The Robot of the Future That's About to Explore the Deep Past of Mars
Equipped with a marshmallow-shaped lump of plutonium for energy and rock-vaporizing lasers for eyes, NASA's Curiosity rover is en route to an ancient crater on the red planet.
I want to tell you about a special place on the surface of Mars. Back in the solar system's early days, a large object slammed into the red planet, leaving behind a hundred-mile crater -- a dent large enough to withstand three billion years of erosion. The Gale Crater, as the site is known, is almost as wide as Earth's Chicxulub crater, the impact zone of the asteroid that is thought to have killed the dinosaurs. Because Mars orbits close to the solar system's main asteroid belt, it's not unusual to find impact craters there; just recently, a group of geophysicists counted more than 600,000.
But Gale is no ordinary crater. Over eons, thin layers of sediment have accumulated at its center, forming a lumpy, striated mountain that towers three miles high, so high that its peak crests above the lip of the crater. The mountain's rocky layers make up a geologic time capsule, a fine-grained record of Martian history that stretches back more than a billion years. If we could examine those layers up close, we could look deep into the Martian past, perhaps deep enough to see what the planet was like when it had an atmosphere and liquid water.
And we recently sent a super-futuristic robot there to do just that.
On its way to the Gale Crater, right now, is NASA's Curiosity rover, the most sophisticated robot in the history of space science: a dune buggy equipped with a set of tools and instruments to shame Inspector Gadget. Curiosity can vaporize rock, analyze soil samples, gauge the weather, and film in HD. It's due to touch down in the Gale Crater on August 5, completing an eight-month journey through the local solar system. Once it lands, the rover will begin a slow ascent up Aeolis Mons, the mountain in the crater's center, probing its layers for signs that Mars once supported life. It will also collect new data about the surface of Mars, which NASA will use to determine the feasibility of future manned missions there.
A few weeks ago I visited NASA's Jet Propulsion Laboratory in Pasadena, California, to talk with Michael Mischna, a planetary scientist who works on the Curiosity team. What follows is our conversation about Curiosity's mind-blowing technologies and what those technologies might tell us about the history of Mars.
This mission features a lot of next-level engineering, but its landing technology might be its most impressive feature. Can you describe how Curiosity will land on the Martian surface?
Mischna: Curiosity, like most of the spacecraft that have gone to Mars, will arrive in a capsule; as it approaches the surface, the friction of Mars' atmosphere will slow the spacecraft and trigger a special steering system onboard. That's one of the unique things about this mission -- the capsule isn't just a ballistic object plummeting through space. It actually moves in response to precise conditions in the atmosphere. After the capsule slows to a certain speed, a parachute will deploy to slow it down even further. The capsule's heat shield will drop off the bottom and expose the rover to the Martian atmosphere, after which the rover will detach from the parachute and plummet to the ground on this cool thing we call a sky crane. A sky crane is a rocket system in the underbelly of the spacecraft; its rockets fire towards the ground, allowing the spacecraft to slow and then hover 30 to 50 meters from the surface. Then cables come down and Curiosity, the rover, is lowered to the ground. When the rover senses touchdown on the surface, the cables detach and the sky-crane system launches off into the distance.
Another thing to note -- there are going to be orbiters passing overhead at the time of entry. When the Phoenix Lander landed on Mars back in 2008, we were able to capture remarkable images of it rocketing through the atmosphere, with its parachutes deployed. We're going to try to do the same thing with this one, but it's a hit or miss thing because it requires a high level of precision to pull that kind of thing off.
You say the sky crane launches off in the distance; is that because you want to avoid damaging the rover?
Mischna: Exactly. You want to make sure that it lands sufficiently far away so that nothing happens to the rover. The gravity on Mars is about one-third of the gravity we have here on Earth, which means you don't need as much rocket power to make the spacecraft hover -- but it's still a lot of power. To get something of this size to the Martian surface, you have to come up with a whole new landing system because the air bags, which we used on previous rovers, just aren't going to work. It would be like dropping a piano wrapped in bubble wrap and expecting it to land without damage. We had to completely rethink our approach to landing, but that's a good thing, because we hope this will be the first of many projects like it, eventually leading to human exploration on Mars. And if you're going to send humans to Mars, you're going to need to bring a lot of heavy equipment -- places to live, food, water, etc.
Curiosity is about 10-15 feet long, roughly the size of a Mini Cooper, and yet it has a wide range of scientific instruments onboard. In fact, the official name of this rover is the Mars Science Laboratory. What are its most impressive instruments?
Mischna: The remote sensing mask is really extraordinary. It has two rectangular eyes -- a primary imaging camera with a bunch of different filters and focal lengths, and another large, circular camera that can fire a laser that turns rock into vapor, which is picked up by another camera that interprets its composition. So, yeah, this rover can go around firing laser beams at rocks and other materials to find out what they're made of; I'd say that's one of its most impressive instruments.
