June 2007
Second
Venus ;yby
January 2008
First
Mercury ;yby
October 2008
Second
Mercury ;yby
September 2009
Third
Mercury ;yby
April 2011
Start of yearlong
data collection
March 2011
Orbit insertion
around Mercury
Mercury emerged. But each theory predicts a different composition for surface
rocks, so analyzing those rocks could
point to the early solar system processes
most important for the planet’s formation and evolution.
Identifying those mechanisms may
offer insight about the formation of
planets beyond the solar system, particularly those born close to their parent
stars. “Almost everything we learn will
be relevant for the interaction of close-in
extrasolar planets with their host stars,”
Solomon says.
Magnetic measurements made during
orbit will further illuminate Mercury’s
interior, including how heat transfers
between layers of material at the planet’s
core. The craft’s flybys already have confirmed a small but persistent magnetic
field, suggesting that Mercury’s core is
divided into two parts — a hot outer portion, made of molten iron, circulating
around a cooler inner core of solid iron.
The churning of the planet’s liquid outer
core could act as a “dynamo,” driving the
magnetic field.
Like the magnetic field of Earth, the
only planet known for sure to have a
liquid-iron dynamo, Mercury’s field
broadly resembles that of a dipole, a
bar magnet with a north and south
pole. But the strength of the dipole field
near Mercury’s core is only about one-thousandth the intrinsic strength of
Earth’s field, and Solomon and his colleagues would like to find out why.
MESSENGER’s magnetometer will
look for deviations from the field that a
perfect bar magnet would generate. By
comparing the observed deviations with
those predicted by different dynamo
theories, scientists hope to zero in on
the model that best explains how the
planet’s weak field comes about.
“The hardest question to answer is
probably the origin of the magnetic
field,” says Francis Nimmo of the University of California, Santa Cruz.
Inside to out
Researchers do know that Mercury’s
persistent magnetic field is part of a
larger story involving a delicate balance
between heating and cooling inside
the planet.
On the one hand, an active dynamo
requires a source of internal heat to
keep the planet’s outer core molten. On
the other hand, both the MESSENGER
and Mariner 10 flybys suggested that
the planet has cooled substantially over
time. Huge cliffs, or scarps, mark the
tops of faults that mar Mercury’s surface,
meaning cooling has caused the planet to
contract like a shriveling raisin.
MESSENGER flybys revealed that the
planet has shrunk about one-third more
than previously estimated, pointing to
a higher rate of cooling. Accounting for
both the internal heating required for
the dynamo and the cooling revealed by
the scarps “has been a conundrum that’s
interested me since Mariner 10,” says
Solomon. MESSENGER’s examination
of the scarps from orbit “will provide
us with a story not only on the amount
of the shrinkage of the planet but when
it happened relative to other geological
processes — the whole history of cooling.”
The rate and timing of the cooling
should also hold clues to how Mercury’s
outer core retains enough heat to remain
molten and thus keep the dynamo and
magnetic field alive.
Though Mercury’s active magnetic
field sets the pockmarked planet apart
from Earth’s moon, the two bodies share
some secrets: Both may possess pockets
of frozen surface water and therefore
hold clues to the abundance of water in
the solar system.
Like some areas of the moon, some
of Mercury’s craters lie in permanent
shadow and may be cold enough to trap
ice deposits delivered by asteroids and
meteorites over millions to billions of
years. Radar studies from Earth have
already suggested the presence of frozen water, and MESSENGER will seek
confirmation by looking for a signature
of hydrogen in the polar regions.
Finding ice on the planet closest to
the sun would further feed the blaze of
renewed interest that MESSENGER
has ignited in Mercury. After presenting scientists with a storehouse of new
knowledge, the spacecraft will end its
mission on March 18, 2012—unless
NASA decides to extend the craft’s tour
for an additional year.
Encores aside, MESSENGER will
run out of fuel in a few years and crash
into Mercury’s surface, melding with
the planet it will have so thoroughly
explored. s
Explore more
s Visit the MESSENGER mission
website at messenger.jhuapl.edu
May 21, 2011 | SCIENCE NEWS | 29
