Curiosity

by Rod Pyle

  This accurate, if wordy, explanation came from the promotional materials from the Mars Habitability Conference held at UCLA about six months after the landing of Curiosity. I was privileged to attend, and some of the smart folks we have met here—including Ashwin Vasavada—were present. Of course, more has been learned since the conference was held in February 2013, but the idea of habitability remains the same—when, if ever, could Mars have supported life, microbial or otherwise?

  Habitability, then, is the driving force behind the mission of Mars Science Laboratory. This is also what specifies the difference between a mission such as MSL's and that of Viking in the 1970s. Where the latter had operated on the assumption—or hope—that there might be microbial life on the surface that could be measured via a series of relatively simple chemistry-based tests, MSL operates on a series of new assumptions and hypotheses, developed over the intervening decades by further observations of Mars and of Earth. The Martian data has been gathered both from orbit (Mars Reconnaissance Orbiter, among others) and the surface (Phoenix and also the MER rovers). Intensive observations and explorations of our own planet have also greatly expanded our understanding of these processes.

  A few examples of Earth-based data, and how it might apply to Mars, are in order. As regards life, and what it might look like, one need go no farther than the life-forms found since the time of Viking on Earth's ocean floor. I previously mentioned geothermal vents—the “black smokers,” hot, mineral-rich torrents of ultra-hot water that bubble up through the ocean floor. These vents are usually found at the boundaries of tectonic plates or other areas that are volcanically active. The life-forms found adjacent to geothermal vents range from bacteria to worms, and all can exist in conditions previously thought impossible—the water can get up over 800˚F. So the water is hot, there's no light down there, and the creatures present ultimately derive their nourishment from the mineral content of the upwelling water. Remarkable as that sounds, they are just one form of what are now known as extremophiles, forms of life that defy previous expectations. I mentioned some found in places like Antarctica as well, but there are also extremophiles in the Atacama Desert (some of which is officially the “deadest” place on Earth), protected by thin layers of rock yet still able to draw energy from the weak sunlight that penetrates. The life-forms in the Atacama are especially interesting, since that region comes about as close to Mars on Earth as one can get. It's deadly dry, gets tons of UV exposure from the ruthless sun, and even has nasty perchlorate in the soil—just like Mars. Yet life manages to find a way, even there.

  It's interesting that while NASA will not refer to MSL as a life-science mission or “life-detection mission,” it happily calls it “the first astrobiology mission since Viking.” The distinction is small but important, if unintentionally misleading. MSL is not seeking life (though mission scientists would be ecstatic to find some definitive sign of it), but potential habitability, past or present, and possible biosignatures from current or former living things. These would likely take the form of isotopes of organic carbon.

  More specifically, as spelled out by Michael Meyer, MSL's lead man at NASA, during a public presentation in 2012, the goal of MSL and Curiosity is to “explore a region of Mars and determine if that area was ever able to support microbial life and assess its potential for the preservation of biosignatures.” That last part is important to us because for the most part, any biosignatures existing will come from rocks and soil (there has been hope of finding a biological source of methane on Mars, but this has, to date, been unsuccessful).

  Fig. 11.1. EXTREMOPHILES: This image of a “black smoker” geothermal vent on the ocean floor is an example of the new ways we have learned that life can adapt. The vents can emit dark, mineral-rich water heated to over 800°F. Below the plume are the various worms and other life-forms that live directly or indirectly off the vent's emissions. Image from NOAA Okeanos Explorer Program, INDEX-SATAL 2010.

  This idea formed the backbone of a program strategy called “the Exobiology Strategy for Mars Exploration,” which was defined in 1995. It underwent several revisions, many due to the work of the MER rovers Spirit and Opportunity. Even as MSL was being proposed, approved, developed, and built, this idea was in flux. And it will likely change again, based on the scientific results from Curiosity's explorations. Part of the process was to limit NASA's Mars exploration goals, to reign in overly general expectations and the expenses that they could incur. These goals were refined from the broad proclamation of exploring overall Mars habitability, Mars polar habitability, and its recent climate, to this simpler, more focused directive for MSL: assess the habitability of Gale Crater, past and present. It is a more-manageable set of expectations for a such a mission.

  This mission, once it was pinned to Curiosity, was expected to last at least one Martian year, or about twenty-three months. JPL’ers have learned much since Pathfinder and the MER rovers, and they have refined and evolved not only the techniques for the design and construction of the new rover but also their expectations of (and confidence in) their machines.

  As we know, once the landing-site selection process had concluded, Gale Crater was the target. Ninety-six miles in diameter, it is dominated by Mount Sharp at the center, 3.5 miles high—three times as high as the Grand Canyon is deep! Mount Sharp may look at first glance like central peaks found in lunar craters such as Tycho, but that is where the similarity ends. Tycho's peak, and most similar peaks within craters, are thought to be created during the impact that causes the crater to form. Shock and energy returning from the impact causes the central peak to rise. Mount Sharp is nothing like that.

