On the horizon

Gravitational field all shook up

Last December's tragic earthquake off Sumatra left its mark well beyond Earth's surface. It also shifted the shape of the planet's gravitational field. That's the conclusion drawn by Italian scientists at the University of Milan. They note that the quake generated an undersea ledge in the northeast Indian Ocean some 20 feet high and more than 620 miles long. It would take six years' worth of melting in the Patagonian ice fields of South America to alter the planet's gravitational field to the same degree the quake did in the blink of an eye. The researchers describe their findings in EOS, a weekly publication of the American Geophysical Union.

By human standards the change is imperceptible. But the European Space Agency is set to launch a satellite next year that should be able to map the quake's imprint on Earth's gravity. ESA's Gravity Field and Ocean Circulation Explorer is expected to accurately estimate how much mass moved with December's quake.

The rock band ZZ Top got it right when they sang "every girl's crazy 'bout a sharp dressed man" - at least for male songbirds. While their brilliant plumage entices potential mates, many believe it also serves as a bull's-eye for predators.

But apparently, male songbirds can strut their stuff without attracting the attention of a hungry hawk, say a research team from Sweden's Uppsala University.

While songbirds can see ultraviolet reflections, some avian predators cannot. Using computer simulations of how bird vision works, the team found that songbirds in Swedish forests have key markings that look relatively dull to avian predators, whose eyes are most sensitive to the violet end of the visible spectrum. The results, the team says, indicate that sensory systems need to be taken into greater account when studying animal behavior, ecology, and evolution. The results appear in the current edition of the Proceedings of the National Academy of Sciences.

Researchers at the University of California at Los Angeles have designed a small device that generates neutrons whose traits signal "fusion reactions." It is a far cry from the huge reactors researchers hope will one day generate electricity. But the UCLA team suggests that its apparatus could lead to propulsion units for tiny spacecraft. Others say that with refinement, the device could replace the bulky, energy-hungry neutron sources used to screen luggage.

The team heated a lithium-based crystal in a vacuum chamber holding deuterium gas, a heavy form of hydrogen. As the crystal got hot, it produced an electric field. The field grew so large that it stripped electrons from the deuterium atoms and sent the remaining protons hurtling toward a deuterium target. Some of the high-speed protons fused with protons in the target, producing a large stream of high-energy neutrons.

The team reported its results in Thursday's edition of the journal Nature.

Bluefin tuna are highly prized and heavily fished. Despite international conservation efforts, their population has continued to shrink since the 1970s. But researchers tracking the bluefin are providing insights that may lead to improved management strategies.

A team led by Barbara Block at Stanford University's Hopkins Marine Station tagged 772 Atlantic bluefin in the western Atlantic between 1996 and 2004 to get a better handle on population structure and migration patterns.

The team found two distinct populations - one that spawns in the Gulf of Mexico and one that spawns in the Mediterranean Sea. But the two populations mix significantly in the northern Atlantic as adolescents and adults.

The results indicate that bluefin tuna are no respecters of the boundary that splits the Atlantic into a region regulated by Europe and another by the United States and Canada.

The team, whose work appears in Thursday's edition of the journal Nature, suggests that the work can help highlight regions where tuna fishing should be limited during spawning season.