Does the Moon Have a Titanium Heart?

March 12, 2002 | It’s taken cosmochemists decades to convince themselves that the Moon has a small metallic core (Sky & Telescope, August 1999, page 17). But that conclusion is not without its problems. For example, a recent reanalysis of "moonquakes" recorded on seismometers left by Apollo astronauts argues for a core with a density no greater than about 4.7 grams per cubic centimeters — well below the density of the two leading candidates: pure iron or an iron-sulfur mix. One alternative, put forward yesterday by Mark A. Wieczorek and Maria T. Zuber (MIT) at a meeting of planetary scientists in Houston, Texas, is that the Moon’s center consists of molten silicate highly enriched in titanium. This metal is well known to lunar geochemists: it is common (and sometimes abundant) in the thick lava flows that cover the maria. If the lunar exterior was once completely molten, Wieczorek and Zuber reason that large amounts of dense, titanium-bearing silicates might have "rained out" to form a core some 800 kilometers across.

You can read more about this intriguing proposition in their abstract for the Lunar and Planetary Science Conference:

A Big Chunk of Mars

March 11, 2002 | Japanese geologists have plucked the second-largest known Martian meteorite from the windswept ice fields of Antarctica's Yamato Mountains. Called Y-000593, the 13.7-kilogram (30.3-pound) stone was among the 3,550 meteorites collected by members of the Japanese Antarctic Research Expedition during their 2000-2001 field season. A 1.3-kg stone found in the same vicinity, Y-000749, appears to be another piece of the same infalling object, which apparently broke apart as it passed through Earth's atmosphere. Based on analyses by Naoya Imae and his colleagues, these igneous rocks are roughly 2 billion years old and were blasted from the Martian surface by an impact some 10 million years ago.

Details of the discovery, reported this week at a meeting of planetary scientists in Houston, Texas, are at

SETI Project Hears Farthest Spacecraft

March 11, 2002 | The SETI Institute's Project Phoenix has started another three-week observing run at the giant Arecibo radio telescope, scanning nearby stars for signs of broadcasts by intelligent aliens. Project scientists tested the system on a known source of intelligent signals from beyond the solar system — the Pioneer 10 spacecraft. In honor of Pioneer's 30th anniversary in space, on March 2nd NASA briefly activated its transmitter. Although the transmitter emits only a few watts, at Arecibo the signal "came booming in like a Valkyrie," writes Seth Shostak of the SETI Institute. The craft was 79 astronomical units away (almost 12 billion kilometers, or 11 light-hours), twice the average distance to Pluto and beyond most of the Kuiper Belt.

Shostak's report is at

GRACE to Track Earth's Gravity Glitches

March 11, 2002 | In a technological tour de force, NASA and the German Space Agency plan to launch a pair of satellites this Saturday to track extremely tiny irregularities in Earth's gravitational field. The Gravity Recovery and Climate Experiment (GRACE) will consist of two craft flying in formation about 220 kilometers apart in low orbit, with a ranging system that can track changes in their separation to an accuracy of 10 microns. This should allow mapping of fluctuating ocean levels, underground water tables, atmospheric motions, and other slight gravitational influences from below. The project is also a forerunner for future space-based interferometers and gravitational-wave detectors that will depend on microscopic measurements of separations between spacecraft far apart.

More information is on the GRACE Web site at


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