January 14, 2008

The Enduring Mysteries of Mercury

is the smallest, densest and least explored planet around the sun. More than
half of it is virtually unknown.

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

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

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

Mercury's hidden side

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.

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?

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.

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

Is Mercury shrinking?

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.


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

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?

incredibly tenuous atmosphere is unstable, with gases regularly escaping the
planet's weak gravity. How Mercury's atmosphere gets constantly replenished is

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

Why is Mercury magnetic?

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.

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 after all.

Why all that metal?

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.

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.

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.