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This is Talk of the Nation: Science Friday. I'm Ira Flatow. A bit later in the hour, we'll be talking about living computers and the science behind the crystal skulls of the new "Indiana Jones" movie. But up first, the latest reports out of NASA say that its latest Mars spacecraft is healthy and on track to arrive this Sunday at its appointed landing site, a spot near the Martian north pole. And if everything goes smoothly and the Phoenix spacecraft survives its entry and descent and landing, scientists will put it to work searching for signs of past life and water on the Red Planet.

It has a robotic arm that will scrape and dig up the soil, scoop up samples for instruments on the lander to analyze. And eventually, scientists hope it will dig down deep enough to hit pay dirt, and in this case, it's more like pay water, water ice buried under the arctic plane. The soil and the water ice, they suspect, could tell the story of water on the planet and may even contain evidence of past life in the form of organic molecules. We're joined now by the mission's top scientist with an update on the spacecraft's progress and a preview of its three-month mission.

Our number is 1-800-989-8255, 1-800-989-TALK. You can go over to our website at sciencefriday.com for more information. Also on Second Life, you can find Science Friday's island. Peter Smith is the principle investigator for the NASA's Phoenix Mars mission and a senior research scientist in the Lunar and Planetary Laboratory at the University of Arizona in Tucson. He joins us today from the Jet Propulsion Laboratory in Pasadena. Welcome back to Science Friday, Dr. Smith.

Dr. PETER SMITH (Principal Investigator, Phoenix Mission, NASA; Senior Research Scientist, University of Arizona): Well, thank you, Ira. It's a pleasure to be back.

FLATOW: Getting a little nervous?

Dr. SMITH: Yes.

(Soundbite of laughter

Dr. SMITH: I woke up at four o'clock this morning, stared at the ceiling for a couple of hours. I'm a little nervous

FLATOW: Hey, I remember being there, I mean, for one of the early missions, the Viking mission, back in the '70s. And this sort of reminds me of that mission

Dr. SMITH: Yes, it does. And it's very similar in that we're using thrusters again to go to the surface and not airbags

FLATOW: Why are you doing that? The rovers all came down bouncing on airbags. They're coming down with the rockets.

Dr. SMITH: Yeah, that's the best way to go, in my opinion. That's a lot more dignified when you arrive on another planet, and frankly, it costs a lot of weight to carry that airbag system along, and the thrusters are much more efficient, so we're going with thrusters

FLATOW: OK. We will watch for it. When is the most critical or dangerous or nail-biting part of the mission

Dr. SMITH: Well, it's that last seven minutes when we hit the upper atmosphere at 12,000 mph, and go through what's basically a metamorphosis as our spacecraft goes from a cruise vehicle into a landed science lab

FLATOW: And tell us about this science that's on there. Go through some of the experiments and what you hope to accomplish

Dr. SMITH: Well, there are three types of science we're doing. The first, we're a weather station on Mars. We can study clouds, pressure, temperature, winds and even composition of the atmosphere. And secondly, we have geologists on the surface and we look at the distribution of rocks and the shapes of the patterns in the soil, particularly the ice-form patterns, and we try to understand the formation of this interesting terrain in the northern plains

And finally, we're a science laboratory and we delivery samples from the surface up to three instruments on the deck, a set of ovens that can bake the materials and drive off gases which we analyze, a water system where we compare water to soils and we can tell the saltiness of the Martian soil and acidity, and finally, a microscope station. So, we have lots of capability and it's through this laboratory analysis of the soils that we can tell the history of the ice under the surface.

FLATOW: Well, let's talk about that. Why did you choose the polar regions there?

Dr. SMITH: Well, NASA has had a theme of "follow the water" for many years now, and it was only in 2002 that water was discovered away from the polar caps that you could have a chance of landing on and digging down to. In other words, it's right near the surface. So, we're following the water and we're also taking the next step to see if there's a habitable zone associated with this water ice

FLATOW: When you say habitable, does that mean somewhere below the surface

Dr. SMITH: Yes. Down at the ice-soil boundaries is where we're looking in particular

FLATOW: So how far down will you be scooping

Dr. SMITH: Well, it's probably only a few inches, maybe five or six inches

FLATOW: And do you get a change, then, from the surface soil to something else

Dr. SMITH: Yeah. That's the thing. In an ideal mission, you would start with the globally distributed dust that we know is covering most of Mars and you see it in the major dust storms that happened periodically. So that'll be right on the surface. We'll analyze that. And then as we go down beneath the surface, we'll look at the modification of the volcanic soils by the action of liquid water. In other words, has the ice ever melted and changed the chemistry and mineralogy of the soil

FLATOW: And you'll be able to tell that with the laboratory on board

Dr. SMITH: That's correct

FLATOW: How long until you get any results

Dr. SMITH: Well, we get images right away, and then we need about a week to get our instruments prepared and our arm positioned to actually dig the trench. And then about a week or two into the mission, we start analyzing our first samples, those will be the surface samples, and then probably by the end of the first month, a month and a half, we will have dug down at least near an ice layer and we'll be analyzing a new set of samples that are associated with the ice.

