A.J.S. Rayl • Mar 03, 2004
Mars Exploration Rovers Update: Opportunity Finds Evidence of Past Liquid Water
The Mars Exploration Rovers have sent home the first real prize of the mission - evidence of past liquid water on the Red Planet. Opportunity -- which landed five weeks ago inside a small crater near exposed bedrock -- has found evidence that Meridiani Planum was once "drenched with water," and, thus, was once suitable for life as we know it, mission scientists announced at a press conference held NASA Headquarters yesterday.
The rover's examination of the outcrop -- which has been dubbed Opportunity Ledge -- detected evidence of sulfates and other minerals, including a rare mineral known as jarosite, that form in the presence of water, as well as other geologic structures that form via aqueous action.
"Ever since Opportunity touched down on Meridiani Planum on the night of on January 24, and we first saw this marvelous outcrop literally in front of us, we have been trying to puzzle out what [it] is trying to tell us," began Steve Squyres, of Cornell University, the lead scientist on the rovers' scientific payload. "For the last two weeks, we've been attacking this outcrop with literally everything we have. Over last couple of weeks, the puzzle pieces have been falling into piece and the last puzzle piece fell into place a few days ago, and we have concluded the rocks here were once soaked in liquid water."
In fact, scientists on the MER team believe there was so much water involved in the geologic processes in the area that life could easily have existed there, although they have yet to figure out whether that water flowed underground or pooled above on the surface.
"We believe at this place on Mars, for some period of time, it was habitable," said Steve Squyres, lead investigator for the rovers' science instruments. "That doesn't mean life was there. We don't know that."
If life had been present when these layered rocks were formed, however, these types of rocks are ideal, he noted, for preserving any fossils that might remain embedded there.
The mission objective of both Opportunity, and her twin, Spirit, at Gusev Crater on the other side of the Red Planet, is to explore the areas around their landing sites for evidence in the rocks and soil that would reveal whether those areas ever had environments that were watery and possibly suitable for sustaining life. But this was a habitable place on Mars at one point in time."
Members of the news media had been speculating for the last week that the scientists had found the much-sought-after evidence of water, the elixir of life. But the team members remained adamantly noncommittal, as reported here in the last MER update. The finding announced confirms that speculation and satisfies, in effect, the rover's primary mission objective.
Since Opportunity landed, the robot field geologist has dug a trench on the crater floor and examined various patches of the soil, but has spent most of her research efforts on the layered outcrop of rock that borders much of the 'bowl' in which she is working. From the first images of the rover's landing site that came streaming into the Jet Propulsion Laboratory (JPL), where the two rovers were designed and built, the scientists were "blown away." It was the first time an outcrop of bedrock had ever been found on Mars.
"There are two puzzles we are working on," Squyres told the assemblage in Washington DC and countless others in virtual attendance at the special press conference. One puzzle, he said, revolves around the question -- were these rocks altered by liquid water? "The answer to that we believe, definitively, is yes." Whether these layered rocks were actually laid down in liquid water, the second puzzle or question, they do not know, Squyres said. "We have some tantalizing clues and we may have something for you in a week or two, but we don't have the answer to that one yet." Therefore, the MER science team cannot say for sure whether the water flowed underground or whether some kind of lake or pond or even ocean once covered the immediate Meridiani Planum area.
For weeks now, science team members have been debating three primary hypotheses for the formation of these perfectly round little BB-like particles. "We thought they might be lapilli -- little volcanic hailstones that form when you have an eruption of volcanic ash and it coagulates and falls out. And another possibility is that these were droplets of volcanic glass or droplets formed by an impact - an energetic event that sprays liquid rock into the air and it freezes and falls out [and forms] little round objects," Squyres elaborated.
After a rigorous examination, the team concluded the spherules are actually concretions, compact little masses of mineral matter that are most often spherical or disk-shaped and are embedded in a host rock of a different composition.
"We've look that these things with our color camera, taken dozens of images with the microscope, and we have sliced through them and looked at them in cross-section - and all these things together have led us to the conclusion that these are probably concretions," said Squyres.
These hard, round spheres of sedimentary rock cement are carried into place by ground water. Concretions -- which vary in size, shape, hardness, and color, from objects that require a magnifying lens to be clearly visible to huge boulders 10 feet in diameter and weighing several hundred pounds -- are found all over the Earth and are the most varied-shaped rocks of the sedimentary world. They form when a considerable amount of cementing material precipitates locally around a nucleus, such as a leaf, piece of shell, or fossil. "We believe that's what we're seeing here. If so, it points to water," Squyres said.
