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BBC The Planets Atmosphere

without it, the Earth would not be a home to us A billion kilometers from the Earth, a small probe uncoupled from an interplanetary spacecraft and headed for its target. It had a date with Jupiter. In an encounter that would break all records for speed and violence, the probe accelerated towards the largest planet in the solar system, plunged into its fearsome clouds and was swallowed by its atmosphere. Rain on our faces, wind in our hair, the very air we breathe. Atmosphere is what turns our planet into our world. To understand the other planets, we have to understand their atmospheres. The journey of discovery began here on Earth. The first man to explore the limits of the atmosphere was retired US Air Force colonel Joe Kittinger. At the age of 72, he is still flying. In 1960, he attempted something no one had tried before: to leave the atmosphere behind. This was the first time anybody had ever been to this height of altitude. It is the first time man had been in an actual space environment, so I had some concern about what was going to happen. Project Excelsior was an attempt to send a man to 100,000 feet in a helium balloon. If successful, Joe Kittinger would be the first human in space. 63,000 feet, that's where a man, unprotected, his blood would boil actually boil because of lack of pressure. So, for man, space is 63,000 feet. So any place above that, you die very quickly without a pressure suit. The balloon was overhead. And at the signal, they cut the straps holding down, the gondola down and off I lifted. I had confidence in myself, in the equipment and the team, but there's always that unknown, there's always that something that you may not have covered. At 50,000 feet, Kittinger hit a snag. The glove on his rudimentary space suit sprung a leak. Now, what that meant was that the blood would pool in my hand, and just continuously pool and pool and pool. But I didn't tell my flight surgeon, because I didn't want to worry him and I also felt that if I told him, he would make me abort the flight. And I felt that reasonably certain I could survive, not having a pressure suit glove on my right hand. So I continued on. Overhead, the sky was black as it could be. I couldn't see any star; an hour and a half after lift-off, Kittinger reached 103,000 feet. I was up on a porch that was the highest step in the world, looking down on our planet. I could see the clouds below and the atmosphere, the haze layer. It was a very profound feeling that I had, the realization of really just how hostile that environment is, and it's only 20 miles above our Earth. The air outside looks the same, but there's no air there. It's like being in an environment that's cyanide - and you take one whiff and you're dead. Just 20 miles down, there was safety and comfort, and an environment that man is used to. Kittinger spent just 11 minutes in that netherworld, on the brink of space. And then he prepared to return. So I took that final look around, I said my silent prayer, I hit the button to start the cameras working, and I jumped from the gondola. I fell face to Earth for a little ways. And I had really no sensation of falling, because I had no visual reference of anything, so I thought I was really suspended in space. And I turned over my back about this time and looked up, and the balloon was racing into the heavens. I mean, at a fantastic rate. To me, it was just flying away. And what it was, the balloon was standing still, and I was the one that was falling so rapidly. Kittinger fell to Earth at the speed of sound, but with no air around him it was a silent fall. I had no ripple of the fabric on the pressure suit. It was a very weird sensation. After four minutes of falling through space, Kittinger began to re-enter the protective blue haze, and was hit by the familiar world of cloud, sky and air. Joe Kittinger had gone beyond the edge of the sky, and lived to tell the tale. 15 minutes before, I'd been at the edge of space, and now to be out in the Garden of Eden. We don't really appreciate what a beautiful planet we have. The atmosphere is a tiny layer; it's insignificant, in terms of mass, compared to the mass of the planet. And yet it is so key to our experience and our very existence here on this planet. Dave Grinspoon is an atmospheric scientist. When I first got excited about the planets when I was young, it was the pictures of the surface of a planet; it's the most obvious, tangible thing that grabs you. But then, as I got into studying planets, I learned about some of the, sort of, frontiers of knowledge, some of the real mysteries that were crucial, to understanding exactly how the solar system got to be the way it is, how Earth achieved its unique status. And I found that a lot of the problems that interested me in that way had to do with the atmosphere, with the evolution of the atmosphere. As far as early astronomers knew, other planets were just like the Moon: airless balls of rock. But in 1761, a Russian star-gazer observed an unusual event. From an observatory in St Petersburg, Mikhail Lomonosov saw the planet Venus pass across the face of the sun. He noticed that the edge of the planet wasn't