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BBC The Planets Terra Firma

where the history of Earth is written in its rocks We live on an active planet. The rock itself is alive. The Earth spits out hot lava, creating new land. It is a sight we can only watch in awe. I find it absolutely overwhelming to look inside a volcano like this and see hot lava oozing out. We can see that the Earth is active. It's like the lifeblood of the planet. How does this all play out on the other planets? What does it look like there? Would there be hot lava like this? Volcanoes? Would we see things like ocean basins? We had absolutely no clue. Over hundreds of millions of years, mountain ranges have risen up, entire continents have drifted apart. Is this how other worlds would be? Earth's celestial partner, the Moon, could hardly be more different. Nothing has happened on this dusty world for billions of years. Its surface shows only the scars of countless impacts by asteroids. The story of the other planets must also be locked on their surfaces. But they are so far away that they are just dots of light in the sky. Astronomers gazed at those dots, but only on one of them could they make out anything that looked remotely like the landforms of the Earth. It was the red disc of Mars. Over a century ago, an Italian astronomer, Giovanni Schiaparelli, began to chart the dark and light regions of Mars. His maps were the best we had until space probes came along. In 1964, NASA launched Mariner 4. Its mission was to fly past Mars and try to send back close-up pictures. The mission was a technical success, but the fuzzy images showed nothing particularly interesting - just craters like those on the Moon. Astronomers were convinced that the probe had looked in the wrong place. Brad Smith had spent years gazing at the Red Planet. He thought he knew where the interesting terrain was. For the next mission, NASA asked him to guide them to it. I had been observing Mars for quite some time and we had noticed that there were certain regions that were very changeable. They changed with the seasons. They seemed to have colours to them. And we thought those would be particularly interesting. So I set up the targeting so that Mariner 6 and 7 would look at these particular areas during the approach. As Mariners 6 and 7 raced towards Mars, they saw what appeared to be mountains, dark plains, deep canyons. But when the close-ups came back, Brad and NASA were once again disappointed. Unfortunately, the surface was very heavily cratered. There were dark and light areas to be sure, but it was still a lot like looking at the Moon.' Convinced that there had to be more than just craters on Mars, NASA went back again. Only this time the element of chance had been removed. Mariner 9 was not a quick fly-by. It was designed to go into orbit around Mars, and photograph every square foot of the planet. But as the probe closed in, a large dust storm began to stir. I was watching the planet, hoping it wouldn't be a bad one. But within just a matter of days, a brilliant yellow cloud developed. And we knew it was a bad dust storm. And it spread out over the entire planet, covering up everything. Mariner sat out the storm. And then it saw something poking up through the dust. We saw these four dark spots. And we weren't quite sure what they were. And one of our team members had pointed out there were things that looked like the tops of volcanoes. And he suggested that, in fact, these were volcanoes that were so high that they were actually poking up through the dust. As the dust receded, the four spots showed themselves. They were volcanoes. Giant ones. The biggest of them was christened Olympus Mons. It was 15 miles high, three times the size of Everest. Mars was not just another moon. It was a real world. And geologists were eager to draw comparisons with the Earth. Jim Head went straight to the biggest volcanoes he knew, on the island of Hawaii. I remember when Olympus Mons first came out of the clouds on Mariner 9. It was absolutely spectacular; I couldn't wait to get back here to Hawaii to get some sense of the perspective and scale from something I knew about. And here we are on the edge of this volcano, this incredibly large volcano, one of the largest on the Earth. And, in fact, it's like tiny compared to Olympus Mons. To travel from the central crater of Olympus Mons to its outer edge is a journey of 300 miles. In between, there's an area the size of France covered with crumbling red lava. But the volcanoes weren't all. As the dust withdrew, Mariner 9's cameras took thousands more pictures. A whole new world was taking shape. NASA called in a new type of scientist - planetary geologists. When this thing began to shape up, and we began to see all these features, that was the first time that this whole planet Mars had ever really begun to take shape in sense of knowing what the physical features were the geology. None of that stuff had ever been seen before. It was a historical moment. When you finally start, like with an orange, peeling the peel off and exposing the planet for what it is. As Mariner's cameras tracked across the belly of Mars, a giant fissure appeared. Each day, we get a new set of images. And as they mosaic, then we said "Look at that, where is it going?" And it stretched clear across roughly an eighth of the planet. They had discovered the biggest geological feature ever seen. Vallis Marineris is 4,000 miles across and maybe 100 miles at its widest and as much as six miles deep. On the scale of the United States, I think it is ..., over there would be San Francisco, and over here would be New York, something like