Mars Exploration Rovers Update: Spirit Cruises to New Target, 'White Boat' Opportunity Uncovers Mystery Spheres
Posted by A.J.S. Rayl
10-02-2004 11:00 CST
Both Mars rovers -- Spirit and Opportunity -- are roving on the Red Planet and doing exactly what they were programmed to do as robot field geologists, explore their surrounding areas.
Each rover is now driving to defined targets, closely inspecting chosen patches of soil and rocks to gather clues necessary to uncover the geological history of Gusev Crater and Meridiani Planum, and determine if either environment ever featured a body of water and was capable of supporting life as we know it.
Spirit finished her study of Adirondack last weekend, and has been allowed to shift to her 'AutoNav' system so that she can make some of her own driving decisions. Today, this rover broke the record for the farthest distance driven in one sol [day] on Mars, traveling 69.6 feet (21.2 meters). This distance shattered the Sojourner's previous record of 23 feet (7 meters) in one sol, back in 1997.
On the other side of the planet, her twin, Opportunity, has returned a host of data on the intriguing, tiny rounded spheres discovered embedded in the rock outcrop and dotting the surface nearby in Meridiani Planum where she landed last month.
From Gusev Crater
Last weekend, Spirit completed her study of Adirondack by using her rock abrasion tool to grind off the surface of a patch about 1.8 inches (45.5 millimeters) in diameter and 0.1 inch (2.65 millimeters) deep. It was the first artificial hole ever drilled in a rock on Mars, giving Spirit yet another 'first' in an ever-growing list of 'firsts.'
Opening a window
"It has really opened up a window into the interior," said MER lead scientist Steve Squyres, of Cornell University.
Spirit's ensuing examination of the freshly exposed interior included taking pictures with the microscopic imager, and measurements with the mini-thermal emission spectrometer, as well as the Mössbauer and alpha particle x-ray (APXS) spectrometers. The data returned and analyzed so far has indicated that the rock is volcanic basalt.
"What [we're seeing] is a beautifully cut, almost polished rock surface and it looks very much like an image of volcanic rock," Squyres said. "In fact, when we look at this with the APXS and with the Mössbauer spectrometer, we find compelling compositional evidence that it is volcanic basaltic rock."
After completing her examination of the rock late Sunday, Spirit drove right over Adirondack - not to be rude, but efficient - arriving at her next target about 21 feet (6.37 meters) away, a rock called White Boat.
Baby, you can drive yourself
Along the way, Spirit tested out her autonomous navigation ability -- a built-in navigation software and hazard avoidance system that enables her to make her own decisions about how to get to a specified point of interest. It marked yet another 'first' for the Mars Exploration Rover.
"We're in a new phase of the mission," said Mark Maimone, rover mobility software engineer, at a morning news briefing yesterday at the Jet Propulsion Laboratory (JPL). "We're going to let the rover decide how to get to where it's going."
Spirit switches to her autonomous mode when she receives the commands from the mission team on Earth that instruct her to do so. In that series of commands, the rover is directed to drive to a specific destination. As the rover proceeds, she evaluates the terrain with stereo imaging as she goes, choosing the best way to get to her destination, while avoiding anything she identifies as an obstacle that might be in her path.
This autonomous navigation ability "opens up new opportunities and lets us drive farther distances," explained Maimone. Most significantly, it frees Spirit up from the step-by-step navigation commands that have directed her since egress from her lander January 15.
Last night, this robot field geologist was commanded to drive farther on a northeastward course toward Bonneville Crater, about 820 feet (250 meters) away, where she will study the rocks thrown outward by the crater-forming impact. "This is the beginning of a very long drive, and we're looking forward to letting Spirit do her thing and deciding for herself decide for herself if it's safe and how far to go," Maimone said.
The general plan for this weeks calls for Spirit to begin her examination of the rock White Boat today, and continue on the route to Bonneville, stopping along the way, here and there, to study rocks of interest.
From Meridiani Planum
MER mission team members also announced at the news briefing the results of the triangulated data that has allowed them to pinpoint the location of Opportunity's landing site crater. Just as with Spirit, the team relied on radio-tracking data, descent images, post-landing images from the surface, and orbital images to home in on the rover's exact position at Meridiani Planum.
