# News this Week

Science  07 Mar 2008:
Vol. 319, Issue 5868, pp. 1318
1. NATURAL DISASTERS

# Ecologists Report Huge Storm Losses in China's Forests

1. Richard Stone*
1. With reporting by Li Jiao in Beijing.

GUANGZHOU, CHINA—From delicate orchids and magnolias to rare Chinese yews and Kwangtung pines, the flora of Guangdong Nanling National Nature Reserve is considered so precious that ecologists call the reserve “a treasure trove of species.” But winter storms have reduced the biological hot spot to a splintered ruin. Snow, sleet, and ice laid waste to 90% of the 58,000-hectare reserve's forests, says He Kejun, director of the Guangdong Forestry Administration in Guangzhou.

Nanling Reserve is one of scores of fragile ecosystems, from Anhui Province in the east to Guangdong Province in the south, that took a beating from storms in late January and early February that set records for snowfall and low temperatures in some areas. Last week, China's State Forestry Administration (SFA) announced that the storms damaged 20.86 million hectares—one-tenth of China's forests and plantations—roughly equivalent to the number of hectares that were reforested between 2003 and 2006. SFA pegs the losses at $8 billion. “The severe storms did a massive amount of harm,” says Li Jianqiang, a plant taxonomist at Wuhan Botanical Garden. “This scale of damage has never happened before.” He Kejun and others say it will take decades for the hardest-hit ecosystems to recover. The ecological and economic toll rivals that of devastating floods along the Yangtze River in 1998 that inundated 25 million hectares of farmland. For broadleaf evergreen forests, “this is bigger than the Yangtze disaster. It's unique in the history of south China,” says Ren Hai, an ecologist with the South China Botanical Garden (SCBG) in Guangzhou. SFA and other agencies have dispatched scientists to take stock and formulate restoration plans. “The government is acting very, very fast,” says Ren. In southeastern China's worst winter in 5 decades, snow and ice knocked out power and paralyzed roads and rail lines at the height of the year's busiest travel season—the Spring Festival, when many Chinese return to their hometowns. The storms pummeled 21 of 33 provinces and regions, claiming 129 lives. Some 485,000 homes were destroyed and another 1.6 million damaged, displacing nearly 1.7 million people, according to central government statistics. Agriculture officials estimate that 69 million livestock—mostly chickens and ducks—froze to death. Storm-related losses exceed$21 billion. As Science went to press, electricity had still not been restored to some remote areas.

Scenes of scrums at train stations and vehicles adrift on highways were splashed across the news in China and abroad last month. Meanwhile, outside the spotlight, an ecological calamity was unfolding. In Jiangxi Province, for example, entire bamboo forests were reduced to matchsticks; fast-growing bamboo can regenerate in several years. In Guangdong, officials estimate that more than 700,000 hectares of forest and plantations are damaged severely, with losses approaching $1 billion. Other provinces enduring extensive forest damage are Anhui, Guangxi, Guizhou, Hubei, Hunan, and Sichuan (see map). The carnage was not limited to natural ecosystems. “Exotic species were harmed more than native species,” says Ren. In northern Guangdong Province, plantations of slash pine (Pinus elliottii), an import from the southern United States, splintered under wet snow, and extensive stands of Australian gum trees “are almost all going to die,” Ren predicts. At Wuhan Botanical Garden in Hubei Province, the roof of a greenhouse housing Asia's largest assemblage of aquatic plants caved in under heavy snow. “A unique collection has been lost,” says Wuhan botanist Li Xiaodong. SCBG scientists maintain long-term experimental plots at Nanling that will allow them to gauge ecosystem damage and recovery. At the moment, the picture is bleak. Nanling's entire forest between 500 meters and 1300 meters in elevation was wiped out, says He. “Before the storm, we could hear birds singing in the reserve. Now it is mostly silent,” he says. Many bai xian, or silver pheasants—Guangdong's official bird—succumbed to the severe weather, and carcasses litter Nanling's trails, says He. One worry, he says, is that epidemics will erupt this spring in the storm-sapped animal populations and among migratory birds. With support from Guangdong Province's government, SCBG plans to send teams of scientists to several of the most devastated forests to survey damage and to set up test plots that will track everything from species composition to the susceptibility of the degraded forests to insect pests and fires. The storm damage lends urgency to a new national strategy for plant conservation released last week by SFA, the Chinese Academy of Sciences, and the State Environmental Protection Agency. Under the manifesto, crafted with help from Botanic Gardens Conservation International, a Richmond, U.K., nonprofit, China has pledged to launch a nationwide survey of species and habitats, construct a national herbarium, crack down on illegal logging, and establish by 2010 a system to monitor and protect China's 31,000 plant species, more than half of which are native. Some 5000 plant species in China are threatened with extinction. As damage assessments proceed, SFA has established a disaster relief technology group and will hold an emergency meeting later this month to plan for restoration. Botanical gardens are doing their part, too. “We must work hard to save vegetation and lessen the extent of damage,” says Ren. “We want to find a way to help natural ecosystems recover with minimal human disturbance.” That is a tricky balancing act. At Nanling, managers are barring local residents from entering to remove downed timber. Although salvage logging could reduce wildfire risk, it could exacerbate erosion, further degrading ecosystems. The bulk of the restoration work is likely to focus on economic recovery: rehabilitation of plantations. The storm's aftermath should also spur long-term research on plant cold tolerance, says Li Jianqiang. The immediate task is picking up the pieces after the worst winter in recent memory. “We cherish our endangered species,” says Li. But for some of the precious plants at Wuhan Botanical Garden and in southern China's battered reserves, he says, “there is nothing we can do to save them.” 2. ANIMAL RIGHTS # Brazilian Scientists Battle Animal Experimentation Bans 1. Martin Enserink Brazilian scientists are fighting a series of local attempts to ban animal experimentation that they say could cripple scientific research. At the top of their list: a controversial law passed 2 months ago by the city of Rio de Janeiro that prohibits all animal experiments at private companies. Researchers are hoping that a comprehensive federal bill addressing animal experimentation, which has been lingering in the Brazilian Congress for 12 years, will put a stop to such local bans. The battle in Rio, a major biomedical research hub, has gone on for more than 2 years. In 2006, the city council passed an animal-welfare law, introduced by actor-turned-politician Cláudio Cavalcanti, that would have banned all animal experiments in the city. Mayor César Maia vetoed the bill. A second version, passed in September 2007, made an exception for universities and public organizations such as the Instituto Oswaldo Cruz (Fiocruz), a major vaccine producer. Maia vetoed that one, too. But the council overrode his veto on 26 December. The law has not taken effect yet, and the mayor does not appear in a hurry to enforce it, says animal physiologist Luis Eugênio Mello of the Federal University in São Paulo, president of the Federation of Brazilian Societies of Experimental Biology. But if enforced, the ban could force several Rio biotech companies out of business. “It's a crazy law,” says Eduardo Krieger, a former president of the Brazilian Academy of Sciences. A comparable far-reaching bill was approved in December by Florianópolis, the capital of the southern state of Santa Catarina; that law was replaced by the city's mayor in February with much less stringent regulations. Similar legislative plans are afoot in other cities. Brazil's scientists contend that regulating animal research should not be a local issue and are arguing for a federal law. Indeed, such a bill was introduced in 1995 by Chamber of Deputies member Sérgio Arouca, who was once director of Fiocruz; it would ban animal experiments if other alternatives are available, require ethics committees to approve studies, and set up a national council to issue guidelines. But the bill never came to a vote, and Arouca died in 2003. Researchers say “Arouca's law” would protect them from a wave of municipal or state initiatives, and they have been lobbying hard to get the bill to a vote. Animal-rights activists oppose it, however. Ethics panels, which already exist at the majority of research institutions, are dominated by scientists and rubber-stamp proposals, says George Guimarães, director of Ethical Vegetarianism, Animals Rights Defense and Society, a São Paulo-based group. The researchers' lobbying appears to have paid off, says Mello, with “support from left to right” in Congress. And recently, Brazil President Luiz Inácio Lula da Silva, who has made advancing research a national priority, named Arouca's law among his legislative priorities. Guimarães agrees that the federal bill is now likely to pass. But a legislative stalemate in the Congress, unrelated to the bill, could make it hard to pass any laws at all in 2008, Mello warns. Cavalcanti says that he wants Rio's mayor to enforce his law; he will also reintroduce the proposal for a total ban this year. Officials at Fiocruz, a big yellow fever vaccine producer, have warned that such a move could imperil routine quality testing of vaccines. But Cavalcanti says that he does not believe animal research can benefit human health. A single-issue politician, he has offered to become a guinea pig himself if it can help save animals. “This is my mission, my only reason for living,” he says. Scientists should do more to counter the cruel image of animal studies that activists have promoted and explain why such work is necessary, says Walter Colli of the University of São Paulo. “We are guilty of not having done enough to influence public opinion,” he says. “The average citizen is confused.” 3. PHYSICS # Test of Hawking's Prediction on the Horizon With Mock 'White Hole' 1. Adrian Cho Physicists can't travel to a black hole to see how it ticks, but they have taken a big step toward creating something similar in the lab. Using an optical fiber and laser light, Ulf Leonhardt of the University of St. Andrews in the U.K. and colleagues have simulated a “white hole”—essentially a black hole working in reverse—as they report on page 1367. The model might soon mimic perhaps the most tantalizing property of a black hole: the “Hawking radiation” that should emanate from it. Others have cooked up analogs, but “this is probably the first one that has some correlation to the Hawking effect,” says Grigori Volovik, a physicist at Helsinki University of Technology in Finland, who is working on a model in liquid helium. “This is some kind of a breakthrough.” Formed from the collapse of a star, a black hole is like a funnel-shaped pit in the fabric of space and time. Light or anything else that ventures into the funnel cannot get back out once it passes a point of no return known as the event horizon. In contrast, a white hole would resemble a mountain in spacetime so steep than nothing could reach the summit. It would have an event horizon that marks the point of closest approach. Unlike black holes, white holes should be unstable, and none are thought to exist. In principle, simulating an event horizon is simple. Consider a river filled with fish swimming upstream at maximum speed. If the fish reach a point where the water flows as fast as they can swim against it, they will pile up there (see figure, below). That point simulates a white-hole horizon, and theorist William Unruh of the University of British Columbia in Vancouver, Canada, and colleagues have experimented with fluid analogs. Leonhardt and colleagues took a different tack. They fired a pulse of infrared light down an optical fiber. Crucially, the intensity of the pulse itself altered the speed at which light could travel in the fiber. The researchers then shot in light of a second, longer infrared wavelength that moved slightly faster than the pulse. As that light caught up to and “ascended” the pulse, it slowed until at some point its speed exactly matched that of the pulse. That spot on the pulse simulated the event horizon, and light accumulating there was compressed to a slightly shorter wavelength. That squeeze made the light travel slower than the pulse, so it effectively rolled back down the peak and fell behind. The researcher detected the telltale wavelength shift and other evidence that the light was piling up on the horizon. The next goal is to see Hawking radiation, Leonhardt says. Thanks to quantum mechanics, the vacuum roils with photon pairs that normally pop in and out of existence too quickly to be observed. But near a black hole, one particle might emerge beyond the event horizon and fall in while its partner emerges outside and escapes, as theorist Stephen Hawking argued in 1975. So a black hole ought to glow like an ember, although far too feebly to be seen through the cosmic microwave background. The white-hole analog ought to radiate, too, Leonhardt says. Like warped spacetime near a black hole, the wildly varying speed of light in the fiber can rip particles out of the vacuum. So the pulse should shine faintly but detectably in ultraviolet wavelengths. Seeing that glow would be key, Unruh says, because in spite of its fame, Hawking radiation remains unconfirmed. If physicists spot its equivalent in an analog, Unruh says, “then you get a lot, lot more faith that the prediction is solid.” Volovik agrees: “If they really see Hawking radiation, I think Hawking will finally get his Nobel Prize.” 4. LARGE FACILITIES # NSF Delays Three Projects to Get Better Handle on Costs 1. Jeffrey Mervis After a decade of making their case to the U.S. National Science Foundation (NSF), scientists planning a major project for remote monitoring of the oceans thought they had cleared the final hurdle in December. That's when an external panel blessed the$331 million venture, called the Ocean Observatories Initiative (OOI), and told NSF officials “to enter into the detailed design and construction phase” to build it. “We were ready to go, and the reviewers agreed,” says Steven Bohlen of the Consortium for Ocean Leadership in Washington, D.C., which is managing the project.

