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| SCIENCE TRIBUNE | Thursday, April 24, 2003, Chandigarh, India |
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An
evaporating planet Fireflies
to fight cancer?
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Harnessing
sun light as a gas fuel
There are three fundamental requirements for any system for converting and storing solar energy. First, sunlight must be efficiently absorbed to produce excited electron states in the light-absorbing material, the photo-catalyst. Second, to obtain directed work, either chemical or electrical in form, the photo-electron and its accompanying electron vacancy must be separated in space to prevent their waste energy. Third, the photo-charge must be energetically and kinetically able to perform a desired chemical transformation, for instance splitting water. Furthermore, these charges must not result in undesirable end products, such as heat, or chemically transform or otherwise degrade the photocatalyst. Satisfying all of these requirements simultaneously is a tall order. A popular approach is to use semiconductors as the light absorbers. Semiconductors generally have broad, strong optical absorption characteristics, meeting the first requirement for solar-energy conversion. They also generally satisfy the second requirement, because effective charge separation is facilitated by electric fields at the interface between a semiconductor and selected liquid electrolytes. The electrolyte provides the chemical feedstock that will be transformed into the fuels formed by the photochemical reaction and ensure that electrical charge can flow through the liquid to complete the electrical circuit of the water-splitting electrolysis cell. Figure shows strategy for converting energy from the sun, with the semiconductor-electrolyte cell in the centre. The key requirement is the third one: whether the photo-charge can split water efficiently. In 1972, Back described the sunlight-assisted electrolysis of water using crystalline Ti-oxide photo-electrodes, and prompted a flurry of research into photo-chemical water splitting. In consequence, several metal oxides, including SrTi-oxide and KTa-oxide, were identified as being able to sustain the light-driven, unassisted photo-electrolysis of water into hydrogen and oxygen. Unfortunately, a fundamental problem has thwarted the practical implementation of such water-splitting systems. Although SrTi-oxide and KTa-oxide effectively convert absorbed photons into stored chemical fuel, the band gaps of these metal oxides — that it, the energies at which light starts to be absorbed — are too large to allow the efficient absorption of most of the photons in the solar spectrum. The overall energy conversion efficiency of such system is only 2%. Compounds such as CdTe (a chalcogenide) or Inp (a semiconductor) have smaller band gaps that are better matched to the spectral distribution of sunlight reaching the Earth, but these materials corrode when used when used as photo-electrodes in aqueous solution. When the band gap of various metal oxides is made smaller, as it is in Zn-oxide or Fe-oxide, the potential of the photo-electron in the semiconductor becomes more positive than the potential needed to reduce water to hydrogen, and the reaction becomes thermo-dynamically unfavourable at NTP. The constraint is imposed by the interplay between the optical, electronic and chemical properties of the light absorbing materials that have been tested. The introduction of Ni into InTa-oxide extends the light absorption of the composite In(1-x)Ni(x) Ta-oxide photo-electrode family into the visible enough energy to reduce water to hydrogen. The photo-electron vacancies then oxidise water to oxygen, completing the circuit and driving the water-splitting reaction. In these system wavelengths as long a 420 nm can power a water-splitting process. The system is stable, because the amount of hydrogen and oxygen produced exceeds the amount of reducing equivalents in the photo-electrode sample. The optimum band gap for a single threshold device to convert sunlight into stored energy is between 1.1 eV and 1.7 Ev for water splitting because of the energetic constrains. So advances in this area will be essential of developing an integrated photo-catalytic system for converting and storing solar energy. It is encouraging that all of the parts of a water-splitting process exist in biological systems: Photosynthesis in plants produces oxygen from water; hydrogenase enzymes reduce water to hydrogen; and the chlorophyll-derived energy-harvesting and charge-separating components of plants and bacterial photosynthesis provide a model for efficient light absorption and charge separation structures that can drive fuel-forming photochemical reactions. However, plants are far from being optimal machines for converting solar energy: only 4% of the total sunlight energy falling onto a leaf is converted into stored free energy by photosynthesis. The biological process inspires artificial photosynthetic systems, but the goal is to outperform it. The idea is promising, but can we beat the nature? The writer is from the Physics
Department, Kurukshetra University. |
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An evaporating planet For the first time, astronomers using NASA’s Hubble Space Telescope have observed the atmosphere of an extrasolar planet evaporating off into space. Much of the planet may eventually disappear, leaving only a dense core. The planet is a type of extrasolar planet known as a "hot Jupiter." These giant gaseous planets orbit their parent stars very closely, drawn to them like moths to a flame. The scorched planet, called HD 209458b, orbits only four million miles (7 million km) from its yellow, Sun-like star. The Hubble observations reveal a hot and puffed up evaporating hydrogen atmosphere surrounding the planet. This huge envelope of hydrogen resembles a comet with a tail trailing behind the planet. The planet circles the parent star in a tight, 3.5-day orbit. Earth also has an extended atmosphere of escaping hydrogen gas, but the loss rate is much lower. An international team of astronomers, led by Alfred Vidal-Madjar of the Institut d’Astrophysique de Paris, CNRS, France, reported this discovery in Nature Magazine. "We were astonished to see that the