The Enduring Mysteries of Comets

The Enduring Mysteries of Comets

For millennia, comets were believed to be omens of doom. Instead, solving the mysteries regarding these "dirty snowballs" could help reveal the part they played in the birth of life on Earth, as well as secrets concerning the rest of the galaxy.

Did comets help create Earth's seas?

For years scientists thought comets slamming against the newborn Earth helped deliver water to a once dry planet. But roughly a decade ago this view was shaken by the discovery that the water in comets and Earth's oceans did not match up in terms of hydrogen isotopes.

Calculations then showed it was highly improbable that enough icy rocks from the suspected homes of comets — the Kuiper belt past Neptune and the Oort cloud past that — could have collided with Earth to supply its oceans.

In the last two years, however, researchers have discovered comets in the outer part of the asteroid belt. These "main-belt comets" may have the right levels of hydrogen isotopes, and are perhaps close enough to Earth to have realistically brought us the seas that life emerged from.

"No one knows for certain yet where Earth's oceans came from," said University of Hawaii astrophysicist David Jewitt. "Earth's oceans are likely a mixture of water from all sorts of places, but the main-belt comets are very likely one of them."

Where do comets come from?

The suspected homes of comets include the Oort cloud, the Kuiper belt and now the asteroid belt. But are there more reservoirs of comets yet to be found?

The Oort cloud is a theoretical cloud of icy rocks roughly 4.6 trillion miles (7.5 trillion kilometers) from the sun thought to be the source of long-period comets — that is, ones that take more than a few centuries to complete their orbits. It was once thought the original home of short-period comets as well, until calculations suggested that was impossible.

About 20 years ago, the Kuiper belt roughly 4.6 billion miles (7.5 billion kilometers) from the sun was then proposed to be the home of short-period comets. "But measurements taken in the last few years raise some doubts about that," Jewitt explained. "Maybe there are other reservoirs of comets yet to be discovered."

Secrets regarding the birth of the solar system?

Comets were long thought to be primordial relics, pristine leftovers from the protoplanetary disk that once surrounded the newborn sun. As such, it was supposed they might hold secrets untouched for billions of years regarding the birth of our solar system.

Increasingly, however, it looks as if the comets we see are anything but unspoiled. Instead, "there is good evidence that many of them are nearly burned-out hulks, with neither the size, mass, shape nor spin they might have had before entering the solar system," Jewitt said.

Still, "since comets are icy, they're not entirely cooked, and we may learn a lot regarding the formation of the solar system from chemicals trapped in their ice," he added.

Comets so close to the sun?

The main-belt comets are themselves a mystery. Until their discovery, researchers had largely supposed no comets could have lasted that close to the sun without getting baked away after a few centuries or millennia.

Dirt coatings on main-belt comets could have protected them from sunlight for billions of years. Every now and again boulders a yard or larger tumbling around the asteroid belt might hit these comets, uncovering their ice and triggering the plumes of gas and dust that got them discovered in the first place.

"We expect to soon find many hundreds or thousands of main-belt comets," Jewitt said.

Interstellar comets?

As our solar system formed, calculations predict the gravitational pull of the planets would have scattered 90 to 99 percent of all comets that once orbited the sun away toward the stars, never to be seen again. "If every star does that, you would expect some of their comets to come toward us, but no such object has ever been seen," Jewitt said.

Still, as astronomical telescopes and techniques improve, Jewitt remains optimistic that such interstellar comets will be detected fairly soon. These comets would prove quite distinctive, zipping at great speeds and following trajectories completely unlike the orbits our comets follow.

"We could see interstellar comets for the first time in the next few years," Jewitt predicted. "It would be great if we saw one, especially so if we had the wherewithal to launch a mission to one, to get samples and study the diversity of comets in an interstellar and galactic context. But we have to find one first."

