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Posted by baalke on May 3, 2007, 5:01 pm
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http://www.jpl.nasa.gov/news/news.cfm?release=2007-050
NASA Antenna Cuts Mercury to Core, Solves 30 Year Mystery
Jet Propulsion Laboratory
May 03, 2007
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. The research appears in this week's issue of the journal
Science.
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
------------------------------------------------------------------------
Media contacts: 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
2007-050
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