Curious Mars


CuriousMars: Opportunity Hunts for Clay Minerals on Matijevic Hill

Opportunity used its Pancam to take the images combined into this self-portrait days before settling into its winter parked position, at Greeley Haven, named for planetary geologist and MER science team member Ronald Greeley (1939-2011). Heading into its fifth Martian winter, the rover's solar panels were dustier than ever, but the winds of the Martian spring have since cleared much of the accumulation. Image credit: NASA/JPL/Cornell/ASU


The Mars rover Curiosity has been commanding headlines with its every move from its historic, breathtaking landing last August to its first major discovery of an ancient stream bed formed by fast flowing water. In recent week's rumors, speculation, and wild conspiracy theories about what Curiosity has found have demonstrated, if nothing else, just how much the public is interested in Mars.

Curiosity is this week zeroing in on its first drilling target, but driving downhill toward it slower than planned because of difficult terrain, said Rick Welch mission manager at the Jet Propulsion Laboratory (JPL).

But on the other side of the planet, the Mars Exploration Rover (MER) Opportunity has been quietly soldiering on in her younger sibling's shadows.

Opportunity has all but been forgotten by the media, even as it explores the most scientifically rich, potentially life spawning environment that it has traversed since bouncing to a stop inside a small crater in January 2004 -- and it may not be long before this long-lived robotic explorer reclaims the science spotlight with a headline of her own.

Opportunity became the sole Mars Exploration Rover in May 2011 when the last attempt to communicate with her twin Spirit failed to elicit a response. Spirit had become ensnared in a sand patch and was forced to spend the rovers' fourth Martian winter without being able to angle her solar arrays to the Sun. She sent a communique before ostensibly slipping into hibernation in March 2010. It would be her last. About one year later, in March 2011, after some 1300 electronic attempts to reestablish communication with Spirit, NASA declared that rover's mission over.

Now nearing her ninth anniversary on January 24, the veteran MER Opportunity is roaming the "sweet spot" of clay mineral hunting grounds on Matijevic Hill, located on a broad ridgeline named Cape York. The feature is part of the rim of the 22-kilometer (13.7-mile) diameter Endeavour Crater that is more than 3 billion years old.

Opportunity pulled up to the rock-strewn rim of Endeavour in early August 2011, after a three-year, 21.5-kilometer (13.35-mile) trek across the Meridiani Plains from Victoria Crater, a 730 meter (2,400 ft.) diameter hole in the ground that the rover had studied for two years.

Guided by rover driver Frank Hartmann, it crossed a geologic boundary at Spirit Point, an area named for her twin sister, to the cheers of everyone on her team following the robot's adventures. "Now that we've made it," said Steve Squyres, the MER principal investigator, of Cornell University, "it feels like a new mission all over again." And, for all intents and purposes, it is.

This crossing represented the first time the MER mission has had the chance to examine rocks and crust material from Mars' Noachian Period, some 3.7 billion to 4.1 billion years ago. The Noachian Period was a time when Mars was probably warmer, wetter and more Earth-like that the desolate desert it is today.

The Noachian Period is characterized by meteorite and asteroid impacts, erosion, valley formation, volcanic activity, the possible presence of abundant surface water, and weathering of surface rocks to produce abundant clay minerals known as phyllosilicates.

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Artist Ittiz used knowledge of Martian geologic history and data from the Mars Orbiter Laser Altimeter (MOLA), an instrument on the Mars Global Surveyor (MGS) that was decommissioned in 2007, to create this impression of what a warm and wet Mars might have looked like 4.1 billion to 3.7 billion years ago during the Noachian Period. Because Late Hesperian features - outflow channels - are shown, this is not an exact impression of Noachian Mars. However, from space the overall appearance of the planet may have looked similar to this depiction. Image credit: Ittiz

Since arriving at Endeavour, "it's been one thing after another," Squyres said in a recent interview. During the past year and a half, Opportunity and her team have seen things never seen on Mars before beginning with the very first rock the rover examined there. The footstool-sized Tisdale2, on the western side of Cape York proved to be "different from any rock ever seen on Mars," Squyres said. It had a composition similar to some volcanic rocks the rover had studied, but science team members found much more zinc and bromine than they'd typically seen in rocks on Mars.