Portrait of a Meltdown

Portrait of a Meltdown: Many factors led to 2007's record low in Arctic sea ice

A variety of climatological factors converged this year in a perfect storm that dramatically melted the Arctic Ocean's ice cover to a record low. The abrupt downturn could be a harbinger of ice-poor summers for decades to come.  

GOING DOWN. The long-term decline in the extent of the Arctic Ocean's end-of-summer sea ice is shown superimposed on a graphic depicting this year's record-low ice coverage.

 In late summer, scientists reported that Arctic sea ice had shrunk to cover only about 4.2 million square kilometers (SN: 10/13/07, p. 238). That area is about 38 percent below the long-term average for late-summer ice coverage. Moreover, it's a striking 23 percent below the previous record low, set just 2 years ago. An adverse combination of factors contributed to this year's steep decline, researchers noted last week at a meeting of the American Geophysical Union in San Francisco.

First, a long-term trend in thinning and shrinkage of Arctic ice set the stage for this year's meltdown, says Jinlun Zhang, an oceanographer at the University of Washington in Seattle. End-of-summer ice coverage has been declining by about 11.4 percent per decade since 1979. Also, average ice thickness decreased by about 1.13 meters, or 22 percent, between 1981 and 2000.

Second, Zhang notes, unusually strong summer winds pushed much of the ice out of the central Arctic, leaving a large area of thin ice and open water. Third, a decrease in cloud cover in the Arctic—a trend suspected but not confirmed earlier this year (SN: 6/16/07, p. 382)—allowed more sunlight to reach the ocean. Because open water absorbs more of the sun's radiation than snow-covered ice, it significantly boosts warming trends both for the ocean and for the atmosphere above it (SN: 11/12/05, p. 312). This so-called ice/albedo feedback accelerated this year's melting, says Zhang.

In parts of the Arctic Ocean this year, sea surface temperatures were 3.5°C warmer than average and a full 1.5°C warmer than previously recorded highs, says Michael Steele, also of the University of Washington in Seattle. All that warm water chewed away at Arctic ice from below. In some parts of the Beaufort Sea, north of Alaska and western Canada, ice that started the summer 3.3 m thick ended up measuring just 50 centimeters, says Donald K. Perovich, a geophysicist at the U.S. Army Cold Regions Research and Engineering Laboratory in Hanover, N.H.

About 70 cm of that shrinkage resulted from melting of the ice's upper surface—a typical amount for the summer, says Perovich. However, a whopping 2 m or so of that erosion, about five times the normal summer loss, occurred from below.

The thinning conceals the true extent of ice loss, says Perovich. "There's a lot less ice there than we think," he notes. "And the farther we go down this path, the harder it is to get back."

Indeed, the Arctic meltback may be self-perpetuating, says Steele. In some areas, the average date for winter freeze-up is now 2 months later than usual. The extra heat absorbed during summer months will suppress ice thickness by as much as 75 cm, about half the growth in thickness during an average winter.

Has the meltdown in the Arctic reached a point of no return? Many scientists, including Perovich, speculate that it has. "Years from now, we'll look back at 2007 and be amazed," he says. 

BioChips Offer Animal-Friendlier Drug-Testing Technology

BioChips Offer Animal-Friendlier Drug-Testing Technology

Biochips promise to deliver better drug and chemical testing, cutting costs and nixing the need for most animal testing

 
METACHIP: A MetaChip is a glass slide dotted with 20-nanoliter droplets of a solution containing human liver enzymes; researchers can test toxicity of compounds by introducing these chemicals into the solution droplets and seeing how they react.

 
DATACHIP: DataChip is a glass slide lined with droplets containing cell cultures from the bladder, kidney or liver; scientists can test a chemical's safeness by putting drops of it onto the slide and measuring the culture's growth or shrinkage over time.

 
RESULTS: Once a DataChip has been prepared, researchers use an optical assay system< consisting of a camera connected to a fluorescent light source to take a digital image of the cell culture and highlight living (green) and dead (red) cells.

