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队友们，节日快乐，放假无忧！
阅读还是要坚持的。文章题目选中后就能看到。
今天文章的内容比较简单，长度也比较精悍。加油!
Article I
Earth's Center Is 1,000 Degrees Hotter Than Previously Thought, Synchrotron X-Ray Experiment Shows
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【Time1】
Scientists have determined the temperature near the Earth's centre to be 6000 degrees Celsius, 1000 degrees hotter than in a previous experiment run 20 years ago. These measurements confirm geophysical models that the temperature difference between the solid core and the mantle above, must be at least 1500 degrees to explain why the Earth has a magnetic field. The scientists were even able to establish why the earlier experiment had produced a lower temperature figure. The results are published on 26 April 2013 in Science.
The research team was led by Agnès Dewaele from the French national technological research organization CEA, alongside members of the French National Center for Scientific Research CNRS and the European Synchrotron Radiation Facility ESRF in Grenoble (France).
The Earth's core consists mainly of a sphere of liquid iron at temperatures above 4000 degrees and pressures of more than 1.3 million atmospheres. Under these conditions, iron is as liquid as the water in the oceans. It is only at the very centre of the Earth, where pressure and temperature rise even higher, that the liquid iron solidifies. Analysis of earthquake-triggered seismic waves passing through the Earth, tells us the thickness of the solid and liquid cores, and even how the pressure in the Earth increases with depth. However these waves do not provide information on temperature, which has an important influence on the movement of material within the liquid core and the solid mantle above. Indeed the temperature difference between the mantle and the core is the main driver of large-scale thermal movements, which together with the Earth's rotation, act like a dynamo generating the Earth's magnetic field. The temperature profile through the Earth's interior also underpins geophysical models that explain the creation and intense activity of hot-spot volcanoes like the Hawaiian Islands or La Réunion.
【300 words】
【Time2】
To generate an accurate picture of the temperature profile within the Earth's centre, scientists can look at the melting point of iron at different pressures in the laboratory, using a diamond anvil cell to compress speck-sized samples to pressures of several million atmospheres, and powerful laser beams to heat them to 4000 or even 5000 degrees Celsius."In practice, many experimental challenges have to be met," explains Agnès Dewaele from CEA, "as the iron sample has to be insulated thermally and also must not be allowed to chemically react with its environment. Even if a sample reaches the extreme temperatures and pressures at the centre of the Earth, it will only do so for a matter of seconds. In this short timeframe it is extremely difficult to determine whether it has started to melt or is still solid."
This is where X-rays come into play. "We have developed a new technique where an intense beam of X-rays from the synchrotron can probe a sample and deduce whether it is solid, liquid or partially molten within as little as a second, using a process known diffraction," says Mohamed Mezouar from the ESRF, "and this is short enough to keep temperature and pressure constant, and at the same time avoid any chemical reactions."
【210words】
【Time3】
The scientists determined experimentally the melting point of iron up to 4800 degrees Celsius and 2.2 million atmospheres pressure, and then used an extrapolation method to determine that at 3.3 million atmospheres, the pressure at the border between liquid and solid core, the temperature would be 6000 +/- 500 degrees. This extrapolated value could slightly change if iron undergoes an unknown phase transition between the measured and the extrapolated values.
When the scientists scanned across the area of pressures and temperatures, they observed why Reinhard Boehler, then at the MPI for Chemistry in Mainz (Germany), had in 1993 published values about 1000 degrees lower. Starting at 2400 degrees, recrystallization effects appear on the surface of the iron samples, leading to dynamic changes of the solid iron's crystalline structure. The experiment twenty years ago used an optical technique to determine whether the samples were solid or molten, and it is highly probable that the observation of recrystallization at the surface was interpreted as melting.
"We are of course very satisfied that our experiment validated today's best theories on heat transfer from the Earth's core and the generation of the Earth's magnetic field. I am hopeful that in the not-so-distant future, we can reproduce in our laboratories, and investigate with synchrotron X-rays, every state of matter inside the Earth," concludes Agnès Dewaele.
【220words】
Article II
Bad beehaviour
The strange case of the bandit bumblebees
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【Time4】
TO MOST people, bumblebees are charming, slightly absurd creatures that blunder through garden and meadow with neither the steely determination of the honeybee nor the malevolent intention of the wasp. If you are a plant, though, things look rather different—for from the point of view of some flowering plants many bumblebees are nothing more than thieves. They rob them of their nectar and give nothing in return.
Nectar robbery, in which a bumblebee carves a hole in the side of a flower as a bank robber might cut his way into a vault, was discovered by Charles Darwin. This technique lets bees get at the nectar of flowers whose shapes have evolved to encourage their pollination by insects with long tongues, which can reach down narrow tubes.
Some bumblebees do have such tongues. But some do not. Short-tongued bees are, however, unwilling to deny themselves the bounty of nectar inside these flowers. Hence the hole-cutting. By breaking in in this way, though, a bumblebee nullifies the 100m-year-old pact between flowering plants and insects: that the plant feeds the insect in exchange for the insect pollinating the plant.
The question about nectar robbery that has intrigued biologists from Darwin onwards is whether the behaviour is innate or learnt. Darwin, though he originated the idea that many behaviour patterns are products of evolution by natural selection, suspected that it is learnt. Insects, in other words, can copy what other insects get up to. Only now, though, has somebody proved that this is true.