This was their first in situ confirmation that reaching Endeavour really was the equivalent of a second landing site for Opportunity. It's a finding that they are still analyzing and considering in relationship to other findings since then, that have turned the story of Endeavour Crater into a cloak-and-dagger Martian mystery.

As Opportunity roved on, driving north/northwest along the western edge of Cape York toward its winter parking slope, it made its first major discovery at Endeavour -- gypsum, a striking, light-tone vein cutting through the bedrock over which she was driving. Squyres and the team named it Homestake.

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Within three months of arriving at Endeavour Crater, Opportunity discovered in a striking light-toned vein "running" through bedrock. Image credit: NASA/JPL-Caltech/Cornell/ASU/Stuart Atkinson

A soft sulfate mineral composed of calcium sulfate dehydrate, gypsum as far as we know, only forms in the presence of water. It was, as Squyres declared, "the single most powerful piece of evidence for liquid water on Mars" found to that date. "The morphology of this just screams water," said Squyres, who along with Ray Arvidson, the MER deputy principal investigator, of Washington University St. Louis, and others, made the announcement at the American Geophysical Union's fall meeting in 2011. More than that, gypsum speaks of past water with a pH suitable for life as we know it.

From Homestake, Opportunity headed north and east in December 2011, up onto Shoemaker Ridge, which forms the spine of Cape York. There, the rover parked to wait out MER's fifth Martian winter on the northern slope of Greeley Haven. It was the first time this rover had to park for the harsh season, but the robot field geologist never stopped working.

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This is a portion of a 360-degree scene which combines the 817 images Opportunity took with its panoramic camera (Pancam) during its fifth Martian winter. The rover spent that winter parked at Greeley Haven, which is located on the northern end of the Cape York segment, on the western rim of Endeavour Crater. Image credit: NASA/JPL-Caltech/Cornell/ASU

By the time the season lifted, it had produced another stunning, scientifically informative panorama, named, like the winter stop, after Ron Greeley, a member of the MER science team, Arizona State University professor and highly respected planetary science mentor to many in the planetary science community. Greeley passed away unexpectedly in October 2011.

With the Martian winter slowly on the run for spring, Opportunity roved on last May and headed down the gentle slope of Greeley Haven. It then stopped to check out a few targets and went on to round the northern tip of Cape York. By August, the rover was making her way south along the inboard or eastern side of the crater rim, beginning her hunt for the phyllosilicates. The plan was for the rover to take lots of images with the Pancam and navigation camera (Navcam) so the scientists could zero in on outcrops that may be good places to search for the phyllosilicates or anything else that caught their collective eye.

Specifically, the robot field geologist and her team are looking for smectite, a kind of clay mineral. But the objective is not to discover phyllosilicates, but groundtruth them, as Squyres has so often pointed out. "We know the phyllosilicates are there."

They know because a team of scientists, which included both Squyres and Arvidson, detected the mineralogic signature of phyllosilicates, particularly smectite, in data acquired by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), an instrument onboard the Mars Reconnaissance Orbiter designed to search for signs of past and present water on the planet.

James Wray, then a graduate research fellow at Cornell University, now an assistant professor at Georgia Tech, along with Alfred McEwen, of the University of Arizona, Frank Mustard, of Brown University, Scott Murchie, of Johns Hopkins University's Applied Physics Laboratory, and Squyres and Arvidson published their detection of phyllosilicates and hydrated minerals at Endeavour Crater in the Geophysical Research Letters journal in November 2009, hypothesizing that they "formed under wet, non-acidic conditions early in the planet's history."

The observed phyllosilicates, the MER/CRISM investigators wrote, formed in water that was more neutral, as opposed to the highly acidic water for which both Spirit and Opportunity found evidence in their primary and missions and beyond.

Although numerous types of extremophiles have been found on Earth surviving in seemingly uninhabitable geochemical environments that would be deadly to most life on terra firma, the evidence for Cape York's neutral to alkaline water is, simply put, much more conducive, most biologists believe, to the emergence of life. It is a more hospitable haven for "life to find a way" Mars scientists believe.

Like the hematite signature detected first by an infrared spectrometer onboard the orbiters Mars Global Surveyor (MGS) then Mars Odyssey at Meridiani Planum drew the MER mission to choose this landing site for Opportunity, the phyllosilicate and hydrated minerals that the Mars Reconnaissance Orbiter's CRISM detected at Endeavour Crater, are like "mineralogical beacons" that you can see from space, said Squyres.

