Fast-track Planet Building

August 30, 2002 | Two new assessments suggest that the inner solar system assembled much faster than had been previously believed. Teams of researchers led by Qingzhu Yin (Harvard University) and Thorsten Kleine (University of Münster) conclude that Earth had already formed its core — and thus grown to a substantial size — 29 million years after the solar system's birth. That's about half as long as geochemists had assumed, and this compressed time-scale applies to other terrestrial bodies as well, with Mars forming in roughly 13 million years, for example, and the large asteroid 4 Vesta in only 3 or 4 million. The revised chronology derives from assays of the radioactive isotope hafnium-182 and its decay product, tungsten-182. Since the half-life of hafnium-182 is only 9 million years, and because virtually all of a planet's tungsten ends up in its core, hafnium-tungsten ratios provide a very sensitive geochemical clock for core formation. Based on their isotopic studies of meteorites, the two teams conclude that much less hafnium-182 existed in the solar nebula than had been thought, and so a major revision of the core-formation timetable was needed to explain all the tungsten-182 now present in Earth's crust.

The analyses by the Yin and Kleine teams, as well as a commentary by cosmochemist Alastair G. W. Cameron, appear in Nature for August 29th.

LINEAR's Comet Cousins

August 28, 2002 | Even after discovering more than 80 comets, the Lincoln Near Earth Asteroid Research (LINEAR) project can still generate some surprises. On
the night of August 25th, one of the automated telescopes swept up a pair of faint objects in a single frame of its electronic camera. Separated by about ¾°, the call went out from the International Astronomical Union's (IAU) Minor Planet Center for additional observations. The next night, several observers reported seeing comas around the objects. Turns out that these new comets, designated C/2002 Q2 and C/2002 Q3, are related. The two are passing through the solar system in similar parabolic paths. At
discovery, the duo were separated in space by about 2 million kilometers. The pair is heading nearly due south, having already come closest to the Sun a week prior to discovery. Brightening to perhaps no more than 17th magnitude in
early September, they will remain out of reach for visual observing with most amateur equipment.

For more information, see the IAU's Observable Comets page.

Contour Watch Winds Down

August 27, 2002 | Spaceflight engineers have little hope for recovering the Comet Nucleus Tour (Contour) spacecraft, though they will continue to listen periodically for its signal through December. The comet-bound craft was last heard from on August 15th, just before a solid-fuel rocket was to propel it from its temporary Earth orbit into interplanetary space. Telescopic images later showed two objects leaving Earth along (but a little behind) the predicted trajectory. "Obviously, we had a big problem," comments Robert Farquhar, the Contour mission manager at Johns Hopkins University's Applied Physics Laboratory. Although the cause of the mission-ending malfunction may never be determined with certainty, for now the APL team is presuming either that the Star 30 motor ruptured just before the end of its 50-second-long firing, or that heat from the rocket triggered the failure of some other component (such as the tanks of hydrazine fuel used for smaller maneuvers).

The Contour team hopes to convince NASA to build a replacement craft, which might be launched as soon as 2006. However, this effort would overlap — and compete for scarce funds — with the proposed New Horizons mission to Pluto, which APL is also building. Meanwhile, NASA administrator Sean O'Keefe has tasked Theron Bradley Jr., the space agency's chief engineer, to lead the investigation into Contour's loss. Bradley is expected to report his panel's findings in six to eight weeks.

Updates about the ill-fated mission's status appear at http://www.contour2000.org; for a description of New Horizons, go to http://pluto.jhuapl.edu/.

Earth's Oldest Known Impact

August 27, 2002 | After combing through ancient marine sediments in South Africa and Western Australia, geologists have discovered evidence for four globe-rattling impacts that occurred between 3.2 and 3.5 billion years ago. Gary R. Byerly (Louisiana State University, Baton Rouge) and three colleagues have not found the craters themselves, which likely have been obliterated over the eons, but rather the "smoking guns" represented by layers of small spherules that were blasted from each target zone and subsequently rained out of the sky over wide regions. The oldest layer, which Byerly's team pegs at 3.470 ± 0.002 billion years old, contains shocked grains of the mineral zircon, as well as an abundance of the element iridium (which is relatively common in meteorites) roughly 10 times higher than that in typical crustal rocks. The thickness and wide distribution of the spherule layers suggest each blast was 10 to 100 times more powerful than one 65 million years ago that formed a 180-kilometer-wide crater now buried beneath Mexico's Yucatán Peninsula. Only about 160 confirmed impact sites are known on Earth; the previous record-holder for age, at 2 billion years, is the 300-km-wide Vredefort crater in South Africa.

Byerly's team has published its findings in the August 23rd issue of Science. A good summary is found online in a Stanford University press release. A comprehensive list of terrestrial impact craters is maintained at http://www.unb.ca/passc/ImpactDatabase/.

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