Most of the social conversation is positive, but there's a consistent undercurrent that's not so positive: what has Curiosity accomplished, really? She's been on Mars for a year, driven a kilometer and a half, drilled two holes. What does the mission have to show for all this time and all the effort and all the money that it took to get Curiosity there? Where's the science? I hear this from a few members of the public, but I've been hearing it a lot from other scientists.
I'll be defending Curiosity below, but I'll begin by agreeing that there's not much science to show yet -- at least not compared to what we are going to get. In the last few days, I've seen several mission scientists and engineers respond to questions like: what has Curiosity's biggest accomplishment been so far? And pretty much everybody has answered that the mission's greatest scientific accomplishment to date is that it has shown Mars was habitable -- that there was a time when there was an environment with liquid water at a friendly pH that persisted for at least a little while. The habitable environment is the one that created the flat-lying, fine-grained rocks visible at Yellowknife Bay.
NASA / JPL / MSSS / Damia Bouic
Curiosity panorama at "Grandma's House," sol 137
On sol 137 (December 24, 2012), Curiosity sat inside a depression named "Yellowknife Bay." The "shore" of Yellowknife Bay is a layer of rock that makes a distinctive step down. This view is composed of left Mastcam (Mastcam-34) images.
So Curiosity has found rocks that record habitable environments on Mars. This is awesome. But, with much respect to all my friends on the Curiosity mission and to the rover herself, they really doesn't deserve credit for that discovery. They didn't up and discover this environment -- surprise, look what we found! -- after the rover landed on Mars. It's the people who picked the landing site who made that discovery. Those people include many members of the Curiosity science team, but it was led by outsiders, and included the entire community of Mars scientists. Curiosity confirmed the habitable environment, and ground truthing is important, but it's not so much a discovery as a massive relief that the people who are interpreting the orbital data know what they are talking about.
In fact, that's really what we've learned so far: that after five decades of work we are really beginning to get Mars. It's a complicated place. The Mariner and Viking missions had shown us that Mars was a fascinating world that had once seen liquid water, but was so heavily cratered that it must be long dead. It was possible that there was just one paroxysm of wetness on a world otherwise as dead as the Moon. Mars Global Surveyor ushered in a much more nuanced view of Mars' history -- it showed us a world with a lengthy and varied geologic history, one that didn't have a one-way arrow from "Earth-like" to "blah." Mars had regional geology, different stories in different places, different amounts of water and weather acting differently on different rocks on different time scales. Did any of these places host conditions that could support life?
We honed our questions, and sent new spacecraft with new instruments designed to get around the dust that obscures much of Mars' surface and search for rocks that could tell us about past water. After the debacles of Mars Polar Lander and Mars Climate Orbiter, we got Odyssey, and then Spirit and Opportunity and Mars Express. Each one of these brought on new capabilities, building on Mars Global Surveyor's success to show us different ways in which Mars had been more interesting, more diverse, wetter, but only in certain places, in very ancient rocks. When Phoenix landed, it proved that we understood what Odyssey and Mars Express had been telling us, that the water that had once flowed across Mars' surface was now lurking, just beneath the red dust on its surface. And that the chemistry of Mars' rocks did very interesting things when wet.
Mars Reconnaissance Orbiter was truly revolutionary. Its HiRISE camera and CRISM spectrometer gave us a view of Mars that's more detailed, really, than what we have even on our own planet. Our own world's geology is mostly obscured by inconvenient oceans or hidden under a moldy crust of plant and animal matter. Mars' geologic story is often hiding under windblown dust, but wherever the wind removes dust, rather than deposits it, we see its rocks laid bare. Odyssey showed us where those spots were, and Mars Express examined them and found chemical beacons. We targeted those places with Mars Reconnaissance Orbiter. In some of them, we could see beautiful layered rocks in HiRISE, in the same places that CRISM told us there were sedimentary minerals: clays, sulfates, salts. These minerals need water to form. We know Mars has energy, we know it receives carbon compounds from space, and with the HiRISE and CRISM discoveries of layered rocks bearing water-rich minerals, we knew there had been environments where all three existed in the same place. Mars Reconnaissance Orbiter achieved a major goal of the Curiosity mission before the rover had even launched.
Which is great, because with the confidence that we were going to be sending the rover to the right kind of place, we could then refine our questions further. What were these watery environments like? Clement or not? Long-lasting, or not? If they were nice places that could've supported life for a long time, are they the kinds of rocks that might record ancient life, or would fragile organic material have been destroyed in the rock-making process? Curiosity's landing site was selected through a process open to the entire Mars scientific community, drawing on five decades of orbital and landed data, to answer these open questions. Ultimately, she was sent to a place where we saw sedimentary rocks and sedimentary minerals in CRISM and HiRISE data from Mars Reconnaissance Orbiter: Gale crater.
