imposed from above。 While there are deeper regularities in the Universe than the simple circumstances we generally describe as orderly； all that order； simple and plex； seems to derive from laws of Nature established at the Big Bang （or earlier）； rather than as a consequence of belated intervention by an imperfect deity。 〃God is to be found in the details〃 is the famous dictum of the German scholar Abu Warburg。 But； amid much elegance and precision； the details of life and the Universe also exhibit haphazard； jury…rigged arrangements and much poor planning。 What shall we make of this： an edifice abandoned early in construction by the architect？
The evidence； so far at least and laws of Nature aside； does not require a Designer。 Maybe there is one hiding； maddeningly unwilling to be revealed。 Sometimes it seems a very slender hope。
The significance of our lives and our fragile planet is then determined only by our own wisdom and courage。 We are the custodians of life's meaning。 We long for a Parent to care for us； to forgive us our errors； to save us from our childish mistakes。 But knowledge is preferable to ignorance。 Better by far to embrace the hard truth than a reassuring fable。
If we crave some cosmic purpose； then let us find ourselves a worthy goal。
CHAPTER 5 IS THERE INTELLIGENT LIFE ON EARTH？
They journeyed a long time and found nothing。 At length they discerned a small light； which was the Earth 。 。 。 ＇But＇ they could not find the smallest reason to suspect that we and our fellow—citizens of this globe have the honor to exist。
—VOLTAIRE； MICROMEGAS。 A PHILOSOPHICAL HISTORY （1752）
There are places； in and around our great cities； where the natural world has all but disappeared。 You can make out streets and sidewalks； autos； parking garages； advertising billboards； monuments of glass and steel； but not a tree or a blade of grass or any animal—besides； of course； the humans。 There are lots of humans。 Only when you look straight up through the skyscraper canyons can you make out a star or a patch of blue—reminders of what was there long before humans came to be。 But the bright lights of the big cities bleach out the stars； and even that patch of blue is sometimes gone； tinted brown by industrial technology。
It's not hard； going to work every day in such a place； to be impressed with ourselves。 How we've transformed the Earth for our benefit and convenience！ But a few hundred miles up or down there are no humans。 Apart from a thin film of life at the very surface of the Earth； an occasional intrepid spacecraft； and some radio static； our impact on the Universe is nil。 It knows nothing of us。
YOU'RE AN ALIEN EXPLORER entering the Solar System after a long journey through the blackness of interstellar space。 You examine the planets of this humdrum star from afar—a pretty handful； some gray； some blue； some red； some yellow。 You're interested in what kinds of worlds these are； whether their environments are static or changing； and especially whether there are life and intelligence。 You have no prior knowledge of the Earth。 You've just discovered its existence。
There's a galactic ethic； let's imagine： Look but don't touch。 You can fly by these worlds； you can orbit them； but you are strictly forbidden to land。 Under such constraints； could you figure out what the Earth's environment is like and whether anyone lives there？
As you approach； your first impression of the whole Earth is white clouds； white polar caps； brown continents； and some bluish substance that covers two thirds of the surface。 When you take the temperature of this world from the infrared radiation it emits； you find that most latitudes are above the freezing point of water； while the polar caps are below freezing。 Water is a very abundant material in the Universe； polar caps made of solid water would be a reasonable guess； as well as clouds of solid and liquid water。
You might also he tempted by the idea that the blue stuff is enormous quantities—kilometers deep—of liquid water。 The suggestion is bizarre； though； at least as far as this solar system is concerned； because surface oceans of liquid water exist nowhere else。 When you look in the visible and near…infrared spectrum for telltale signatures of chemical position； sure enough you discover water ice in the polar caps； and enough water vapor in the air to account for the clouds； this is also just the right amount that must exist because of evaporation if the oceans are in fact made of liquid water。 The bizarre hypothesis is confirmed。