hard and crisp like the Moon. Venus seemed fuzzy. As the planet crept past the sun, he was astonished to see it surrounded by a thick halo. It was a sure sign of an atmosphere. For two hundred years, what lay below the clouds remained a mystery. Was Venus a world like Earth? The Russians sent a probe to find out. As Venera 4 headed for a splash-down on Venus, the designers' main worry was their spacecraft might sink and radio contact would be lost. In October 1967, radio dishes across Russia were trained on Venus, eager for news from the world beneath the clouds. Under a pressure 15 times that on Earth, and when it was still 15 miles up, Venera 4 was crushed. The probe didn't splash into an ocean. It never even came close. What kind of hell lay below the clouds of Venus? The Russians were determined to land a probe there. They tested their probes to the limit. The Soviets set about recreating Venus on Earth. To mimic the severe atmospheric conditions, they built the world's biggest pressure cooker. After four years, they had a craft they hoped was tough enough to survive the crushing inferno of Venus: Venera 7. Just before Christmas 1971, Perminov and his team saw the probe's faint signal reporting touch-down. They had made it. The first glimpses we got of what it really looks like on the surface were from those Russian Venera pictures of small pieces of, sort of, strange volcanic landscapes on Venus. Those picks were mostly of the ground. But just in the upper corner of the picture, you could see a little bit of sky, because it's cloudy sky. I've always thought that was really neat, that you were there on the surface of Venus, and you were catching this little glimpse of what it looked like to actually look up to see the sky of Venus. But what had the probe detected as it fell through that sky? If you were plummeting down through the atmosphere of Venus, first of all you'd have to make it through the clouds which are quite extensive. They are not like clouds on Earth. They, first of all, cover the entire planet, and that card deck is more than ten-miles thick, so it would take you a long time just to get down through the clouds. When you're actually in the clouds themselves by the way, they're very diffuse; it's more like the fog. But what would it be like to stand on the surface of Venus? Now the first thing that would happen with you would be you'd scream, and nothing else would happen. Because you would be instantly consumed by the hot high pressure and the noxious atmosphere. But assuming you had a good environmental suit and you walked out onto the surface of Venus, I think the first thing you would notice would be the murky red light. On Venus, in the middle of the day, it's about as light as on a deeply overcast day on Earth. So you'd never see the sun from the surface of Venus, but there's sunlight filtering through the clouds. Venus is a world of unchanging and extreme weather. In the high clouds there is a constant drizzle of sulphuric acid. And far below, close to the surface, the probes detected an unending stream of electrical discharges. Why there is lightning so far below the clouds, where it's too hot for rain is a mystery. This hot, high-pressure, corrosive atmosphere held one more surprise. In 1990 r., an American spacecraft pierced Venus's clouds with radar. It saw that some of the highest mountain tops were very bright, as if they were covered in something reflective. The high mountain peaks all around the planet seemed to be coated with some kind of metallic or shiny frosting, something that reflects a lot of radar energy, and we don't know what it is. One idea is that it's coated with tellurium, which is a trace metal on Earth, but may be common enough on Venus, and seems to have just the right properties. So below a certain temperature, which would mean above a certain altitude, it may be frosting all of the peaks of Venus, the high peaks. Mountain tops on Earth are covered with frozen water. On Venus, the snowcaps are more exotic. It seems there's metallic snow on the highest peaks of Venus, which is a kind of strange thing to think about. Venus turned out to be stranger than was ever imagined. Would our other near neighbor be more similar to Earth? In 1971, two Russian spacecraft were on their way to Mars. As the probes barreled in, Mars was suddenly enveloped in a giant dust storm. After transmitting for just 15 seconds, the craft fell silent. The flight controllers had no doubt that the violent dust storm was to blame. But this was the first picture from the surface of Mars. Incomplete and fuzzy - but there was a horizon, and beyond it, the shadow of a dark sky. Five years later, another spacecraft headed for touchdown on the surface of Mars. This time it was American. It was called Viking. OK, maybe we could take this opportunity to summarize. Would you like to date, we have, of course, landed on the surface of Mars and taken pictures both immediately after landing, and again since then. At the present time, we are slowly watching the build-up of data that's going to be built into a color picture. And the way we're going to do it is bring up, first the red, which you see now. Here comes the green, it'll be the second, and then finally the blue. When