So Bohlen and his colleagues were shocked last month when NSF omitted building funds for OOI and two other long-running projects on the verge of construction—the $100 million National Ecological Observatory Network (NEON) and the$123 million Alaska Region Research Vessel (ARRV)—from its 2009 budget request to Congress. It's part of a new policy aimed at eliminating cost overruns that occur after construction is under way. Those overruns have not only forced NSF to borrow from other accounts, but they can also lead to last-minute changes that weaken a project's scientific capabilities. Under the previous policy, a project was approved based chiefly on its scientific merit; it might be years before NSF arrived at a final price based on all relevant factors. Now, NSF is requiring a firm cost estimate before asking Congress for construction funds.

Scientists whose projects face delays of a year or more aren't pleased about the sudden policy shift. But they understand why NSF is asking them to go back to their calculators. “It's not good news for the science,” says Terry Whitledge, director of the Institute of Marine Science at the University of Alaska, Fairbanks, which manages the ARRV project for NSF. “But I think that NSF is probably doing the right thing in the current budget environment. And their message to all of us is clear: Don't come back to us down the road with a higher number.”

The new rules mark the latest attempt by NSF, an agency known for its expertise in small science, to get a better handle on an expanding portfolio of large projects. The facilities can cost upward of $500 million to build and tens of millions of dollars a year to operate, a sizable commitment for a$6 billion agency. The policy affects four projects now in NSF's major research equipment and facilities construction account—including the Advanced Technology Solar Telescope in Hawaii, which is listed as a new start for 2009 although it is currently still in the design phase—as well as all future proposals.

The large-facilities account was created more than a decade ago to segregate big-ticket items such as ships and telescopes from the agency's bread-and-butter research and education programs. But the community was so unhappy with how NSF's oversight body, the National Science Board, approved and ranked projects that it complained to Congress, which ordered up a review by the National Academies' National Research Council. Its 2004 report recommended that the process be more rigorous and transparent (Science, 16 January 2004, p. 299). NSF Director Arden Bement says the agency has embraced those suggestions by setting up a new administrative office and monitoring each project more closely.

Even so, last month Bement went one step further. Although NSF has spent millions on each project to help scientists lay the groundwork, Bement says he won't ask for construction funds until each has passed a final project review that includes a firm cost estimate and a detailed analysis of environmental and regulatory issues. “It's a huge culture change for the foundation,” says a White House official familiar with how NSF manages its large projects.

A more thorough vetting could shorten the actual construction time, Bement says, and reduce the chances that a project would need to be “descoped” to stay within its budget. He says the new rule also brings NSF practices closer to those at other federal agencies, such as the Department of Energy and NASA, which have more experience building and managing large scientific facilities and instruments.

