"Venus Unveiled"

PBS Airdate: October 17, 1995

She is a goddess in nearly every culture, but to science, Venus is the planet of mystery. Until recently, virtually nothing was known about Venus - even though it is the nearest planet to Earth. It is impossible to see the surface of Venus through its dense layer of clouds. Nearly identical to the Earth in size, Venus once held out the tantalizing possibility of being Earth's twin - possibly even a source of life. But no longer.

The spacecraft Magellan just recently completed a four year tour of Venus - piercing its clouds with radar and mapping the surface in exquisite detail. What Magellan saw beneath those clouds was a tortured and alien landscape - one of the most mysterious places in the entire solar system.

SEAN SOLOMON: To study the planets takes us to the very root of some very fundamental philosophical questions, like who are we, and why are we here, and what is our place in the cosmos.

NARRATION: All the planets in our Solar System were formed four and a half billion years ago, out of the mass of gas, dust and ice swirling around the Sun. Farthest away is Pluto. Then come the watery worlds of Uranus and Neptune - then the huge gas giants, Saturn and Jupiter. Closest to the Sun are the four tiny rock planets - Mars, Earth, Venus and Mercury. The Earth and Venus are the only planets in our Solar System that are geologically active. What other secrets do they share?

ROBERT GRIMM: There are many things that can be learned from all the planets...about how atmospheres work...and the behavior of climate through time, but for geology the key planet is Venus. Venus is about the same size as the Earth, it's right next door to us in the solar system, it should be made of about the same stuff, the same proportion of rock and metal as the Earth. It should be our twin.

NARRATION: Because we can't see its surface, Venus has always inspired curiosity and speculation.

PATRICK MOORE: I have been observing Venus ever since 1934, when I was a boy of eleven, and in that time I've seen the whole gamut of emotions. Theories have come, theories have gone. Venus has changed: either it was a watery world, or it was a carboniferous world, or a desert. I've seen it all.

Right up to the start of the space age, Venus was still described as 'the planet of mystery.' The closest of all the planets and yet we knew nothing about the surface at all, so you could really put there what you liked and no-one was going to say you were wrong.

NARRATION: And that's exactly what happened until 1610 when Galileo - using the newly invented telescope - discovered that Venus appears to wax and wane in the sky just as the Moon does. From this observation, he confirmed the heretical theory that the planets revolve around the sun. But in the centuries since Galileo, astronomers have imagined everything on Venus - from canals to continents. Thanks to science and science-fiction alike, many fantastic hopes for Venus remained alive well into this century.

PATRICK MOORE: Even in my lifetime there have been four possible models of what the surface of Venus might be like. And the first of these was the most attractive one of all I think, a kind of carboniferous model with a primitive life, a kind of idyllic Venus: warm, but not too warm, with luxuriant vegetation. Swamps everywhere and possibly things like dragonflies and amphibians.

On the other hand it was thought there might be oceans there, a kind of marine Venus. If there are oceans on Venus, the carbon dioxide in the atmosphere would have got into the water and fouled it and you'd have oceans of soda water.

Number three was an oil-rich Venus: seas of oil - "Venus is probably endowed with oil beyond the dreams of the richest Texas oil king."

And then number four of course was the dust theory, the dust-desert theory - according to which Venus was a desiccated planet with no water anywhere. A kind of raging dust storm, a kind of inferno. The conventional idea of hell.

NARRATION: In the early days of space exploration, there were hopes that life would be found on Venus.

NASA FILM NARRATOR: "Tantalizing, exasperating Venus. A single significant experiment can confirm old or create new theories."

CARL SAGAN: Many theories of the Venus environment have been suggested. However, new information eliminates at least some of these theories. Measurements with radio telescopes show that there is a region on Venus where temperatures are greater than 600 degrees Fahrenheit. It is just possible that the surface temperature could then be almost earth-like, and life as we know it could exist there. However, it is more likely that if there is life on Venus, it is probably of a type that we can not now imagine.