The journey of a drug from lab to pharmacy is usually long and pricey, typically taking a decade or more and gobbling up hundreds of millions of dollars. Pharmaceutical and chemical companies are willing to make these major investments in time and money on chemical compounds that promise to become the next Viagra, Prozac or other blockbuster medication. Often, however, these experiments are scuttled late in the game because toxic side effects surface.

Drugmakers may soon have a new tool to assess safety much earlier in the process, saving them money and time and negating the need to test early-stage compounds on live animals. Researchers at Rensselaer Polytechnic Institute in Troy, N.Y., the University of California, Berkeley, and Solidus Biosciences, Inc. (a biotech company located at the Rensselaer Incubator Program for start-up businesses), report in Proceedings of the National Academy of Sciences USA that they have developed biochip technology that promises to reveal the potential toxicity of chemicals and drug candidates during early experiments.

These biochips—called MetaChip and DataChip—mimic what the body does when it ingests a drug. MetaChip is actually a glass slide dotted with 20-nanoliter droplets—each 20 billionths of a liter—of a solution containing human liver enzymes; researchers can test toxicity of compounds by introducing these chemicals into the solution droplets and seeing how they react. DataChip is also glass slide, but it is lined with droplets containing cell cultures from the bladder, kidney or liver; scientists can test a chemical's safeness by putting drops of it onto the slide and measuring the culture's growth or shrinkage over time. The two biochips can also be used in tandem—a MetaChip can be turned over and applied directly to a DataChip to see how the materials interact.

"We started with the aim of reducing the cost of developing new drugs by enabling toxicity assays much earlier in the drug development process," says Douglas Clark, a U.C. Berkeley professor of chemical engineering and a Solidus Biosciences co-founder. "We wanted drug candidates to be screened for toxicity at the same time they are screened for efficacy. Ultimately, that will lower the cost of failure and that will lower the cost of drugs brought to market."

MetaChip and DataChip are now a reality because scientists are able to isolate and generate p450 liver enzymes as well as make three-dimensional cell cultures in droplets. It is difficult to quantify how much biochips will speed up development and cut costs, says Solidus co-founder Jonathan Dordick, a Rensselaer professor of chemical and biological engineering. "The safest thing to say is that about 70 percent of drug failures occur due to toxicity. If we can catch these sooner, those drug compounds would never make it to clinical trials. It costs hundreds of millions of dollars per chemical compound to take a drug through the discovery process."

And these costs do not take into account the large number of animals needlessly subjected to potentially toxic substances even though humans may respond differently. "There's always a question you have to ask," Dordick says, "and that is whether testing on an animal is predictive of how a human will react."

Demand for biochip technology will no doubt rise in response to a European Union ban on testing on animals set to take effect in March 2009. "I'm not suggesting that we'll eliminate animal testing in the pharmaceuticals industry, but it can be done later in the testing process," Dordick says, after MetaChips and DataChips identify which chemical compounds are safe enough to make it to the stage where such testing might be useful. Solidus is working to commercialize its chips and is close to signing a contract with a "large cosmetics company," Clark says, declining to name the firm.

The technology required to create MetaChips and DataChips is largely available today. The use of biochips requires a high-throughput microarray spotter machine to place the liquid enzyme dots on the slides. The next step involves an optical assay system consisting of a camera connected to a fluorescent light source to take a digital image of the cell culture and highlight living and dead cells. Dordick says the ultimate goal is to create one machine that can carry out both functions. Researchers say the biochips may also be used to target different drugs to different groups of patients. "Ultimately, each person would have their own DataChip or MetaChip that contains their own genetic information," Dordick says, noting that most drugs on the market today are "one size fits all."

