Studies find hardy Earth microbes may resist conditions on Mars

 A hardy bacteria common on Earth was surprisingly adaptive to Mars-like low pressure, cold and carbon dioxide-rich atmosphere, a finding that has implications in the search for extraterrestrial life.

The bacteria, known as Serratia liquefaciens, is found in human skin, hair and lungs, as well as in fish, aquatic systems, plant leaves and roots.

"It's present in a wide range of medium-temperature ecological niches," microbiologist Andrew Schuerger, with the University of Florida, told Reuters.

Serratia liquefaciens most likely evolved at sea level, so it was surprising to find it could grow in an experiment chamber that reduced pressure down to a Mars-like 7 millibars, Schuerger said.

Sea-level atmospheric pressure on Earth is about 1,000 millibars or 1 bar.

"It was a really big surprise," Schuerger said. "We had no reason to believe it was going to be able to grow at 7 millibars. It was just included in the study because we had cultures easily on hand and these species have been recovered from spacecraft."

In addition to concerns that hitchhiking microbes could inadvertently contaminate Mars, the study opens the door to a wider variety of life forms with the potential to evolve indigenously.

To survive, however, the microbes would need to be shielded from the harsh ultraviolet radiation that continually blasts the surface of Mars, as well as have access to a source of water, organic carbon and nitrogen.

NASA's Curiosity Mars rover is five months into a planned two-year mission to look for chemistry and environmental conditions that could have supported and preserved microbial life.

Scientists do not expect to find life at the rover's landing site - a very dry, ancient impact basin called Gale Crater located near the Martian equator. They are however hoping to learn if the planet most like Earth in the solar system has or ever had the ingredients for life by chemically analyzing rocks and soil in layers of sediment rising from the crater's floor.

So far, efforts to find Earth microbes that could live in the harsh conditions of Mars have primarily focused on so-called extremophiles which are found only in extreme cold, dry or acidic environments on Earth.

Two extremophiles tested along with the Serratia liquefaciens and 23 other common microbes did not survive the experiment, which not only replicated Mars' low pressure, but also its cold temperature and carbon dioxide-rich atmosphere.

A follow-up experiment on about 10,000 other microbes retrieved from boring 40 to 70 feet into the Siberian permafrost found six species - all members of the genus Carnobacterium - that could survive and grow in the simulated Mars chamber, located at the Space Life Sciences Laboratory adjacent to NASA's Kennedy Space Center in Florida.

The next step is to see how the microbes fare under even more hostile conditions, such as higher salt levels, more radiation and less water.

Related studies to analyze the genetics and metabolism of the common bacteria Serratia liquefaciens also are under way.

"In the search for life on another planet, we have to start with something that we at least have access to. We don't have a Martian bacterium we can experiment with, not yet, so we keep trying to see if some of our own hardy micro-organisms have the ability to grow at another location," Schuerger said.

"If we can never find a microbe that can grow under conditions on another planet, then it starts implying that life may not exist on that other location," he said.

The studies appear in the December 19 edition of the Proceedings of the National Academy of Sciences and this week in the journal Astrobiology.

Did Mars's Magnetic Field Die With a Whimper or a Bang?

Giant asteroids may have wiped out Mars's magnetic field. The energy released by massive collisions upset the heat flow in the planet's iron core that produced the magnetism, according to a new study. The finding offers a solution to the mystery of the disappearing magnetic field and sheds light on early Earth conditions.

A planet's magnetic field results from a process called convection. Within the core, molten iron rises, cools, and sinks. The convection induces a magnetic field, in a system known as a dynamo.

Like Earth, early Mars had a magnetic field and perhaps an atmosphere conducive to liquid water. But magnetic analysis of the martian surface indicates that when Mars was a mere 500 million years old, its magnetic field withered away. Without this shield, streams of ionizing particles spewing from the sun strip away a planet's atmosphere, killing any life that may have emerged or forcing it underground.

