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et；　with　fine　detail　down　to　about　100　meters；　the　goal…line…to…goal…line　distance　in　an　American　football　stadium。　More　data　were　radioed　home　by　Magellan　than　by　all　other　planetary　missions　bined。　Since　much　of　the　ocean　floor　remains　unexplored　（except　perhaps　for　still…classified　data　acquired　by　the　U。S。　and　Soviet　navies）；　we　may　know　more　about　the　surface　topography　of　Venus　than　about　any　other　planet；　Earth　included。　Much　of　the　geology　of　Venus　is　unlike　anything　seen　on　Earth　or　anywhere　else。　Planetary　geologists　have　given　these　landforms　names；　but　that　doesn't　mean　we　fully　understand　how　they're　formed。

Because　the　surface　temperature　of　Venus　is　almost　470°C　（900°F）；　the　rocks　there　are　much　closer　to　their　melting　points　than　are　those　at　the　surface　of　the　Earth。　Rocks　begin　to　soften　and　flow　at　much　shallower　depths　on　Venus　than　on　Earth。　This　is　very　likely　the　reason　that　many　geological　features　on　Venus　seem　to　be　plastic　and　deformed。

The　planet　is　covered　by　volcanic　plains　and　highland　plateaus。　The　geological　constructs　include　volcanic　cones；　probable　shield　volcanos；　and　calderas。　There　are　many　places　where　we　can　see　that　lava　has　erupted　in　vast　floods。　Some　plains　features　ranging　to　over　200　kilometers　in　size　are　playfully　called　〃ticks〃　and　〃arachnoids〃　（which　translates　roughly　as　〃spiderlike　things〃）—because　they　are　circular　depressions　surrounded　by　concentric　rings；　while　long；　spindly　surface　cracks　extend　radially　out　from　the　center。　Odd；　flat　〃pancake　domes〃—a　geological　feature　unknown　on　Earth；　but　probably　a　kind　of　volcano—are　perhaps　formed　by　thick；　viscous　lava　slowly　flowing　uniformly　in　all　directions。　There　are　many　examples　of　more　irregular　lava　flows。　Curious　ring　structures　called　〃coronae〃　range　up　to　some　2；000　kilometers　across。　The　distinctive　lava　flows　on　stifling　hot　Venus　offer　up　a　rich　menu　of　geological　mysteries。

The　most　unexpected　and　peculiar　features　are　the　sinuous　channels—with　meanders　and　oxbows；　looking　just　like　river　valleys　on　Earth。　The　longest　are　longer　than　the　greatest　rivers　on　Earth。　But　it　is　much　too　hot　for　liquid　water　on　Venus。　And　we　can　tell　from　the　absence　of　small　impact　craters　that　the　atmosphere　has　been　this　thick；　driving　as　great　a　greenhouse　effect；　for　as　long　as　the　present　surface　has　been　in　existence。　（If　it　had　been　much　thinner；　intermediate…sized　asteroids　would　not　have　burned　up　on　entry　into　the　atmosphere；　but　would　have　survived　to　excavate　craters　as　they　impact　this　planet's　surface。）　Lava　flowing　downhill　does　make　sinuous　channels　（sometimes　under　the　ground；　followed　by　collapse　of　the　roof　of　the　channel）。　But　even　at　the　temperatures　of　Venus；　the　lavas　radiate　heat；　cool；　slow；　congeal；　and　stop。　The　magma　freezes　solid。　Lava　channels　cannot　go　even　10　percent　of　the　length　of　the　long　Venus　channels　before　they　solidify。　Some　planetary　geologists　think　there　must　be　a　special　thin；　watery；　inviscid　lava　generated　on　Venus。　But　this　is　a　speculation　supported　by　no　other　data；　and　a　confession　of　our　ignorance。