you see the last one coming down, you're now going to see the surface of Mars as best we can configure in color at the present time. You can see there are a number of different... Yeah! If you douse the lights. I wouldn't even see all the colors. Look at that! That's pretty spectacular. And it does have a reddish hue to it. And look at that sky. Light blue sky. But there'd been a mistake. Because they expected the sky to be its familiar color, the engineers had unwittingly filtered in too much blue. The next day, they showed Mars the way it really is. This world has a pink sky. Its thin atmosphere is tinged with red dust. Mars is a desert world. From day to night, the temperature swings by hundreds of degrees. On Mars, the colors of the heavens are turned on their heads. Red skies at noon and blue skies at sunset. So, how did Mars and Venus, both rocky planets like the Earth, get such different atmospheres from our own? It's a puzzle that atmospheric scientists are just beginning to solve. A big part of what I study is trying to understand the divergence of Earth's atmosphere from our neighboring terrestrial planet atmospheres, Mars and Venus. And I say divergence, because as far as we can tell the three probably started out very similar, much more similar than they are today. Mars, Venus and Earth were worlds of seething lava, pummeled by meteorites and surrounded by a veil of steam. Mars just didn't have enough gravity probably to hold on to that entire atmosphere. But while the Martian air was drifting away, Earth and Venus remained covered in a thick layer of fog. After that phase, Earth settled down and formed oceans. OK. It rained. The same thing may have happened on Venus. Venus probably had oceans when it was young, but that steam in the atmosphere, with the enhanced sunlight, from being closer to the sun, led to what we call a "runaway greenhouse". Venus never lost its steamy clouds, and like a thermal blanket, they trapped the heat in forever. The hotter it got, the more steam boiled off the oceans. The planet became locked in a global greenhouse effect, and it meant disaster. And eventually, we think Venus's oceans just kept boiling, and boiled away. Now on Earth, we have a little bit of greenhouse effect, mostly because of the small amount of CO2 and water vapors in our atmosphere. And it turns out that's a good thing. The greenhouse effect gets a bad rap these days because of global warming. And we are worried about having too much of greenhouse effect. But it's interesting that if we didn't have one at all, we wouldn't be able to live here on this planet. We get about 30 degrees of warming from Earth's greenhouse, which is just enough to keep it in the temperate range that it's in. For planetary scientist Andy Ingersoll, the delicate balance of Earth's climate is fascinating. I would say that we've learned that there's a fairly narrow habitable zone around every star. Mars is too cold, Venus is too hot. Earth is just right, and the extremes are frightening. The dominant weather on Venus seems to be just the circulation of the atmosphere. It'd be probably pretty boring if you could stand the heat, day after day of temperatures hot enough to melt lead. That's predictable. On Mars, the dominant weather that you'd notice would be the day night cycle, sort of like living in a desert or semi-desert area on Earth. It's another sunny day and it will get cold at night. And that's predictable. And it's somewhere in between that... The Earth is sort of in between, and that's where the weather is unpredictable. There is one other world with weather, and it's written on a colossal scale. The giant planet Jupiter. The whole planet is atmosphere because it's a gas ball. And there we have examples of storms that last for 300 years, or maybe even longer than that. I knew there were 300-year-old storms, and I had this picture of these storms calmly spinning. And when Voyager got there, and started taking close-up pictures, we realized that at the smaller scales, it was chaos. Everything was changing. The little tiny storms were coming and going every few days, and yet we had these large storms that were very stable. And it really deepened the mystery of why the large storms could have endured. Jupiter's Great Red Spot has been around for three centuries, but its future is by no means certain. The Great Red Spot is two or three times the size of the Earth, but it's only one of a whole class of large storms. The next largest class was three white ovals that formed about 60 years ago and had been sort of keeping their distance from each other. But last year two of them merged, so it was a kind of historic event for Jupiter weather men. What forces could be driving the giant storms? Voyager could only look at Jupiter's surface. To find out more, a probe would have to dive right in to those swirling clouds. NASA put one of its most experienced engineers to work on the design. For Jupiter, the central problem was that the planet is so massive, so gigantic, that in falling into it from a great distance, as a probe does, as a probe approaches a planet it certainly just falls into the gravity field, and falling into Jupiter brought you up to speeds of sixty kilometers per second.