The observations were made by David Goulson (then at the University of Stirling, now at the University of Sussex), and his colleagues. To test his ideas he had to go from Britain to Switzerland, for only there could he find a flower of the correct shape to conduct the study.
His crucial observation was that when the flowers of an alpine plant called the yellow rattle are robbed, the entry holes—because of the structure of the flower—tend to be unambiguously on either the right-hand side or the left-hand side. Moreover, preliminary observation suggested that the holes in flowers in a single meadow are often all made on the same side. This led him to speculate that bumblebees in a particular area do indeed learn the art of nectar robbery from one another, and then copy the technique with such fidelity that they always attack a flower from the same side.
【404 words】
【Time5】
Crime and nourishment
His team monitored 13 alpine meadows during the summers of 2009 and 2011. They painstakingly recorded the sites of robbery holes in yellow-rattle flowers, and studied the behaviour of 168 bumblebees. They tried to follow each bee until it had visited 20 flowers, though they lost sight of some insects before they had reached this score. If they could, they then captured the insect so as not to follow it again on another occasion.
Dr Goulson found, as he reports in Behavioral Ecology and Sociobiology, that two short-tongued bumblebee species which live in the area, Bombus lucorum and Bombus wurflenii, demonstrated handedness when they robbed flowers. Moreover, if one species was behaving in (say) a left-handed manner in a particular meadow, the other was likely to do the same. This suggests that one species can learn from another—a trick previously thought to be confined to vertebrates.
Handedness in any given meadow, Dr Goulson found, increased as the season progressed. But each summer appeared to start as a blank slate. The handedness that developed in a meadow in 2009 did not predict its handedness in 2011.
The most reasonable explanation, Dr Goulson argues, is that each year a few bumblebees which have learnt the trick of nectar robbery in the previous season come out of hibernation and start robbing flowers again. By chance, they make more holes on one side of the flowers than the other, and as the habit is picked up by other, newly hatched bees, a preference for left or right spreads by a process of positive feedback. The bees have, in other words, created a simple culture. It is a criminal culture, admittedly. But no one ever said that nature was pretty.
【289 words】
Obstacle
Article III
Strange New Bursts of Gamma Rays Point to a New Way to Destroy a Star
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A team led by the University of Warwick has pinpointed a new type of exceptionally powerful and long-lived cosmic explosion, prompting a theory that they arise in the violent death throes of a supergiant star.
These explosions create powerful blasts of high energy gamma-rays, known as gamma-ray bursts, but while most bursts are over in about a minute, this new type can last for several hours.
The first example was found by astronomers on Christmas Day 2010, but it lacked a measurement of distance and so remained shrouded in mystery with two competing theories put forward for its origin.
The first model suggested it was down to an asteroid, shredded by the gravity of a dense neutron star in our own galaxy, the second that it was a supernova in a galaxy 3.5 billion light years away, or in the more common language of astronomers at a redshift of 0.33.
A new study by a team of scientists led by Dr Andrew Levan at the University of Warwick finds several more examples of these unusual cosmic explosions and shows that the Christmas Day burst took place in a galaxy much further away than the two theories suggested.
This research is to be presented at the GRB 2013 Symposium in Nashville, Tennessee on Tuesday 16 April.
Using data from the Gemini Telescope in Hawaii, the scientists calculated that this ultra-long gamma-ray burst had a redshift of 0.847. This gives it a location of approximately half-way to the edge of the observable universe, or 7 billion light years away.
Armed with its location, Dr Levan's team, which included scientists from an international collaboration, has developed a new theory to explain how it occurred.
They suggest this kind of burst is caused by a supergiant, a star 20 times more massive than the sun, which evolves to become among the biggest and brightest stars in the universe with a radius of up to 1 billion miles -- up to 1,000 times that of the sun.
They believe the ultra-long durations of the Christmas gamma-ray burst and two other similar bursts are simply down to the sheer size of the supergiants exploding in a supernova.
Most stars that create gamma-ray bursts are thought to be relatively small and dense, and the explosion that destroys them punches through the star in a matter of seconds. In the case of these new ultra-long bursts the explosion takes much longer to propagate through the star, and so the gamma-ray burst lasts for a much longer time..
Dr Levan said: "These events are amongst the biggest explosions in nature, yet we're only just beginning to find them.
"It really shows us that the Universe is a much more violent and varied place than we'd imagined.
"Previously we've found lots of gamma-ray events with short durations, but in the past couple of years we've started to see the full picture."
Nial Tanvir, a professor at the University of Leicester, and second author of the study added: "We believe that powering the explosion is a newly formed black hole in the heart of the star.
"Predicting the detailed behaviour of matter falling into a black hole in these circumstances turns out to be very difficult, and from a theoretical point of view we didn't initially expect explosions at all.
"The amazing thing is that nature seems to have found ways of blowing up a wide range of stars in the most dramatic and violent way."
The more common type of gamma-ray burst is thought to be caused when a Wolf-Rayet star in the final phase of its evolution collapses into a black hole at its own core.
Matter is drawn into the black hole, but some of its energy escapes and is focussed into a jet of material which blasts out in two directions forming copious gamma-rays in the process.
These jets are ejected extremely quickly (close to the speed of light), otherwise the material would fall into the black hole from which it can't escape. For this reason they last only a few seconds.
However, a gamma-ray burst in a bigger star the size of a supergiant needs to power through a larger reservoir of material, hence its longer duration.
【701 words】
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发表于 2013-4-29 23:48:10
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