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Mercury Surprises
Mercury Surprises
30 Apr 2009
(Source: NASA)

Dwayne Brown
Headquarters, Washington

Paulette Campbell
Johns Hopkins University Applied Physics Laboratory, Laurel, Md.

RELEASE: 09-092


WASHINGTON -- A NASA spacecraft gliding over the surface of Mercury
has revealed that the planet's atmosphere, the interaction of its
surrounding magnetic field with the solar wind, and its geological
past display greater levels of activity than scientists first
suspected. The probe also discovered a previously unknown large
impact basin about 430 miles in diameter -- equal to the distance
between Washington and Boston.

Analyses of these new findings and more are reported in four papers
published in the May 1 issue of Science magazine. The data come from
the Mercury Surface, Space Environment, Geochemistry, and Ranging
spacecraft, known as MESSENGER. On Oct. 6, 2008, the probe flew by
Mercury for the second time, capturing more than 1,200
high-resolution and color images of the planet. The probe unveiled
another 30 percent of the planet's surface that had never been seen
by previous spacecraft, gathering essential data for planning the
remainder of the mission.

"This second Mercury flyby provided a number of new findings," said
Sean Solomon, the probe's principal investigator from the Carnegie
Institution of Washington. "One of the biggest surprises was how
strongly the dynamics of the planet's magnetic field-solar wind
interaction changed from what we saw during the first Mercury flyby
in January 2008. The discovery of a large and unusually well
preserved impact basin shows concentrated volcanic and deformational

The spacecraft also made the first detection of magnesium in Mercury's
thin atmosphere, known as an exosphere. This observation and other
data confirm that magnesium is an important constituent of Mercury's
surface materials.

The probe's Mercury Atmospheric and Surface Composition Spectrometer
instrument detected the magnesium. Finding magnesium was not
surprising to scientists, but seeing it in the amounts and
distribution observed was unexpected. The instrument also measured
other exospheric constituents, including calcium and sodium.

"This is an example of the kind of individual discoveries that the
science team will piece together to give us a new picture of how the
planet formed and evolved," said William McClintock, co-investigator
and lead author of one of the four papers. McClintock, who is from
the Laboratory for Atmospheric and Space Physics at the University of
Colorado at Boulder, suspects that additional metallic elements from
the surface, including aluminum, iron and silicon, also contribute to
the exosphere.

The variability that the spacecraft observed in Mercury's
magnetosphere, the volume of space dominated by the planet's magnetic
field, so far supports the hypothesis that the great day-to-day
changes in Mercury's atmosphere may be a result of changes in the
shielding provided by the magnetosphere.

"The spacecraft observed a radically different magnetosphere at
Mercury during its second flyby compared with its earlier Jan. 14
encounter," said James Slavin from NASA's Goddard Space Flight Center
in Greenbelt, Md. Slavin is a mission co-investigator and lead author
of one of the papers. "During the first flyby, important discoveries
were made, but scientists didn't detect any dynamic features. The
second flyby witnessed a totally different situation."

The spacecraft's discovery of the impact basin, called Rembrandt, is
the first time scientists have seen terrain well exposed on the floor
of a large impact basin on Mercury. Landforms such as those revealed
on the floor of Rembrandt usually are buried completely by volcanic

"This basin formed about 3.9 billion years ago, near the end of the
period of heavy bombardment of the inner solar system," said Thomas
Watters from the Smithsonian Institution in Washington, a
participating scientist and lead author of one paper. "Although
ancient, the Rembrandt basin is younger than most other known impact
basins on Mercury."

Half of Mercury was unknown until a little more than a year ago.
Globes of the planet were blank on one side. Spacecraft images have
enabled scientists to see 90 percent of the planet's surface at high
resolution. The spacecraft's nearly global imaging coverage of the
surface after the second flyby gives scientists fresh insight into
how the planet's crust was formed.

"After mapping the surface, we see that approximately 40 percent is
covered by smooth plains," said Brett Denevi of Arizona State
University in Tempe, a team member and lead author of a paper. "Many
of these smooth plains are interpreted to be of volcanic origin, and
they are globally distributed. 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 more information from a third flyby of the planet
on Sept. 29.

"The third Mercury flyby is our final dress rehearsal for the main
performance of our mission, the insertion of the probe into orbit
around Mercury in March 2011," said Solomon. "The orbital phase will
be like staging two flybys per day and will provide the continuous
collection of information about the planet and its environment for
one year. 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."

The MESSENGER project is the seventh in NASA's Discovery Program of
low-cost, scientifically focused missions. The Johns Hopkins
University Applied Physics Laboratory of Laurel, Md., designed, built
and operates the spacecraft and manages the mission for NASA's
Science Mission Directorate in Washington. Science instruments were
built by the Applied Physics Laboratory; Goddard; the University of
Michigan in Ann Arbor; and the University of Colorado in Boulder.
GenCorp Aerojet of Sacramento, Calif., and Composite Optics Inc. of
San Diego provided the propulsion system and composite structure.

For more information about the Mercury mission, visit:

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Last Updated: 30 Apr 2009