One way the DataChip can be expanded is to include different cell types representing different organs. In addition to the aforementioned cells, Clarks says researchers are now developing solutions containing cardiac, neural and skin cells. The company is also looking for ways to do more than toxicity testing on its chips. In the cosmetics industry in particular, the chips might someday be used in lieu of guinea pigs—both the actual and human ones—to determine toxicity as well as whether certain chemicals may cause allergic reactions or irritation in certain skin types.

Closest Whale Cousin—A Fox-Size Deer?

Closest Whale Cousin—A Fox-Size Deer?

Researchers split on closest evolutionary kin to whales and dolphins 

 
INDOHYUS SKULL shares similarities with whales and dolphins, particularly in the structure of its inner ear. Researchers say this raccoon-size, 48-million-year-old wading mammal may be the closest known relative to seafaring mammals.

 
GONE SWIMMING: Indohyus likely spent most of its time wading in shallow water, as in this artist's depiction, perhaps coming to shore to feed on plants.

A group of researchers says that the closest known evolutionary cousin of whales, dolphins and porpoises is not the hippopotamus, as conventional wisdom has it, but an extinct deer-like animal roughly the size of a raccoon. In a new study, the team finds that a fossilized specimen of the extinct, 48-million-year-old mammal Indohyus bears several telling similarities to whales, including dense limb bones for ballast and a middle ear structure found only in the cetaceans, or sea-dwelling mammals, which is thought to help them hear underwater.

"What we think happened is that the ancestors of both Indohyus and whales were animals that looked like a tiny deer," says Hans Thewissen, professor of anatomy at Northeastern Ohio Universities Colleges of Medicine and Pharmacy, who led the study, published in Nature. The modern creature that most resembles Indohyus, however, is the African mousedeer (or chevrotain), which lives on the forest floor but scurries into the water to take cover from predators [see video of eagle pursuing chevrotain]. Similarly, Thewissen says, the common ancestor of whales and Indohyus may have been a herbivore (plant-eaters) that took to water to hide out, but eventually switched to a swimming, meat-eating lifestyle, which it passed down to modern cetaceans.

Other experts, however, caution that although the scenario is possible, the ancestry analysis is based on incomplete data. Researchers "really thought the book was closed on this," says Annalisa Berta, an evolutionary biologist at San Diego State University. "To suggest that this fossil somehow is closer than hippos, that's a big deal—I'm just not convinced."

Whatever its relationship with whales, Indohyus was probably not a direct predecessor of them, Thewissen says, because the specimen, unearthed 30 years ago in Kashmir, dates to roughly two million years after the earliest known cetacean fossils.

Over the past 15 years, researchers have uncovered a series of fossils intermediate between whales and land animals, but were still missing a link to landlubbing beasts, which Thewissen says Indohyus now provides.

The disagreement reflects the fact that researchers weren't around to watch successive populations of animals budding off from those that came before, resulting in a bushy pattern of new species over the millennia. Instead they have to infer the bush's branch points. They do this by categorizing extinct and living species together into the evolutionary bush that has the fewest cases of unrelated species evolving the same trait, which is in general less likely than one species passing down a unique trait to another species.

Indohyus, for example, has a half-walnut-shaped tympanic (a bony casing around the middle ear bones), with thicker walls on the sides than in front. The same thickening is found in all fossil and modern cetaceans but not in other mammals, the group notes. It also has dense bones that would have weighed it down in water. But in contrast to the pointy molars of dolphins or killer whales, however, its equivalent nchompers are squarish like those of hippos, possibly for grinding plant matter, the group suggests.

The researchers used a computer program to test the possible evolutionary bushes that could have yielded cetaceans along with artiodactyls—the mammalian order made up of two-toed—which include Indohyus and hippos.

The new analysis does not yet unseat the hippo as cetaceans' kissing cousin, because it only takes into account anatomical features, not molecular ones, says Maureen O'Leary, a professor in the department of anatomical sciences at Stony Brook University on Long Island, N.Y. She says that her own categorization of artiodactyls supports the hippo as the closest relative to cetaceans, but notes that it did not include the features uncovered by the Ohio team.

