Sky & Telescope - November 2021 - 8

NEWS NOTES
MARS
NASA's Insight Reveals First Look Inside Mars
NASA'S INSIGHT MISSION has
reported the first direct observations
of another rocky planet's interior
structure, publishing a trio of studies
in the July 23rd Science. The results
- a surprisingly thin crust, a onelayer
mantle, and a large core - will
help scientists understand how Mars
formed and evolved.
Measuring marsquakes wasn't just
a matter of sending a seismometer to
Mars; in a sense, the scientists also
had to wait for the planet to come
to them. While Insight recorded 733
marsquakes since early 2019, the vast
majority were weak, surface tremors.
" Of course, when you're in the middle
of the situation, it can feel as if 'Oh
my god, it's not working!' " says Simon
Stähler (ETH Zurich, Switzerland).
This illustration shows shear seismic waves
reflecting off Mars's iron-nickel core, which
provide an estimate of the core's size. The
strength of the reflected waves revealed the
core to be in a liquid state, through which
shear waves cannot propagate.
The team waited several months
after the seismometer's deployment
before a shake occurred deep enough
to probe the planet's interior, says
seismometer co-investigator Brigitte
Knapmeyer-Endrun (University of
Cologne, Germany).
That quake already showed what
additional deep ones would confirm:
The Martian crust was thinner than
expected. At first, analysis suggested
that the crust had two layers and
was only 20 kilometers (12 miles)
thick beneath the Insight lander.
" This was thinner than any models
had predicted, " Knapmeyer-Endrun
says. Additional events and analysis
revealed that the crust could have an
additional layer, making it twice as
thick (39 km) and somewhat closer
to expectations.
A thinner crust would be " very
surprising indeed, " says Doris Breuer
(German Aerospace Center), who was
not involved in the study. " A thin crust
would raise many questions about how
it was formed and what this means
overall for the thermochemical evolution
of Mars. "
Insight data can't distinguish
between the two possibilities yet,
though Knapmeyer-Endrun hopes
additional data and analyses will help
resolve the ambiguity.
Meanwhile, Amir Khan (ETH
Zurich, Switzerland) and colleagues
found that the mantle of Mars is
simple and deep, extending 400 to
600 km down. Compared with Earth's
two-layer mantle, Mars's single-layer
version is less insulating, so the core
loses heat more quickly.
Nevertheless, Insight data confirm
that the core is fully liquid. With the
analysis of six marsquakes coming in
at just the right angles toward Insight,
Stähler and colleagues found that the
core is on the larger end of what previous
indirect measurements had suggested,
between 3,580 and 3,740 km
across. Its seismic shadow blocks
quakes from the active Tharsis Rise on
the other side of the planet.
Such a large core probably contains
a high proportion of lighter elements.
It also probably doesn't have the solid
inner part that Earth's core does. On
Earth, the cooling inner core is the
power source of the magnetic dynamo:
As it crystallizes, it releases heat that
churns up the outer core. Without an
inner core, the global magnetic field
on Mars most likely came from the
release of the intense heat of the planet's
formation, which is why it only
lasted a few hundred million years.
NASA recently extended the Insight
mission through the end of next year
(S&T: May 2021, p. 10), past the
current Martian winter, when marsquakes
are difficult to detect due to
high winds. During that time, the
scientists expect Insight to double the
number of high-quality, low-frequency
quakes that enable them to probe the
planet's interior.
¢MONICA YOUNG
8
NOVEMBER 2021 * SKY & TELESCOPE
CHRIS BICKEL / SCIENCE
Sky & Telescope - November 2021

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