A MESSENGER spacecraft image of Mercury
Credit: NASA/APL

For more than three decades, the origins of Mercury's smooth plains and the source of its magnetic field have been a matter of debate. Now, analyses of data acquired by the MESSENGER spacecraft during its January 2008 flyby of Mercury have shown that volcanoes were involved in plains formation and suggest that Mercury’s magnetic field is actively generated in the planet's core.

Furthermore, scientists took their first look at the chemical composition of Mercury's surface. MESSENGER probed the composition of the planet's tenuous atmosphere, sampled charged particles (ions) near the planet, and revealed 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 on 4 July.

The controversy over the origin of Mercury's smooth plains was sparked by the results of the Apollo 16 Moon mission in 1972, which suggested that some lunar plains came from material that was ejected by massive impacts and then formed smooth "ponds."

When the Mariner 10 spacecraft imaged similar structures 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 Mercury's surface through its reflectance and color variation, surface chemistry, high-resolution, 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 basin 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 our 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 larger than 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.

The flyby also made the first-ever observations of the ionized particles in Mercury's unique exosphere. The exosphere is an extremely thin 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.

Further Reading

Mercury
http://www.nineplanets.org/mercury.html
NASA
www.nasa.gov/
MESSENGER
http://messenger.jhuapl.edu/

Aymen Mohamed Ibrahem
Senior Astronomy Specialist