pale blue dot -carl sagan- 第42节

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