Opportunity -- where are you?
Radio signals gave the team a preliminary location for Opportunity about 35 minutes after landing, and additional information from communications with the Mars Odyssey orbiter soon narrowed the estimate, said Tim McElrath, deputy chief of the navigation team.
As Opportunity neared the ground, winds changed its course from eastbound to northbound, according to analysis of data recorded during the landing, to wind up in the tiny crater that she is about to crawl out of. "It's as if the crater were attracting us somehow," said Andrew Johnson, an engineer on the Descent Image Motion Estimation System (DIMES) camera team. The DIMES systems -- which were installed on the bottom of both MER rovers' landers -- estimate the spacecraft's horizontal motion during the landing.
The spacecraft bounced 26 times and rolled for more than 1 minute for about one-eighth of a mile or 219 yards (200 meters) before coming to rest inside the small crater, which is about 72 feet (22 meters) in diameter. As Squyres put it January 24, the night Opportunity landed: ""We scored a 300-million-mile, interplanetary hole in one."
JPL geologist Tim Parker was able to correlate a few features on the horizon above the crater rim with features identified by Mars orbiters, while imaging scientist Justin Maki, also of JPL, identified the spacecraft's jettisoned backshell and parachute in another Opportunity image showing the outlying plains. "This was a difficult location effort, because the crater is so small that we can't identify features on the rim that we were triangulating to, and compare views," explained Parker.
The location determined from the triangulation of data proved to be almost right on the mark when the tour de force image arrived from the Mars Orbital Camera (MOC) onboard Mars Global Surveyor (MGS). The MOC image actually shows the Opportunity lander as a bright little splotch in the crater. A darker feature near the lander may be the rover. "I won't know if it's really the rover until I take another picture after the rover moves," said Michael Malin of Malin Space Science Systems.
MGS passes over Opportunity's landing site twice a day, morning and afternoon, and images from the MOC are generally about 4.9 feet (1.5 meters) per pixel [picture element], although they can utilize a super-resolution feature on the camera and reduce that to 1.6 feet (.5 meters) per pixel, to really home in on an object, said Malin, a member of the rovers' science team and principal investigator for the MOC.
In the final report, the team announced that Opportunity's crater is at 1.95 degrees south latitude and 354.47 degrees east longitude, the opposite side of the planet from Spirit's landing site, which is at 14.57 degrees south latitude and 175.47 degrees east longitude.
Slip, Sliding Away
Meanwhile, the second Mars Exploration Rover was getting on with her job of collecting science in her little landing crater.
Yesterday, engineers woke up Opportunity with a lighthearted wake-up tune, Paul Simon's "Slip Sliding Away." They chose the song because this Mars rover had experienced quite a bit of slippage in the loose soil on her way to the outcrop last weekend. That's why it took her just a little bit longer to get to Stone Mountain, the rock at the edge of the outcrop formerly known as Snout. ["We tend to come up with names quickly in 'the heat of battle,'" Squyres noted of the name change. And, obviously, the initial names don't always stick.]
"We actually had quite a bit of slippage," Maimone told The Planetary Society. "In fact, the crater where Opportunity is, is quite a bowl and as we were going farther up the side of the bowl, we were slipping more and more. During one of the set of moves there, we drove 97 centimeters [about 3 feet] slipping on the order of 40 to 50 percent. We told it to go so far, and it actually went a little more than half that distance."
Considering that the soil appears to be composed of very loose sand-like materials the slipping came as no real surprise. The MER team has spent months testing the rovers' capabilities in all types of soil and sand and are prepared for pretty much any kind of ground materials they have been able to envision encountering either in Gusev Crater or on Meridiani Planum.
For Opportunity, driving to the outcrop was a bit like "trying to walk up hill on a sand dune," Maimone said. "It takes more energy and it takes you a while to get there." So what they have to do on the ground for now, he explained, "is manually look and see how far the rover has gone and determine where it really is. We have tried to characterize it and compare it with tests we did here on Earth in different soil and we've measured the slip on those and we're getting the right model."
That said, Maimone also explained that the rovers each feature software technology that allow them to incorporate their cameras and determine the distance traveled. This feature, he said, will be tested out on Opportunity at some point in the next few days. "That's going to actually let the rover look with its cameras and figure out how far it's really traveled, and then use that information to tell itself how far it's gone, so even though it may be slipping, pretty soon it will be smart enough to know how far it's gone," and when it reaches its destination.