that. So it would span the United States. And this little canyon right here is the size of the Grand Canyon. The Vallis Marineris probably cracked open when the four giant volcanoes to its north pushed up and stretched the very skin of the planet. Martian geology is written on a scale that dwarfs the Earth's. Giant features like Vallis Marineris and Olympus Mons had been steadily growing in the same place for billions of years. It suggested that the surface of the planet did not move, unlike the drifting continents of the Earth. So it was a very active planet, it was a planet that had real geology, active geology, going on, even in modern geologic history. But was it all history? To find out, a probe had to land on the planet's surface. Gerry Soffen was the mission scientist. But the whole point of Viking was to be the first time you really explored the surface of the planet. The Mariners had actually orbited or flown by the planet, but the idea of actually getting a lander on the surface of Mars... It was as important as Columbus's voyage. I was absolutely terrified that we wouldn't land successfully because I had spent so much of my life aimed at this great moment in history. "15,480 feet per second." "3,000 feet." On July 20th, 1976, Viking dropped into Mars's upper atmosphere and began its descent. When the landing took place, it was like...it was two minutes ago. I can see every single face and the expectation. The signals from Viking took 18 minutes to travel back to mission control. There was nothing to do but wait. When the touchdown came, I just exploded inside. We have touchdown. Yeah! Looking good. The idea of being there in history, when the first landing on Mars took place, was thrilling. And now the real waiting began. What would the surface of Mars look like? Was it going to be like some place in the US - Was it going to be a desert? Was it going to be sandy, dusty? But all we knew is it was going to be red. An hour after Viking landed, the first black and white pictures began to creep back. Line by line, the surface of Mars was revealed. Then came color pictures. The surface was littered with rocks, some of them dark and porous, clearly volcanic. They must have been hurled there by volcanoes. We knew there were volcanoes on Mars, but to actually see a piece of the volcano was astonishing. But when had the pieces landed there? Viking carried a device to find out whether Mars was still active - a seismometer to tell if the ground was shaking. Viking listened and waited but felt nothing. For all its spectacular volcanoes and canyons, it seemed that geological activity on Mars was a thing of the past. While the Americans were putting all their efforts into Mars, the Russians headed for Venus. This world is almost as large as the Earth, and geologists always thought it would be our twin. But Venus was no easy target. The surface was hidden by a thick blanket of cloud, and below that serene exterior, the conditions were hellish. The pressure of the atmosphere had already crushed three Soviet probes. The Russians had found to their cost that the surface temperature was nearly 500 degrees centigrade. In 1975, they tried again, and equipped their probe with a camera. They hoped it would cling on long enough to send back just one picture of the surface. Mission chiefs didn't want anyone to know that it might fail. Seconds after landing, signals showed Venera 9's systems were intact. On the surface, the temperature was hotter than an oven. Would the probe survive to send back the image? The camera had captured a blurred view of some rocks. It was the first-ever image of the surface of Venus. But it was only a tantalizing glimpse. With their next probe, the Russians hoped for something better. On landing, all systems radioed back OK, but there was no image of the surface. Sasha Basilevsky was on the team that tried to work out what had gone wrong. We had a technical meeting and there was discussion, and the chief designer at that time (said), "You know, I have an idea that we have landed in something very sticky and viscous." And a young, nasty voice told, "Yes, sir, in the shit." But Venera had not sunk. The intense heat on the surface had melted the lens cap on to the camera. Three years later, another pair of probes headed for Venus. This time, they took beautiful pictures of a lava-filled patch of ground. But when the probe tried to sample Venusian rock, that lens cap came back to haunt them. In Venera 14 as in Venera 13, they had a special device to measure electric properties and mechanical properties of the surface. This arm goes and puts the device on the surface and measures. And Venera 14 did it in a perfect way but it just got the cap. So, we measured the mechanical properties and electrical properties of that thing they brought from Earth. On the planet's surface, the probes could only survive for an hour or two at most. Somehow another way had to be found to see through the clouds. In 1989, NASA launched Magellan. Magellan wouldn't take pictures but would scan the planet with radar to make up the contours of the surface, cutting through the clouds as if they weren't there. We were able to actually come around the globe every day many times and build up a picture of the global geology of Venus. Magellan began to send back reams and reams of data - and a whole new generation of geologists set to work on it. When the first image taken came back from Magellan, I went in about 4 in the morning and looked at the first strip of Magellan data, this first track down the planet. To see the planet surface being revealed in such incredible detail, and to say "I'm one of the first people