NEON's checkered history highlights the problems Bement is trying to correct. NSF first requested money for NEON—some $12 million—in its 2001 budget request. Then-NSF Director Rita Colwell called it “a continental-scale research instrument consisting of 10 geographically distributed observatories, networked via state-of-the-art communications, for integrated studies to obtain a predictive understanding of the nation's environments.” But the initial design was reworked substantially after scientists raised numerous objections. Last spring, the community came up with the current version, which features a network of 20 core sites and 40 “relocatable” sites. The core sites are expected to provide a 30-year longitudinal record of myriad factors, whereas the other sites will focus on narrower scientific questions and capture more transient environmental events. Despite those ups and downs, its price tag never varied. David Schimel, who runs the Boulder, Colorado-based consortium responsible for building NEON, says the original$100 million figure announced in 2001 “was not based on anything.” The new policy, he says, allows project leaders to do it right. “NSF has actually done us a huge favor by unshackling us from that $100 million estimate,” he says. “Now we can start over and come up with a new, more realistic baseline. Needless to say, the new figure will be higher.” Just as important as the initial construction cost, says Schimel, is the estimated$30 million a year needed to operate and maintain the network. “That's the real constraint,” he says. “We don't want to gut the community's research budget [at NSF] by building a facility that's too costly to operate.” Project scientists are hoping to incorporate several features to reduce labor and maintenance costs in the final design, he says. Accordingly, NSF's 2009 budget request includes $26 million for NEON from its research account, in part to fund the additional work needed to come up with a more efficient design. Project leaders for OOI, which is expected to cost$50 million annually to operate, say they made similar hard choices in preparing for the preliminary design review NSF conducted in December. The observatories will gather data on coastal, regional, and global scales, and the community has been ruthless in paring each system down to the bone, says Holly Given, the consortium's director of ocean-observing activities. For example, Bohlen notes that only three sites remain from an original plan for 10 blue-water autonomous buoy systems—in the Southern Ocean off the Chilean coast, the North Atlantic near Greenland, and the Gulf of Alaska in the northern Pacific. “As we refined our cost estimates, we had to scale back and concentrate on what was most important scientifically,” he explains.

The cost of some components can't be nailed down until the plans are actually sent out for bids, Bohlen says. Referring to the five sets of seabed cables that will connect instruments continuously monitoring the Juan de Fuca Ridge off the coast of Washington state, he notes that “the market price for those materials and sensors, plus labor, can vary a lot.” NSF is seeking $10.5 million in 2009 for OOI for continued planning. Scientists involved in the Alaska research vessel are acutely aware of how the economy can wreak havoc on carefully laid scientific plans. Whitledge estimates that NSF's new policy will add 12 to 18 months to the project's scheduled solicitation of bids in 2010—at a price yet to be calculated. “Shipyard costs have been going up by 20% a year,” Whitledge says, because of the rising cost of steel and other raw materials and industry's demand for new and refurbished exploration ships triggered by$100-per-barrel oil prices. A delay also means a longer wait for data on the impacts of climate change in the Arctic, he notes. The ship will replace the Alpha Helix, a 40-year-old research vessel that the university retired in 2004.

Bement doesn't pretend to have all the answers for managing large facilities. “I'm convinced that we can do a lot better,” he told Representative Alan Mollohan (D-WV), chair of the House panel that sets NSF's budget, during a hearing last week on NSF's 2009 budget request. But the problem clearly has his full attention. When Mollohan asked about one project, Bement brushed aside the chair's suggestion that he turn to one of his aides for the details. That won't be necessary, Bement replied: “I know the answer. I get a report every month. And I read them.”

5. RESEARCH FUNDING

# U.S. Biomedicine's Mother Ship Braces for Lab Closings

1. Jocelyn Kaiser

Distress signals are emerging from the intramural program at the U.S. National Institutes of Health (NIH) in Bethesda, Maryland, as funding troubles begin to pinch. Most institutes are affected, but the pain is acute at the National Institute of Child Health and Human Development (NICHD), where up to 12 intramural labs—run by 16% of 74 tenured staff—could be shuttered. “This is a completely new category of nightmare,” says an NICHD investigator who asked not to be named. Compared with a poor review in the extramural world, in which a researcher can try for a new grant, closing an intramural lab means going “from full funding to zero,” he says.

NICHD's troubles reflect the impact of 5 years of flat budgets on the $2.8 billion NIH intramural program. The campus has seen a net loss of 114 of 1252 principal investigators (PIs), or 9%, since 2004 when a period of rapid growth halted. Half of the decline came in the past year, according to NIH data. “There's no way with conservation of matter to do anything else,” says NIH Deputy Director for Intramural Research Michael Gottesman, who nevertheless thinks the program is still “a reasonable size.” He adds that the squeeze “is not unique to NIH or any organization,” although extramural research seems less constrained. The number of funded extramural PIs has hovered around 26,300 for the past 4 years, according to NIH. Gottesman points out that the intramural program has downsized before, after a 1994 blue-ribbon panel called on NIH to cut less productive programs and create a formal tenure system. The number of PIs dropped from roughly 1584 in 1990 to 1206 in 2000. Growth resumed from 2000 to 2002 (see graph). But when it stopped, many of NIH's 21 intramural programs had a hard landing. Scientific directors saw budgets lag behind inflation while costs increased. As a result, at the National Cancer Institute, the tally of PIs has dropped by 65 to 253, a decline of about 20% since 2003, says Center for Cancer Research Director Robert Wiltrout. The institute has been more aggressive in closing labs after a leader retires or receives low marks on a site visit. And some top scientists have simply left. The diabetes institute closed several labs in 2006 to help trim 7% from its operating budget, says National Institute of Diabetes and Digestive and Kidney Diseases senior scientist Alan Schechter. Last year, a shortfall in lab operating funds at NICHD forced scientists to curtail experiments and travel (Science, 18 May 2007, p. 968). The possible cuts in personnel at NICHD, however, appear to be more drastic than any before. Soon after the final NIH budget passed Congress in mid-December, Owen Rennert, NICHD's scientific director, met with program chiefs and “tentatively outlined … some areas that could be reduced” to free up$15 million, he told Science by e-mail. In January, a few PIs were told their labs were to be closed this year. After staff protested to Gottesman, Rennert relented. In a 30 January e-mail sent to Science and circulated to his staff, he wrote that “no decisions have been reached.” Any cuts in programs, he wrote, would be based on reviews by outside scientists over the next 2 years and factors such as publications and relevance to NICHD's mission. Still, anxiety is running high throughout NICHD's labs.

Some don't blame Rennert; he has made “Herculean efforts” to avoid lab cuts until now, says one senior scientist. But PIs who are due for their 4-year site visit in April are now bracing for the worst. Those whose labs are closed won't be out of a job, Gottesman says, but will have to join someone else's lab or become an extramural grants administrator. University positions seem out of the question for anyone but superstars. Even if they survive, NICHD scientists worry about the impact: “You'll have to be more focused and not take as many risks,” says fruit fly geneticist Judith Kassis.

It seems unlikely that NIH's intramural researchers will get much sympathy from outside, where funding is also tight. Yale University cell biologist Barbara Ehrlich, a member of NICHD's board of scientific counselors, says that although some of its investigators are “just spectacular,” others “haven't kept up as much.”

And Gottesman says there's a bright side to this “pruning”: It has freed up some money to strengthen other labs, including big teams doing cutting-edge science. (The overall number of intramural scientific staff, about 6000 to 7000 M.D.s and Ph.D.s, is probably stable, he says.) “For the remaining scientists, this is still a terrific place.” He admits, however, that “everybody says that I'm a Pollyanna.”

6. SPACE PHYSICS

# Antimatter Experiment May Be Too Costly for NASA to Launch

1. Andrew Lawler

NASA says it is willing to fly a $1.5 billion experiment designed to detect antimatter. But Congress would have to come up with as much as$4 billion to make it happen, the agency says. Supporters of the Alpha Magnetic Spectrometer (AMS) dispute those cost estimates but face an uphill struggle to get the 7000-kg probe into orbit.

In a 17-page report to Congress that was released last month, NASA paints a sobering picture of what it would take to attach the instrument to the international space station. Samuel Ting, the physics Nobelist at the Massachusetts Institute of Technology in Cambridge who has championed the project, says the 16-nation AMS collaboration has no money to buy another ride into space. That leaves the fate of AMS, and its quest to understand why there is more matter than antimatter in the universe, with Capitol Hill.

