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Mercury's hidden side revealed

The giant Caloris impact basin is at upper right in

this image of Mercury, captured by NASA's Messenger

probe. Click on the image for a bigger version.

In the wake of this week's successful flyby, the team behind NASA's Messenger probe has released the first picture of a side of the planet Mercury never seen before – a moonlike landscape covered with craters and bright material turned up by impacts.

Yet another image presents a new, up-close view of a double-ringed crater named Vivaldi, which was last seen during the Mariner 10 flyby more than 30 years ago. From here on out, images from the Messenger flyby should be dribbling out on a regular basis via the science team's Web site.

Until now, Mariner 10's flybys in 1974 and 1975 have provided the definitive views of surface details for our solar system's closest-in planet (and the smallest, if you don't count places like Pluto or Eris). Mariner missed seeing more than half the planet, however. This time around, the $427 million Messenger mission - which was launched back in 2004 - will be seeing the whole picture, during three flybys as well as a yearlong orbital mapping phase that's due to begin in 2011.

Monday's initial flyby came as close as 124 miles (200 kilometers) to the surface, zipping past at 16,000 mph (25,000 kilometers per hour). The spacecraft's camera captured about 1,200 images, and the science team is now checking through the best of the bunch, one frame at a time, mission spokeswoman Paulette Campbell told me from the team's headquarters at Johns Hopkins University's Applied Physics Laboratory in Maryland.

The first look, taken from a distance of 17,000 miles (27,000 kilometers) and released Tuesday, reveals one of the prime targets for the imaging team: the 800-mile-wide (1,300-kilometer-wide) Caloris impact basin, one of the biggest and youngest craters in the solar system. Mariner 10 spotted the eastern side of Caloris, but the western portions have never been seen before.

"The new image shows the complete basin interior and reveals that it is brighter than the surrounding regions and may therefore have a different composition," Messenger scientists said in their image advisory. "Darker smooth plains completely surround Caloris, and many unusual dark-rimmed craters are observed inside the basin."

This image of Mercury's surface, taken by NASA's

Messenger probe on Monday, shows the double-

ringed Vivaldi crater in shadow at upper right. Click

on the image for a bigger version.

Today's picture, the second in Messenger's series, shows the 125-mile-wide (200-kilometer-wide) Vivaldi crater, which boasts a double ring and was glimpsed by Mariner 10. This view - taken from a distance of 11,000 miles (18,000 kilometers) - shows new details, including a broad depression overlapped by Vivaldi's lower left rim.

Right now, there are more questions than answers about what Messenger is seeing - but that's what this mission is all about. In addition to paying tribute to the messenger of the Roman gods, the probe's name is a tortured acronym for MErcury Surface, Space ENvironment, GEochemistry and Ranging. To cover all those bases, the spacecraft is carrying a dual imaging system (juggling wide-angle and narrow-angle cameras), four spectrometers, a laser ranging device, a magnetometer and a Doppler radar to gather data about Mercury's mass distribution.

By the time Messenger is done, sometime in the next decade, scientists should be able to resolve some of Mercury's mysteries - such as why the planet is so dense, where its vanishingly thin atmosphere comes from, and whether or not frozen water lurks in deep shadowed craters. The answers should start coming in by Jan. 30, when the science team is due to share their first impressions in a NASA news briefing. Until then, be sure to check the Web sites at APL as well as NASA for your daily dose of Mercury.

Update for 9:30 p.m. ET Jan. 16: Oops, they did it again ... The Messenger team has released yet another picture of Mercury's surface, showing a weird-looking terrain of craters and cliffs. And then there's this picture of a never-before-seen crater with bright rays and dribbling craters extending outward. How long will it be before someone spots the "Face on Mercury"?

Update for 1:30 p.m. ET Jan. 17: ... And again! Today's additions include an even clearer view of a double-ring crater and a look at Mercury's horizon. From here on out, you're on your own to keep tabs on the messages from Messenger.

Update for 9:15 p.m. ET Jan. 18: Some Cosmic Log correspondents have been asking about the oval appearance of most of the craters in the pictures released so far. My guess was that the ovals came through because the spacecraft was taking its pictures from an angle rather than from directly overhead. Ralph McNutt, the Messenger mission's project scientist, confirmed my surmise in a voicemail:

"The craters that look like they're oval in shape really are the result of the oblique viewing perspective. We'll be working through some of that as the processing continues."

McNutt reminded me to tune in on Jan. 30 for the big news briefing, and so I'll remind you as well.