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| Sunday, 4 September 2005 |
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Enceladus, delights and baffles Space scientists say their discoveries about Saturn's moon Enceladus are stunning, if just a little baffling. Using the instrument-packed Cassini probe, they have confirmed that the 500km-wide world has an atmosphere.
"There were signs from a long time ago that Enceladus was a strange moon," said Dr Carolyn Porco, leader of Cassini's imaging team, "but it is just so gratifying and fabulous to see all the results come together and clearly point to a specific region on the surface which seems to be the origin of a lot of that peculiarity." The moon has become a major target of interest since the Cassini mission to the Saturn system arrived just over a year ago. Enceladus orbits the ringed planet at a distance of approximately 237,400km and is described as the most reflective object in the Solar System; its icy surface throws back about 90% of the sunlight that hits it. The spacecraft made a special low pass of the moon on 14 July, crossing a mere 173km above the surface at its closest approach. This allowed Cassini to make observations of unprecedented detail; and they backed up data obtained by the probe's magnetometer instrument on previous flybys that hinted at the presence of a water vapour atmosphere. But that was just the start of what is now proving to be a fascinating and evolving story. "We confirmed the signature that there was an atmosphere but it is a strange atmosphere," Professor Michele Dougherty, from the UK's Imperial College and the lead scientist for the magnetometer instrument, said. "It seems to be concentrated at the south pole and the best way to match our observations is that you have almost a cometary jet coming off the south pole." High-resolution imagery shows the southern polar region to be relatively smooth - usually a good indicator of recent activity - but cut by a number of long, dominant cracks. These are the so-called tiger stripes. They are about 130km long and roughly parallel to one another, spaced about 40km apart. Cassini's composite infrared spectrometer shows the region to be much warmer than expected. Whereas temperatures near the equator are a frigid 80 Kelvin (minus 193C), the south polar average reaches 85K (minus 188C). Small areas of the pole, concentrated near the tiger stripe fractures are even warmer: well over 110K (minus 163C) in some places. "The amount of heat there is really hard to understand as being due to just sunlight warming the surface," said Dr John Spencer, from the Southwest Research Institute in Colorado, US. "It shouldn't be that warm at the pole. It would be like flying past the Earth and finding that Antarctica was warmer than the equatorial regions. "This is only the second place in the Solar System beyond Earth that we've seen signs of heat coming out of the interior - the other being Jupiter's moon Io." The scientists think the cracks may act like vents, spewing out water vapour and very fine water-ice particles. Some have suggested there could be ice geysers and even ice volcanoes at the stripe locations - but these have not been imaged directly. The puzzle for researchers is how to explain such an energetic system on Enceladus. As the moon moves around an eccentric orbit of Saturn, gravitational forces should subject the tiny world to some tidal heating. Radioactive isotopes in its rocky core may also be a source of some warming. But scientists are struggling to make the numbers add up and are frankly baffled as to why the activity they see should be so concentrated in just the one region. "One of the most fascinating aspects of Enceladus is that it's so very small as icy moons go, but so very geophysically active," said Dr Bob Brown, from the University of Arizona, US, and team leader for Cassini's visual and infrared mapping spectrometer. "It's hard for a body as small as Enceladus to hold onto the heat necessary to drive such large-scale geophysical phenomena, but it had done just that. ############### Gravity-defying Geckos teach scientists a lesson Five years ago, researchers at the University of California, Berkeley; Stanford; and Lewis and Clark College found the secret: 500,000 minute hairs cover the sole of each foot, and the tip of each hair splits into hundreds more.