The　thick　atmosphere　moves　sluggishly；　because　it's　so　dense；　though；　it's　very　good　at　lifting　and　moving　fine　particles。　There　are　wind　streaks　on　Venus；　largely　emanating　from　impact　craters；　in　which　the　prevailing　winds　have　scoured　piles　of　sand　and　dust　and　provided　a　sort　of　weather　vane　imprinted　on　the　surface。　Here　and　there　we　seem　to　see　fields　of　sand　dunes；　and　provinces　where　wind　erosion　has　sculpted　volcanic　landforms。　These　aeolian　processes　take　place　in　slow　motion；　as　if　at　the　bottom　of　the　sea。　The　winds　are　feeble　at　the　surface　of　Venus。　It　may　take　only　a　soft　gust　to　raise　a　cloud　of　fine　particles；　but　in　that　stifling　inferno　a　gust　is　hard　to　e　by。

There　are　many　impact　craters　on　Venus；　but　nothing　like　the　number　on　the　Moon　or　Mars。　Craters　smaller　than　a　few　across　are　oddly　missing。　The　reason　is　understood：　Small　asteroids　and　ets　are　broken　up　on　entry　into　the　dense　Venus　atmosphere　before　they　can　hit　the　surface。　The　observed　cutoff　in　crater　size　corresponds　very　well　to　the　present　density　of　the　atmosphere　of　Venus。　Certain　irregular　splotches　seen　on　the　Magellan　images　are　thought　to　be　the　remains　of　impactors　that　broke　up　in　the　thick　air　before　they　could　gouge　out　a　crater。

Most　of　the　impact　craters　are　remarkably　pristine　and　well　preserved；　only　a　few　percent　of　them　have　been　engulfed　by　subsequent　lava　flows。　The　surface　of　Venus　as　revealed　by　Magellan　is　very　young。　There　are　so　few　impact　craters　that　everything　older　than　about　500　million　years*　must　have　been　eradicated—on　a　planet　almost　certainly　4。5　billion　years　old。　There　is　only　one　plausible　erosive　agent　adequate　for　what　we　see：　vulcanism。　All　over　the　planet　craters；　mountains；　and　other　geological　features　have　been　inundated　by　seas　of　lava　that　once　welled　up　from　the　inside；　flowed　far；　and　froze。

*　The　age　of　the　Venus　surface；　as　determined　by　Magellan　radar　imagery；　puts　an　additional　nail　in　the　coffin　of　the　thesis　of　Immanuel　Velikovsky—who　around　1950　proposed；　to　surprising　media　acclaim；　that　3；500　years　ago　Jupiter　spat　out　a　giant　〃et〃　which　made　several　grazing　collisions　with　the　Earth；　causing　various　events　chronicled　in　the　ancient　books　of　many　peoples　（such　as　the　Sun　standing　still　on　Joshua's　mand）；　and　then　transformed　itself　into　the　planet　Venus。　There　ire　still　people　N％　ho　take　these　notions　seriously。

After　examining　so　young　a　surface　covered　with　congealed　magma；　you　might　wonder　if　there　are　any　active　volcanos　left。　None　has　been　found　for　certain；　but　there　are　a　few—for　example；　one　called　Maat　Mons—that　appear　to　be　surrounded　by　fresh　lava　and　which　may　indeed　still　be　churning　and　belching。　There　is　some　evidence　that　the　abundance　of　sulfur　pounds　in　the　high　atmosphere　varies　with　time；　as　if　volcanos　at　the　surface　were　episodically　injecting　these　materials　into　the　atmosphere。　When　the　volcanos　are　quiescent；　the　sulfur　pounds　simply　fall　out　of　the　air。　There's　also　disputed　evidence　of　lightning　playing　around　the　mountaintops　of　Venus；　as　sometimes　happens　on　active　volcanos　on　Earth。　But　we　do　not　know　for　certain　whether　there　is　ongoing　vulcanism　on　Venus。　That's　a　matter　for　future　missions。

Some　scientists　believe　that　until　about　500　million　years　ago　the　Venus　surface　was　almost　entirely　devoid　of　landforms。　Streams　and　oceans　of　molten　rock　were　relentlessly　pouring　out　of　the　interior；　filling　in　and　covering　over　any　relief　that　had　managed　to　form。　Had　you　plummeted　down　through　the　clouds　in　that　long…ago　time；　the　surface　would　have　been　nearly　uniform　and　featureless。　At　night　the　landscape　would　have　been　hellishly　glowing　from　the　red　heat　of　molten　lava。　In　this　view；　the　great　internal　heat　engine　of　Venus；　which　supplied　copious　amounts　of　magma　to　the　surface　until　about　500　million　years　ago；　has　now　turned　off。　The　planetary　heat　engine　has　finally　run　down。