Despite the loose soil, Opportunity pressed on and drove another 13 feet (4 meters) yesterday, to a second point in what is a counterclockwise survey of the rock outcrop along the inner wall of the rover's landing-site crater.
The outcrop -- dubbed Opportunity Ledge -- is the first outcrop ever seen and explored on Mars, or any other planet for that matter, and it represents a veritable bonanza for geologists working on the mission.
Blueberries in a muffin
Pictures taken at the first point in the outcrop survey have revealed perfectly rounded gray spherules, or tiny spheres, within the layered rocks and also loose on the ground nearby that have the scientists excited.
"We had a big weekend -- probably the biggest 2 or 3 days for science since we landed," enthused Squyres, as he prepared to display a series of "tantalizing" new pictures from the PanCam and microscopic imager.
"The deeper we get into Meridiani, the more it's reminding me of a mystery novel. When you start into a mystery novel, you start getting clues and you get them one at a time, chapter by chapter. Some of the clues mean something. Some of them are probably red herrings -- and you don't know which is which. We're working our way through these," he explained.
The outcrop is "tan or buff-colored," composed of "finely-layered" materials, and in the process of being eroded by windblown sand. "The thickness of the individual layers is a few millimeters," Squyres specified. "[The outcrop] is very, very finely grained and then embedded in it -- like blueberries in a muffin - are these little spherical grains -- I call them spherules -- because we don't know what they are."
The spherules are different in color -- "very, very gray," and very different from the stuff that was in the matrix [whole of the outcrop]," Squyres continued as he showed a microscopic image that showed the gray spherules in various stages of being released from the rock. "This is wild looking stuff. The rock is being eroded away and these spherical grains are dropping out," he noted.
"What's happened is [this]: [the outcrop] is sitting there for a very long period of time and has been sandblasted . . . the wind blows and the grains are striking it. Some portions of the rock are softer and some portions are harder, and the portions that are softer get worn away more rapidly. The intricate texture is telling [us] something about how well this stuff is stuck together -- geologists use the word indurated.
The gray spherules, Squyres added, "seem to be pretty tough." In many cases, as the rock erodes away, these "little blueberries in a muffin" drop out and roll down the slope of the crater.
The new data has helped the science team to winnow down its list of hypotheses about what the outcrop rock is made of, and what the tiny spherules might be. "For the matrix [outcrop], the tan colored rock itself, there are really only two ideas that we think are still holding up, Squyres said. Those theories are that the outcrop is either made up of volcanic ash or Martian windblown dust -- "the same dust you see everywhere else on the planet, compacted into sedimentary rock."
As for the gray spherules, "there are three hypotheses still standing, but one is fading fast," Squyres said. Those three hypotheses are that these perfectly round little spheres are:
- Lapilli, the tiny round spheres that are formed when suspended ashes from a volcanic eruption agglomerate and stick together;
- Tiny spheres that form when molten rock is sprayed into the air by a volcano or a meteor impact;
- Concretions formed by minerals coming out of a fluid, such as water diffused through rock.
"The one [hypothesis] that is fading fast is the idea that these are lapilli," offered Squyres. "We go back and forth on this to be honest. The thing about lapilli -- though they can be very round and just this size -- they tend to be made of the same stuff as the matrix of the stuff they're embedded in."
That's not what the evidence shows at the outcrop. In fact, a false color image "emphasizes that they are different in color and that's a hint that they may be different in composition too," he explained. That duly noted, however, Squyres added that they have not yet been able to complete separate Mössbauer measurements on the spherule and the matrix -- "on the blueberries and the muffin," as he described it. "We are going to do that and I think that will nail down whether or not these two are made of the same stuff or not, but the fact that their colors, their spectra are so different suggests that the little spherules are made of something different from the matrix stuff.
The notion that the spherules are formed from molten rock is a strong, perhaps the strongest contender in terms of the hypotheses right now. "Spherical grains can form when molten rock is sprayed into the air and freezes while it's still in mid-air -- solidifying [into] 'rocklets.' "Then, with these frozen 'rocklets' in the air you get these little glass beads that fall down to the surface."
