“There is no foreign land; it is the traveller only that is foreign.”
—Robert Louis Stevenson
Stevenson, a Scot, was exploring California in 1880 with his new bride. I’m one of a team of more than 500 travelers exploring Mars from California with the most sophisticated robot ever sent to another planet. As I write, Curiosity is pounding a hole into a rock in Gale crater. That Neanderthal feat may not seem like proof of its sophistication. But it is. It took us ten years of engineering on Earth and six months of preparation on Mars to get to that rock. Drilling a two-inch-deep hole into it and extracting a baby-aspirin-size piece will take weeks more. We’re doing it all to look for chemical evidence that Mars is not so different from Earth—that it too was once hospitable to life.
I’m a geologist, and I do fieldwork on Earth. I usually head out with only a handful of other people. We drive into remote areas with four-wheel-drive trucks or get dropped off by small airplanes or helicopters. Then we walk a lot. To plan a field campaign takes months, not a decade, and when I want to sample a rock, I reach into my rucksack, grab my rock hammer, and knock off a piece. Sampling takes minutes, not weeks. Back in the lab we analyze samples in a few days rather than the months it takes Curiosity. On Earth as on Mars, doing fieldwork well takes a great deal of practice—but on Mars it’s at a different level.
For starters, we need a bunch of brilliant engineers just to figure out how to wield the hammer or the drill. At Caltech’s Jet Propulsion Laboratory they practiced for years on Curiosity’s twin sister, testing tens of thousands of lines of computer code that command the seven-foot robotic arm to make sure they could execute the hundreds of motions required to place a 65-pound drill as gently as a feather on a target the size of a pea. We drilled scores of real rocks, and then we made fake rocks and drilled those too, because we worried that the rocks might be different on Mars. We were certain the weather would be different. The daily 180-degree-Fahrenheit temperature swings on Mars would cause the whole rover, including the drill bit, to expand and contract. So we had to figure out how to keep it from getting stuck. We worried too whether the powder produced by drilling would clump and clog the tiny tubes and sieves of our onboard chemical lab. We sweated a lot of details.
Then, after we endured the famous “seven minutes of terror” as the sky crane set Curiosity down lightly on Mars, we went through six months of hand-wringing. We had to go easy with our brand-new $2.5 billion vehicle. When I swing my hammer on Earth, every once in a while I miss and whack the hand that holds the chisel. Band-Aids and time usually solve the problem. On Mars we really don’t want the drill or the percussion hammer to hit the rover, ever. The arm was built with as little slop as possible in the joints, and those thousands of lines of software were checked and checked again—but we still didn’t know exactly how it would all work on Mars until we tried it. For one thing, gravity there is about one-third as strong as it is on Earth. And so the dozens of activities we had practiced already in California, we practiced again on Mars, in very small steps. If working on Mars weren’t so amazing, it would be enough to make you scream sometimes. But after six months we were ready to drill a rock.
So what is this precious powder we come in search of, like early explorers to the Spice Islands? Curiosity is looking for evidence that life could once have existed on Mars—for environments that could have supported microbes and for organic molecules the microbes might have made. We’re not searching for life itself; that would take instruments even more advanced than Curiosity’s. Its job is to help us figure out where a future mission should look for life.
A habitable environment includes three important ingredients: water, a source of energy, and the chemical building blocks of life, such as carbon. Earlier missions proved that Mars was once wet. Orbiters photographed ancient river valleys; rovers found minerals that contained water in their crystal structure. Curiosity is testing for the other two ingredients of habitability. Since the surface of Mars today is not hospitable, we’re hunting for ancient rocks that preserve records of a wetter, more Earth-like environment. We’re expecting to find such rocks in the stacked sediment layers of Mount Sharp, at the center of Gale crater. But we stumbled on some not far from our landing spot, and so we’re drilling there first.
We have to drill to find the good stuff. Drilling gets at material inside the rock that is less degraded and more likely to contain a faithful record of an ancient environment. From the study of Earth’s ancient environments, the major focus of my research for more than two decades, I’ve learned how difficult it is to discover such a record—and especially to find organic molecules that may have been made by ancient organisms.
Even on Earth, which we know was teeming with microbial life billions of years ago, we find the traces in only a few locations. The paradox is that water, an essential ingredient for life, can also destroy organic carbon molecules. In just the places where we might look for life, places where water has flowed through sand or silt, precipitating minerals that bind the particles into rock, the water has often erased the organic traces of life—with rare exceptions. On Earth we’ve learned how to hunt for those exceptions. It’s a long shot, but we’re hoping Curiosity will find organic molecules on Mars. They can be made by nonliving processes too, so finding them wouldn’t prove there was once life on Mars. But it would tell us where to look.
We’ve already proved, with the first rock we drilled, that Mars was once habitable. A flat mudstone, shot through with veins of a mineral that formed in water, the rock looks like something from a mining district. Curiosity’s analysis showed that the water was not too acidic for life—it would have been drinkable. It contained sulfur compounds that on Earth are an energy source for some microbes. It contained a carbon source too. We still can’t say that the pond our rock formed in, maybe three billion years ago, was inhabited—only that it could have been.
We didn’t need a gas chromatograph, though, to sense that Gale crater is full of promise. We just needed to look at the photographs. Within a month of landing, we realized that Curiosity had touched down on an ancient streambed. The stones looked like the ones I’d sent skimming across the creek behind my house in Pennsylvania, back when I was a boy.
Images of distant and unknown places have long inspired explorers and the public too. The photographs made during the Hayden expedition to Yellowstone were an essential reason it was selected as America’s first national park in 1872. Photographer William Henry Jackson captured the public’s imagination and support by confirming the existence of western landmarks previously regarded as glorified myths: the Grand Tetons, Old Faithful, and strange pools of boiling-hot mud. Half a century later photographer Ansel Adams began his long career of delighting the public with luminous pictures of parks that many would never visit.
Curiosity’s photos are like that—inspiring but also familiar. Our robot is no Ansel Adams, and Gale crater is not the next national park, but its strikingly Earth-like appearance in Curiosity’s postcards has delighted the public and all of us at the Jet Propulsion Lab too. From the day we landed, this place looked different from all the others we’d visited on previous missions to Mars. From the summit of Mount Sharp to the highlands of the crater rim to the close-up of those stones shaped by water in an ancient stream, the images have reminded us of home.
It’s a strange and potent thought to have about another planet. Soon after you read this, we should be on our five-mile way across the crater to the mountain. As a traveler on Mars now, I’m feeling the truth of Stevenson’s statement: This land is not so foreign. It’s a beautiful place to go for a drive.