NASA Film Narrator: "Why do we explore space? Because we wonder if we really are alone. Or if on some other beautiful world, perhaps in some other corner of the universe, there are other thinking beings asking the same questions that we ask."

NARRATION: Some people believe those questions have already been answered.

ALAN MOSELEY: Blessed are the planetary ones who have at this time sacrificed peace, sacrificed friendship, sacrificed their very salvation.

The Aetherius Society was founded in 1955 by an Englishman, George King. He was contacted while in his flat by intelligences from other planets within this solar system and he was told that he was to become the voice of interplanetary parliament to make their message known to the world. Jesus is from Venus in our understanding. So is the intelligence we refer to as Buddha. More and more information is going to come to light which definitely will reveal the fact that there is advanced life, intelligent life, within this solar system and on the planet Venus definitely.

AETHERIAN WOMAN: We pray that your light may shine through us all.

ALAN MOSELEY: There would be nothing that science could bring back to us that would prove to us, in the Aetherius Society, that life did not exist on Venus. These intelligences don't live in a carbon based body like we do. They don't breathe the same type of atmosphere as we do. They are masters of matter.

PATRICK MOORE: It was the space age which changed all our ideas about Venus. From having no information at all we suddenly had a lot, and many people didn't really like what they found.

NARRATION: Venus was the target of the very first interplanetary probe in 1962. Far from finding Earth's twin, Mariner 2 revealed a planet that is extremely hot - far hotter than could be explained simply by its distance from the sun. It was the height of the Cold War, and Venus was also becoming a focus of the Soviet Space Program. They were determined to land on Venus.

ROALD SAGDEEV: There was always a sense of competition. The Soviet government could easily support such mission - from the point of view of propaganda and from the point of view of flexing the muscles. Venus . . . was considered by them as a battle ground of the Cold War. One of the battle grounds. We scientists were not interested very much in flexing the muscle. We were taking this chance to do our own science and it was terribly exciting.

NARRATION: Despite the excitement, the goal remained elusive. One lander was sent after another, each built to withstand greater heat and pressure. The Soviets were puzzled when nothing survived the atmosphere of Venus.

After endless tests and failures, they discovered why. As each probe descended through the clouds, it sent back information revealing a parched planet, choked with carbon dioxide, temperatures hot enough to melt lead, and atmospheric pressure that would crush a jumbo jet. And those mysterious clouds were actually made of super-corrosive sulphuric acid.

Eventually, a few heroic super-probes made it to the surface and survived there for up to sixty five minutes before being destroyed. Finally in 1975, a camera aboard a Soviet lander survived long enough to send back the first photograph of the surface of Venus.

ROALD SAGDEEV: We were expecting at the beginning to find a twin sister planet of our Earth. Everything which was different surprised us. If one would compare the base knowledge of Venus before the space age and after this cavalcade of the spacecrafts landing on Venus, I would say we discovered completely new world.

PATRICK MOORE: If you go to Venus you'd be promptly fried, poisoned, squashed and corroded. I remember...giving a talk, I think, in London University in the 1950's and I said then that, quite possibly, Venus, as a potential colony, might be more promising than Mars. How wrong I was!

NARRATION: With data from more and more missions, scientists had to draw up new images of Venus. At 900 degrees, Venus has the hottest surface temperature of any planet. The pressure there is the same as that at ocean depths of about 3000 feet on Earth. There seemed no chance of life. How had Venus, so close to Earth in size and distance, become so different?

ELLEN STOFAN: When you look at Venus and the Earth, they formed at about the same place in the solar system, they're made of about the same materials, they're about the same size. To a scientist this tells you, 'okay you started at the same point, now shouldn't you end up at relatively the same point?' And instead, the two planets have gone off in different directions. Why did that happen?

ROBERT GRIMM: It's just as if we put two pies in the oven to bake, and they should have had the same recipe, they should have been put in identically, they were cooked for the same time, out they come and they're different. What happened?