There were actually several places Curiosity could've gone to find these things. It was pretty much guaranteed, then, that Curiosity would land in a place where she would "discover" evidence for a habitable environment. So I'm not giving the Curiosity team credit for that one. I'm just thanking them and breathing a huge sigh of relief that we seem to understand the orbital data well enough to have gotten that right. It's surprisingly rare, in planetary science, to be right.
NASA / JPL-Caltech / MSSS / Tanya Harrison
A mosaic of Mars Reconnaissance Orbiter Context Camera images covering all of Gale Crater. The whole crater is about 150 kilometers in diameter.
There are three reasons we haven't started yet. There's one that you can justifiably be annoyed about. But the other two explain why you're going to have to wait for the science, and also why the wait will be so, so worth it.
Reason #1: They weren't ready to do all the science (or driving) when Curiosity landed.
They are still, now, a year into the mission, bringing new capabilities online. For example, the rover's top one-day driving record to date is just over 100 meters. The most common distance these days is around 60. Curiosity's predecessor Opportunity, working with more sophisticated autonomous navigation software but one gimpy ankle and wheels only half the diameter of Curiosity's, has achieved more than 170 meters in a single sol. There's no reason Curiosity can't do better, and they're working toward enabling the same software features in Curiosity that have been installed on Opportunity. But they couldn't do that and work through all of the first-time activities of the soil and rock sampling challenges at the same time. Curiosity can do more, a lot more, than Spirit or Opportunity could, but each of its capabilities has to be laboriously tested and verified before it can be used on Mars, and time moves at the same rate for all rovers.
Still, the mission could have been more ready with the software needed for operations before they landed, with a lot more time spent with the testbed model of Curiosity back on Earth. (That rover's name is "Maggie," by the way.) Several people have told me that, in the time before landing, the mission chose to shift engineers away from the development of landed operational capability, toward ensuring that the landing would succeed. It's hard to fault them for that, but it's frustrating to have the most capable-ever robot sitting on the surface of Mars for a year and not be able to use it to its fullest. We'll get there, with time. Depending on what you call "there." They may never stop developing new capabilities for this awesome machine. They're still coming up with new tricks for Opportunity.
Reason #2: Curiosity is not where the science is going to happen -- yet.
We're actually lucky to have the one geology paper we've got so far. Unlike all previous landed Mars missions, Curiosity was always expected to land in a not-very-interesting place (modulated, of course, by the fact that it's on Mars) but within reach of a much more interesting place than we've ever seen before. Curiosity's legendarily complex landing system was designed to allow the landing site selection committee to pick a small flat spot that was safe to land in, located near some dramatic geology that was completely unsafe for landing. Curiosity is the first Mars lander that had this luxury.
At Gale crater, it's never been about doing science right at the landing zone. The cool stuff is located outside the "landing ellipse," the safe zone in which Curiosity set down. We have to wait for Curiosity to exit the landing zone before she enters the science zone. So you can continue to be frustrated about it taking Curiosity a long time to exit the landing zone for reason #1, but not about the fact that the vast majority of the rocks we're going to be seeing for the next several kilometers of driving are scientifically boring. This was how the mission was going to be, all along.
It's time to drive, drive, drive. I know it must be making some science team members crazy not to be stopping and poking at this or that bright or dark or otherwise unusual-looking rock. But the jumbled cobbles of uncertain provenance that Curiosity is now driving by will not be terribly rich scientifically. Not compared to the stuff at the base of the mountain. At least, that's what the orbital data is telling us. We must hope we're right about that. Curiosity's confirmation that there was a habitable environment here suggests that we're right about that, so it's worth it to hit the road.
Curiosity's landing ellipse
A "+" marks the spot within Gale crater at which Curiosity was targeted. Considering all the uncertainties inherent in the landing, engineers were confident it would land within the black ellipse, which is 20 kilometers long and 7 kilometers wide. Gale crater is 154 kilometers wide.
Reason #3: Curiosity is not a flyby mission.
