SCIENCE science.org 17 DECEMBER 2021 • VOL 374 ISSUE 6574^1431NASA lander
uncovers the Red Planet’s core
The interior of a rocky planet is a kind of time machine: Its dense
core, viscous mantle, and hardened crust can reveal how it co-
alesced, churned, and settled into what it is today. Until this year,
scientists have had access to just two such time capsules: Earth
and, briefly during the Apollo missions, the Moon. Now, for the first
time, instruments aboard NASA’s InSight lander are bringing Mars’s
planetary core into focus.
When InSight arrived on the Red Planet in 2018, Mars seemed
reluctant to expose its buried secrets. InSight’s heat probe failed to
penetrate the planet’s surprisingly sticky sediments, despite repeated
attempts. The lander’s hypersensitive seismic station, designed
to monitor the underground rumblings that could help chart the
planet’s interior, never picked up a marsquake powerful enough to
do the job. And dust piled on the lander’s solar panels, slowly erod-
ing their power output.
But within 1 year, InSight picked up a handful of moderate
quakes, including several stemming from Cerberus Fossae, a fissured
region 1600 kilometers away. When combined with estimates of
the interior’s composition, those readings helped chart the planet’s
depths. Offsets in the quakes’ seismic waves revealed that the mar-tian crust is layered and less than 40 kilometers thick—thinner than
Earth’s continental crust. That thin shell would have let Mars quickly
shed its early internal heat.
Looking deeper, InSight found the martian mantle lacked the
insulating lower layer seen in Earth’s. The mantle was also shallow,
squeezed between the crust and an unusually large, liquid core that
occupies more than half of Mars’s width. Given the planet’s mass,
scientists concluded that the core’s density is low, and that a mixture
of light elements such as sulfur likely keep its iron and nickel liquid,
despite the planet’s rapid heat loss—much as salt prevents icing. The
researchers published their findings in Science this year.
Armed with these new data, scientists will be puzzling over Mars’s
history for years to come. Questions include whether the planet once
had something resembling plate tectonics and when its liquid core
stopped churning, shutting off the magnetic field it once generated.
InSight may have still more stories to share: In August and Sep-
tember, the lander heard its largest marsquakes yet. But as red dust
continues to build on its solar panels, its time is growing short. It is
estimated to run out of power by the end of 2022.
Until then, InSight will wait, and listen. —Paul VoosenNASA lander
uncovers the Red Planet’s core
Waves from marsquakes
showed the planet has a thin
crust, a shallow mantle, and
an unusually large liquid core.ILLUSTRATION: C. BICKEL/
SCIENCE