NARRATION: In 1978 the United States launched Pioneer Venus the most ambitious probe yet. Pioneer discovered a clue to the mystery of why Venus is so hot. Venus is closer to the sun than the earth. That by itself would increase its surface temperature by 90 degrees. But Venus is about 800 degrees hotter. In the atmosphere, Pioneer found evidence that although Venus may once have had oceans, it had lost much of its hydrogen - an essential component of water. This could have had a devastating impact on it's climate.

ANDREW INGERSOLL: I was doing some simple models of the Earth's climate, and I decided to run an experiment with this model. Move the earth to the orbit of Venus and see how the climate changed. And I got this amazing result. That as you, that basically the model went crazy.

As you move the Earth towards the sun it gets warmer and water evaporates from the oceans and water in the atmosphere acts as blanket, it traps the heat and causes the surface to get even warmer. And that of course causes more water to evaporate, and the whole process runs away - and I called it the runaway greenhouse.

NARRATION: Once the water was gone, Venus was really in trouble. Made of the same materials, Venus and the Earth contain roughly the same amounts of carbon dioxide - the so-called "greenhouse gas." On the Earth, most of this gas is dissolved in the seas and is kept there as limestone. On Venus, it's a different story. Early in its life, water on the surface evaporated. Carbon dioxide built up in its atmosphere. It formed a barrier to heat - like the glass windows of a greenhouse. Carbon dioxide is transparent to incoming radiation from the sun, which warms the surface of the planet and is re-emitted in the form of heat. Some of that heat radiation escapes back into space, but the carbon dioxide also absorbs some, trapping it in the planet's atmosphere. As more radiation from the sun comes in, the carbon dioxide absorbs more heat, and the surface temperature rises. As the carbon dioxide layer increases, so does the temperature.

ANDREW INGERSOLL: We've learned that natural history is chaotic. Small difference in the starting conditions can produce enormous differences later on, and Venus and the Earth are a perfect example of that. You've got a hellish place over there and you've got this Garden of Eden here.

NARRATION: While Pioneer helped explain the extremely hot and dry atmosphere of Venus, it caused a storm among geologists. Using radar to see through the clouds, Pioneer sent back crude maps of the surface of Venus. These maps sparked a debate about heat within the planet - the driving force of all geology. Over time, a planet must lose as much heat as it generates. But no one agreed on how Venus was losing the heat from its interior.

SEAN SOLOMON: There were two theories that evolved - both of them based on our understanding of how the earth works. There was the idea that Venus had some kind of plate tectonics.

NARRATION: Plate tectonics is the key to the Earth's geology. The surface of the earth is divided into huge moving plates constantly colliding and separating. Where the plates move apart, large amounts of magma well up from deep inside the earth and form new crust. Almost all of the heat generated within the Earth is lost along the vast boundaries of plates through plate tectonics. Many thought Venus might be losing its heat in the same way.

SEAN SOLOMON: The second theory was that Venus would lose its heat not by plate tectonics but by great outpourings of lava and individual volcanic centers.

NARRATION: To lose enough heat through volcanoes, there would have to be a great number of them - all continually active and erupting.

SEAN SOLOMON: Both of these theories evolved very much by analogy to what we understood on the Earth. What we needed was a close-up view of the surface so that we could see the individual geological features. The only way we could get that was to put another spacecraft in orbit around Venus that would give us images globally of the surface.NARRATION: That spacecraft was to be VOIR - the Venus Orbiting Imaging Radar. Packed with scientific equipment it would map the surface in fine detail, and answer many geological questions. The possibilities were exciting, and expensive.

JOHN GERPHEIDE: We have this wonderful, fantastic mission that's going to go to Venus and tell us all about that planet. We had the spacecraft approved, the funding was about to appear, the excitement was rampant here at the lab, Reagan got elected. He came into power, he came into office, and suddenly the funding disappeared. We were distraught. Our challenge then was to make that mission cheap. We went to the junk yard, picked up a lot of spare parts, and said we will use those parts in building this new mission. Parker was the spacecraft manager. He went to that junkyard and found a lot of stuff. Tell them about it.

