New observations from a spacecraft orbiting Mercury have revealed that the
tiny, pockmarked planet harbors a highly unusual interior — and the craft’s
glimpse of Mercury’s surface topography suggests the planet has had a very
dynamic history.
The observations were taken by a probe called MESSENGER (short for Mercury
Surface, Space Environment, Geochemistry and Ranging), the first ever to
enter orbit around Mercury. MESSENGER reached Mercury’s orbit in March 2011,
and has since circled the planet twice a day, collecting nearly 100,000
images and more than four million measurements of Mercury’s surface.
A team of scientists from institutions including MIT, the Carnegie
Institution of Washington, Johns Hopkins University’s Applied Physics
Laboratory and NASA’s Goddard Space Flight Center have analyzed the data and
precisely mapped the planet’s topography and gravitational fields. From the
gravity estimates, the team found that Mercury likely has a highly unusual
interior structure — an exceptionally large iron core overlain by a solid
layer of iron sulfide and a thin outer shell of silicate mantle and crust.
From topographic measurements, the team mapped out a large number of craters
on the planet’s surface, making a surprising finding: Many of these have
tilted over time, suggesting that processes within the planet have deformed
the terrain after the craters formed.
The researchers detail their findings in two papers published in the journal
Science.
“Prior to MESSENGER’s comprehensive observations, many scientists believed
that Mercury was much like the moon — that it cooled off very early in solar
system history, and has been a dead planet throughout most of its
evolution,” says co-author Maria Zuber, the E.A. Griswold Professor of
Geophysics at MIT. “Now we’re finding compelling evidence for unusual
dynamics within the planet, indicating that Mercury was apparently active
for a long time.”
Mercurial mission
Getting into orbit about Mercury was no easy feat, mostly because of its
proximity to the sun. Any spacecraft heading toward the planet speeds up,
drawn in by the sun’s powerful gravitational field. To counteract the sun’s
pull and slow MESSENGER down, the MESSENGER team programmed the probe to fly
by Venus twice, and Mercury three times, before slowing down enough to be
captured in Mercury’s orbit with the help of a main engine burn.
After entering Mercury’s orbit, the spacecraft began measuring the planet’s
surface elevations via laser altimetry. Through radio tracking, the probe
estimated the planet’s gravity field. Throughout the one-year mission, the
MESSENGER spacecraft battled tides from the sun, which tugged the probe out
of its optimal orbit, as well as what Zuber calls “sunlight pressure” —
photons or packets of light from the sun that exerted pressure on the
spacecraft. The team periodically adjusted the probe’s orbit and made
precise corrections to its measurements to account for the sun’s effects,
mapping out the gravity field as well as the elevation of the surface of
Mercury’s northern hemisphere.
Inside and out
The team’s measurements revealed surprising findings both in the planet’s
interior and on its surface. From the probe’s gravity estimates, the group
inferred that Mercury likely has a huge iron core comprising approximately
85 percent of the planet’s radius. (Earth’s core, by comparison, is about
half the planet’s radius in size.) This means that Mercury’s mantle and
crust occupy only the outer 15 percent or so of the planet’s radius — about
as thin as the peel on an orange, Zuber says.
The researchers also reasoned, given Mercury’s gravity field, that just
above the outer molten layer of the planet’s core may be a solid layer of
iron and sulfur — a type of layered structure not known to exist on any
other planet.
“If the iron and sulfur model is correct, it would have implications for how
the dynamo inside Mercury produces the planet's magnetic field,” says Gerald
Schubert, professor of earth and space sciences at the University of
California at Los Angeles, who did not participate in the research. “The
dynamo generation process might work differently in Mercury compared with
Earth.”
Co-author Dave Smith, a research scientist in MIT’s Department of Earth,
Atmospheric and Planetary Sciences, says the scientific process that led to
the team’s results was a journey in itself.
“We had an idea of the internal structure of Mercury, [but] the initial
observations did not fit the theory so we doubted the observations,” Smith
says. “We did more work and concluded the observations were correct, and
then reworked the theory for the interior of Mercury that fit the
observations. This is how science is supposed to work, and it’s a nice
result.”
Through laser measurements of the planet’s surface, researchers mapped out
multiple geologic features in Mercury’s northern hemisphere, finding the
range of elevations to be smaller than that of Mars or the moon. They also
observed something unexpected in Mercury’s Caloris basin, the largest impact
feature on Mercury: Portions of the floor of the crater actually stand
higher than its rim, suggesting that forces within the interior pushed the
crater up after the initial impact that created it.
Zuber and her team also identified an area of lowlands approximately
centered on Mercury’s north pole that could conceivably have migrated there
over the course of the planet’s evolution. Zuber explains that a process
called polar wander can cause geological features to shift around on a
planet’s surface due to the redistribution of mass on or within a planet by
geodynamical processes.
One such process of transporting mass in a planet’s interior is convection
within the mantle. Viscous material within the mantle circulates and can
push fragments of crust up and out, shifting terrain around the globe. Given
Mercury’s extremely thin mantle, as revealed by MESSENGER, Zuber says it’s
challenging to understand how convection operated to raise broad expanses of
terrain to the elevations observed.
“It’s interesting to think what might be causing the observed deformation,”
Zuber says. “It appears there are some very unusual dynamics going on inside
Mercury.”
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