The fact that the spherules are randomly distributed serves as another hint that they were probably formed in water, he added. If they were of volcanic or impact origin, the spherules would likely concentrate in rock layers exposed at the time of those events.
The second piece of evidence, Squyres said, emerged in close-up pictures of the layered rock. Images show that the El Capitan rock is pocked with indentations or voids -- not even a half an inch long that may have once contained salt minerals. "We found that in some places it was shot through with some very weird looking holes, not round, but tabular in shape. Where they intersect the surface, they are long and thin, typically maybe a centimeter long and maybe a millimeter or two wide. And they are shot all through the rock in all kinds of angles."
The science team believes that these voids were once filled with vugs formed by crystals of salt minerals that aggregated within a rock in salty water. "If you have water in solution and crystals precipitate from that -- if the crystals are tabular, flat in form, as they grow, either by pushing the rock aside or replacing the rock, they [become] tabular crystals within the rock. Then, either the water's chemistry changes and they dissolve away or they're eroded away and leave these little tabular holes of voids within the rock. So these we think these holes are the molds of crystals that were once there."
The third piece of evidence comes from the alpha particle x-ray spectrometer (APXS), the German built and designed instrument that measures how much of each chemical element is in rocks or patches of soil. "When we first looked at this rock, it looked like it had a lot of sulfur in it, way more sulfur than has ever been found in a Martian rock," Squyres noted.
That analysis, however, was from the outside of the rock. "We [thought] we might be seeing a coating on the outside of the rock because the APXS can't see very deep inside the rock," Squyres explained. So, Opportunity used the rock abrasion tool - affectionately known as the RAT -- to grind 4 millimeters into the surface of the rock, then the rover put the spectrometers there. "And, we found an enormous quantity of sulfur in this rock, too much to explain by any other mechanism [except] this rock being full of sulfate salts. That's a telltale sign, we believe, of liquid water," he said. The team also had Opportunity use her mini- thermal emission spectrometer (mini-TES), which records the thermal spectra of various rocks and soils to determine the types and amounts of minerals that they contain, and also found evidence of sulfate salts.
"We knew sulfur was high from Viking and inferred at that time this could be salts," added Viking veteran Benton C. Clark III, of Lockheed Martin Space Systems Astronautics, and a member of the MER science team. But when previous mission teams analyzed rocks on Mars at Pathfinder and more recently at Gusev Crater, they didn't find high concentrations of sulfates. "But at outcrop things are different. At Guadalupe [the upper portion of the El Capitan section of the outcrop that Opportunity has been studying], we have the all-time record on Mars -- almost 5 times the amount of sulfur as in the soil," he said.
The team has interpreted the reading to be coming from a compound of sulfate. "The salt we think is probably most prevalent is magnesium sulfate -- known as Epsom salts -- with the only difference being the Epsom salt [here on Earth and available at your local drugstore] has water in it and we think [the compound on Mars] is more dehydrated, called Kieserite," Clark continued. "That, plus the chlorides from the chlorine we saw, adds up to salt concentration that may be as high as 40% of outcrop. This is an astounding amount of salt and can no longer considered to just be volcanic construct of some kind. The only way to form such large concentrations of sulfur on Earth is to have it in water and have the water evaporate. We have further evidence that that is what happened as well."
Unlike other areas on Mars that have been studied, the chlorine levels did not change much. "This was quite a shock," said Clark. "At both Viking sites and at Pathfinder, whenever the sulfur [readings] went up, the chlorine went up as well in the soil. They kind of stayed together. Here it's different. The chlorine stayed the same. The bromine stayed up at this landing site only in certain materials. At Guadalupe, we have the highest level of sulfur, and down at McKittrick [the lower portion of El Capitan, below Guadalupe] the highest level of bromine, what scientists call an evaporated sequence. It happens whenever you have a salt-rich briny material -- salty water that could as salty as Dead Sea. Then this water evaporates slowly, and the salts begin to precipitate out. With each different type of salt precipitating out at different times and in different way, you build up a sequence," he explains.
The fourth piece of evidence was uncovered when Opportunity put her Mössbauer spectrometer on the RAT hole she had grinded into the rock. The Mössbauer, another instrument that was designed and built in Germany, can determine the composition and abundance of iron-bearing minerals to a high degree of accuracy, and the findings revealed evidence for a mineral called of jarosite.