GARY PARKER: I'm not sure I'd call it a junkyard, really, it was the store house if you will we had for stuff that was left over from previous missions. We had a high gain antenna and other low gain antennas and by combining leftover equipment with the spare equipment, we were able to put together a spacecraft at this much much cheaper price.

NARRATION: It took 200 engineers to downscale the mission into the more modest Magellan spacecraft.

JOHN GERPHEIDE: We thought that if we could save a hundred million, Magellan had a reasonable chance of being funded and authorized. We got the hundred million out and they said 'that's fine gents, now get another hundred million out.' So this went on for a period of about three months...where we incrementally cut off hundred million chunks to get the price down to two hundred and fifty million. . .

NARRATION: After being cut to the bare bones, this hybrid space craft was left with only one scientific instrument: a powerful radar sensor to map the surface of Venus.

JOHN GERPHEIDE: ...and then headquarters says you're in!

NARRATION: On May 4th, 1989, Magellan was launched aboard the space shuttle Atlantis.

STEPHEN SAUNDERS: A launch is kind of like a birth, in a way, because it separates lot of stuff that went before from all the good things that are yet to come. And I remember standing there in the viewing area and watching Shuttle slowly rise up and after it was out of sight beyond a few scattered clouds, I realized that I hadn't heard a thing, and tears were streaming down my face, and I thought, you know it's really in the best, safest place it can be now, out in space, where nobody can drop a hammer on the solar panels or burn the batteries or anything, and that was an exciting event.

NARRATION: Because of where Venus was in its orbit, the trip would take well over a year. Magellan would have to circle the sun one and a half times before falling into orbit around Venus.

ELLEN STOFAN: Magellan had a 15 month cruise before it got to Venus. Everybody was tremendously excited that we were finally going to get those answers that everybody had been waiting for. The summer right before the Magellan got to Venus, everybody laid out their best theory of what was going to happen in an issue of a scientific journal. We were all told, 'okay, say what you think the data is going to prove,' and everybody did, and so in anticipation of that you've now published what you think is going to be the right answer, and now you're just waiting and waiting for this data to come back.

NARRATION: Not everyone agreed on what that data would show.

SEAN SOLOMON: I like to think I went in with an open mind but I was favouring a lot of deformation, a very active planet, a lot of young volcanism and the surface churning up left and right wherever we would look.

ELLEN STOFAN: I think a lot of us in the scientific community, based on the earlier data sets, especially the Pioneer Venus data, thought that there was still a possibility that there might be plate tectonics on Venus.

STEPHEN SAUNDERS: I really expected that Venus would be a very active place and a geological analogy might be a simmering pot of porridge in the sense that over periods of time it's very active and bobbing up and down here and there with a lot of activity.

NARRATION: Magellan would provide the most detailed maps ever made of the surface of Venus. Every orbit, Magellan's radar pierced the thick clouds - imaging the landscape below in narrow strips about 15 miles wide and 10,000 miles long. By bouncing radio waves off the surface and recording their echoes, geographic features could be distinguished. An altimeter then made precise measurements of surface elevations so the terrain could be mapped. At the end of each imaging session, Magellan turned around and sent this data back to earth in digital form.

It was received by NASA's global network of giant tracking antennas in California, Australia and Spain.

SEAN SOLOMON: All of us were on pins and needles awaiting the first data that would allow us to see what the surface really looked like. We all had our own ideas of how Venus worked. Many of them were in conflict. We were simply terribly anxious that mission would work at all and very eager to see who if anybody would be proven right.

NARRATION: The numerical information was transformed into long, thin images, referred to as "noodles," which would be assembled into a map of the entire surface.

ELLEN STOFAN: A couple of us went in at about four in the morning to look at those first strips and you were just looking at them and realizing I'm seeing these parts of the planet that have never been seen by another person and so there's a sense of awe just from that you're an explorer, you're reaching into new worlds that no one's seen before. But in addition, this is a little nerve wracking because is it going to prove everything I've said and published is completely wrong?

SEAN SOLOMON: We were all proven wrong. The Venus surface turned out to be more mysterious than any of us had anticipated and all of our ideas, which after all had been based on our understanding of how the Earth worked, were tossed out the window.