The hairs are so elastic that they can bend or squish to conform to microscopic nooks and crannies under the creature's feet, even on the glass walls of an aquarium. As a result, the tiny hairs touch so much surface area so closely that weak forces of attraction between molecules in the hairs and in whatever surface the animal is walking on add up and become sufficient to let the gecko hang on. The connection breaks when the gecko shifts its foot enough to change the angle between the hairs and the surface. The discovery intrigued scientists, who immediately realized that if synthetic gecko-foot hairs could be made, they might be a great adhesive - strong, glue-free, dry, reusable and unlike suction cups, capable of working in a vacuum like outer space. Engineers envisioned robotic instruments that could climb walls or grab objects without dropping them, and rovers that could maneuver rugged terrain on distant planets. Such adhesives could also be used to stick components together in electronic devices. The National Science Foundation took these ideas so seriously that it gave a $400,000 grant to scientists at the University of Akron and Rensselaer Polytechnic Institute to try making imitation gecko feet. In a recent issue of the journal Chemical Communications, the team reported that it had indeed produced synthetic hairs, with 200 times the sticking power of the ones made by nature. Although the scientists have tested only minute amounts of the material, they estimate that if its properties hold up on a larger scale, a dime-size patch of it could support 2 to 22 pounds, depending on how densely the hairs were packed. "Think of it almost like nano-Velcro," said Ali Dhinojwala, an associate professor of polymer science at the University of Akron. The synthetic hairs - one ten-thousandth the width of a human hair - are made of highly flexible carbon cylinders, or nanotubes, embedded in a plastic base like bristles in a hairbrush. The tubes are strong and practically unbreakable, Professor Dhinojwala said, adding that other groups had tried making the tubes of plastic, but it turned out to be too weak. ############### Small satellite to tackle big biological issues If humans are to embark outward beyond low Earth orbit, research scientists are faced with a set of perplexing challenges. Chief among them is a better grasp of the biological effects that occur from exposure to the space environment - particularly from radiation and reduced gravity. Data gleaned from ground and in-space studies can help create countermeasures to the injurious impacts from long-duration space stays on the moon, Mars and elsewhere. There are plenty of large problems out there to solve. And that's where small-satellite technology might play a big role. The concept of the GeneSat-1 was showcased at the 19th Annual Conference on Small Satellites. Small, fast, cheap: These watchwords meld together to shape the idea behind GeneSat-1, a 22-pound (10-kilogram) nanosatellite that's actually the product of three smaller "cubesats" fastened together. While diminutive in size, GeneSat-1 would be a fully-automated, miniaturized spaceflight system that provides life support and nutrient delivery and performs assays for genetic changes in the bacterium Escherichia coli. For the moment, GeneSat-1 is a work in progress and could be wrapped up by year's end. But the small satellite is "temporarily on hold," pending launch availability and funding, explained John Hines, the project manager at NASA's Ames Research Center in California. To date, roughly $6 million has been spent on the work over the last two and a half years. Behind the GeneSat-1 idea is an integrated analytical fluidics card assembly. This little piece of mini-hardware - about the size of a playing card - is outfitted with a pump, valves, microchannels, filters, membranes and wells. These are the elements that maintain the biological viability of the selected microorganism. A built-in thermal control system keeps biological specimens at just-right temperatures as GeneSat-1 slips through the space environment. Once in orbit, samples of E. coli are dispensed into assay wells of the fluidics card. Sugar water is released to activate the experiment, with a suite of sensors monitoring the bacteria. Expressions of genetic signals are to be detected by the ultra-small optical system. Gene expression is a process in which a gene is profiled by revealing information encoded within the gene into protein or RNA. ############## Lasers trigger cleaner fusion Russian scientists have managed to use lasers to create a billion-degree nuclear fireball. The resulting fusion reaction is far cleaner than the kind currently being investigated to generate nuclear power. Sadly, the team's efforts are no good for power generation at the moment as the laser takes so much energy to run. But achieving this kind of laser-driven fusion in the lab will give scientists a better way to investigate the phenomenon, which could one day be used to create cleaner energy. Currently, the main contender for generating fusion power uses strong magnetic fields to confine a fiery plasma of atomic nuclei: fusion experts hope that the International Thermonuclear Experimental Reactor (ITER), to be built in Cadarache, France, will fuse deuterium and tritium nuclei together in this way to create energy. But this reaction also produces copious amounts of neutrons. When these neutrons hit the reactor walls they generate radioactive isotopes that will eventually have to be disposed of and although this radioactive waste is cleaner than the by-products created by fission the reaction used by today's nuclear power plants it isn't perfect. Thus some physicists have suggested using a different fusion process instead which forces protons and boron nuclei together in a reaction that generates virtually no neutrons. Although this sounds safer, kick-starting proton-boron fusion requires temperatures of a billion degrees, more than ten times the heat needed by the deuterium-tritium reaction. "Deuterium-tritium fusion is the reaction of choice simply because it's easier to achieve," explains Gennady Shvets, a physicist at the University of Texas at Austin. Now a team of Russian scientists have topped the billion-degree mark in a system that does not need huge magnets to confine the reaction. "We have achieved a neutronless proton-boron reaction for the first time using a laser," says Vadim Belyaev, a physicist from the Central Research Institute of Machine Building, Koralev, Russia. The team blasted polythene pellets containing boron atoms with laser pulses that last for just over a trillionth of a second (10-12 seconds). This creates an intensely hot plasma where protons from the polythene merge into boron atoms, which then fall apart to release a stream of helium nuclei, also known as alpha particles. Lumbering alpha particles tend to stay within the reaction mixture rather than escaping to make surrounding equipment radioactive. Crucially, the team detected no neutrons coming from the reaction at all. The success opens the door to an ecologically pure technology of nuclear energy production: An added advantage to the system is that the charged alpha particles could be directly tapped as a source of electric current. A power plant based on ITER would simply use the heat from fusion to turn
electrical turbines, much as coal-fired power stations do today. |
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They have also seen a "hotspot" at the icy moon's south pole, which is
riven with cracks dubbed "tiger stripes". But the US and European scientists
told a London meeting they could not yet explain fully the energetic
processes driving all the activity on Enceladus. 
