Sky & Telescope - August 2023 - 19

NASA / JPL-CALTECH
light elements were available for planet formation before the
solar wind blew them away. Or maybe Mars accreted smaller
planetary bodies from the outer solar system, which brought
the light elements with them.
A third explanation is the presence of a molten layer of
rock at the bottom of the mantle, which would seismically
behave like the fluid core, making it look larger than it really
is. This idea is still being explored, though.
Hit Me Baby One More Time
On Christmas Eve of 2021, Mars shook more violently than
usual. SEIS detected a 4th-magnitude marsquake, among the
most powerful recorded. It was the first event to produce surface
waves. The epicenter wasn't in Cerberus Fossae, though,
but 3,500 km northeast from Insight in Amazonis Planitia, a
low, flat plain bordering the vast polar basin.
Two months later, scientists poring over images from the
Mars Reconnaissance Orbiter (MRO) spotted a big blotch
in this same region. Images revealed a 150-meter-wide
crater, surrounded by a large blanket of ejected material
that stretched up to 37 km away. Such an impact, scientists
estimated, must have been produced by a meteorite 4 m to
12 m in diameter. The crater was 21 m deep and excavated
water ice that had lain buried underground, visible as white
spots in the satellite image that disappeared over time as the
ice sublimated.
Liliya Posiolova (Malin Space Science Systems), the operations
lead for MRO, remembered that Insight had reported a
large seismic event and realized this impact could be the culprit.
Daily weather images taken by another camera on MRO
confirmed the date of the impact.
Inspired, Insight researchers looked for similar events
in their records, finding that a 4.1-magnitude event that
occurred on Sol 1000 (September 18, 2021) was a good
candidate. When MRO looked at its epicenter, 7,500 km
from Insight, they found a cluster of new craters, the largest
130 m wide.
The surface waves produced by these impacts traveled
through the shallower part of the crust to reach Insight.
Surprisingly, the waves traveled at roughly the same speed
through the northern and southern hemispheres. The topography
of the two Martian hemispheres is radically distinct,
with a low-lying north that might have hosted oceans and a
rugged south made of highlands. On Earth, the oceanic and
continental crusts have different densities, something that
doesn't seem to be true on Mars.
Weather, Wind, and Dust
As it sat on Elysium Planitia, Insight did more than sense
quakes. It also produced an unprecedented set of meteorological
records. It measured the atmospheric pressure and wind
speed uninterruptedly for over a Martian year, watching
conditions change from second to second.
But one thing the lander didn't see is dust devils, small
dusty tornadoes that are commonplace in other Martian
TIME TO SLEEP This image from sol 1436 (December 11, 2022)
is one of the last Insight took before it ran out of power. The
seismometer's 69-cm-wide protective dome appears at center.
locations. Insight registered thousands of the pressure drops
associated with dust devils, but not one was caught on camera
- even though vortexes shook the lander and left visible
tracks around it.
" We have many theories, but we really don't know why we
haven't seen a single dust devil, " says José Antonio RodríguezManfredi
(Center for Astrobiology, INTA-CSIC, Spain), principal
investigator for the lander's temperature and wind sensors.
Insight has taken daily images of the ground to track how
the winds move dust and pebbles over time, and while there's
plenty of dust at the landing site, there are still few clues as to
why the dust seems reluctant to become airborne in this area.
" This is one of the things we don't understand yet. "
But there was still enough dust to impact the lander.
After four years on Mars, the dust build-up on Insight's
solar panels finally blocked more light than the craft needed
to recharge its batteries. Ingenious efforts to lower energy
consumption and clean the solar panels were eventually not
enough, and Insight entered a hibernation state called " dead
bus mode, " in which everything is disconnected except the
circuit that charges the batteries. In the unlikely case that a
wind gust clears some of the dust from the solar panels, the
spacecraft is ready to resume charging and send a message
home: " I'm alive! "
Meanwhile, scientists will keep exploring Insight's data.
Currently we are between the second and third generation of
Insight results, says Stähler. The first one, he explains, comprised
the first observations right after landing, the second
included odd or surprising findings, and the third one will
consist of new hypotheses to explain all that doesn't add up.
" We are entering the third generation of Insight results right
now, " he says.
¢ JAVIER BARBUZANO is a freelance science journalist who
covers astronomy and geoscience.
sk yand tele scope .o r g * AUGUST 2023 19
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