pale blue dot -carl sagan- 第34部分

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　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　ultra