whole new side of Mercury has been revealed in pictures taken by NASA's MESSENGER probe, which flew by the tiny planet two weeks ago in the first mission to Mercury in more than three decades.
MESSENGER skimmed only 124 miles (200 kilometers) over Mercury's surface on Jan. 14, in the first of three passes it will make before settling into orbit March 18, 2011.
The photos, released today, include one of a feature the scientists informally call "the spider," which appears to be an impact crater surrounded by more than 50 cracks in the surface radiating from its center.
Scientists are perplexed by this structure, which is unlike anything observed elsewhere in the solar system.
Messenger's Pictures From Mercury Surprise Scientists
The Messenger spacecraft that sped past Mercury on Jan. 14 sent back pictures of a geological formation never seen before in the solar system: a central depression with more than 100 narrow troughs radiating out from it.
Called "The Spider" by scientists analyzing the trove of images and data coming back from Messenger, the puzzling feature is the kind of surprise that researchers live for.
"Messenger has sent back data near perfectly, and some of it confirms earlier understandings, and some of it tells us something brand-new," said principal investigator Sean C. Solomon. "The Spider is definitely in the category of something we never imagined we'd find."
Scientists were also surprised by evidence of ancient volcanoes on many parts of the planet's surface and how different it looks compared with the moon, which is about the same size. Unlike the moon, Mercury has huge cliffs, as well as formations snaking hundreds of miles that indicate patterns of fault activity from Mercury's earliest days, more than 4 billion years ago.
"It was not the planet we expected," said Solomon, of the Carnegie Institution of Washington. "It's a very dynamic planet with an awful lot going on."
Messenger passed by Mercury after a journey of more than 2 billion miles. It will swing by the planet twice more before settling into orbit around it in 2011.
Mercury is among the least understood planets because its proximity to the sun makes it hard to visit and to explore. Among the mysteries researchers hope to unravel is where and how Mercury was formed and the nature of the magnetic fields around it. Earth is the only other planet with such an active magnetosphere.
Solomon said clues into whether Mercury once orbited much farther from the sun, as theorized by many scientists, may emerge as the craft begins to orbit and conducts a chemical analysis of the surface.
Among the early findings is that a crater called Caloris is larger than researchers thought after the Mariner 10 spacecraft sent back the first images of the planet 33 years ago. Scientists now believe it is more than 950 miles wide. The Caloris basin, created by a long-ago asteroid strike, is home to "The Spider."
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Surprise Slosh! Mercury's Core is Liquid
Kitchen physics dictates that a raw egg will spin slower than a hard-boiled one. Scientists using this same logic have discovered the planet Mercury has a fluid core of molten iron.
The finding, detailed in the May 4 issue of the journal Science, solves a 30-year-old mystery but raises another.
To figure out whether Mercury's core was liquid or solid, a team of scientists led by Jean-Luc Margot at Cornell University measured small twists in the planet's rotation. They used a new technique that involved bouncing a radio signal sent from a ground telescope in California off the planet and then catching it again in West Virginia.
After 5 years and 21 such observations, the team realized their values were twice as large as what would be expected if Mercury's core was solid.
"The variations in Mercury's spin rate that we measured are best explained by a core that is at least partially molten," Margot said. "We have a 95 percent confidence level in this conclusion."
A polluted core
Mercury, named after the Roman gods' fleet-footed messenger, is the closest planet to our Sun. One year there is equal to 88 Earth-days. Mercury is thought to consist of a thin silicate mantle encasing an iron core. Because it is so small-its mass is only 5 percent of Earth's-scientists thought it cooled rapidly early in its formation, essentially freezing any liquid core it had into a solid.
But 30 years ago, a flyby of the planet by the Mariner 10 spacecraft detected a weak magnetic field, about 1 percent as strong as Earth's, within the planet. Magnetic fields are generally associated with a dynamic molten core.
Margot and his team speculate that sulfur or some other light element got mixed with Mercury's iron core when the planet was forming and lowered its melting temperature.
"If you had such a lighter element polluting the iron, it could explain why the core has remained fluid up to the present time," Margot told SPACE.com.
"The surprise," Margot added, "is that you don't expect sulfur to condense out at the distance of Mercury from the Sun."
Radial mixing
That unexpected result fails to jibe with standard planet formation theory. That theory asserts that planets form out of the swirling disks of gas and dust that swaddle newborn stars. Within this "protoplanetary" disk, elements condense and solidify out at different distances from the star depending on their densities.
Heavy elements with high melting points, such as iron, nickel and silicon, condensed into solids closer to the star. Out of these solids, planet embryos form. These "planetesimals" sometimes become full-fledged planets. This is why the inner planets in our solar system-Mercury, Venus, Earth and Mars-are formed mostly of these heavy elements. Lighter elements such as sulfur can only solidify farther out from the star, where it is cooler.
The new findings suggests there was some "radial mixing" happening early in the solar system's history, with lighter elements at the fringes of the solar system being transported inward, possibly by planetesimals interacting gravitationally with each other.
"Since you don't expect sulfur to condense in solid form there when [Mercury] formed, it must have been brought in from farther out in the solar system," Margot said.
The mysteries still surrounding Mercury's core might be solved when NASA's Messenger spacecraft makes its first flyby of the planet in 2008.
"It is our hope that Messenger will address the remaining questions that we cannot address from the ground," Margot said.
The researchers made their measurements using the NASA Jet Propulsion Laboratory 70-meter antenna at Goldstone, California, and the Green Bank Telescope in West Virginia. They also sent signals from Arecibo Observatory in Puerto Rico and received them back at Goldstone.
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