The disappearance of the martian magnetic dynamo has puzzled scientists. One theory links it to the Late Heavy Bombardment, a period of 100 million years when asteroids--some hundreds of kilometers across--smashed into Mars and the inner planets. A massive collision could warm Mars's mantle, disrupting core convection. That's because the cooling action of the mantle draws heat from the core, keeping it churning. Without that flow, core convection grinds to a halt.

The theory fits with the observation that only the oldest impact craters on Mars are magnetized. Still, all remained speculation until data came back from the Mars Global Surveyor and other recent spacecraft. Last year, planetary scientists Robert Lillis and Michael Manga, both of the University of California, Berkeley, linked age estimates of impact basins with magnetic field strength to show that the previously established date of heavy bombardment, about 3.9 billion years ago, corresponds to the death of Mars's dynamo.

Could the bombardment have released enough energy to trigger the shutdown? Lillis, Manga, and planetary geophysicist James Roberts of John Hopkins University Applied Physics lab in Laurel, Maryland, have modeled the effects of heat produced by impacts. When they added the heat release from the biggest asteroids to models of mantle convection, they found that the mantle became a heating blanket rather than an ice pack. The extra heat was enough to stop core convection, the team reports in the current issue of the Journal of Geophysical Research – Planets.

Mars was hit by at least five particularly large asteroids during the bombardment. "Any one of the super-giant impacts could have shut off [the dynamo]," says Roberts. Earth likely suffered the same onslaught, but at twice the radius of Mars, it probably had a strong enough dynamo to withstand or recover from huge impacts.

Not all scientists are on board with the analysis. David Stevenson, a planetary scientist at the California Institute of Technology in Pasadena, suggests that although the explanation is plausible, he's not convinced the collisions released enough energy. Furthermore, "the dynamo does not need to have an external influence to stop functioning," he points out, adding that without enough core convection, "it may simply die of its own accord."

Invisibility Cloak for Almost-Visible Light

An invisibility cloak that works for visible light might soon be in sight, now that a team has made one that works for the slightly longer wavelengths of near-infrared light. The cloak is only about a micrometer in size--a little snug for Harry Potter--but other researchers say it's a major advance.

The ages-old fantasy of invisibility became scientifically acceptable in 2006, after work by theoretical physicist John Pendry and colleagues at Imperial College London and, working independently, Ulf Leonhardt of the University of St. Andrews in the United Kingdom. The theorists imagined stretching space so that light rays would flow around an object and rejoin seamlessly beyond it, rendering the object imperceptible (Science, 26 May 2006, p. 1120).

Of course, researchers can't stretch space. But Pendry and Leonhardt showed how to fill a region with a "metamaterial," an assemblage of metallic rods, rings, and other bits that together interact with light to manipulate it in novel ways, to mimic such stretching. Five months later, David Smith and colleagues at Duke University in Durham, North Carolina, built a cloak for microwaves with a fixed wavelength roughly 46,000 times longer than visible light waves (Science, 20 October 2006, p. 403).

Now, Xiang Zhang, an applied physicist at the University of California, Berkeley, and colleagues have fashioned a cloak that works for near-infrared light with wavelengths 1.8 to 2.4 times longer than visible light. Whereas the first cloak was a circular shield that ferried microwaves around something inside it, Zhang's cloak is a so-called carpet cloak. The scheme, first introduced last year by Pendry and Jensen Li, now a member of Zhang's team, consists of a layer of metamaterial covering a flat mirror. The layer has a small space beneath it, like a hump in a carpet, but is tailored in such a way that light continues to reflect as usual, so that something can be concealed in the void.

You can't even hide your pocket change in Zhang's cloak, however. The whole experiment takes place within a slab of silicon 250 nanometers thick, to which the light is confined. The mirror consists of the metal-coated edge of the slab, which reflects light back into the material. A notch in the edge serves as the hiding place. The researchers sculpted the optical properties of the silicon by drilling closely spaced 110-nanometer-wide holes through it. Regions with more holes would have a lower "index of refraction," which means that light travels through them at a higher speed. Light reflected neatly off the flat edge but scattered wildly off an uncloaked notch. When the researchers tailored the pattern of holes above the notch to make a cloak, the light once again reflected as if it were hitting a mirror, the researchers report online this week in Nature Materials. Michal Lipson and colleagues at Cornell University report similar work in a paper posted to the arXiv preprint server (www.arXiv.org) on 22 April.

