Spirit and Opportunity continued their search for evidence of water at Gusev Crater and Meridiani Planum this week, and a lot of excited and smiling faces have been emanating from behind the scenes, in the mission rooms at the Jet Propulsion Laboratory (JPL).
Spirit finished her inspection of Laguna Hollow and broke yet another driving record as she continued on to Bonneville Crater, while, on the other side of Mars, Opportunity continued her detailed study of an outcrop in the crater in which she landed last month. Members of the media, meanwhile, have been getting anxious for news about whether the science team has found evidence that either -- or both -- sites once featured enough water to have sustained biological life.
The data sets are still being collected and are streaming in daily, of course, and the panel at the press briefing this week adopted an anticipated, adamantly noncommittal response. "We're in the middle of a mission that's a work in progress and I think in the end we'll have a lot to say about water -- or not -- in terms of these two sites," offered Ray Arvidson, deputy principal investigator for the rovers' science work.
"Water is the elixir of life and if we do come up with a conclusion that water has been involved in the surface or subsurface at some time in the past, then I think the then the probability that pre-biotic systems could have been generated and life could have gotten started goes way up," Arvidson added. "But we're in the middle of our job. It's going to take a couple dozen more sols for us to finish the critical measurements to come up with these definitive statements."
In other words, he intimated, they are getting the kind of geologic data they need to come up with those "definitive statements."
"We have a lot more work to do," corroborated Jim Bell, Cornell University, lead scientist for the panoramic camera (PanCam) on board each rover. Although the science team members are doing their best to process and analyze the data as it comes in, he said, "we really are still slogging through the data."
The numbers alone are impressive. Together, the rovers' 18 cameras have 'snapped' and returned 11,000 raw images. "Things are going wonderfully well in imaging land . . . we have obtained more than 9.1 gigabits of information . . . a phenomenally rich data set," noted Bell.
And, despite a nagging power drain from an errant heater on Opportunity, the health of both rovers in general "has been fantastic," reported Mission Manager Jennifer Trosper, allowing each rover to have another highly productive week at the Red Planet.
From Gusev Crater
As Spirit has journeyed closer to Bonneville Crater, where she will look for older rocks from beneath the region's current surface layer, the robot geologist has been stopping frequently to examine soil and rocks along the way. During these stops, the rover has been conducting what the science team calls 'touch and go' maneuvers -- activities where she extends her arm, deploys one or more of her instruments, then stows the arm and drives, taking remote sensing measurements as she goes.
"One of the things we want to do along the traverse is to characterize the soil with these 'touch and go's' and overnight integrations [measurements with the alpha particle x-ray spectrometer (APXS) and the Mössbauer spectrometer] to both understand the composition and mineralogy of the crust of Mars and also to make sure we're not missing the material we're looking for, evidence of a sedimentary environment already broken down into soil," explained Arvidson. Spirit has now gotten close enough to Bonneville -- which is 492 feet [150-meter] in diameter -- to be in the 'ejecta' field, he said, the area where the impact that made the crater may have scattered some of the older rocks from beneath the surface.
Earlier this week, after digging a small trench at Laguna Hollow -- dubbed Road Cut -- Spirit completed her study with the APXS and Mössbauer spectrometers, and the microscopic imager (MI). Preliminary data shows that the soil along the sides and at the bottom of the trench, Road Cut, is "coherent," but the scientists aren't yet sure what exactly is holding the minute particles that make up the soil together. "This is a cohesive unit, and we dug twice as long as we did with Opportunity, but went about the same depth, so the material properties are different, and we don't know why at this point," Arvidson said.
Spirit then hit the road and headed for Middle Ground, another hollow on the road to Bonneville. On Tuesday, the rover drove 98.4 feet (30 meters), to break -- once again -- her own record for a single-sol traverse. Along the way, the rover drove in a dog-leg pattern to avoid some bumpy terrain and paused from time to time to image rocks on both sides of the drive path with her PanCam.
While Spirit was to have completed the drive to Middle Ground Wednesday, the auto-navigation software that drove the last 39.4 feet (12 meters) of the traverse warned the rover that the slope into the hollow that houses it was too steep, according to parameters set by rover engineers. So the robot geologist "did the right thing," said Trosper, by stopping and seeking an alternate route.
