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； tidal waves kilometers high； of flowing； red…hot liquid magma welled up from the interior and poured over the surface of the planet； burying everything in their path： mountains； channels； craters； perhaps even the last evidence of much earlier； more clement times。 The geological odometer was reset。 All accessible records of surface geology begin with the last global magma flood。 Before the； cool and solidify； oceans of lava may be hundreds or even thousands of kilometers thick。 In our time； billions of years later； the surface of such a world may be quiet； inactive； with no hint of current vulcanism。 Or there may be—as on Earth—a few small…scale but active reminders of an epoch when the entire surface was flooded with liquid rock。
In the early days of planetary geology； ground…based telescopic observations were all the data we had。 A fervent debate had been running for half a century on whether the craters of the Moon were due to impacts or volcanos。 A few low mounds with summit calderas were found—almost certainly lunar volcanos。 But the big craters—bowl or pan…shaped and sitting on the flat ground and not the tops of mountains—were a different story。 Some geologists saw in them similarities with certain highly eroded volcanos on Earth。 Others did not。 The best counter…argument was that we know there are asteroids and ets that fly past the Moon； they must hit it sometimes； and the collisions must make craters。 Over the history of the Moon a large number of such craters should have been punched out。 So if the craters we see are not due to impacts； where then are the impact craters？ We now know from direct laboratory examination of lunar craters that they are almost entirely of impact origin。 But 4 billion years ago this little world； nearly dead today； was bubbling and churning away； driven by primeval vulcanism from sources of internal heat now long gone。
In November 1971； NASA's Mariner 9 spacecraft arrived at Mars to find the planet pletely obscured by a global dust storm。 Almost the only features to be seen were four circular spots rising out of the reddish murk。 But there was something peculiar about them： They had holes in their tops。 As the storm cleared； we were able to see unmistakably that we had been viewing four huge volcanic mountains penetrating through the dust cloud； each capped by a great summit caldera。
After the storm dissipated； the true scale of these volcanos became clear。 The largest—appropriately named Olympus Mons； or Mt。 Olympus； after the home of the Greek gods—is more than 25 kilometers （roughly 15 miles） high； dwarfing not only the largest volcano on Earth but also the largest mountain of any sort； Mt。 Everest； which stands 9 kilometers above the Tibetan plateau。 There are some 20 large volcanos on Mars； but none so massive as Olympus Mons； which has a volume about 100 times that of the largest volcano on Earth； Mauna Loa in Hawaii。
By counting the accumulated impact craters （made by small impacting asteroids； and readily distinguished from summit calderas） on the flanks of the volcanos； estimates of their ages can be derived。 Some Martian volcanos turn out to be a few billion years old； although none dates back to the very origin of Mars； about 4。5 billion years ago。 Some； including Olympus Mons； are paratively new—perhaps only a few hundred million years old。 It is clear that enormous volcanic explosions occurred early in Martian history； perhaps providing an atmosphere much denser than the one Mars holds today。 What would the place have looked like if we had visited it then？
Some volcanic flows on Mars （for example； in Cerberus） formed as recently as 200 million years ago。 It is； I suppose； even possible—although there is no evidence either way—that Olympus Mons； the largest volcano we know about for certain in the Solar System； will be active again。 Volcanologists； a patient sort； would doubtless wele the event。
In 1990…93 the Magellan spacecraft returned surprising radar data about the landforms of Venus。 Cartographers prepared maps of almost the entire planet； 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 plume