The Incredible Shrunken Mercury

/ Source: Discovery Channel

With its massive heart of iron, deeply wrinkled crust and long, comet-like tail, Mercury stands apart from Earth and our rocky sister planets in the solar system.

Scientists will soon learn if looks indeed are deceiving. A NASA spacecraft called MESSENGER is expected to complete a circuitous, six-year voyage on Thursday and put itself into orbit around Mercury for a year-long study, becoming the innermost planet's first artificial satellite.

Circling as close as  29 million miles from the sun -- compared to Earth's 93 million -- Mercury feels the brunt of the sun's heat and its muscular gravitational grasp. The planet completes an orbit around the sun in 88 days, compared to the 365 days it takes Earth. Oddly though, Mercury rotates so slowly that 176 days on Earth will pass between a single sunrise and sunset. On Mercury time, two years go by every day.

With only a tenuous atmosphere, Mercury's skies are glaringly bright in the direction of the sun, which looms three times larger than it appears on Earth. Away from sun, it is black as night. And it is the only planet in the solar system that shows signs of shrinkage.

Curving lines of cliffs, known as scarps, stretch for hundreds of miles over most of the planet's surface, mini-mountain ranges that speak to a time when a hotter, slightly larger Mercury cooled and shrank.

"Where the scarps cut into pre-existing impact craters, the craters have either become half, or squeezed horizontally," MESSENGER lead scientist Sean Solomon, with the Carnegie Institution of Washington, told Discovery News.

"The interpretation of the scarps is that they are the surface expression of faults, somewhat like the one Earth experienced with the magnitude-9 earthquake on Friday," he said.

On Earth, the faults stem from the huge horizontal motions of tectonic plates inside the planet. They are constantly grinding against one another, though for most of the time, remain locked. When they suddenly release, such as what happened off the coast of Japan on Friday, an earthquake occurs, releasing energy that's been stored up over years or decades.

"The Mercury scarps are smaller scale examples of those kind of fault motions, though they are not tied to plate tectonics, but to the horizontal shortening of the crust almost everywhere we look," Solomon said.

The amount of contraction, however, is not nearly enough to explain Mercury's great density. Scientists think Mercury contains a substantially higher percentage of iron than Earth, Venus and Mars, the solar system's other rocky planets.

Most of Earth's iron resides in its core, which comprises about 30 percent of the planet's mass. Mercury's iron core, by comparison, is believed to be about 60 percent of the planet's mass.

Mercury may have started off with a different blend of materials when the solar system was forming 4.5 billion years ago. Or perhaps it had the same mix, but lost its lighter elements due to extreme heating, or after a massive impact that blasted a huge chunk of Mercury's developing body into space.

"The mystery is how do you assemble a planet that ends up almost two-thirds metal and one-third rock instead of the more common two-thirds rock and one-third metal?" Solomon said.

Over the next year, scientists hope MESSENGER will relay some answers.

After two sneak previews of Mercury, MESSENGER is scheduled to fire its braking rockets for 15 minutes Thursday night and settle into an orbit that will come as close as 120 miles above the planet's surface. In addition to studying the planet's surface composition, instruments will probe Mercury's magnetic field and look for water ice in permanently shadowed craters near its poles.

"This really is like a whole new mission because we are going to be in orbit," said Andy Calloway, the mission operations manager at Johns Hopkins University Applied Physics Laboratory in Maryland. "We think we're ready to go and are looking forward to it."

Hopkins' Applied Physics Lab is hosting a live webcast as MESSENGER arrives at Mercury starting at 7:55 p.m. EDT on Thursday.