The New Face of Mercury

Recent analysis of data collected in January 2008 by NASA’s Mercury Surface, Space Environment, Geochemistry and Ranging spacecraft has shown that Mercury’s volcanoes were involved in plain formation and suggest that its magnetic field is actively produced in the planet’s core. The new results put an end to a 30-years old argument regarding the origins of Mercury’s smooth plains and the source of its magnetic field.

During the analysis, scientists took a first look at the chemical composition of Mercury’s surface. The tiny craft probed the composition of the planet’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 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, opinions were divided. Some scientists believed that the same processes were at work, while others thought Mercury’s plains material came from lava from erupted volcanoes. However, the absence of volcanic vents or volcanic features in the images taken prevented a final answer to the debate.

Recently submitted papers to the magazine Science report on analyses of the planet’s surface made during the Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) mission. The parameters measured are reflectance and color variation, surface chemistry, high-resolution imaging at different wavelengths, and altitude. Eventually, evidence of volcanic vents was found just along the margins of the Caloris basin, which is one of the solar system’s youngest impact basins. It was also found that Caloris has a much more complicated geologic history than previously believed.

After reviewing Mercury’s first altitude measurements, the researchers 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 of Mercury. Another result the flyby revealed was 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. Adding to these surprising results is the fact that Mercury’s core makes up at least 60 percent of its mass, unlike any other known terrestrial planet. In comparison, most planets’ cores are about 30 percent of their mass.

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

Remarking on the published results, principal investigator Sean Solomon of 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.”

TFOT has also covered the first Aurora Borealis observation from space, made by a crewmember aboard Space Shuttle Discovery, and NASA’s detection of a ring of material surrounding the magnetic remains of a blasted star, discovered by the Spitzer Space Telescope. Another related TFOT story is the brightest flare ever seen from a normal star other than our Sun, which was visible to the naked eye.

For more information on Mercury’s latest discoveries, see NASA’s website.

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