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This geological and mineralogical map of Cape York was produced from data acquired by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument onboard MRO. The location of the clay mineral signatures CRISM detected are displayed in red. The route, charted here by Stuart Atkinson, shows that Opportunity is "in the red". Image credit: NASA/JPL/UA /JHUAPL

With the scientists' over-arching objective being to figure out what the Red Planet was like all those billions of years ago, "groundtruthing both phyllosilicates and hydrated sulfates identified from orbit would allow a more refined evaluation of what these mineral detections imply about environmental conditions on ancient Mars," wrote Wray, et al., in 2009. The objective therefore is for us "to see what is there at the place where the phyllosilicates are," said Squyres.

As a co-investigator for CRISM, Arvidson has spent long hours studying the data and the topography of Cape York to become deeply familiar with the specific areas at Endeavour Crater where those phyllosilicates and hydrated minerals are lying in wait. So on August 28, when he and Squyres and the rest of the team saw a wild looking outcrop bearing fins in the images Opportunity sent home, they decided to "slam on the brakes" and make "a hard right turn," as Squyres put it, directing Opportunity to the west and up the slope to the area that is now called Matijevic Hill.

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Opportunity took the images that went into this panorama of the sinuous-shaped Kirkwood outcrop in the area now known as Matijevich Hill with her Pancam. This photograph was processed by the Pancam team at Cornell and Arizona State University (ASU). Image credit: NASA/JPL-Caltech/Cornell/ASU

As the rover approached the wild, fin-bearing outcrop the team calls Kirkwood, they saw something else they'd never seen before - a strange, new kind of spherules, scattered all over the outside and densely embedded in the outcrop. Although these are small apparent concretions, they are not the hematite "blueberries" that Opportunity has found throughout Meridiani and which turned out to be the source of the hematite signature MGS detected.

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As soon as Opportunity headed onto the base of Matijevic Hill in late August 2012 to check out a very "crazy" looking, fin-bearing outcrop called Kirkwood, it discovered densely packed spherules of a kind it had never before seen. The rover took the pictures that went into this close-up mosaicked picture with her microscopic imager (MI) on its Sol 3064 (Sep. 6, 2012). The individual spherules - which MER Principal Investigator Steve Squyres immediately dubbed "newberries" - are up to about 3 millimeters (one-eighth inch) in diameter. The scientists are still working out what, exactly they are. But they do know that they are not hematite blueberries. Squyres noted in his recent AGU presentation that this image may be the most important to date in the hundreds of thousands the rover has returned during her nearly nine years of exploring Mars. Image Credit: NASA / JPL-Caltech / Stuart Atkinson

"This is something different," said Squyres. "We're right on the rim. Rocks on the rim didn't travel far, so if you're looking at rocks transported during the impact event, you would expect to see coarse angular breccias, which is what we saw over at most of Shoemaker Ridge earlier. They are looking sort of like basaltic sandstone, but how that sandstone formed is open to question at this point," Squyres told CuriousMars.

The theories flowed fast and furious, but they remain theories for the moment. "I have no particular favorite interpretation about whether it was the result of an impact, volcanic event, sediment deposited in water, or sediments deposited by wind," Squyres said. "The thing I've been hammering on the team about is to come up with a set of multiple working hypotheses. We're going to take our time and work our way through this."

The team also spotted tiny white veins running through Kirkwood and a totally different looking outcrop behind it, so Opportunity maneuvered her way around the wild, fin-bearing outcrop to look at Whitewater Lake, a flat, light-tone rock that offered the science team members something else they'd never seen before - a strange coating or rind that sort of transforms the slab into a kind of Jackson Pollack-light painting, especially in the false color images the Pancam team and others have produced.

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In the middle of this picture, you can see part of the layered, fin-bearing outcrop called Kirkwood. Just behind it, in the top part of the image is the fainter, circular, "slabby" flatter rock called Whitewater Lake, which Ray Arvidson, the MER deouty principal investigator, believes may be the bearer of the clay mineral signature detected by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) onboard the Mars Reconnaissance Orbiter. Image credit: NASA/JPL-Caltech/Cornell/ASU/Stuart Atkinson

As intriguing as Kirkwood and Whitewater Lake are, the team decided to have Opportunity do the next logical, geologic thing: conduct a survey. "Just like a geologist would do at any new geologic site, we directed Opportunity to walk around and take notes about which outcrops to go back to take detailed measurements," said Arvidson.