Testifying before the House Science and Technology Committee on 13 February, NASA Administrator Michael Griffin said he doesn't oppose launching AMS aboard the shuttle. But for planning purposes, he added, Congress must find the money this year. The agency's report, submitted a week later, notes that all remaining shuttle flights are devoted to completing the station by 2010, after which NASA intends to retire the launcher. Adding an additional flight “would be difficult, costly, and would have a significant negative impact on NASA's exploration program,” the report asserts.

An additional flight in late 2010 would cost between $300 million and$400 million, NASA estimates. The cost would rise 10-fold if the flight were delayed until 2011 because of the need to extend industry contracts. “These costs would come directly at the expense of exploration development activities,” states the study.

NASA is overstating the costs, says Trevor Kincaid, a spokesperson for Representative Nick Lampson (D-TX), who represents the area around NASA's Johnson Space Center and is a strong AMS backer. Lampson puts the price of another shuttle mission at between $150 million and$175 million. A spokesperson for Senator Bill Nelson (D-FL) suggested that a $1 billion supplement to NASA's 2009 budget could cover the additional mission. Last summer, the Senate approved such an increase, but it was dropped from the final 2008 spending bill. “NASA signed on to this, and they should uphold their end of the [international] deal,” adds Kincaid. The only option besides an additional shuttle mission is to put AMS on an expendable rocket. That would cost at least$600 million, says the report, and delay the launch by 2 or 3 years while the spacecraft is modified. That means AMS might not arrive until 2013 or 2014 at a space station that NASA intends to shut down in 2016.

Ting and his collaborators say AMS needs at least 3 years in space to gather data. But the Nobelist says he's not giving up. “My job is to get this finished,” he says.

7. PLANETARY SCIENCE

# Electron Shadow Hints at Invisible Rings Around a Moon

1. Richard A. Kerr

Space physicists poring over Cassini spacecraft data think they have two firsts: the first known natural satellites of a moon, which also form the first rings known to encircle a moon. Unlike the rings around Jupiter, Saturn, Uranus, and Neptune, however, the proposed rings around Saturn's moon Rhea are—so far, at least—invisible.

Even though the proposed rings are physically “very weird and [Cassini scientists] don't have image proof, it's certainly a good batch of circumstantial evidence,” says ring dynamicist Jack Lissauer of NASA's Ames Research Center in Mountain View, California. Ring specialist Jeffrey Cuzzi, also of NASA Ames, agrees, up to a point. “It's clearly something unusual,” he says. But whereas the discoverers “want to say it's unusual rings, I'd want to say it's unusual physics.”

The case for rings depends on shadows Cassini passed through while flying by 1530-kilometer-diameter Rhea in November 2005, as space physicist Geraint Jones, now of University College London, and his colleagues report on page 1380. Rather than recording how the rings block starlight—a common way of spotting them—Cassini serendipitously recorded the dimming of Saturn's trapped energetic electrons that stream by Rhea. The electron dimming extended about 6000 kilometers on each side of Rhea. Something seemed to be absorbing electrons before they got to Cassini. Because other Cassini instruments failed to detect enough gas or dust to do the job, Jones and colleagues inferred that unseen boulders up to about a meter across were absorbing the electrons. The Pioneer 11 spacecraft discovered the G ring of Saturn in 1979 in much the same way.

The clincher for Jones was a set of six dark, narrow electron shadows that Cassini recorded, three on each side of Rhea. The shadows are stunningly symmetrical side to side, as seen in figure 4b of the paper. “I saw [a figure] like that in my first planetary science class,” Lissauer recalls. The year was 1977, and the figure was the now-classic plot of the previously unimagined narrow rings of Uranus occulting a star. Given Cassini's broad electron absorption with narrow spikes, all strikingly symmetrical, a broad disk of debris in Rhea's equatorial plane with three embedded narrow ringlets or incomplete ringlets was “the only reasonable explanation we've been able to come up with,” Jones says. Space physicist Mihaly Horanyi of the University of Colorado, Boulder, agrees. Cassini has provided “a fascinating set of observations from multiple instruments that is indeed best explained by proposing a possible set of rings around Rhea,” he writes in an e-mail.

But ring specialists still have their reservations. Such rings, they say, are possible but improbable. First, just the right sort of impact would probably have been required to blast material off the icy moon and into orbit. Then the ring particles would have had to survive millions if not billions of years being torn apart by the tidal pull of Saturn and worn down to dust by eroding small impacts. Most constraining, perhaps, is the “incredibly low” limit on dust around Rhea set by Cassini's camera, says Joseph Burns of Cornell University, who is on the imaging team. Ring boulders must shed some dust, and even tiny amounts of dust show up when backlit by the sun. “We're going to keep trying harder,” says Burns, to see what may be seen.

8. NEAR-EARTH OBJECTS

# Preparing for Doomsday

1. Richard Stone

Over the next several years, new telescopes will spot thousands of near-Earth asteroids and comets. If one is headed our way, will world leaders be ready to respond?

Over the next several years, new telescopes will spot thousands of near-Earth asteroids and comets. If one is headed our way, will world leaders be ready to respond?

TIESHAN TEMPLE NATIONAL FOREST, CHINA—In the control room of XuYi Observatory, Zhao Haibin sits at a computer and loads the night sky over Jiangsu Province. A faint white dot streaks across a backdrop of pulsating stars. “That's a satellite,” Zhao says. Elsewhere on the screen, a larger white dot lumbers from east to west. It's a main-belt asteroid, circling the sun between Mars and Jupiter.

On a ridge in this quiet, dark corner of southeastern China, about 100 kilometers northwest of Nanjing, XuYi's new 1-meter telescope espies a few dozen asteroids on a good night. Most are known to science. But since China's first telescope dedicated to asteroid detection saw first light early last year, Zhao's team has discovered more than 300 asteroids, including a near-Earth object (NEO), the class of asteroids and comets that could smash into our planet, if fate would have it.

China's asteroid hunters are the latest participants in a painstaking global effort to catalog NEOs. Close encounters with asteroids in recent years—and comet Shoemaker-Levy's spectacular death plunge into Jupiter in 1994—have spurred efforts to find the riskiest NEOs before they blindside us. Tracking potentially hazardous objects—NEOs passing within 0.05 astronomical units, or 7.5 million kilometers, of Earth's orbit—is essential for any attempt to deflect an incoming rock.

The first test of our planet's defenses could be Apophis, an asteroid the size of a sports arena that made the world sweat for a few days in December 2004, when calculations suggested as great as a 1 in 37 chance of an impact in 2029. Although further data ruled out that day of reckoning, another could be looming. In April 2029, Apophis will pass a mere 36,350 kilometers from Earth, inside the orbits of geostationary satellites. If it enters a keyhole—a corridor of space barely wider than the asteroid itself where gravitational forces would give it a tug—it will end up on a trajectory that would assure a collision 7 years later: on 13 April 2036, Easter Sunday. The odds of Apophis threading the needle are currently 1 in 45,000—but dozens of factors influence asteroid orbits. Researchers will get a better look during Apophis's next appearance in our neighborhood in 2012.

By then, a powerful new telescope for detecting asteroids and comets—the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS), expected to be up and running by summer—should have unmasked thousands more NEOs. An even grander project, the 8.4-meter Large Synoptic Survey Telescope (LSST), is expected to be operational in 2014.

The anticipated bumper crop of NEOs confronts society with urgent questions. In the next several years, with increasing rapidity, Pan-STARRS and its ilk will discover potentially dangerous NEOs. Currently, 168 NEOs have a chance of striking Earth in the next century, although the odds are minuscule. By 2018, the risky rock roster could swell more than 100-fold. Additional observations will allow astronomers to refine orbits, and in most cases, rule out a threat. For that reason, astronomers are debating when the public should be alerted to hazards, to minimize false alarms.

