Mars Exploration Rovers Update: Opportunity Emerges from Crater and Onto the Shoreline of What Once Was a Salty Sea
Posted by A.J.S. Rayl
2004/03/23 11:00 CST
After nearly two months of study, Opportunity has emerged from her landing crater at Meridiani Planum and onto "the shoreline of what was once a salty sea suitable for life," scientists announced at a special press conference held at NASA headquarters in Washington D.C., yesterday.
"This was a habitable environment on Mars . . . a shallow sea . . . very suitable for life," said Mars Exploration Rover principal investigator Steve Squyres, of Cornell University.
The news built on and added to the discovery announced three weeks ago that Meridiani Planum had been "drenched" with water at some point in the distant past, perhaps as long as 4 billion years ago. That announcement was based on the first in situ evidence of chemicals and minerals that indicate water once flowed on the Red Planet. The science team members said then that they did not know whether the water was pooling above the surface in a lake, sea, or ocean, or flowing from reservoirs beneath the surface.
But the chemical and visual clues streaming into the Jet Propulsion Laboratory (JPL) were revealing signatures of salt water and distinctive layering patterns in rocks that indicated they were generated by slow-moving water in a body of water on the surface. Despite those clues -- and their own inklings -- that the water at Meridiani was probably present in the form of a salty sea, team members decided to secure peer review on their evidence and hypothesis before making the announcement of their latest discovery.
"Three weeks ago, we provided evidence that the rocks at Meridiani Planum once had water seep slowly through them, changing their chemistry, changing their texture in distinctive ways we could look at. What's happened since then is we have found what I believe is strong evidence that the rocks themselves are sediment that was laid down in liquid water," said Squyres. "It's a fundamental distinction. It's like the difference between water you can draw from a well and water you can swim in."
Sedimentologist Dave Rubin, of the U.S. Geological Survey at the Pacific Science Center in Santa Cruz, California, who led the independent peer review panel of six scientists, supported the team's hypothesis. "The best explanation for those rocks is their explanation -- sedimentary structures being deposited by water," he said.
Although the water evaporated millennia ago, scientific evidence that Mars once boasted watery environments will forever change the image of this now cold and desolate planet -- and it will change the way young readers take in science fiction classics, like Ray Bradbury's The Martian Chronicles, by offering up the Red Planet in a new, reality-based, context to consider amidst the fantasy.
While the latest discovery adds knowledge to the water story at Meridiani Planum, many unknowns remain. The MER team does not yet know, for example, exactly how long ago the sea existed or for how long, how deep it was, or whether life ever evolved there.
Still, water is the elixir of life and where there is water, science tells us, life evolves. The question of whether life could have evolved on another planet, of course, has always been the big question, the Holy Grail for astrobiologists and planetary scientists alike. The notion that life may have evolved on Mars -- the only other planet in our solar system in the so-called 'zone of habitability' -- is particularly intriguing, and will spark, no doubt, many thought-provoking conversations around the world even though evidence for such life has yet to emerge.
"We don't know that life was there, but we have an environment that was suitable for life to form," Squyres said. If life did evolve at Meridiani Planum, it is possible though that fossils might be preserved in the rocks there, he added. "If you have liquid water, and you precipitate minerals from that liquid water, the process of precipitation can actually trap inside the crystals, inside the minerals grains that are grown whatever is in the water chemically, biochemically or biologically, and that is evidence that can be trapped and can be preserved over long periods of time."
At this point, the general consensus is that any such fossils found in outcrop rocks at Meridiani Planum would be microscopic and undetectable with Opportunity's toolkit. But the search has only just begun. Opportunity is heading now for the much larger Endurance Crater, a little less than a half-mile [about 750 meters] away, which appears, in images taken by the Mars Orbital Camera (MOC) onboard Mars Global Surveyor (MGS), to feature extensive layering formations that will allow the science team to "look farther into the history of Mars," as Squyres put it. Meanwhile other missions to Mars are in the development stages.
Even without having all of the pieces to the puzzle, the findings are undeniably significant because the discovery does indicate, for the first time, that a planet other than Earth once had a habitable environment for life to have formed, and because the current environment holds the potential for the preservation of evidence of that life. The discovery, said Ed Weiler, an associate administrator at NASA headquarters, "has profound implications for astrobiology."
On a personal front, the MER team's findings "the stuff of dreams, geologically and scientifically," according to Jim Garvin, lead scientist for Mars and lunar exploration at NASA Headquarters. "We're seeing the kinds of things that as geologists many of us have lived to see."
The team ultimately drew its conclusion that Opportunity is now roving along what was once an ancient seabed from two kinds of evidence -- chemical, and visual identification of morphological characteristics of water laid sediments.
