Mercury's Shifting,
Rolling Past - Simulation Reveals Possible Cause Of Mercury's Distinctive
Features
(18 March 2008) Patterns of
scalloped-edged cliffs or lobate scarps on Mercury's surface are thrust faults
that are consistent with the planet shrinking and cooling with
time.
However, compression occurred in the planet's early
history and Mariner 10 images revealed decades ago that lobate scarps are among
the youngest' features on Mercury. Why don't we find more evidence of older
compressive features?
Scott D. King, professor of geosciences at
Virginia Tech, reports in Nature Geoscience this week that mantle convection -
loss of heat from the mantle through the crust has also played a role in the
formation of lobate scarps on Mercury.
The gravity and topographic data
from the MESSENGER (Mercury Surface, Space Environment, Geochemistry, and
Ranging) mission will test his hypothesis. In the meantime, King has created
numerical simulations of the three-dimensional nature of convection within
Mercury's silicate mantle. The computations were done using the Virginia Tech
geoscience department's High-Performance Earth Simulation System, a high-speed,
high-capacity 768-core Dell computing cluster.
Scientists have offered a
number of explanations for global contraction on Mercury, such as cooling and
core formation, tidal effects due to gravitation interactions with the Sun,
impacts, and mantle convection.
"The idea that contraction due to
cooling is the cause of these features has been around for a long time and
makes a lot of sense," King said. "But the apparent pattern and the orientation
of these features is puzzling. I can't really rule out the idea that this is
just an artifact of the one hemisphere we have seen and the one camera/sun
angle that we have pictures from. But the orientation of these features seems
to require something additional, which I think is mantle
convection."
King noted that the upwellings from mantle convection on
Mercury takes the form of long, linear rolls in distinctive clusters and
directionality, rather than a random pattern associated with upthrusts from
global compression acting alone.
"The pattern of convection I see in my
Mercurian convection models is different from Venus, Mars, and Earth because
the mantle is so much thinner -- or the iron core is so much larger relatively
speaking," King said. "On Venus, Earth, and Mars, the hot material coalesces
into cylindrical plumes, not linear sheets. That could influence the tectonics
at the surface and the convection within the iron core, which is most likely
what is responsible for Mercury's magnetic field," he said.
"The timing
and orientation of these features are controlled by convection and not global
contraction," King said. "Because the model suggests that mantle convection is
still active today, gravity and topography data from the Messenger mission may
be able to confirm the model."
King adds that the scarps almost
certainly stopped deforming several billion years ago. "The planet has cooled
so much and the lithosphere is so thick that even if mantle convection still
exists today, it will not modify the surface further."
He concludes, "I
think that if we want to figure out how the Earth got to be the way it is, we
need to understand how the other planets got to be the way they are
too."
Messenger made its first early pass of Mercury in January. It will
enter orbit around the inner planet in March 2011.
(source: Virginia
Tech)
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