  Instead, it was laid down over the 3.5-billion-year life of Gale. It has sedimentary and cross-bedded layers that have been seen both from orbit and from Curiosity's long-range cameras. It appears that this enormous mound formed when water flowed into the crater and, possibly aided by wind-blown deposits, filled it with sediment. As the water dried, it left layers behind, which built up over billions of years to approach the larger earthly mountains in height, and fill the crater. The remaining central location is due to wind patterns coming over the edge of the crater, scouring away material in an ever increasingly complex pattern of swirls. The net result: the perimeter material is gone, and a huge mound of it remains at the center.

  Among the layers of Mount Sharp are clays and other mineral types indicative of water in the past. This, along with the other data flowing in from Curiosity, sealed the case for water in Gale, and lots of it, and over a very long period. Mars was, at one time, very, very wet. And water, of course, is a key part of the ancient habitable environment story. Mount Sharp, whether or not it contains evidence of organic materials, appears to tell that story across time, layer by ancient layer.

  Of course, there is more to Gale Crater than just Mount Sharp. One of the reasons it was ultimately chosen as the MSL landing site was the rich variety of geological and topographic features within the crater, which offer many other opportunities for discovery.

  The landing zone, ultimately named Bradbury Landing after the famed science-fiction author who died shortly before Curiosity arrived at Mars, is near the terminus of an alluvial fan. These features occur when water washes down a hillside over a long period of time, depositing rocks, sand, and silt at the base of the incline. It's a great place to study materials that have migrated from higher elevations, and some of the stuff observed there will have traveled many, many miles, both horizontally and vertically. It's like having rock and soil samples from the entire region collected in a relatively small area for you to inspect at your leisure. And that's just one feature of Gale.

  There are also areas that incorporate a characteristic called “high thermal inertia.” This simply refers to the way heat, absorbed by rocks or soil during the day, gradually radiates away during the night. Gravel, sand, and loose soil lose heat quickly, solid rock and hardened sediments do so much more slowly—they tend to retain heat. It's similar to the way a con
crete house insulates better than a wooden one, and why sidewalks exposed to the sun stay hot well into the night after a long summer day, while nearby loose dirt cools rapidly. With infrared cameras that can image temperatures visually, the geologists have built up thermal maps of the region over time from orbit. By interpreting these maps, they can determine with some accuracy the probable structure of the terrain on the floor of Gale Crater, as the temperatures on Mars can change from day to night by a couple hundred degrees Fahrenheit or more.

  Now there are nuances to this as well. Areas that are made up of what is called conglomerate, which is a kind of natural, sedimentary cement that binds gravels and clays and what have you, also behave like solid rock and hold heat longer. This thermal behavior, if seen in an area that appears to be comprised of water-borne deposits as opposed to solid rock, can be an indicator of something interesting to go and investigate.

  It's all about the rocks, you see…and sand, gravel, clay, dirt, and more. They are the timekeepers, and living record, of Mars's history.

  So, when you are seeking a “habitable environment,” it is presumed that you need water (as would be found in regions like the one just described), carbon, a friendly atmosphere, and a form of energy to allow life to exist and possibly flourish. Indications of some of all of these may show up in layered rock formations.

  Carbon compounds can be identified by the Curiosity, utilizing ChemCam, from a distance. This can aid in deciding what to drill or scoop for a sample. Once the sample is inside the rover's laboratory, SAM can determine the molecular weight and charge information, which can aid in identifying the carbon's likely origins—organic or inorganic.

  Carbon can be a bit of a red herring on Mars, however. Even organic forms of carbon can be the result of a meteor fallen from space—the carbonaceous chondrite variety, as opposed to resulting from life processes. These meteorites, rife with water and organic compounds, are remnants of the formation of the solar system. They wander through interplanetary space for billions of years until they encounter something to stop them—in this case, the gravitational field of Mars. While the carbonaceous chondrite variety comprise just a small percentage of all meteors, they fall to the surface of that planet all the time, so it follows that much of the carbon present there will be meteoritic in origin.

  One more thing about organics and rocks—unique circumstances must occur for organics to be preserved and observable in sediments. Mars takes the full brunt of the sun's radiation—including ultraviolet and cosmic radiation. It has a thin atmosphere as well as a negligible magnetic field, which stops little of either (Earth has a dense atmosphere and a strong magnetic field, both of which protect us from the worst of the sun's intense energy). Martian soil also contains perchlorate, a chemical that is not helpful to detecting organics and that is toxic and corrosive to potential life-forms or their remains. And while perchlorate can serve as food for some microbes observed on Earth, on Mars the presence of perchlorate in a sample that it is heated (as most experiments on Martian soil have done) can destroy organic compounds.

  Mars hides its secrets well. Just ask the guys looking at the rocks.

  Clearly, understanding the story of sedimentation on Mars is a vitally important part of unraveling its history. But not that long ago, planetary scientists weren't even sure if there were any sedimentary deposits on Mars. The Vikings were static landers that remained right where they set down, using robotic arms that could reach out only in a ten-foot-or-so arc with their scoops. Pathfinder was limited to a thirty-foot radius from the lander. But the fleet of orbiters that have been staring at Mars since Mariner 9, culminating with the Mars Reconnaissance Orbiter, offered spectacular visual resolution that showed a lot more detail than ever before. What had to be inferred earlier could now be seen directly. But even then the jury was out—is what we saw below the result of volcanic activity, as most believed, with some wind and water mixed in, or had major sedimentary events that had taken place?