It also has a meteorology station that will measure the temperature, pressure, wind and relative humidity of the Martian surface. It has a 7-foot robotic arm with a number of different components at the end of it, including scoops and percussion drills that allow it to chip off and pick up rock samples. It also has an alpha proton x-ray spectrometer, which can identify minerals in surface rocks by firing x-rays into them. It has a microscopic camera called MAHLI, the Mars Hand Lens Imager, that images microscopic features in the soil, which can tell you how it evolved over time. There is another instrument called DAN that fires neutrons into the surface, in order to detect water underneath it. Last but not least, there's SAM, which is an acronym for Sample Analysis from Mars. This is the rover's real workhorse; it takes soil samples, drops them into an oven where they are baked to over 1000º Celsius and then senses the gases that leach off, which can tell you the composition of the rock.
Altogether this rover has an order of magnitude more science instrumentation than anything we've ever sent to Mars, and it's all designed to be a comprehensive habitability investigation. It's meant to find out out whether Mars was ever conducive to life, whether life existed in the past on Mars, and whether life could exist there now.
The Mars rovers are less than two decades old, and they've evolved a lot over that time, especially in the way that they're powered. Can you describe that evolution? How was the first Mars rover powered and what is Curiosity's power source?
Mischna: The first and second generation rovers -- Pathfinder, followed by Spirit and Opportunity -- were all solar powered. They had solar panels that would receive sunlight that charged the spacecraft's batteries, which would in turn power the instruments. The challenge with the solar panels is that, for one, the sun's not always up; because of this, the early rovers had to be small and they didn't always have enough power to do experiments. Also, dust from the atmosphere would settle on the solar panels, covering them so that they either failed to work, or worked, but at a reduced capacity. In some cases, Martian dust storms would cover the panels completely, and we'd have to wait for a Martian tornado to come and blow the dust off the panels.
"We've got this little marshmallow-shaped block of plutonium that decays, creating heat that generates electricity that charges the rover's battery."
Curiosity is revolutionary, because it has a nuclear-power source. We've got this little marshmallow-shaped block of plutonium that decays, creating heat that generates electricity that charges the rover's batteries. This process produces a lot more power than any of the previous rovers, enough energy, in fact, to run all of the instruments in the daytime or the nighttime. And unlike the solar panels, it doesn't matter if it's a cloudy day on Mars or a dusty day, this power source is going to provide a constant level of energy.
What's so special about the Gale Crater, where Curiosity is headed? Why are planetary scientists so keen to study that particular location on Mars?
Mischna: The Gale Crater is a remarkable place because at its center is a mound of material that is about 3 miles high -- higher than the Grand Canyon is deep -- and all along this mound are what we think are layers of material; layers of sediment and rock that were deposited on top of each other over time. By looking at these layers, we'll be able to infer what the climate on Mars must have been like. For example, we might see materials like clay, which requires water to form. On Earth you only get clay in the presence of water, so if you see clay on Mars, you can be pretty sure that there was water at the time that it was deposited. The Gale Crater is the deepest stack of sedimentary layers that we have ever seen from orbit. We're going take Curiosity and we're going to drive up that stack as far as we can go, and we're going to look at what each of those layers is telling us about the Martian past.
Do we have a guess at what amount of time that mound represents, geologically?
Mischna: I can only give you an approximate guess, and you can't hold me to it, but we think it's on the order of a billion years or more, which is quite a long stretch of Martian history.
What do we know about the difference between Mars' geology and the geology of Earth? What have previous missions taught us about Mars' past?
Mischna: Well, Mars has two disadvantages that Earth doesn't have; first, it's close to the asteroid belt, which is between Mars and Jupiter. So, geologically, Mars has probably experienced more impacts than Earth, because it's closer to this large belt of asteroids that constantly strike it.
Also, because Mars is so small, it cooled much quicker than Earth. Earth still has a molten core and a magnetic field, but Mars doesn't have either. One consequence of that is that it doesn't have plate tectonics like Earth does; we have earthquakes because all these plates along Earth's surface are sliding and grinding against each other. You don't have that on Mars because the whole planet is basically a static ball of cooled rock. Earth's magnetic field acts as a shield from the solar wind; solar wind is constantly bombarding the Earth with energetic particles, but our magnetic field deflects them away. Because Mars doesn't have a magnetic field, the solar wind strikes its atmosphere and ejects single particles of carbon dioxide from Mars' atmosphere. It's a super slow process, but over 4 billion years, Mars' atmosphere has slowly vanished, one molecule at a time.
