Messages from Mercury

Flyby reveals clues to the planet’s past and present

Mercury, the solar system’s forgotten planet, is finally getting its place in the sun.

HOT STUFF These images of Mercury were taken by the MESSENGER spacecraft during its mid-January flyby. The data reveal signs of volcanic activity on the planet, provide additional evidence that Mercury’s magnetic field was generated by its core and offer the first look at the planet’s surface composition. NASA, JHUAPL, Carnegie Institution

The MESSENGER spacecraft flies past Mercury twice, revealing the origin of the planet’s magnetic field, hinting at the presence of early volcanic activity and providing the first looks at the planet’s surface composition
LAVA WAS HERE The Caloris basin on Mercury (pictured), and other surface features of the planet, are revealed in new detail thanks to images from the MESSENGER craft. The new data suggest the once-overlooked planet has a magnetic field likely generated internally, and possibly has a volcanic history. AAAS/Science

An analysis of data collected during the January flyby of the spacecraft MESSENGER — which will begin a year-long orbit of Mercury in 2011 — has revealed the origin of the planet’s magnetic field, discovered evidence of early volcanic activity and provided a first look at the planet’s surface composition.

Researchers describe their findings in the July 4 Science.

Although Mercury bears a superficial resemblance to Earth’s pockmarked moon — both bodies have been pummeled by rocky debris — volcanism appears to be widespread on the planet, notes Mark Robinson of ArizonaStateUniversity in Tempe. For instance, the ancient Caloris Basin, big enough to hold Arizona, Nevada and California, is filled with smooth plains that appear to have been created by the belching of vast amounts of lava.

In addition, Jim Head of BrownUniversity in Providence, R.I., and his collaborators found evidence of volcanic vents along the edges of Caloris. The data “open up a whole new realm” about how volcanic activity may have shaped the planet, he says.

The only previous mission to examine Mercury close-up — Mariner 10 in the mid-1970s — found no evidence of volcanism. MESSENGER, which came within 200 kilometers of the planet in January and imaged about half of the region not seen by Mariner 10, can see much finer detail.

For 30 minutes of the two-day flyby, MESSENGER recorded the first observations of ionized particles in Mercury’s exosphere, the planet’s tenuous atmosphere. The craft also examined Mercury’s magnetosphere, the vast magnetic bubble surrounding both the planet and exosphere. The wind of charged particles from the sun, as well as micrometeoroids that strike Mercury, lift particles from the surface and inject them into the exosphere and magnetosphere.

The abundances of silicon, sodium and sulfur in the magnetosphere are too high and their ionization states, or charges, too low to be solar wind particles, notes Thomas Zurbuchen of the University of Michigan in Ann Arbor. The particles indeed came from Mercury’s surface, and the magnetosphere observations therefore provide the first view of the surface composition of the planet, his team reports.

Mercury is riddled with faults, caused by the contraction of the planet as its giant core, which accounts for 60 percent of the planet’s mass, slowly cooled, notes MESSENGER lead investigator Sean Solomon of the Carnegie Institution of Washington (D.C.).

The flyby data indicate that the contraction was at least one-third greater than scientists had estimated. Because Earth’s core occupies much less of the planet’s total mass and volume, its contraction doesn’t sculpt Earth’s surface nearly as much.

On Mercury, huge cliffs mark the tops of crustal faults. “This is the one planet in the solar system where we are seeing the effect of internal cooling on the surface,” Solomon says.

The cooling of the inner core, which added heat to outlying regions, also stirred up material in Mercury’s outer core, fueling the dynamo that generated the planet’s magnetic field, Solomon says. The MESSENGER data adds to evidence that the magnetic field is produced internally and is not a vestige of a field produced early in the planet’s history.

“The determination that the field is of internal origin is significant and an important part of understanding Mercury’s history and planets and dynamos in general,” comments David Stevenson of the California Institute of Technology in Pasadena.

In preparation for its year-long sojourn in 2011, MESSENGER — which stands for Mercury Surface, Space Environment, Geochemistry and Ranging — will fly past Mercury two more times, once this October and once in September 2009. These two flybys will offer views of the side of the planet not yet viewed by MESSENGER.

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