Scientists poring over the first closeup pictures of Mercury in almost 33 years say they're rediscovering a "dynamic" planet brimming with features they've seen nowhere else in the solar system.
The new images were captured Jan. 14 by NASA's Maryland-built Messenger spacecraft, which is being managed by scientists and engineers at the Johns Hopkins University's Applied Physics Laboratory near Laurel.
One of the most puzzling images is that of a 25-mile-wide crater in the middle of Mercury's broad Caloris impact basin. It is surrounded by perhaps 50 troughs radiating from the central crater like spokes on a wheel.
Impact experts have dubbed the odd feature "the Spider," and they say they are unsure of its origins. Somehow, the crust of the planet seems to have been pushed upward and cracked from a central point, like a pane of glass.
"One suggestion for its formation is maybe a volcanic intrusion below the surface," said Louise Prockter, a mission instrument scientist at the Applied Physics Laboratory. "Another is that the basin has undergone a ... rebound after its formation."
The "Spider" was just one of the mission's initial discoveries described at the first formal NASA news conference since Messenger's Mercury flyby.
"We were continually surprised. This was not the planet we expected," said Sean Solomon of the Carnegie Institution in Washington and the mission's principal investigator. "It is a very dynamic planet. There is an awful lot going on."
Messenger's instruments charted the solar wind and charged particles that pummel the planet's surface and tenuous atmosphere, 30 million miles from the sun.
It captured the first high-resolution images of sodium and hydrogen atoms that are blasted off the planet by the sun's energy, like beach sand lofted by a stiff wind, especially from the planet's northern hemisphere. They are blown into a tail that extends into space away from the sun.
Messenger measured Mercury's global magnetic field - the only one except Earth's among the inner, rocky planets - and confirmed that it is dipolar, like Earth's.
The spacecraft's cameras found evidence of volcanic craters and ancient lava flows that have smoothed the planet's surface.
Laser altimeter data - measured along 2,000 miles (20 percent) of Mercury's equator - have charted craters nearly two miles deep and peaks more than three miles high.
Scientists said they are eager to study and map the remaining 25 percent of Mercury's surface that has never been seen when Messenger flies by the planet again in October. It is scheduled to make a third pass in 2009, then spend a year in orbit beginning in March 2011.
"We're chasing some basic questions that have been with us for 20, 30 years," some since NASA's Mariner 10 last visited in 1975, Solomon said.
"I'm looking at a new planet, and it's as exciting as it can be,"said Robert Strom, 74, a planetary scientist on the Messenger team and the only one who also served on the Mariner 10 mission in the 1970s. For schoolchildren following the mission, he advised, "If you want to get high, stay off drugs and get on science."
To see additional photos and a video, go to baltimoresun.com/mercury