[meteorite-list] MESSENGER Spacecraft Reveals More Hidden Territory on Mercury

[Ron Baalke]

Nov. 3, 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: 09-257

MESSENGER SPACECRAFT REVEALS MORE HIDDEN TERRITORY ON MERCURY

WASHINGTON -- 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 Sept. 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." 

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, Md. "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. 

"Now we know Mercury's surface has an average iron and titanium 
abundance that is higher than most of us expected, similar to some 
lunar mare basalts," says David Lawrence, an APL participating 
mission scientist. 

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. 

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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