So it develops slowly. There's no quick answer. By the end of the summer, we think we'll have gathered enough data so that we can start interpreting what we've learned and try and make some you know, real statements about what's going on with this northern planes on Mars

FLATOW: So just so we're sure on this, this is not a rover. This is a stationary base, right

Dr. SMITH: Well, I'll tell you, Ira. If you look at a picture, a high-resolution picture of this territory, it looks the same from one end to the other. So, why would you land one place and want to go to another place? I think we're going to land on a place that's typical of the northern plains and our mission is to go beneath the surface.

FLATOW: What would be a "wow" moment for you? If you discover something, would it be evidence of organic material down there

Dr. SMITH: Evidence of organic material is the big wow moment. That's the Holy Grail of the search for life outside of the Earth. And it doesn't mean that life is there to find organics because it could've come from asteroids and comets, which are known to have impacted the planets or particularly Mars over the last several billion years. So, we expect organic material on Mars and we're curious as to why it's never been found

FLATOW: How does this fit in - we have lot of Mars probes now, visiting and circling and orbiting and rolling around, how does this fit in to that total picture

Dr. SMITH: Well, if you look at the two rovers, for instance, they are studying rocks and the rocks are formed early in the history of Mars. So these are ancient parts of the geological fabric that make up Mars. And what they tell us is that Mars was once, early in its history, a wet place where there was water involved on the surface and a place where there could've potentially been a habitat and maybe even life evolved. Nobody's sure about that.

But we are now looking at the current history on Mars, modern processes taking place today. So, we're really looking at a place where we think there's two good reasons to go to the polar climates and they're the same reasons we go on the Earth. One is that climate history is written into the polar regions, and two is that it preserves organic materials. It's like the deep freezer in your kitchen. That's where you go to store things. So, we're hoping that some of the secrets of Mars are preserved in that ice

FLATOW: So, if there had been some sort of life before and no longer living, it might still have evidence of it there

Dr. SMITH: Yes. And you do see that on the Earth in the ice regions, the permafrost regions in Siberia, you can actually take a little chunk of Siberian soil on ice and analyze it for its DNA structure and recreate pretty much the tree of life on Earth. So, it's amazing how well materials are preserved in these icy regions.

FLATOW: Can you analyze a DNA of something, if you found it there

Dr. SMITH: No, we can't. So, we're looking for a habitable zone, and the question as to whether a habitable zone is actually inhabited will be left for future missions. So, we're a stepping stone in that regard. But as of now, Ira, there is no signpost that tells us where to land to find life on Mars. There's no clues of any sort. So, we're kind of taking the next step, which is not the final step.

FLATOW: The Phoenix Mission builds on to some of previous Mars missions that didn't make it, doesn't it

Dr. SMITH: It certainly does, the Mars Polar Lander and the Surveyor 2001 Lander. In fact, we are the Surveyor 2001 Lander. It was built in the year 2000, ready to fly and its mission was canceled and it's stored in a box. And we inherited that box and all the good things and bad things that went with it

FLATOW: So you just renamed it?

Dr. SMITH: We renamed it. That's correct.

FLATOW: So, it's - it's parts that were left over is actually the whole lander?

Dr. SMITH: It's just the whole lander. It was already built and partly tested, and so we were using that. But because its sister spaceship, the Mars Polar Lander, had crashed on landing, we knew there were flaws inside of it. So, it's like looking for hay in a needlestack there. It was - we had to find all the flaws that were inside of the spacecraft and correct them.

FLATOW: How many did you find

Dr. SMITH: Oh, probably around 20.

FLATOW: Wow! It's lucky you found those

Dr. SMITH: Yeah. You know, the real question is did we find all of them? And we think we did, but Sunday will be the test of that hypothesis.

FLATOW: Give us the timeline. What's going to happen on Sunday

Dr. SMITH: Well, about seven minutes before we get to the planet, we'll be feeling the gravitational pull of Mars.

FLATOW: What time of the day would that be here on Earth

Dr. SMITH: Well, in the pacific time, it would be about 4:39, and we would finally be landed safely on the surface at 4:53. Now, you have to remember that it actually happens 15 minutes before on Mars. So, whatever happened has already happened when we start getting the signals. So, we transform from a cruise stage, which is what we've been for the last 10 months. And we release the cruise-stage solar panels and guidance devices and we start our entry, and then seven minutes later, we hit the upper atmosphere at 12,500 miles an hour. And for three minutes, we're slowing down.

And as we slow down through the upper atmosphere, we heat up to 2,600 degrees. We have to be prepared to withstand that heat. And then we're going down at about a thousand miles an hour. We release our parachute, but because Mars has a very thin atmosphere, the parachute will not slow us down enough to land. We slow down to about 150 miles an hour, and we can't land at that speed, as you might imagine. So...

FLATOW: And you put the brakes on

Dr. SMITH: Thirty-seven seconds before getting to the surface, we release from the back shell that's holding us to the parachute and we drop in free-fall, and the only thing stopping us from crashing is 12 thrusters that fire and a guidance device that keeps us steady. And then we land at five miles an hour and our legs are like shock-absorbers and they prevent us from damaging the spacecraft. So that's the plan

FLATOW: All right, Dr. Smith. We'll be keeping our fingers crossed for you and watching around after five o'clock pacific time, see what happens.

Dr. SMITH: Please, wish us luck

FLATOW: Good luck.

Dr. SMITH: Thank you, Ira.

FLATOW: Peter Smith, principal investigator for NASA's Phoenix Mars Mission and senior research scientist on the Lunar and Planetary Laboratory at the University of Arizona in Tucson.

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