Mars Exploration Rovers Update: Spirit Finishes Work at Bonneville Crater Opportunity Completes Study at Eagle Crater
Posted by A.J.S. Rayl
27-03-2004 11:00 CST
The Mars Exploration Rovers each focused their research efforts this week studying the rim of a crater. At Gusev, Spirit continued her study of Bonneville Crater, while at Meridiani Planum Opportunity spent most of her time driving around the southern and eastern portions of Eagle Crater conducting a soil survey of five targets.
For both rovers, the mission objective is to explore their respective crater-pocked sites on Mars for evidence in the rocks and soil about whether past environments there were ever watery and possibly suitable for sustaining life -- and two weeks ago, Opportunity met that objective, as reported here and around the world.
The second Mars Exploration Rover commanded the spotlight again this week delivering more 'wow' water news on Tuesday at a news briefing staged at NASA headquarters in Washington D.C., where the science team extended their hypothesis that the rock outcrop the rover has been studying in Eagle Crater not only once featured water, but in fact boasted a shallow, salty sea.
"Exploring the outcrop has been our job from January through a few sols ago and the scientific payoff has been extraordinary -- the chemistry, mineralogy, the textures all indicate deposition of those sediments in a kind of shallow, open water environment, although a very acidic one," summarized deputy principal investigator Ray Arvidson at the weekly news briefing held at the Jet Propulsion Laboratory (JPL), where both rovers were designed and built. The plan for Opportunity now is to begin a trek across the vast Meridiani plain to a larger impact area, called Endurance Crater, a little less than a half-mile [700 kilometers] away, "to look for other pieces of evidence that will put this picture of this shallow, kind of salty ocean in a broader perspective," he said.
Although Opportunity has already completed her mission objective by returning the stuff of which geologists dream -- the first in situ evidence of flowing water on the surface of Mars, no one is giving up on Spirit or Gusev Crater.
Spirit has investigated numerous intriguing targets around the rim of Bonneville Crater in the last couple of weeks, and conducted various remote sensing observations of the interior, but the team hasn't found what it's looking for there. "The ejecta from Bonneville didn't get excavated from deep enough to get below the volcanic layer, that we think is a rock cap," explained Arvidson, a geologist from Washington University in St. Louis.
So, once the robot geologist finishes an examination of Mazatzal, a light-colored rock on the crater's rim, she will turn east and begin the longest journey yet, toward a range of peaks 1.3 miles [2.3 kilometers] away -- the Columbia Hills, named in honor of the seven astronauts who lost their lives in a tragic disaster aboard the space shuttle Columbia last year. "The Columbia Hills [represent] an island of older rock surrounded by a younger volcanic layer that surfaces the plain Spirit has been crossing. There are some indications that there are layered rocks there, and the morphology is certainly different," Arvidson explained. And, they may well hold the clues to the water story at Gusev that the science team is seeking.
Although the "90-day warranty" is about to run out, the rovers aren't. As of this week, Spirit has racked up an impressive 492 meters at Gusev, setting the all-time global mileage record on the Red Planet. Although Opportunity's odometer reads 180 meters, she, too, has passed Sojourner's record of 102 meters, and she's logged ever centimeter of it in the 72-foot Eagle Crater into which she rolled to a stop after landing.
In fact, the way things look right now both vehicles will be able to rove on for some time; therefore, both Spirit and Opportunity will be going into extended missions. Spirit, actually, will begin her extended mission on Sol 91 -- April 5, one week from Monday, long before she gets anywhere near the Columbia Hills. Opportunity will follow three weeks later, probably, as just as she is deep into investigating Endurance Crater. The transition into those extended missions, according to Matt Wallace, Opportunity mission manager, "is going to be relatively seamless."
There are "compelling reasons" for both rovers to extend their missions, noted Arvidson. "For Opportunity, we've just explored this one 22-meter diameter [72-foot] crater, and we want to get to Endurance Crater, which has a greater vertical section of the outcrop, and also see if there are other indications, along the way, of the extent of the blueberries. For Spirit, our best bet to attack the question of whether there was a lake in Gusev at one point is to expeditiously get to the Columbia Hills and if all goes well in 60 sols from when we finish our work on Bonneville, we will be exploring those hills."
As Spirit and Opportunity rove seamlessly into their extended missions, the MER team members will begin to transition themselves, off Mars time and back to Earth time. Since the two rovers landed -- Spirit on January 3, Opportunity on January 24 -- the rover team members have been following the Mars diurnal cycle, which is 24 hours and 39 minutes, just a little longer than Earth's 24-hour cycles. Working in shifts around the clock, they evaluate the downlinked data, determine the next sol's activities, uplink the new commands, and then repeat the process. It is a schedule both the Spirit and Opportunity teams have carried out day after day for the last 2 to 3 months on both vehicles. Considering that Mars local time at Gusev and at Meridiani are about 12 hours apart, one wonders how any of the team members ever really knew, outside of their own sub-teams, who was coming and who was going.
