[meteorite-list] NASA Reveals New Discoveries From Mercury (MESSENGER)

[Ron Baalke]

July 3, 2008

J.D. Harrington
Headquarters, Washington 
202-358-5241
j.d.harrington at nasa.gov

Mike Buckley
Johns Hopkins University Applied Physics Laboratory, Laurel, Md.
240-228-7536
mike.buckley at jhuapl.edu

RELEASE: 08-166

NASA REVEALS NEW DISCOVERIES FROM MERCURY

GREENBELT, Md. -- Scientists have argued about the origins of 
Mercury's smooth plains and the source of its magnetic field for more 
than 30 years. Now, analyses of data from the January 2008 flyby of 
the planet by the Mercury Surface, Space Environment, Geochemistry 
and Ranging (MESSENGER) spacecraft have shown that volcanoes were 
involved in plains formation and suggest that its magnetic field is 
actively produced in the planet's core.

Scientists additionally took their first look at the chemical 
composition of the planet's surface. The tiny craft probed the 
composition of Mercury's thin atmosphere, sampled charged particles 
(ions) near the planet, and demonstrated new links between both sets 
of observations and materials on Mercury's surface. The results are 
reported in a series of 11 papers published in a special section of 
Science magazine July 4.

The controversy over the origin of Mercury's smooth plains began with 
the 1972 Apollo 16 moon mission, which suggested that some lunar 
plains came from material that was ejected by large impacts and then 
formed smooth "ponds." When Mariner 10 imaged similar formations on 
Mercury in 1975, some scientists believed that the same processes 
were at work. Others thought Mercury's plains material came from 
erupted lavas, but the absence of volcanic vents or other volcanic 
features in images from that mission prevented a consensus. 

Six of the papers in Science report on analyses of the planet's 
surface through its reflectance and color variation, surface 
chemistry, high-resolution imaging at different wavelengths, and 
altitude measurements. The researchers found evidence of volcanic 
vents along the margins of the Caloris basin, one of the solar 
system's youngest impact basins. They also found that Caloris has a 
much more complicated geologic history than previously believed. 

The first altitude measurements from any spacecraft at Mercury also 
found that craters on the planet are about a factor of two shallower 
than those on Earth's moon. The measurements also show a complex 
geologic history for Mercury. 

Mercury's core makes up at least 60 percent of its mass, a figure 
twice as large as any other known terrestrial planet. The flyby 
revealed that the magnetic field, originating in the outer core and 
powered by core cooling, drives very dynamic and complex interactions 
among the planet's interior, surface, exosphere and magnetosphere. 

Remarking on the importance of the core to surface geological 
structures, Principal Investigator Sean Solomon at the Carnegie 
Institution of Washington said, "The dominant tectonic landforms on 
Mercury, including areas imaged for the first time by MESSENGER, are 
features called lobate scarps, huge cliffs that mark the tops of 
crustal faults that formed during the contraction of the surrounding 
area. They tell us how important the cooling core has been to the 
evolution of the surface. After the end of the period of heavy 
bombardment, cooling of the planet's core not only fueled the 
magnetic dynamo, it also led to contraction of the entire planet. And 
the data from the flyby indicate that the total contraction is a 
least one-third greater than we previously thought."

The flyby also made the first-ever observations of the ionized 
particles in Mercury's unique exosphere. The exosphere is an 
ultrathin atmosphere in which the molecules are so far apart they are 
more likely to collide with the surface than with each other. The 
planet's highly elliptical orbit, its slow rotation and particle 
interactions with the magnetosphere, interplanetary medium and solar 
wind result in strong seasonal and day-night differences in the way 
particles behave.

For more information, visit:

http://www.nasa.gov/messenger

or

http://messenger.jhuapl.edu/mer_flyby1.html

	
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