The Mars Exploration Rovers are each entering new chapters in their extended missions that are already returning more intriguing discoveries, but they are both beginning to slow down now as the Martian winter closes in on them.
Spirit, now within a stone's throw of the Columbia Hills, has uncovered more evidence of past water activity at Gusev Crater in the form of salty sulfates. "We have found more evidence of salts, more evidence for the action of water -- much more compelling evidence than we have found anywhere else at Gusev," rover lead scientist Steve Squyres, of Cornell University, announced at a press conference yesterday, held at the Jet Propulsion Laboratory (JPL) where the two robots were designed and built.
On the other side of Mars, at Meridiani Planum, Opportunity is preparing to rove about 20 feet or so into Endurance Crater to explore exposed layered rock that should give scientists a better idea of what Mars was like farther back in the past. "You look down the slope, the stuff's right there," Squyres said as he presented an image of the crater taken by Opportunity with her panorama camera (PanCam). "Not only can we get to this [band of] rock, we can look at the story it has to tell about the environment . . . and actually read the record of environmental change on Mars."
"Inside the Endurance Crater waits the possibility for the most compelling science investigations Opportunity could add to what it has already accomplished," added JPL's Firouz Naderi, manager of NASA's Mars Exploration Program.
Although both rovers are in "great shape," according to Jim Erickson, deputy project manager for the rovers, "the lower solar power that we're getting right now is leading to leading to a sort of stand-down in how much we do in operations both scientifically and how much driving we can do."
Spirit, which launched one year ago tomorrow, has driven more than 2 miles [3.2 kilometers] inside the vast Gusev Crater region that she has been exploring since January. A trench the rover dug last month exposed soil with relatively high levels of sulfur and magnesium, said Johannes Brueckner, of Max-Planck-Institut fuer Chemie, Mainz, Germany. Spirit's alpha particle X-ray spectrometer (APXS) -- an instrument that measures the elemental chemistry -- showed concentrations of these two elements varied in parallel at different locations in the trench, suggesting they may be paired as a magnesium sulfate salt.
"The most likely explanation is water percolated through the subsurface and dissolved out minerals," Squyres added. "As the water evaporated near the surface, it left concentrated salts behind. I'm not talking about a standing body of water like we saw signs of at Eagle Crater, but we also have an emerging story of subsurface water at Gusev."
The strategy now, Squyres said, "is to continue on a sort of regular basis to dig trenches and make measurements like the one we made with the APXS and especially to do it once we get up into the hills. Getting up into the hills and digging some trenches there is going to be very revealing as well. So, it's an unfolding story -- this most recent [trench] is our most compelling one yet, but we still have a ways to go before we understand this completely. The bottom line is that this is clear evidence that water has played a role."
The rover has also returned data that hints of bedrock in the Columbia Hills. "We found [bedrock] at Eagle Crater and we found that at Endurance Crater and we're seeing that now in the Columbia Hills," said Squyres. Bedrock, of course, preserves evidence of what happened at a given location in the past, "because you know the rock came from that place," noted Squyres. "We're certainly seeing different materials there than we saw on the plains, but it's way too early now to know what story they have to tell."
Opportunity, meanwhile, has been studying the rim of the stadium-sized Endurance Crater since mid-May and returned various observations of the depression that the MER team has been scrutinizing to evaluate potential science benefits of entering the crater and the traversability of its inner slopes.
Although the rover was initially directed to traverse the entire rim of Endurance, she was ordered last week to turn around and return to Karatepe, the area on the southern rim featuring the gentlest slope into and out of Endurance, and one of the first areas at the rim she studied. That signaled the media and those following the mission that something was up, and sure enough, the current powers-that-be at NASA announced late last Friday that they were giving the green-light for Opportunity to rove into Endurance Crater. The MER team is maintaining full confidence that Opportunity will have no troubles completing her task inside the crater, but there is a possibility that the rover won't be able to get back out.
"We'll take an incremental approach, edging our way down to the target," Richard Cook, project manager for the rovers said last week. That plan was set in motion yesterday morning. In fact, as the press conference was underway, Opportunity was testing the slope so to speak, driving into the crater just far enough to get all six wheels in. Then, she was to immediately back out and up onto the rim so that she and the engineering team could look at her tracks and check her traction.