Living and working on Mars time has been "fairly trying" for the teams, Wallace admitted. Now, for the longer terms of the mission, the goal is to "squeeze" the cycle down, and "compress" it to fit within a 7 a.m. to 10 p.m., Earth day. "That will help ease some of the pain on our hard-working team, and we think we can do that with the efficiencies we've created in the cycle and without losing a whole lot relative to mission capability," he added.
Spirit and Opportunity are still scheduled to get that new flight software mentioned previously in these MER Updates, although that uplink has been delayed for a couple of weeks. "The people we need to complete the testing, validation, and preparation of that load are also the people who are also running two vehicles, so it's been a balancing task and the upload has been pushed out a little bit as result," Wallace explained. "But that is coming along and we're looking forward to the new capabilities we'll have with that." Essentially, the new flight software will give the team more flexibility with the rovers, and give the rovers more autonomous navigation ability, resulting in increased efficiency all the way around for the rest of the robots' lifetimes.
Since the health reports for both rovers remains "excellent," the exact durations of the extended missions is, for now, "indeterminate," said Wallace. While no one is hazarding a definitive date for the impending demise of either rover, the official projection is now anticipating both vehicles will 'live' at least 240 sols. In any event, the agenda at the moment calls for Spirit and Opportunity to complete their research, at the Columbia Hills and Endurance Crater respectively. Then, the MER team "will stop and evaluate the health of the vehicles," Wallace said. "Right now," he added, "there is a lot of strong interest in continuing to upgrade these remarkable machines for quite some time."
SPIRIT FROM GUSEV CRATER
Sols 76 - 82
After completing her investigation of a sand drift named Serpent, Spirit drove last weekend toward the rock Mazatzal, an intriguing, light-toned, scalloped rock on the southern rim of the crater, her next target of focused study and the largest rock at the rim of Bonneville Crater. The name Mazatzal comes from a mountain range and rock formation that was deposited around 1.2 billion years ago in the Four Peaks area of Arizona.
On Monday - Spirit's Sol 77, the rover analyzed a soil patch, Soil 1, with her microscopic imager (MI) and Mössbauer spectrometer, and conducted some observations of the atmosphere, and the area around Mazatzal with her panorama camera (PanCam) and mini-thermal emission spectrometer (mini-TES). Then, she made her final approach to Mazatzal. The science team had selected this rock as a target for further study, because of its lighter color, which caused it to stand out from the majority of rocks around Bonneville's rim. The rock has been carved, they believe, by the steady winds that they know come from the northwest into the top area of the Bonneville Crater rim.
"Mazatzal -- which is about a meter or a little more across -- is a rock that we think has been very much shaped by the wind," confirmed Arvidson. "If it was on Earth would be called ventifact." [By definition, a ventifact is a pebble or rock that has been shaped by wind-blown sand.] "It's a rock that is big enough to be largely underneath the ground -- it's not going to move, so it feels the kind of dominant wind direction, and the sand and dust that have blown against it and has shaped these flukes or ventifacts," Arvidson continued. "Yet, it's bright, what we call a light-tone rock, and when we saw this off at a distance, we knew almost immediately we needed to get to it. We wanted to understand if the light tone was all the way through the rock. And if not, is it a dust cover or a weathering product? And what would we see as we get into the rock using the rock abrasion tool (RAT)?" And that was the grand plan for the week -- to RAT Mazatzal.
Spirit completed her Monday by taking more PanCam and mini-TES observations of the atmosphere, as well as other targets near the Mazatzal site, including Sandbox, Saber, and Dark Sands.
On Tuesday - Sol 78, Spirit took some early morning soil and atmosphere measurements with her mini-TES, then began analyzing Saber and Sandbox with mini-TES, and took some PanCam images of Skull and Saber. The robot field geologist then pulled out her MI to take pictures of three targets on Mazatzal, named Arizona, Illinois, and New York, scoping out the best area to RAT. Before her sol was over, the rover also conducted an integration with her Mössbauer spectrometer of New York, and attempted to get measurements with her alpha particle x-ray spectrometer (APXS), but the instrument door did not open, so the team rescheduled that experiment for Thursday.
