NASA Antenna Cuts Mercury to Core, Solves 30 Year Mystery
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.
Image right: Diagram showing the interior structure of Mercury. The metallic core extends from the center to a large fraction of the planetary radius. Radar observations show that the core or outer core is molten. Image credit: Nicolle Rager Fuller, National Science Foundation + Larger view+ High resolution JPEG (553Kb)
"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.
Image left: Artistic rendering of the observational geometry. A radar signal (yellow) is transmitted from the Goldstone antenna in California. Radar echoes (red) are received at the Goldstone antenna and at the Robert C. Byrd telescope in Green Bank, West Virginia. Image credit: Bill Saxton, NRAO/AUI/NSF + Larger view + High resolution JPEG (911Kb)
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).
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