Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://star.arm.ac.uk/~meb/ras_leaflets2012/LifeInTheUniverse.2012.pdf Äàòà èçìåíåíèÿ: Tue Mar 6 03:05:52 2012 Äàòà èíäåêñèðîâàíèÿ: Tue Oct 2 06:59:30 2012 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: molecular cloud

Life on

he whole collection of life on Earth ­ the biosphere ­ is extremely varied, and yet there are great similarities at and below the level of the individual cell. All life on Earth is based on similar large molecules. Of central importance is DNA, a huge, complicated molecule that contains all the information necessary for building an organism. DNA is an organic compound, which means that it contains the chemical element carbon. Many other organic compounds are also necessary for life, such as proteins. No other element comes near carbon's facility to form molecules complex enough for life. Organisms construct the molecules of life in various ways. In green plants the first stage is photosynthesis, in which radiation from the Sun is used to make organic molecules from molecules like water and carbon dioxide. Oxygen is released into the atmosphere as a by-product. Other organisms do things differently. For example, animals eat the organic compounds contained in plants and in other animals ­ food. Another requirement for all life on Earth is water, which acts as a solvent, as a medium for carrying materials from one place to another, and as a reactant in important biochemical reactions. Without liquid water, there would be no life. Life on Earth can thus be described as carbon­water life. The great similarity at the subcellular level of all life on Earth, from single-celled creatures to humans, points to a common ancestor ­ a common origin. A typical scene of life on Earth: colonies of single-celled creatures in water. (DErEk MArTIN)

Today

Earth

T


Origin
T

The

Earth
The Earth was heavily bombarded from space until about 3900 million years ago. (JuLIAN BAuM)

of

Life on

he Earth formed about 4600 million years ago. Surface conditions were hostile to life as the Earth was bombarded from space by material left over from the formation of the planets. This sterilized the Earth until the bombardment dwindled about 3900 million years ago. Quite what happened next nobody knows, but perhaps after only a further 50 million years the first living cells appeared in the ancient oceans that by then covered much of the Earth's surface. The evidence is in the oldest rocks where substances are found that could well have been produced only by biological processes. If this is so, then it only took a short time for life to emerge after the heavy bombardment stopped. The oldest fossils about which there is relatively little controversy date from about 3500 million years. Important early fossils are the remains of stromatolites, which were colonies of single-celled creatures. All life at that time was single-celled.

Present-day colonies of single-celled creatures, forming stromatolites in the Bahamas. Fossilized stromatolites have been found from nearly 3600 million years ago. (rOBErT F DILL)


Evolution
The
end of heavy first Earth bombardment living formed cells? first eukaryotic cells millions of years ago

of

onEarth
first multi- life on cell dry organisms land dinosaurs vanish 0

Life

A

4600

new type of cell appeared 2000­2500 million years ago: the eukaryotic cell (`youcarry-otic') is distinguished by having a nucleus. It is the basis of all multi-cellular organisms, though such organisms didn't appear until much later, about 700 million years ago. And it was only 450 million years ago that life spread to the land. On land and in the seas, species came and went. Sometimes climate change was the cause. The most famous group of extinct species is the dinosaur, which vanished 65 million years ago. Our own species emerged a mere 0.1­0.2 million years ago. Single-celled creatures have dominated the biosphere from the beginning. The Earth's atmosphere has also changed. Most notably for life, photosynthesis became such a prolific source of oxygen that today oxygen accounts for 21% of the atmosphere. It has become essential for all multi-cellular organisms on Earth. However, many single-celled organisms do not require oxygen; indeed, it is poisonous to them.
Some Edmontosauruses emerging from a forest about 70 million years ago. These vegetarians are just one of a huge range of species of dinosaur. (JOHN S WATSON)


