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Did Exploding Stars Help Life On Earth Thrive?
ScienceDaily (Apr. 24, 2012) — Research by a Danish physicist suggests that the explosion of massive stars -- supernovae -- near the Solar System has strongly influenced the development of life. Prof. Henrik Svensmark of the Technical University of Denmark (DTU) sets out his novel work in a paper in the journal Monthly Notices of the Royal Astronomical Society.
When the most massive stars exhaust their available fuel and reach the end of their lives, they explode as supernovae, tremendously powerful explosions that are briefly brighter than an entire galaxy of normal stars. The remnants of these dramatic events also release vast numbers of high-energy charged particles known as galactic cosmic rays (GCR). If a supernova is close enough to the Solar System, the enhanced GCR levels can have a direct impact on the atmosphere of Earth.
Prof. Svensmark looked back through 500 million years of geological and astronomical data and considered the proximity of the Sun to supernovae as it moves around our Galaxy, the Milky Way. In particular, when the Sun is passing through the spiral arms of the Milky Way, it encounters newly forming clusters of stars. These so-called open clusters, which disperse over time, have a range of ages and sizes and will have started with a small proportion of stars massive enough to explode as supernovae. From the data on open clusters, Prof. Svensmark was able to deduce how the rate at which supernovae exploded near the Solar System varied over time.
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Comparing this with the geological record, he found that the changing frequency of nearby supernovae seems to have strongly shaped the conditions for life on Earth. Whenever the Sun and its planets have visited regions of enhanced star formation in the Milky Way Galaxy, where exploding stars are most common, life has prospered. Prof. Svensmark remarks in the paper, "The biosphere seems to contain a reflection of the sky, in that the evolution of life mirrors the evolution of the Galaxy."
In the new work, the diversity of life over the last 500 million years seems remarkably well explained by tectonics affecting the sea-level together with variations in the supernova rate, and virtually nothing else. To obtain this result on the variety of life, or biodiversity, he followed the changing fortunes of the best-recorded fossils. These are from invertebrate animals in the sea, such as shrimps and octopuses, or the extinct trilobites and ammonites.
They tended to be richest in their variety when continents were drifting apart and sea levels were high and less varied when the land masses gathered 250 million years ago into the supercontinent called Pangaea and the sea-level was lower. But this geophysical effect was not the whole story. When it is removed from the record of biodiversity, what remains corresponds closely to the changing rate of nearby stellar explosions, with the variety of life being greatest when supernovae are plentiful. A likely reason, according to Prof. Svensmark, is that the cold climate associated with high supernova rates brings a greater variety of habitats between polar and equatorial regions, while the associated stresses of life prevent the ecosystems becoming too set in their ways.
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He also notices that most geological periods seem to begin and end with either an upturn or a downturn in the supernova rate. The changes in typical species that define a period, in the transition from one to the next, could then be the result of a major change in the astrophysical environment.
Life's prosperity, or global bioproductivity, can be tracked by the amount of carbon dioxide in the air at various times in the past as set out in the geological record. When supernova rates were high, carbon dioxide was scarce, suggesting that flourishing microbial and plant life in the oceans consumed it greedily to grow. Support for this idea comes from the fact that microbes and plants dislike carbon dioxide molecules that contain a heavy form of carbon atom, carbon-13. As a result, the ocean water is left enriched by carbon-13. The geological evidence shows high carbon-13 when supernovae were commonest -- again pointing to high productivity. As to why this should be, Prof. Svensmark notes that growth is limited by available nutrients, especially phosphorus and nitrogen, and that cold conditions favour the recycling of the nutrients by vigorously mixing the oceans.
Although the new analysis suggests, perhaps surprisingly, that supernovae are on the whole good for life, high supernova rates can bring the cold and changeable climate of prolonged glacial episodes. And they can have nasty shocks in store. Geoscientists have long been puzzled by many relatively brief falls in sea-level by 25 metres or more that show up in seismic soundings as eroded beaches. Prof. Svensmark finds that they are what can be expected when chilling due to very close supernovae causes short-lived glacial episodes. With frozen water temporarily bottled up on land, the sea-level drops.
The data also support the idea of a long-term link between cosmic rays and climate, with these climatic changes underlying the biological effects. And compared with the temperature variations seen on short timescales as a consequence of the Sun's influence on the influx of cosmic rays, the heating and cooling of Earth due to cosmic rays varying with the prevailing supernova rate have been far larger.
The director of DTU Space, Prof. Eigil Friis-Christensen, comments: "When this enquiry into effects of cosmic rays from supernova remnants began 16 years ago, we never imagined that it would lead us so deep into time, or into so many aspects of the Earth's history. The connection to evolution is a culmination of this work."
