Messenger from Mercury

Orbiter returns images of odd landforms on innermost planet

Hot and heavy little Mercury is warming up to NASA’s MESSENGER probe and revealing its true planetary colors — in enhanced-color images. Among the spacecraft’s finds are bizarre landforms (shown here in blue) tucked inside impact craters on the planet’s surface.

PLANET POCKMARKS Irregular hollows on Mercury’s surface appear as odd blue pockmarks in this enhanced-color image from MESSENGER. NASA, JHU APL, Carnegie Institution of Washington

David Blewett of the Johns Hopkins University Applied Physics Laboratory and his colleagues report these puzzling scarlike hollows in the Sept. 30 Science, which features seven papers describing the compact world. The pits resemble sunken Swiss cheese holes — smooth, rimless depressions that vary in size between several meters and a few kilometers across. Irregularly shaped, the clustered hollows don’t look purely volcanic in origin, but instead make some areas appear as though the planet’s innards are leaking away.

That’s exactly what scientists speculate might be happening. They propose that impacts exposed unstable material from Mercury’s interior to the planet’s harsh surface. Whether baked off by the sun, blasted away by the solar wind, blown skyward by volcanoes or simply burped out, the stuff disappeared, leaving these pits behind.

But that’s not all MESSENGER has seen — another report shows that Mercury’s surface itself comprises different ingredients than terrestrial bodies like Earth and the moon. And, the charged planet — though possessing a magnetic field and surrounded by swirling electrons — doesn’t wear radiation belts like Earth’s.

MESSENGER has circled the solar system’s smallest planet since March, carrying an array of instruments that peer at and beneath Mercury’s surface in an attempt to become more familiar with this dense, mysterious world.

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