hydrogen atmosphere of this planet extends over 124,000 miles (200,000 km)," says Vidal-Madjar. HD 209458b is too close to the star for Hubble to photograph directly. However, astronomers could observe the planet indirectly since it blocks light from a small part of the star during transits across the disk of the star, thereby dimming it slightly. Light passing through the atmosphere around the planet is scattered and acquires a signature from the atmosphere. In a similar way, the Sun’s light is reddened as it passes obliquely through the Earth’s atmosphere at sunset. Astronomers used Hubble’s Space Telescope Imaging Spectrograph (STIS) to measure how much of the planet’s atmosphere filters light from the star. They saw a startling drop in the star’s hydrogen emission. A huge puffed up atmosphere can best explain this result. The planet’s outer atmosphere is extended and heated so much by the nearby star that it starts to escape the planet’s gravity. "The atmosphere is heated, the hydrogen escapes the planet’s gravitational pull and is pushed away by the starlight, fanning out in a large tail behind the planet - like that of a comet," says Alain Lecavelier des Etangs at the Institut d’Astrophysique de Paris, CNRS, France. Astronomers estimate the amount of hydrogen gas escaping HD 209458b to be at least 10,000 tons per second, but possibly much more. Hot Jupiters orbit
precariously close to their stars. They are giant gaseous planets that
must have formed in the cold outer reaches of the star system and then
spiraled into their close orbits. |
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Fireflies to fight cancer? Could the gentle firefly turn out to be a potent weapon against cancer? In a new study, researchers from London inserted the firefly gene that activates bioluminescent light into modified cancer cells, hoping to set off a chain of events that has a proven track record at fighting the disease. This light source, known as Luciferin, caused the modified cancer cells to glow much like it does with the firefly. When a photosensitising agent was added, the combination proved lethal. "The cells produced enough light to trigger their own death," said Dr. Theodossis Theodossiou of the National Medical Laser Centre, University College London. University College London scientists and colleagues at the Ludwig Institute for Cancer Research published their results today in the journal Cancer Research. This firefly technique (BioLuminescence Activated Destruction of cancer, or BLADe) may add a further layer of depth to photodynamic therapy, an effective treatment that uses bursts of light to attack tumors that sit near the skin’s surface or on the lining of internal organs. As part of the therapy, cancer cells are treated with a photosensitiser and then exposed to lasers or another external beam. The light triggers the production of active oxygen species that can destroy cancer cells. External light sources, however, can only pass through a small amount of tissue to get to the tumour. In an attempt to treat deeper malignancies, the BLADe team inserted the light source into the disease itself. Cancer cells were modified to express the firefly luciferase gene and then incubated with luciferin in the lab. The cells essentially became miniature lamps, giving out light that could trigger their own destruction. After a photosensitiser was added, the cells produced toxic substances that forced them to commit suicide. "The light is generated within the tumour cell, so there’s no need for outside penetration," said study co-author John Hothersall of the Institute of Urology and Nephrology, University College London. The researchers are pursuing efforts to one day test the firefly-inspired treatment in patients. Already, a separate team has shown that it’s feasible to deliver the luciferase gene to prostate cancer cells. As a mobile light source, the firefly gene may have far-reaching applications. "Luciferase could be transferred
to primary tumours, and from there it could migrate to cancer cells that
spread," said Dr Theodossiou. |
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UNDERSTANDING
THE UNIVERSE I have learnt from someone that the high power transmission lines used to carry electricity from one station to very far off places usually have very strong electric field around them. Why is this field so strong that even vehicles and individuals near a high-tension transmission line can discern it?
When the transmission line carries an alternating current, as is usually the case, there is an additional loss of energy. Alternating electric current produces electromagnetic waves that carry away some of its energy. Because of this it is sometimes preferable to use direct current transmission lines, particularly for long distance transmission, even though the convenience of easy stepping down of voltage through use of transformers has to be sacrificed. You might be wondering "why should we have high voltage transmission when it causes such problems?" The reason is simple. The objective is to transfer power. This is the product of voltage and current. If the voltage is low the current will have to be increased proportionately to get the same energy throughout. But high current would require thicker conductors. There are important trade offs here to keep the cost of transmission at a minimum level. Perhaps low voltage transmission might be an option when super-conducting lines become possible at affordable cost. What is the physical difference between an electron and a proton that they attract each other, as their charges are equal but opposite and their masses are also the same." The basic reason that they attract each other is that they have opposite charges. No, their masses are not the same, but even if they were they would still attract each other. The electric force is usually called the Coulomb force. It is attractive between like charges and repulsive between unlike charges and varies inversely as the square of the distance between them. You perhaps know that a hydrogen atom
consists of a proton and an electron. If there were no other
considerations the electron would go on spiraling towards the proton
till they merge. This does not happen. In ordinary language we say
that the orbit of the electron around the proton is stabilised because
of quantum considerations.