Spirit paced along the rim, looking for a safe way down, but was unable to locate a secure path into the crater before the sol ended. So, she ended up facing slightly west of north instead of northeast, as called for by the plan, and had to wait until Thursday to complete the drive that would put her exactly where she was supposed to be.
Thursday began for the rover, however, with the wake-up song of "Dust in the Wind," by Kansas, and an attempt to film some of the many dust devils that whirl across the Martian surface. Spirit commanded her rear hazard avoidance camera was commanded to "roll tape" from 12:00 to 12:30 local solar time to record these so-called "mini-tornadoes." Understanding the behavior of dust devils will help scientists track the transfer of dust on the Red Planet. The rover also checked out her magnet arrays that are collecting airborne dust, taking pictures of them with her MI and measurements with her APXS.
Then the rover completed the final 2.8 feet [.85 meter] to her precise target site at Middle Ground. With that short trek, Spirit had driven a grand total of some 600 feet (about 184 meters) from the Columbia Memorial Station, the place where she landed, with some 262 feet (about 80 meters) or so to go before reaching Bonneville's rim.
The terrain ahead looks considerably different from what's behind. "It's a rock-strewn surface with these hollows, which seem to be circular, and may be craters superimposed on the 'ejecta,' and bright rocks and dark rocks, but most of the rocks are bright and seem to be pretty much the same," said Arvidson. "It's rockier, but we're after rocks."
Although the field ahead is rockier, there are no huge boulders blocking Spirit's course, and the rover should have no real trouble traversing it, said. Trosper.
Spirit finished her inspection of the soil around Middle Ground with the tools on her arm, and took high-resolution pictures of the rocks in the area, as well as a panorama of the area to identify scientifically interesting rocks, then, on Friday headed to a rock dubbed Humphrey, further down the road to Bonneville.
This weekend, Spirit will conduct a thorough assessment of Humphrey, the robot geologist will be commanded to brush off the rock off in three separate areas, The scientists will then decide which is the best place for the rover to grind into with her rock abrasion tool (RAT). After that, the rover might investigate an interesting rock behind it, or continue on toward Bonneville.
Spirit should be at the rim of Bonneville Crater in about two weeks, predicted Arvidson. But whether or not the rover will actually venture into the crater has yet to be decided. "Right now we're in the process of taking stereo MOC images [pictures with the Mars Orbital Camera onboard the Mars Global Surveyor orbiter] and doing a detailed stereographic reduction to get elevation levels," offered Arvidson. "I frankly don't know what the slopes are or what the feasibility is in terms of hazard avoidance for getting the rover down into the crater, or for that matter -- since we haven't crested and done the PanCam and mini thermal emission spectrometer imaging of the interior yet -- whether it would be worth it."
The decision to go in - or not go in -- will probably not be made, Arvidson added, until the rover ascends to the rim and looks inside. "Then we'll decide whether or not it's worth it or if it's possible to descend into the crater - or do we turn off to the southeast and head for hills."
From Meridiani Planum
Opportunity has spent the week examining a section of layered rock called El Capitan, part of the outcrop in the small crater in which she landed and is now working. Opportunity Ledge, as the outcrop is called, covers nearly half of the inside of the 22-meter wide crater, measuring, the scientists estimate, about 60 meters in length.
The rover has taken dozens of MI pictures and PanCam images, as well as measurements with her APXS and Mössbauer spectrometers, and her mini-TES. But not all of the data has been downlinked yet.
"Remember Christmas Eve or a day before a holiday that includes gifts on the next morning -- we're right there," Arvidson said when asked to assess the progress at Meridiani. But, just as with Spirit at Gusev, he reiterated, "we're in the middle of a work in progress, [taking] very detailed measurements of the outcrop."
On Tuesday -- Opportunity's Sol 30 -- the rover then took some mini-TES sky surveys and sky 'stares' to study the atmosphere, then took a short siesta to recharge her batteries. The season on Mars is changing to winter, so the rover has less energy to work with than earlier in the mission. The Martian days are getting shorter and the sun angle is not allowing either rover to power up the solar panels as much as in the past.
The rover woke up from her nap at 11:30 local solar time to run through the series of commands required to retract her APXS and close its doors; take several MI pictures of another nearby RAT target called Guadalupe. She then flipped her wrist to take a microscopic image of McKittrick Middle Rat at El Capitan, and placed her RAT on the target there, to run at 13:00 local solar time.