Spirit also used her PanCam and mini-TES to check out another interesting feature, Orange Beach, as well as make some more observations of Bonneville Crater. Following an afternoon nap, the rover woke up to capture some mini-TES ground and sky images, then positioned and set up her PanCam for taking some more nighttime pictures of Mars smaller moon, Deimos, to add to her collection.
On Wednesday - Sol 79, Spirit completed programmed sequences using her RAT to brush and other instruments to analyze the New York and Illinois sections of Mazatzal, then imaged her handiwork with the Pan Cam and MI, and, later, performed another Mössbauer integration on New York.
Spirit actually performed two brushes on Mazatzal, "a light brush, and a heavy brush," Arvidson explained. "One was for 5 minutes at 500 rpm, and the other was at 3000 rpm," he said. "What we found at the end was that both brushes removed a bright coating at least to NavCam and PanCam, and what we saw was a dark rock underneath." Yet, he added, it was not clear from the Mössbauer and APXS, measurements done on the rough surface if anything else has changed." So the team decided to have Spirit stick around a little longer to do some grinding with the RAT and see what that might turn up. "Neither the APXS or Mössbauer spectrometer see much of a difference and that's why we're grinding more deeply into the rock."
Spirit devoted most of her Sol 80, Thursday, to acquiring the APXS measurements of Illinois and New York at Mazatzal that she had tried to do on Tuesday, and getting ready to RAT again on Friday, Sol 81.
After wrapping up the APXS integration on New York Friday morning, Spirit positioned herself for a serious grind into New York. In fact, the rover spent nearly three hours and 45 minutes grinding into the 'big apple' on Mars, producing, in the end, an impression that is .15 inches [3.79 millimeters] deep. Mazatzal is "one hard rock," as Arvidson described it, and the team decided they wanted Spirit to grind deeper, after completing analyses with the Mössbauer and APXS. "We still don't think [the hole] is deep enough, so we want to put the RAT in the hole on Sol 83 [Sunday] and go down to 8 millimeters.
What they're trying to figure out, said Arvidson is whether the rock is "fresh" underneath and just coated with dust, or if it a rock that's "weathered down" as a function of time, and therefore has a vertical profile to reveal. "We want to use our tools to understand that process, because it's telling us about the weathering environment in the current situation on Mars, the current environmental conditions. With Mazatzal, we're in the middle of the experiments with the world's deepest RAT. If we get down to 8 millimeters, we hopefully will get beneath these weathering layers, to the extent they exist, and see what the deep interior of the rock is like there [as compared to] three or four layers above that -- and try to characterize whether or not these volcanic glasses that we think are in Mazatzal get devitrified, broken down, and blown away."
So, after completing the analyses of her work today with the MI, Mössbauer, and APXS, Spirit will prepare to RAT Mazatzal one more time tomorrow.
In a few sols, the rover will finish up her study of the Mazatzal are and start the traverse to the Columbia Hills. With stops for "traverse science" along the way, the trip of about 1.3 miles [2.3 kilometers] to the near edge of the range of peaks will probably take 60 to 90 sols, Arvidson said. Truth told, they expect Spirit will be able to make the 2.35 kilometer journey in 60 sols including stops along the way, but they have built in 30 sols of contingency to allow for stops that might turn into lengthier investigations or potential glitches that might hold up the rover progress.
"We don't want to do it blindly, so we're kind of dividing this mission, including the extended mission, into 3 parts -- one part is the Bonneville ejecta work we're just completing; the second is the traverse science to get across the plains to the Columbia Hills; and the third part will be exploring the hills," offered Arvidson.
"Because the Columbia Hills is an older piece of crust," and "an island in this set of basaltic flows," he continued, "it may tell us about the earlier history of Gusev Crater." They're looking to test the notion that there were lakes or shallow seas in Gusev, " back long before this basaltic cap came in with late-stage lava flows."
Although the journey to the Columbia Hills will be a long one, the trip will not be "boring," promised science team member Larry Crumpler, of the New Mexico Museum of Natural History, who has been leading traverse science planning for Spirit's trip eastward. "It won't be like some bad road trip. Between stops, we'll actually get out and do touristy things along the way, and learn some things," he elaborated.
The team is already "planning out exactly what sort of science we're going to do in order to maximize the amount of time we spend on the plains before we get there," Crumpler said. "When a field geologist is in the field, he doesn't just willy-nilly walk from one outcrop to another, but he stops along the way and looks at his feet -- or she looks at her feet -- and identifies things that are in the landscape. That's kind of what we're planning on doing here -- marching out across the plains and doing a series of experiments along the way at a regular series of intervals, what we call traverse science. This might include things such as deploying the instruments to surface -- doing a quick Mössbauer spectrometer or APXS integration, looking at the chemistry of soils, then the next morning, doing a quick look with the MI and stowing instruments and driving on to next site."
