Sky & Telescope - August 2023 - 16

Mars Missions
MARSQUAKE This magnitude-3.3 quake
shook Insight's seismometer on sol 235.
The first set of wiggles are the P waves,
the second the S waves.
Ready to Rumble
As the mole struggled, the seismometer - the real bread and
butter of the mission - listened to the murmurs from the
Martian underground.
Insight's Seismic Experiment for Interior Structure (SEIS)
is the all-terrain vehicle of seismometers. " We were able to
provide a seismometer as good as the best portable seismometer
on Earth, except that it's able to survive the space environment,
very cold temperatures, radiation, et cetera, " says
Philippe Lognonné (Institute of Earth Physics of Paris), SEIS's
principal investigator.
SEIS, like other seismometers, uses a system of pendulums
to measure movement caused by seismic waves, which occur
when something makes the ground vibrate. It's
extremely sensitive, able to detect movements
smaller than the size of an atom - so sensitive,
in fact, that it needed special shielding from
ambient noise and temperature changes in order
to operate.
SEIS found that, seismically speaking, Mars
ful and less frequent than earthquakes. On Earth, moving tectonic
plates constantly produce quakes. But Mars has a single
plate, a thick crust that has been cooling and contracting for
billions of years like a raisin. The built-up tension occasionally
breaks the crust and creates thrust faults, where one side of the
fault jumps on top of the other. This process accounts for onethird
of the total seismic energy released on Mars.
Quakes on Mars also last longer than on Earth. While
4.7
is very different from Earth. The first thing scientists
noticed is that it's very quiet. On Earth,
oceans create a constant seismic buzz that masks faint quakes
at long distances. But on Mars, the seismometer could pick
up quakes 100 times weaker than it would on Earth.
Researchers could not take full advantage of the seismic
silence, though. Mars is very windy, and the seismometer -
although covered by many isolating layers and a wind dome
- was essentially exposed to the elements. As the wind hit
the lander, it wiggled and rattled, transmitting vibrations to
the ground. Luckily, the wind died out at night almost like
clockwork, leaving a window of six to eight quiet hours when
most of the quake detections occurred.
The first quake appeared on Sol 128, after two months of
nail-biting wait. It was a faint, 2nd-magnitude tremor, which
would be undetectable to human beings on Earth. Before the
end of the mission, SEIS recorded 1,319 quakes, with magnitudes
ranging from 1 to 4.7. Only a handful of them were
above magnitude 4, powerful enough to jingle windchimes
but probably not to wake you up at night.
The number of quakes is at the lower end of pre-Insight
expectations, showing that marsquakes are both less power16
AUGUST 2023 * SKY & TELESCOPE
Magnitude of
largest marsquake
Insight observed
(on sol 1222, or
May 4, 2022)
earthquakes last seconds or minutes at most, marsquakes
typically last tens of minutes. Moonquakes, detected by
seismometers deployed during the Apollo missions, linger for
hours after the initial shake. The key difference is the attenuation
produced by a warm interior and liquid-filled pore
spaces, two factors that hinder seismic waves. Mars
occupies a middle ground between Earth and the
Moon in this regard.
Like on Earth, quakes generate two types of seismic
waves. Surface waves travel on the surface and
can only reveal details about the crust and upper
layers. Body waves travel through the interior of the
planet and tell us about its deep structure. They
come in two flavors: primary or P-waves, which
are compressional waves and travel faster, and secondary or
S-waves, which oscillate perpendicular to the direction of
propagation and are slower. Researchers can use the difference
between the speeds and arrival times of P-waves and
S-waves to find the source of quakes. The waves also reveal
the properties of the rocks they passed through.
Cerberus Fossae
To the scientists' astonishment, Insight's marsquakes had a
favorite source: Cerberus Fossae.
Cerberus Fossae is a fault network 1,200 kilometers long,
located 1,500 km east of Insight's landing spot. It's one of the
youngest features on the Martian surface, thought to have
started forming just 10 million years ago. Scientists suspect
ongoing volcanism in the area. Satellite images have revealed
lava fields and ash deposits, some of them younger than
50,000 years - not even the blink of a geologic eye.
The flanks of Cerberus's faults don't slide laterally past
each other, like in California's San Andreas zone, but seem to
be pulling away from each other, opening gaps in the terNASA
/ JPL-CALTECH
Sky & Telescope - August 2023

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