  Atmosphere without it, the Earth would not be a home to us
 
Clouds on EarthClouds on Earth
  The study of Saturn and Titan is a major target of this mission
The study of Saturn and Titan is a major target of this mission
  Four and a half billion years ago, the planets formed from a giant cloud of dust and ice
Four and a half billion years ago, the planets formed from a giant cloud of dust and ice
  The probe began drifting gently down into the planet's atmosphere
The probe began drifting gently down into the planet's atmosphere
 
And if you want to figure that in miles per hour, it's a big number. 120, 140 thousand miles per hour, or something like that. In the 1960s, Al Seiff had designed the heat shields that saved pioneers like John Glenn and Neil Armstrong from burning up on re-entry. Seiff had a hand in almost every spacecraft NASA ever built. Spacecraft is stable. Galileo is on its way to another world. But Jupiter was his biggest challenge. We worked, as it turned out, for nearly 20 years in getting that experiment mounted and ready to go, and ready to function and then finally to fly way out into the solar system to that distance. And of course it was probably the least certain of success of any of the missions, because of the challenging entry conditions. In December 1995, Galileo began its kamikaze dive into Jupiter. The heat shield itself was made out of carbon phenolic. And carbon heats up to about 4,000 Kelvin and then it starts to vaporize, and that's exactly what this shield did. The heat shield was about 80% vaporized, and about 20% remaining. It was hoped that Galileo would detect clouds and rainfall, and uncover the secrets of Jupiter's weather. But Andy Ingersoll was frustrated. I was surprised at how little water they found, because water is so important to the weather on Earth, and it presumably was important to the weather on Jupiter. Although Jupiter is covered with clouds, Galileo had the bad luck to fall into a narrow gap between them. Appears that Galileo probe went into a desert of Jupiter, and that if we'd gone somewhere else, we would have found rain. As it sank ever deeper into the atmosphere, Galileo detected the winds picking up and the weather becoming more and more turbulent. The probe seemed to be approaching the source of the giant storms. But it could give no more clues. Three hours after it began its descent, Galileo ventured too far into Jupiter's boiling interior. There was no buoyancy to stop it from going down and down, and so these very high temperatures in the interior ultimately caused the probe to melt and vaporize, and today the Galileo probe is a part of the atmosphere of Jupiter. We have actually slightly contaminated this giant planet.
Was there another world that could compare to the Earth? For centuries, astronomers had watched a tiny object circling near the rings of Saturn. It was bright, like Venus. It almost seemed to glow. Could this world have an atmosphere too? Astronomers were captivated. It was the moon Titan. Modern-day planetary scientists like myself, know that we can hypothesize and come up with theories we want, but eventually we'll get to where we're going and will be able to see exactly what the truth is. The first close-up pictures of this enigmatic moon came in 1980, when Voyager flew by. I mean it was clear that it was a satellite with an atmosphere, which made it unique, because so far up until that point we associated atmospheres with planets. The composition of Titan's atmosphere, the fact that it's 90% to 95% molecular nitrogen, along with the fact that its atmosphere is pretty dense, surface pressure is comparable to that here, on the Earth a little bit more - makes it kind of an analogue with the Earth, which is terribly surprising, because no one expected years ago you'd find an analogue of the Earth out at the distance of Saturn. The surface of Titan remained hidden from Voyager's cameras. But since then, the Hubble Space Telescope has managed to peer through the thick clouds. There are tantalizing signs of what look like continents and oceans. Some say this mysterious moon has seas of liquid methane. It is a world planetary scientists are more eager than ever to explore. And we, in a matter of, you know, just a few years, once we get to Titan and once we get to Saturn, will... will find out a great deal about what makes this object tick. And it's the Cassini mission, which was launched in October of '97. And I, of course, was there. And it was the most spectacular thing I'd ever witnessed. It was also very emotional because I and my colleagues have spent seven years developing instrumentation in the space craft itself to go back to Saturn, and now we have a six-year wait for it to get there. Cassini will arrive at Titan in November 2004, and release a probe that will embark on a journey down to the shrouded surface of this distant moon. Parachutes will deploy, the air shell will be jettisoned and the probe, which is instrumented with six scientific investigations, will begin a two-and-a-half-hour descent through the atmosphere. On the way down it will make measurements of the pressure, the temperature, the composition of the atmosphere, and it will continue down until it finally breaks through what we think to be the lower cloud deck. The probe will slowly sink into an alien atmosphere, taking pictures throughout its descent. We don't know what kind of material it might land in. The surface of Titan is likely to be ice, covered with debris that has rained out of the atmosphere. There may also be liquid on the surface, seas and lakes. With pools of methane, oily fog, maybe even rain, Titan could bear a frigid resemblance to the Earth. Here is a body with a surface and an atmosphere. You could, if properly equipped, you could walk on the surface of Titan, and, you know, you could see the results of wind erosion and flowing liquids, and seas and waves and winds. It has almost a feeling of home to it, and I think that's a tremendously powerful feeling for us. Titan has brought back the spirit of the early days of planetary exploration. There's a new world out there to discover. Our current knowledge of Titan is comparable to our knowledge of Venus in the '50s and '60s. In those days, when we were trying to study Venus, we kind of imagined Earth-like scenarios, and I suppose we're doing the same for Titan. We're imagining lakes, although we imagine they're made of hydro-carbons, but still we, sort of, feel comfortable with that concept that it may turn out Titan is dry as a bone, just like Venus. You learn humility. You've studied the Earth and think you understand it, and then you say "OK, I have got all these principles, I'm gonna apply them to another planet. Let's go out there." You're always surprised. It's always not what you expected, and that really teaches you how little you know, and how much there is to know.