Astro News Briefs: May 20–26

The Closest Brown Dwarf to a Star

May 24, 2002 | Hunts for extrasolar planets have turned up an interesting trend: a dearth of brown dwarfs in close orbits around Sun-like stars. But this "brown-dwarf desert" doesn't seem to apply to stars much smaller and dimmer than the Sun. Using an adaptive-optics imager on Mauna Kea's 8-meter Gemini North telescope, Laird Close and others from the University of Arizona surveyed 64 low-mass stars and found very dim companions very near a dozen of them. "We find companions to low-mass stars are typically only 4 astronomical units [about 600 million km] from their primary stars, and this is surprisingly close together," says team member Nick Siegler. Surveys made with the Hubble Space Telescope have found a similar trend for dim pairs to be close, while more massive stars tend to have wider stellar companions. The closest pairing found by Laird's team involves the red dwarf LHS 2379A, located 46 light-years away in the constellation Crater. A brown dwarf hovers just 3 astronomical units from the star in the plane of the sky (their actual separation may be somewhat greater if the companion is a little in front of or behind the primary).

Details and images from Laird's team can be found at:

Europan Waters May Run Deep

May 24, 2002 | Planetary astronomers have been tantalized for two decades by the prospect that a liquid ocean of organic-charged water lies beneath the icy exterior of Jupiter's moon Europa. But efforts to answer the basic question "How far down?" have been inconclusive, frustrating efforts to plan future spacecraft that could tap into the reservoir by boring down from the surface. A new analysis of crater depths by Paul M. Schenk (Lunar and Planetary Institute), which appears in the May 23rd issue of Nature, is further cause for despair. He compared impact features on Europa with those on neighboring Ganymede and Callisto, which are also though to have subsurface oceans. Small craters have a simple bowl shape, but those more than 30 km across on Europa and 150 km across on Ganymede undergo a transition to nests of concentric ripples. Schenk believes this rippling results from the incoming projectile penetrating deeply enough to reach the water — 80 km down for Ganymede, and at least 19 km for Europa.

A fuller description of Schenk's analysis, along with supporting images, appears at:

Did an Impact Help The Dinosaurs?

May 20, 2002 | Most paleontologists now accept the premise that an enormous impact on Earth led to the extinction of the dinosaurs (and most other species) 65 million years ago. But new research, published in the May 17th issue of Science, suggests that a similar but much earlier mass-extinction event created a evoutionary "window of opportunity" for dinosaur species. Fossil evidence shows that large theropods and other dinosaurs proliferated within just 100,000 years of the Triassic-Jurassic boundary and went on to dominate Earth's life for 135 million years. Paul E. Olsen (Columbia University) led a team that analyzed samples from more than 70 sites, most in eastern North America. These revealed a previously undetected spike in the abundance of iridium precisely at the boundary, 202 million years ago. Because iridium is rare in Earth's crust, Olsen's team suggests that it was delivered by an impacting comet or asteroid — though a volcanic source has not been completely ruled out.

Although online access to Science is restricted, a detailed summary appears in Nature magazine's online Science Update.

Martian Plains May Have Been All Wet

May 20, 2002 | Two years ago, based on a spectral maps obtained by Mars Global Surveyor, it looked as if large tracts of Mars's flat northern plains were covered by a silicon-rich, metal-poor rock known as andesite. On Earth, andesite occasionally erupts from volcanoes that have tapped into relatively shallow reservoirs of melted crustal rocks. But it seemed odd to have so much andesite on Mars, even though the analysis team of Josh Bandfield, Vicky Hamilton, and Philip Christensen (Arizona State University) stood behind their conclusion. Last week, however, Michael B. Wyatt and Harry Y. McSween Jr. (University of Tennessee) offered an alternative explanation. They argue that the spectral signature is instead a match to volcanic basalt that has weathered in the presence of water. Although the presence of basalt makes more geochemical sense (it's the dominant rock type in Mars's southern hemisphere), Wyatt and McSween are left to explain where the liquid water came from to alter the original rock. Since the signature is strongest where a Mediterranean-size sea may have once existed, they suggest that massive sheets of lava may have erupted onto Mars's surface — but beneath the ocean — early in the planet's history. Wyatt and McSween report their analysis in the March 16th issue of Nature.

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