Dark-Matter Experiment

For 75 years, scientists have vainly searched for particles of dark matter, the invisible substance believed to pervade deep space and to glue galaxies together. Next year, in a vat of chilled liquid buried deep in a cave in central Italy, the universe may finally be ready to give up this great secret. An international team of physicists is preparing XENON100, a simple experiment with a huge ambition: to record the moment when a bit of dark matter—known as a weakly interacting massive particle, or WIMP—smacks into the nucleus of an atom of liquid xenon, triggering a flash of light and an electric charge. “We definitely have a chance to see these events,” says Columbia University physicist and XENON team leader Elena Aprile.

According to the latest theories and observations, the universe has about six times as much dark matter as the atomic matter that makes up our ordinary world. But even though uncounted billions of dark-matter particles pass through Earth (and right through you, in fact) every second, they cannot be seen; they have no electric charge and interact so infrequently with atomic matter that the only way we can hope to find them is by laying a clever trap.

Currently there are about 10 teams of scientists devising experiments to discern the elusive moment when a stray WIMP nudges an atom of ordinary matter, but the latest version of XENON will be by far the most sensitive. All the experiments are lodged far below Earth’s surface to shield the detectors from background radiation. The Italian Gran Sasso National Laboratory is located nearly 4,600 feet beneath the top of a mountain, in caverns off a highway tunnel.

XENON100 is a scaled-up version of XENON10, one of Aprile’s earlier dark-matter experiments. It uses liquid xenon, an inert gas at room temperature, to catch WIMPs. The detector is a stainless-steel cylinder surrounded by a protective “castle” made of two kinds of lead and a layer of polyethylene to screen out residual background interference. Inside, 330 pounds of xenon will be chilled to –140 degrees Fahrenheit. Xenon’s attractive property is that it gives off a brief flash of light if a WIMP bumps into the nucleus of one of its atoms. A set of sensors on the cylinder bottom records this signal, while sensors on top detect the minuscule release of electrons liberated by the WIMP. By reading the two signals and measuring the time interval between them, researchers can fix the point of impact within the cylinder in three dimensions.

Dark matter is not necessarily composed of WIMPs—theorists have identified a host of other possible dark-matter particles—but they are the leading candidates because their presence would close a loophole in the reigning theory of particle physics, called the standard model. To address this discrepancy, scientists have proposed that all particles have large-mass counterparts, or superpartners. The neutralino, even with a mass at least 50 times that of a proton, would be the lightest of these. It is a prime WIMP candidate.

If XENON100 uncovers the long-rumored neutralino, it will mark another huge step in science’s grand humbling of humanity. “Copernicus discovered we’re not the center of the universe,” says Yale physicist Daniel McKinsey, a member of the XENON10 team. “If we find dark matter, we will discover that we are not even made of the stuff that composes most of the universe.”

Guy Gugliotta

What lives in the ocean? In 2000, this deceptively simple question spawned a $650 million study to catalog all sea life: plants, animals, bacteria, and fungi. “We simply had no concept of the diversity of life in the ocean,” says Rutgers University biologist Fred Grassle, who chairs the Census of Marine Life’s scientific steering committee. “Whether it was coral reefs, the deep seafloor, or even what lives in people’s backyards along the shore—so little was known.” By helping researchers identify threatened species and habitats, the census will allow better protection of the ocean’s resources. Newly discovered creatures could also provide a gold mine of natural chemical compounds useful as pharmaceuticals or for industrial applications.

Found by the Census of Marine Life:
The amphipod Eusirus holmii, a small shrimplike
crustacean.

Image courtesy of Russ Hopcroft

It takes time to cover something as large as the world’s oceans, so the 2,000-plus census takers from more than 80 countries have broken up their assignment into 17 more manageable subgroups—like coral reefs, continental shelves, and mid-ocean ridges—that should provide a good overview of what’s out there. Teams of marine scientists are towing nets to scoop up plankton, tagging large predators to track their migrations, sequencing the DNA in seawater to hunt for microbes, and trawling the seafloor for bottom dwellers.

The scale and scope of the Census of Marine Life go beyond any previous biological survey, but 7 years into the 10-year initiative, Grassle reports that work is progressing on schedule. About 5,300 previously unknown organisms have already been identified, and every new sighting is logged into the census’s freely accessible Ocean Biogeographic Information System (www.iobis.org), which boasts more than 13 million observations of 80,000 species. Scientists have been bowled over by the diversity that has turned up in unexpected places, from crustaceans and worms on the deep seafloor to carnivorous sponges in the Antarctic Ocean.

The long-term challenge, says marine ecologist Paul Snelgrove of Memorial University of Newfoundland, will be to continue investigating the ecological significance of the newfound organisms after the primary census is completed in 2010. “The first 10 years have been very much focused on discovery,” he says. “The next step is to ask, what do these species do and how important are they to the way the earth works?”

Jennifer Barone

Artificial Life

In the mid-1990s, Craig Venter rose to fame by claiming that he and his colleagues would decipher the human genome long before a huge team of government scientists would. He at least managed a tie: Both groups have provided increasingly accurate versions of the genome since 2000, and Venter has just published the first genome sequence from one person (himself) that includes all the chromosomes inherited from his parents. As important as sequencing the human genome has been, however, Venter is overseeing another experiment that could someday eclipse it. Scientists at the J. Craig Venter Institute and Venter’s biotech firm, Synthetic Genomics, are trying to make a genome from scratch. “I plan to show that we understand the software of life by creating artificial life,” Venter declares in his new memoir, A Life Decoded.