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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¡ªon¡¡timescales¡¡that¡¡are¡¡geologically¡¡in¡¡the¡¡near¡¡future¡ªthat¡¡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¡¡plume
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