Key Genes That Switch Off With Aging Highlighted as Potential Targets for Anti-Aging Therapies
ScienceDaily (Apr. 19, 2012) — Researchers have identified key genes that switch off with aging, highlighting them as potential targets for anti-aging therapies.
Researchers at King's College London, in collaboration with the Wellcome Trust Sanger Institute, have identified a group of 'aging' genes that are switched on and off by natural mechanisms called epigenetic factors, influencing the rate of healthy aging and potential longevity.
The study also suggests these epigenetic processes -- that can be caused by external factors such as diet, lifestyle and environment -- are likely to be initiated from an early age and continue through a person's life. The researchers say that the epigenetic changes they have identified could be used as potential 'markers' of biological aging and in the future could be possible targets for anti-aging therapies.
Published April 20 in PLoS Genetics, the study looked at 172 twins aged 32 to 80 from the TwinsUK cohort based at King's College London and St Thomas' Hospital, as part of King's Health Partners Academic Health Sciences Centre.
The researchers looked for epigenetic changes in the twins' DNA, and performed epigenome-wide association scans to analyze these changes in relation to chronological age. They identified 490 age related epigenetic changes. They also analysed DNA modifications in age related traits and found that epigenetic changes in four genes relate to cholesterol, lung function and maternal longevity.
To try to identify when these epigenetic changes may be triggered, the researchers replicated the study in 44 younger twins, aged 22 to 61, and found that many of the 490 age related epigenetic changes were also present in this younger group. The researchers say these results suggest that while many age related epigenetic changes happen naturally with age throughout a person's life, a proportion of these changes may be initiated early in life.
Dr Jordana Bell from King's College London, who co-led the study said: 'We found that epigenetic changes associate with age related traits that have previously been used to define biological age.
'We identified many age-related epigenetic changes, but four seemed to impact the rate of healthy aging and potential longevity and we can use these findings as potential markers of aging. These results can help understand the biological mechanisms underlying healthy aging and age-related disease, and future work will explore how environmental effects can affect these epigenetic changes.'
Dr Panos Deloukas, co-leader of the study from the Wellcome Trust Sanger Institute, said: 'Our study interrogated only a fraction of sites in the genome that carry such epigenetic changes; these initial findings support the need for a more comprehensive scan of epigenetic variation.'
Professor Tim Spector, senior author from King's College London, said: 'This study is the first glimpse of the potential that large twin studies have to find the key genes involved in aging, how they can be modified by lifestyle and start to develop anti-aging therapies. The future will be very exciting for age research.'
Hinode and SOHO Paint an Asymmetrical Picture of the Sun
ScienceDaily (Apr. 21, 2012) — Approximately every 11 years the magnetic field on the sun reverses completely -- the north magnetic pole switches to south, and vice versa. It's as if a bar magnet slowly lost its magnetic field and regained it in the opposite direction, so the positive side becomes the negative side. But, of course, the sun is not a simple bar magnet and the causes of the switch, not to mention the complex tracery of moving magnetic fields throughout the eleven-year cycle, are not easy to map out.
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Mapping such fields, however, is a crucial part of understanding how -- and, in turn, when -- the sun will exercise its next flip. This flip coincides with the greatest solar activity seen on the sun in any given cycle, known as "solar maximum."
While the cycle unfolds with seeming regularity every 11 years, in two upcoming papers scientists highlight just how asymmetrical this process actually is. Currently the polarity at the north of the sun appears to have decreased close to zero -- that is, it seems to be well into its polar flip from magnetic north to south -- but the polarity at the south is only just beginning to decrease.
"Right now, there's an imbalance between the north and the south poles," says Jonathan Cirtain, a space scientist at NASA's Marshall Space Flight Center in Huntsville, Ala., who is also NASA's project scientist for a Japanese solar mission called Hinode. "The north is already in transition, well ahead of the south pole, and we don't understand why."
One of the two papers relies on Hinode data that shows direct observations of this polar switch. The other paper makes use of a new technique observing microwave radiation from the sun's polar atmosphere to infer the magnetic activity on the surface. The asymmetry described in the papers belies models of the sun that assume that the sun's north and south polarities switch at the same time. In addition, both papers agree that the switch is imminent at the north pole, well in advance of general predictions that solar maximum for this cycle will occur in 2013. Lastly, the direct Hinode results also suggest a need to re-examine certain other solar models as well.
