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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