Extracts from the book
by Carl Sagan, Jonathon Norton Leonard and the Editors of LIFE
Life Science Library"This book, from its conception to final editing was under the professional direction of Carl Sagan"
Contents[Impact craters] [Life on Earth] [Water & life on the Moon] [Water & life on Mars] [Other planetary systems] [SETI] [A Universe teeming with life] [Source]
Impact Craters on Earth[following discussion of impact craters on the Moon]
It is natural to ask why the moon should have suffered so many blows from space while the earth, probably its close companion for several thousand million years, seems to have escaped. The answer is that the earth did not escape; it merely covered most of its scars. As soon as scientists began to search seriously for impact craters on the earth, they found plenty of them. The famous Canyon Diablo [Meteor or Barringer] crater of the American State of Arizona is only the most obvious and best preserved one.
Maps and aerial photographs have led the searchers to many other intriguing circular
formations. In the very old rocks exposed in Canada, for example, several large, almost perfectly circular lakes have been identified. Their features can be explained only as the result of meteorite impacts. Similar structures exist in many other parts of the earth. Drilling shows that these "astroblemes" (star-scars) usually contain the broken rubble that is to be expected in a meteorite crater. They are often surrounded by bed-rock full of cone-shaped fractures made by the shock wave. There is seldom any trace of the meteorite itself, but this is not surprising. Unless it hits at unusually low speed, a meteorite will vaporize and spurt out of the crater as gas.
The largest structure on earth that is suspected of being part of an astrobleme is near the Nastapoka Island chain on the eastern shore of Hudson Bay in Canada. These islands form a beautifully circular arc with its centre of curvature out in the bay itself. If they are segments of an ancient rim, the crater must have been 275 miles in diameter, larger than all but a very few craters on the moon. Not far away to the east, the curving shores of the Gulf St Lawrence outline a somewhat smaller circle.
Life on Earth[Illustration associated with discussion of the early conditions on Earth, first organisms and evolution]
A CLUE TO PLANETARY LIFE maybe offered by these fossils, discovered in ancient rock by scientists. The primitive fossil above, the oldest yet found, dates back 3,1 00 million years and yet its double-layered cell wall is characteristic of modern bacteria. The more visible fossil below is a rod-shaped bacterium that lived about 2,000 million years ago. Life on other planets may have evolved-or may now exist-in similar rudimentary form.
Water and Life on the MoonThe moon is known to be waterless only on the surface. Astronomers seriously believe
that about 100 feet below may lie a layer of porous material saturated with ice like the permafrost that is common in the earth's Arctic regions. This frozen layer may have been formed by water vapour, outgassed from the moon's interior and turned to ice when it reached the extremely cold regions just below the surface. Water in liquid form may be found a little deeper down and nearer the warm interior of the moon. Lunar colonists would use this treasure, to them more precious than diamonds, not only for drinking, but also as a raw material from which to extract oxygen for breathing, and perhaps for use as a rocket propellant. Liquid water trapped under a seal of permafrost is about the only possible place where indigenous life might be found on the moon. There is little chance that life could develop on the barren surface under violent sun, but subsurface environments offer a faint possibility. If underground water does exist, it probably contains many materials, perhaps including carbon compounds that were carried up with the water vapour from the deep interior. Life may have evolved somehow in this ancient soup as it did in the earth's oceans, but underground in the absence of sunlight, the available energy for life is sparse. Life on the moon is not at all likely, but no prudent scientist would exclude it entirely.
No one can guess in advance what secrets the samples will reveal. Analysis may determine the age of the moon and tell whether it was formed in the same part of the solar system as the earth. The samples may have a layered structure that marks the passage of time like the rings in tree trunks; it may be possible to tell from these how conditions in space have varied since the youth of the moon. The layers may point, for instance, to periods when dust was unusually plentiful in the system, dimming the light from the sun, as has been suggested in theories of the earth's recurrent ice-ages.
It is not unlikely that organic matter of some sort (but not necessarily formed by living
organisms) will be found in samples of lunar material. If so, it might provide clues to the origin of terrestrial life. If it shows traces of once-living organisms, they may have come from the earth and have been tossed to the moon by an ancient asteroid impact. Hardy spores may be found that have been kept dimly alive below the moon's permafrost layer. Perhaps they can be revived to show what life was like on the earth thousands of millions of years ago. Or perhaps they originated beneath the moon's surface and represent a kind of life never known on the Earth.
None of these eventualities is likely, but no possibility, however remote, will be
neglected when the first ancient sample of the moon is brought down to earth.
Compiling the first comprehensive geologic moon map, lunar geologist Eugene Shoemaker checks detail maps on the wall and floor.
