UC Berkeley News


Research Roundup
Milky Way's warp, a threat to the marbled murrelet, and methanogens on Mars

26 January 2006

Milky Way's warp explained

The most prominent of the Milky Way's satellite galaxies - a pair of galaxies called the Magellanic Clouds - appear to be interacting with the Milky Way's ghostly dark matter to create a mysterious warp in the galactic disk that has puzzled astronomers for half a century.

The warp extends across the entire 200,000-light-year diameter of the Milky Way, with the Sun and Earth sitting somewhere near the crease. Leo Blitz, professor of astronomy, and his colleagues Evan Levine and Carl Heiles, have charted this warp and analyzed it in detail for the first time - finding that the atomic gas layer is vibrating like a drum, and that the vibration consists almost entirely of three notes, or modes.

Astronomers previously dismissed the Magellanic Clouds as a probable cause of the galactic warp because their combined masses are only 2 percent that of the disk - too small to influence a massive disk equivalent to about 200 billion suns during the clouds' 1.5-billion-year orbit of the galaxy.

Blitz helped create a computer model that takes into account the Milky Way's dark matter, which, though invisible, is 20 times more massive than all visible matter in the galaxy combined. The motion of the clouds through the dark matter creates a wake that enhances their gravitational influence on the disk. When this dark matter is included, the Magellanic Clouds, in their orbit around the Milky Way, very closely reproduce the type of warp observed in the galaxy.

Though no one knows what dark matter really is - the current consensus is that it is exotic matter rather than normal stars too dim to see - astronomers are now taking it into account in their simulations of cosmic dynamics, whether to explain the lensing effect galaxies and galaxy clusters have on the light from background galaxies, or to describe the evolution of galaxy clusters in the early universe.

Blitz, Levine, and Heiles are continuing their search for anomalies in the structure of the Milky Way's disk. Weinberg hopes to use the Berkeley group's data and analysis to determine the shape of the dark-matter halo of the Milky Way.

- Robert Sanders

Another threat to the marbled murrelet

The effects of overfishing may have driven the marbled murrelet, an endangered seabird found along the Pacific Coast, increasingly to rely upon less-nutritious food sources, according to a new study by Berkeley biologists. This factor could exacerbate the survival challenges the bird is already known to face from severe reductions in its preferred habitat, the old-growth coastal forests of California and the Northwest.

The results, to be published online by early March in the journal Conservation Biology, suggest that feeding further down the food web may have played a role in low levels of reproduction observed in contemporary murrelet populations, and has likely contributed to the seabirds' listing as an endangered species, the researchers say. They decided to embark on this study when they noticed that the population of 600 marbled murrelets they were studying in central California had very low levels of reproductive success. Steve Beissinger, professor of conservation biology in the Department of Environmental Science, Policy, and Management and principal investigator of the study, says: "Even in good years, only 50 percent of the seabirds were nesting, and for those that did, many nests failed for a variety of reasons."

The researchers measured stable carbon and nitrogen isotopes preserved in the feathers of 136 murrelets collected in the Monterey Bay region between 1895 and 1911, comparing them with feathers collected between 1998 and 2002 from 201 murrelets in the same region. More-nutritious food items, such as sardines, anchovies, and squid, are distinguished in the feathers by higher levels of nitrogen isotopes. Less-valuable food sources, including marine invertebrates such as krill, are signaled by greater levels of carbon isotopes. The research team found that the proportion of the most-nutritious food sources was significantly higher for the murrelets of a century ago.

"When murrelets have to rely upon less-nutritious foods, they need to spend more time diving underwater to catch and eat them, and have less energy to make and lay eggs," said Beissinger. "The availability of food commonly affects the reproductive success of seabirds."

Ben Becker, lead author of the study, said it illustrates an additional factor related to the decline of the murrelet: "Not only are they losing their nesting habitat, they're also experiencing a degraded prey environment."

The U.S. Fish and Wildlife Service recently decided to start the process to remove the marbled murrelet from the Endangered Species list, denying it protection even though the bird has not recovered throughout its range. The researchers argue that such a move would be premature. "The threats that have caused marbled murrelets to decline throughout their range have not been remedied," says Beissinger.

- Sarah Yang

Methanogens on Mars?

A study of methane-producing bacteria frozen at the bottom of Greenland's two-mile-thick ice sheet could help guide scientists searching for similar bacterial life on Mars.

Methane is a greenhouse gas present in the atmospheres of both Earth and Mars. If a class of ancient microbes called Archaea are the source of Mars' methane, as some scientists have proposed, then unmanned probes to the Martian surface should look for them at depths where the temperature is about 10 degrees Celsius warmer than that found at the base of the Greenland ice sheet, according to lead researcher P. Buford Price, a professor of physics. This would be several hundred meters - some 1,000 feet - underground, where the temperature is slightly warmer than freezing and such microbes should average about one every cubic centimeter, or about 16 per cubic inch.

Methanogens (methane-generating Archaea) could just as easily be detected around meteor craters where rock has been thrown up from deep underground. "There are oodles of craters on Mars from meteorites and small asteroids colliding with Mars and churning up material from a suitable depth," says Price. "So if you looked around the rim of a crater and scooped up some dirt, you might find them if you landed where the methane oozing out of the interior is highest."

Variations in methane concentration in ice cores, such as the 3,053-meter-long core obtained by the Greenland Ice Sheet Project 2, have been used to gauge past climate. In that core, however, some segments within about 100 meters of the bottom registered levels of methane as much as 10 times higher than would be expected from trends over the past 110,000 years.
Price and his colleagues have shown that these anomalous peaks can be explained by the presence in the ice of methanogens. Common on Earth in places devoid of oxygen, they could easily have been scraped up by ice flowing over the swampy subglacial soil and incorporated into some of the bottom layers of ice.

These methanogens were found in the same foot-thick segments of the core where the excess methane was measured. The researchers calculated that the measured amount of Archaea, frozen and barely active, could have produced the observed amount of excess methane in the ice.

"We found methanogens at precisely those depths where excess methane had been found, and nowhere else," Price says. "I think everyone would agree that this is a smoking gun."

- R.S.