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 spacecraft flown by the Earth a hundred million years ago； in the age of the dinosaurs when there were no humans and no technology； you would still have seen oxygen and ozone； they chlorophyll pigment； and far too much methane。 At present； though； your instruments are finding signs not just of life； but of high technology—something that couldn't possibly have been detected even a hundred years ago：
You are detecting a particular kind of radio wave emanating from Earth。 Radio waves don't necessarily signify life and intelligence Many natural processes generate them。 You've already found radio emissions from other； apparently uninhabited worlds—generated by electrons trapped in the strong magnetic fields of planets； by chaotic motions at the shock front that separates these magnetic fields from the interplanetary magnetic field； and by lightning。 （Radio 〃whistlers〃 usually sweep from high notes to low； and then begin again。） Some of these radio emissions are continuous； some e in repetitive bursts； some last a few minutes and then disappear。
But this is different： A portion of the radio transmission from Earth is at just the frequencies where radio waves begin to leak out of the planet's ionosphere； the electrically charged region above the stratosphere that reflects and absorbs radio waves。 There is a constant central frequency for each transmission； added to which is a modulated signal （a plex sequence of ons and offs）。 No electrons in magnetic fields； no shock waves； no lightning discharges can generate something like this。 Intelligent life seems to be the only possible explanation。 Your conclusion that the radio transmission is due to technology on Earth holds no matter what the ons and offs mean： You don't have to decode the message to be sure it is a message。 （This signal is really； let us suppose； munications from the U。S。 Navy to its distant nuclear…armed submarines。）
So； as an alien explorer； you would know that at least one species on Earth has achieved radio technology。 Which one is it？ The beings that make methane？ Those that generate oxygen？ The ones whose pigment colors the landscape green？ Or somebody else； somebody more subtle； someone not otherwise detectable to a spacecraft plummeting by？ To search for this technological species； you might want to examine the Earth at finer and finer resolution—seeking； if not the beings themselves； at least their artifacts。
You look first with a modest telescope； so the finest detail you can resolve is one or two kilometers across。 You can make out no monumental architecture； no strange formations； no unnatural reworking of the landscape； no signs of life。 You see a dense atmosphere in motion。 The abundant water must evaporate and then rain back down。 Ancient impact craters； apparent on the Earth's nearby Moon； are almost wholly absent。 There must； then； be a set of processes whereby new land is created and then eroded away in much less time than the age of this world。 Running water is implicated。 As you look with finer and finer definition you find mountain ranges； river valleys； and many other indications that the planet is geologically active。 There are also odd places surrounded by vegetation； but which are themselves denuded of plants。 They look like discolored smudges on the landscape。
When you examine the Earth at about 100…meter resolution； everything changes。 The planet is revealed to be covered with straight lines； squares； rectangles； circles—sometimes huddling along river banks or nestling on the lower slopes of mountains； sometimes stretching over plains； but rarely in deserts or high mountains； and absolutely never in the oceans。 Their regularity； plexity； and distribution would be hard to explain except by life and intelligence； although a deeper understanding of function and purpose might be elusive。 Perhaps you would conclude only that the dominant life…forms have a simultaneous passion for territoriality and Euclidean geometry。 At this resolution you could not see them； much less know them。
Many of the devegetated smudges are revealed to h