It was a strange and alien place we were looking at. There was a river formed by running lava longer than the Nile. There were huge mountain belts. The sides of the mountains ran up so steeply that they were unlike anything we had seen on Earth. We looked searchingly for features that we could identify, that looked familiar, that looked like something close to home. We were just blown away by how different that planet looked.

NARRATION: Among all the strange and inexplicable features, there seemed to be none of the usual geological signs of heat loss: no tell-tale ridges and fault lines of plate tectonics, and no large active volcanoes. Most of the volcanoes, in fact, looked like they had been dormant for hundreds of millions of years. So Venus had to be losing internal heat in some unknown and un-Earth-like way.

The key to solving the puzzle turned out to be hidden in an unexpected place on the planet's surface. Like the moon, all solid bodies in the solar system are hit by meteorites. Such collisions scar the surface with impact craters which can tell us about its age. Older land has more craters because it has had more time to accumulate meteorite strikes. Fresh land is formed when volcanoes erupt, coating the surrounding landscape in a new skin of lava. This younger land will have fewer impact craters. When a team of specialists sat down to count, measure and plot every crater on the surface of Venus, they were in for the surprise of their careers.

ROBERT STROM: This is the surface of Venus after it has been fully mapped by the Magellan mission. And when we plotted all of the craters on the surface, lo and behold, it was extremely uniform, they were extremely uniformly distributed across the surface. Just totally random. Truly astounding.

NARRATION: Such an even distribution of impact craters across the whole planet could only mean one thing: there was no old or new land. The whole surface was the same age. Judging from the number of craters, it was young for a planet - only about 500 million years old.

GERALD SCHABER: It was quite amazing.

ROBERT STROM: It was incredible to me. I had never seen anything like this in the thirty years that I've been looking at solid bodies in the solar system. I've counted craters from Mercury to Triton at Neptune and this is the first time I've ever seen a crater population that was completely random. It just absolutely blew my mind. No place else in the solar system.

One simple way of explaining this is to have a complete resurfacing of Venus and then starting all over again, building up the cratering record. What this actually means: is that Venus has turned itself inside out. How do you do that? You know, how do you do that?

NARRATION: For a planet to suddenly resurface itself was unheard of. More proof would be necessary to convince the scientific establishment.

ROBERT STROM: It was very hard for the community to swallow this because it was so unusual and it had never happened on any other planet.

So what we did was to run fifty thousand Monte Carlo simulations of random point distributions. Now Monte Carlo simulation is named after the casinos in Monte Carlo where gambling is a chance. In other words, it's a random process and that's what these are. So these were computer simulations of....random distributions of points. So we ran fifty thousand of these and you could not tell the difference between the random distributions generated by the computer and the surface of Venus.

GERALD SCHABER: We challenged the scientific community when we published this paper to identify which of these six simulations was really truly Venus. Five of these are Monte Carlo simulations and one of them is Venus and no-one could tell them apart.

ELLEN STOFAN: I couldn't pick out which one was Venus, and it really dawned on me that the crater population really was random. We'd never seen this on any other planet before. Every other planet has areas that are older and areas that are younger so that there's a difference in the crater population, and now here's Venus and the whole surface is basically the same age. This is really confusing.

NARRATION: Explaining the distribution of craters would require a new theory for how Venus works.

DON TURCOTTE: Basically the theory that evolved in my mind at that time was that this planet today is basically dead, as far as its surface is concerned. But the net result is that the interior is heating up and getting hotter and more active like a pot of porridge if you turn up the heat.

And eventually the surface will become disrupted and it will catastrophically sink into the interior. And then there will be a period of totally catastrophic surface volcanism with a time of flame and you have virtually a complete magma ocean on the entire surface of the planet. This extracts so much heat from the interior of the planet that the interior cools off until the point that it is sufficiently cool that again the planet dies and starts to form a solid surface, very quiet and peaceful for 500 million years, and looks like the planet is totally dead with no volcanism, earthquakes or other activity of that sort.

