A.J.S. Rayl • Feb 05, 2004
Mars Exploration Rovers Update: Spirit Suffers Minor Set-Back
Opportunity Finds Features "Unlike Anything Seen on Mars" and Begins Trek to Outcrop
It's been another week unlike any other week on Mars.
Opportunity has examined a patch of soil up close at Meridiani Planum, discovering what lead scientist Steve Squyres called "features unlike anything that's ever been seen on Mars before." And, earlier today, passed her driver's test, taking a 10-foot cruise toward the rock outcrop at the rim of the crater in which she came to rest on January 24.
On the other side of the planet, at Gusev Crater, Spirit suffered a slight setback, delaying her return to work until today.
Opportunity at Meridiani Planum
Now that Opportunity has successfully rolled off the lander and tested all her instruments, the investigation of Meridiani has really gotten underway. Earlier this week, the second Mars Exploration Rover examined, up close, a patch of soil in the small crater in which she landed -- a tiny spot in a 3-centimeter by 3-centimeter square -- is located just in front her.
Opportunity moved her arm into position, rotating the turret to aim the microscopic imager (MI) down toward the patch. The MI was designed to emulate a geologist's hand lens -- "a 10-power hand lens that geologists take out into the field to look at rocks and soils up close," says Ken Herkenhoff, lead scientists for the instrument. But, unlike the standard geologist's field lens, this one has a camera. The MI was the last of 20 cameras -- each rover is equipped with 10 -- to be used on the mission.
From the images, which Opportunity sent home Tuesday, the team was able to distinguish at least three components in the soil there: a very fine-grained type of red sand; particles that appear to be a form of basaltic sand; and a couple of strikingly spherical pebbles that immediately grabbed their attention.
"We knew from NavCam [navigation camera] images that we were dealing was a soil that had two different kinds of materials in it at least, maybe more," said Squyres. "We saw a bunch of really fine-grained stuff that we couldn't tell much about. We saw these coarser grains on top -- and when we first looked at it, it looked sort of like gravel. Then, as we took a closer look, we noticed some of these grains look awfully darn round. . . . every characteristic -- a shape, a texture, a color -- has a story to tell about where that material came from and how it formed," he noted.
Those 'stories' are exactly what Opportunity is charged with finding.
The color, shape, or texture of a particle or material is not enough to reveal origin with certainty, but distinctive characteristics can lead scientists to certain basic knowledge and down certain hypothetical roads. In the case of this Martian soil sample, "[t]he variety of shapes and colors indicates we're having particles brought in from a variety of sources," noted Herkenhoff, who is a member of the U.S. Geological Survey's Astrogeology Team in Flagstaff, Arizona.
As for the unusually round spheres in the soil, "a number of straightforward geological processes can yield round shapes," said Hap McSween, a rover science team member from the University of Tennessee, Knoxville.
One process is accretion under water -- "the idea that grains on the sea floor or in living water roll around and sometimes they accrete or grow by adding layers of material," he explained McSween. These grains -- called ooids -- are a distinctive type of sand that usually forms on sea floors. In the process of being buried under other sediments, these ooids can fuse into a solid mass of sedimentary rock called oolite. If these round spheres are oolites, that would be a major finding, for the simple reason it would mean there was once a large body of water at Meridiani.
"We got excited by the possibility we might have found oolites, but, McSween said, detectable 'holes' in the particles imaged in the Martian soil would seem to indicate that the alternative possibilities of meteor impacts or volcanic eruptions are more likely origins of these teeny round balls.
"Large meteors, when they impact the surface of a planet, melt some of the target rock and this material is sprayed out as a fine jet of little droplets of liquid and these droplets, as fly through the air, develop aerodynamic shapes, intriguing shapes -- like dumbbells, and teardrops, sometimes buttons," McSween explained. "I have seen all three of those shapes in looking at some of these microscopic images. Then they cool quickly into a glass, but if the target rock has water in it, that may form gas and that could [produce] the holes we see."
The notion that these round spheres emanated from erupting volcanoes is also plausible, McSween added. "Sometimes erupting volcanoes have fire fountains that spew out liquid droplets, and those could possibly be the things we see. But more likely, with violent eruptions you have ashes that are buoyed by hot gases, and as they are suspended they begin to coagulate together and they produce these little rounded pellets that we call lapilli," he explained. "I think it's possible that these things may be lapilli. It's also intriguing to think that many of these features may relate to the outcrop that we see and have paid so much attention to."
"We're looking for correlations now," Squyres confirmed. In the meantime, as those and other theories are being debated, the investigations, barely underway, continue.
