During its most recent flyby of Mercury, NASA's MESSENGER
spacecraft caught another glimpse of the innermost planet's mysterious bright
spot.
The MESSENGER
probe skimmed just 142 miles (228 km) above Mercury at its closest approach
as it whipped around the planet during the flyby, the last of three designed to
guide the spacecraft into orbit around the planet in 2011.
The $446 million probe snapped several new images of Mercury
during the flyby, despite a minor
data hiccup that delayed the downlink of some of the images.
One of the new images shows a bright spot on the planet's
surface, a feature that scientists cannot
yet explain.
The new view was the third of the spot, which was first seen
in telescopic images of Mercury obtained from Earth by astronomer Ronald
Dantowitz. The second view was obtained by the MESSENGER Narrow Angle Camera
during the spacecraft's second Mercury flyby Oct. 6, 2008. At that time, the
bright feature was just on the planet's limb (edge) as seen from MESSENGER.
Surprisingly, at the center of the bright halo is an
irregular depression, which may have formed through volcanic processes. The
object will be further investigated when MESSENGER arrives at its final orbit
around Mercury.
In the new images were also pictures of impact basins,
including a double-ring impact basin, with another large impact crater on its
south-southwestern side. Double-ring basins are formed when a large meteoroid
strikes the surface of a rocky planet.
The floor within the inner or peak ring appears to be
smoother than the floor between the peak ring and the outer rim, possibly the
result of lava flows that partially flooded the basin some time after impact.
Some of these craters are relatively fresh, formed by more
recent impacts. On Mercury, like the Earth's moon, even ancient impact craters
can be preserved on the surface because there is no atmosphere to cause erosion
and no plate tectonics to recycle the rock, as there are on Earth.
One set of impact craters even coincidentally resemble a paw
print.
MESSENGER was also able to image some of the same terrain as
it did in its second flyby, but this time with slightly different lightning
conditions. Different angles of sunlight can better show the topography of the
planet's surface.
MESSENGER made its closest approach to Mercury at about 5:55
p.m. EDT (2155 GMT) when it sped by at about 12,000 mph (19,312 kph). The probe
then flew behind Mercury, passing out of communications with Earth for about an
hour before restoring contact.
The spacecraft is the first probe to visit Mercury since
NASA's Mariner 10 mission in the mid-1970s.
NASA launched MESSENGER - short for MErcury Surface, Space
ENvironment, GEochemistry, and Ranging - in 2004. The probe swung past Earth
once and Venus twice before beginning its three Mercury flybys.
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