Bell has been involved in Mars telescopic and spacecraft exploration for many years. He was involved with the Mars Pathfinder rover in 1997, and is involved in the NASA missions to send two new rovers to Mars next year. He talked with CNN Science and Technology producer Marsha Walton.

Q: In your Science article, you say "These results, even after a month of mapping, are stunning." Why such excitement?

A: The measurements that Odyssey obtained very early in its mission were designed primarily to check out the instrument, make sure it was operating correctly, take an initial sort of coarse, rough survey of the composition of the planet. So no one expected this kind of result to come out so early in the investigation. Mars continues to surprise us every chance it gets. Lo and behold this very strong signature of hydrogen from the planet was detected very early on. We didn't think the instrument would have the sensitivity to detect this signal so early. But also the amount of hydrogen on the planet was much larger than we expected.

Q: What's most intriguing about these findings?

A: We've known for a while that there's water ice on the Martian surface, that there's water vapor in the Martian atmosphere. People have hypothesized that there's got to be ice underground. And now to have some actual measurements, some actual data that appear to confirm this hypothesis, is really exciting and there are obviously some important long term implications for my kids or your kids to have this critical resource, water, waiting for them on another planet.

Q: What further tests are underway?

A: The spacecraft will operate for at least another few years. We'll get better and better maps, not only of hydrogen, which is what we think is responsible for this ice signature underground, but also for other chemical elements -- silicon, aluminum, iron, titanium -- all of the elements that went into forming rocks on planets like Mars and the earth. And also, the spacecraft has some very good cameras onboard. And so in addition to getting measurements of the chemistry of the surface, we're getting pictures of the geology, the different kinds of land forms -- in both colors like we're used to seeing in visible wavelengths and also in the infrared, measuring the heat coming off the planet and looking at different wavelengths of different infrared energy.

Ice, ice, everywhere...?

Q: Could the hydrogen signature be anything other than ice?

A: There are two main contenders to explain the hydrogen that's been detected. One, of course, is water; the other possibility is that we are looking at some rocks and minerals that have hydrogen in their structure. For example, clay is a kind of mineral that has water in its structure. Water is actually built into clay, that's why you can mold it and make sculptures and things like that. That water has hydrogen in it. But what's been detected by the spacecraft is hydrogen in abundances that far exceed what most people reasonably believe could be explained by minerals.

Q: Why is the ice near the poles?

A: Mars is just like the earth in that it's tilted on its axis. It has seasons like the earth does, and the regions closest to the poles are colder on Mars just like they are on earth. These colder areas are places where ice can remain stable, where it won't be evaporated or melted by sunlight.

Q: What are the best theories about why large amounts of water went underground?

A: Mars was perhaps a lot like the earth a long time ago, early in its history. The atmospheric pressure may have been higher, the temperature may have been warmer, and liquid water may have been stable on its surface. We see evidence for valleys carved into the rock, for tributaries, all the hallmarks for liquid water flowing on a planet. But Mars today is a very, very cold, very dry, very hostile environment and liquid water is not stable. Liquid water cannot exist on Mars today because the temperature is too cold, and the atmospheric pressure is too thin. So the question is, if Mars was like the earth a long time ago, if it had a lot of liquid water ... responsible for creating some of the land forms that we see, and perhaps being related to Mars once having been a hospitable environment for life, where did all that water go? That's the $64 million question in Mars science today.

What we're seeing with the Odyssey discovery is some of the first evidence that there really is ice trapped underground. We don't know how much; we don't get a very good measurement of the total quantity from the Odyssey data. In fact, that's why I wrote in the piece that this may be the tip of the iceberg. There may be an enormous quantity. We won't know until we send more sophisticated robots and eventually people to Mars and drill into the subsurface and make a direct measurement.

Taking advantage

Q: Do you think it is solid ice? Or would it be mixed with rocks and other debris?

A: We know that Mars, like the moon, has been hammered by asteroids and comets over time. And this jumbles up the surface. And it mixes up whatever is in the subsurface together, and so we know there's a lot of rock, we know there are volcanoes, we know there's a lot of dust from the dust storms.

Q: How might astronauts someday take advantage of this discovery?

A: That's the most important long term implication. If there is water there waiting for people and it's accessible, meaning it's within the upper most part of the surface, not way, way down deep, then that provides a wonderful natural resource for the first explorers that go there sometime in the next few decades. First of all, we need water to drink. We can use deposits of ice to extract oxygen from the water, and water can be used to create rocket fuel for the return trip home or for other excursions around the planet.

Q: How will Odyssey help in determining things like landing sites for future missions?

A: In the near term, we're just deciding where to send the two rovers that are launching next year. And unfortunately, those are both restricted to landing sites near the equator, because of the orbital dynamics and the launch vehicle used. So we can't get to the high latitudes that show the most evidence for this, these ice deposits.

Future rovers and landers will likely be able to go to high latitudes. So we could send a lander in 2007 or 2009, with a drill, send it to a place with water ice close to the surface. That would be a spectacular confirmation of this announcement. We'd be able to drill down, take a picture of the ice, analyze it, perhaps the ice will tell us something about the past climate of Mars, just like the ice in Antarctica and Greenland does on earth.

Q: What's it like studying something you can't see or touch?

A: I have an advantage over many of my colleagues who study quasars or black holes and galaxies. Conceivably, we could go to Mars in our lifetimes. It's almost a certainty for the generation that follows us. Every time we send up one of these missions we are reaching out and touching the planet with our robotic emissaries, we are collecting information and exploring the best way we can with the resources we get from NASA and the taxpayers and the Congress. I don't feel shortchanged at all.

http://www.cnn.com/2002/TECH/space/05/30/coolsc.mars.ice/index.html