When Opportunity measured the chemical composition of various rocks in the outcrop bordering the inside rim of Eagle Crater, where she fortuitously landed January 24, the robot geologist found both chlorine and bromine. Those findings immediately suggested a salt-water environment to the science team members, and also bolstered the idea that rock-forming particles precipitated from surface water as salt concentrations climbed past saturation while the water was evaporating.
"When we looked with our alpha particle x-ray spectrometer (APXS) at the composition of the rock in this outcrop, we found the amount of the element bromine present varied tremendously from one place to another within the rocks," said Squyres. "The proportion of bromine to other chemically similar elements, like chlorine, varied by a factor of 10 from place to place. That sounds a little obscure, but that's a fairly fundamental finding towards this characteristic of what you see in rocks on Earth that were formed by evaporation of seawater." In other words, this was a chemical clue that some kind of evaporative process had been responsible for forming the drops.
The chemical evidence alone, however, was not enough for the team to conclude "with great confidence" that the sediments were deposited in liquid in water," Squyres said. "So we decided to go looking for more evidence."
The additional evidence they were seeking would be in striations or layering characteristic of sediments formed in moving water, what geologists call crossbedding. Certain detectable patterns would be preserved in the rocks if, in fact, they had been formed in moving water. "With our panorama camera (PanCam), we looked off in distance and saw rocks that looked like they had these distinctive forms," recounted Squyres.
The resolution of the PanCam, even from a range of only a few meters away, though, was not good enough to be certain, so they set out to look at targeted rocks in more detail. "It took a lot of work . . . many Martian days to do this, said Squyres, acknowledging science team member John Grotzinger for the plan that guided the team.
"We weren't sure as we got closer if this crossbedding would still be there, but it turned out indeed we were able to demonstrate that [it was]," Grotzinger added, picking up the story. "Some patterns seen in the outcrop that Opportunity has been examining might have resulted from wind, but others are reliable evidence of water flow," he continued.
To the trained eye, patterns formed by wind and patterns formed by water are distinctive.
The layers in rocks formed by wind-blown sediments are, for the most part, flat and uniform, informed Grotzinger, a sedimentary geologist at the Massachusetts Institute of Technology (MIT). "If you just get grains that settle out under the influence of gravity, you're going to get flat layers on a flat surface. If you then make a cross-section and look at these flat layers, you see parallel lines [that] are non-unique. They don't give us a fix on what the general environment looks like," he explained.
"But when these grains start to move in the presence of a current, they organize in a way that forms small little mounds that migrate downstream in the current," Grotzinger continued. "As they migrate downstream, they sort of avalanche -- the grains overtake each other, and that avalanching then produces layering at an angle to the surface that they're moving on. This is crossbedding."
Grotzinger's plan to confirm the existence of crossbedding involved sending Opportunity to the rocks named Last Chance and Upper Dells to study their textures up close and in exacting detail. Beginning at Last Chance, the rover snapped a series of 152 microscopic images -- "each image just the size of a postage stamp," noted Squyres. Then, they mosaiced them together to create "one super image," as Grotzinger described it. Then, the team repeated the process at Upper Dells.
"It's a real testimony to the hardware built and especially the operators, the people who figured out how to move arm to take those incredible mosaics -- it really is a fantastic piece of work by the engineers at JPL, Squyres added.
The effort paid off. There, in the specially created, very high-resolution, close-up images, the MER team members what they were looking for, the inclined layers or crossbedding features needed to support their hypothesis that the rocks Opportunity had been studying had been formed in a water environment.
"The crossbedding patterns in these finely layered rocks indicate the sand-sized grains of sediment that eventually bonded together were shaped into ripples by water," Grotzinger said. "We can actually see lamination -- fine-layering -- about a millimeter-scale thick. The grains that make up this lamination are discrete and they line up next to each other and they're up to about a millimeter in diameter and they get about as fine as 0.1 millimeters."
Along with the crossbedding features, the team also found signs of festooning, small smile-shaped curves produced by currents of water shifting the sediments beneath the surface. "This is the key attribute of that highly sinuous crest line that characterizes the types of ripples that form in water," Grotzinger said. "You can see these expressions [in the images] these smiles opening upwards, these open concave geometries."
"These are the hints, the textures that we see in cross-section, that we then infer a three-dimensional geometry from, and, through the experiments that have been done here on Earth and the natural environments that we can observe on Earth, we feel very confident that this adds up to a story about ripples moving in water, rather than in wind, at least 2 inches [5 centimeters] deep, possibly much deeper, and flowing at velocities of 4 to 20 inches [10 to 15 centimeters] per second, maybe a mile an hour, gentle flows moving by," he concluded.