The theory that these may be concretions is also a prime contender -- and these sedimentary objects form in a process involving water. On Earth, geologists discovered long ago that concretions form when fluids -- water -- carrying dissolved stuff precipitates through a rock. "This stuff diffuses through the rock, and precipitates around the nucleation sites, then grows into these spherical granule," Squyres said.
The most varied-shaped rocks on the sedimentary scene, concretions are formed and found in many places on our home planet. By definition, a concretion is a compact mass of mineral matter, usually spherical or disk-shaped, and embedded in a host rock of a different composition. These little spheres tend to form when a considerable amount of cementing material precipitates locally around a nucleus, often organic material, such as a leaf, or piece of shell or fossil.
Concretions vary in size, shape, hardness, and color -- from tiny little balls that require a magnifying lens to be clearly visible to huge bodies 10-feet in diameter and weighing several hundred pounds. Most importantly with regard to the quest at Meridiani, however, these hard, round masses of sedimentary rock 'cement' are ferried to their hiding places by groundwater.
Although the team is only on chapter two of the Mystery at Meridiani novel, "we think we should be able to test all of those hypotheses," Squyres said.
"I don't believe that the only spherules that we're seeing in the soil came from the outcrop," Squyres told The Planetary Society later. "I really think there is another source higher up."
They'll find out soon enough, and the traverses across the featureless, flat topography of Meridiani Planum is going to make for "smooth sailing" for the rover. At this point, Squyres pointed out, the only thing you can see for hundreds of meters is the backshell and harness. "If we were to drive in that direction, the first obstacle we would get to is the backshell."
The trip out of her landing crater will come once this rover completes her work on Opportunity Ledge.
Shoot 'n Scoot
For now, the agenda for the next week or so, is for Opportunity to complete the thorough survey of the outcrop, following follow a systematic plan. The rover will progress from point to point -- arriving, shooting pictures of the terrain, and acquiring new scientific measurements of the rocks, then moving on to the next chosen location where she will follow the same procedure. She will continue to investigate Opportunity Ledge in that manner until all levels of the outcrop up, down, and across have been investigated.
"We 're calling it a shoot'n scoot -- where we shoot a bunch of pictures and scoot to the next site about 3 meters over, shoot a bunch more pictures, then scoot again and do that for several sols, working our way across the outcrop," said Squyres.
All along the way, the team will instruct Opportunity to take high-resolution PanCam images of the entire outcrop, as well as taking Mini-TES measurements and using the other spectrometers and instruments as deemed necessary.
"We're going to find a couple of the best places -- a place where finely layered stuff in this matrix is really well exposed so we can go in there with the RAT and grind away at this stuff and then see what those layers are really like . . . and a place where there are a whole bunch of these spherules and if we could RAT those, see what they look like in cross-section and then stick the Mössbauer on them to figure out what they're really made of," Squyres added. "That's stuff that is yet to come."
Although the spectrometers data is still being analyzed, Squyres did say yesterday that the APXS measure on the rock outcrop indicates that there's a lot of sulfur, "more sulfur than we've found in any other location on Mars." It's yet another clue in the Mystery at Meridiani, but like many of the other recent clues, they don't know what it means.
And what about the hematite -- the mineral that on Earth usually forms in water -- which is what originally enticed Mars scientists and drove them to pick Meridiani Planum as a choice landing site?
"When we look hard at the outcrop we don't see high concentrations of hematite, so the matrix itself doesn't appear to be hematite bearing," Squyres said. "That does not rule the possibility that the spherules contain hematite - they could on the basis of Mini-TES." But the Mini-TES measurements are definitive, he added, because they have not yet found a place filled with spherules that Mini-TES can homes directly in on.
"The key to answering that [hematite question] is going to be to use the PanCam on a lot of these spherules," Squyres contended. "There's no question though that the highest concentration is actually above the outcrop -- and we don't know what's up there," he said. "Everything that we're seeing so far is either the outcrop itself or stuff that has fallen down. The evidence suggests that the highest concentration of hematite is in the stuff up above the outcrop layer, which we're not going to see until we start going at it."Both Spirit and Opportunity were designed and built at JPL, a NASA facility managed by the California Institute of Technology.
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