Researchers have been able to use seismic data to look inside Mars for the first time. They measured the crust, mantle and core and narrowed down their composition.
From:
ETH Zurich
July 22, 2021 -- Since early 2019,
researchers have been recording and analysing marsquakes as part of the InSight
mission. This relies on a seismometer whose data acquisition and control
electronics were developed at ETH Zurich. Using this data, the researchers have
now measured the red planet's crust, mantle and core -- data that will help
determine the formation and evolution of Mars and, by extension, the entire
solar system.
Mars once completely molten
We know that Earth is made up of shells:
a thin crust of light, solid rock surrounds a thick mantle of heavy, viscous
rock, which in turn envelopes a core consisting mainly of iron and nickel.
Terrestrial planets, including Mars, have been assumed to have a similar
structure. "Now seismic data has confirmed that Mars presumably was once
completely molten before dividing into the crust, mantle and core we see today,
but that these are different from Earth's," says Amir Khan, a scientist at
the Institute of Geophysics at ETH Zurich and at the Physics Institute at the
University of Zurich. Together with his ETH colleague Simon Stähler, he
analysed data from NASA's InSight mission, in which ETH Zurich is participating
under the leadership of Professor Domenico Giardini.
No plate tectonics on Mars
The researchers have discovered that the
Martian crust under the probe's landing site near the Martian equator is between
15 and 47 kilometres thick. Such a thin crust must contain a relatively high
proportion of radioactive elements, which calls into question previous models
of the chemical composition of the entire crust.
Beneath the crust comes the mantle with
the lithosphere of more solid rock reaching 400-600 kilometres down -- twice as
deep as on Earth. This could be because there is now only one continental plate
on Mars, in contrast to Earth with its seven large mobile plates. "The
thick lithosphere fits well with the model of Mars as a 'one-plate
planet'," Khan concludes.
The measurements also show that the
Martian mantle is mineralogically similar to Earth's upper mantle. "In
that sense, the Martian mantle is a simpler version of Earth's mantle."
But the seismology also reveals differences in chemical composition. The
Martian mantle, for example, contains more iron than Earth's. However, theories
as to the complexity of the layering of the Martian mantle also depend on the
size of the underlying core -- and here, too, the researchers have come to new
conclusions.
The core is liquid and larger than
expected
The Martian core has a radius of about
1,840 kilometres, making it a good 200 kilometres larger than had been assumed
15 years ago, when the InSight mission was planned. The researchers were now
able to recalculate the size of the core using seismic waves. "Having
determined the radius of the core, we can now calculate its density,"
Stähler says.
"If the core radius is large, the
density of the core must be relatively low," he explains: "That means
the core must contain a large proportion of lighter elements in addition to
iron and nickel." These include sulphur, oxygen, carbon and hydrogen, and
make up an unexpectedly large proportion. The researchers conclude that the
composition of the entire planet is not yet fully understood. Nonetheless, the
current investigations confirm that the core is liquid -- as suspected -- even
if Mars no longer has a magnetic field.
Reaching the goal with different
waveforms
The researchers obtained the new results
by analysing various seismic waves generated by marsquakes. "We could
already see different waves in the InSight data, so we knew how far away from
the lander these quake epicentres were on Mars," Giardini says. To be able
to say something about a planet's inner structure calls for quake waves that
are reflected at or below the surface or at the core. Now, for the first time,
researchers have succeeded in observing and analysing such waves on Mars.
"The InSight mission was a unique
opportunity to capture this data," Giardini says. The data stream will end
in a year when the lander's solar cells are no longer able to produce enough
power. "But we're far from finished analysing all the data -- Mars still
presents us with many mysteries, most notably whether it formed at the same
time and from the same material as our Earth." It is especially important
to understand how the internal dynamics of Mars led it to lose its active
magnetic field and all surface water. "This will give us an idea of
whether and how these processes might be occurring on our planet,"
Giardini explains. "That's our reason why we are on Mars, to study its
anatomy."
https://www.sciencedaily.com/releases/2021/07/210722163028.htm
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