Eventually, an asteroid with our name on it will come into focus, forcing an unprecedented decision: whether to risk an interdiction effort. “The very concept of being able to slightly alter the workings of the cosmos to enhance the survival of life on Earth is staggeringly bold,” says Russell Schweickart, chair of the B612 Foundation, a Sonoma, California, nonprofit that lobbies for NEO deflection strategies. We have the means to deflect an asteroid—indeed, “it's really the only natural hazard that we can possibly prevent,” says NEO specialist David Morrison, an astrobiologist at NASA's Ames Research Center in Mountain View, California.

There is one “fatal missing element,” says Schweickart, who in 1969 piloted the lunar module for the Apollo 9 mission: “There is no agency in the world charged with protecting the Earth against NEO impacts.” He and others hope to change that.

## Wake-up calls

Like any natural disaster, impacts occur periodically; gargantuan impacts are so rare that their frequency is hard to fathom. Every 100 million years or so, an asteroid or a comet a few kilometers or more in width—a titan like the rock thought to have wiped out the dinosaurs 65 million years ago—smacks Earth. “This is not just getting hit and killed,” says Edward Lu, a former astronaut who now works for Google. “You're on the other side of the Earth and the atmosphere turns 500° hotter. Lights out.”

Reassuringly, no doomsday asteroid identified thus far is on track to intersect Earth's orbit in the next century. Less reassuring, an unobserved, long-period comet from the Oort cloud could swoop in with little warning. Although the odds of this happening in anyone's lifetime are on the order of winning the Powerball lottery, a megaimpact's annualized fatality rate is likely to rival those of earthquakes or tsunamis, says Clark Chapman, an astronomer at the Southwest Research Institute in Boulder, Colorado.

Near-Earth asteroids tens to hundreds of meters in diameter are far more numerous— there may be as many as 3 million in the solar system—and they cross Earth's path more frequently. The iconic Meteor Crater in northern Arizona was gouged by a 50-meter-wide hunk of iron and nickel 50,000 years ago. In 1908, a fireball scorched and flattened trees over 2100 square kilometers of taiga in Siberia's Tunguska region—the devastating footprint, many experts say, of a modest asteroid that exploded in midair.

Recent supercomputer modeling has downsized the Tunguska rock. An asteroid just a few dozen meters wide, fragmenting explosively with a yield of 3 to 5 megatons—a fraction of earlier estimates—could have done the trick, Mark Boslough and David Crawford of Sandia National Laboratories in Albuquerque, New Mexico, report in an article in press in the International Journal of Impact Engineering. If this is correct, the expected frequency of Tunguska-sized impacts changes from once every couple of millennia to once every couple of centuries. “Smaller objects may do more damage than we used to think,” says Chapman.

Today the impact threat may seem obvious, but for decades it was largely ignored. Aerodynamicist Anatoly Zaitsev, director general of the Planetary Defense Center in Moscow, sounded the alarm in a landmark report delivered to Soviet leaders in 1986. “They just laughed,” he says. Then on 22 March 1989, an asteroid several hundred meters across whizzed by Earth at about twice the distance to the moon; astronomers didn't spot Asclepius until it had already passed.

Asclepius was a shot across the bow, prompting the U.S. Congress to query NASA about whether the agency had a plan for the next killer asteroid. A parade of committees followed, after which Congress in 1998 ordered NASA to tally and track at least 90% of NEOs that are more than 1 kilometer wide. NASA launched the Spaceguard Survey, named after a survey in Arthur C. Clarke's 1972 novel Rendezvous with Rama. To date, Spaceguard and other efforts have identified more than 700 of an estimated 1000 or so NEOs in this category. Then in 2005, Congress called on NASA to expand the search by 2020 to cover 90% of NEOs at least 140 meters in diameter—the approximate minimum size to damage an area at least as large as a state or seaboard. NASA expects Spaceguard II to spot 21,000 potentially hazardous NEOs and forecasts a 1-in-100 chance that such a rock will hit Earth in the next 50 years.

The uncertainties are huge. Main-belt asteroids can knock into each other, turning a benign rock into a malignant projectile. And with only a fraction of NEOs having been identified so far, what we don't know can hurt us. Astronomer Brian Marsden, director emeritus of the International Astronomical Union's Minor Planet Center, the clearinghouse for asteroid and comet orbits, figuratively sums up the situation: “The ones to worry about are those that were discovered yesterday and have a very high probability of hitting us the day after tomorrow. Those, plus the ones we've never even seen yet!”

Night has fallen on an early December evening near Tieshan Temple, which, according to local lore, was the home of China's first monk. The sky above the national forest is pitch-black but overcast. On nights like this, asteroid hunters know how to kill time. In a chilly, cigarette smoke-filled lounge down the hall from XuYi's control room, Zhao and his colleagues play cards and sip from tall, clear plastic bottles packed with green tea leaves, hoping that the weather forecast is wrong and the skies will clear.

Zhao has worked at Purple Mountain Observatory, which operates XuYi, since graduating from Nanjing University in 1996. He has a comet named after him, but his biggest thrill came last spring, when he found an NEO.

On most nights, the telescope is pointed away from the sun, toward main-belt asteroids outside Earth's orbit. More elusive objects between Earth and the sun can be discerned in the right conditions. With a clear sky and a new moon, just after nightfall or before sunrise, Zhao aims the telescope at a 60° angle to the sun, where faint NEOs, like a crescent or gibbous moon, reflect sunlight in phases. During the telescope's first year, his team got fewer than a dozen opportunities to gaze sunward. One was 7 May, when they scored their NEO.

Tonight, just after midnight, the clouds have dispersed enough for viewing. Zhao's team swings into action, pointing the telescope at a 2-degree-square patch of sky. As dawn breaks, they will e-mail the data to Purple Mountain's Nanjing headquarters for analysis.

Zhao's team is working fast to stake NEO claims before Pan-STARRS, the first Spaceguard II facility, starts gobbling up the heavens. The telescope on Mount Haleakala on Maui Island, Hawaii, has a charge-coupled device camera with 1.4 billion pixels—the highest resolution in the world—that acquires images every 30 seconds.

Pan-STARRS, which saw first light last August, will usher in a new paradigm in observational astronomy (Science, 12 May 2006, p. 840). “It's a set of surveys that will be analyzed in a wealth of different ways,” says Kenneth Chambers, an astronomer with the Institute for Astronomy (IfA) at the University of Hawaii, Manoa, who is leading a consortium of 300 scientists whose institutions have paid for first crack at Pan-STARRS gold. Some will map the Milky Way or look for distant quasars. Others will hunt for asteroids. “The astronomical community is not ready for the fire hose of data that's going to hit them,” Chambers says.

Once Pan-STARRS begins taking data in earnest this summer, NEO finds should come thick and fast. According to IfA astronomer Robert Jedicke, who led development of the software that will cull NEOs from the data deluge, Pan- STARRS will be 10 times more effective at spotting NEOs than all current surveys combined. “Are there many more objects like Apophis out there? This is something that Pan-STARRS will answer,” says IfA Director Rolf-Peter Kudritzki.

Magnificent feats of detection are also expected from LSST, which will have 24 times greater survey power than Pan-STARRS. Like its Hawaiian rival, the $389 million project has broad science objectives, including studying dark energy and dark matter and mapping the Milky Way. Unlike Pan-STARRS, LSST data will be available immediately to any researcher. Construction is expected to begin in 2011 at Cerro Pachón, Chile. When completed, LSST will cover the entire available sky every 4 nights with a 3.2-billion-pixel camera. Project scientists have teamed up with Google, Microsoft, and others to develop algorithms for processing the masses of data. After 10 years of operation, LSST should have plotted rough orbits for 82% of potentially hazardous NEOs larger than 140 meters, with only the risk assessments requiring human input, says LSST Director J. Anthony Tyson, a physicist at the University of California, Davis. Funding is not assured. Tyson has lined up$45 million so far from private sources, including two gifts announced in January that will help pay for the mirror: $20 million from Charles Simonyi, chief executive of Intentional Software, and$10 million from Microsoft's Bill Gates. The tycoons, says Tyson, “are excited about the LSST acting as a peripheral device for the Internet and thus bringing the universe to everyone's computer.” Much of LSST's construction funds are expected from the U.S. National Science Foundation, which will hold a Major Research Equipment and Facilities Construction review on the project this autumn.