Measuring the magnetic activity near the poles isn't easy because all of our solar telescopes view the sun approximately at its equator, offering only an oblique view of the poles, when they require a top-down view for accurate magnetic measurements. Hinode can observe this activity annually with its high resolution Solar Optical Telescope that can map magnetic fields when observing them from near the equator. The microwave radiation technique described in the second paper makes use of the discovery in 2003 that as the sun moves toward solar maximum, giant eruptions on the sun, called prominence eruptions -- which during solar minimum, are concentrated at lower solar latitudes -- begin to travel toward higher latitudes near the poles. In addition, the polar brightness in the microwave wavelengths declines to very low values.
"These prominence eruptions are associated with increased solar activity such as coronal mass ejections or CMEs, so CMEs originating from higher latitudes also point to an oncoming solar maximum," says Nat Gopalswamy. Gopalswamy is a solar scientist at NASA's Goddard Space Flight Center in Greenbelt, Md. who is the first author on the microwave observations paper, which was accepted by The Astrophysical Journal on April 11, 2012. "When we start to see prominence eruptions above 60 degrees latitude on the sun, then we know that we are reaching solar maximum."
To look at the prominence eruptions toward the poles, Gopalswamy and his team used observations from Japan's Nobeyama Solar Radio Observatory telescopes and the joint ESA/NASA mission the Solar Heliospheric Observatory (SOHO). They watched the sun in the microwave wavelengths -- which are used to observe the area of the sun's atmosphere just above the surface, known as the chromosphere. Gopalswamy created precise techniques to use such microwave radiation to measure the intensity of magnetic activity on the sun's surface at the poles. By mapping the brightness of the microwave radiation throughout the chromosphere, the scientists showed that the intensity at the north pole has already dropped to the threshold that was reached in the last solar maximum cycle, suggesting the onset of solar max there. This is backed by the fact that prominence eruptions are also occurring at high latitudes in the north. Eruption activity in the south half of the sun, however, is only just beginning to increase -- the first CME occurred there in early March 2012.
The Hinode data also shows this discrepancy between the north and the south. The Hinode results are reported by a Japanese team, led by Daikou Shiota a solar scientist at RIKEN Institute of Physics and Chemical Research, and were recently submitted to The Astrophysical Journal for publication. Shiota and his team used Hinode to observe the magnetic map of the poles every month since September of 2008. Early maps showed large, strong concentrations of magnetic fields that are almost all magnetically negative in polarity. Recent maps, however, show a different picture. Not only are the patches of magnetism smaller and weaker, but now there is a great deal of positive polarity visible as well. What once pointed to a strongly negative north pole, is now a weakly magnetized, mixed pole that will become neutral -- which occurs at solar maximum -- within the month according to the team's predictions.
"This is the first direct observation of this field reversal," says Cirtain. "And it is extremely important to understanding how the sun's magnetism generates the solar cycle."
Ted Tarbell is the principal investigator for Hinode's Solar Optical Telescope at Lockheed Martin in Palo Alto, Calif., and he points out that the direct measurements showed the progress of the pole reversal, and highlights the earlier portion of the cycle in 2008. Typical models of the magnetic flip, suggest that as active regions rotate around the equator, their higher, trailing edge -- which is almost always the opposite polarity from the pole in their hemisphere -- drift upward, eventually dominating the status quo and turning positive to negative or negative to positive. The Hinode data show that this transition at the north began before such drifting had a chance to occur.
"This is one of the most interesting things in this Hinode paper to me," says Tarbell. "How did the polar reversal start so early, even though the onset of the solar cycle, that is, increased activity at lower latitudes, hadn't begun yet?"
Tarbell thinks these observations mean that this model, too, may need to be re-examined.
Such adjustments to models are of course expected whenever new and better data is collected. Indeed, David Hathaway, who is a solar scientist at NASA's Marshall, and who is a co-author on the microwave observations paper with Gopalswamy, points out that the idea that asymmetries exist in the sun is not completely new. Other work has recently emphasized symptoms of this asymmetry, measuring, for example, more sunspots in the northern hemisphere than in the south at the moment. "But most of the well-developed models don't incorporate the asymmetry in them," Hathaway says. "More complicated models that incorporate asymmetries do exist, but they have other ways in which they fail to match observations."
Continued study on these differences, using the best observatories as well as new techniques for analysis will help expand and improve our understanding of the sun, its 11-year cycle, and the great eruptions that occur on its surface.
Scientists will also keep their eye on the current cycle -- numbered Solar Cycle 24 -- because a polar switch at the north that is sooner than was expected also implies this may be a fairly small cycle in terms of the number of sunspots and amount of solar activity.
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发表于 2012-4-25 03:02:42
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发表于 2012-4-25 08:47:30
), this study is astonishing! Scientists need to come up with new model to explain this, to significantly modify our current models.