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NEW PRODUCTS & DISCOVERIES DNA persists in soil
for millennia
Minuscule samples of sediment from New Zealand and Siberia have yielded bits of DNA from dozens of animals and plants, some long extinct. This genetic material, which includes the oldest DNA sequences yet found that can be traced to a specific organism, could help scientists reconstruct ancient ecosystems in those regions. Nearly every cell of an organism carries DNA, the genetic information that researchers can use to identify species. Scientists usually study DNA extracted from living tissue or from preserved remains, says Eske Willerslev, a molecular biologist at the University of Copenhagen. However, his new research suggests that some soils may hold stockpiles of ancient DNA even if they don’t include identifiable fossils. For part of the project, Willerslev and his colleagues analysed samples of permafrost drilled from several sites along a 1.2-kilometer stretch of Siberia’s Arctic coast. The sediment cores, up to 31 meters long, included material dating from modern times to about two million years ago. The cores contained ice, soil, pollen, and plant rootlets, as well as small groups of unidentifiable cells. Two-gram samples of sediment up to 30,000 years old included DNA from eight living and extinct animal species, including lemmings, hares, horses, reindeer, bison, musk oxen, and woolly mammoths. DNA extracted from sediment as old as 400,000 years matched the genetic signatures of at least 28 modern and ancient species of trees, shrubs, herbs, and mosses. Researchers didn’t find DNA in the older sediment samples, says Willerslev. Although the scientists don’t know how animal DNA ended up locked in permafrost, Willerslev speculates that the genetic material came from cells that creatures shed in their feces. The scientists also looked at samples of silt taken from a cave in New Zealand and sand taken from within and around an ancient bird’s bone unearthed from a coastal dune there. From these 600-to-3,000-year-old sediments, the team identified at least 29 plant species, three types of extinct flightless birds called moas, and an extinct parakeet. They report their results in an upcoming issue of Science. "This is a startling finding, if it’s true," says David M. Lambert, a molecular biologist at Massey University in Palmerston North, New Zealand. Similar but preliminary efforts by his group haven’t yielded penguin DNA from Antarctic soils or moa DNA from New Zealand sediments, he notes. By extracting and analysing the DNA in small amounts of sediment, scientists might determine what animals have been present in a particular area, contends Hendrik N. Poinar of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. However, at some sites, the mixing of soil layers over time could complicate attempts to reconstruct ancient ecosystems, he adds. Cannibal dinosaur New fossil evidence suggests a distant cousin of the Tyrannosaurus rex that roamed the plains of Madagascar millions of years ago regularly dined on its own kind to survive during hard times. The discovery is the strongest evidence yet that some carnivorous dinosaurs were cannibals. Dinosaur experts say it sheds light on the hardships predators faced in the late Cretaceous period when dinosaurs vanished, possibly as a result of asteroid impacts, widespread climate change and disease. "This is the first strong, convincing evidence of cannibalism within theropod dinosaurs," said Thomas Holtz, a paleontologist at the University of Maryland who was not part of the study. Scientists working in Madagascar uncovered evidence of cannibalism in fossilized bones of Majungatholus atopus, a toothy beast the size of a small school bus that was the top hunter 70 million years ago on the island off East Africa. The research appears in the current issue of the journal Nature. Majungatholus is a distant relative of T. rex, the fierce hunter that ruled what is now North America. During the late Cretaceous, Madagascar was semiarid and subject to severe climate swings that led to dramatic fluctuations in essential resources. Fossil evidence showed dinosaurs and other creatures were victims of massive die-offs. When food and water were scarce,
scientists believe Majungatholus fed on the remains of other dinosaurs
like titanosaurs- gigantic, long-necked plant-eaters. AP |
A
stable and efficient material that uses sunlight to split water into
hydrogen has and oxygen gas would be an immense blessing. Water and
sunlight are both renewable resources, and cheap. One of the end
products, hydrogen gas, is a clean-burning fuel that produces water as
the waste product.
It
is true that high voltage transmission lines have high electric field
around them. They might carry voltage as high as 250,000 volts. This
is so high that there is a breakdown of air resistance, particularly
when the humidity is high. When you are close to a high-tension cable
you can hear a singing noise. Sometimes you also get a smell, perhaps
of ozone. This is due to a corona discharge — essentially leaking of
electrons from the high voltage cables or the air surrounding them.
This discharge also produces electromagnetic radiation, namely radio
waves, that disturb the reception in a nearby radio set. Such a radio
noise is also produced from the spark plugs of vehicles if they are
not well tuned and do not have a proper condensers to quench the
discharge in the spark plug.