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The enduring mysteries of Mercury

More than half of Mercury  is virtually unknown. NASA's Messenger probe will be the first spacecraft to image the whole planet.
Mercury, the solar system's most inner planet, still hides many secrets NASA hopes to solve with Messenger.
Mercury, the solar system's most inner planet, still hides many secrets NASA hopes to solve with Messenger. NASA / JPL
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Mercury is the smallest, densest and least explored planet around the sun. More than half of it is virtually unknown.

Insights into this mysterious world of extremes could shed light on how planets were made in our solar system, astronomers say.

NASA's Messenger probe will be the first spacecraft to image the whole planet, making its initial flyby of Mercury Jan. 14 as part of a long process to settle into orbit.

"With Messenger, many of Mercury's secrets will now be revealed," said NASA's planetary science division director James Green. A list of some of these is below.

Mercury's hidden side
The only spacecraft to ever visit the solar system's innermost world — NASA's Mariner 10 — mapped less than 45 percent of Mercury's surface, a heavily cratered landscape. This means more than half the planet is unknown to us, save for relatively poor observations from Earth-based radars.

"We can't get cocky about what the other side of Mercury looks like. So far, every solar system body has looked very different from every other one," said Faith Vilas, director of the Multiple Mirror Telescope (MMT) Observatory at Mt. Hopkins, Ariz. "We're expecting some major surprises from it."

Ice near the sun?
On the closest planet to the sun, where temperatures can reach more than 800 degrees Fahrenheit (425 degrees Celsius), there might surprisingly be ice. Ice is highly reflective to radar, and Earth-based radar suggests deposits of frozen water might be hidden in deep, dark craters at Mercury's poles that have never seen sunlight. This water might have come gassing up from within the planet or from meteorite impacts.

Messenger will search for hydrogen at the permanently shadowed floors of polar craters. If the spacecraft discovers any, Messenger may have found ice amidst an inferno.

Is Mercury shrinking?
Mercury could be shrinking as its core slowly freezes. Pictures from Mariner 10 revealed the planet's surface appears to have buckled from within, resulting in gigantic cliffs more than a mile high and hundreds of miles long biting into Mercury. Messenger will look for any evidence of such crumpling on the world's hidden side and will also study the planet's metal core by analyzing that world's magnetic field.

Do a band of small asteroids dubbed "vulcanoids" lie inward of Mercury's orbit, hidden in the glare of the sun?

Messenger has a chance of spotting these asteroids as it approaches Mercury, although its opportunities are limited. To keep the sun from frying it, Messenger hides itself behind a sunshade pointed at the sun at all times, and its scientific instruments are pointed away from the sun. Nevertheless, scientists will use Messenger "to chase down any hints there might still be a modern population of vulcanoids," said the Messenger mission's principal investigator, Sean Solomon.

Where does Mercury's atmosphere come from?
Mercury's incredibly tenuous atmosphere is unstable, with gases regularly escaping the planet's weak gravity. How Mercury's atmosphere gets constantly replenished is unclear.

Researchers suspect the hydrogen and helium in Mercury's atmosphere is continuously brought there by the solar wind, the supersonic stream of charged particles from the sun. Other gases might have evaporated off Mercury's surface, seeped from inside the planet or been brought in by vaporized meteorites. Messenger will closely study the planet's atmosphere to pinpoint how it gets generated, Vilas said.

Why is Mercury magnetic?
A completely unexpected discovery Mariner 10 made was that Mercury possessed a magnetic field. Planets theoretically generate magnetic fields only if they spin quickly and possess a molten core. But Mercury takes 59 days to rotate and is so small — just roughly one-third Earth's size — that its core should have cooled off long ago.

To solve this mystery, Messenger will probe Mercury's magnetic field. There was some thinking that the field might have become inactive, but last year, scientists discovered Mercury seems to have a molten core after all, so the planet might still be actively generating a magnetic field.

Why all that metal?
Mercury is extraordinarily dense, leading researchers to estimate that its iron-rich core potentially makes up about two-thirds of the planet's mass, a startling figure double that of Earth, Venus or Mars. In other words, Mercury's core might take up roughly three-quarters of the world's diameter.

One theory explaining this bizarre density is that huge impacts billions of years ago might have stripped Mercury of its original surface, Vilas explained, collisions that also shifted the planet toward the sun to its current location. Another theory suggests Mercury simply formed where it now lies.

To see which theory concerning Mercury's origins might be right, Messenger's battery of miniaturized scientific instruments will scope out the planet's geology. Understanding how Mercury formed will shed light on how all the planets evolved, Solomon said.