But it then heats up in the interior until you're ready for another catastrophe. As the interior heats up, eventually this process repeats. The whole surface founders and sinks into the interior and this episodicidity repeats again.

When this idea was first presented, some of the noted scientists, some of whom are on this program, laughed openly in the audience.

SEAN SOLOMON: My reaction was this is one of the most implausible theories I've ever heard.

DAN MCKENZIE: When I first heard what Don had to say, I really didn't like it very much. I don't like catastrophic explanations, basically. There used to be lots of such things for the earth, and most of them have come undone, and it seemed to me just exactly the same thing going on on Venus.

NARRATION: Don Turcotte had struck a raw nerve. His theory reopened a one hundred and fifty year old debate at the heart of geology. Early geological theory was influenced by the Bible and the idea that everything you see on Earth - mountains, canyons, oceans - were formed suddenly, often through Biblical catastrophes like Noah's flood.

"Catastrophism" dominated science until it was challenged in the Nineteenth century by the theory of "uniformitarianism". This argued that all geological processes work slowly and continuously, that landscapes take millions of years to form, with no dramatic global upheavals, and that what you see today is simply part of an ancient and on-going process.

Since this theory best explains how the earth works, all geologists are now trained as uniformitarians.

DAN MCKENZIE: I'm fundamentally a uniformitarianist. I think that that's the right way to start, and so far from what I've seen on Venus, there's nothing which convinces me that I'm wrong in taking that approach. I may be, but I'm to be convinced.

NARRATION: Dan McKenzie's model doesn't allow for the planet to swing between extremes from being dormant to violently active. Instead, everything must behave in a steady, uninterrupted way. He believes that heat generated within the planet must be escaping across a very thin outer skin - or lithosphere.

DAN MCKENZIE: I think the planet is now losing heat really quite rapidly, probably as rapidly as the earth. Venus has all kinds of features on the surface which are just like the features on the Earth which are active now and I bet it's active.

NARRATION: In contrast to McKenzie's thin lithosphere planet which allows heat to escape continuously, Turcotte's catastrophic model of Venus predicts that for five hundred million years, the surface of the planet looks dead. The lithosphere thickens, trapping and bottling up the heat inside until suddenly the whole planet convulses, releasing its heat, before shutting down again. Knowing which theory best explained Venus depended on whether its lithosphere was thick or thin.

SEAN SOLOMON: There was a great debate over the thickness of the lithosphere on Venus There were two opposing camps, one favouring a very thin lithosphere, a lot of heat coming out, one favouring a thick lithosphere. The key data to resolve that question, everyone thought, was the gravity data.

NARRATION: Every part of a planet has a minutely different gravitational pull dependent on the density of the rock at each place. When a spacecraft passes close enough to a mountain, it's very slightly speeded up by the additional gravitational pull of the mass of the mountain.

In theory, this gravity data could reveal what lies underneath the surface; whether the weight of the mountain is being passively supported on a thick lithosphere or actively pushed up on a thin lithosphere by a plume of hot rocks.

But Magellan had not been designed to gather this data. Its orbit was elliptical, so most of the time it was too far from the surface to sense minute changes in gravitational pull. Magellan would have to slow itself down into a tight circular orbit - closer to the surface of Venus. And the only way to do that was to use the atmosphere of Venus as a braking mechanism. But this maneuver was risky - and if something went wrong, it could destroy the whole spacecraft.

Tony Spear: When we first brought it up to NASA as a concept for an extended mission, they thought we were crazy. You know, they looked at us like we were nuts. like Dan....was nuts but Dan persisted.

DAN LYONS: When we first started firing the thrusters to lower the space craft down into the atmosphere, I realized at that point it could very easily burn up in the atmosphere and people...would look at me like I was crazy. It worked beautiful and things went much smoother than many of the other phases of the mission.

TONY SPEAR: Magellan was very robust and we were able to work around all the many problems we had and then in the extended mission we got this beautiful gravity data - pretty much free.

NARRATION: As it turned out, the gravity data was open to interpretation.

