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See Inside April 2008

Mercury Rising

Pockmarked Mercury may superficially look like Earth’s moon, but close-up images show that it is anything but moonlike. NASA’s MESSENGER spacecraft transmitted these and other images during its first flyby of the scorched planet on January 14. Scientists last saw such detail when Mariner 10 zipped past 33 years ago, and thanks to better equipment and different lighting angles, MESSENGER has gleaned new information.

Equipped with 11 color filters, MESSENGER’s eyes can see beyond what human vision can detect. In the false-color photograph, constructed from three different color-filtered images, younger craters—those no more than 500 million years old—reveal themselves with a faint bluish tinge. The craft has also spied many new “scarps,” or faults, extending hundreds of kilometers. Such a scarp runs vertically along the right edge of the image (bottom), which encompasses a horizontal extent of about 200 kilometers. Scarps probably formed when Mercury’s interior cooled and the planet shrunk, cracking the surface.

MESSENGER—which stands for Mercury surface, space environment, geochemistry, and ranging—also determined that Mercury’s giant Caloris basin, among the biggest impact craters in the solar system, spans 1,500 kilometers—nearly one third of the planet’s diameter and 200 kilometers more than previous estimates. Inside the basin and unseen by Mariner 10 is a structure scientists are calling “the spider”—made of troughs radiating outward that probably mark the areas where the basin floor broke apart after its formation.

The craft will revisit Mercury this October and again in September 2009 before settling into an orbit around the planet in March 2011. It will continue to snap pictures and laser-map the topography, as well as take readings of the planet’s magnetosphere.

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