Water and Life on MarsMars lives in a bad neighbourhood of the solar system, where flying rocks are a frequent hazard. Between the planet's orbit and that of Jupiter lies the asteroid zone, where thousands of objects, presumably debris from small broken planets, revolve around the sun. There are probably 50,000 asteroids big enough to be seen with large telescopes. Most of them stay in their proper zone, but a considerable number wander eccentrically. Together with the visible asteroids move uncounted millions of meteorites too small to be seen but large enough to blast conspicuous craters in any planet they hit. The earth and moon have suffered many of these impacts, and Mars, being closer to the asteroid belt, must have had many more. It has been estimated that during any selected period Mars must have been hit by about 25 times as many meteorites as hit the moon. If the planet's surface were really ancient and free of significant erosion, it should be much more pitted than the moon's. Since it is not, erosion must be very effective on Mars and its surface must be younger than the moon's. Astronomers have calculated that none of the features that we see can be older than 800 million years, a small fraction of the age of the planet itself, nearly 5,000 million years. Something must have eroded the ancient craters of Mars's youth, and the most likely agents are water or wind. Though the planet has no oceans now and lacks a substantial atmosphere, it may have had both in the past. If it had, it may have been much more favourable for the development of life than it is now.
Not much is definitely known about the early history of Mars, so conjectures are necessarily vague...
The combined action of ultra-violet light and rusting iron may have been slow, so those who like to think that Mars was once more favourable for the appearance of life can look back hopefully to the youth of the planet. At that time it may have had. a good deal of water on its surface and a climate warm enough to keep at least some water in the liquid state. If ammonia, methane and other reducing gases were present in its atmosphere, the. stage would have been set, as it was on the earth, for the appearance of life.
Any kind of life that got a start on Mars would have slowly evolved by natural selection to keep up with changes in the environment. Life is enormously resourceful, and there is no reason to think that life originating on Mars would be less adaptable than the earth's. If free oxygen were never present in the atmosphere, life could thrive without it. Many of the earth's organisms do. If liquid water became scarce, Martian organisms might have come to retain a supply in their tissues and add to that precious hoard by acquiring other forms of water from the environment. .
Many common earthly micro-organisms can survive under Martian conditions. Scientists have put samples of soil in containers called "Mars jars", where the atmospheric temperature, composition, pressure and dryness are close to those of Mars. Some of the micro-organisms in the samples always survive. Living things are able to adapt themselves to extreme conditions. Many of the earth's micro-organisms, for example, can withstand almost any degree of cold while they are in a dormant state. Some insects winter-proof themselves with glycerol, a common antifreeze used
in car radiators. There is no conclusive reason why Martian organisms should not extend this principle, adding so much antifreeze to their tissues that they can live and reproduce in the extremely cold temperatures occurring on Mars.
More radical adaptations are possible, granted the origin of life on Mars and its subsequent evolution. If finely powdered limonite is a major constituent of Martian deserts, it may have become a medium of life. Each molecule of iron oxide in limonite is combined with about two water molecules. Any organisms that have found a way to extract water from limonite could live in the Martian deserts as if they were in oceans.
The possibility that Mars may support life is intensely interesting to scientists in fields outside astronomy. Many biologists feel that their science has potentially broader limits than the study of life on earth. On the biochemical level, all the earth's multitudinous organisms have much more striking similarities than differences. All are believed to be descendants of a single remote ancestor whose basic chemistry they still retain. They all store the coded information that enables them to reproduce their species in large, coiled molecules of nucleic acid. They all use similar proteins (the enzymes) to promote and control the chemical processes that take place in their tissues. Not a single deviation from this general pattern has been found on the earth, though there are literally millions of closely related chemical compounds that might be used in the processes of life. Why this curious narrowness? ask the biologists. They are eager to find out whether only one kind of life is possible or whether there may be others. Perhaps the
earth's present type is the lone survivor of many less efficient kinds of life that appeared on the early earth and were eliminated by competition.
Other Planetary SystemsOther stars can have planets too, and positive evidence is accumulating that many of them do.
Why some stars wobbleNo telescope can directly detect a planet revolving around a star; even the nearest stars are too far away. But planets can be detected by indirect methods. The most successful technique so far requires many accurate pictures which can be compared to reveal the star's motion. All near-by stars show "proper motion"-that is, motion of their own, not apparent motion that is due to any effect of the earth's movements on observations. If a star's proper motion wobbles slightly, the star must have with it at least one companion that cannot be seen. The wobble appears because the star and its unseen companion are actually revolving around a common centre, like partners in a waltz, but only the large
star can be seen. If the mass of the large star is known, as is sometimes the case, the mass and orbit of the companion can be calculated.
