Mercury Flyby Reveals New Oddities
NASA's MESSENGER spacecraft caught this view of the planet Mercury during an Oct. 6, 2008 flyby.
Credit: NASA/JHUAPL.

Scientists are tackling new oddities at the planet Mercury via images beamed home by a NASA spacecraft that zoomed past the small, rocky world earlier this month.

NASA’s MESSENGER spacecraft returned 1,287 new views of Mercury during an Oct. 6 flyby, the second this year, which revealed nearly a third of the planet that had never seen before. The result: visions of Mercury’s western hemisphere showing an area that is 30 percent smoother than the planet’s eastern half.

“We need to think hard about why that’s actually the case,” said Maria Zuber, a MESSENGER co-investigator at MIT in a Wednesday briefing.

During this month’s encounter, MESSENGER’s cameras caught an odd feature on Mercury, a so-called “wrinkle ridge” some 1,968 feet (600 meters) high, about twice the height of similar features seen on Mars, suggesting the planet has contracted in on itself considerably as it cooled, Zuber added.

The new photos show some empty craters on Mercury lying next to others nearby that are filled in by vast, solidified lava flows. One such crater was filled with so much solidified lava that, on Earth it would bury the Baltimore-Washington, D.C., metropolitan area in a layer 12 times the height of the 555-foot (169-meter) Washington Monument.

“That’s an awful lot of volcanic material in one place for such a little planet,” Zuber said. “A lot has been going on inside of it to cause melting that then extruded to the surface.”

Images from MESSENGER’s first swing past Mercury earlier this year yielded evidence that ancient volcanic activity, not space rock impacts, shaped the planet’s smooth plains. Altogether, MESSENGER's two flybys and NASA's earlier Mariner 10 mission have mapped about 95 percent of Mercury's surface, reserachers said.

Other instruments aboard MESSENGER found strong interactions between Mercury’s magnetic field and the sun’s solar wind, which led to supercharged energy exchanges equivalent to the output of about 100 medium-sized power plants on Earth, researchers said.

“That was just a knock-your-socks-off observation,” said Brian Anderson, MESSENGER’s deputy project scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. “We weren’t expecting it to be that intense at all.”

The spacecraft also spotted magnesium in Mercury’s wispy, tenuous atmosphere for the first time. Infrared views of the planet’s surface revealed more views of material that currently defies identification, but shows up dark blue in enhanced images.

“We really want to get into orbit so we can get some geochemical measurements of this material so I can stop saying blue material,” said Mark Robinson, a MESSENGER co-investigator at Arizona State University, adding that the odd stuff could be some sort of opaque mineral.

MESSENGER, short for MErcury Surface, Space ENvironment, GEochemistry, and Ranging, is the first spacecraft to take a close look at Mercury since NASA’s Mariner 10 probe zipped by the planet three times in 1974 and 1975.

But unlike Mariner 10, the $446 million MESSENGER mission is destined to circle Mercury for at least a year once it enters orbit on March 18, 2011. The spacecraft launched in August 2004 and flew by Earth once and Venus twice before zipping past Mercury for the first time on Jan. 14 to use each planet’s gravitational pull to tweak its flight path for the 2011 rendezvous.

The Oct. 6 Mercury flyby was MESSENGER’s second of three planned swings past the planet. The third is set for September 2009.