Mercury's
surface is not only peppered with impact craters, but also wrinkled with
mysterious chains of cliffs.
Scientists
think the "lobate scarp" cliffs — some 2 miles (3.2 kilometers) high
and hundreds of miles long — were created as Mercury's crust bunched up around
its shrinking interior, something like a dried-out piece of fruit. A new
theory, however, suggests that rising sheets of hot mantle rock popped out the
planet's characteristic ridges, helping to create the cliffs.
"There's
a preferred north-south alignment to these scarps," Scott King, a
planetary geophysicist at Virginia Tech University, told SPACE.com.
"If you just have a shrinking sphere, there's no reason they should be
aligned. It should be fairly random."
Instead of
just a shrinking crust, King thinks linear sheets of rock heaved on the
planet's crust from below, pushing up the cliff-like features. He detailed his
computer-modeled hypothesis in the March 16 online edition of the journal Nature
Geoscience.
"It's
a very plausible idea," said Sean Solomon, principal investigator for
NASA's MESSENGER spacecraft at the Carnegie Institution of Washington, who was
not involved in the study. "It gives a nice set of predictions about what
we might see, so it's highly testable."
New
views
Prior to
MESSENGER's January 2008 flyby of Mercury, Mariner 10 was the planet's most
recent visitor — but that was 33 years ago.
Mariner 10
captured only half the surface of the solar system's innermost planet, King
said, preventing a final verdict on whether or not the cliffs have an
orientation across the planet. That scenario improved dramatically with new
MESSENGER data.
"We
saw a third of the part we had never seen after this recent flyby," King
said, noting that the new views also had different sun angles to highlight new
cliff regions. "It still looks like the idea of an orientation is going to
hold up."
King said
that after the MESSENGER (which stands for MErcury
Surface, Space ENvironment, GEochemistry and Ranging) spacecraft reaches
stationary orbit around the planet in March 2011, scientists will know for
certain if the scarps are aligned planet-wide.
"Until
then, we won't have 100 percent coverage," King said. For now, the craft
is zooming near Venus's orbit and is closing in on Mercury for a second pass in
October 2008 and a third in September 2009.
Mantle
push-ups
Assuming
the scarps are indeed lined up, King thinks a thin yet active mantle layer
beneath Mercury's crust may be to blame for the cliff-like features.
"It has
a very large iron core compared to Earth, Venus and Mars," King said of
Mercury's metallic heart. "The rock above it is confined to a real thin
shell."
Because
there's hardly any room for hot mantle rock to snake toward the planet's
surface, King's models suggest that the material is forced into a rolling pattern of
linear or sheet-like plumes. On Earth — where mantle space abounds
beneath the crust — rising rock is mostly squeezed into cylindrical plumes.
"The
dynamics are much different for Mercury. I've done a number of models, and this
roll pattern almost always shows up," King said. "The stresses
created by that on the crust can be enormous."
Still
active?
Although
both King and Solomon agree that Mercury's cliffs probably stopped forming a
few billion years ago, King's models suggest that convection activity might
still be roiling beneath the planet's surface.
"You
bottle up a lot of energy when you create a planet, and it takes a long time
for that heat to get out of the system," King said. "It's still
debatable whether or not that convection continues on today."
Solomon
said instruments on MESSENGER that can monitor changes in mass should be able
to tell if hot rock is still moving beneath Mercury's cold, hardened shell.
"If
new images show an orientation to the lobate scarps, Scott [King] may have
described convection of Mercury's mantle in the past," Solomon said.
"If that convection is still going on today, we should be able to use
MESSENGER to detect that activity in the planet's gravity field and topography."
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