This method of detecting the companions of stars by the wobble they cause can be used only on stars that are the sun's closest neighbours, and it has not detected objects smaller than Jupiter. Its success, nevertheless, has been extraordinary. Barnard's star, the nearest single star to the sun, turns out to have an invisible, or dark companion with only 1.5 times the mass of Jupiter. This is the smallest planet discovered beyond the solar system so far, but 3 out of the 12 stars nearest to the sun are already known to have some sort of dark companion. (Note: in "Cosmos" (1980) Carl Sagan notes that "Two such quests [for extra-solar planets] have been performed for planets around Barnard's star, and both have been by some criteria successful... But unfortunately the two sets of observations seem mutually incompatible...unambiguous demonstration awaits further study.")
It is safe to assume that if a star has one planet it very likely has others formed by
the same process, and that stars too distant to be examined for planets are likely to have them too.
In fact, it appears that planetary systems are common possessions of stars. It is probable that the Milky Way galaxy with its 100,000 million stars has several hundred thousand million planets, and there may be comparable numbers in other galaxies.
How many of these innumerable planets may have life?
The answer to this fascinating question depends on a large number of unknown factors. Planets that are similar to the earth in mass, surface temperature, atmosphere and history are probably relatively rare, but this strict set of conditions may not be necessary. If it turns out that life can exist on planets like Mars or Jupiter, the number of favourable habitats will be enormously increased. It is also possible that life can appear on planets that do not resemble any examples in the solar system. But even if strict conditions are necessary, the statistics are strongly in favour of life.
The earth's modern organisms have some 4,000 million years of history behind them, during which evolution explored innumerable blind alleys. Every large, advanced creature is the product of literally thousands of millions of mutations, most of them imperceptible, which had the effect of giving the creature's ancestors, age after age a survival advantage over their competitors. Under slightly different conditions, or as the result of mere accidents, a different form might have won the battle for continuance. Many evolutionists believe that if the primitive earth were to evolve life all over again, there would be no appreciable chance that an animal physically resembling man, or even any of the mammals, would evolve.
Intelligent life on other planets (SETI)The question of whether alien planets have intelligent beings on them is a still-more tangled one. It is by no means certain that intelligence of human quality is the normal culmination of evolution. Some conditions may be favourable to life but not to high intelligence. The earth's oceans, for instance, have no appreciably intelligent creatures, except for mammals such as porpoises and seals that have returned to the water. To judge by this analogy, which is risky, an alien planet that is completely covered by water will probably have no animals more intelligent than the earth's fish.
The human combination of large brain and tool-holding hand is even more fortuitous. A long series of special circumstances was necessary to develop it. If any of them had been lacking, the earth might have continued for thousands of millions of years more, perhaps for the life of the solar system, without achieving really high intelligence.
On the other hand, intelligence undoubtedly has important survival value. Evolution on the earth has generally moved in the direction of more highly developed brains. Fish have better brains than the marine worms from which they evolved, and amphibians, reptiles and mammals have successively better ones. If man had not developed his large brain, some other mammal, perhaps the raccoon, might have done so in a few tens of millions of years.
If wonderful civilizations exist among the stars, it is only natural that human beings would want to visit them, or at least to communicate with them. Both these enterprises are fantastically difficult. Even the nearest stars are so enormously far away that to reach them not only must space be overcome, but also time. If a space ship sets out for near- by Barnard's star, about six light-years away, at the steady speed of 100,000 miles per hour, the voyage will take 40,000 years, and before it is fairly begun the crew will die of old age.
Three hundred years ago no one on earth had even seen the crudest working steam-engine. Now the earth has spacecraft that journey to Mars. Three hundred years hence the men of the future may well have discovered some unsuspected way to travel to the stars.
Until that time approaches, the most promising way to make contact with high civilizations on alien planetary systems is to listen for radio messages from them. Though difficult, this is by no means impossible. Radio telescopes no larger than those existing today on the earth could communicate with similar telescopes on planets tens of light-years away. Within that distance are thousands of stars, many of which are sure to have planets, and it is quite possible that radio messages from civilizations on some of them are reaching the earth now. The first sign that life exists among the stars may be radio signals that mark meaningful pulses on the recording tape of an earthly radio telescope.
A Universe Teeming with LifeDespite its 10,000-million-mile diameter, the solar system is dwarfed by the
Milky Way galaxy to which it belongs. But the Milky Way, containing 100,000 million
stars, is only a mote in the universe.
There are thousands of millions of such galaxies, most with their own myriad stars
having their own planetary systems. If only 1/10,000 of 1 per cent of these planets
harbour a civilization - and this is a very conservative estimate - the universe must teem with more than 100 million million civilizations.
These observations and conjecture are not too startling at the end of the 1990s. They were written, however, in 1966! (the youthful picture of Gene Shoemaker should have given a strong hint)