A.J.S. RaylFeb 20, 2004

Mars Exploration Rovers Update: Spirit To 'Trench' Laguna Hollow; Opportunity Uncovers Shiny Spherules, Takes on El Capitan

The rovers have been kicking up dirt and roving to new destinations this week, as they pick up the pace and take care of their geological business on Mars.

As Opportunity inspected the hole she dug Monday in Meridiani Planum, uncovering some more mysterious "shiny" spherules, the rover also set her sights on the next target -- El Capitan. On the other side of the planet, at Gusev Crater, Spirit "wiggled" her wheels at Laguna Hollow and prepared to dig a little trench of her own in the circular depression.

From Gusev Crater

For the last couple of weeks, Spirit has been roving toward Bonneville Crater, a large crater spotted right after the rover landed January 4, and making stops at chosen targets along the way.

The robot field geologist completed her study of Halo Wednesday, during her Sol 45. After collecting measurements with the alpha particle x-ray spectrometer (APXS), the Mössbauer spectrometer, miniature thermal emission spectrometer (mini-TES), and taking images with the microscopic imager (MI) and the panoramic camera (Pan Cam) the rover stowed her arm for the northeast drive toward her present location, a circular depression the scientists dubbed Laguna Hollow.

"We're at interesting point within Gusev Crater, in the sense that we're now well into what we call a geologic traverse from the landing site up to the rim of [Bonneville] crater," Dave Des Marais, a rover science team member from NASA Ames Research Center, said at a Mars Exploration Rovers (MER) news briefing held yesterday at the Jet Propulsion Laboratory (JPL). "As we do this traverse, there are segments of the landscape we cannot see from the place from which we started," he said. "Just within the last two sols, we've come over sort of a break in the slope."

Once there, the view exposed Laguna Hollow in the foreground, a depression that has fine-grained material and in the background a rock field with "a whole bunch of interesting targets," as Des Marais put it.

The first 19 meters of the drive toward Laguna Hollow was commanded using 'go-to waypoint' commands, with the hazard avoidance system turned off. This mode - used for the first time during this sol -- provides automatic heading correction during a blind drive. That day, the rover put another 74.5 feet (22.7 meters) on her wheels.

Laguna Hollow drew the MER science team, because it features a fine-grained deposit that may be able to reveal something new about the dust in the Martian atmosphere and, "potentially to the salts and other fine-grained materials that might be there," Des Marais said.

"In going to such a place where you see a lot of fine-grained material and not too many rocks, we hope to get a more pure concentrate of this and, therefore, understand more of how it's made," he continued. "So when we drove into this hollow, we thought we'd do a little trick with the wheels, then back up and turn, so we could analyze it more closely with the IDD."

According to plan, Spirit "wiggled" her wheels to perturb that fine dust-like soil there, then adjusted her position in order to put the perturbed area in reach of her arm, and conducted a mini-TES scan.

The scientists also quickly noticed that some of the soil in Laguna Hollow stuck to the rover's wheels. "That was interesting," said Des Marais. "The surface layer seemed to be rather different from the layers underneath it -- it tended to stick to the wheel. Immediately you say -- why is this surface material potentially so sticky? It could be that it is a very fine-grained dust. Fine dust can be coherent when it's compressed. But it could also have salt in it or, for that matter, a brine or a little bit of water to give it moisture. In any case, we're interested in why that surface material is sticking to the wheels."

Des Marais was quick to say the team does not expect to dig deeper and find a puddle of water, but "it wouldn't take much water, just enough to moisten the soil."

Pictures of the surface of the trench there also show pebbles arranged in clusters or lines around lighter patches that Des Marais described as "miniature hollows." Such patterns do have Earth analogues.

"It could be similar to what we see on the Earth of what we call patterned ground - a situation where you have a very fine-grained surface layer that, for whatever reason, expands and contracts, and as it does that periodically it forms these polygon type cracks," Des Marais elaborated. "You can get that with freeze-thaw type activity at higher latitudes, such as in tundra. You can also get that in a salt flat where the salt by warming or by wetting or drying expands and contracts and forms a very characteristic polygon pattern. You can do it with mud cracks in a mudflat -it gets wet and then when it dries these cracks form."

To find out whether they were looking at evidence of very fine-grained material or evidence of salt, the team decided to dig a trench. "If we're looking at salt that's moving up and down with the assistance of water, we might expect to see a concentration of salt near the surface, and as we go deeper perhaps less of a concentration," Des Marais explained. "This might be due to a brine moving to the surface, and as the water evaporates the salt becomes concentrated there, at the surface. By doing this profile, we hope to understand a little more perhaps about the history of this deposit and perhaps more about the details of its composition."

During her Sol 46, which ended yesterday at 11:17 a.m. PST, Spirit marked the halfway milestone in her primary surface mission, Sols 2 through 91. The rover started her day by conducting some remote sensing experiments of the big crater rim and imaging the surrounding soil with her panoramic camera (Pan Cam) and miniature thermal emission spectrometer (mini-TES). After completing these tasks, the rover took a break with a nap lasting slightly more than an hour.