"Traverse science is where we're using the vehicle as a mobility system," Arvidson added. "It's also used to extract topographic information, because you've got suspension angles -- the yaw, pitch, and roll of the vehicle as it's moving, so you get very exquisite information about material, properties and topography. And, the PanCam and mini-TES will be used in a reconnaissance mode to map what's around us, so we don't pass things of interest, but rather stop at the appropriate time, and there will probably be, as Larry mentioned, the occasional sols where we deploy the instrument deployment device (IDD) and make chemical, and microscopic and mineralogical measurements to insure that we properly characterize what we're driving past."
It should come as no surprise to anyone following this mission that the team has already selected a few targets to inspect along the route, based on photographs taken from the Mars Orbital Camera (MOC) onboard Mars Global Surveyor (MGS). "We've been looking at the MOC images and identifying some of the things we might look at along the way," Crumpler said, specifically several small craters and a dark streak apparently left by a whirlwind that removed dust.
"Once we get to the Columbia Hills, then the story develops into the typical story of the geologist examining an interesting outcrop or interesting area where there is a more focused field of study," Crumpler proffered. "And then we have many, many tens of sols beyond that that we hope to spend at Columbia Hills examining the nature of that geologic material."
Indeed, this is Spirit's greatest hope for uncovering evidence for water. This "island" of older rock may be harboring the secrets to the water that most Mars scientists believe must have existed at some point in Gusev's past. "After whatever produced these hills and uplifted them and eroded them, subsequently, the surrounding areas were covered over by volcanic rock. In fact, the ejecta from Bonneville looks to be basaltic rock of various types with various degrees of weathering, so that is probably a cap rock over the older materials that we just can't get to. The ejecta from Bonneville didn't get excavated from deep enough from beneath this volcanic cap rock. The Columbia Hills "look different morphologically," Arvidson informed. "And there's some kind of vague indication that there are striations that run across the hills at an angle relative to horizontal. So these hills could have the older sedimentary record. They could have ejecta from Gusev itself, or they could have some surprises that we just won't understand until we get there."
OPPORTUNITY FROM MERIDIANI PLANUM
Sols 56 - 62
After examining a soil patch named Brian's Choice with her Mössbauer spectrometer, APXS, and MI last Sunday - Sol 56, Opportunity attempted to exit her landing crater, but she didn't make it on the first try. The rover followed a sequence of drives turning first to the north, then turning east with the intention of driving directly up the northeastern slope and out of Eagle Crater. "What happened was we encountered a fairly soft part of the crater at relatively high slopes, at about 17-18 degrees, and the wheels kept slipping," explained Wallace. "Essentially, we ended up with a 100% slip and so we did not drive up and out of that crater as expected last weekend."
Opportunity wound up spending the rest of her Sunday sol taking another drive, about 12.5 meters, along the inner slope of the crater to position herself to egress at a different place. There, the rover took pictures with her navigation camera (NavCam) and measurements with her mini-TES and then shut down for the day.
Monday - Sol 57, was a new day and both Opportunity and the team were ready to attempt egress again. "Rather than a frontal assault, we took an oblique or diagonal path this time and were able to continue the traverse up the southeastern part of the crater rim to successfully drive up and out of the crater, and about 7 or 8 meters beyond the rim," recounted Wallace.
It's almost hard to believe that this robot geologist has spent her entire mission to date studying the 72-foot [22-meter] diameter crater into which she just happened to roll into after landing January 24. But after 57 sols inside Eagle Crater, Opportunity has finally emerged onto the plains of Meridiani.
Although the results of Opportunity's soil survey completed last weekend are still being analyzed, the team has discovered that the rover's northward traverse around the crater to the targets -- Goal 5, Neopolitan, Mud Pie, Meringue, and Black Forest - was an investigation that was well worth the effort.
"There are 5 spots we selected on the southeastern side of the crater, because basically they looked different, from PanCam, NavCam, and mini-TES," explained Bethany Ehlmann, a science team collaborator, from Washington University. "They seemed to be distinct soil units, so our goal was to traverse around the crater looking for these differences as we went. At each spot, we went down with the IDD took a MI, then did a Mössbauer spectrometer integration on the soil, and also a shot with PanCam of what we were looking at so we have this entire data set for each of these spots that we're in the process of analyzing."