## Gauging risks

In the early 1990s, as astronomers intensified their search for NEOs, IfA's David Tholen upbraided colleagues for turning a blind eye to asteroids lurking inside Earth's orbit. He was concerned that an inner-orbit NEO at its farthest point from the sun could hit our planet. “For years, I wanted to do something about that,” says Tholen. But he lacked the means. “Other folks had great cameras. I was envious.” In 1997, he finally got time on a decent telescope. Aiming it low on the horizon just after nightfall or before dawn, his group over 3 years discovered four asteroids in this blind spot—including a whopper that is 5 kilometers wide.

Riding high, Tholen won a grant for a more intensive search campaign. But his team struggled with technical glitches, and by their final year of funding in 2004, he says, “we hadn't found a single asteroid.” He redoubled his efforts, booking time at observatories around the world. In June 2004, he was juggling nights on two telescopes. Then in the early evening of the 18th, at Kitt Peak National Observatory near Tucson, Arizona, Tholen, Roy Tucker, and Fabrizio Bernardi hit pay dirt: They got a first glimpse of Apophis.

“Apophis demonstrated that we know very little about the region of space near Earth,” says Boris Shustov, director of the Institute of Astronomy in Moscow. Anxiety will mount when Apophis chugs back into range in 2012. Ironically, the best instrument for refining the asteroid's orbit—the world's most powerful planetary radar at Arecibo Observatory in Puerto Rico—may be switched off in 2011, the victim of budget cuts. Even without Arecibo, optical measurements almost certainly will reduce or rule out the impact risk. For that reason, NASA has no plans to send a probe to Apophis, and the European Space Agency has shelved a mission (see sidebar, p. 1329).

A chilling reassessment of Apophis could change the political landscape fast.

Suppose that observations forecast a 1-in-1000 impact risk in 2036. “That risk is really low, but if it hits, it's really bad,” says Lu. “How much is it worth to us to have peace of mind?”

The “threshold of pain,” as Lu calls it, may depend on who would be affected—and what resources they have. Based on current calculations, the line where Apophis might hit—the so-called risk corridor—runs from Kazakhstan through Siberia, over the northern Pacific, and across Costa Rica, Colombia, Venezuela, and the south Atlantic. Who would mount and pay for a deflection mission? All countries along the corridor? Just Russia, vulnerable to a direct hit, or the United States, vulnerable to a towering tsunami? The United Nations? What if a mission failed, deflecting Apophis to another point on the risk corridor, converting an “act of God” into an act of humankind? Who would be liable?

As experts grapple with these questions, some are trying to rouse political leaders. With outside advice, the Association of Space Explorers, an organization of astronauts and cosmonauts based in Houston, Texas, is drafting an NEO Deflection Decision Protocol to present to the U.N.'s Committee on the Peaceful Uses of Outer Space in 2009. “Apophis should unite our efforts to deal with the threat,” says Shustov, who is leading an effort to develop Russia's first national R&D program on NEO hazards.

Shustov's nightmare is that leaders will drag their feet until the threat of a direct hit becomes real. But an asteroid need not impact to cause chaos. Each year, military satellites detect several 1-kiloton explosions of asteroids in the upper atmosphere, and every several years, a much larger explosion of 10 kilotons or more, says Sandia's Boslough. “They are quite frightening to people on the ground.” A bus-size meteoroid would explode in the stratosphere with the energy of a small atomic bomb, producing a blinding flash much brighter than the sun, says Chapman. “Military commanders in a region of tension might regard it as the hostile act of an enemy and retaliate,” he says. A 25-kiloton airburst occurred over the Mediterranean Sea on 6 June 2002. Imagine, Chapman says, “if that had happened instead in the vicinity of Kashmir, where tensions between India and Pakistan were elevated.”

While this scenario may argue for giving NEO sightings wide publicity, some experts think that detailed predictions—particularly risk corridors—should be withheld from the public. They want to avoid a “Chicken Little” phenomenon of repeatedly sounding alarms that are later downgraded or called off. NASA has not released Apophis's risk corridor in 2036. (The B612 Foundation provided the diagram above.) “We do not generally release these kinds of diagrams when they relate to future and ongoing risk assessments,” says Steven Chesley, an NEO specialist at NASA's Jet Propulsion Laboratory in Pasadena, California.

Others believe in full disclosure. “People don't like secrecy. It breeds distrust,” says Chapman. “When the facts are finally revealed, people wonder whether to believe them and wonder about what else might be still under wraps.” NEO impact forecasts, he says, should be treated like hurricane forecasts, allowing people to respond.

Like the first hurricane of the season, the first test of our planetary defenses may be an asteroid whose name starts with the letter “A.”

9. NEAR-EARTH OBJECTS

# The State of Our Planet's Defenses

1. Richard Stone

Experts can't say exactly when the next Earth-bound asteroid will heave into view, but they are confident that humanity has the tools to defend itself.

Experts can't say exactly when the next Earth-bound asteroid will heave into view, but they are confident that humanity has the tools to defend itself. There are several deflection scenarios; in most, the straightforward objective would be to change an asteroid's speed so that it arrives too early or too late to hit Earth.

In 2005, former astronaut Edward Lu of Google and astronomer-astronaut Stanley Love, a mission specialist on the shuttle flight last month that delivered the Columbus Laboratory to the space station, proposed a “gravity tractor”: a spacecraft that hovers in front of or behind an asteroid, using its minuscule gravitational force to slightly accelerate it or slow it. The gravity tractor could divert an asteroid from a keyhole, a narrow swath of space where gravitational forces would yank an asteroid onto a trajectory in which it would hit Earth a few years later.

Steering an asteroid clear of a keyhole would require less energy and thus is much easier to accomplish than diverting an asteroid on a direct course for Earth. A 1-ton gravity tractor would have to hover more than 3 years near a Tunguska-size, 45-meter-wide NEO on a collision course to change its orbit enough to bypass Earth, says Russell Schweickart, chair of the B612 Foundation. It would take less than 40 days to divert the much-larger Apophis from a keyhole on its close encounter with Earth in 2029, he says.

Another way to fiddle with Apophis's speed would be to spray it with material that changes the amount of sunlight it absorbs or reflects. If such a mission were mounted by 2018, just a few-percent change in its energy balance over 18 years would assure that Apophis misses Earth, says Jonathan Giorgini, a senior analyst at NASA.

Coaxing an asteroid to miss a keyhole won't necessarily eliminate the risk. Space is littered with keyholes and resonant return points that can sling an object back at Earth. To counter that possibility, scientists are devising ways to slap a transponder on a target asteroid or otherwise send back telemetry revealing whether a follow-up mission is necessary.

Last week, The Planetary Society in Pasadena, California, announced the winners of a competition to design a mission that would tag a potentially hazardous near-Earth asteroid to better track its orbit. The top prize of $25,000 went to two companies—SpaceWorks Engineering Inc. in Atlanta, Georgia, and SpaceDev Inc. in Poway, California—for Foresight, a$137 million spacecraft that would shadow Apophis for almost a year, taking its measure with a multispectral imager and a laser altimeter. The European Space Agency (ESA) has a similar mission on the drawing board: Don Quijote, which for €150 million would send a spacecraft to rendezvous with Apophis and precisely measure its position, mass, and other parameters. But Don Quijote is on hold. “Without an imminent impact threat, ESA is focusing on other priorities,” says the agency's Ian Carnelli.