"We've decided on a very cautious, straight-in straight-out approach designed to minimize the risk," said Erickson. "As we speak the rover is actually cresting the hill, going down into the crater and after a few remote sensing observations is going to back right out. We'll take a good hard look at where we actually drove on the way out and make sure that we understand exactly what happened to the rocks and soil that the crater has and that the wheels of the rover actually came into contact with." Barring any surprises or untoward events, Opportunity will enter the stadium-sized crater "about Tuesday," he added, for two to three weeks of scientific studies.
The strategy for driving on the crater's inner slope is to keep wheels on rockier surfaces instead of sand, explained rover-mobility engineer Randy Lindemann. The team ran trials with a test rover on a "tiltable platform" specifically built to simulate Karatepe's surface conditions, complete with flat stones, sand, and some pseudo 'blueberries,' the tiny spherules that bear hematite. "The tests indicate we have a substantial margin of safety for going up a rocky slope of 25 degrees," Lindemann said. Opportunity's observations from the rim at the top of the planned entry route show a slope of "slightly less than 20 degrees," according to Erickson.
For Opportunity, the trip in and out of the 140-yard-diameter [130-meter-diameter] crater is all about traction. "When you talk about risk, our concern was only the ability of the rover to drive back out again," elaborated Lindemann. "We were not concerned about the slopes and conditions that we knew we were trying to going over. We were not concerned about rover flipping over or the rover sliding uncontrollably down in the crater or anything like that. This was a direct assessment of the traction capability of rover more than anything else."
The target for inspection within the crater -- which is only about 16 to 23 feet [5 to 7 meters] into the crater, according to Cook -- is an exposure of rock layers that lie beneath a layer that corresponds to rocks Opportunity previously examined in the shallower Eagle Crater, where she fortuitously bounced to a stop January 24. Once inside Endurance, Opportunity will use the tools on her robotic arm to analyze the exposed layers for several days, and then drive in reverse, backing up and out of the crater. Reaching lower-priority targets, such as various wind ripples at the bottom of the crater, would entail driving on sand, presenting a greater risk of not getting out again.
The sulfur-rich rock layer that Opportunity found in Eagle Crater yielded evidence earlier this year that a body of gently flowing water once covered the area, an environment that may have been habitable for life. "We've read the last chapter, the record of the final gasps of an evaporating body of water," Squyres noted previously. "What came before? It could have been a deep-water environment. It could have been sand dunes. It could have been a volcano. Whatever we learn about that earlier period will help us interpret the upper layer's evidence for a wet environment and understand how the environment changed."
Opportunity's observations from the rim of the crater already have shown the composition of this deeper layer of rocks differs from the layers studied at Eagle Crater. "If there was a change in rock type, there was a change in environment," Squyres explained. "This unit will tell us what came before the salty water environment the Eagle Crater unit told us about. We want to get to the contact between the two units to see how the environment changed. Is it gradual? Is it abrupt? Even if the lower layers formed under dry conditions, they may have been exposed to water later." The water's effect on these different layers could have left telltale evidence of that interaction.
"The biggest risk that we've got here is the risk that we'll get stuck in candy store for the rest of the mission," said Squyres matter-of-factly. "It's a good place to be. The fundamental thing that we would lose if we were not to be able to get out of the crater is we would lose the opportunity to travel kilometers to the south to in search of the very same stuff we know we can get here That's why I feel so good about taking this risk -- if you want to call a risk - it's a pure win-win situation. We can't lose."
The rovers' combined findings are slowly beginning to paint a "fascinating" picture that is beginning to "connect together" the stories of Gusev and Meridiani Planum, Squyres told the assembled throng of reporters at JPL yesterday. "At Gusev, we have evidence for percolation of water, maybe a very small amount of water, through the sub-substance. Now this is speculation, but what we're seeing at Meridiani may be a similar sort of process, one that extended to a much greater degree -- lots of water flowing through the subsurface, dissolving out lots of sulfates, salts, iron, magnesium, iron sulfates, calcium sulfates, all kinds of stuff that comes up to the surface. Instead of just a tiny bit of [water] evaporating on the surface, it comes up and actually blows out across the surface and you've got a lake or sea or puddle. You have flowing currents of water causing the ripples, and you've got precipitation of all kinds of salts because you've got lots of water and it's dissolved lots of things out of the rocks. Then it goes away and instead of leaving a little rind of magnesium sulfate salts below the surface, you actually build up layers of salty sulfate rock which is what [we've seen] at Endurance Crater."