Wonders of Life. Andrew Cohen
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Название: Wonders of Life
Автор: Andrew Cohen
Издательство: HarperCollins
Жанр: Прочая образовательная литература
isbn: 9780007452682
isbn:
LIGHT SPECTRA
Coloured scanning electron micrograph of a section through human skin. The darker layer contains cells called melanocytes, which produce melanin.
The use of pigments such as melanin evolved very early in the history of life on Earth – forming a fundamental component of life. They are the way that life interacts with light and protects itself from harm.
Melanin is found in virtually all animals. In humans, this dark pigment is found in a type of cell under our skin called melanocytes. As high-energy ultraviolet photons rain down on our skin, they have the potential to damage the sensitive molecules that lie beneath. DNA is particularly susceptible to damage from UV, with potentially deadly consequences, and melanin is the first line of defence. The secret to its ability to shield cells from the damaging effects of high-energy photons lies in its molecular structure. Melanin is a complex molecule able to form polymers with varying structures depending on their location in the body. Its active heart, however, is a series of rings of carbon atoms bound together by a sea of mobile electrons. When a high-energy photon from the Sun hits one of the electrons, it doesn’t break the molecule apart. Instead, the energy is dissipated in around a pico-second, which is very fast indeed. In a million millionths of a second, the potentially threatening photon has been adsorbed and all its energy has been converted to heat. The melanin molecule survives intact to fight another day. Melanin is so efficient that over 99.9 per cent of the harmful UV radiation is adsorbed in this way, protecting cells from damage.
Melanin in its many forms is ubiquitous in nature; it is even found deep in the human brain, where its function is unknown. Even microorganisms such as bacteria and fungi employ melanin to protect themselves from UV radiation. This suggests that the use of pigments such as melanin evolved very early in the history of life on Earth – forming a fundamental component of life. They are the way that life interacts with light and protects itself from harm. While this would probably have been irrelevant for the very first life forms on Earth, which most likely lived deep in the oceans around hot-water vents, the dangers of UV light would have been one of the first challenges faced by life as first it rose to the ocean surface and then eventually colonised the land.
This image of the Sun, taken on 5–6 June 2012 by NASA’s Solar Dynamics Observatory, shows the transit of Venus, an event that will not happen again until 2117.
The Sun’s chromosphere is the source of ultraviolet radiation. It is thought that, in the first few billion years of its life, the Sun was seven times brighter in the ultraviolet.
Four billion years ago our planet was under siege. Bombarded by the rocky remnants of the Solar System’s foundation, our world was a tortured land of barren rock and dust-filled skies.
The early Earth was not a place that we would recognise as home – 4 billion years ago, our planet was under siege. Bombarded by the rocky remnants of the Solar System’s formation, our world was a tortured land of barren rock and dust-filled skies. The days were short, sweeping by in just five hours as the Earth spun frantically on its axis. Each morning this desolate landscape would have been met with the sight of a rising sun very different from the one we see today. Hanging in the sky was a sun in its infancy. If there had been human eyes to view it, it would have appeared only 70 per cent as bright as it is today, and Earth would have been in a kind of perpetual twilight. This raises an interesting question, because there is strong geological evidence that the temperatures on Earth were very similar to those today, and certainly permitted liquid water to exist on the surface. The reason for the relative stability of Earth’s climate as the Sun brightened is still a matter of research, although it is thought that a combination of higher concentrations of greenhouse gasses such as CO2 in the atmosphere and, perhaps, less cloud cover resulting in less sunlight being reflected back out into space, kept surface temperatures high.
The relative lack of brightness, however, was deceptive. Beyond the visible and into the UV, the infant Sun was dazzling. This is because the Sun’s outer layers were much hotter than they are today, energised by the star’s higher spin rate giving rise to intense electromagnetic heating. Hotter surfaces radiate more of their energy in the high-energy, short-wavelength part of the spectrum – in other words, they are brighter in the ultraviolet.
It is thought that the young Sun was seven times brighter in the ultraviolet during the first few billion years of its life. The UV flux at the top of the Earth’s atmosphere would have been similar to that experienced by Mercury today, a planet around 100 million km closer to the Sun. The composition of the young Earth’s atmosphere is not well known, but it is unlikely that it was able to absorb such high levels of UV radiation. This suggests that it would have been necessary for life to deal with an intense UV onslaught, which may in turn have driven the evolution of pigments at a very early stage in its history.
The young Sun was seven times brighter during the first few billion years of its life. It would have been necessary for life to deal with an intense UV onslaught, which may in turn have driven the evolution of pigments at a very early stage in its history.
A coronal mass ejection blasts off the surface of the Sun in the direction of the Earth, and is deflected by Earth’s magnetic field.
The aurora borealis (northern lights) occurs when the solar wind – charged particles from the Sun – is drawn by the Earth’s magnetic field to the polar regions.
On 18 March 2011, after a seven-year journey around our Solar System, NASA’s Messenger space probe became the first spacecraft to orbit the tortured inner planet of Mercury. Six days later it reactivated its dormant instrumentation, switching on its powerful cameras and returning the first photograph ever taken from Mercury’s orbit. This pioneering spacecraft has sent back thousands of images from the closest planet to the Sun, revealing in extraordinary high definition its complex surface, pitted with craters. But these images also reveal a monochrome world; there is little colour to decorate its dusty, damaged surface.
On its journey to Mercury, the Messenger spacecraft also flew by another of the inner planets – Venus. Again, despite all the acuity of modern technology, this is a planet painted with a limited pallet. A yellow fug shrouds another monochrome planet; a surface with texture but little colour.
Rocks on Mars (left, taken by the Curiosity Rover), compared with rocks on Earth (right). Both images exhibit rounded gravel fragments, suggesting that Mars, like the Earth, once had surface water.
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