Let me explain what I mean by that. When you don't know anything about a new world, just zipping past it and shooting a few photos completely changes our scientific understanding. That kind of reconnaissance has been done for Mars. There will be moments of "flyby science," like when Curiosity drove right past a rock that clearly contained river-rounded pebbles. She didn't even need to take out her robotic arm (which was good, because she couldn't use it yet; see reason #1). That's an instant discovery that could be turned right into a peer-reviewed paper and published in Science as soon as they did their due diligence describing what they found and explaining the context (nothing like it had been seen on Mars before, but there are plenty of analogs on Earth), and the implications (there were streams here, ankle-to-hip-deep, running for several kilometers). Whenever I'm asked what Curiosity's greatest scientific accomplishment to date is, I talk about these running rivers on Mars. (And how it's funny that it's the second place off of Earth where we've found rounded river rocks, the first being Titan.)
However, almost none of Curiosity's science is going to come so easily. What Curiosity has to do now is to perform a geologic survey and figure out the stratigraphy. What are the rocks here? What are they over there? Is it the same sequence over here as over there, or different? What are the rocks made of now? Is it the same suite of minerals that were there when the rocks were laid down, or have they changed? What are the natures of the contacts that separate the different rock layers? Are they gradational, or are there gaps? How big of a gap is it -- how much time is missing from the record? How has the whole package of rocks been altered physically or chemically since it formed? Was there more than one episode of geologic change? When did that alteration happen? Curiosity has to drive on all the rocks to tell their story. This will not be a fast process. It will take time, and it will be an awful lot of fun to pick up the clues.
We've gotten a preview of how this is going to work with Oppportunity. After three years of roving across the (formerly) trackless wastes of Meridiani Planum, Opportunity arrived at a region of wholly new geology. Opportunity proceeded by doing what any good field geologist would do: she performed a survey of the available outcrops, an initial reconnaissance and circuit of the rocks, and then zeroed in on a few key locations for detailed examination. That process took most of two years. I've only seen one peer-reviewed publication out about that phase of the mission so far. Now Opportunity is arriving at the next bit of Endeavour's rim, Solander Point. I fully expect it to be two years again before we begin to see peer-reviewed publications about this new place. And a lot of the early conclusions in those first papers will turn out to be wrong, or at least to need refinement in subsequent work. We'll be seeing results of the current phase of the Opportunity mission coming out for many years to come. The same will be true of Curiosity, once she's had a chance to do her fieldwork. But she's only just begun the last leg of her long trip from port Kennedy to her field site.
At Monday's celebratory event at the Jet Propulsion Laboratory, project scientist John Grotzinger remarked that it may well take until the end of the nominal mission, a year from now, for Curiosity to get to those exciting-looking rocks at the foot of the mountain. The views will be awesome, and we can hope that there will be some instant discoveries, some flyby science that jumps out at us in the images of those rocks like the river-rounded pebbles did. But the real scientific payoff will not begin to come until Curiosity has spent years studying those rocks. Not to mention the fact that any results from Curiosity's fiendishly complex gas chromatograph mass spectrometer can't really be trusted until they've been experimentally reproduced in the laboratory back on Earth.
So, for all the scientists who are looking over the Curiosity team's shoulders, asking "are we there yet?" The answer is "no" and maybe also "sit down and be quiet." We have a long, long way to go. It's going to be a grueling road trip, with not a lot of scientific reward along the way.
When I was a kid, growing up in Fort Worth, Texas, my dad used to take me and my brother on these amazing summer road trips. We'd go west to the Rocky Mountains, and spend weeks seeing canyons and pueblos and mines and peaks and rock shops and art galleries. I loved those trips, but the first day was hard. When you're starting in east Texas, it doesn't matter whether you head for the pueblos and canyons of New Mexico or the mines and mountains of Colorado; you still have five hundred miles of west Texas to get through, five hundred miles of Big Sky Country, open land dotted only occasionally with cows, mesquite trees, very lonely farms, "towns" consisting of a truck stop and a Dairy Queen, and rare pronghorn antelope. It has its own special beauty, and driving through it a half dozen times has given me a real appreciation for how vast and mostly empty our country is. But there's nothing much to stop for, and you wouldn't want to anyway, because any stop just delays your arrival at the sights that it's really worth getting out of the car to see.
That's the phase of the mission that Curiosity is in right now. Amazing sights await us. Spectacular landscapes, and rocks older than anything we've ever touched on Mars before. They'll tell us a long, long story, and it will take us a long time to figure out what that story is. Hopefully it will be a thrilling one. But first, we've got to get there.
NASA / JPL / MSSS
Mount Sharp (Mastcam-100 panorama, sol 45)
Mount Sharp, also called Aeolis Mons, is a layered mound in the center of Mars' Gale Crater, rising more than 5 kilometers above the crater floor, where Curiosity has been working since the rover's landing in August 2012. Lower slopes of Mount Sharp are the major destination for the mission.