The spectrometers further reveal that the air on this world is one fifth oxygen； O2。 No other planet in the Solar System has anything close to so much oxygen。 Where does it all e from？ The intense ultraviolet light from the Sun breaks water； H2O； down into oxygen and hydrogen； and hydrogen； the lightest gas； quickly escapes to space。 This is a source of O2； certainly； but it doesn't easily account for so much oxygen。
Another possibility is that ordinary visible light； which the Sun pours out in vast amounts； is used on Earth to break water apart—except that there's no known way to do this without life。 There would have to be plants； life…forms colored by a pigment that strongly absorbs visible light； that knows how to split a water molecule by saving up the energy of two photons of light； that retains the H and excretes the O； and that uses the hydrogen thus liberated to synthesize organic molecules。 The plants would have to be spread over much of the planet。 All this is asking a lot。 If you're a good skeptical scientist； so much O2 would not be proof of life。 But it certainly might be cause for suspicion。
With all that oxygen you're not surprised to discover ozone （O3） in the atmosphere； because ultraviolet light makes ozone out of molecular oxygen （O2）。 The ozone then absorbs dangerous ultraviolet radiation。 So if the oxygen is due to life； there's a curious sense in which the life is protecting itself。 But this life 〃night be mere photosynthetic plants。 A high level of intelligence is not implied。
When you examine the continents more closely； you find there are； crudely speaking； two kinds of regions。 One shows the spectrum of ordinary rocks and minerals as found on many worlds。 The other reveals something unusual： a material； covering vast areas； that strongly absorbs red light。 （The Sun； of course； shines in light of all colors； with a peak in the yellow。） This pigment might be just the agent needed if ordinary visible light is being used to break water apart and account for the oxygen in the air。 It's another hint； this time a little stronger； of life； not a bug here and there； but a planetary surface overflowing with life。 The pigment is in fact chlorophyll： It absorbs blue light as well as red； and is responsible for the fact that plants are green。 What you're seeing is a densely vegetated planet。
So the Earth is revealed to possess three properties unique at least in this solar system—oceans； oxygen； life。 It's hard not to think they're related； the oceans being the sites of origin； and the oxygen the product； of abundant life。
When you look carefully at the infrared spectrum of the Earth； you discover the minor constituents of the air。 In addition to water vapor； there's carbon dioxide （CO2）； methane （CH4）； and other gases that absorb the heat that the Earth tries to radiate away to space at night。 These gases warm the planet。 Without them； the Earth would everywhere be below the freezing point of water。 You've discovered this world's greenhouse effect。
Methane and oxygen together in the same atmosphere is peculiar。 The laws of chemistry are very clear： In an excess of O2； CH4 should be entirely converted into H2O and CO2； The process is so efficient that not a single molecule in all the Earth's atmosphere should be methane。 Instead； you find that one out of every million molecules is methane； ail immense discrepancy。 What could it mean？
The only possible explanation is that methane is being injected into the Earth's atmosphere so quickly that its chemical reaction with Oz can't keep pace。 Where does all this methane e from？ Maybe it seeps out of the deep interior of the Earth—but quantitatively this doesn't seem to work； and Mars and Venus don't have anything like this much methane。 The only alternatives are biological； a conclusion that makes no assumptions about the chemistry of life； or what it looks like； but follows merely from how unstable methane is in an oxygen atmosphere。 In fact； the methane arises from such sources as bacteria in bogs； the cultivation of rice； the burning of vegetation； natural gas from oil wells； and bovine flatulence。 In an oxygen atmosphere； methane is a sign of life。
That the intimate intestinal activities of cows should be detectable from interplanetary space is a little disconcerting； especially when so much of what we hold dear is not。 But an alien scientist flying by the Earth would； at this point； be unable to deduce bogs； rice； fire； oil； or cows。 Just life。
All the signs of life that we've discussed so far are due to paratively simple forms （the methane in the rumens of cows is generated by bacteria that homestead there）。 Had your spacecra