After waking, Spirit continued her observations of the ground and sky with the mini-TES. At about 1:34 p.m. Gusev Crater time, the rover was analyzing a patch of the atmosphere with the mini-TES as, simultaneously, Mars Global Surveyor's thermal emission spectrometer (TES) was looking down through the same section of atmosphere. This synchronized observation -- similar to the two observations the rover previously conducted with the European Space Agency's Mars Express -- will allow scientists to gain a more thorough understanding of Martian atmospheric conditions.

Spirit was slated to take stereo microscopic images of the target Trout in Laguna Hollow yesterday afternoon -- the first time the microscopic imager (MI) will take pictures at Gusev Crater without the Mössbauer instrument first touching the surface of the soil. The observation will provide pictures of undisturbed soil. After that, the rover was to be commanded to perform a calibration activity by imaging a location in the sky with the MI and the navigation camera simultaneously to be followed by an overnight Mössbauer spectrometer integration.

After a brief sleep, Spirit was to wake at about 2:00 a.m. Gusev Crater time on her Sol 47 to end the integration, collect the data and turn on the arm heaters. Then, she was to change the tool from the Mössbauer to the alpha particle x-ray spectrometer (APXS), and begin observations with that instrument. Finally, the rover will leave the APXS powered on and go back to sleep around 2:30 a.m. The robot geologist will also gather APXS and collect that data, as well as perform some early mini-TES soil properties observations.

Once those assignments are completed and the trenching and final examination of Laguna Hollow is done, Spirit will continue her journey to Bonneville Crater, now estimated to be about 443 feet (135 meters) away. With 420 feet (128 meters) on her odometer, the rover is about to mark another half-way point.

"What's exciting about this is that as we [get closer], we will see a change in the materials - we are clearly now entering closer into the domain into the impact and the material that's been thrown out from that impact, and some of that material will be from the subsurface," pointed out Des Marais. "We'll be doing stratigraphy by traversing towards a crater and seeing the deposits that were perhaps more deeply placed and have been brought to the surface by the impact."

At last report, Spirit was to spend her day today finishing the APXS recording, collect that data, perform some early mini-TES soil properties observations, and then to dig a trench. The scientists hope that by peering underneath the soil in this hollow they may be able to answer questions about what caused the distinctive traits on the soil surface, thereby adding another piece to the puzzle of what happened long ago at Gusev Crater, Mars.

From Meridiani Planum

As Opportunity took a good look with the MI and APXS at the walls and floor of the hole she dug on Monday inside the small crater in which she landed, she uncovered more alien surprises.

"When we looked at the soil at Meridiani Planum, we initially recognized we had two different materials that we were dealing with, at least," recounted Steve Squyres, Cornell University, lead scientist for the science instruments. "One is the very fine-grained stuff, sand and even finer grained than that. The other is the really coarse-grained stuff, the things that are millimeters in size that we're calling granules, some of which are spherical -- what we call spherules, and what are also referred to as blueberries. The question was -- what's the distribution of these things within the soil? At the surface it's easy, you look around with Pan Cam and see what's there . . . but we had no idea what lay underneath. So that's one of the pieces of the puzzle we wanted to go after."

To get a look below the surface, Opportunity dug a trench that is about 20 inches (50 centimeters) long by 8 inches (20 centimeters) wide by 4 inches (10 centimeters) deep, and then used her arm to do "a complicated little dance," as Squyres described it, to acquire microscopic images and other data.

Opportunity manipulated her robotic arm to take pictures with the MI at five different locations within the trench, and to take spectrometer readings of two sites, one at the bottom and one along the wall. "We've given the arm a very strenuous workout," informed Eric Baumgartner, JPL, lead engineer for the rover's arm.

The rover's -- and her handlers' -- ability to so accurately place the instruments at times seems nothing less than remarkable. Of course the rovers are state of the art robots. "The accuracy of this system is unparalleled for Mars, and for probably even terrestrial autonomous robotics systems," said Baumgartner. "We've been able to place the instruments to within a level of accuracy of 5 millimeters, less than a quarter of an inch . . . and in terms of repeatability - if we move it away and bring it back down to the surface -- we can do that to within less than a third of a millimeter or 1/100th of an inch."

The scientific peeks into the interior of Mars revealed more findings. "What's underneath is different than what's at the immediate surface," said Albert Yen, JPL, MER science team member, as he presented an MI picture of the crater floor. "You can see some of the spherules, likely coming from the outcrop, and other granules that could be shedding from higher elevations above, and they're in the field of view in this sand-sized matrix," he continued. "The fact that you can see the sand-sized particles in this image show that they're about 100 microns or so in dimension. And, that's entirely consistent with them being brought in by the wind."

An MI picture of a part of the 'wall' of the trench revealed more of the strange spherules, embedded in the matrix of a fine-grained soil, so fine-grained in some places that the instrument couldn't resolve the individual particles. "What's interesting here is that the spherules appear polished, shiny." Although the team is not quite sure what that means, it could be an effect caused by environmental processes, Yen said later.

