July 17, 2010: The first spacecraft designed by NASA to orbit Mercury
is giving scientists a new perspective on the planet's atmosphere and
evolution.
Launched in August 2004, the Mercury Surface, Space Environment,
Geochemistry and Ranging spacecraft, known as MESSENGER, conducted a
third and final flyby of Mercury in September 2009. The probe
completed a critical maneuver using the planet's gravity to remain on
course to enter into orbit around Mercury next year.
Data from the final flyby has revealed the first observations of ion
emissions in Mercury's exosphere, or thin atmosphere; new information
about the planet's magnetic substorms; and evidence of younger
volcanic activity than previously recorded. The results are reported
in three papers published online in the July 15 edition of Science
Express.
The distribution of individual chemical elements that the spacecraft
saw in Mercury's exosphere varied around the planet. Detailed
altitude profiles of those elements in the exosphere over the north
and south poles of the planet were also measured for the first time.
"These profiles showed considerable variability among the sodium,
calcium, and magnesium distributions, indicating that several
processes are at work and that a given process may affect each
element quite differently, " said Ron Vervack, lead author of one of
the papers and the spacecraft's participating scientist at the Johns
Hopkins University Applied Physics Laboratory (APL), in Laurel, Md.
Emission from ionized calcium in Mercury's exosphere was observed for
the first time during the flyby. The emission was concentrated over a
relatively small portion of the exosphere, with most of the emission
occurring close to the equatorial plane.
During its first two flybys of Mercury, the spacecraft captured images
confirming that the planet's early history was marked by pervasive
volcanism. The spacecraft's third flyby revealed a new chapter in
that history within an impact basin 180 miles in diameter that is
among the youngest basins yet seen. The basin, recently named
Rachmaninoff, has an inner floor filled with smooth plains that
differ in color from their surroundings. These sparsely cratered
plains are younger than the basin they fill and apparently formed
from material that once flowed across the surface.
"We interpret these plains to be the youngest volcanic deposits we
have yet found on Mercury, " said Louise Prockter, one of the
spacecraft's deputy project scientists at APL and lead author of one
of the three papers. "Other observations suggest the planet spanned a
much greater duration volcanism than previously thought, perhaps
extending well into the second half of solar system history."
For the first time, the spacecraft revealed substorm-like build-up, or
loading, of magnetic energy in Mercury's magnetic tail. The increases
in energy measured in Mercury's magnetic tail were very large. They
occurred quickly, lasting only two to three minutes from beginning to
end. These increases in tail magnetic energy at Mercury are about 10
times greater than at Earth, and the substorm-like events run their
course about 50 times more rapidly.
Magnetic substorms are space-weather disturbances that occur
intermittently on Earth, usually several times per day, and last from
one to three hours. Earth substorms are accompanied by a range of
phenomena, such as the majestic auroral displays seen in the Arctic
and Antarctic skies. Substorms also are associated with hazardous
energetic particle events that can wreak havoc with communications
and Earth-observing satellites.
"The extreme tail loading and unloading observed at Mercury implies
that the relative intensity of substorms must be much larger than at
Earth, " said James A. Slavin, a space physicist at NASA's Goddard
Space Flight Center in Greenbelt, Md., and a member of the
spacecraft's science team and lead author of another paper.
The new measurements give fresh insight on the time duration of
Mercury's substorms. Scientists await more extensive measurements
when the spacecraft is in orbit.
"Every time we've encountered Mercury, we've discovered new
phenomena, " said Sean Solomon, the mission's principal investigator
at the Carnegie Institution of Washington. "We're learning that
Mercury is an extremely dynamic planet, and it has been so throughout
its history. After MESSENGER has been safely inserted into orbit
around Mercury next March, we'll be in for a terrific show."
In addition to flying by Mercury, the spacecraft flew past Earth in
August 2005 and Venus in October 2006 and June 2007. Approximately 98
percent of Mercury's surface has been imaged by NASA spacecraft.
After this spacecraft goes into orbit around Mercury for a yearlong
study of the planet, it will observe the polar regions, which are the
only unobserved areas of the planet.
The spacecraft was designed and built by APL. The mission is managed
and operated by APL for NASA's Science Mission Directorate in
Washington.
For more information about the mission, visit: http://www.nasa.gov/messenger
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