Mars: Upstairs, Downstairs

Pasadena, Opportunity mission Sol 5, Spirit mission Sol 26

The mission scientists for the twin Mars rovers meet and work on different floors at JPL. The Gusev site dominates planning on the fourth floor. The Meridiani site occupies thought on the fifth floor. Even the chairs are color-coded so that whether day or night on Mars, whether looking at pictures from opposite sides of the planet, the scientists can focus on the task in front of them: to follow the water. Both teams want to know whether there was once liquid water on Mars and if so what happened to all of it.

Rock, Paper, Scissors…

Spectacular stitch pattern left in fine Meridiani soil from airbag seams

For Opportunity, it is the 5th martian day since landing, or Sol 5. For Spirit, it is Sol 26. Since their eighteenth martian day, Spirit scientists have been catching up with the wealth of pictures and chemical spectra that are still just a fraction of the mission analysis they ultimately hope to return from Mars.

The prevailing theory as to why the Spirit rover has taken a scientific pause may even have something to do with this early haul of data: too many files have accumulated in the rover’s flash memory for proper commandability. The Spirit engineering team plans to delete some of the cruise files, those collected in transit from Earth to Mars over seven months, later today. The Opportunity engineering team will do the same before the twin of Spirit, which includes the same software architecture, reaches its own limits in two weeks.

Scissors Cut Pictures

With the latest addition of high-resolution panoramic images, or octets that cover 45 degrees of a complete horizon, Meridiani is starting to be comparable to what scientists on the fourth floor–the Gusev floor–also have in front of them. Both missions still await enough data transfers to assemble a true high-resolution picture of their respective sites. While the Spirit site is better characterized because of its nearly three week headstart, most of the earliest images focused on the hills on the horizon to look for evidence of layering and the sheared soil at its feet, the so-called ‘magic carpet’. Dr. Jim Bell, lead scientist for the Pancam instrument, noted that Gusev still needs high-resolution study of a crater within its driving range, particularly as scientists have become more intrigued with the outcrops and bedrock at the Opportunity site. Bell said there was some speculation as to whether some layering may be visible near that Gusev crater too, and imagined ‘the crater could become a near-term target when Spirit is back on track.’

Scientific Papers Will Be Made from Dust

This "mud-like" Gusev material near the base of the Spirit lander is called the Magic Carpet. It appears to have curled when the airbags planed the cohesive soil.Credit: NASA/JPL/Cornell

One common feature of both Gusev and Meridiani is the relatively little dust on the rocks compared to other places on Mars. This is less a matter of luck than good planning. Since both Viking sites and the Pathfinder mission couldn’t see much of a true rock surface, at least not without the interference of a dust coating, one site selection criterion for both MER missions was less dust. Where dust might obscure a rock’s true composition, their instrument package would need to be able to penetrate it. Neither mission group has yet had the chance to exercise the Rock Abrasion Tool (or RAT), but this diamond drill has the ability to put scientists up-close with a clean view at either location. One reason given for the low amount of dust coating at Meridiani and Gusev is the influence of wind. Mars may be dusty, but relatively fine dust with the consistency of ‘kitchen flour’– according to science team member Matt Golombek–doesn’t accumulate at these sites compared to the previous three landing locations.

Before the interruption of science at the Spirit site, the long-term plan had already taken shape. The rover first sampled the soil near its base station. Unlike the fine silt seen at the Meridiani site, the Gusev crater soil seems stangely cohesive, almost ‘mudlike’ when sheared by the rover’s airbag retraction. Bell described the effects of ‘bizarre cohesive force…The soil is different at Magic Carpet. When the rover planed it like a piece of wood, it curled. …[But Opportunity is] like when you run a rake, like a Japanese sand-painting. Meridiani doesn’t have the same coherent layer.’

"There is alot of sulfur and chlorine at the Spirit site," said Bell. "And less salt [to hold the soil together] at Opportunity."

The difference in salts, or charged ionic particles, may reveal why Gusev smears into some strange curl, but Meridiani soil can be smoothed over and even hold a seam-pattern from an airbag stitch. Meridiani soil is not as cohesive.