In another MI picture, they found that the sand-sized grains were 'cemented' together. "An idea that has been around since Viking time is that water vapor exchanging between the atmosphere and the subsurface can mobilize salts in the upper few centimeters or more of the surface, depositing a weak cement that can hold these grains together and may be what we're seeing," suggested Yen.

When they get the APXS down and get that data back, you can bet team members will be looking carefully for signs of any salt products that might be holding these grains together. And, once all the data from the APXS and the Mössbauer spectrometer are analyzed, scientists will be able to determine what elements and what iron-bearing minerals are present. Then, the differences between the subsurface and the surface will be easier to interpret, Yen said.

For now, "the bottom line," Yen said, "is that what's underneath is different from what is at the immediate surface, and this trenching has exposed a region that is completely different."

"One thing that may be giving us a clue here about what is going on with the materials is that when you look at the concentration of the granules -- the coarse grains that are way too big to be transported by being lofted by the wind, you see a high concentration of those at the surface and then a significantly lower concentration down below," Squyres offered.

"What might be going on here at least in the floor of the crater [is that] you could have had a process in the past that created a mixture of the very fine-grains with a small number of these granules embedded in it," he continued. "Then as the wind blows across, two things will happen -- sand sized stuff that bounces along the surface can be transported in from elsewhere and will just form a thin layer at the surface. The other fine-grained stuff can get lofted into the atmosphere and completely be carried away - gone," he explained.

"As that takes place, the surface will go down and down and down, and those granules -- the coarser grained stuff, the millimeter-sized stuff -- will get concentrated at the surface, because the fine stuff can blow away and the coarse stuff can't," Squyres continued. "And you'll get what geologists call a lag deposit at the surface. So what we may be seeing at the surface is a combination of a lag deposit of granules, plus a small amount of sand that's been blown in horizontally from someplace else," he suggested.

Time -- and findings from new targets -- will no doubt reveal more clues in coming sols.

The rovers are now operating it would appear on the assumption that there is no time for a hiatus of any kind. Consider that as Opportunity was digging and examining her trench this week, this robot field geologist was also transmitting some high-resolution pictures and information -- data taken from her survey last week of the rock outcrop along the inner wall of the small crater where she landed and is now working, so that the science team could decide where to go next.

Based on the outcrop survey, the team chose a feature they call El Capitan. "We've planned our assault on the outcrop," said Squyres.

The science team is hoping that Opportunity's investigation of the outcrop will answer three questions, Squyres said:

El Capitan -- which is about 30 feet (9 meters) away -- was a natural choice of targets, because it's a place where the outcrop is exposed "in virtually in its entire stratigraphic section - the whole stack of rocks seems to be well exposed here," said Squyres. "If you just look at it, eyeballing it, you get a sense that there are different kinds of materials here. The lower portion is gently sloped and you can see layers exposed very finely in it and then the upper portion is much steeper - and in fact it's overhanging in places. There's a sense that the rock has weathered differently in different places. So one of the nice things about going to El Capitan is that by going to this particular location from a single rover parking spot is that we can reach both the lower and upper unit with the arm."

The team hopes to have Opportunity use her arm and all its capabilities to start to address those three questions.

"We want to zero in on a place where there's lot of those spherical granules and look at them in detail with MI. We want to grind into the interior of the rock to find out what the composition is," said Squyres.

"You'll recall a number of sols ago, down at the end of the outcrop, we did sort of a real quick and dirty reconnaissance of the arm at the place we call Stone Mountain, and we saw some of the spherules that were weathering out of the outcrop there," Squyres continued. "We made a measurement with the APXS that showed there was an enormous amount of sulfur on the surface. The APXS doesn't see deeply into the rock, so what we want to do is grind away the outer layers with the RAT, and put the APXS in there to see if it's sulfur rich all the way down. Then, we'll use the Mössbauer to see what minerals were there. So, we're going to do all of that on both the upper and lower units at El Capitan."

The plan calls for Opportunity to make a few intentional "stutter steps" on her way to El Capitan, stopping to take a few front hazard avoidance camera images and navigation camera images to plan for final approach and robotic arm activities. The rover will stop about 6 or 7 feet (a couple of meters) short of her destination to take images with her PanCam, and gather science measurements with her mini-TES. The rover will then make a short, closer approach to El Capitan, to poise herself to use the RAT, and other instruments.

1 Billion Bits And Counting

By all accounts and accomplishments so far, the MER mission has already achieved success status. Together, Spirit and Opportunity have returned 10 gigabits -- or one billion bits -- of good data. That's enough data to keep many scientists busy for years to come.

Most of the rover's science data is being returned to Earth through a network of orbiters at the Red Planet -- primarily NASA's Mars Global Surveyor (MGS) and Mars Odyssey, and also including ESA's Mars Express, which has returned data collected in collaborative experiments with Spirit.

"Since we can reach the orbiters at almost satellite-speed of 250 kilobits per second, this transfer of data is much more efficient," telecommunication system engineer Andrea Barbieri, JPL, pointed out.

By sending data the short distance to the orbiters, the task of getting the data from orbit back to the DSN then falls to those orbiters. "This is much more efficient, because the orbiters have larger solar arrays and larger antenna [and] are not so energy and bandwidth constrained as the rovers," he said.

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