"We don't quite know what these things are yet, but we are making some significant progress in narrowing down the possibilities, Squyres offered. "I'm interested in finding a place where there's as high as possible a concentration of these little pebbles, especially the round guys, and then slapping the Mössbauer spectrometer and the alpha particle X-ray Spectrometer (APXS) and the MI down and find out just what the pebbles are made of. We're also going to dig a hole -- in the next not too many sols -- we're going to drive to a spot and dig a hole with the wheels and then see what's below.
What they have discovered about that one small patch of soil is this: "There are several different components to the soil and they're distributed in uneven ways around the crater," summarized Squyres said.
"It's a little bit of a cliché, but I sometimes liken this process to the parable of the blind men and the elephant," Squyres continued. "We've just seen this first part of our elephant but there are many, many other parts to it. What we've seen here is one small part of the story. What we've got to do is fold that in with what we're going to see in other patches of soil, in that rock outcrop, and in that stuff above the outcrop -- there's a lot of stuff yet to come. We're seeing little bits and pieces of this mystery. It's going to take us a while to piece this together."
The team has also discovered that the ground immediately surrounding Opportunity in the crater is an area in which there is not a lot of hematite, the mineral that drew NASA to Meridiani Planum.
On Earth, this iron oxide usually forms in association with liquid water, so the hematite could well be the key to unlocking the greater mystery of whether Meridiani Planum was once a vast ocean or ever boasted a watery environment that could possibly sustain life.
On Monday -- the rover's tenth day at Meridiani, the robot geologist collected measurements with its mini-thermal emission spectrometer, known as Mini-TES. The colorful map produced from this data reveals the concentration of hematite within the crater, and is, Squyres noted, "the first mineral map ever done on another planet."
The soil patch examined with the microscopic imager, as it turns out, is in an area that is particularly low in hematite. The map indicates that there are higher hematite concentrations in a layer above the outcrop of bedrock and on the slope just under the outcrop.
"[We're] seeing a tremendous variation in the concentration of hematite at this site," said Squyres. "It is fairly low in that rock outcrop. It is very high in the material above the rock outcrop, and in some of material directly below it [and] … where we now sit, you see much less of the hematite. We know from this marvelous hematite concentration map that we're looking in a place where we don't expect to see much hematite. In order to see the hematite we're going to have to move, and that's what's coming next."
Interestingly, there appears to be no hematite in the bounce marks. These areas -- where the lander package bounced as it came to a stop after touching down - appeared to have been impacted and altered by the bounces in that the color of the soil seemed to change dramatically, and the marks left definite impressions. Somehow, the impact of the bounces "made the hematite disappear," Squyres said.
Since the team members have not yet looked at a bounce mark up close, they are "long on theory and short on facts," as Squyres put it. But one idea that seems to be popularly argued is that the hematite is carried in some of the coarse grains -- "maybe the really round grains" said Squyres -- and the impacts of the bounces effectively pushed the coarser grains down into and beneath the finer grained materials, in such a way that the Mössbauer spectrometer - the instrument used to identify the types and quantities of iron- bearing minerals in a target -- can no longer detect them.
Opportunity's Mössbauer measurements did find a strong signal in the soil patch for olivine, a common ingredient in volcanic rocks that Spirit also found in significant amounts in Gusev Crater. A few more days of analysis may be needed to determine whether any fainter signals found are actually from hematite, according to Franz Renz, a science team member from the University of Mainz, Germany.
Today, Opportunity struck out toward the outcrop, driving some 10 feet (3 meters), about halfway to the destination. "We're going to head toward the right hand side of outcrop, do some soil investigations, then go right up to the outcrop and work across it, shooting with the PanCam, right to left as we go," said Squyres.
The series of high-resolution images from the panorama camera, along with the data from the Mini-TES and Mössbauer spectrometers, will allow the scientists, "to really characterize this outcrop," Squyres added. "We're initially heading into a place that is hematite poor -- and then as we work across the face of the outcrop, we are going to be moving into materials that are progressively more and more rich in hematite. We're going to be seeing other pieces of this very complicated scientific elephant that we're trying to piece together here."
Exploring the outcrop -- the first outcrop ever found on Mars -- is a major expedition and the Opportunity science team has been preparing for it, by watching its flash memory files, the system that glitched on Spirit and caused that rover to break down about two weeks ago.
"One of the things the team has been doing is taking fewer pictures than we'd like to take, and taking fewer spectra, and leaving lots and lots of room in our flash memory, because when we get to that outcrop we're going to hammer on this thing with PanCam in a very big way," Squyres said. "We're going to take hundreds and hundreds of megabits worth of data on that outcrop and we're going to fill up that flash real quick."