Golombek compared Meridiani soil to the consistency of ‘kitchen flour’. "There is a crust at Gusev, or duricrust," said Golombek, "but no evidence of that at Opportunity." When the face plate of one instrument–the Mossbauer–was pressed to the soil at Gusev, this crust barely moved. What happens when that qualitatitve experiment is done at Opportunity will likely be different, since scientists already are debating two possible origins for the gravel-like dark grey granules that seem to "bleed" a brighter red color when disturbed.

"Like in the Mojave desert," said Golombek, "on fresh lava flows, pebbles can be kept on the surface by freeze-thaw cycles, and dust infiltration below." While having denser rock on top of less dense dust might at first seem counterintuitive, the net effects can be compared to shaking a box of cereal, in which lighter flakes fall to the bottom, and prop up what is actually heavier flakes and even fruit pieces. Mars ‘pebbles on top of dust’ story may share this surprising soil physics. "We will want to hit this with mini-TES", said Golombek, to test in infrared for densities and thermal properties of the soil.

This dramatic difference in soil color is one feature that occupies the soil properties group located on the fifth floor, among the Meridiani scientists. "We don’t know yet," said Golombek, if the dark granules are aggregates that are weathered on their surface but redder inside or a top layer that is grey, and a fine dust layer underneath that is red. In either case, when the airbags disturb this grey veneer, the surface color at Meridiani takes on a deep red or maroon shade.

Rocks Uncut So Far

The rock coverage at both sites was hoped to be low, less than eight percent in coverage. Because these large rovers (weighing nearly 390 pounds on Earth) were built to explore, the twenty percent rock coverage at previous Viking and Pathfinder sites would diminish their mobility on those surfaces. But while Gusev exceeded the selection team’s expectations, with less than three percent coverage of its relatively flat, smooth terrain, Meridiani scientists could hardly have been prepared for the nearly complete absence of large rocks above soil at the Opportunity site.

Top vertical slice through bedrock at Meridiani shows evidence for layering; Spirit’s Adirondack rock, bottom, an early science target

To investigate further after the soil analysis, Spirit’s first rock target, called Adirondack, seemed a good candidate for exercising the four major diagnostics for chemical and microscopic analysis. Whether the nearby crater called Sleepy Hollow remains a science destination after starting up again, the chance to go towards the eastern hills could occupy the Spirit maneuvers for quite some time. That was the plan: to understand this ancient 150 kilometer wide crater, and its past that shows signs of once being filled with water.

Mars Time

In contrast, the Opportunity team is just considering what to do after validating its mobile geological lab. Golombek said "there is agreement that we landed in a unique spot. We need to know the soil character, so we will take chemical measurements using mini-TES, take high-resolution pancam and nav[igation] cam images. There is interest in trenching the soil" [up to 20 centimeters down] "to understand the material properties such as the angle of repose. Remote sensing shows tantalizing targets on the horizon. Particularly the descent images show crater floors which are dark, basaltic and granular on top. Combined with the brighter, redder underneath and an almost whitish upcrop at crater rims, the sense is that Meridiani [as a whole] is different from where we are now."

Where Opportunity is now is inside a 20 meter crater. Where it goes once it pops above the crater rim, depends on agreements between the fourth and fifth floors at JPL. Mission manager, Jennifer Trosper, said "There is still lots of shuffling between Spirit and Opportunity teams. From entry to egress, more than half the team is needed for Opportunity, to get stood up and rolled off the lander. We have people in reserve, called ‘strategic people’ and the anomaly team for Spirit involves 15 to 20 people focused on software. They will get another 10 people for a return to its nominal timeline. We need to add science and engineering teams to get Spirit back on track."

So while science teams are thinking about opposite sides of the planet, the engineering teams have a keen interest in understanding both rovers’ ability for what they call ‘trafficability’. How to keep their landers going to the horizon is about mobile geology, doing what the real geologists on the fourth and fifth floors at JPL see and want to touch remotely on the large, clear images laid out now on their conference tables.

Related Web Pages

Mars Exploration Rovers
Spirit Condition Serious
Water Signs
Microscopic Imager
Gusev Crater
Pancam– Surveying the Martian Scene
Mössbauer spectrometer
Alpha Proton X-ray Spectrometer
Mars Rover: The Owner’s Manual