he evolution of life on Earth will continue into the future. Within the next 100 million years it is likely that the Earth will be struck by a large comet or asteroid that will destroy many species. But many new ones will emerge as evolution continues. Whether our own species will survive is uncertain, but as the Sun ages it gets brighter, and by 1000­4000 million years from now the Earth will be too hot to support life of any sort. Though the Earth will then be uninhabitable, Mars could have become suitable for carbon­ water life, and so our species, or its descendants, might find The Earth, devoid of oceans and atmosphere, with refuge there for a while. As the the Sun as a red giant. (JuLIAN BAuM) Sun gets even brighter, Mars will be replaced by the large satellites of Jupiter and Saturn as potential abodes of life. By about 6000 million years from now the Sun will be a red giant, fiercely heating an Earth long parched of atmosphere and oceans. Water will only be able to exist as a liquid in the outer reaches of the Solar System, and therefore Pluto might have become inhabitable. After a further 1000 million years or so the Sun will cast off a shell of hot gas, that could well destroy any remaining life in the Solar System. The remains of the Sun will then shrink to a feeble white dwarf, and the surviving planets will freeze. The Solar System will then surely be devoid of life, unless a super-civilization had found a means to survive, perhaps through a nuclear energy technology.

Endof Life in the
The

System

Solar

T


Universe

Life in the
the

re we alone? Is the Earth the only place in the universe where life exists? Many astronomers believe that there is life out there, perhaps even intelligent life. But where is it? Is there life comparatively nearby, in our Solar System, or do we have to look to planets beyond ­ to the planetary systems being discovered around other stars? The key is to look first at life on Earth ­ the one place where we know there is life in the universe.

A

What life might look like on an alien world. (JuLIAN BAuM)

This is one of a series of leaflets prepared by the Education Committee of the Royal Astronomical Society. It may be copied for educational use. The first edition was produced with the aid of a grant from COPUS, the Committee on the Public understanding of Science. Written by Barrie W Jones (Open university). Designed by Paul Johnson (www.higgs-boson.com). © rAS 2004. Published by the rAS, Burlington House, Piccadilly, London W1J 0BQ, uk. +44 (0)20 7734 3307. reprinted 2012 by Armagh Observatory with funding from the Northern Ireland Department of Culture, Arts and Leisure (DCAL). www.ras.org.uk


T

here is one way in which the painstaking search for extrasolar planets, and for life on them, could be bypassed, and this is through contact with an extraterrestrial intelligence. This would show at once that life does indeed exist elsewhere, and that it has given rise to intelligent species. There is no evidence at all that the Earth has ever been visited, but the search for extraterrestrial intelligence (SETI) also involves searching for signals. Across interstellar space, radio waves are particularly good for communication, and for several decades radio astronomers on Earth have had the technology to send signals to the stars. Extraterrestrials with our sort of technological intelligence could have had this capability for millions of years. Since 1959 astronomers have conducted over 100 searches for radio signals that could not be natural. In recent years they have been joined by astronomers searching for optical laser beams. So far no `intelligent' signals have been detected, but there's a lot more searching to be done, and it will be done. If in a few decades time there is still silence, then we would have to conclude that even if life is common in our Galaxy, intelligent life that attempts interstellar communication with radio waves or lasers is probably rare.

Extraterrestrial
Intelligence

The

Search

for



The Lovell radiotelescope at Jodrell Bank in the UK. It is 76.2 metres in diameter and has been used to search for signals from extraterrestrial intelligence. (IAN MOrISON)

If you have a computer you can help with SETI by allowing your machine to be used to analyse signals received by radiotelescopes. For more details see setiathome.ssl.berkeley.edu


Life

System
Mars Earth
Venus

in the

Elsewhere

Solar


Pluto

1500 million km

Neptune

150 million km

uranus Saturn Jupiter


The Solar System, face-on to the orbits of the planets around the Sun, which is at the centre.