who have ever looked at this piece of ground on Venus," you felt like such an explorer. The first images showed that Venus had many similarities to Earth. There were large mountain ranges, some of them almost similar to the Himalayas. There were long faults on the planet that looked maybe similar to faults we see on the Earth. There were volcanoes, lots and lots of volcanoes on the surface, very large ones, much larger than some on the Earth, others on a similar scale. But then an alien landscape emerged. There were these huge circular features. By huge, I mean 250-300 kilometers across. They were encircled by ridges. They were high, they were sort of mountainous. They tended to have volcanoes all over the surface. And we just said, "What are these features?" How could they have formed? We'd never seen anything like them. The 3-D images revealed giant blisters that had oozed lava from every crack. And the surface of Venus seemed to be cut with channels. They looked like long rivers going across the surface of the planet, but we know with Venus' incredibly high surface temperatures, there's no way that water could have formed those channels. So they had to have been formed by lava. There were other volcanic features that looked like pancakes. They had very steep sides and very flat tops. And literally looks like somebody threw a bunch of pancakes out onto the surface of the planet. There were other kinds of volcanoes that looked like little squashed bugs, like ticks. Everywhere you look, you see some sort of volcanic feature, a flow, a small volcano, a weird channel - it's just dominated by volcanism. And that's just something we weren't It's intriguing to look at this surface that you say should be so much like the Earth. It's not. How did it get this way? It's just a puzzle. That's what makes Venus so interesting. But when had this volcanic surface formed? Geologists tried to find out by counting impact craters. When we got the global picture together, we started counting craters and looking for areas that had a high density of craters, and an area that would have a low density indicating old and young ages. When we look at the Moon, look at Mars, we look at Mercury, you have areas where you say, "OK, this area has more craters, it's older. This area has fewer craters, it's younger. The amazing part was that it looked like the craters were almost randomly distributed across the surface. And you say, "Wait a minute. The whole planet can't be the same age - it's the size of the Earth. How could you have a planet the size of the Earth where the entire surface forms at the same time? It just didn't make sense. What could have happened for a planet to create its entire surface at one time? Well, it was really mystifying, because nobody could really be sure since we hadn't seen anything like this. What was going on? One of the main ideas that came out was the idea that Venus may have actually been catastrophically resurfaced a few hundred million years ago. The idea that Venus boiled over in a planet-wide flood of lava is still hotly debated. But if it did, and has now cooled down, will it ever erupt again? The most exciting thing would be to find an erupting volcano on Venus, to say "Here we have proof, Venus is still geologically active.ĒIt really is the planet most like the Earth, and here's proof of it. "It's not a dead planet, it's still alive, and itís still active." That's something that I would give a lot, awful lot, to be able to find. Magellan scanned the planet for four years, but found no fresh lava flows. Venus may well be alive, but there is still no sign of it. Of course if you orbit around Earth for a year, you might not see any volcanic activity at all. So it's really hard to tell whether it's actually going - it's like trying to find the smoking gun or the smoking volcano. It's not easy to find. Despite being laid bare by Magellan, Venus remains a planet shrouded in mystery. Was Earth the only place where geologists would find active volcanoes? It was beginning to look that way. The only other rocky planet is Mercury, but it's a small world, barely bigger than our moon. And its surface is just as cratered and dead. There is some solid rock out here. On its way past Jupiter, Voyager flew by the planet's moons. We expected small objects the size of the Moon or smaller to be pretty lifeless, geologically, and expected them to be holding records of the very early solar system, the impact processes, and fairly esoteric kinds of questions that solar system geologists might be interested in. Not something the general public would care a whole lot about. The first thing we ran into course was Callisto. That was pretty much what we had thought one of these moons would look like. Callisto is dark and icy. Like Mercury, its cratered surface hasn't changed for billions of years. The next moon, Ganymede, is the largest in the solar system, but it too held few surprises. Circling closest to the giant planet is Io, a world about the size of our moon. As we looked at it from a great distance, we saw a lot of dark spots on the surface which we thought maybe were impact craters. Voyager took a few pictures, sailed past Io, and the scientists focused on Jupiter itself. Meanwhile, one of the engineers busied herself with some routine spacecraft maintenance. I came in about 9 o'clock that morning to the navigation area. And the tape with the pictures the spacecraft had taken the day before was on my desk. I put them on the computer system and I displayed them. And I could see that Io or the moon of Io was a crescent, as very often our own moon is a crescent in the night sky. And I went and enhanced