pale blue dot -carl sagan-第38节
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most recent 0。l percent of Martian history。
Some of the minerals they contain show clear evidence of having once been in water； warm liquid water。 These hydro…thermal minerals reveal that somehow； probably all over Mars； there was recent liquid water。 Perhaps it came about when the interior heat melted underground ice。 But however it happened； it's natural to wonder if life is not entirely extinct； if somehow it's managed to hang on into our time in transient underground lakes； or even in thin films of water wetting subsurface grains。
The geochemists Everett Gibson and Hal Karlsson of NASA's Johnson Space Flight Center have extracted a single drop of water from one of the SNC meteorites。 The isotopic ratios of the oxygen and hydrogen atoms that it contains are literally unearthly。 I look on this water from another world as an encouragement for future explorers and settlers。
Imagine what we might find if a large number of samples； including never melted soil and rocks； were returned to Earth from Martian locales selected for their scientific interest。 We are very close to being able to acplish this with small roving robot vehicles。
The transportation of subsurface material from world to world raises a tantalizing question： Four billion years ago there were two neighboring planets； both warm； both wet。 Impacts from space； in the final stages of the accretion of these planets； were occurring at a much higher rate than today。 Samples from each world were being flung out into space。 We are sure there was life on at least one of them in this period。 We know that a fraction of the ejected debris stays cool throughout the processes of impact； ejection； and interception by another world。 So could some of the early organisms on Earth have been safely transplanted to Mars four billion years ago； initiating life on that planet' Or； even more speculative； could life on Earth have arisen by such a transfer from Mars？ Might the two planets have regularly exchanged life…forms for hundreds of millions of years？ The notion might be testable。 If we were to discover life on Mars and found it very similar to life on Earth—and if； as well； e were sure it wasn't microbial contamination that we ourselves had introduced in the course of our explorations—the proposition that life was long ago transferred across interplanetary space would have to be taken seriously。
IT WAS ONCE THOUGHT that life is abundant on Mars。 Even the dour and skeptical astronomer Simon Newb （in his Astronomy for Everybody； which went through many editions in the early decades of this century and was the astronomy text of my childhood） concluded； 〃There appears to be life on the planet Mars。 A few years ago this statement was monly regarded as fantastic。 Now it is monly accepted。〃 Not 〃intelligent human life；〃 he was quick to add； but green plants。 However； we have now been to Mars and looked for plants—as well as animals； microbes； and intelligent beings。 Even if the other forms were absent； we might have imagined； as in Earth's deserts today； and as on Earth for almost all its history； abundant microbial life。
The 〃life detection〃 experiments on Viking were designed to detect only a certain subset of conceivable biologies； they were biased to find the kind of life about which we know。 It would have been foolish to send instruments that could not even detect life on Earth。 They were exquisitely sensitive； able to find microbes in the most unpromising； arid deserts and wastelands on Earth。
One experiment measured the gases exchanged between Martian soil and the Martian atmosphere in the presence of organic matter from Earth。 A second brought a wide variety organic foodstuffs marked by a radioactive tracer to see if there were bugs in the Martian soil who ate the food and oxidized it to radioactive carbon dioxide。 A third experiment introduced radioactive carbon dioxide （and carbon monoxide） to the Martian soil to see if any of it was taken up by Martian microbes。 To the initial astonishment of； I think； all the scientists involved； each of the three experiments gave what at first seemed to be positive results。 Gases were exchanged； organic matter was oxidized； carbon dioxide was incorporated into the soil。
But there are reasons for caution。 These provocative results are not generally thought to be good evidence for life on Mars： The putative metabolic processes of Martian microbes occurred under a very wide range of conditions inside the Viking landers—wet （with liquid water brought from Earth） and dry； light and dark； cold （only a little above freezing） to hot （almost the normal boiling point of water）。 Many microbiologists deem it unlikely that Martian microbes would be so capable under such varied conditions。 Another strong inducement to skepticism is that a fourth experiment； to look for organic chemicals in the Martian soil； gave uniformly negative results despite its sensitivity。 We expect life on Mars； like life on Earth； to be organized around carbon…based molecules。 To find no such molecules at all was daunting for optimists among the exobiologists。
The apparently positive results of the life detection experiments is now generally attributed to chemicals that oxidize the soil； deriving ultimately from ultraviolet sunlight （as discussed in the previous chapter）。 There is still a handful of Viking scientists who wonder if there might be extremely tough and petent organisms very thinly spread over the Martian soil—so their organic chemistry could not be detected； but their metabolic processes could。 Such scientists do not deny that ultraviolet…generated oxidants are present in the Martian soil； but stress that no thorough explanation of the liking life detection results from oxidants alone has been forthing。 Tentative claims have been made of organic matter in SNC meteorites； but they seem instead to be contaminants that have entered the meteorite after its arrival on our world。 So far； there are no claims of Martian microbes in these rocks from the sky。
Perhaps because it seems to pander to public interest； NASA and most Viking scientists have been very chary about pursuing the biological hypothesis。 Even now； much more could be done in going over the old data； in looking with Viking…type instruments at Antarctic and other soils that have few microbes in them； in laboratory simulation of the role of oxidants in the Martian soil； and in designing experiments to elucidate these matters—not excluding further searches for life—with future Mars landers。
If indeed no unambiguous signatures of life were determined by a variety of sensitive experiments at two sites 5；000 kilometers apart on a planet marked by global wind transport of fine particles； this is at least suggestive that Mars may be； today at least； a lifeless planet。 But if Mars is lifeless； we have two planets； of virtually identical age and early conditions； evolving next door to one another in the same polar system： Life evolves and proliferates on one； but not the other。 Why？
Perhaps the chemical or fossil remains of early Martian life can still be found—subsurface； safely protected from the ultraviolet radiation and its oxidation products that today fry the surface。 Perhaps in a rock face exposed by a landslide； or in the banks of an ancient river valley or dry lake bed； or in the polar； laminated terrain； key evidence for life on another planet is waiting。
Despite its absence on the surface of Mars； the planet's two moons； Phobos and Deimos； seem to be rich in plex organic matter dating back to the early history of the Solar System。 The Soviet Phobos 2 spacecraft found evidence of water vapor being out…gassed from Phobos； as if it has an icy interior heated by radioactivity。 The moons of Mars may have long ago been captured from somewhere in the outer Solar System； conceivably； they are among the nearest available examples of unaltered stuff from the earliest days of the Solar System。 Phobos and Deimos are very small； each roughly 10 kilometers across； the gravity they exert is nearly negligible。 So it's paratively easy to rendezvous with them； land on them； examine them； use them as a base of operations to study Mars； and then go home。
Mars calls； a storehouse of scientific information—important in its own right but also for the light it casts on the environment of our own planet。 There are mysteries waiting to be resolved about the interior of Mars and its mode of origin； the nature of volcanos on a world without plate tectonics； the sculpting of landforms on a planet with sandstorms undreamt of on Earth； glaciers and polar landforms； the escape of planetary atmospheres； and the capture of moons—to mention a more or less random sampling of scientific puzzles。 If Mars once had abundant liquid water and a clement climate； what went wrong？ How did an Earthlike world bee so parched； frigid； and paratively airless？ Is there something here we should know about our own planet？
We humans have been this way before。 The ancient explorers would have understood the call of Mars。 But mere scientific exploration does not require a human presence。 We tan always send smart robots。 They are far cheaper； they don't talk back； you can send them to much more dangerous locales； arid；
