pale blue dot -carl sagan-第42章
按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
It is the nearest planet whose surface we can see with a small telescope。 In all the Solar System； it is the planet most like Earth。 Apart from flybys； there have been only two fully successful missions to Mars： Mariner 9 in 1971； and Vikings 1 and 2 in 1976。 They revealed a deep rift valley that would stretch from New York to San Francisco； immense volcanic mountains； the largest of which towers 80；000 feet above the average altitude of the Martian surface； almost three times the height of Mount Everest； an intricate layered structure in and among the polar ices； resembling a pile of discarded poker chips； and probably a record of past climatic change； bright and dark streaks painted down on the surface by windblown dust； providing high…speed wind maps of Mars over the past decades and centuries； vast globe…girdling dust storms； and enigmatic surface features。
Hundreds of sinuous channels and valley networks dating back several billion years can be found； mainly in the cratered southern highlands。 They suggest a previous epoch of more benign and Earthlike conditions—very different from what we find beneath the tenuous and frigid atmosphere of our time。 Some ancient channels seem to have been carved by rainfall； some by underground sapping and collapse； and some by great floods that gushed up out of the ground。 Rivers were pouring into and filling great thousand…kilometer…diameter impact basins that today are dry as dust。 Waterfalls dwarfing any on Earth today cascaded into the lakes of ancient Mars。 Vast oceans； hundreds of meters； perhaps even a kilometer； deep may have gently lapped shorelines barely discernible today。 That would have been a world to explore。 We are four billion years late。*
* Although in a few places； such as the slopes of the elevation called Alba Patera； there are multibranched valley networks that by parison are very young。 Somehow； even in the most recent billion years； liquid water seems to have flowed here and there； from time to time； through the deserts of Mars。
On Earth in just the same period； the first microorganisms arose and evolved。 Life on Earth is intimately connected； for the most basic chemical reasons； with liquid water。 We humans are ourselves made of some three…quarters water。 The same sorts of organic molecules that fell out of the sky and were generated in the air and seas of ancient Earth； should also have accumulated on ancient Mars。 Is it plausible that life quickly came to be in the waters of early Earth； but was somehow restrained and inhibited in the waters of early Mars？ Or might the Martian seas have been filled with life—floating； spawning； evolving？ What strange beasts once swum there？
Whatever the dramas of those distant times； it all started to go wrong around 3。8 billion years ago。 We can see that the erosion of ancient craters dramatically began to slow about then。 As the atmosphere thinned； as the rivers flowed no more； as the oceans began to dry； as the temperatures plummeted； life would have retreated to the few remaining congenial habitats； perhaps huddling at the bottom of ice…covered lakes； until it too vanished and the dead bodies and fossil remains of exotic organisms—built； it might be； on principles very different from life on Earth—were deep…frozen； awaiting the explorers who might in some distant future arrive on Mars。
METEORITES ARE FRAGMENTS OF OTHER WORLDS recovered on Earth。 Most originate in collisions among the numerous asteroids that orbit the Sun between the orbits of Mars and Jupiter。 But a few are generated when a large meteorite impacts a planet or asteroid at high speed； gouges out a crater； and propels the excavated surface material into space。 A very small fraction of the ejected rocks； millions of years later； may intercept another world。
In the wastelands of Antarctica； the ice is here and there dotted with meteorites； preserved by the low temperatures and until recently undisturbed by humans。 A few of them； called SNC （pronounced 〃snick〃） meteorites* have an aspect about them that at first seemed almost unbelievable： Deep inside their mineral and glassy structures； locked away from the contaminating influence of the Earth's atmosphere； a little gas is trapped。 When the gas is analyzed； it turns out to have exactly the same chemical position and isotopic ratios as the air on Mars。 We know about Martian air not just from spectroscopic inference but from direct measurement on the Martian surface by the Viking landers。 To the surprise of nearly everyone； the SNC meteorites e from Mars。
* Short for Shergotty…Nakhla…Chassigny。 You can see why the acronym is used。
Originally； they were rocks that had melted and refrozen。 Radioactive dating of all the SNC meteorites shows their parent rocks condensed out of lava between 180 million and 1。3 billion years ago。 Then they were driven off the planet by collisions from space。 From how long they've been exposed to cosmic rays on their interplanetary journeys between Mars and Earth； we can tell how old they are—how long ago they were ejected from Mars。 In this sense； they are between 10 million and 700；000 years old。 They sample the 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 hist