Experts praise the work. "I think it's incredible," says Pendry, who has laid much of the conceptual groundwork for cloaking in the past few years. "It amazes me how quickly the experimental groups can react to theoretical suggestions." "It's a huge step," agrees Duke's Smith. "I think it's very possible that this will be pushed into the visible." The carpet cloak mimics the bowing of plane into a hump, which is much less radical than the stretching of a line into a cylinder in the original cloak, he notes. As a result, it's possible to make one using only insulating "dielectric" materials such as silicon or glass and no metals, which tend to strongly absorb visible light. That, in turn, should make it easier to design carpet cloaks and mathematically related devices that work at shorter wavelengths, Smith says: "This should open people's eyes" to the possibilities.

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The Curse of the Mummies' Arteries

In the ancient tomb paintings of the Nile Valley, Egypt's nobility often appears lithe, beautiful, and, above all, healthy. But researchers have long doubted that life at the top of the social pyramid in ancient Egypt was quite so rosy. At least as early as 1500 B.C.E., Egyptian physicians observed symptoms of angina, heart attacks, and congestive heart failure in patients and recorded them in medical papyri. Yet they gave little indication of how often they saw such cases.

Now a team led by cardiologists Adel Allam of the Al Azhar Medical School in Cairo and Gregory Thomas of the University of California, Irvine, has conducted the broadest and most detailed study yet of atherosclerosis—a hardening of the arteries that causes both strokes and heart attacks—among ancient Egypt's upper classes by building on earlier work and performing CT scans on 52 of their mummies. In a study presented at the scientific session of the American College of Cardiology today in New Orleans, the team found that 44 of the mummies still possessed identifiable cardiovascular tissue, and of these 45% exhibited definite or probable hardening of the arteries.

"We were a bit surprised by how just how much atherosclerosis we found on ancient Egyptians who were young," says team member James Sutherland, a radiologist at the South Coast Radiological Medical Group in Laguna Hills, California. "The average age of death was around 40."

Researchers have long known that factors as such as smoking, a genetic predisposition to early heart disease, a calorie-rich diet, and a lifestyle lacking in exercise all increase the risk of atherosclerosis today. But how prevalent were these factors in ancient Egypt? According to ancient hieroglyphic inscriptions, wealthy ancient Egyptians relished such calorie-rich fare as cakes sweetened with honey. But they did not smoke tobacco and, in an age before automobiles, they likely got more exercise than many of us do today. "So we think there must be other risk factors that we are missing," says Thomas.

One such risk might have been a high exposure to bacterial infection and infectious disease. Parasitic diseases such as malaria and schistosomiasis are endemic in the Nile Valley, and the ancient Egyptians had little means of treating them. "So all that infection might have really revved up the [ancient Egyptians'] inflammatory response," says team member Michael Miyamoto, a cardiologist at the Mission Internal Medical Group in Mission Viejo, California. Inflammation helps to remove the agents of infection and promote healing, but older individuals may have paid a steep price for it: high levels of inflammatory response can contribute to the development of atherosclerosis.

The team now plans to test this hypothesis. Since microscopic signs of inflammation might well be absent from ancient, desiccated tissue, the researchers intend to look for indirect evidence of inflammation by examining CT scans of the mummies for the extent of chronic infections, such as bone infections or periodontal disease.

Guido Lombardi, a mummy researcher and paleopathologist at Cayetano Heredia University in Lima, Peru, is impressed by the study. "The team found mummies that were intact, with arteries that were readily identifiable. … I have no doubt that [their diagnoses] are right," he says.

Moreover, Frank Rühli, an anatomist at the Centre for Evolutionary Medicine at the University of Zurich and the co-director of the Swiss Mummy Project, which is examining mummies in Swiss collections by medical imaging, is intrigued by the team's hypothesis that high inflammation levels may have posed a major risk for vascular disease in ancient Egypt. "I think the whole issue now is to find out what the risk factors were and how they changed over time," he says