On Mars the whole atmosphere is carbon dioxide, which, in large quantities, creates a greenhouse effect -- so we think that long ago Mars must have had a really warm atmosphere. But you start chipping away at it bit by bit and the planet just gets colder and colder because you're taking away that warming blanket of CO2. And so today you've just got this cold, barren place, with temperatures that can dip below -130º Celsius. Mars is remarkably different from Earth.
How much of the Martian atmosphere is left?
Mischna: There are roughly 6 millibars of atmosphere left on Mars, which is a little less than 1 percent of Earth's atmosphere. And we think that Mars might have started off with the same atmosphere that Earth has today --
How do we know that? How do we know what Mars' atmosphere was like in the distant past?
Mischna: Well, for one, the presence of water on the surface requires a heavier, thicker atmosphere, because a thin atmosphere simply can't support water. It's a physical property of water that it needs to have a thick atmosphere in order to exist.
We know that the atmosphere must have been thicker, because of the morphology of the surface, because we can see canyons that could only have been formed by rivers and streams. We also have a spacecraft launching in 2013 that will allow us to see, to a high degree of accuracy, how many atmospheric molecules are stripped away every day by the solar wind, and we can use that data to work backwards through time. If you're losing one per second now, you can look back 10 billion seconds, and that will give you an idea about what the atmosphere was like at that time.
Will this mission be our most extensive exploration of another planet to date? After this mission, how will our knowledge of Mars compare with our knowledge of Venus or Saturn?
"In some ways we know Mars better than we know Earth."
Mischna: Mars is the by far our best known planet, our best-observed planet apart from Earth. In some ways we know Mars better than we know Earth. For instance, we don't know what's underneath Earth's oceans -- part of Earth's surface is mysterious to us. But we know the entire surface of Mars. What's interesting about this spacecraft is that we're using it to construct a timeline. We're not just mapping latitude and longitude; we're mapping time as well.
We've sent far fewer spacecraft to places like Venus or Mercury, and even fewer still to Saturn and Jupiter. The environments on those planets are much harsher, and so it's much more difficult to get in close to get a good look at things. For instance, we parachuted a capsule down to the surface of Saturn's moon Titan, and we got a couple of decent pictures, which scientists have been poring over for years. But with Mars it's a different story -- we've got tens of thousands of pictures of Mars, and each one tells us a different story. There is so much stuff about Mars that we can learn; it will be decades before we can even come close to that on any other planets.
Previous rovers have produced some iconic images -- Spirit, for instance, captured the sunset on Mars. Can we look forward to anything like that?
Mischna: We don't know what we're going to find, but we're going to be in a crater, and we're going to be looking with high-definition cameras, so I'm sure there's going to be some dramatic shots of the surrounding environment. The orbiters don't let us see at the human scale, so there's a chance that, with a closer look, we may find unique rock structures or something even more interesting.
One of Curiosity's mission priorities is to gauge the feasibility of a manned mission to Mars. But if we can build a robot with this kind of skill set, why send humans at all?
"The instruments on Curiosity are really great, but the human brain is the best instrument of all."
Mischna: Because humans can think -- humans can think on the fly. That's why we sent humans to the moon, because a human can pick up a rock and be able to tell right away whether it's an interesting rock, a rock that needs further exploration. With a rover like this, you can pick up a rock and analyze it, but you might burn a week or two figuring out if there's any scientific value in it. A human can look at a rock and be able to tell, instantly, whether it deserves further study. The instruments on Curiosity are really great, but the human brain is the best instrument of all. The human brain can solve problems intelligently and quickly, whereas robots rely on us here on Earth -- and that takes a lot of time.
There have been reports in the last week or so that soil samples taken by Curiosity may be contaminated by Teflon as a result of pieces of its drill breaking off. Is that going to throw off the data at all?
Mischna: The short answer is that it's not an ideal situation, but it's manageable. On Earth we can test the consequences of having Teflon in a sample, especially heated samples, where a gas may release that could present itself as an artificial signal. You might get an artificial indicator of some gas as opposed to another. But, it appears that there are methods of using the instruments that can minimize that contamination and so, presently, we're working on testing which of those mechanisms will minimize the possibility of contamination. And we're also figuring out how to recognize when a contamination does occur, so that we can sort of subtract it from our signals.
Assuming all goes well with Curiosity's landing, does your team at JPL have a special celebration planned? Or would a celebration jinx it?
Mischna: The celebration is going to be to getting our asses to work, because, for us, that's when the real fun begins. This is a two-year mission; we're going to be working pretty much every day for those two years and, knock on wood, for many years after that. We've invested too much time and spent too much money to celebrate the landing. There will be a few cheers, a few shouts, but the science is really what we live for -- the big celebration will come when the data starts to roll in.