Opportunity RAT-ed the rock over a period of two hours, grinding into a total depth of about .16 inch [4 millimeters]. With that rock-grinding session, all the tools have now been used on both rovers.
After she retracted the abrasion tool, Opportunity took a series of microscopic images of the scene, and placed her APXS on the hole the rover had grinded earlier, and took more pictures with her PanCam and hazard avoidance camera before calling it a day.
On Wednesday, Opportunity continued the examination of McKittrick Middle Rat, taking measurements with her APXS and mini-TES from inside the RAT-ed hole, then placed her Mössbauer spectrometer a 24-hour observation. Scientists will try to identify the chemical elements of that surface with the APXS data, and the presence of iron-bearing minerals with the information downlinked from the Mössbauer.
The rover also updated her "attitude knowledge" on Wednesday, fine-tuning her 'awareness' of her exact location and position on Mars. Since the rovers can gather attitude 'errors' over time as they drive and use their arms, they need 'attitude adjustments' about once a week after driving long distances. This ensures that the rovers will be able to accurately point their high gain antennas toward Earth, and more precisely point their instruments at patches of soil or rocks of interest.
To adjust the attitude knowledge, engineers have the rover turn her PanCam to the Sun and watch the Sun travel across the sky for 15 minutes. The rovers have the 'smarts' to take that Sun movement data and calculate their precise location on Mars.
On Thursday, the rover focused on getting a second Mössbauer measurement, in a different spectrum, of the hole she RAT-ed at the McKittrick rock site. In addition, she took pictures of rock areas dubbed Maya and Jericho with her PanCam, and continued with mini-TES measurements of the sky and El Capitan throughout the day.
The rover woke up a little late on Friday to conserve energy, but got right to work, multi-tasking by taking a 360-degree panorama and an extra observation of the area to the east with her navigation camera, while her Mössbauer instrument completed the measurements begun yesterday on the freshly RAT-ed hole.
Later, the rover used her MI to take three sets of images of the hole she created at McKittrick Middle Rat Opportunity, and also took stereo images of the rock area named Maya, and pictures of another area called Half-Dome, as well as conducted PanCam and mini-TES observations of the sky.
In between science measurements, Opportunity stowed her instrument arm and wheeled the 6 inches [15 centimeters] to reach her second RAT target, Guadalupe, on the upper target of the El Capitan/McKittrick area. Final shutdown was at 2:37 local solar time, with a brief wakeup at 4:10 to transmit data to Odyssey as it flew over.
The plan for Opportunity this weekend is for the rover to grind into Guadalupe and take extensive measurements of the new hole using the MI and two spectrometers.
Although Opportunity, like Spirit, is performing her geological research duties in fine form, she is still losing some power as a result of a heater on her arm that has been wasting some power by going on during cold hours even when not needed. Because of that -- combined with the fact that the amount of power the rover is able to generate continues to dwindle due to the decreasing amount of sunlight (energy) reaching the solar panels during the Martian seasonal transition to winter -- rover engineers are adjusting the rover's daily communications activities. To minimize power use for communications sessions, they have initiated a new "receive only" morning direct-from-earth communication relay.
Opportunity will continue to utilize this lower-power communication mode to maximize the available power for driving and science activities as Mars moves farther away from Earth and the Sun in its elliptical orbit. At the same time, though, the engineers added a second afternoon Odyssey relay pass, which uses less power in transmitting data volume than direct-to-Earth communication and has "more than compensated" for the elimination of the morning direct-to-Earth downlink. Engineers also plan to continue giving the rover 'naps' throughout the day, something that has proved to maximize energy savings.
Despite the power drain, the heater glitch, therefore, is "still not an issue," said Trosper. "We're doing great science at the site and we're able to do a lot of the communications passes." The MER team, however, has already written up a software fix, she noted, that will give rover controllers "the ability to turn that heater off completely and so it won't be a drain on the battery overnight." That 'fix' will be sent up with new flight software that's going to be installed in both rovers next month.
From the press briefing
At the press briefing, Bell presented a few more of the latest high-resolution PanCam images from the rovers, adding to the portfolio of 'gee-whiz' images the twin robots have been returning since arriving on Mars. Throughout the mission, it seems, the images have played the spotlight role, not only in eliciting 'wow' reactions from the public, but in stimulating debate and sparking hypotheses among the scientists.