Mercury

asteroid belt inner edge of Edgeworth­kuiper belt of comets

F

or life as we know it, our search for life beyond the Earth is guided by the question ­ can liquid water exist there? Some planets are too hot; some are too cold. The surface of Venus, at 460 °C, is far too hot, but until about 3500 million years ago it is possible that the surface was sufficiently cool for liquid water to exist. It is conceivable that life got going on Venus, only to be snuffed out as the temperatures rose, but the chance of any fossil evidence is very slim. Titan, the giant satellite (moon) of Saturn, at ­179 °C, is too cold for life as we know it. Nevertheless, its thick atmosphere of nitrogen and methane, largely hiding its surface, might in some ways resemble that of the Earth before life emerged here. Titan might therefore tell us about the origin of life on our own planet. The arrival of the Huygens probe provides an opportunity for collecting a lot more data about the surface of the satellite. For more information see www.esa.int/science/huygens. Apart from the Earth, only two bodies in the Solar System look promising as abodes of life, past or present. These are Mars and one of the Artist's impression of the Huygens probe on the surface of Titan. (ESA) large satellites of Jupiter, Europa.


M

ars is a small planet, just over half the Earth's diameter. It has a thin atmosphere with a surface pressure only 0.6% that of the Earth and consisting largely of carbon dioxide (CO2). At best the surface temperature can just exceed 0 °C, but it plunges a hundred degrees or more at night. There is frozen water at the surface ­ in the polar caps and in frosts. There is probably a lot of water in the form of subsurface permafrost, and it is possible that in the porous rocks at a depth of a kilometre or so, there is liquid water where living organisms might exist today. On Earth there is life in porous rocks, sustained by energy from chemical reactions between minerals, and a similar energy source could support subsurface life on Mars. A search for life on the surface of Mars was made in 1976 by Viking Landers 1 and 2 that analysed the dust and sand in their vicinity at two well-separated sites. They searched for evidence of chemical processes important for life on Earth. The results were inconclusive, though the failure by another experiment to detect organic compounds seems to rule out life at the surface of Mars today. It is also possible that there is no life under the surface. But even if there is no life on Mars today, what about in the past ­ what about fossils? Many scientists think there is a good chance of finding fossils on Mars, because of evidence that, perhaps as long as 3800 million years ago, Mars was warmer than today. The best evidence is networks of channels that look as if they were carved by flowing water. A typical network on Mars consists of branching channels each a few kilometres

Ma

Life

Mars, showing clouds (at the edges), and the north polar ice-cap at the top. (AurA/STScI)

Water frost in rock shadows at the surface of Mars, as imaged by the spacecraft Viking Lander 2 in 1976.
(NASA/JPL)


rs?

on

wide and hundreds of kilometres long. The networks are particularly abundant on the older regions of Mars , indicating that they were created early in Martian history. The water could have arrived as rain, or might have been present as liquid water near the surface. In either case, the atmospheric temperatures and pressures would need to be higher than today. The conditions were different on Mars in the distant past, probably because the carbon dioxide atmosphere was a lot more massive than today. Carbon dioxide gas acts like a blanket around a planet, enabling the Sun to raise the surface temperature through the greenhouse effect. Gradually, the carbon dioxide would have been removed to form carbonate rocks, and so the temperatures slowly fell. If the warm conditions lasted more than 100 million years, life might have begun, only to be frozen to death later. But fossils would probably have survived. So have we found any fossils from Mars yet? There is a handful of meteorites that have been found on Earth for which there is good evidence of a Martian origin ­ bits blasted off Mars by impacts. One of these, catalogued as ALH84001, is believed by some scientists to contain evidence of past life on Mars, but the balance of opinion is that it does not. Nevertheless, Mars deserves further exploration and it is certain to get it. Missions that could find fossils on Mars might be some way off, as is the search for life at considerable depths, but the search for life near the Martian surface will be advanced by a number of space missions, including an ESA mission planned for early in the next decade.