At the time the rocks were forming, the environment at Meridiani Planum could have been a salt flat, or playa, sometimes covered by shallow water and sometimes dry, Grotzinger said. Such transient environments on Earth, either at the edge of oceans or in desert basins, can have currents of water that produce the type of ripples seen in the Mars rocks. "By comparison to analogues on Earth, these bodies of water move around and are small and are susceptible to climate change," Rubin added.
An Earth analogue for that kind of Martian environment that the scientists believe existed long ago at Meridiani Planum, Rubin suggested, would be the Qaidam Basin, a sunken valley topped with a crust of salt, and dotted with dunes, on the northern part of the Tibetan Plateau, surrounded by the Altun Mountains to the north and the Kunlun Mountains to the south.
"It's a very dry place," said Rubin. "It might get an inch of rain a year. But, if you dig down under the surface in places, there is soft, wet, sandy, salty sediment -- and the particles in the dunes are not a usual sand, but little pellets of clay cemented together by salt that the wind picked up from these flat salty basins and piled up. It doesn't have any water standing there now, because most of the time it's dry at the surface. But this is the kind of transitional environment that I would imagine those kinds of rocks formed in."
When he first received the various images of Last Chance and Upper Dells to review, Rubin said he was "astonished" to see sedimentary structures on Mars just like those on Earth. "You can go out to your nearest beach or creek and take a shovel and dig in and see some of these same kinds of structures," he said, presenting an image of ripples from the Colorado River on Earth, showing same cross-lamination found on Mars and being describing as 'smiles.' "You don't have to have a river as big as the Colorado River to make these -- they can form in a little creek, and it doesn't have to take a long time, but it does require flowing water," he added.
As he considered the MER science team's hypothesis, Rubin said he pondered all the possibilities. "If I was going to try and come up with a counter example -- and I looked through my whole slide collection -- probably the best I could come up with it isn't very good. But it would involve very small, wind-blown bedforms and probably the best way to keep wind-blown bedforms small would be to have water just beneath the surface. So, even in the best counter examples I could come up with, there would probably be water at the surface, if not above the surface. In any case, I think their interpretation is the best explanation."
The unequivocal evidence needed to answer the big question, of course, would be fossils. In the Earth-analogue environments, there could be some kind of microscopic fossils, such as massive algae, according to Rubin. "And maybe some of the rocks that have these same kind of environments also have dinosaur footprints," he added. "But there's no reason to thing there'd be anything like that here."
"If there were dinosaur footprints at Meridiani, we would see dinosaur footprints and if there were giraffes on the horizon we'd see them," Squyres chuckled, then added: "These are the kinds of rocks that are exceptionally good at preserving evidence of microbial life and it could be preserved within the salt crystals that are deposited. This rock is shot through with these lovely hematite concretions -- the things that we call 'blueberries,' the little spherical objects -- concretions are marvelous at preserving fossils, but all of those are at micro-scales. The payload on this rover was designed to seek evidence of former habitable environments. It's done that exceptionally well. We have found the right stuff on Mars to go to and study in depth with a payload like the one that Mars Science Laboratory [tentatively slated for launch in 2009] will have, and with the kind of capabilities that exist in terrestrial laboratories when we bring the stuff back. So, I don't expect to find microbial fossils, because they're too small, or and I certainly don't expect to find dinosaur tracks."
Miles to go . . .
As exciting and Mars-changing as these discoveries are, many unknowns remain and the whole water story on the Red Planet will not be written for some time.
If Meridiani did once boast a salty sea -- transitional or not -- there is no visible remnant of a rim of any kind, no demarcation from the vast flat plains that Opportunity is exploring, described by so many team members as a 'parking lot.' At the moment, however, that lack of a defined outline isn't something that's really raining on anyone's parade.
"Even though it's flat and doesn't have a clear rim around it, things can erode away and the topography can change over time," Squyres responded. "Piecing together the total geographic puzzle is going to be something that [will] keep us occupied for a good long time. What we see on the surface is what we see on the surface -- and it cries out to be explained. Now with respect to whether we are talking about water that was up to your neck ,or your ankles with briny pools or a deep sea, we'll have a better handle on that once we've explored this region over a larger area."
How, then, do they theorize, then, that Opportunity is parked on the shoreline?
"The reason I feel good saying that is, in a situation like this, where you have evaporation taking place, all you have to do is sit in one place long enough and the water will come through," Squyres maintained. "In other words, the shoreline might be there and you're in deep water, but then it gets shallower and shallower and whoosh it goes by and you're sitting on dry stuff. And then it builds up with more water and the process repeats. We're talking about water that comes and goes and so the location of the shoreline will move with time." Therefore, he rationalizes, "at some point in time, it is likely that there was a shoreline here."
