investigator
Bruce Banerdt of JPL. "It's more like trying to follow a trail of tricky
clues than having the answers presented to us in a nicely wrapped-up package."
The Wind May Hide Quakes
Once InSight started detecting
quakes, they became so regular that, at one point, they were happening every
day. Then, in late June of this year, the detections essentially stopped. Only
five quakes have been detected since then, all of them since September.
Scientists believe
Mars' wind is responsible for these seismically blank 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
domed wind and heat shield. But the wind still shakes the ground itself and
creates literal noise that covers up quakes. This could also have contributed
to what seems like the long seismic silence before InSight's first quake, since
the spacecraft landed while a regional dust storm was settling down.
"Before landing, we had to guess at how the wind would affect
surface vibrations," Banerdt said. "Since we're working with events
that are much smaller than what we'd pay attention to on Earth, we find that we
have to pay much closer attention to the wind."
Surface Waves Are
Missing
All quakes have two sets of body waves, which are waves that
travel through the planet's interior: 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 getting to Mars, InSight's
seismologists expected these waves to offer glimpses as deep as 250 miles
(about 400 kilometers) below the surface, into a sub-crustal layer called the
mantle. But Mars continues to offer mysteries: Despite hundreds of quakes, none
has included surface waves.
"It's not totally unheard of to have quakes without
surface waves, but it has been a surprise," Panning said. "For
instance, you can't see surface waves on the Moon. But that's because the Moon
has far 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 for
over an hour. The lack of surface waves on Mars may be linked to extensive
fracturing in the top 6 miles (10 kilometers) below InSight. It could also mean
that the quakes InSight detected are coming from deep within the planet, since
those wouldn't produce strong surface waves.
Of course, untangling 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 Mission Directorate. InSight is part of NASA's Discovery Program,
managed by the agency's Marshall Space Flight Center in Huntsville, Alabama.
Lockheed Martin Space in Denver built the InSight spacecraft, including its
cruise stage and lander, and supports spacecraft operations for the mission.
A number of European partners,
including France's Centre National d'Études Spatiales (CNES) and the German
Aerospace Center (DLR), are supporting 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 for 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 provided the Heat Flow and Physical Properties
Package (HP3) instrument, with significant
contributions from the Space Research Center (CBK) of the Polish Academy of
Sciences and Astronika in Poland. Spain's Centro de Astrobiología (CAB)
supplied the temperature and wind sensors.
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