Artist's impression of an ESA sample return mission, originally planned as part of the Aurora programme. (ESA)

Valley networks in the Xanthe Terra region of Mars, probably created by liquid water a long time ago when the climate was more Earth-like. The white rectangle is 10 km wide. (NASA/JPL)


Europa?
E
uropa is one of the four large satellites of the giant planet Jupiter, the largest planet in the Solar System. These four worlds are comparable in size with the smaller planets such as Mercury and Pluto. Europa is the second out from Jupiter, and much of its interior consists of the sort of rocky materials that make up the Earth's interior. Its surface, however, is ice ­ frozen water ­ and it is remarkably smooth. It is almost certain that this smooth icy crust is floating on a planet-wide ocean of water several kilometres deep. The floor of this ocean might be punctured by volcanic vents releasing heat from a warm interior. The interior is warmed, and the ocean sustained, with the aid of tidal heating ­ the gravitational flexing of the whole body of Europa by Jupiter. Two essential conditions for life seem to be present in Europa's oceans: liquid water, and an energy source ­ volcanic vents. Furthermore, simple carbon compounds are common in the Solar System, so it is very probable that Europa was endowed with adequate quantities to sustain life. We need to land on Europa and drill through the ice, to see if aquatic life-forms are present.
A close up of Europa, showing ice rafts refrozen into the surface. The frame is 28 km across. (NASA/JPL)

Life on

Europa is a large satellite of Jupiter. Its diameter is 3130 km and it has a smooth, icy surface. (NASA/JPL)


Extrasolar
T

Planets

he Sun is a star, and over 100 other stars are already known to have planetary companions. These planets are giants, with masses no less than several tens of the mass of the Earth. This is because massive planets are easier to discover than the smaller Earth-size bodies that might well be present. Among the currently known systems, Earth-size planets could be present in about half of them at just the right distance from the star to bear life. Any such `Earths' A design for the InfraRed Space Interferometer await improvements in our (IRSI), also called Darwin, to explore extrasolar techniques for their discovery and planets. Each telescope is a few metres across. (ALCATEL) subsequent investigation. No images of extrasolar planets have yet been obtained, because the light from the star drowns the planet's much feebler light. They have been discovered so far almost entirely through their gravitational influence on the motion of the star they orbit. A huge step forward will be to obtain images of these far-off planets and space telescopes are being designed to do just this. These will operate at infrared wavelengths, where the planet's light is less swamped by the light of its star. The telescopes need to operate in space to get above the effects of our atmosphere, and also to facilitate cooling the telescopes to increase their sensitivity. Arrays of telescopes will be used, because these provide images with sufficiently fine detail for the planet to be seen separately from its star. See ast.star.rl.ac.uk/darwin for further details. For an up-to-date list of exoplanets see exoplanets.org


Extrasolar

Biospheres

radiant power (arbitrary units)

I

t is unlikely that a giant planet can support life. 1.0 The only places with CO2 suitable temperatures for liquid water are at certain O3 levels within the deep 0.5 atmospheres. But these levels are in turbulent motion and there are no stable environments. 0 Earth-sized planets are far 6 7 8 9 10 12 15 20 50 better prospects. As soon wavelength/µm as such planets are found The infrared spectrum of the Earth's atmosphere, showing and astronomers can absorption due to atmospheric constituents. The ozone distinguish their light from (O3) absorption indicates the presence of molecular the light of their stars, it will oxygen (O ), the huge quantities of which are sustained 2 be possible to investigate by photosynthesis in the Earth's biosphere. them for biospheres. Careful analysis will enable astronomers to estimate the planet's surface temperature, and if it were in the range for liquid water it would be encouraging. If water vapour were detected in an atmosphere too, then this would be even more encouraging. Liquid water would then be a possibility at the surface and so, with carbon common in the universe, the planet would be a potential habitat for carbon­water life. If the atmosphere were also shown to contain a large proportion of oxygen then most astronomers would conclude that an extensive biosphere was supported by photosynthesis. At sub-cellular level the biosphere would broadly be life as we know it, though the actual forms of any multicellular creatures could be quite alien. If, however, oxygen was not detected, we should not conclude that a biosphere was absent. It probably took the Earth's biosphere 2000 million years before photosynthesis had produced sufficient oxygen for it to be appreciable in the atmosphere. Also, much single-celled life on Earth does not perform photosynthesis, and some single-celled creatures do not depend on organisms that do.