"It's an iron sulfate hydrate, and a pretty unusual mineral," informed Squyres. Although fairly rare, it is a mineral that can be found on Earth and was something previous Mars scientists predicted would be found on the Red Planet. "Roger Burns, a late colleague of ours, is someone who always talked about jarosite [as] the kind of thing that if you went to the right place and looked around properly you might actually be able to find it," he noted. "We found a lot of jarosite in this rock, and it is a sulfate mineral that [requires] water around to make it," he said. With Earth as the only analog, the presence of jarosite on Mars points to an acid-rich lake or, possibly a hot springs environment.
"So the combination of probable concretions; these things we think are crystal molds; the sulfur, the sulfates; and, in particular, the jarosite -- if you put that story together, it's hard to avoid the conclusion that this stuff was deposited in liquid water," summed up Squyres.
"Each of [the pieces of evidence] are strong in own right," corroborated geologist John Grotzinger, MIT, and a member of the MER science team. "Added up together they make a very strong case that adds up to the water story."
The quantity of sulfates -- "an astounding amount of salt," as Clark put it -- is the key piece of evidence that this area was once drenched in water, said Squyres later. "With [this] quantity of sulfate, you kind of have to have a lot of water involved," said Squyres. "The only way you can form such large concentrations of salt is dissolve it in water and allow the water to evaporate," Clark added.
If the water had pooled on the surface at Meridiani Planum, the science team members are not hazarding high bets on what shape it took. "It's difficult to point to a basin on this site where water was," Squyres pointed out. "But Mars can change its topography. This stuff may be fairly old, stuff that was laid down along time and exhumed by material on top getting eroding away. So the jury is still out on that particular one."
When pressed, Squyres offered two hypothetical scenarios of water at Meridiani. "One possibility is that we had an eruption of volcanic ash, and the ash settles out -- and maybe there were other eruptions and layers of fine-grained rock build up. These are fundamentally basaltic in character, but you've got lots of pore space in it. Then subsequent to that, water percolates through that rock and it deposits sulfates. It changes the chemistry, and the spherules grow and the crystals that form the vugs in the voids and then go away. It's a process of alteration of this ash as the water percolates through.
"A totally different scenario is that we had a salty sea at this location, and you had water in that sea and had currents and maybe even had waves," Squyres continued. "We've seen things that look like they might be the result of water sloshing around and we want to take a look at that. Then as that stuff evaporates, crystals of salts, sulfate salts are deposited from that and they settle out and more sulfur is deposited and maybe that happens multiple times and you build up layers and layers of these salts, which then subsequently can have water percolate through them and cause recrystallization and so forth. Those are two very different scenarios and both of those are fundamentally possible based on what we see right now."
The plan now calls for Opportunity to head to an area the MER team has named Big Bend to try and determine which of those two possible scenarios might be correct.
The robot field geologist is also scheduled to intensively study a rock called Last Chance, which appears to feature layers in a pattern geologists call crossbedding. "If it is crossbedding, it requires that sediment particles be moved in a flowing current - and that could be air, water, or volcanic gases," explained Grotzinger. "All of these options are open right now. But this is a tantalizing hint something might be consistent with water, so we've designed some experiments to test this."
The scientists declined to even speculate on how long ago the water may have been there on Meridiani Planum. "It's very, very difficult to infer age simply by looking at pictures, by measuring this kind of composition," said Squyres. "What you really need is samples brought back." That said, he added: "If you ask what kinds of rocks on Earth really well-preserve evidence for very ancient life, for biochemistry - one of the very best kind of rocks for doing this is rocks that contain minerals precipitated from water. That's because what happens is not only did the water provide the medium in which that biochemistry or life takes place, but as minerals precipitate they can trap the evidence of that and preserve it very well for very long periods of time. So these are very, very interesting rocks."
Neither Opportunity nor Spirit will return any samples of 'evidence' back to Earth, but will remain on the Red Planet as space debris, after they stop functioning. A sample return mission is on the NASA agenda though at some point in the future, and President Bush proposed sending manned flights to the moon and Mars in his new vision presented earlier this year.
As for the hematite -- the gray mineral that was detected from orbit studies and which drew them to Meridiani? "I have always looked at as a chemical beacon [that] something interesting, chemically, happened here," said Squyres. "It drew us to this place - and certainly is a lot of hematite. But I don't think we're going to have a good answer on the hematite until we get out of this crater - once we climb out and get out there, then I think we'll see more hematite and figure out the relationship - so stay tuned. We're just getting started and there may be much better stuff out there."
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