DON TURCOTTE: I certainly hoped that it would demonstrate that the lithosphere was thick but it really demonstrated it far beyond my wildest expectations.

DAN MCKENZIE: It seems to me the data is quite clear and that is nonsense. The lithosphere is thin not thick.

STEPHEN SAUNDERS: I think the thermal lithosphere is two or three hundred kilometers thick.

ELLEN STOFAN: I think it's relatively thin on the order of a hundred kilometers or less.

ROBERT GRIMM: As much as two hundred kilometers thick.

DON TURCOTTE: The thickness of the lithosphere on Venus today is close to three hundred kilometers.

DAN MCKENZIE: It's quite clear from the gravity data that it is not three hundred kilometers thick, that it is no more than a hundred kilometers over the whole planet. The reason why I think Venus is losing the heat is because I can see the convection in the gravity data and it's...thermal convection which is very vigorous, it's at least as vigorous as the Earth, and it is bringing up a great deal of heat which is somehow being lost. Now exactly how it's lost we don't know but we can actually see the heat moving.

DON TURCOTTE: Of course a good geophysicist can model gravity in such a way that he or she can get any answer that they're looking for.

SEAN SOLOMON: Gravity data have an inherent ambiguity in the sense that there are a variety of models for the interior of a planet that can satisfy the same gravity field so even though we know the gravity field much better now on Venus it hasn't answered fully the debate that's been going on.

NARRATION: To help solve the mystery of Venus's lithosphere, other scientists thought that those incredibly steep mountains they had seen in the Magellan images might hold clues.

On Earth, high mountain ranges actually spread out over millions of years with the downward pull of gravity unless other geological forces push them up. Geologists thought that on Venus, where the temperature is 900 degrees - or half way to the melting point of rock - this sagging process should happen even faster.

ELLEN STOFAN: Prior to Magellan, due to the fact that we knew it was so hot on Venus, we thought that the rocks at the surface would behave more plastically, more like silly putty than like solid rock in the way that we think of it like the rocks that I'm sitting on. Now that would mean that high mountain ranges on Venus, unless something was holding them up, would start relaxing away just due to the force of gravity - they would sort of flow outward trying to flatten themselves out under this great force.

NARRATION: But after 500 million years, the mountains on Venus are not relaxing. So what is holding them up?

STEVE MACKWELL: We went and took some rocks that were very very similar to the rocks that you find on Venus and we baked them out so they were bone dry because we know that the surface of Venus is dry. It's so hot there's no water there. The rock is dry.

And then we measure how strong they are after they've been baked out with samples that haven't been baked out. And that way, we can compare how strong they are on Venus with how strong they are on earth.

NARRATION: The baked Venus-like sample and the unbaked earth-rock are then subjected to intense heat and pressure as on the surface of Venus. The behavior of the rocks under such extreme conditions was startling.

STEVE MACKWELL: You know, it was a great surprise to see how much difference in strength there was. It says straight away that the rocks that make up the surface of Venus are much stronger than everybody had thought before.

NARRATION: The dried-out Venus-like sample on the left still maintained its original shape after heat and pressure had been applied, but the un-dried earth rock on the right deformed under the intense load.

STEVE MACKWELL: What this means is that the presence of water within the rock causes a weakening of the minerals within the rock. And if you take that water out of the rock, the minerals are much stronger and the rock becomes much more rigid. They are about up to a factor of 10 times stronger than they are on earth. All of a sudden, we're left with a situation where we understand why these mountains are so high, how these slopes stay so steep on the surface of that planet, in a way that we never understood before.

NARRATION: What did this mean for the thickness of Venus's lithosphere?

ELLEN STOFAN: Mackwell's data really shows that the mountains don't have to slump away when you have a thin lithosphere. And this is just really critical and exciting for supporting a thin lithosphere idea on Venus.

SEAN SOLOMON: Now that we know how strong the rocks of Venus can be, even under the hot conditions at the surface of Venus, it helps us also to understand how Venus could have a thick lithosphere.

