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Satnews Daily
November 03, 2009
MESSENGER Almost Delivers Mercury Completely
A NASA spacecraft's third and final flyby of Mercury
gives scientists, for the first time, an almost complete view of the
planet's surface and provides new scientific findings about this
relatively unknown world.
The Mercury Surface, Space Environment, Geochemistry and Ranging
spacecraft, known as MESSENGER, flew by Mercury on September 29. The
probe completed a critical gravity assist to remain on course to
enter into orbit around Mercury in 2011. Despite shutting down
temporarily because of a power system switchover during a solar
eclipse, 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.
Approximately 98 percent of Mercury's surface now has been imaged by
NASA spacecraft. After MESSENGER goes into orbit around Mercury, it
will see the polar regions, which are the only unobserved areas of
the planet.
"Although the area viewed for the first time by spacecraft was less
than 350 miles across at the equator, the new images reminded us that
Mercury continues to hold surprises," said Sean Solomon, principal
investigator for the mission and director of the Department of
Terrestrial Magnetism at the Carnegie Institution of Washington.
Many new features were revealed during the third flyby, including a
region with a bright area surrounding an irregular depression,
suspected to be volcanic in origin. 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.
"This double-ring basin, seen in detail for the first time, is
remarkably well preserved," said Brett Denevi, a member of the
probe's imaging team and a postdoctoral researcher at Arizona State
University in Tempe. "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. 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."
Photo on right: Selected features revealed during MESSENGER’s third flyby. This enhanced-color view was created with a statistical technique that highlights subtle color variations seen in the 11 filters of MESSENGER’s wide-angle camera that are often related to composition. Merged with images from the higher-resolution narrow-angle camera, the two sets of observations tell the story of the geology of the area and the compositional differences of the features observed. This region, viewed in detail for the first time during the third flyby, appears to have experienced a high level of volcanic activity. The bright yellow area near the top right is centered on a rimless depression (Image 4.4) that is a candidate site for an explosive volcanic vent. The 290-km-diameter double-ring basin in the center of the image has a smooth interior (Image 4.5) that may be the result of effusive volcanism. Smooth plains, thought to be a result of earlier episodes of volcanic activity, cover much of the surrounding area. Image resolution is 1 km/pixel.
One of the spacecraft's instruments conducted its most extensive
observations to date of Mercury's exosphere, or thin atmosphere,
during this encounter. The flyby allowed for the first detailed scans
over Mercury's north and south poles. The probe also has begun to
reveal how Mercury's atmosphere varies with its distance from the
sun.
"A striking illustration of what we call 'seasonal' effects in
Mercury's exosphere is that the neutral sodium tail, so prominent in
the first two flybys, is 10 to 20 times less intense in emission and
significantly reduced in extent," says participating scientist Ron
Vervack, of the Johns Hopkins University Applied Physics Laboratory,
or APL, in Laurel, Maryland. "This difference is related to expected
variations in solar radiation pressure as Mercury moves in its orbit
and demonstrates why Mercury's exosphere is one of the most dynamic
in the solar system."
The observations also show that calcium and magnesium exhibit
different seasonal changes than sodium. Studying the seasonal changes
in all exospheric constituents during the mission orbital phase will
provide key information on the relative importance of the processes
that generate, sustain, and modify Mercury's atmosphere.
The third flyby also revealed new information on the abundances of
iron and titanium in Mercury's surface materials. Earlier Earth and
spacecraft-based observations showed that Mercury's surface has a
very low concentration of iron in silicate minerals, a result that
led to the view that the planet's crust is generally low in iron.
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.