Signs of TB in Ancient Skull Support Theory on Vitamin D

Signs of TB in Ancient Skull Support Theory on Vitamin D

 
Photographs by, left, Marsha Miller/University of Texas, Austin; right, John Kappelman

A replica of the fossil found in a quarry in Turkey, right. A stylus points to lesions from tuberculosis.

In the disease-scarred bones of a Homo erectus from Turkey, scientists have found evidence of a peril that human ancestors encountered in their migrations out of Africa: tuberculosis.

Paleontologists examining small lesions etched inside the 500,000-year-old skull said this was the earliest known sign of a form of tuberculosis that attacks the meninges, the membranes surrounding the brain. Previously, the earliest physical traces of TB were only a few thousand years old, in mummies from Egypt and pre-Columbian Peru.

The discovery was not surprising, as recent genetic research has indicated that TB pathogens existed in the time of protohuman species. And the Peruvian evidence showed that the disease was introduced into the Americas from Asia, perhaps as much as 15,000 years ago.

But the discovery’s importance, scientists say, is the support it gives to the theory that dark-skinned people who migrate out of tropical climates tend to have lower levels of vitamin D, a condition that can adversely affect the immune system as well as the skeleton.

While the presumably dark skin of human ancestors protected them from the intense ultraviolet radiation from the African sun, the adaptation became a liability when they moved into the temperate latitudes of Eurasia, as the pigment melanin blocked much of the attenuated sunlight. The reduction of absorbed vitamin D from sunlight compromised their immune systems.

The findings are reported in the current issue of The American Journal of Physical Anthropology by a team of American, Turkish and German researchers led by John Kappelman, a geologist and paleontologist at the University of Texas, Austin.

Dr. Kappelman said in a telephone interview that the lesions near the base of the cranium were “dead ringers” for bone scars seen in modern specimens as a result of Leptomeningitis tuberculosa, the bacterium that causes a fatal disease of the brain not as common as the one that attacks the lungs. The analysis was made by Michael Schultz, an anatomist at Göttingen University in Germany.

Calling the discovery “fantastic and significant,” Nina G. Jablonski, an anthropologist at Penn State, agreed that it provided strong support for the relationship of skin color to vitamin D deficiencies. She was not involved in the discovery but has made an extensive study of the dark skin-vitamin relationship in modern humans.

In Africa, Dr. Jablonski noted, sunlight is so strong that even with dark skins as protection against its deleterious effects, enough light is absorbed to provide high levels of vitamin D. The risk of vitamin D deprivation increased as dark-skinned people moved to temperate zones, then as now. In the ancestral migrations, she said, it probably took hundreds of generations for immigrants to evolve lighter skins for absorbing more sunlight.

Homo erectus is widely believed to be among the first hominids to leave Africa, and Turkey was probably one of their avenues of travel. But the specimen studied by Dr. Kappelman’s team was the first early hominid to be found in Turkey. It was uncovered more than two years ago in a travertine quarry near Denizli, in western Turkey.

Dr. Kappelman conceded that in the absence of a full skeleton, there might be some dispute that this is a Homo erectus and not some slightly more evolved hominid species.

But the skull has erectus characteristics, he said, and appeared to be that of a young male.

The scientists said the shape and location of the lesions linked them to TB. The tiny disease bumps pressed on the membrane and that pressed against the bone, leaving the impressions. The fact that the lesions had time to leave imprints suggested to scientists that the TB was not especially virulent, probably afflicting the victim with an illness that lingered months before death.

In their journal article, the researchers said that the new evidence “permits us to speculate that a case of TB was exacerbated by the reduced level of ultraviolet radiation encountered during the expansion of a low-latitude population of dark-skinned Homo into temperate Turkey.”

The researchers further pointed out that this was not likely to be the earliest case of tuberculosis in hominid ancestors or their close kin, apes and other primates, which are also afflicted by forms of TB.