PBS高端访谈:揭秘矮行星--谷神星的秘密

 　　JUDY WOODRUFF: Let's look at a space mission that's generating lots of interest and excitement. It's the mission to orbit the dwarf 1 planet Ceres by the NASA spacecraft named Dawn.

　　It arrived in March, and as the spacecraft has gotten closer in recent weeks, it's getting a better view of the planet, leading to some intriguing 2 questions.
　　MARC RAYMAN, Dawn Mission Director: Our team sat down with Dawn's mission director in California and put together this video.
　　We launched Dawn from Cape 3 Canaveral in September 2007. In my view, Dawn is exploring two of the last uncharted worlds in the inner solar system. In more than 57 years of space exploration, it's the only spacecraft ever to orbit two extraterrestrial destinations.
　　I'm Marc Rayman, the chief engineer and mission director for the Dawn mission.
　　Vesta and Ceres are the two most massive objects in the main asteroid 4 belt. There are millions of objects orbiting the sun there between Mars and Jupiter. But Ceres itself contains about a third of the mass of all of those objects.
　　Ceres is the largest object in the main asteroid belt. In fact, it's the largest object between the sun and Pluto 5 that a spacecraft had not visited prior to Dawn, so it's almost 600 miles across. This is a big place. It's more than one million square miles. It's got 38 percent of the area of the continental 6 United States.
　　Ceres and Vesta, they are remnants from the epic 7 in our solar system of the formation of planets, the dawn of our solar system. And so scientists want to study these bodies because they may tell us more about the conditions and the processes that were acting 8 at the times planets formed.
　　As Dawn got closer to Ceres, one of the first things we saw were those bright spots. And it's impossible not to be mesmerized 9 by these glowing beacons 10 shining out from the unfamiliar 11 lands ahead.
　　The reason these are so bright is that they reflect so much more light than the rest of Ceres does, maybe five times or more as much light as the rest of Ceres. And so the contrast is just stark 12.
　　There are many possibilities for what the bright spots might be. They might be ice in some form. They also might be the remnants from ice that was on the surface and sublimated 13. That is, it's almost as if the ice evaporated into space and left behind the materials that had been dissolved in it, so essentially 14 salts that could be very, very reflective.
　　And while we don't know yet what they are, as we continue to get in closer and get measurements, not only with the camera, but with the other sensors 15 on the spacecraft, we will figure out what they are.
　　What it is about them, whether it's a difference in the chemical compositional nature of this material or something about its structure, its makeup 16, why it reflects so much more light, we don't know. And it makes you want to send a spacecraft there to find out. And, by golly, that's what we're doing.
　　At Ceres, we fly the spacecraft in four different orbits in order to study this alien world. So, we started out in an orbit 8,400 miles high, and then eventually, we will fly it down to only about 230 miles, which is actually a little bit closer to the surface of Ceres than the International Space Station is to the surface of Earth.
　　Part of what is so exciting is, not only does each orbit answer questions for us, but it raises new questions. And then we can answer those questions by going lower and getting more detailed 17 data. It's been a remarkable 18, ambitious interplanetary adventure.
　　JUDY WOODRUFF: Our own Jenny Marder helped produce that piece. And visited NASA for the story. And she joins me now.
　　So, Jenny, thank you for being here.
　　Really interesting. And you just — you heard the mission director say, this is really about learning about the origins of our solar system.
　　JENNY MARDER: That's right.
　　Ceres is known as a protoplanet, and that means that scientists believe it was in the process of forming, like a full-pledged planet, and its formation was suddenly halted, likely by the tremendous gravity of Jupiter.
　　So what you're really seeing is a planet in the process of forming which is frozen in time, so that it's really a window into the beginning of the solar system.
　　JUDY WOODRUFF: And, as we just saw in that description in the pictures, the spacecraft Dawn is getting closer and closer. We heard him talk about the bright spots. What is the spacecraft seeing now?
　　JENNY MARDER: The spacecraft is maneuvering 19 now from its third to its second orbit. Each orbit brings it closer and closer to the surface of Ceres.
　　So the images that we're seeing, we're seeing images of the craters 20, of valleys, a three-mile-high cone 21 that sort of looks like a volcano on Earth. And we're seeing these bright spots which are very interesting. In August, we will start getting a new crop of photos. These are images coming from Dawn at the third orbit.
　　Those photos will be three times sharper than the images we're seeing now. And then, in early December, when it gets to its closest altitude, those will be 12 times sharper than what we're seeing now and actually 850 times sharper than what we were seeing back in January, when Dawn first started capturing these images.
　　JUDY WOODRUFF: So, still trying to figure out if there could have been life or could be life on this dwarf planet?
　　JENNY MARDER: Yes, that's a really interesting question.
　　It's a question that this mission likely won't answer, because they're not collecting anything from the surface of the planet. They will just — they're just orbiting it. However, Ceres does seem to possibly, very possibly have the ingredients for life.
　　JUDY WOODRUFF: Because of those bright spots?
　　JENNY MARDER: At least as we know it on Earth, life requires three ingredients, food, water, and energy.
　　And the energy could come from radioactive nuclear decay from the planet's interior. The planet is very dark, which they think indicates carbon-rich compounds on the surface. And then the third ingredient is water. And it looks like it's likely that these bright spots are evidence of water, of a liquid water mantle 22 underneath 23 the surface.
　　JUDY WOODRUFF: Well, it sounds like the scientists are very excited about this.
　　JENNY MARDER: Yes.
　　What's so cool about this is, is it's incredible detective story that we're just seeing unfolding in real time. And as we get closer and closer to the surface of Ceres, this planet just comes increasingly into focus.
　　JUDY WOODRUFF: Jenny Marder, thank you very much.
　　JENNY MARDER: Thank you.
　　JUDY WOODRUFF: And you can learn more about the Dawn mission to Ceres by going to our Web site, PBS.org/NewsHour.