  Mike Malin, whom we have met briefly as the creator of the cameras for Curiosity and other Mars machines, tipped the balance in this debate. Malin has a deep passion for Mars and, along with his close associate geologist Ken Edgett (whom we will get to know better soon), has devoted untold hours looking at images of Mars, mostly from his own cameras. In fact, hundreds of thousands of orbital images of Mars have been returned from his cameras alone. Malin could be likened in enthusiasm to Percival Lowell, the late Victorian who devoted thousands of hours to observing Mars with his telescope—except that Malin neither allowed his imagination to run away from him, nor did he map his own eye's retinas as Lowell may have. But Malin did make an intuitive leap, as recounted by Grotzinger. The paper that Malin and Edgett authored about their observations caught Grotzinger's attention because it contained a theory ripe for a sedimentologist like him to latch onto and start looking deeper.

  Fig. 11.2. SEDIMENTATION SQUARED: This Mars Reconnaissance Orbiter image shows an area on Mars called Arabia Terra. This is an extreme example of weathered sediment—few are as obvious as this. But the extreme nature of the terrain does show how sedimentary layering can be seen from orbit once one understands what one is looking at. Image from NASA/JPL-Caltech.

  Grotzinger started the conversation that day discussing “hazing” in science, which I thought was a great term for the way scientists question each other. More specifically, he was discussing how, when there is an established and largely accepted body of theory, there is pressure to “get on the bandwagon” and just go along with prevailing theory, unless there is clear and compelling evidence to the contrary. That evidence is what Malin and Edgett had labored so many hours to find, and Grotzinger recounted their quest.

  “Those guys spent a lot of time looking at Mars; they really were looking incredibly carefully in an attempt to make a distinction between things that were obviously volcanic terrains associated with obviously volcanic features, as opposed to these giant Noachian [an ancient geologic period on Mars ending about 3.6 billion years ago] craters that were filled up with something that looked layered when there was no other crater nearby.” These features looked like evidence of sedimentation…but were they? If they were, this would indicate a lot of water doing a lot of work at one time or another in Mars's history, and probably very long ago. Following this line of thought, Eberswalde Crater caught Malin's attention.

  “In 2003, the Eberswalde Crater paper was published by Malin and Edgett, [and] that was a barn burner,” Grotzinger recalled. “But the way that science works is that when there's a reigning paradigm, people don't spend a lot of time questioning it, they would rather run with it.” Malin and Edgett were swimming upstream, and the current was running against them in the form of contrary opinion. But Malin was used to going against the prevailing current—decades before, when he first suggested to NASA that high-resolution cameras orbiting Mars could return valuable data, he was told that they already had all the images they would ever need from the Viking orbiters. Well, as you can guess, whoever at NASA said that was wrong. Viking was like a cheap pocket camera compared to the high-powered optics that Malin had in mind.

  But back to hazing in science: “We all do it. We all do it to each other. It's an inherent fallibility of science. Every time there is a band wagon, you will always see somebody saying ‘Why can't this person get on board?’ But one time in ten, that person is at the leading [edge] of [a] revolution, and that is where Malin and Edgett were. They said ‘These features are inconsistent with being just lava flows; there is more here than meets the eye,’” Grotzinger said. And the more people looked, the more it seemed that what they saw in some areas could be the result of millions of years of sedimentation, and those sediments could yield a goldmine of information about how much of Mars was formed, where the water had been, and what it had been doing when it was there.

  Another revelation resulted from Malin's interpretations: it appeared that not all watery activity had been ancient. Some of these changes looked relative
ly recent to his eye. The idea of water existing as a liquid on modern Mars was a revelation (and in some quarters, heresy). Besides being a shock, given the incredibly low atmospheric pressure, it also implied possible supplies of water that could be utilized by organisms today. Note that nobody is waving their arms and proclaiming, “Hey! We found a place where there should be life!” It's simply one more link—albeit a more current one—in the chain of events that could contribute to a habitable Mars.

  The main point was this: the discovery of sedimentary rocks on Mars, among its other effects, was primary in shifting the search for life from the Viking model (“Are there critters here?”) to the MSL model (“Was there an environment on Mars that could have, at one time, supported critters?”).

  Being able to drive a rover up to exposed sedimentary layers—“outcrops,” in the parlance—would satisfy a lot of needs. First, it gives you a visual record of geologic time, especially in a place like Mount Sharp where the sedimentary layers reach very high (note, however, that this is all relative time, so you also need to look for signs of ancient events that might “fix” some part of the layer to a specific period).

  Second, sedimentary layers tell the story of water through a combination of things frozen in time. There are the sizes of the objects in the layer—silt says one thing, sand another, pebbles and larger items still another. Generally they indicate the speed and force of the water. The chemistry of the layer tells you other things—how wet, how salty, and so forth.