Lecture: Development of views about Mars: Narrator: Listen to part of a lecture in an astronomy class. Professor: I'm sure y'all have been following the news about Mars. A lot of spacecraft have been visiting the planet recently-some have gone into orbit around it, while others have landed on it. And, they've sent back a ... an abundance of data that's reshaping our knowledge ... our vision of the planet in a lot of ways. Is there anything that you've been particularly struck by in all the news reports? Female Student: Well, they seem to mention water a lot, which kinda surprised me, as I have this picture in my head that Mars is dry ... sorta dry and dead. Professor: You're not the only one. You know, for centuries, most of our knowledge of the planet came from what we saw through telescopes, so, obviously, it was pretty limited-and our views of the planet were formed as much by writers ... as they were by serious scientists. When the first science-fiction stories came out, Mars was described as being a lot like Earth except Male Student: I know: the planet was red and, uh, the people were green. I've seen some of those old movies, what were they thinking? I mean really ... they ... Professor: Well, it seems silly to us now, but those ideas were quite imaginative and, occasionally, scary in their time. Anyway, we began to rethink our image of Mars when the first spacecraft flew by the planet in 1965 and sent pictures back to Earth. Those pictures showed a planet that looked a lot more like our Moon than Earth-lots of craters and not much else. It was bitterly cold, it had a very thin atmosphere, and that atmosphere was mostly carbon dioxide. So the view of Mars after this first flyby mission was that dry, dead planet that Lisa mentioned. But, then there were more visits to the planet in the 1970s-and this time the spacecraft didn't just fly by; they orbited ... or landed. This allowed us to receive much more detailed images of the planet and it turned out to be a pretty interesting place. Mars had ... has a lot more than craters-it has giant volcanoes and deep canyons. It also showed signs of dried-up riverbeds and plains that had been formed by massive floods. So we concluded that there must have been water on the planet at one time-billions of years ago. Now, what does it take for water to exist? Male Student: You need to have a warm-enough temperature so that it doesn't freeze. Professor: That's one thing-and the other is that you need enough atmospheric pressure, thick-enough air so that the water doesn't instantly vaporize. The Mars we see today doesn't have either of those conditions-it is too cold and the air is too thin-but a long time ago, there may have been a thicker atmosphere that created a greenhouse effect that raised temperatures-and maybe that combination produced water on the surface of the planet. So maybe Mars wasn't just a dead, boring rock; maybe, it was, uh, a fascinating fossil that was once alive and dynamic-worthy of exploration, Now let's jump forward a few decades to the beginning of this century, and a new generation of orbiters and landers that have been sent to Mars. Of course, the scientific instruments now surveying Mars are far more sophisticated than the instruments of the '70s, so we're getting all kinds of new data for analysis. And, not surprisingly, that data is challenging our notions of what Mars is like. Lisa, you mentioned that a lot of the news reports talked about water-do you remember any of the details? Female Student: Well, they were showing these pictures of these long, uh, cuts in the ground, which would be gullies here; I mean on Earth. They say that since, uh, gullies are usually formed by water, it seems like they might be evidence that water still exists on Mars, but I didn't get how that worked. Professor: I'm not surprised. There're a lot of theories ... a lot of speculation ... and some argue the formations aren't caused by water at all. But there're some ingenious theories that assume that there's a lot of water right under the planet's surface that somehow is causing the gullies to form. If we could only get a lander there ... but the gullies aren't in places where we can send landers yet. Anyway, if there is some kind of water activity, it may change our view of the planet once again ... to something that's not dead, not even a fossil, but rather a planet like Earth that undergoes cycles-think of our ice ages-over long periods of time. Maybe Mars could sustain water again at some distant date.