A NASA spacecraft has spotted what appears to be changing
seasons on Mercury and found much more iron on the surface of the small, rocky planet
than previously thought.
The MESSENGER
probe made the observations during its third
flyby of Mercury on Sept. 29, when it took a host of measurements and
images of the innermost planet's surface and atmosphere. Only about half of the
planned measurements were made because of a data
glitch that affected the spacecraft during the flyby.
The $446 million probe's third flyby brought it within 142
miles (228 km) of Mercury's surface to cover more uncharted terrain, leaving 98
percent of the planet now mapped. The flyby was also a gravity assist meant to
guide the spacecraft into orbit around the planet in 2011.
Tenuous exosphere
Mercury's atmosphere is what scientists call an
"exosphere," and is made up of atoms kicked up from the surface. It
is very
tenuous and has a very low density, meaning atoms in the atmosphere rarely
run into each other. It also has a tail that streams away from the planet in
the opposite direction of the sun.
MESSENGER looked at differences in three atoms in the
exosphere — sodium, calcium and magnesium — between the probe's three flybys.
They detected much less sodium during the third flyby than they had during the
second.
"While this is dramatic, it isn't totally unexpected,"
Vervack said. This is because radiation pressures from the sun change as
Mercury moves through its orbit, which changes the amount of sodium liberated
from the surface.
In essence, Mercury's atmosphere experiences seasonal
effects during the planet's orbit.
Calcium and magnesium showed less variation between
"seasons" than did sodium, showing that different atoms "are
going to have their own unique seasonal varations," Vervack said.
Understanding these seasonal differences will help
scientists understand how surface material is lost and how the surface has
changed over time.
Mercury's atmosphere is "the end product of a few
billion years of these processes, they never stop," said mission scientist
Ronald Vervack, Jr., of The Johns Hopkins University Applied Physics Laboratory
(JHUAPL) in Laurel, Md.
Surface surprises
The MESSENGER flybys also provided the first direct
measurements of the amounts of certain elements on Mercury's surface.
Mercury's surface has long thought to be deficient in the
heavy metals iron and titanium on the basis of earlier observations, an ironic
make-up considering that its heavy iron core makes up an estimated 60 percent
of the planet's mass and makes it the densest of the solar system's rock
planets.
MESSENGER's observations show that Mercury's surface
actually possesses relatively high numbers of these elements — similar to the
concentrations in the moon's nearside maria basalts — which could mean that
models on the planet's formation and evolution will need to be revised.
"That's a pretty exciting result for us," said
David Lawrence, also of JHUAPL.
The third flyby also yielded new images of the surface that
fill in as-yet unobserved areas. The spacecraft's cameras and instruments
collected high-resolution and color images unveiling another 6 percent of the
planet's surface never before seen at close range.
"We've done a good job on filling in most of that
map," said Brett Denevi, imaging team member and postdoctoral researcher
at Arizona State University in Tempe.
The flyby garnered images of a feature seen before, but not
in enough detail to characterize it. It appears to be a
bright area surrounding an irregular depression, with steep sides and an
odd shape, "all of which are hallmarks of something like a volcanic
vent," Denevi said.
Other images revealed a double-ring impact basin
approximately 180 miles across. The basin is similar to a feature scientists
call the Raditladi basin, which was viewed during the probe's first flyby of
Mercury in January 2008.
"One similarity to Raditladi is its age, which has been
estimated to be approximately one billion years old. Such an age is quite young
for an impact basin, because most basins are about four times older,"
Denevi said. "The inner floor of this basin is even younger than the basin
itself and differs in color from its surroundings. We may have found the
youngest volcanic material on Mercury."
The spacecraft has completed nearly three-quarters of its
4.9-billion-mile journey to enter orbit around Mercury. The full trip will
include more than 15 trips around the sun. In addition to flying by Mercury,
the spacecraft flew past Earth in August 2005 and Venus in October 2006 and
June 2007.
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