SPACE GEOSCIENCE ARTICLE
Mercury has a molten core
entered by MIKE BALDWIN
news release by DC Agle/Carolina Martinez; NASA Jet
Propulsion Laboratory
05.03.2007: Pasadena, CA:
Researchers working with high-precision planetary radars,
including the Goldstone Solar System Radar of NASA's Jet
Propulsion Laboratory, Pasadena, Calif., have discovered
strong evidence that the planet Mercury has a molten core.
The finding explains a more than three-decade old planetary
mystery that began with the flight of JPL's Mariner 10
spacecraft.
Launched in Nov. 1973, Mariner 10 made three close approaches
to Mercury in 1974 and 75. Among its discoveries was that
Mercury had its own weak magnetic field - about one percent as
strong as that found on Earth.
"Scientists had not expected to find a magnetic field
at Mercury," said Professor Jean-Luc Margot of Cornell
University, Ithaca, N.Y., leader of the research team.
"Planetary magnetic fields are associated with molten
cores, and the prevailing theory was the planet was too
small to have a molten core."
Scientists theorized that Mercury consisted of a silicate
mantle surrounding a solid iron core. This iron was considered
solid - or so the theory went - because small planets like
Mercury cool off rapidly after their formation. If Mercury
followed this pattern, then its core should have frozen long
ago.
Many believed the Mercury mystery would only be resolved if
and when a spacecraft landed on its aggressively toasty
surface. Then, in 2002, scientists began pointing some of the
most powerful antennas on our planet at Mercury in an attempt
to find the answer.
"On 18 separate occasions over the past five years, we
used JPL's Goldstone 70-meter [230-foot] antenna to fire a
strong radar signal at Mercury," said Planetary Radar
Group Supervisor Martin Slade of JPL, a co-author of the
paper. "Each time, the radar echoes from the planet
were received about 10 minutes later at Goldstone and
another antenna in West Virginia."
Measuring the echo of particular surface patterns from the
surface of Mercury and how long they took to reproduce at both
Goldstone and the Robert C. Byrd Green Bank Telescope in West
Virginia allowed scientists to calculate Mercury's spin rate
to an accuracy of one-thousandth of a percent. The effect was
also verified with three more independent radar observations
of Mercury transmitted from the National Science Foundation's
Arecibo Observatory in Puerto Rico.
With these data the science team was able to detect tiny
twists in Mercury's spin as it orbited the sun. These small
variations were double what would be expected for a completely
solid body. This finding ruled out a solid core, so the only
logical explanation remaining was that the core - or at the
very least the outer core - is molten and not forced to rotate
along with its shell.
Maintaining a molten core over billions of years requires that
it also contain a lighter element, such as sulfur, to lower
the melting temperature of the core material. The presence of
sulfur supports the idea that radial mixing, or the combining
of elements both close to the sun and farther away, was
involved in Mercury's formation process.
"The chemical composition of Mercury's core can
provide important clues about the processes involved in
planet formation," said Margot. "It is fundamental
to our understanding of how habitable worlds -- planets like
our own -- form and evolve."
Mercury still has its share of mysteries. Some may be solved
with the NASA spacecraft Messenger, launched in 2004 and
expected to make its first Mercury flyby in 2008. The
spacecraft will then begin orbiting the planet in 2011.
“It is our hope that Messenger will address the
remaining questions that we cannot address from the
ground,” said Margot.
The study's other co-authors include Stan Peale of the
University of Santa Barbara in California; Raymond Jurgens, a
JPL engineer, and Igor Holin of the Space Research Institute
in Moscow, Russia.
The Goldstone antenna is part of NASA's Deep Space Network
Goldstone station in Southern California's Mojave Desert.
Goldstone's 70-meter diameter antenna is capable of tracking a
spacecraft traveling more than 16 billion kilometers (10
billion miles) from Earth. The surface of the 70-meter
reflector must remain accurate within a fraction of the signal
wavelength, meaning that the precision across the
3,850-square-meter (41,400-square-foot) surface is maintained
within one centimeter (0.4 inch).
For more information about NASA and agency programs on the
Internet, visit:
http://www.nasa.gov
REFERENCE: DC Agle/Carolina Martinez 818-393-9011/354-9382 Jet
Propulsion Laboratory, Pasadena, Calif.; Dwayne Brown/Tabatha
Thompson 202-358-1726/3895 NASA Headquarters, Washington; May
13, 2007. Image credit: Nicolle Rager Fuller, National Science
Foundation.
This information is presented for educational purposes under
the provisions of the Fair Use Act of 1976.
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