A modest-sized asteroid spotted too late for subtle maneuvering could be rammed with a kinetic impactor—a missile, or a spacecraft similar to NASA's Deep Impact probe that crashed into comet 9P/Tempel in 2005. According to a report last year in the journal Science and Global Security, a single strike, with 5 years' lead time, should safely divert an asteroid up to 250 meters wide; many larger asteroids could be deflected with multiple strikes and longer lead times. A gravity tractor in place before the strike could both provide telemetry and give the asteroid an extra nudge, if necessary.

Some scientists, meanwhile, favor a nuclear detonation, the force of which could obliterate smaller asteroids—with the hope that any fragments still on target for Earth would burn up in the atmosphere—or alter the trajectory of larger ones. Schweickart, for one, views nukes as a last resort. An asteroid for which other technologies would fail, he contends, comes along only once every 100,000 years or so. However, notes Harvard University astronomer Brian Marsden, “if the warning time is such that an object will hit us in a matter of months or even weeks, the nuclear option is the only one we really have. If the warning time is only days, I really don't know what we would do.”

10. PALEONTOLOGY

# Experts Find No Evidence for a Mammoth-Killer Impact

1. Richard A. Kerr

A devastating cosmic collision 13,000 years ago continues to play well in the media, but specialists are challenging the grounds for thinking it happened.

A devastating cosmic collision 13,000 years ago continues to play well in the media, but specialists are challenging the grounds for thinking it happened

It looked impressive as slide after data-laden slide flashed on the screen last spring. Nearly a dozen debris markers, found at 26 sites from the U.S. West Coast to Belgium, testified to a huge impact followed by a continent-spanning wildfire. The catastrophe had taken place a geologic instant ago—closely coinciding with the disappearance of North America's mammoths and the continent's earliest human culture (Science, 1 June 2007, p. 1264). Then came the 26-author paper last October in the Proceedings of the National Academy of Sciences (PNAS), not to mention the hourlong National Geographic Channel documentary running on cable since last October, with more coverage on the way from the History Channel and PBS's prestigious program NOVA.

Although cosmically blasted mammoths may make good copy, many impact specialists have lately swung from leeriness to thorough disbelief. “The whole thing is contrived,” says geochemist and impact specialist Christian Koeberl of the University of Vienna, Austria. “Their data don't agree with anything we know about impacts. It just doesn't make any sense. Occam's razor has been put safely in a drawer somewhere.”

One problem is that no one has “any of the classic evidence of an impact,” says impact specialist David Kring of the Lunar and Planetary Institute in Houston, Texas. Spurred by the 1980s debate over what killed off the dinosaurs, “the community learned a lot about what the threshold of evidence is” for confirming an impact, he explains. But taking all the evidence offered by the group proposing the mammoth-killer impact, “you end up with [markers] that are not diagnostic of impact,” says impact specialist Bevan French of the National Museum of Natural History in Washington, D.C. Proponents, meanwhile, are defending some of their published claims and giving ground on others but promising ultimate vindication.

## Diamonds not forever

Everyone agrees on one point at least. “Obviously, something really interesting happened 13,000 years ago,” as Kring puts it. It was 12,900 years ago, to be precise, that a world staggering out of the last Ice Age suddenly plunged back into a millennium of near-glacial climate before emerging into the current warmth. It was also about then—emphasis on the uncertainties summed up by “about”—that the mammoths and other great beasts disappeared from North America. And the Paleo-Indian Clovis culture vanished from the archaeological record around then, too.

The PNAS authors have a cosmic explanation for the coincidence of climate shift, extinctions, and cultural oblivion: A body or clump of bodies from outer space ravaged North America. By exploding over or actually hitting the great ice sheet in the north, their reasoning goes, the impactors could have shifted climate into the chill of the so-called Younger Dryas (YD) period. And the blast or blasts, as well as the resulting continent-wide wildfire, would have sufficed to wipe out or at least seriously weaken man and beast.

Headed by nuclear chemist Richard Firestone of Lawrence Berkeley National Laboratory in California and retired geophysical consultant Allen West of Dewey, Arizona, the 26 PNAS co-authors present what they argue is debris from the impact: metallic bits, an abundance of the exotic element iridium, nanodiamonds, and molecular “buckyballs” filled with extraterrestrial helium. And the wildfire would have left charcoal, soot, carbon spherules, and glasslike carbon. Along with the impact debris, these components appear in a thin layer of sediments—the YD boundary layer—that was laid down near the beginning of the cold snap and the end of the mammoths.

That sort of litany impressed the largely nonexpert crowd at last May's Joint Assembly of the American Geophysical Union (AGU) in Acapulco, Mexico, but the few experts there were nonplussed. Now, in the wake of the detailed PNAS paper, the experts are able to take a more critical look. For starters, they are pointing out that the carbon-rich debris says nothing about the cause of the fires. Fire happened back then, notes geologist Nicholas Pinter of Southern Illinois University (SIU) in Carbondale, especially once humans arrived. Critics are equally quick to set aside the helium-filled buckyballs or fullerenes reported in the PNAS paper by geochemist Luann Becker of the University of California, Santa Barbara (UCSB). Throughout a half-dozen years of effort, no one else has replicated the isolation of fullerenes with helium (Science, 14 May 2004, p. 941).

Then there are the nanodiamonds. Zillions of diamond bits a few nanometers in size sound exotic enough. Many meteorites are filthy with them, so the impactor could have brought them in. Nanodiamonds have in fact been reported in the debris of the dinosaur-killing impact 65 million years ago.

At the AGU meeting, paleoceanographer and PNAS third author James Kennett of UCSB reported that UCSB colleagues had “conclusively” shown the presence of nanodiamonds in sediments from the YD boundary layer. They used transmission electron microscopy (TEM), the gold standard for nanodiamond identification. However, no TEM results appeared in the PNAS paper. Instead, a sample of glassy carbon recovered from the YD boundary had been sent to a commercial laboratory for analysis using carbon-13 nuclear magnetic resonance (NMR). The NMR analysis showed that the “sample contains nanodiamonds, which are inferred to be impact-related material,” the paper states.

Experts asked to comment on the findings disagree. “Their NMR data do not provide evidence for nanodiamonds,” says geochemist George Cody of the Carnegie Institution of Washington's Geophysical Laboratory in Washington, D.C., who in 2002 was the first to use NMR to identify nanodiamonds in meteorites. “I would never have claimed that [their NMR spectrum] had anything to do with nanodiamonds.”

Under the proper analytical conditions, says Cody, nanodiamonds produce a narrow NMR peak centered at a chemical shift of 34 parts per million. The PNAS spectrum is broad and centered at 38 parts per million, too broad and too far afield to be nanodiamonds, he says. In any case, the analytical conditions used were wrong for detecting nanodiamonds, Cody adds; no peak would have appeared even if they were there.

## Mundane metals?

Another claimed marker of the YD impact—the element iridium—is coming under attack as well. An iridium “spike” was the first clue to identifying the impact that caused the Cretaceous-Tertiary (K-T) mass extinction 65 million years ago. The metallic element is scarce in Earth's crust but relatively abundant in meteorites, so like nanodiamonds, any excess might have arrived via asteroid or comet.

Firestone and colleagues reported elevated iridium of a few parts per billion (ppb)—comparable to K-T sediments—in some sediment samples from the YD boundary but not in sediments above or below. They found tens to more than 100 ppb of iridium in microscopic particles—both rough grains and once-melted spherules—magnetically separated from some of those sediments. And they cite an earlier report in Nature of “large increases” in iridium “during the Younger Dryas as recorded in the GRIP (Greenland) ice core.” The iridium came from beyond Earth in an impactor, the group concluded.

Other researchers aren't sure where the iridium came from, if it's there at all. As to the ice core record, “I was surprised to see such an interpretation of our results in Nature,” says Paolo Gabrielli, first author of the Nature paper and now at Ohio State University in Columbus. “My paper does not report any large increase of iridium in the Younger Dryas. So it has nothing to do with an extraterrestrial impact.” Firestone disagrees: “I interpret his results differently than he does.”