STEVE MACKWELL: The problem is that to have such strong rocks making up the surface of a planet, it's a bit like having a rigid top on a cake you're baking. You know, you try and look at the cake, you tap the surface, but it doesn't tell you whether the interior of the cake is liquid or solid. And so to try and come up with arguments about what's going on inside Venus on the basis of what you see on the surface may be very misleading. And so, I don't think our data clearly establishes who's right in the thin-thick lithosphere argument.

NARRATION: While the rock data was inconclusive, all those impact craters had begun to change the minds of some scientists.

The problem is that most of the craters are pristine, untouched since the day they were formed. There's been virtually no volcanic activity spilling fresh lava over them or tectonic movement crushing or distorting them. So how could this be explained by a uniformitarian model where geological activity should be continuous?

ROBERT GRIMM: I had always had this one picture of Venus: a picture of Cleopatra Patera, an impact crater on top of the mountain range Maxwell Montes. Here's this mountain range that's miles and miles high, it's intensely deformed, it's been broken and crunched up, and yet on top of it is this perfectly pristine, preserved impact crater. This is a huge problem.

NARRATION: It was a problem that the uniformitarians couldn't explain. On an active planet, many of the craters would also be severely distorted - not perfectly preserved.

ROBERT GRIMM: I'd been looking at this picture for months and months off and on and finally it just wore me down, it just ground me down and...finally I just had to say look, I was wrong. This business of uniformitarianism just isn't working and I just got to change and to go with what I think is right now and and admit it and that is that Venus has undergone a catastrophic change in the last few hundred million years.

NARRATION: But the craters have not changed everyone's mind.

DAN MCKENZIE: At the moment catastrophism is the only explanation for the craters which anyone has offered which works. I don't like catastrophism. I don't know what the answer is. I don't know how to explain that by uniformitarian argument but I haven't given up trying.

NARRATION: Unable to resolve the debate, and with its power fading, on October 12th, 1994 Magellan plunged into the atmosphere of Venus.

FRANK MCKINNEY: We all knew it had to come to an end. The spacecraft has done a tremendous job. I think we all had a tremendous amount of pride with the mission, with the data that we'd been able to return. But when the space craft was supposed to have come from behind the planet the last time and we couldn't lock up on it...we all just looked at each other, all of our eyes got a little wet, and I think we kind of hugged each other for the last time and everybody kind of turned and walked away knowing that it was over.

TONY SPEAR: That startling announcement on television that hey, Magellan had gone in, caused me to get up and I saluted and tears came out of my eyes.

STEPHEN WALL: I feel incredibly sad that we won't get back anything else from Magellan and there were a lot of people who were very sad and very emotional over the fact that this satellite, this friend of ours, was gone. I think of it as a friend. I think of it as something that has not only helped our careers along but in a lot of ways has really made our career like a friend would do.

NARRATION: Although Magellan is gone now, speculation about Venus continues - just as it always has.

GERALD SCHABER: The big question now that many people are looking at in the planetary community is, is Venus dying, is it dead, or is it going to recycle and come back to life in the future?

SEAN SOLOMON: The only way we can resolve those kinds of questions is to go back to Venus.

MIKHAIL MAROV: It would be good if we could cooperate with Americans, you know, not just to compete.

DAN MCKENZIE: I would love there to be another probe to Venus in my lifetime and the thing I would really like it to do is to put down seismometers so that we could see if there were Venus-quakes.

NARRATION: But will there be another mission to Venus?

ROALD SAGDEEV: I think the end of Cold War would complicate even American space programme. It is not anymore driven by syndrome of the space race. Whether they will be able to excite tax payers with more and more projects, more and more launches, it's doubtful.

NARRATION: The dazzling light of Venus continues to beckon to scientists. The more we know, the more enticing the mystery of our closest planetary neighbor. While the Magellan mission answered many of our questions about Venus, it also gave us many more to ask.

DON TURCOTTE: Well, I would say that I really do hope I have solved the problem of Venus. One might push me to say I have, but there is always that queasy feeling that that kind of statement in science might come back to haunt one.

THE END.