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Lunar Prospector Probes Moon's Core Mysteries

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Science  04 Sep 1998:
Vol. 281, Issue 5382, pp. 1423-1425
DOI: 10.1126/science.281.5382.1423

Most planetary scientists suspect that far beneath the cold craters and dusty seas of the moon lies an important clue to its fiery past—a dense core. Now the modestly outfitted Lunar Prospector spacecraft has gathered gravitational and magnetic data that offer support to those suspicions and hint at the size of this small metallic nugget. The new findings, presented with other Prospector results on page 1475 of this issue, fit with a theory that the core is the remnant of a violent crash between the infant Earth and a Mars-sized body that spawned the moon 4.5 billion years ago.

The discoveries lengthen a string of successes by NASA's desk-sized $63 million robot, launched in January, which spotted signs of frozen water at the moon's poles last winter (Science, 13 March, p. 1628). Skimming just 100 kilometers above the moon's surface, Prospector has put together detailed magnetic maps as well as the most thorough lunar gravitational atlas to date, which reveals hidden concentrations of mass. “These [gravity field] images are remarkable in their clarity,” says geophysicist Gregory Neumann of the Massachusetts Institute of Technology and NASA's Goddard Space Flight Center in Greenbelt, Maryland. “They resolve aspects of the moon that we couldn't see before.”

Researchers charted the moon's gravitational peaks and valleys by using Earth-based radio telescopes to track gravity's subtle tugs on Prospector. The resulting map let them calculate how mass is parceled out in the moon's interior five times more precisely than before, says team leader Alex Konopliv of NASA's Jet Propulsion Laboratory in Pasadena, California. The calculations indirectly constrain the core's radius to between 220 and 450 kilometers—toward the small end of the range if the core is pure iron, and toward the larger end if it is made of a less dense alloy such as iron sulfide.

Such a core would hold 1% to 4% of the moon's mass, says the team. That's good news for proponents of the leading theory of the moon's birth: that it coalesced from the debris of an impact between a Mars-sized protoplanet and the half-formed Earth, 50 million years after the solar system arose. “The giant-impact theory has no problem explaining a core of that size,” says planetary scientist Robin Canup of the Southwest Research Institute in Boulder, Colorado. The iron came mostly from the shattered core of the impactor, according to models by Canup and astrophysicist Alastair Cameron of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. Other scenarios of lunar origin, such as coformation with Earth, capture of a large asteroid, or fission from the young Earth's mantle, call for a larger iron core or none at all, Canup notes.

Lunar Prospector's gravity survey also exposed several new “mascons,” dense blobs of rock, beneath impact basins that are not filled with smooth lava. This suggests that the stronger pull of gravity over these areas results from plugs of dense material that rose toward the surface from the mantle, rather than from lava fills, Konopliv says. “That's a surprising result,” says planetary geophysicist Roger Phillips of Washington University in St. Louis. “It means the moon's outer layers must have cooled off rapidly, within half a billion years, to become rigid enough to support mantle upwarpings.”

Refining the moon's early thermal history may help scientists understand how long the core itself stayed hot. Prospector data suggest it remained molten until at least 3.6 billion years ago, when the era of giant impacts on the surface came to an end. The probe's magnetometer picked up traces of magnetism locked into patches of crust—perhaps sites where big impacts shocked the rocks strongly enough for them to capture an imprint of the magnetic field. The strength of this fossil field implies that at the time, the dynamo action of flowing metal in the core was generating a magnetic field perhaps as strong as Earth's today.

One patch of lunar crust is so intensely magnetized, in fact, that it deflects the solar wind's charged particles away from the surface, just as Earth's own magnetic field does. The phenomenon, first seen in less detail by the Explorer 35 orbiter in 1967, could be “a signature of a strong dynamo field from a molten iron core in the past,” says physicist Robert Lin of the University of California, Berkeley. However, magnetic-field expert Norman Ness of the University of Delaware, Newark, cautions that the crustal imprints could have come from a strong interplanetary magnetic field rather than one internal to the moon.

The gravity and magnetic data seem to “tell a consistent story” about the moon's core, says planetary scientist Lon Hood of the University of Arizona, Tucson, a co-author on both studies. Future probes such as next year's Japanese Lunar-A mission, which will take seismic x-rays of the moon by implanting seismometers on opposite sides, may offer the definitive word on the core's mass and size. Still, Prospector scientists say that NASA's first moon mission since Apollo has taken more than a small step forward in lunar exploration.

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