Science - USA (2022-05-27)

IMAGE: NASA/JPLCALTECH

SCIENCE science.org 27 MAY 2022 • VOL 376 ISSUE 6596 905

I

t’s never easy saying goodbye. But more

than 3 years after NASA’s InSight lander

touched down on Mars, planetary scien-

tists are preparing for the demise of the

mission, which has helped reveal how

the planet is put together. Last week,

NASA officials said dust accumulating on the

probe’s solar panels has reduced power flows

to a trickle, forcing them to shut it down by

the end of the year.

Launched in 2018, InSight (Interior Explo-

ration using Seismic Investigations, Geodesy

and Heat Transport) has recorded a host of

accomplishments, including measuring the

magnetic field of Mars and detecting more

than 1300 quakes, which have enabled re-

searchers to map its structure. It has pro-

vided “an important first step in studying the

interior of Mars,” says Paula Koelemeijer, a

seismologist at the University of Oxford.

InSight landed in November 2018 on

the Elysium Planitia, a broad, flat plain on

the martian equator. Planners picked the

spot because they hoped its relatively calm,

quiet weather would allow one of the land-

er’s primary instruments, an ultrasensitive

seismometer, to detect seismic waves from

distant marsquakes as they bounced around

the interior of the planet.

After researchers switched on the instru-

ment in February 2019, however, they heard

nothing for 2 months. “We were rather ner-

vous,” recalls John Clinton, a seismologist

from ETH Zürich and a co-investigator on the

mission. One fear was that any marsquakes

were too faint to be detected. But those wor-

ries were allayed in April 2019 when InSight

recorded its first quake. Since then, more

than 1300 have been documented by Clin-

ton’s Marsquake Service, an online database

that catalogs InSight’s daily seismic data.

Many of the quakes appear to originate

in the nearby Cerberus Fossae region, but

their causes remain largely unknown. Possi-

bilities include remnant volcanic activity on

the largely quiescent planet, geologic stress,

and meteorite impacts. Still, by tracking the

arrival of seismic waves and working out

how fast they traveled through interior lay-

ers, researchers have been able to assemble

a more detailed picture of the planet’s core,

mantle, and crust. “We now have a map of

the inside of Mars,” says Bruce Banerdt of

NASA’s Jet Propulsion Laboratory, the lead

on the mission.

There have been surprises. Some re-

searchers expected to find a relatively mea-

ger and solid core, for example. Instead,

“It’s a really big core,” proportionally as

big as Earth’s, says planetary scientist Julia

Semprich of the Open University, and like

Earth’s outer core, it is molten. But it is not

clear yet whether Mars has a solid inner

core like Earth’s. And researchers are still

analyzing data from a radio instrument de-

signed to track the wobble of Mars’s spin

axis, which can offer other clues to the size

and consistency of the core.

The planet’s mantle, the layer sandwiched

between the planet’s core and crust, appears

to consist of just a single rocky layer, rather

than two like Earth. And the thin martian

crust has either two or three layers, with pos-

sible evidence for water buried inside.

Most of the quakes detected by InSight

have been relatively modest, up to a magni-

tude 4. But on 4 May, in the lander’s waning

days, it registered a rare and welcome event:

a magnitude 5 quake. Throughout the mis-

sion, scientists had been hoping to capture

such a tremor, which was “as large as all of

the other quakes seen until then combined,”

says Brigitte Knapmeyer-Endrun, a planetary

seismologist from the University of Cologne.

Researchers are still analyzing the waves

from the powerful quake, but they hope

it will deepen their understanding of the

planet’s structure. “There’s going to be lots

of information as we get our teeth into it,”

Banerdt says.

The $425 million mission has also docu-

mented a planetary magnetic field that is

10 times stronger than expected. That sug-

gests magnetized rocks sit deep under the

surface, storing a record of its lost mag-

netic field.

It hasn’ t been all plain sailing. A heat probe

designed to measure subsurface tempera-

tures couldn’t penetrate the tough martian

skin. “That was the biggest disappointment

of the mission,” Banerdt says.

A plan to have a second seismometer run-

ning in parallel with InSight’s also failed to

materialize. Europe’s ExoMars mission was

supposed to deliver the twin sensor, which

would have enabled researchers to “see

events in different regions,” Knapmeyer-

Endrun says. But ExoMars has faced delays.

InSight is now generating just one-tenth as

much power as at the mission’s start. NASA

says it will cease science operations by July,

then switch off the lander at the end of this

year unless a passing dust devil brushes off

its solar panels. That could enable the lander

to “boot itself up and come back to life,”

Banerdt says.

Even as InSight falters, researchers are

eyeing how they might follow its example by

listening for quakes on other bodies. Some

are planning a seismometer network for the

Moon, for example, and NASA’s Dragonfly

mission is scheduled to take a seismometer to

Saturn’s moon Titan in 2026. Venus or Jupi-

ter’s icy moon Europa are also attractive tar-

gets. The InSight mission, Banerdt says, “has

shown seismology for the incredibly capable

technique it is.” j

Jonathan O’Callaghan is a journalist in London.

An artist’s conception of NASA’s InSight lander, which

is scheduled to be shut down by the end of 2022.

PLANETARY SCIENCE

By Jonathan O’Callaghan

End draws near for lander that

mapped the interior of Mars

Mars InSight listened to quakes to

give us our first look inside another planet