"We have scored a 300-million-mile, interplanetary hole-in-one and we are actually inside a small impact crater!" Steve Squyres, lead scientist for the Mars Exploration Rover mission exclaimed yesterday of Opportunity's landing spot. "I don't know what the odds would be of hitting a crater like this, but it's just sensational."
Although it did appear from the very first images returned that the second Mars Exploration Rover had landed inside a small crater when it arrived at Meridiani Planum last Saturday night, that belief was confirmed with further analysis yesterday.
The air-bag enshrouded lander package carrying Opportunity did not stop in its upright position like Spirit did, but it immediately retracted its airbags and righted itself, as it was designed to do. Opportunity then deployed its solar arrays and began taking pictures of its surroundings, sending the first postcards home, just four hours after its touchdown.
Yesterday and this morning, Opportunity continued returning images of what Squyres and the other scientists are calling "a bizarre alien landscape." Today, the robot field geologist was slated to complete the 'mission success' 360-degree color, full-resolution PanCam panorama that will encompass the rover's surroundings all the way out to the horizon.
"I keep thinking this can't possibly get any better and it just does," Squyres said.
The real appeal is just how different this place looks from all the others sites on Mars where humankind has set down landers - with it vast gray-red-brown terrain of plains, and, most notably, an outcropping of bedrock.
"Every place we've been to before is littered with boulders, and loose debris carried in from some place else, by floods, or blown in," reminded Squyres. The color images recently returned from Opportunity reveal this site, however, to be geologically different, darker than the surface of either the Viking sites, the Pathfinder/Sojourner site, or at Gusev, where Opportunity's twin, Spirit, landed January 3. And this is the only place on Mars so far in which they have found bedrock.
"We knew going in that there were two fundamental geologic units here [in Meridiani Planum]; one of them is a thick sequence of layered rocks fairly light in tone," explained Squyres. "We don't know what they are. And, draped on top of that is a thin veneer or coating of what appears to be some kind of fine-grained material and that's the stuff we think contains the hematite. My fondest hope after looking at pictures from orbit before we landed was that we would land someplace where we would be close enough to a crater, that we would have a chance of traversing to it, and actually getting to the layered material."
As fate would have it, the crater in which Opportunity is currently resting is "roughly 20 meters in diameter," estimated Squyres, and "probably 2 meters" deep, meaning essentially that it shouldn't take much of an effort for Opportunity to be able to drive out of it. "And in the wall of the crater, is this exposed, light-color layered rock unit, the one we were hoping we could get to and its right there in front of us exposed in the walls of the crater . . . and then we have this fine-grained stuff, which [might] be hematite bearing unit all around us. And when you look off in the distance you can see that wonderful rock outcrop. We have both of those geologic units right here, within 10 meters of where we are. In this site alone, can see ejecta and into crater itself, we can see deeper into Mars. So in principle, we could spend most of the mission just in this little crater and do wonderful things. But there's much better stuff to be had out there."
In fact, the bedrock, which was visible in the very first black and white images Opportunity sent home, is the geological formation that immediately gripped the scientists for it is the Holy Grail for planetary scientists.
"The thing about bedrock is you know where it came from," Squyres explained. "We knew going into Gusev pretty messed up placed geologically . . there were a lot of processes that really mixed things up. We see rocks there, but we don't really know where they came from - many were probably carried there, by water or lava that flowed up. It's going to be tough puzzle to sort out what happened at Gusev. But the beauty of bedrock is you know this is home neighborhood of these rocks. That's just a tremendously valuable thing. Instead of being busted up into pieces, you've actually got these layers . . . and bedrock is wonderful for trying to unravel the geologic history."
While Squyres wasn't will to hazard a guess as to whether the outcropping was formed by sedimenting accumulation or by the accumulation of volcanic rock, Opportunity has the tools to find out.
The site offered such a welcoming vista that the scientists have all but figured out the route they send Opportunity on. "The way I envision this mission going is this: we'll drive off lander, look at the soil, investigate this hematite material. We'll then go to the layered rock material and explore it in some detail because it's right there in front of us ready ripe for the picking. We'll look at that carefully and we will understand that geologic unit. Then we climb out of crater, take a look around, and then we head to the big [crater.] And it's going to be a wonderful adventure."
Squyres, however, admitted today that Meridiani features "a number of tempting targets" to which Opportunity can go after roving out of the crater it's in. "We are eagerly awaiting for Mike Malin to take some better images of where we are -and after taking a good look at the soil and the outcrop, we'll make a decision.
