WASHINGTON, D.C.--The untrained eye might have trouble distinguishing the latest images from Mercury from familiar images of Earth's moon, but the pictures are surprising experts. The planet "was not the place we expected," says Sean Solomon, the principal investigator of the MESSENGER mission. The first close look at Mercury in 33 years, yielding images released here today at a NASA press conference, has revealed a side never seen: much more volcanism, deep-penetrating impact craters, and a unique feature in the solar system--"The Spider."
Mercury had its last visitor in 1975, when the Mariner 10 spacecraft returned images suggesting that lava once flowed across the surface, at least in places. But volcanism "wasn't accepted by everyone," says MESSENGER team member Louise Prockter of Johns Hopkins University's Applied Physics Laboratory in Laurel, Maryland. The new images have removed the doubt. She pointed to impact craters hundreds of kilometers across with floors so smooth that they must have been partially filled by lava. Team member Robert Strom of the University of Arizona in Tucson also found that the side of Mercury seen by Mariner 10 turns out to be more heavily cratered by impacts than the side seen for the first time by MESSENGER. That means that lava has extensively flooded the MESSENGER side. "There's been a lot of volcanic activity on Mercury," says Strom.
The moon has its volcanic flooding too--witness the dark "seas" or maria of the moon--but MESSENGER found a mercurial variation on such light-dark patterning. Caloris is a huge--1550-kilometer-wide--impact basin glimpsed by Mariner 10 but now seen in its entirety by MESSENGER. On the moon, such giant impact basins were often filled with dark lava to form maria, but Caloris has the opposite pattern. Its interior is lighter and is surrounded by a darker ring. Perhaps the Caloris impact excavated deep, lighter-colored rock and left it at the surface, says Solomon, who works at the Carnegie Institution of Washington's Department of Terrestrial Magnetism in Washington, D.C. Including smaller craters with distinctive dark rims, "we've got a variety of natural drill holes into Mercury's interior," says Solomon.
Then there's “The Spider.” More than 50 troughs radiate from near the center of Caloris where a 40-kilometer-wide crater has formed. Whether this inner crater has anything to do with the radiating troughs, Prockter can't say; no one has ever seen anything like the spider. One possibility is that the formation of Caloris somehow created a plume of molten rock that rose beneath the basin's center, bulging the basin floor upward and cracking the crust to form the troughs. The crater would then have been a coincidental impact.
More surprises may be in store as planetary scientists pore over the images, which are providing the first look ever at one-third of Mercury. They'll be busier still when MESSENGER returns in October for another look at Mercury on its way to entering orbit in 2011.