|
+ Play
Audio
|
+ Download Audio | +
Email to a friend | +
Join mailing list
July
3, 2008: Mercury's magnetic field is "alive."
Volcanic vents ring the planet's giant Caloris Basin. And
Mercury has shrunk in on itself more than previously suspected.
These
are just a few of the new discoveries by NASA's MESSENGER
spacecraft, which flew past Mercury on January 14, 2008. The
results are described in a series of 11 papers published in
a special July 4th issue of Science magazine.
Six
of the papers in Science report studies of the planet's
surface--its colors, mineralogy, and the shape of its terrain.
For instance, the color enhanced image below reveals evidence
of volcanic vents along the margins of Caloris basin, one
of the Solar System's largest and youngest impact basins:
Above:
A color image of the Caloris basin and adjacent regions. Orange
hues just inside the Caloris basin rim mark the locations
of features thought to be volcanic. Courtesy of Science/AAAS
[Larger
image] [more]
"By
combining Mariner 10 and MESSENGER data, the science team
was able to reconstruct a comprehensive geologic history of
the entire Caloris basin interior," says James Head of
Brown University, lead author of one of the Science
reports. "The basin was formed from an impact by an asteroid
or comet during a period of heavy bombardment in the first
billion years of Solar System history. As with the lunar maria,
a period of volcanic activity followed, producing lava flows
that filled the basin interior. This volcanism is responsible
for the comparatively light, red material of the interior
plains intermingled with [newer] impact crater deposits."
Finding
volcanic vents around Caloris resolves an old debate among
planetary scientists: Are smooth plains on Mercury, such as
the interior of Caloris basin, caused by erupting lava or
some other process? Lava has won the day.
 Right:
Near the rim of Caloris basin, this broad, smooth dome or
shield-like feature is interpreted to be a volcano. The bright
halo surrounding the kidney-shaped depression is probably
an explosive volcanic eruption deposit. Courtesy of Science/AAAS
[Larger image]
[diagram]
One
of the most exciting results announced in Science
involves Mercury's magnetic field. Until Mariner 10 discovered
Mercury's magnetic field in the 1970s, Earth was the only
other terrestrial planet known to have a global magnetic field.
Earth's magnetism is generated by the planet's churning hot,
liquid-iron core via a mechanism called a magnetic dynamo.
Researchers have been puzzled by Mercury's field because its
iron core was supposed to have cooled long ago and stopped
generating magnetism. Some researchers have thought that the
field may have been a relic of the past, frozen in the outer
crust.
MESSENGER
data suggest otherwise: Mercury's field appears to be generated
by an active dynamo in the planet's core. It is not a relic.
"MESSENGER's
measurements indicate that, like Earth, Mercury's magnetic
field is mostly dipolar, which means it has a north and south
magnetic poles," says lead author Brian Anderson of the
Johns Hopkins University Applied Physics Laboratory (APL)
in Laurel, Md. "The fact that it is dipolar, and that
we did not find the signature shorter-wavelength anomalies
that would signify patches of magnetized crust, supports the
view that we’re seeing a modern dynamo. We are eager for the
October flyby and the year in orbit to see if this is the
case elsewhere on the planet and confirm that the field comes
from the core."
 Right:
A crater deformed by a lobate scarp. Click to view more
examples. Courtesy of Science/AAAS
Mercury's
core makes up 60% of its mass, which is at least twice as
large as any other planet. Cooling of this outsized core has
led to a remarkable contraction of the planet, revealing itself
in the form of cliff-like "wrinkles" called lobate
scarps (pictured right). MESSENGER Principal Investigator
Sean Solomon, at the Carnegie Institution of Washington, explains:
"The
dominant tectonic landforms on Mercury are lobate scarps,
huge cliffs that mark the tops of crustal faults that formed
during the contraction of the surrounding area. They tell
us how important the cooling core has been to the evolution
of the surface. After the end of the period of heavy bombardment,
cooling of the planet's core not only fuels the magnetic dynamo,
but also led to contraction of the entire planet. And the
data from the flyby indicate that the total contraction is
a least one third greater than we previously thought."
The
flyby also made the first-ever observations of charged particles
in Mercury's unique exosphere. The exosphere is an ultrathin
atmosphere where the molecules are so far apart they are more
likely to collide with the surface than with each other. Material
in the exosphere comes mainly from the surface of Mercury
itself, knocked aloft by solar radiation, solar wind bombardment
and meteoroid vaporization:
"MESSENGER
was able to observe Mercury's exosphere in three areas—the
dayside, the day/night line, or terminator, and its 25,000
mile-long (40,000 km) sodium tail," says author Bill
McClintock of the University of Colorado. "Atoms of hydrogen,
helium, sodium, potassium, and calcium have been seen in the
exosphere, and many other elements almost certainly exist
there. These atoms are accelerated away from Mercury by solar-radiation
pressure and form a long tail of atoms flowing away from the
Sun. But their abundances differ depending on whether it's
day or night, effects from the magnetic field and solar wind,
and possibly the latitude."
"Mercury's
exosphere is remarkably active," he marvels.
Another
significant scientific surprise involves Mercury's magnetosphere--the
bubble of magnetism surrounding the planet. Thomas Zurbuchen
of the University of Michigan explains: "Mercury's magnetosphere
is full of many [kinds of charged particles], both atomic
and molecular. What is in some sense a 'Mercury plasma nebula'
is far richer in complexity and makeup than the Io plasma
torus in the Jupiter system." The composition of the
nebula doesn't match that of the solar wind, leading researchers
to conclude "that this material came from the planet's
surface. This observation means that this flyby got the first-ever
look at surface composition."
 Right:
Data from MESSENGERS FIPS sensor reveal the composition of
Mercury's plasma nebula. Courtesy of Science/AAAS [Larger
image] [more]
"When
you look at the planet in the sky, it looks like a simple
point of light," remarked MESSENGER Project Scientist
Ralph McNutt, of APL. "But when you experience Mercury
close-up through all of MESSENGER's 'senses' seeing it at
different wavelengths, feeling its magnetic properties, and
touching its surface features and energetic particles, you
perceive a complex system and not just a ball of rock and
metal."
"It's
remarkable that this rich lode of data came from two days
of imaging, just 30 minutes of sampling the planet's magnetosphere
and exosphere, and less than ten minutes carrying out altimetry
and collecting other data near the time of its closest approach,"
adds Solomon. "MESSENGER's flyby was a huge success."
And
it was just the beginning. Two more flybys are scheduled for
Oct. 2008 and Sept. 2009. Then, MESSENGER will actually go
into orbit around Mercury in 2011. Exciting times lie ahead.
Stay tuned to Science@NASA for updates.
SEND
THIS STORY TO A FRIEND
Editor: Dr.
Tony Phillips | Credit: Science@NASA
|