If Opportunity's six-meter drive to the rocky outcrop is successful, the team will attempt a 'trenching' operation sometime next week. In that operation, ground controllers will command one front wheel to rotate to dig the hole, while the other five wheels are commanded to stay still. The moving wheel will spin and kick up the top soil, exposing the materials below the surface, and the rover will then image and examine the area. It will be another milestone for the MER mission, marking the first time any robot on Mars has used its wheels to scoop and spin out material from below the surface to examine the soil.
Spirit at Gusev Crater
On the other side of Mars, Spirit hit a bump on the road to recovery Monday when mission controllers sent commands for the rover to begin studying the rock Adirondack. The rover it seems was unable to get her bearings, so her 'electronic brain' automatically put every other command on hold.
"We were going to continue our arm operations on Adirondack, and take some microscopic images, and spectra," recounted Mark Adler, deputy mission manager. "Unfortunately, at the beginning of that day, we tried to do a 'Sun find' and that did not succeed ... and, as a result, the subsequent activities were not allowed to continue, because the vehicle was not certain what it's attitude was."
The team and Spirit recovered from that event relatively quickly, successfully completing a Sun find -- an exercise in which the rover orients itself -- later in the day. This latest failure, Adler said, "may be related" either to some activities that were ongoing on the spacecraft at the time, or to a file within the flash file system that was corrupted. "This may be related to the flash file system issue," he said.
Scientists believe too many files in the flash memory system and the way those files interacted with the random access memory (RAM) system caused the rover to abruptly stop work January 21. Spirit had briefly resumed science operations before the MER team once again halted the work to prepare for the intricate process of deleting old files from the rover's flash memory and reformatting the system.
On Tuesday, Spirit's Sol 31, the team carried out those preparations and sent the rover to bed early that night, canceling both overnight data relay passes. Spirit was allowed to 'sleep' through the night for her big day of flash memory erasing the reformatting yesterday, Adler said.
"The poor rover was woken up by its alarm clock at 6 a.m., much earlier than it's been woken up before," Adler noted. "We then rebooted into cripple mode that does not use the flash memory, so that we could do our operations."
During the four-hour process -- in which all 224 megabits of the flash memory used for the file system were deleted -- the team also checked on the flash hardware "to make sure all the hardware and all the flash chips are working," Adler added. "We're still not certain that there wasn't some hardware problem that contributed to what we found," Adler offered. "We don't think so but to be safe, we're going to make sure we check all the flash memory and make sure it's good."
The erase-and-reformat operation performed on Spirit is, Adler confirmed, "an operation you don't just do willy-nilly." In fact, the MER team spent four days testing the operation with a surrogate rover in the testbed.
"It's not an operation that we do lightly," Adler expounded. "We did go through a lot of testing to make sure that the operation [does] exactly what we expect. We reconstructed the environment in the testbed that the vehicle is going to be operating in, and we verified also that there are no other side effects that the flash format erase operation could have on the rest of the vehicle."
Spirit's flight software images, for example, are stored in another area of flash, separate from the flash file system. "We verified that that there's no way for those copies of the flight software to be corrupted in any way," Adler pointed out. In fact, he added, the sequence they developed has a checks and balances system that will abort the operation if it encounters a side effect or something else untoward. Provided the erasing-reformatting procedure of the rover's flash memory and subsequent rebooting went as tested and expected, Spirit should by now be restored to good-as-new condition.
This morning, Spirit was scheduled to get back to her study of the rock Adirondack, with the same agenda laid out for her last Monday. "We plan to put the RAT down and brush the rock, then take the microscopic imager and look at brushed area, then put one or both spectrometers down to get some measurements so we can see how much effect the brushing had," said Adler.
If all that goes well today, said Adler, "we'll do a full 'RAT' and grind operation" on Friday, "and then again image it with MI and [conduct] spectrometer operations." This will mark the first time either of the two rovers has used its rock abrasion tool -- affectionately known as the RAT -- which grinds away the outer layer of rock to allow scientists to peer at what's inside. The process on Adirondack, Adler said, "might take a few hours."
By Saturday, Spirit will probably hit the road and stay on that road for a good long while, Adler added. "We'll probably [go out] around the north side of the lander to get into a position where we can do a straight shot toward Bonneville Crater," he said. "That's when we start the long traverse operations and those will take many, many sols. As we get closer, we'll adjust plans accordingly to investigate materials we're going to look for."
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