Sky & Telescope - June 2020 - 8

SEIS
(seismometer)

HP3 (heat probe)

SOLAR SYSTEM

NASA's Insight Detects Marsquakes
MARS REGULARLY QUIVERS with

barely discernible quakes due to fracturing in its crust as the interior cools.
That's the takeaway from the first
10 months of observations by NASA's
Insight lander.
Insight is the equivalent of a planetary X-ray, with instruments designed
to reveal the Martian interior. The
team reports results from some of these
instruments in five papers published in
the March 2020 Nature Geoscience.
Insight's seismometer, called the
Seismic Experiment for Interior Structure (SEIS), had detected 174 seismic
events as of the paper's writing. Most
of these are tiny and shallow, but 24
propagate much deeper and have magnitudes between 3 and 4. That's still
weak - you probably wouldn't feel them
if you were standing on the surface.
These 24 marsquakes seem to come
from deeper in the planet than earthquakes do, maybe 30 to 50 kilometers
(20 to 30 miles) down instead of 5 to
10 km, says SEIS principal investigator
Philippe Lognonné (University of Paris).
The quakes also lasted a long time,
from 10 to 20 minutes each. For comu A test using an engineering model shows
how the Insight lander will use its robotic arm to
press on "the mole" to help it dig.

8

JUNE 2 02 0 * SK Y & TELESCOPE

p Artist's illustration of NASA's Insight lander,
with its seismometer (under dome at left) and
heat probe (center) deployed.

parison, one of the longest earthquakes
ever recorded on Earth lasted between 8
and 10 minutes, but most earthquakes
are smaller and fade much faster, attenuating within tens of seconds as they
travel through Earth's partially molten
interior. Marsquakes' longer duration
suggests seismic waves pass more easily through the Martian interior than
Earth's. Lognonné says this indicates
that Mars's upper mantle is less melted
than Earth's is. But seismic waves fade
more quickly on Mars than on the
ultra-dry Moon, so the Martian crust

might have a trace of moisture in it, he
adds. For example, there may be thin
films of water in the rocks' pores.
Surprisingly, Insight hasn't seen any
tremors stronger than magnitude 4.
Because larger quakes carry information about what lies deeper down in the
mantle, the team might have to rely
more heavily on Insight's other instrument packages to probe the nature of
the planet's core.
Signals from three of the 24 deep
marsquakes were clear enough to trace
them back to their sources. Two appear
to have come from Cerberus Fossae, a
set of steep troughs in the plains southeast of the dormant volcano Elysium
Mons. The third comes from a bit farther east. Cerberus Fossae is a region of
recent (within the last 10 million years)
geological and volcanic activity. It's
possible there's a magma chamber deep
underground, which would contract as
it continues to cool, causing the crust
above it to crack. There's no evidence
yet for magma plumes that could be to
blame, but concentrations of radioactive elements might keep pockets of the
subsurface warm.
While SEIS is yielding a slew of
results, another Insight instrument is in
dire straits. The Heat Flow and Physical
Properties Package (HP3), fondly known
as "the mole," started digging in February 2019, intending to draw a string
of temperature sensors underground
to measure heat flow from the planet's
interior. But the probe only made it
some 35 cm (14 inches) down before
it got stuck. A slew of tests determined
that the soil there isn't as sandy as at
other landing sites. Instead of falling
back into the hole, providing traction
for the probe as it dug, the soil clotted,
leaving the mole with nothing to push
against as it tried to hammer deeper.
The team is now prepping to do what
it had hoped never to do, Bruce Banerdt
(JPL) says: Use the lander's robotic arm
to push the mole down. The researchers
think that, if they can press the probe
far enough to bring its top end level
with the ground, friction might win out
and enable the mole to keep digging.
■ CAMILLE M. CARLISLE

N ASA / JPL- CA LTECH (2)

NEWS NOTES



Sky & Telescope - June 2020

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