the brightness, and there appeared beside Io an object, a huge object that looked like something I couldn't recognize and could never have expected. And it completely captured my attention. I wanted to know so badly what that was. I just asked myself "My goodness, what is that?" And the answer that occurred to me first was it looked like another moon peeking out behind Io. But there was no other moon, and no fault in the camera. Linda Hyder concluded this object had to be part of Io. And in fact, that was very hard to accept because the size of this object was enormous with respect to the size of Io. And when I explored it, I was able to find that this large, strange object it was exactly coincident and fell over a heart shape feature on Io. What I had discovered a huge plume of a volcanic eruption rising 270 kilometers over the surface of Io and raining back down onto it. So I had discovered the first-ever volcanic eruption ever seen on another world besides the Earth. We didn't really expect to find active volcanic eruptions throwing material from a volcanic vent to an altitude of a couple of hundred miles, 300 kilometers above the surface. This stuff goes up with the velocity of a high-powered rifle. Of course, it comes back on the surface with the same velocity. So, a healthy place to stand would not be the surface of Io. The entire surface of Io is covered with volcanoes. This moon is awash with multi-colored lava flows. But why is Io an active world? It's only the size of our moon. It should be cold inside. Head of the Voyager camera team was Brad Smith. Till now, he had concentrated on Jupiter's atmosphere. Now Io drew his attention. Io is a very small body. It doesn't have enough of the radioactive materials that heat up the rock as happens on the Earth. So we didn't expect any kind of volcanism on Io. The explanation was to be found in our own moon. And a NASA scientist in California had predicted it. Well, as you can see, the power of the Moon's gravity can move oceans on the Earth. Imagine what the power of Jupiter - what just 300 times the mass of the Earth - can have on Io. Now Io, as it circles Jupiter, approaches Jupiter closer at one point than at another. What this does is it changes the gravitational force from Jupiter and results in a giant squeeze. Physicist Ray Reynolds had realized that friction from the constant wrenching by Jupiter was heating up the interior of Io to incredible temperatures. We plugged in the numbers into our equations, and, lo and behold, we come out with thousands of degrees near the surface of this satellite. Well, this immediately raised visions in our mind of volcanoes going off. But nobody was prepared for the ferocity of the sulphur-spewing volcanoes that Voyager found. Io is just enormously volcanically active, more active than the Earth and any other body in the solar system. There is nothing that even comes close to it. Its surface is completely covered with volcanic debris. Finally, geologists had found a world that was alive and changing before their eyes.


  BBC The Planets Terra Firma where the history of Earth is written in its rocks
 
Interesting geology on worlds
Interesting geology on worlds
  Expect the unexpectable
Expect the unexpectable
  Mariner 9 had gone in search of geological life
Mariner 9 had gone in search of geological life
  Geologists are now returning to the Earth's neighboring planets
Geologists are now returning to the Earth's neighboring planets
 
Draw a map of Io, and it will be obsolete the next day. But Io wasn't the only surprise Voyager found among Jupiter's moons. Next to it was the bright disc of Europa. Europa was surprisingly smooth. That is there was little or no topography on it at all. Scaled down, it would be as smooth as a billiard ball. And why it should be? Why there was no topography, we could only guess. Close up, the surface was baffling. When we look at Europa, we see a startling lack of craters. And we also see that there are large linear features that look like cracks on the surface. Reynolds thought the cracks might be a result of the constant squeezing from Jupiter's gravity, just like at Io. And there was a more intriguing possibility. Europa's surface is made of ice, and that would be much easier to melt than Io's solid rock. Our calculations indicated that there was a good possibility that there could be an ocean beneath a thin ice layer. Voyager's images were too crude to prove or disprove the idea of a subterranean ocean. But 20 years later, another spacecraft, Galileo, is back for a closer look. Close up, Europa's surface looks like a crazy paving of ice. There are giant icebergs and a network of cracks where it seems that hot water has welled up from below and then frozen instantly in place, just like lava from a volcanic fissure on Earth. Instead of hot rock that's coming out, it's water, its liquid water. But the principle is the same. There's a fluid down underneath the crust. And from time to time, this pushes up and flows onto the surface much in the same way that hot lava flows onto the surface of the Earth. No actual eruption of ice has yet been spotted, but Europa has opened up the possibility of completely new kinds of geological activity. As space probes ventured further out, they found other frozen worlds. Among the moons of Saturn and Uranus, there were signs of strange geological events. But they all seemed to have occurred eons ago. Then, a decade after its encounter with Jupiter, Voyager met Triton, moon of Neptune. Triton, as we got closer and closer, it became evident that it was going to be a very tiny object. And it was covered with very, very bright material.
