Spirit's landing site more complex than first thought
Posted: January 7, 2004

More ultra-sharp views of the floor of Gusev Crater show a surprisingly rolling, rock-strewn landscape that bears little resemblance to the relatively smooth, wind-swept lakebed investigators thought they saw in initial, low-resolution images. Lakebed deposits may well be present, a mission scientist said today, but they may be more difficult to find than first thought.

"This is not your typical lakebed, if it's a lakebed," said Ray Arvidson, deputy principal investigator for the Mars Exploration Rover project. "A lakebed is typically flat with very fine-grain sediments. That's not what we're looking at. We're looking at a surface that's rock-strewn, a surface that probably has a number of secondary craters that have excavated rocks. So it's not a primary depositional surface of a lakebed you would see on your way, for example, from Los Angeles to Las Vegas.

"If there are lake sediments there, they've been chewed up and rocks have been brought in, either from the bottom or laterally by some set of processes. Our job, once (Spirit rolls off its lander and onto) the surface, is to use this integrated payload to find the evidence, among everything else that's been going on, as to whether or not, in fact, there were lacustrine, or lake, environments in the past there. I suspect we'll find it, but it's going to take a while."

Before such questions can be answered, however, Spirit must exit its lander and bring its instruments to bear on the martian surface. Officials said today roll off likely won't happen before next Wednesday and, depending on how things go, possibly later.

Rover checkout activities are going relatively smoothly, said mission manager Arthur Amador, but a few issues have cropped up to slow things down. One potential problem - current spikes in the motor used to elevate the rover's high-gain antenna - has been resolved. Engineers tested the operation of that motor yesterday and found no problems. They suspect a bit of debris in the motor drive train was responsible for the current spikes seen during deployment operations. They believe the antenna is in good health and ready for routine operations.

Another issue, however, requires more work. Airbag material bunched up under one of Spirit's landing petals poses a potential obstruction to the rover's preferred straight-ahead lander exit path. The lander's retraction system was operated overnight to pull the material in and help engineers determine if it was hung up on anything. Playing it safe, they planned to instruct Spirit's computer to lift the landing petal in question about 20 degrees late today and then retract the airbag some more. The procedure is known as a "lift-and-tuck" operation.

Engineers had planned to begin unfolding the rover's folded undercarriage today, using a screw jack system to push Spirit's body upward. That work likely will slip another day because of the airbag retraction work.

"We really do want nothing more than to get this puppy off of the lander," said Art Thompson, a lead engineer. "Right now, from an engineering perspective, we are a lander-centric mission. Our whole world is the lander that we're sitting on. It affects us thermally, it affects basically all the decisions we're making from an engineering perspective.

"But we are chomping at the bit to get this puppy off the lander and get the scientists working and actually get to drive this vehicle. Because that's really what we're hear for. At this point, if we continue with our nominal plan, we would expect to egress sometime around sol-12, possibly next Wednesday. So we're probably seven days away from getting this vehicle on the surface and getting it to drive."

In the meantime, Spirit's main Panoramic Camera will continue collecting frames that will be stitched together to form a full-color, high-resolution mosaic of the rover's landing site. Half of the frames needed to make the so-called minimum mission success mosaic have been shot and are loaded in the rover's computer memory awaiting downlink to Earth. Jim Bell, the camera's developer, said he hopes to have the complete panorama on the ground by late this weekend or early next week. The downlinking will be aided by the resumption of high-gain antenna operations.

Spirit and an identical rover, Opportunity, were built to find out how long water may have persisted on the surface of Mars in the distant past, a key factor in whether or not life ever evolved on the red planet. A huge channel cuts through the southern rim of Gusev Crater, where Spirit landed, and scientists believe the 100-mile-wide crater must have held a vast lake at some point.

Initial low-resolution navigation camera images of the landing site did not appear inconsistent with a dried-up lakebed. "I don't know if these are sedimentary rocks, if they're lava that's been deposited over it. But if you asked me ahead of time what's a dry lakebed on Mars going to look like, I'd have said a lot like this," principal investigator Steve Squyres said Sunday.

But as more and more high-resolution images come down from Spirit's Panoramic Camera, the rover's immediate vicinity looks less and less like an undisturbed lakebed.

"It looks like there was a lake at some point in Gusev Crater," Arvidson said. "The question is, is the landform we're seeing today the surface of that lakebed? And I suggest not.

"It could have had a lacustrine, or marine, environment, you could have material being brought in as deltaic deposits from the side, the very fine-grained material suspended forming these very finely layered, laminated sediment. The environment may have changed tectonically, Mars may have fractured, lavas may have come up and filled over the lakebeds and then everything stopped. Mars became dry, volcanism stopped.

"The cratering continued and you have these secondary and primary craters that are stirring things laterally," he said. "It may have produced the rocks that we see and then in addition, I think there's some deposition of wind-blown material. It looks to me and to some other team members like we have a desert pavement, kind of a classical, terrestrial desert pavement out there. We see that all across the deserts of the Earth. ... That's an area or set of areas where the cobbles are born on the surface, an old lava flow, for example, that's broken up, and the sand and the dust actually works its way underneath and the cobbles rise up and there's one or two meters of dust or silt underneath. In the southwest, a lot of the dust is buried beneath desert pavements. It's counter intuitive but it's been demonstrated by measurement and observation that that happens."

Other possible scenarios that might explain Spirit's landing site include glaciation and an inflow of rocky debris from catastrophic floods.

The panorama Spirit's pancam system is in the process of making will play a crucial role in determining where the rover goes once it drives off onto the surface. In an initial pancam "postcard" downlinked earlier this week, scientists were excited to see what appears to be a nearby secondary impact crater with exposed rock in its slightly elevated rim. But in part of the panorama downlinked overnight, their interest was piqued by hills to the southeast.

"We're looking out across the landscape at very high resolution, looking at a large number of rocks and soil deposits, out to those hills, those peaks in the distance," Bell said, describing the latest pictures. "Those hills are approximately two kilometers or so off in the distance. They're something like 50 to 100 meters high. They are obviously a potential drive target that many people on the science team are thinking about."

"We have our favorites and it's toward the southeast and it's not that far away," Arvidson said. "But all this is being debated. Realize, this is an excellent science team. But it's like herding a set of cats, they're all trained to be totally independent and vocal. But they are kind of moving in the direction of very detailed characterization of the scene before we go bopping off on a long traverse."

Before Spirit ventures more than a few feet away from the lander, however, its instruments will be trained on nearby rocks and soil to begin that characterization process as quickly as possible. Asked how long it might take the science team to find lakebed deposits, Arvidson said "it could be next week, or it could be longer."

"This is terrain that's both familiar and yet it's a little bit alien," he said. "We haven't seen a place like this in the two Viking sites and Pathfinder. We may be looking at an area that's volcanic terrain, hard volcanic rocks that have been kicked up by craters, both primaries and secondaries. We may be looking at deposits that are brought in by river systems. We really need to get down off the lander and digging around, making measurements of the rocks and the soils with this wonderful integrated payload that we have. ... Are we in a volcanic plain? Is there evidence of the ... lakebed deposits that may be kicked up and exposed at some scale? We need our engineer buddies to get us off of this lander and onto the surface."

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