Each of these distinctive five soil patches shows a diversity of particle sizes and shapes on the surface, as well as some similarities to the composition of the other targets. "We see the spherules and the size of the spherules from the MI of the soil and [those] in the outcrop are pretty much the same," Ehlmann said. "Another particle size that we see in the outcrop are the millimeter-sized, or less, grains that make up the laminations in each part of the rock, and we're seeing things of comparable size in the MI images of the soil targets. Some of the particles do seem to look the same in that they are probably coming from the outcrop . . . [but] on the upper crater unit, particularly on our last stop, at an area called Black Forest, we found these pretty large spherules, blueberry-sized or larger, and more angular fragments that don't look like anything we've seen around the outcrop, but might be coming up on the plains."
The one common denominator in each of the soil patches examined was the consistency in the finer-sand underneath with the larger particles overlying that sand. "What we're seeing here is a pattern in that each of these show that underneath these larger particles, is this fine-grained sand and we think it is probably basaltic," Ehlmann said.
The range of different particles on these different soil patches is, basically, the result of "the effects of differences in wind speed," Ehlmann added. "In some patches more than others, winds have removed small particles and left large particles behind," she said.
The data sets from these five soil targets were collected for a two-fold purpose, Ehlmann continued. "First, we are trying to understand what are the source(s) of all these different looking particles. Do they come from outcrop? The plains? The other is thing we're trying to understand are the processes that have been going on here to sort these grains into the different units we're seeing. So the end result is to try and understand why we see the hematite [signature] on plains, why do we see it in some places in crater and not others, and where doe sit all come from? What is the relationship of the rocks in the outcrops to what we're seeing in the materials we find on the surface?"
We know now, from Opportunity's previous investigations with the Mössbauer spectrometer, which is designed to identify iron-bearing minerals in rocks and soils, that the hematite signature -- first observed from orbit by the thermal emission spectrometer (TES) onboard MGS and which lured the MER team to Meridiani -- appears to be coming from the gray spherules that have come to be known and loved as 'blueberries.' At yesterday's press briefing, the lead scientist for the instrument, Goestar Klingelhoefer, of the University of Mainz, Germany, reported that the Mössbauer has discovered the highest concentration of hematite ever found on Mars, on one of the patches near the top of Eagle Crater's rim.
The team has also discovered through the Mössbauer work that the blueberries are plentiful in some soil patches higher on the inner slope of the crater than they are near the center of the crater. "We have a higher hematite signal at the crater rim in comparison to the center of the crater," confirmed Klingelhoefer.
Back at Meridiani, after driving out of Eagle Crater, Opportunity, on Tuesday - Sol 58, turned around and drove right back to the rim of the crater and perched above a target the scientists call Bright Material. The location also turned out to be "a remarkably good bluff" for the rover to begin taking a series of high-resolution, color images of the entire area, crater and the vast plain beyond with her PanCam. Opportunity ended her day by using her mini-TES to do some remote sensing experiments of the plains.
On Wednesday - Sol 59: Opportunity turned her Mössbauer spectrometer on Bright Material and conducted some more remote sensing observations, then she settled in for a good long rest.
The rover placed her APXS on Bright Material to investigate the chemical composition, then continued taking the images for the PanCam mosaic from her then-resting position on Thursday - Sol 60. The MER team has already dubbed this high-resolution, color panorama to come the 'Lion King Panorama,' because it is offers an expansive view of Opportunity's 'domain' from an elevated vantage point, similar in a way to Mufasa's view from Pride Rock in the Disney's animated Lion King movie. Not surprisingly, the team woke the rover Thursday to Elton John's "Circle of Life," the hit song from the film. The multiple numbers of images are in the process of being downlinked. "But it will take a while to get down, because data volume is significant," said Wallace.
On Friday - Sol 61, Opportunity took a short drive to the north to another target of darker material the team has decided to investigate this weekend, before she heads up to a spot on the north edge of the crater rim to begin her half-mile [700 meter] trek to Endurance Crater. Along the way to Endurance Crater, Opportunity, just like her twin en route to the Columbia Hills, will be conducting traverse science. "We will verify or refute the idea that these blueberries are strewn across the plain and are causing the hematite signal from orbit," said Arvidson.
Images from the MOC on MGS hint of that Endurance Crater features numerous striations and layers that the science team members hope is a thicker set of bedrock layers that can reveal more about the duration of Meridiani Planum's wet history. "We expect or hope that Endurance Crater has a much greater vertical section of these sedimentary rocks," said Arvidson. "One of the things we want to find out, for example, is whether or not this section repeats periodically - that would indicate these water bodies were there and then dried up and came back, or that the edge of the water lapped on and left, and then lapped on again and left. We need more of these outcrops extended aerially to really understand the lateral extent of these water bodies and the way they might interconnect, and therefore the environmental history of Meridiani Planum."To be continued . . . . next week.