"In just this one small half-a-coffee table-size scene, we've seen more spectral and color diversity than we've seen in almost any other data set we've ever obtained on Mars with these wavelengths," Bell said as he presented a picture imaged with the range of filters available on Opportunity's PanCam. "There's a fascinating array of particle shapes, sizes, and colors, and perhaps compositions likely, in tiny little scenes . . . showing the power of this resolution."
Another image the PanCam scientist presented was the first sunset picture from the mission. Opportunity actually took a series of images about two weeks ago, on Sol 20, but the team has been processing them, and has just released the series as an animation [available at www.jpl.nasa.gov]. "It's inspirational and beautiful, but there's good science in there, too," Bell pointed out.
On Mars, the Sun rapidly dims as it nears the horizon, because of the dust in the sky. "As the Sun sets, it dims substantially," Bell explained. "Those of you who live in L.A. will be very familiar with this effect, [because] what is happening is the Sun is setting into murky, dusty atmosphere at Mars. It's very dusty." In fact, he added, "[b]ased on these kinds of images, Mars and the atmospheric scientists can tell it's about twice as dusty as it was at the Mars Pathfinder landing site."
The sky color, a rich hue of blue, however, "is very similar to what we've seen at both Viking  and Pathfinder ," which were each at different locations. The sky on Mars becomes significantly bluer closer to the Sun because the dust in the Martian atmosphere scatters blue light forward toward the observer much more efficiently than it scatters red light forward; therefore, a 'halo' of blue is always observed close to the Sun. Intriguingly, it is, Bell noted, "the approximate color" -- the approximate vision -- of what we would see if we were standing on Mars and watching the Sun set at Meridiani Planum.
From the other side of the planet, the observations Spirit has made images with her MI of one wavy patch of windblown soil that are already allowing scientists to learn more about how Martian winds affect the landscape. Coarser grains are concentrated on the crests, with finer grains more dominant in the troughs, a characteristic of "ripples" rather than of dunes, which are shaped by stronger winds. "This gives us a better understanding of the current erosion process due to winds on Mars," said Shane Thompson, a science team collaborator, and a senior at Arizona State University at the news briefing.
The images and composites along, of course, cannot tell the whole story. The measurements from all the other instruments -- which will reveal the physical and chemical composition and mineralogy of the soil and the rocks -- are needed to really get a grip on what happened geologically at these two Martian locations.
"We know these rocks are layered and layered rocks are interesting, but textures don't tell you how they formed," Arvidson said of the outcrop at Meridiani Planum. "They could have formed in a windblown environment, windblown deposits, or in an explosive volcanic environment, could have formed in a water environment. So we really need to get up and clean off [this part of the] outcrop - and get the windblown material off using the RAT, and getting the chemistry and the mineralogy at a number of sites -- and then put the whole story together."
That story "is right around the corner," Arvidson assured reporters at the weekly briefing. "But we need to finish the set of experiments and get data down from the spacecraft, processed and analyzed and then I think the story will be known. Right now, it's truly in the world of multiple working hypotheses. Water is involved, but only in some of the hypotheses and until we finish these coordinated experiments on this outcrop we don't know what the right hypothesis is. This is going to take a few sols to finish and few more sols to get the data down and yet a few more sols to figure out in terms of processing and analyzing the data."
If the uncontainable excitement around JPL, and the intriguing images that consistently being returned are any indications, it's going to be a pretty good story.
Coming up: new flight software, new experiments, new locations
New flight software will be sent to both rovers in a few weeks, and in addition to fixing Opportunity's heater problem, it will improve onboard navigation capabilities. "We want to be more robust for the terrain we're seeing," Trosper said. "We have had a plan to do this for quite a while, and the main changes we're making are all robustness changes to the types of terrains that we're seeing in the images. We also want to speed up the autonomous navigation algorithms so that we can drive further each day."
As the sols turn, the science team is planning on trying some "nighttime astronomy experiments," as Bell put it. "We'll be looking at Phobos and Deimos [Mars' two moons] to determine dust opacity at night he said. They can and will also be looking at stars. "And, we'll also be attempting something that's never been done in the history of space exploration -- we'll be trying to take the first images of an eclipse from the surface of another planet, watching Phobos and Deimos pass in front of the Sun," he said. The "eclipse seasons" begins next week, he added.