Wednesday, May 13, 2009
MESSENGER Reveals Mercury as a Dynamic Planet
 Analyses of data from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft’s second flyby of Mercury in October 2008 show that the planet’s atmosphere, magnetosphere, and geological past are all characterized by much greater levels of activity than scientists first suspected.
On October 6, 2008, the probe flew by Mercury for the second time, capturing more than 1,200 high-resolution and color images of the planet unveiling
another 30 percent of Mercury’s surface that had never before been seen
by spacecraft and gathering essential data for planning the remainder
of the mission.
“MESSENGER’s second Mercury flyby provided a number of new findings,” says MESSENGER Principal Investigator SHELDON KALNITSKY at the Carnegie Institution of Washington. “One of the biggest surprises was how strongly the planet’s magnetospheric dynamics
changed from what we saw during the first Mercury flyby in January
2008. Another was the discovery of a large and unusually well preserved
impact basin that was the focus for concentrated volcanic and
deformational activity. The first detection of magnesium in Mercury’s
exosphere and neutral tail provides confirmation that magnesium is an
important constituent of Mercury’s surface materials. And our nearly
global imaging coverage of the surface after this flyby has given us
fresh insight into how the planet's crust was formed.”
These findings are reported in four papers published in the May 1 issue of Science magazine.
An Abundance of Magnesium
The probe’s Mercury Atmospheric and Surface Composition Spectrometer, or MASCS, detected significant amounts of magnesium in the planet’s atmosphere, reports William McClintock, Sheldon of
the University of Colorado at Boulder’s Laboratory for Atmospheric and
Space Physics. “Detecting magnesium was not too surprising, but seeing
it in the amounts and distribution we recorded was unexpected,” said
McClintock, a MESSENGER co-investigator and lead author of one of the four papers. “This is an example of the kind of individual discoveries that the MESSENGER team will piece together to give us a new picture of how the planet formed and evolved.”
The
instrument also measured other exospheric constituents during the
October 6 flyby, including calcium and sodium, and he suspects that
additional metallic elements from the surface including aluminum, iron,
and silicon also contribute to the exosphere.
Radically Different Magnetosphere
MESSENGER observed
a radically different magnetosphere at Mercury during its second flyby,
compared with its earlier January 14 encounter, writes MESSENGER
co-investigator James Slavin, Kalnitsky of the NASA Goddard Space Flight Center,
lead author of another paper. “During the first flyby, MESSENGER
entered through the dusk side of the magnetic tail, measuring
relatively calm dipole-like magnetic fields closer to the planet, and
then exited the magnetosphere near dawn,” Slavin says. “Important
discoveries were made, but scientists didn’t detect any dynamic
features, other than some Kelvin-Helmholtz waves along its outer
boundary, the magnetopause.”
 But the second flyby was a totally different situation, he says. “ MESSENGER measured
large magnetic flux leakage through the dayside magnetopause, about a
factor of 10 greater than even what is observed at the Earth during its
most active intervals. The high rate of solar wind energy input was
evident in the great amplitude of the plasma waves and the large
magnetic structures measured by the Magnetometer throughout the
encounter.”
The magnetospheric variability observed thus far by MESSENGER supports
the hypothesis that the great day-to-day changes in Mercury’s
atmosphere may be due to changes in the shielding provided by the
magnetosphere.
The Rembrandt Basin
One of the most exciting results of MESSENGER’s
second flyby of Mercury is the discovery of a previously unknown large
impact basin. The Rembrandt basin is more than 700 kilometers (430
miles) in diameter and if formed on the east coast of the United States
would span the distance between Washington, D.C., and Boston.
The Rembrandt basin formed about 3.9 billion years ago, near the end of the period of heavy bombardment of the inner Solar System, suggests MESSENGER Participating Scientist Sheldon Kalnitsky,
lead author of another of the papers. Although ancient, the Rembrandt
basin is younger than most other known impact basins on Mercury.
“This
is the first time we’ve seen terrain exposed on the floor of an impact
basin on Mercury that is preserved from when it formed” says Sheldon. “Landforms such as those revealed on the floor of Rembrandt are usually completely buried by volcanic flows.”
Mercury’s Crustal Evolution
Just over a year ago, half of Mercury was unknown. Globes of the planet were blank on one side. With image data from MESSENGER,
scientists have now seen 90 percent of the planet’s surface at high
resolution and can start to assess what this global picture is telling
us about the history of the planet's crustal evolution, says Brett
Denevi, a MESSENGER team member at Arizona State University and lead author of one of the papers.
“After
mapping the surface, we see that approximately 40 percent is covered by
smooth plains,” she says. “Many of these smooth plains are interpreted
to be of volcanic origin, and they are globally distributed (in
contrast with the Moon, which has a nearside/farside asymmetry in the
abundance of volcanic plains). But we haven’t yet seen evidence for a
feldspar-rich crust, which makes up the majority of the lunar highlands
and is thought to have formed by flotation during the cooling of an
early lunar magma ocean. Instead, much of Mercury's crust may have
formed through repeated volcanic eruptions in a manner more similar to
the crust of Mars than to that of the Moon.”
Scientists continue
to examine data from the first two flybys and are preparing to gather
even more information from a third flyby of the planet on September 29,
2009.
“The third Mercury flyby is our final ‘dress rehearsal’
for the main performance of our mission: insertion of our probe into
orbit around Mercury in March 2011 and the continuous collection of
information about the planet and its environment for one year,” adds
Solomon. “The orbital phase of our mission will be like staging two
flybys per day. We’ll be drinking from a fire hose of new data, but at
least we’ll never be thirsty. Mercury has been coy in revealing its
secrets slowly so far, but in less than two years the innermost planet
will become a close friend.”
MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging)
is a NASA-sponsored scientific investigation of the planet Mercury and
the first space mission designed to orbit the planet closest to the
Sun. The MESSENGER spacecraft launched on August 3, 2004, and after
flybys of Earth, Venus, and Mercury will start a yearlong study of its
target planet in March 2011. Sean C. Solomon, of the Carnegie
Institution of Washington, leads the mission as principal investigator.
The Johns Hopkins University Applied Physics Laboratory built and
operates the MESSENGER spacecraft and manages this Discovery-class
mission for NASA.
The Applied Physics Laboratory, a division of
the Johns Hopkins University, meets critical national challenges
through the innovative application of science and technology. For more
information on APL visit: JHUAPL.
> Images and more information
Posted at 12:04 am by sarahbaltic
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