Impact specialist Philippe Claeys of the Free University of Brussels in Belgium can't find any iridium at all in the four sediment samples of the YD boundary West sent him for analysis. The PNAS group eventually reported that two of the samples contained elevated iridium easily detectable by Claeys's method; the magnetic fraction of the third sample had extreme iridium concentrations. But Claeys reported to West that he could detect no iridium higher than 0.5 ppb in any of the samples. West blames the “nugget effect,” in which a few microscopic sediment particles highly enriched in iridium account for most of the iridium in an analyzed sample; samples that happen to have few nuggets look barren. Claeys, however, says he intentionally used large enough samples to avoid the nugget effect.

Archaeologist Vance Haynes, professor emeritus at the University of Arizona, Tucson, is finding likely looking magnetic spherules in the darnedest places. He has spent 30 years studying Clovis sites, many of which the Firestone group sampled. As a check on his own ongoing independent analysis of YD samples, he collected a modern sample. “I got 300 grams of dust off the roof [of my house], and it's full of magnetic microspherules,” he says. Whether they are the melted, iridium-rich micrometeorites that continually drift down from the upper atmosphere or the product of high-temperature industrial processes such as coal burning, he doesn't yet know. Either way, they could be trouble. The cosmic dandruff of microspherules could have salted sediments forming 12,900 years ago with iridium, while the humanmade variety might have settled on modern outcrops before sampling.

Chemical analyses of the magnetic particles do not point to impact, Koeberl says. The elemental analyses make little geochemical sense, he says. In particular, the magnetic particles are far too rich in titanium to be extraterrestrial. He rejects the suggestion in the PNAS paper that such odd geochemistry points to “a new and unknown type of impactor.” Meteoriticist Theodore Bunch of Northern Arizona University in Flagstaff, the fifth PNAS author, agrees that the magnetic fraction has problems. What its chemistry means, “I don't know,” he says, speaking for himself. In any case, “it detracts from the main thing.”

The main thing now is nanodiamonds, according to Bunch and other PNAS authors. The initial UCSB detection of nanodiamonds came too late for their paper, says Firestone. Now West is using TEM and has found three different types of nanodiamonds in the YD layer but failed to find any above or below it. “Some people just can't stand the idea of something falling out of the sky,” he says, but “they can't explain all of these [impact] markers, and diamond is the hardest to explain away.”

West and colleagues expect to publish on nanodiamonds, but their critics are still waiting to be impressed. Pinter and Scott Ishman, his micropaleontologist colleague at SIU, wrote in a detailed critique in the January issue of GSA Today that such “spectacular stories to explain unspectacular evidence consume the finite commodity of scientific credibility.” The problem, Pinter says, is that “there's a wide fringe beyond the impact community” where the criteria for impact identification laid out in the literature are not rigorously followed. Whether another try at nanodiamonds will meet the standard is anybody's guess.

11. PLANT SCIENCES

# Corn Genomics Pops Wide Open

1. Elizabeth Pennisi

The sequencing of maize genomes and the development of new strains are enabling faster exploitation of this key crop's natural diversity.

The sequencing of maize genomes and the development of new strains are enabling faster exploitation of this key crop's natural diversity

A decade ago, sequencing the maize genome was just too daunting. With 2.5 billion DNA bases, it rivaled the human genome in size and contained many repetitive regions that confounded the assembly of a final sequence. But last week, not one but three corn genomes, in various stages of completion, were introduced to the maize genetics community. In addition, researchers announced the availability of specially bred strains that will greatly speed tracking down genes involved in traits such as flowering time and disease resistance. These resources are ushering in a new era in maize genetics and should lead to tougher breeds, better yields, and biofuel alternatives. “We're sitting on very exciting times,” says Geoff Graham, a plant breeder at Pioneer Hi-Bred International Inc.

The world's biggest crop, maize (Zea mays) comes in all shapes and sizes. Indeed, the genomes of any two varieties can be as different as chimp and human DNA. Cataloging, understanding, and harnessing this variation to improve crop yields have been longtime goals for researchers.

Toward that end, in 2005, the U.S. National Science Foundation (NSF) and the U.S. departments of Agriculture (USDA) and Energy (DOE) provided $30 million to a consortium headed by Richard Wilson at Washington University in St. Louis, Missouri, to tackle the genome of a well-studied maize strain called B73. Rod Wing of the University of Arizona, Tucson, provided 15,000 mapped segments of the corn's DNA for sequencing, and at a meeting* last week in Washington, D.C., Wilson described B73's draft genome. About 6500 of the segments Wing provided are completely finished and most of the rest are well under way. Even at this stage, “we believe the quality and coverage will enable new discoveries,” says Wilson. Maize researchers agree. B73's full sequence “is going to underpin all the research that we do in maize genomics,” predicts Patrick Schnable of Iowa State University in Ames. Take the quest to improve the potential of corn and perennial grasses as biomass for biofuels. A key goal is to increase sugar content and sugar's availability for conversion to biofuels. “We need to greatly increase mass per acre,” says Nicholas Carpita, a plant cell biologist at Purdue University in West Lafayette, Indiana. He and his colleagues have compared the rice and Arabidopsis genomes with the B73 DNA already deposited in the public database GenBank. They found more than 1400 corn genes involved in building plant cell walls—the ultimate energy sources—and are homing in on those that affect biomass quantity and quality. “The maize genome allowed us to get to [those] genes,” he says. And the B73 genome isn't the only one in the works. With$9.1 million from the Mexican government, Jean-Philippe Vielle-Calzada of the National Laboratory of Genomics for Biodiversity in Irapuato and his colleagues have decoded a native “popcorn” strain grown at elevations above 2000 meters. Although still in more than 100,000 pieces, the sequence has revealed many new genes, he reported. This variety's genome “will be of tremendous value in terms of understanding the evolution of [maize] domestication,” he says.

In addition, Daniel Rokhsar of DOE's Joint Genome Institute in Walnut Creek, California, and his colleagues have begun to decipher the DNA of a well-studied maize strain called Mo17, using new, cheaper sequencing technologies. If the effort proves cost-effective, NSF and DOE may support the sequencing of additional strains.

But genome sequences aren't the only big news for maize researchers. As part of the Maize Diversity Project, USDA plant geneticist Edward Buckler of Cornell University and his colleagues have bred almost 5000 lines of maize, revealing the full range of the plant's diversity. These lines were derived from crosses between B73 and 25 other inbred maize lines from all over the world; each marriage has given rise to about 200 lines. For the past 2 years, teams have planted these lines in 11 fields across the United States and measured many different traits—height, cob size, flowering time, and so on—for each line.

Using those lines, Buckler's team has also put together a detailed genetic map of the maize genome, which is helping researchers home in on target genes by means of an approach called nested association mapping. “It's an incredible resource … on equal par to having the sequence,” says Cornell's Thomas Brutnell.

Using the map, researchers can easily pinpoint the spots on the genome that underlie variation in a particular trait, then use the genome sequence to figure out which gene is at that spot. “It holds [great] power,” says Jay Hollick of the University of California, Berkeley. “Virtually any trait can be measured.”

Already, Buckler reported, his team has pinned down 50 genes that dictate flowering time. Some lines flower as much as 45 days apart, but no single gene region shifted flowering time by more than 3 days.

Another resource introduced at the meeting will help Buckler and others sort out how genes interact. The agribusiness giant Syngenta announced it was making available 7500 lines of corn, each representing a B73 genome with a single piece of DNA bred into it from one of the 25 strains of the Maize Diversity Project. Taken together, the lines incorporate all the genetic diversity of those strains but make it easier to understand the activity of particular genes. The community has long awaited these tools, says Brutnell: “They are really going to revolutionize the way we do genetics.”

• *“The 50th Annual Maize Genetics Conference,” 27 February-2 March, Washington, D.C.