It's still happening. Water was dumped on Jupiter. Here, then, are the gas giants. Jupiter and Saturn mark the current limit of the planet builders' theories. Far beyond these gargantuan worlds lie the ice giants, Uranus and Neptune. But out here, the accretion theory runs into trouble. The formation of Uranus and Neptune are the greatest mysteries in the formation of the solar system, because everything goes more slowly at greater distances from the sun, so all these processes slow down. When we try to run the same computer programs out there that we did in the terrestrial planet zone, we don't get planets forming. That made it very cold - the coldest thing we've encountered so far throughout the solar system. Triton is so cold that even its thin atmosphere of nitrogen freezes into a solid ice cap every winter. To have expected geologic activity on such a surface would be insane, frankly. But when Voyager got close, it saw there were dark streaks all over the fresh ice cap. Those dark markings had to be sitting on top of that ice. They had to be laid down there in the recent past. That meant something had to be going on to make them an active process. Larry Soderblom spent two months looking at pictures of Triton before he spotted it. He flickered pairs of images taken from slightly different angles to give a 3-D view of the surface. All of a sudden, he saw something that seemed to stand up from the ice. One afternoon, we were stunned to find active geysers shooting up above the Triton ice cap. Geysers - fountains of material, raising five, ten kilometers - miles above the surface. Even in this dark corner of the solar system, the faint heat of the Sun manages to penetrate Triton's ice cap. It warms liquid nitrogen trapped beneath the surface to a point where it bursts out and rises up into space before turning a right angle in Triton's high altitude winds. It's almost as if Triton was the last sentence in the message that we got from the Voyager mission: that no matter where you go in the universe. It changed our whole concept of what volcanism is. Our classical concept because before Voyager coming out of the interior of a planet. But Voyager showed us that there are other materials as well that produce volcanism. On Io, we saw molten sulphur - sulphur dioxide. On Europa, water is an important element, and on Triton, liquid nitrogen may actually be the fluid that is involved in volcanism. So many amazing discoveries across the solar system. In 1997, NASA returned to Mars with a new orbiting spacecraft, that's photographing the surface in incredible detail. Mars Global Surveyor can pick out individual boulders on the ground and can see places where they have rolled down into gullies. Was it wind or small tremors? If there is any activity on Mars, Global Surveyor should find it. What we're seeing now on Mars is like a shot from a helicopter compared to a shot from a space shuttle. The whole sense of Mars is a little bit different when you get down to the size of detail that we can see. And we're just beginning to get into that and get a sense of what's going on Mars. Soon, we will have a map of Mars that's as detailed as those of the Earth. Now, when I come out to Hawaii, my whole perspective has changed. I walk around on the surface here, and, in fact, I'm beginning to see things that are the same scale that I'm seeing on Mars. And it's incredibly exciting. It's bridged the gap. It's brought it together. So it's almost like I'm walking around on Mars. I think this is the way it's worked with all the planets. Initially, were alien objects that you had little knowledge about. But particularly from the geology, you begin to see old friends - volcanoes, and lava flows, sand dunes and so on. And then, little by little, you know, you transport yourself there. You think about there. I wake up at night I have a dream about being on Mars. It's just amazing. One day soon, a probe may also return to Venus, and perhaps survive for long enough in its hostile atmosphere to observe a volcano erupting. We know now that the Earth is just one of a family of active worlds. But geologists will never grasp the true nature of another planet until they have touched the stuff that comes from deep within it. The grand finale is still to come. If all goes to plan, the Galileo probe will end its mission and the millennium with one of the most breathtaking stunts ever performed. With its fuel spent and suffering critical radiation damage, the dying spacecraft will dive headlong into a plume of fiery ash from an erupting volcano on Io, sampling with its last gasp the geologist's Holy Grail: the inside of another world.