#22: Mercury Reveals Its Secrets

The planet comes into focus during NASA's first visit in 33 years.

By Karen Wright|Thursday, December 18, 2008
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Previously unseen terrain on Mercury, with a two-ring impact crater at bottom.
Image courtesy of NASA

In January a jubilant NASA mission crew greeted detailed images of nearly 6 million square miles of the surface of Mercury shot by Messenger, the first probe to visit the poorly understood, innermost planet in 33 years.

During two days of observation, Messenger surveyed vast stretches of Mercury’s surface that had lain in darkness when NASA’s Mariner 10 probe made its last flyby in 1975. In October Messenger swept by again and captured most of the remaining uncharted terrain on the solar system’s smallest planet (little Pluto is now a “dwarf planet”).

“Mercury wasn’t completely a blank slate, but it was nearly so,” says principal investigator Sean Solomon of the Carnegie Institution of Washington. Planetary scientists had long wondered whether its plains were made of hardened lava, for example, and whether its towering escarpments resulted from the cooling and contracting of the planet’s core. Messenger’s data shows evidence of both volcanism and contraction. Researchers also got their first full look inside the Caloris basin, one of the biggest impact craters in the solar system.

Messenger’s flyby trajectories are designed to slow the craft for eventual capture by Mercury’s gravitational field. The probe will swing past Mercury one last time next September before entering orbit in 2011. Planetary scientists can’t wait. “Three decades is a long time to go without seeing a planet that’s often the closest one to Earth,” Solomon says.

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