The reason the MER team and NASA chose Meridiani Planum, however, was because of the enormously large deposit of what the thermal emission spectrometer (TES) onboard Mars Global Surveyor revealed to be hematite a crystalline mineral that usually forms in the presence of water - or, perhaps something unknown right now that has the same signature as hematite. And finding evidence of past (or present) water is the prime directive for both Spirit and Opportunity, which are charged with finding the evidence necessary to determine whether Gusev or Meridiani Planum ever held water and were life sustainable.
Hematite is an iron oxide. "The material [pictured at Meridiani Planum] is quite a bit darker and pretty red," said Doug Ming, an expert on mineralogy of Martian soils, from the Johnson Space Center. "But it only takes a few percent of this hematite to make these soils this color. If you drive through Texas or Oklahoma or Georgia, the primary mineral found in the red soil [in those places] is this mineral hematite."
While the scientists are debating as many as 10 different hypotheses for how this hematite may have formed at Meridiani, three have moved to the top of the list. "One of the leading candidates is that this hematite mineral formed in an aqueous environment, such as a lake," Ming explained. "In these particular environments, the iron hydrolyzes essentially, as hydroxyls begin to agglomerate around the iron and settle down to the bottom of this lake material. Then, over a period of time, [through] metamorphic processes, we get into transforming these iron oxyhydroxides to this mineral hematite. So this might be a reason we see we might see the plating nature of this hematite from the MGS test data," he suggested.
The second potential way the hematite could have formed at Meridiani is by hydrothermal alteration of basaltic glass. "This is quite common here on Earth," informed Ming. "One analog is on the big island Hawaii, on the top of Mauna Kea -- a volcano. [T]here we essentially have water that is interacting with basaltic glass materials. The basaltic glass begins to hydrate and forms a material called palagonite, kind of a vague term, but essentially it's a hydrated glass, and one of the byproducts of that process is nanophase iron oxides. In particular the iron oxide that generally we find in these is hematite. So [the hematite at Meridiani] could have formed by hydrolytic weathering of these basalt materials or under hydrothermal processes."
The third potential way the hematite could have formed is by oxidation of a mineral called magnetite in basalt and lavas. "This could happen by thermal oxidation, where it's heated up and the magnetite then oxidizes and forms hematite," Ming explained.
Either of the first two hypotheses -- both of which involve water -- "would lead you to sustainability," noted Squyres. "If we had some hydrothermal activity, that would indicate that as well. The beauty is we have series of well-defined hypotheses and tools necessary to prove those hypothesize."
"The payload we have [the set of scientific instruments onboard Opportunity] has the unique capabilities to identify these particular phases we might encounter - and not only hematite, but the phases that might occur with it," added Ming. "For example, if we would find the mineral goethite -- an iron oxide hydroxide -- that would be almost a slam-dunk for an aqueous process in a lake environment."
"We don't know for sure if this soil is the hematite-bearing unit," Squyres said. "If you look at craters in this area -- gray areas and dark floor -- the stuff we're on is the dark crater floor material and once we get out, and into terrain we're on something different. We might not find hematite until we climb out [of the crater] and get onto terrain. But Mini-TES will be able to tell us before we get off, and once we get out there [and onto it] the Mössbauer will confirm it."
The big news of today, Squyres reported, was that the Mössbauer on Opportunity had tested out "perfectly." This spectrometer had suffered a glitch during the cruise phase of the mission. "The belief now is that it had something to do with the environment of cruise, the weightless environment. But when we got onto surface and in environment for which instrument was designed, it's perfect."
In addition, Arthur Amador reported that "Opportunity continues to send telemetry that indicates it's in excellent health, with all systems "reporting green across the board." The mobility team has indicated that egress looks good "in the straight ahead direction." The preliminary assessment of the deck height in that forward direction, he said, is between 34-41 centimeters, within the rovers' 'hopping off' capability.
In addition to completing its 'mission success' panorama today, Opportunity will undergo assessments of the engineering and science data received from the overnight and morning in UHF passes. For now, Amador said, "everything's looking really good and we're really looking for to the impact to egress phase."
For now, however, the plan is to take it easy and play it safe with Opportunity until the Anomaly Team repairs Spirit's software glitch.
Although Opportunity's front egress path looks good, the second rover will probably follow a similar pace to its alpha twin - and egress sometime toward the end of second week. "We're going to be taking it slow and carefully," said Mission Manager Jim Erickson, "and we'll be getting off lander when it's time to get off lander.”