Mercury, named after the mercurial god of trade who movedswiftly from place to place, has always been just out of scientists' reach.
The elusive planet is hard to glimpse from Earth, as itbarely rises above the horizon, and it has received far less attention fromNASA probes than its flashy cousins, Jupiter, Saturn and Mars.
Many scientists hope enigmatic Mercury will finally revealits secrets through data sent back by NASA's MESSENGERspacecraft, which flew by the planet Jan. 14, but so far the probe has onlyexposed new mysteries.
The mission uncovered strange differences between Mercury'scraters and the moon's, puzzling facets of its improbable atmosphere andmagnetic field, and an unexplained feature called "thespider," the likes of which scientists have never encountered before.
When ancient Greek astronomers saw it rising briefly atsunrise and sunset, they thought the planet was two different objects, namingthe morning body Apollo and the evening appearance Hermes. It wasn't until the5th century B.C. that Pythagoras suggested the two were one.
For a long time, scientists believed the same side ofMercury always faced the sun, similar to the way the moon orbits the Earth.1960s measurements that the "dark" side of the planet was much hotterthan expected called this notion into question, and subsequent radarobservations of its orbit overthrew the theory.
Mercury's small size (it gained the title of smallest planet inthe solar system when Pluto was robbedof that distinction) and unassuming appearance have led scientists to underestimate the planet. It waslong assumed to be merely a larger version of Earth's moon, where nothing muchwas happening.
That idea flew completely out the window when NASA's newspacecraft recently passed by Mercury. The probe revealed a "whole newplanet," said Robert Strom, a scientist on the mission team.
This bizarre world is the only other inner solar system planetbesides Earth to have a magnetic field. Scientists suspect the magnetospheremight be caused by the equally strange presence of an extremely dense iron corethat accounts for two-thirds of Mercury's mass.
Pockmarked it certainly is — its grayish surface bears thescars of millions of years worth of space rocks slamming into it. But these craters seem different than the Moon's, and somebear features unlike anything seen elsewhere in the solar system.
The new photos also revealed a bizarre crater surrounded byradiating cracks in the ground that the scientists have dubbed "thespider." They are unsure of what could have caused such a formation, butsuggest volcanism might have been involved.
The spacecraft's recent flyby only whetted astronomers'appetites for more information about this peculiar world.
"It was not the planet we expected," said SeanSolomon, the principle investigator of the mission. "It?s changing veryrapidly, with features that haven?t been seen on any other planet. It's a verydynamic planet with an awful lot going on."
"The best is yet to come," mission member Stromsaid. "What you're seeing here is just the tip of the iceberg. I am very excited."
- Top 10 New Mysteries of Mercury
- VIDEO: MESSENGER at Mercury
- IMAGES: Explore the Planet Mercury