The rationale, of course, doesn't preclude the search. "We've done all of this on the basis of an outcrop that is literally no bigger than this stage," Squyres reminded pointing to the small dais on which he was sitting.
Although the team members do not yet have "a good handle" on the extent of the water at Meridiani Planum, Squyres said he is confident that one day they will. "I think via a combination of going to other places within Meridiani and looking at deposits there, and then taking data from spacecraft [to launch in the future] like Mars Reconnaissance Orbiter (MRO), and also MGS already up [in orbit] there, and tying it into the regional picture, we'll get a better handle on that story."
One thing is certain: Mars is clearly a different place now. "If you dumped a big pool of water on the surface of Mars right now, it would freeze pretty quickly," Squyres pointed out. "You would have some very rapid evaporation initially, because the atmospheric pressure is so low, and then you would form a cover of ice that would gradually, progressively freeze and get thicker and thicker until eventually it froze solid. Over time, it might sublimate and evaporate away and the ice would eventually end up at the coldest places on Mars, the poles."
That raises another piece of the puzzle that begs for solution -- what was the weather, the climate like at Meridiani Planum when the rocks were forming? When asked what the team might be able to infer about the weather at Meridiani when the rocks were forming, Squyres said the discoveries Opportunity had made in the little outcrop at Eagle Crater really can't offer much insight about what the climate might be."
At the same time, he noted, deposits like those found by Opportunity could, theoretically, form under a cover of ice. "One way you can make these evaporitic salts is by evaporating the water away and concentrating the brine until the salts fall out," Squyres elaborated.
"Another way you could do it, I think, would be if you had a cover of ice that gets thicker and thicker -- and ice that freezes out tends to be much more pure than the water -- so you can concentrate brine simply by freezing ice," he continued. "If you go to the Arctic and you go snowmobiling on the sea ice, you could actually take that sea ice, which is frozen from seawater, and you could melt it and drink it, because there's no salt in it. You're concentrating the brine beneath it and so you can make rocks like this simply by freezing an ice cover. That's a possibility and you certainly could have currents flowing beneath the ice. I'm really reluctant to draw broad conclusions about changes in the Martian climate from a little tiny outcrop," he said.
"There is still a lot we don't know -- yes, we believe Opportunity is parked on what was once the shoreline of a salty sea, but we don't know how laterally extensive this water was," Squyres admitted. "We don't know how long it was there. We don't know how common this is in other places on Mars. But the neat thing is we have the capability in the future to find out. But we'll be able to take a first shot at it."
Now that Opportunity has rolled up and out of the little crater she's been in, the team is directing the robot field geologist to traverse across the plains, some 700-650 meters, to Endurance Crater [where] they expect to be able -- courtesy MGS, as mentioned above -- to find exposures of many meters of the same kind of rock the rover spent the last two months studying in the Eagle Crater outcrop. "That enables you to look farther and farther into the Martian past, and look at greater slice of Martian history," Squyres pointed out.
"Any geologist will tell you way that the way you draw inferences over what happened over geologic time is to is looking at bigger stack of rocks, farther into the past and that's what we hope to do at Endurance Crater -- to get a glimpse much further back into Martian time. The other thing that can happen -- if you look to the future, 2005 and beyond -- NASA is going to fly [other missions to Mars] so we have the capability to really chase this problem in the future."
For this year, though the Mars Exploration Rovers will continue to command the spotlight. And while Opportunity seems to be stealing the show, it hasn't happened without a fair amount of luck and Spirit, Squyres contended, still holds much promise. "At Meridiani Planum, we were incredibly fortunate to just roll into this little crater -- this tiny little hole in the ground -- 20 meters in diameter -- and then flat plains for as far as the eye can see," he said. "We just managed to have the good luck to fall into that little hole where the outcrop is exposed. Everywhere else for almost as far as you can see, there's nothing like this so we got very lucky."
At Gusev, "we weren't that lucky," Squyres continued. "We came down on flat plain that's littered with boulders that we now know are volcanic in origin. We think the sediments that were laid down in the crater at Gusev may be deep below those." Although Bonneville Crater refused to give up any Martian secrets, the best chance to get down deeper in the geologic history lies, they believe, in the Columbia Hills."The Columbia Hills are 2.5 kilometers away [about1.5 miles] -- a very long distance for this rover," Squyres acknowledged. " But the rovers are behaving so well -- the power systems, the thermal systems are performing so beautifully we think they may actually have a fighting chance of going three, four, five, six times farther than they were designed to go. And maybe, over a period of several months, we can [get] to the Columbia Hills where we have some hope we might be able to find some stuff that's very different from where we're parked right now. Gusev is making us work for it, but I've still got a lot of hope and interest in what Spirit may find."