On March 12, John Grotzinger and a team of NASA scientists made a stunning announcement: Mars once had the right conditions for life, with flowing surface water so benign we might drink it. This finding comes courtesy of the Curiosity rover, which drilled and analyzed a rock sample from an ancient stream bed at Gale Crater on Mars. It’s the first habitable environment we know of, other than on Earth. As the first primitive forms of life were emerging here, it now seems possible life might have been taking hold on Mars, too. John Grotzinger is chief scientist on Curiosity, which has been exploring the Martian surface since Aug. 5, 2012.
Q: Scientists have found evidence of water on Mars before. What about this new finding tells you life could have existed there?
A: We’re excited because we’re getting a peek at what we call “grey Mars,” instead of red Mars. [Curiosity’s drill cuttings were green-grey in colour, not red like the surface of Mars, which is highly oxidized.] We’re seeing not just the presence of water, but water with a chemical composition that looks friendly toward microbial life. This is the kind of water that, if you drank a glass, you wouldn’t keel over and curl up, although I’m not sure I would want to plumb it into an urban district. We also see a diversity of minerals, which vary in their oxidation state. We think of these minerals at Gale Crater as though they were little batteries [which can give energy to microbes].
Q: How much water are we talking about? Was it ankle-deep or hip-deep?
A: There would have been flowing rivers at one point. At the ends of these rivers, there may have been lakes. We don’t know how long the lake was around for; it could have dried up very quickly. But it looks like we found that place.
Q: How long ago was this?
A: Most of us think it’s certainly greater than three billion years. It could be 3.5 billion. This is the same time we see the very oldest records of life on Earth—[when] life may have been evolving on Earth.
Q: So what happened? Why did the habitable environment on Mars disappear?
A: It could be related to the termination of plate tectonics. On Earth, plate tectonics involves churning [of metals] within the mantle and core. When they circulate, it generates a magnetic field, and gives rise to something we call the magnetosphere, which has electrical charges that deflect the cosmic and solar wind that comes to Earth. We already know that, if you stay out in the sun too long, that’s bad for you. If you were on the surface of Mars today, you would not last very long—because of all the harmful cosmic and solar radiation. And so the thought is, maybe Mars lost that protective envelope, and with it, Mars may have lost atmosphere. Mars has a much lower gravity, about one-third of Earth’s. The thought is that the very light gases—things like hydrogen and oxygen—got stripped away into space. The atmosphere on Mars today is much, much thinner than Earth’s: it’s about one-thousandth of Earth’s, but we think there’s a chance that at one time, it may have been as thick as Earth’s. It was apocalyptic climate change on Mars that changed it.
Q: Astronomers now say there are probably hundreds of billions of planets in our galaxy alone. If we know that habitable environments can exist off Earth—even right next door—what does that tell us about the chance there’s life elsewhere in the galaxy?
A: When you can confirm an ancient aqueous environment to be this benign, on a planet that’s as foreign as Mars, you’re left to wonder. These are both terrestrial planets; they both have the same prebiotic chemistry available. It seems like the odds of prebiotic reactions happening that did create life could have more readily happened on Mars than we ever would have guessed.
Q: Based on this, do you think there could be life on Mars today? Is there still water at the surface, or under the surface?
A: We’re not equipped to do a life-detection mission. But there are orbiter missions that have such high-resolution cameras that they’re able to see places that we call “slope streaks.” It looks like, on the sunlit side of some hill slopes, there might be places where water could be emerging today. I think most people would agree the current surface environment of Mars is very inhospitable. But if life once originated on Mars, and then the climate changed so dramatically, maybe these microorganisms took refuge in the subsurface. What we are describing is the kind of environment where very primitive microbes could have lived, whose only energy source is really the rocks themselves: they literally eat rocks. They don’t need sunlight, and they don’t need to be on the surface. They could exist in the subsurface. I think there’s a lot of interest on the part of NASA to try to explore those kinds of environments one day.
Q: On Earth, we’ve found microbes that can thrive in pretty much any environment imaginable, including deep underground.
A: Yeah. Absolutely. In the last 20 years of modern microbiology, we’ve learned about microbes that grow in extreme environments, and we call them extremophiles. They can live in very low pH; very high pH; very hot water, including boiling water. In the deep mines in Canada, in Timmins [Ont.], people have gone down and discovered that miles deep in the Earth, you find microbial communities. That’s the kind of environment we’re trying to describe with Curiosity.
Q: How much of a challenge was it for your team to analyze this rock sample? It was the first time a robot sent from Earth actually drilled on another planet.
A: It was a lot of work. There are so many things you have to check to make sure the rover is functioning, and that all the systems are operating, and that the arm is working and the turret at the end of the arm that holds the drill is working, and that it’s all precisely placed. Then you look for a rock that you think will be scientifically interesting, but that when you drill it, it won’t just turn into quicksand or something. We had to worry about all of these details, and then we had the hand-wringing after drilling the rock. We had to wait a couple of sols [Martian days] until we could get visual confirmation that we had processed a sample. We put maybe a baby Aspirin–size sample into the instruments. It just turns out we hit pay dirt.
Q: Now that NASA has a plutonium-powered, car-sized rover driving around on Mars, how soon will you be able to put some human astronauts there?
A: The key technology for getting humans to Mars is being able to bring them home. It’s a longer trip than the moon, but more importantly, Mars has real gravity. So you have to develop a vehicle that will lift something off the surface. The plan is to first do a Mars sample return: to build another rover, drive it around, collect samples and then bring them back. This would be a set of missions, and [it would] take probably a couple of decades. It’s in the planning stages now.
[We would start by building] basically the same kind of rover as Curiosity, which would not just drill, but collect sample cores, roughly the size of pens or pencils. It would then cache those cores for return to Earth. The cache would be something that looks like a bowling ball. The next decade, we would send a retrieval vehicle that would go pick up the bowling ball and put it into orbit around Mars. And then, a couple years after that, you’d send a retrieval vehicle that would go get the bowling ball and return it to Earth. Those three steps are what should pave the way for human exploration of Mars.
Q: If we brought a core sample from Mars back to Earth, I’d imagine scientists could do much more with it than Curiosity can manage, even with its high-tech tool kit.
A: As soon as you get a sample back to Earth, you’ve got instruments that work at such high levels of accuracy. There would be other elements and minerals you could measure. We’re beginning to get a whiff of organics with Curiosity, but we’re never quite sure: is it contamination we brought with us? The amount we’re seeing is small enough that we have to be very careful with what we say. Whereas, if you bring a sample back to Earth, you could work with tiny amounts to find out whether there is organic matter there, and if it has anything to do with biology.
Q: Organics are a key ingredient to life. If Curiosity makes a confirmed find of organic compounds on Mars, what will that mean?
A: The solar system is full of organics. We just had an announcement of organics on the dark side of Mercury. [The organics they found were similar to tar and coal, and are believed to be delivered by comets and asteroids.] Meteorites come into Earth full of organics. So, if Curiosity finds organics and they aren’t contamination we brought from Earth, then they either came from Mars or somewhere else in the solar system. If we determine they came from Mars, we have to figure out whether they were manufactured by a biological process, or not.
Q: What’s next for Curiosity?
A: Finishing up what we’re doing here. Trying our hand at looking for organics. Then we’re off to Mount Sharp [a 5.5-km-tall mountain of sedimentary rock] to decipher the record of planetary history and detect more, and different kinds, of habitable environments.