[meteorite-list] MESSENGER Spacecraft Reveals a Very Dynamic Planet Mercury

[Ron Baalke]

April 30, 2009

Dwayne Brown 
Headquarters, Washington 
202-358-1726 
dwayne.c.brown at nasa.gov 

Paulette Campbell 
Johns Hopkins University Applied Physics Laboratory, Laurel, Md. 
240-228-6792 
paulette.campbell at jhuapl.edu 

RELEASE: 09-092

MESSENGER SPACECRAFT REVEALS A VERY DYNAMIC PLANET MERCURY

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 
activity." 

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 
flows. 

"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: 

http://www.nasa.gov/messenger   
	
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