The solar system's innermost planet is not a pleasant place. Solar storms constantly bombard its surface, and intense daytime heat sizzles everything except its permanently shaded polar craters. NASA's Mariner 10 flew by Mercury three times in 1974 and 1975, but it has taken NASA 33 years to launch a return trip. 

That's where the Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) spacecraft enters the picture.

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"MESSENGER is basically going to hell in the sense that you can't pick a worse part of the solar system to visit," says Professor Robert G. Strom of the University of Arizona's Department of Planetary Sciences.

Send a mission to Mercury and you risk frying it with solar radiation or wasting tremendous amounts of fuel to correct against the influence of solar gravity.

If that's the case, why did NASA launch MESSENGER in 2004, and why do we eagerly await its orbital insertion in 2011? Unraveling the mysteries of the sun's smallest planet may unlock the secrets of the early solar system.  

Mercury's Iron Core

By planetary standards, Mercury is small. Signs of compression in the planet's plate tectonics even suggest that cooling has actually caused it to shrink over time. What it lacks in size, however, it more than makes up in density.

"Mercury has an extremely high density," Strom says. "In fact, the uncompressed density of Mercury is higher than any other planet or satellite in the solar system: 5.44 grams per cubic centimeter."

Scientists believe that Mercury's core — an immense iron body that accounts for roughly 75 percent of the entire planet's mass — has helped the planet to win the density prize. They also suspect a fluid outer core generates the planet's magnetic field though convection. Of all the inner planets, only Mercury and Earth boast such a field.

Scientists have three theories as to why Mercury has such an unusual inner structure. The selective accretion theory chalks it up to high gas density in the early solar system, during the planet's formation. Post-accretion vaporization places the blame on intense solar radiation bombardments that stripped away most of the planet's mantle. The giant impact theory, on the other hand, attributes that mantle-stripping violence to a planet-sized impact. 

Which theory is correct? Just leave that to MESSENGER.

"Fortunately, each hypothesis predicts a very different mantle composition," Strom says. "We don't know the composition of Mercury well. That's why MESSENGER is loaded with instruments to determine the surface composition. Because once we know that, we can infer which of the three hypotheses is correct."

Once scientists sample Mercury's surface, they'll be able to estimate the planet's overall bulk composition.

"Once we have that, we'll have a much better solar system origin model," says NASA's Pamela Clark, author of "Dynamic Planet: Mercury in the Context of its Environment."

Eventually, we may even gain an improved model for solar system formation in general, Clark says.

Scientists would also like a better understanding of how solar storms push through the electromagnetic field and touch the planet's surface, but this will have to wait for a future mission.

"With an orbiter, we can't really do a three-dimensional snapshot of Mercury's fields and particle environment," Clark says. "I worked with a group that actually proposed a multi-probed flyby to do just that — to resolve it as both a temporal and spacial feature. But we're not going to have that. There's an instrument on MESSENGER that looks at energetic particles, but that's about it." 

Even as NASA prepares to unravel one major mystery about Mercury, others will linger. MESSENGER completed its third flyby on Sept. 29, 2009. NASA expects the spacecraft to enter Mercury's orbit on March 18, 2011.

What's on Merucry? Check back next year to find out.