Given the CRISM map and the scientific bounty of riches that the orbital data indicate are right beneath Opportunity's wheels at Matijevic Hill, it made sense strategically too. So after completing quick close-up looks of Kirkwood and Whitewater Lake, Opportunity headed up the hill from Whitewater Lake in October to begin the survey.

The challenge for the MER scientists will be in identifying the phyllosilicates, the smectite, with certainty. Opportunity's miniature thermal emission spectrometer (Mini-TES), the rover's mineral detecting instrument that could see phyllosilicates right away, is out commission, and has been since a planet encircling dust storm in June-July 2007 "sat" on Opportunity and pummeled her with its notorious, ubiquitous dust.

In addition, the radioactive power source for the rover's Mössbauer spectrometer, which could determine the iron-bearing phases of rover targets, has run down and is no longer practical to use.

Fortunately, Opportunity's other instruments are in good shape and working well, just like the rover itself, more than eight and a half years past their "warranties." The stereo panoramic camera (Pancam) which in effect are the rover's "eyes," took on some dust during the big storm of 2007 but the Pancam team, led by Jim Bell, now of Arizona State University, adapted to the dust's affects during image processing, and over time the Martian winds whisked much of the dust on the camera's lenses away.

In addition to taking radiant color images of the rover's surroundings, each "eye" of the Pancam carries a filter wheel that gives this instrument multispectral imaging capabilities and images taken at various wavelengths that can help scientists correlate with previous images and determine what minerals are present in the Martian rocks and soils the rover is checking out.

Opportunity's rock abrasion tool (RAT), used for both brushing and grinding into targets, and its alpha particle X-ray spectrometer (APXS), which is designed to determine the elemental chemistry of rocks and soils using alpha particles and X-rays, are also functioning exceptionally well.

Still, the groundtruthing of phyllosilicates won't come easy. The MER scientists must now become detectives on Mars, inferring their findings of phyllosilicates from the composition, the overall geologic setting, color and correspondence with other data, in this case data they have from CRISM.

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This map shows Opportunity's route up to its Sol 3135 (Nov. 17, 2012), and the rover's approximate current location on Matijevic Hill, a rise on the inboard side of the Cape York segment of Endeavour Crater's western rim. Image credit: NASA/JPL/UA/MSSS/EduardoTesheiner

Even before Opportunity finished the circuit, the scientists had settled on a number of targets and called an end to the survey in mid-November. Both Kirkwood and Whitewater, or outcrops just like them, were on the list. So once the rover was about 15 to 20 meters (50-65 feet) away from its start-point at Whitewater Lake and surrounded by rocks that appear to be just like Whitewater Lake, the decision to dig in came easy.

Toward late November, Opportunity hunkered down in an area within what Arvidson calls the "sweet spot" for the clay minerals, near a Whitewater Lake-like rock, complete with little veins running through it and splotches of a weird coating or rind. "We're finally there," he told CuriousMars. "And I think we're sitting on them."

Whether the clay minerals are hiding in the coating or are a part of the rock itself is something the MER scientists don't yet know. "But we want to figure out how extensive this Whitewater Lake material is, because it's the one unit that is big enough to hold the phyllosilicate signature that we see from CRISM." said Arvidson. And the plot thickens. Maybe Opportunity has been roving over phyllosilicates for a long time.

In a Geophysical Research Letters paper in press, a team of MER scientists will report on the spectroscopic analysis they conducted of the rim and interior of Endeavour using CRISM data to help identify targets of interest for Opportunity. The team - including Murchie, Wray, Tim Parker and Fred John Calef of JPL, and first author Eldar Zeev Noe Dobrea of the Planetary Science Institute found that the spectral character of both the sulfates and the phyllosilicates in the area are "more diverse" than has been reported to date, with phyllosilicates present on the rim and interior of Endeavour as well as on the surrounding plains.

"It was the signature of clays that brought us to this place and there's every reason to believe that this is a really important and interesting problem," said Squyres, who is also serving as a member of the Sample Analysis At Mars (SAM) instrument team on Curiosity. "We're just going to have spend the time we're going to have to spend. We have a long to-do list here. I don't know what we're going to learn."

