[meteorite-list] MESSENGER Spacecraft Reveals New Information About Mercury

[Ron Baalke]

July 15, 2010

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

Mike Buckley 
Johns Hopkins University Applied Physics Laboratory, Laurel, Md. 
240-228-7536 
michael.buckley at jhuapl.edu 
RELEASE: 10-170

MESSENGER SPACECRAFT REVEALS NEW INFORMATION ABOUT MERCURY

WASHINGTON -- 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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