In　another　provocative　theoretical　model；　this　one　by　the　geophysicist　Donald　Turcotte；　Venus　has　plate　tectonics　like　the　Earth's　but　it　turns　off　and　on。　Right　now；　he　proposes；　the　plate　tectonics　are　off；　〃continents〃　do　not　move　along　the　surface；　do　not　crash　into　one　another；　do　not　thereby　raise　mountain　ranges；　and　are　not　later　subducted　into　the　deep　interior。　After　hundreds　of　millions　of　years　of　quiescence；　though；　plate　tectonics　always　breaks　out　and　surface　features　are　flooded　by　lava；　destroyed　by　mountain　building；　subducted；　and　otherwise　obliterated。　The　last　such　breakout　ended　about　500　million　years　ago；　Turcotte　suggests；　and　everything　has　been　quiet　since。　However；　the　presence　of　coronae　may　signify—on　timescales　that　are　geologically　in　the　near　future—that　massive　changes　on　the　surface　of　Venus　are　about　to　break　out　again。



EVEN　MORE　UNEXPECTED　than　the　great　Martian　volcanos　or　the　magma…flooded　surface　of　Venus　is　what　awaited　us　when　the　Voyager　1　spacecraft　encountered　lo；　the　innermost　of　the　four　large　Galilean　moons　of　Jupiter；　in　March　1979。　There　we　found　a　strange；　small；　multihued　world　positively　awash　in　volcanos。　As　we　watched　in　astonishment；　eight　active　plumes　poured　gas　and　fine　particles　up　into　the　sky。　The　largest；　now　called　Pele—after　the　Hawaiian　volcano　goddess—projected　a　fountain　of　material　250　kilometers　into　space；　higher　above　the　surface　of　Io　than　some　astronauts　have　ventured　above　the　Earth。　By　the　time　Voyager　2　arrived　at　Io；　four　months　later；　Pele　had　turned　itself　off；　although　six　of　the　other　plumes　were　still　active；　at　least　one　new　plume　had　been　discovered；　and　another　caldera；　named　Surt；　had　changed　its　color　dramatically。

The　colors　of　Io；　even　though　exaggerated　in　NASA's　color…enhanced　images；　are　like　none　elsewhere　in　the　Solar　System。　The　currently　favored　explanation　is　that　the　Ionian　volcanos　are　driven　not　by　upwelling　molten　rock；　as　on　the　Earth；　the　Moon；　Venus；　and　Mars；　but　by　upwelling　sulfur　dioxide　and　molten　sulfur。　The　surface　is　covered　with　volcanic　mountains；　volcanic　calderas；　vents；　and　lakes　of　molten　sulfur。　Various　forms　and　pounds　of　sulfur　have　been　detected　on　the　surface　of　Io　and　in　nearby　space—the　volcanos　blow　some　of　the　sulfur　off　Io　altogether。*　These　findings　have　suggested　to　some　an　underground　sea　of　liquid　sulfur　that　issues　to　the　surface　at　points　of　weakness；　generates　a　shallow　volcanic　mound；　trickles　downhill；　and　freezes；　its　final　color　determined　by　its　temperature　on　eruption。

*　Io's　volcanos　are　also　the　copious　source　of　electrically　charged　atoms　such　as　oxygen　and　sulfur　that　populate　a　ghostly；　doughnut…shaped　tube　of　matter　that　surrounds　Jupiter。

On　the　Moon　or　Mars；　you　can　find　many　places　that　have　changed　little　in　a　billion　years。　On　Io；　in　a　century；　much　of　the　surface　should　be　reflooded；　filled　in　or　washed　away　by　new　volcanic　flows。　Maps　of　Io　will　then　quickly　bee　obsolete；　and　cartography　of　Io　will　have　bee　a　growth　industry。

All　this　seems　to　follow　readily　enough　from　the　Voyager　observations。　The　rate　at　which　the　surface　is　covered　over　by　current　volcanic　flows　implies　massive　changes　in　50　or　100　years；　a　prediction　that　luckily　can　be　tested