tailieunhanh - Universe a grand tour of modern science Phần 5

Đa số các nhà thiên văn dùng từ "Vũ trụ" khi nói đến "Vũ trụ quan sát được". Lý do là các hiện tượng vật lý nằm ngoài khoảng vũ trụ quan sát được không có tác động gì đến chúng ta và không có giá trị khoa học. | EXTREMOPHiLES Antarctica seemed more challenging. Penguins huddle together in the winter darkness to minimize their heat loss. On the other hand the nematode Panagrolaimus davidi a worm almost too small to see which lives among algae and moss on ice-free edges of Antarctica regularly freezes solid each winter. It can chill out to minus 35 C with virtually all its metabolism switched off and then revive in the spring. In laboratory tests it can go down to minus 80 C without problems. Investigating the nematode s survival strategy Wharton found that the rate of cooling is critical. It survives the rather slow rate experienced in the wild but fast freezing in liquid nitrogen kills it. Cryptobiosis is the term used for such suspended or latent life. Various animals and plants can produce tough larvae seeds or spores that seem essentially dead but which can survive adversity for years or even millennia and then return to life when a thaw comes or a shower of rain in the desert. To Wharton these cryptobiotic organisms are not true extremophiles. Assessing the ability of larger animals to cope with extreme conditions as compared with what archaea and bacteria can do Wharton judged that only a few groups mainly insects birds and mammals are much good at it. Insects resist dehydration with waxy coats. Warm-blooded birds and mammals contrive to keep their internal temperatures within strict limits whether in polar cold or desert heat. On the other hand fishes and most classes of invertebrate animals shun the most severe habitats the big exception being the deep ocean floor. We think of the deep sea as being an extreme environment because of the high pressures faced by the organisms that live there Wharton commented a quarter of a century after the discovery of the animals of the hydrothermal vents. Now that the problems of sampling organisms from this environment have been overcome we have realized that rather than being a biological desert as had been assumed it is .