One critical item on the agenda is to figure out which rocks came first and from where. "Shoemaker is topographically on top of Kirkwood and Whitewater, so it's uphill from where the rover is now, which means it's younger - if it's a normal, stratigraphic sequence, like a pancake system," Arvidson began. "But, it could be that we're looking at the overturned flap of ejecta, so it's turned upside down."

That means Kirkwood and Whitewater may represent the original crust and Shoemaker Formation is the ejecta from Endeavour. "But, there might be multiple Kirkwood layers," added Arvidson. "We don't know yet and we have a long way to go in tracing the contacts. The evidence so far says either we have repeating sections, so [the rim] is structurally deformed, or the emplacement of Kirkwood-like materials happen periodically. How's that for complexity? We just don't have crisp answers to questions yet, because we're in the middle of all these investigations."

As for Curiosity stealing the spotlight, the MER team seems to understand that its MSL's turn in the Sun. "I use the sibling analogy," said John Callas, MER project manager at JPL, where all the American rovers were 'born.' "Opportunity is the like the older sibling who's watching from the sidelines when the new baby comes home from the hospital. The new baby gets all the attention and is lavished with cute gifts. That's what happens, and we're mature enough to know this happens in planetary exploration. Actually, we're excited with Curiosity's arrival on Mars, because it's not quite so lonely anymore."

The two missions are "now complementary," added Arvidson, because the grand goal is to understand Mars and its past history. "Opportunity is now exploring in an area we think has clay minerals with a restricted suite of instruments and what we learn will 'feed back' into campaigns with Curiosity."

By all accounts, the fact that Opportunity is still roving and in good shape - loss of instruments, an arthritic shoulder, and cranky right front wheel notwithstanding - is impressive. "There was never a pool, but some of the storied engineers here said if Opportunity lasts 6 minutes, it will last 6 months, and if it lasts 6 months, then it will last 6 years - and we all laughed - it'll never go that long," recalled John Grotzinger, an MER science team member and project scientist of the MSL/Curiosity mission.

One of those "storied engineers" was Jake Matijevic, for whom Matijevic Hill was named. Following his work as the manager for the Microrover Flight Experiment or the first American Mars rover, Sojourner, the microwave-sized robot delivered to the Red Planet's surface by Pathfinder in 1997. Jake was assigned to the original MER design team. He then led the MER engineering team for several years before and after their landings. A systems engineer and mathematician, he was the "go-to" guy for everything regarding Spirit's and Opportunity's systems and operations. He was the chief engineer for surface operations systems for Curiosity at the time of his unexpected death in August at the age of 64.

Jake often described Opportunity as "a perfect vehicle." The only "flaw," he recalled, was the heater that stuck in the 'on' position at the beginning of the mission, causing the rover to turn it off every night ever since. "Generally, we did all the right things and it never had the same problems Spirit had," he said. "But Opportunity did not have the rough terrain that Spirit faced at Gusev Crater. Meridiani was considered safer because it was flat, warmer, and less treacherous."

Spirit and Opportunity were designed and equipped to search for evidence of past water, and each of them did that, fulfilling their science objective in their 90-day primary mission. Now, with Opportunity on the hunt for the phyllosilicates, the surviving MER rover is moving beyond its official 'Follow the Water' directive and cruising to the next level of NASA's Mars Program architecture, 'Determine Habitability' - which, actually, is what Curiosity is assigned to do.

With a total price-tag now hovering around $925 million, MER has delivered, and from all indications has miles to go and so much more to contribute before it "sleeps."

As December took hold, Opportunity completed the picture taking for the Matijevic Pan and moved 23 centimeters (about 9 inches) to reach some new surface targets on its Sol 3151 (Dec. 4, 2012), beginning the wrap up of the investigation of this Whitewater Lake-like rock. One objective on the to-do list is to figure out just how extensive the Whitewater Lake is areally -- "because it's the one unit big enough to hold the smectite signature we're seeing from CRISM," as Arvidson put it.

In coming sols, MER science team members are considering having the rover bump up hill just a short way to check out Copper Cliff, an outcrop that could be "crazy like Kirkwood," said Arvidson, or it could be the breccia of the Shoemaker Formation. Either way, it may represent "the contact where Shoemaker ends and the Kirkwood / Whitewater stuff begins," he said.

Finding and defining the contact or boundaries between layers and rocks will be an important key to unlocking the mysteries Mars still holds here at Endeavour Crater. For now, said Squyres, "it's fun to be this confused this late in the mission."

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