Scooping a small hole on the surface of Mars could yield big news about the planet’s past.

By Jay Wrolstad

Lindsey Brock ’04 EP (left) and lab manager Tim Bond (right) worked with Harry Stewart (center) and Robert Sullivan on Mars rover wheel tests in Cornell’s Winter Laboratory.

Sometimes just spinning your wheels is more productive than it sounds. Especially if those whirling wheels churn the virgin soil on a planet millions of miles from Earth. That is the premise promoted by two Cornell colleagues and approved by NASA as worthy of inclusion in the Mars Exploration Rover (MER) mission.

In the experiments developed by Harry Stewart, a geotechnical engineer and associate professor in the School of Civil and Environmental Engineering, and Robert Sullivan, a planetary geologist and senior research associate in the Department of Astronomy, the wheels of the Spirit and Opportunity rover vehicles serve as tools to reveal what lies beneath the topsoil of the Red Planet.

The cutting-edge, high-resolution cameras carried by Spirit and Opportunity, which landed on Mars earlier this year, reveal the texture, shape, and angularity of the soil and mineralogy of the planet, but images provide only part of the story. “Pictures are useful,” Stewart says, “but having a physical measurement is something that’s very important.”

The rovers are essentially six-wheel-drive vehicles, with a motor and navigation system in each hub. Sullivan and Stewart, with enthusiastic support from key rover engineers at the Jet Propulsion Laboratory in Pasadena, Calif., devised a way to lock five of the wheels and rotate the sixth to create a small trench a few inches deep. “We want to delve into the third dimension and take a closer look at the subsurface,” says Sullivan. “You can only tell so much by rolling along the surface and taking pictures.” Digging would give scientists access to soil underneath the surface that may be significantly older than the top layer.

In scrutinizing the composition of the Martian ground, these experiments mesh with the $820 million MER mission’s main objective: to search for and characterize an array of rocks and soils that offer evidence of past water activity on Mars. They add scientific value to the mission without adding more tools to the already crowded arsenal of instruments on the rover vehicles, Stewart says. “

Another measurement here, or another observation there, will add to the knowledge we gain from the mission,” says Stewart, who is director of the civil infrastructure laboratories at Cornell’s School of Civil and Environmental Engineering. His research focuses on soil-structure interaction, soil dynamics, and earthquake engineering. “What we do in civil engineering is learn about soil as an engineering material,” Stewart says. “We do tests on strength characteristics and evaluate soils and their engineering properties. Everything we build on earth is founded in or on soil and rock. It’s the number one construction material by volume in the world.”

Now he’s looking at soil that’s out of this world. In 2001 Sullivan, who has participated in NASA’s Mars research for several years, approached Stewart with the idea for a collaboration to study the physical and mechanical properties of Martian soils. The resulting proposal was one of 28 selected by NASA (from more than 80 submitted) for inclusion in the MER mission.

By now, most people are familiar with the images beamed back to earth by Spirit and Opportunity as the golf cart-sized vehicles cruise uncharted extraterrestrial territory. The rovers have provided panoramic views of the landscape, zoomed in on rock formations, and tested the atmosphere with sophisticated sensors. Sullivan and Stewart were confident that the rovers could go a step further, providing information about the texture and strength characteristics of the Martian soil by putting the wheels in motion.

To prepare their experiments, Sullivan and Stewart brought a Mars Rover wheel to Cornell’s George Winter Civil Infrastructure Laboratory and tested its interaction with various types of soils. Sullivan also visited the Martian terrain proving ground at the Jet Propulsion Lab (JPL), the lead laboratory for NASA’s robotic exploration of the solar system, to gather information on how the wheels interacts with different soil types and sand.

Each of the 10-inch wheels is fashioned from a single piece of aluminum, hollowed out to reduce the weight. Traction is provided by a paddlewheel-type tread that is machined onto the outside of the circular frame. Allowing the wheel to spin in the test bed layered with yellow, pink, and green sand, Sullivan observed how the wheel created piles of soil tailings and where each color of sand was concentrated in the tailings. With this information, scientists can see how deep the rover wheel is when it reveals a specific color and estimate, by comparison, which soils in the tailings come from various depths when the same process is used on Mars—since the soil there won’t be color-coded.

“By examining those tailings on Mars with a microscopic imager, we can determine the characteristics of the larger soil grains such as size and angularity,” Sullivan says.

At the same time, monitoring the motor current driving the wheel as it spins and digs can indicate the strength of the soil; if the motor struggles or spins easily, scientists can determine the relative strength of the soil.

Interpreting the results of the trenching tests is Stewart’s primary role. Stewart notes that if the tracks made by a rover wheel in a variety of soil types found on Earth match those made on Mars, it’s possible to deduce that the soils there have the same textural characteristics as those on earth.

“What’s interesting is that we may be able to expose more fresh soil on Mars in other ways as well,” he says, citing the rover’s rock abrasion tool that grinds off a layer of rock and exposes unweathered material, thereby offering a clearer view of Martian mineralogy for a spectrometer.

Another possible test involves locking the wheels on one side of the rover and rotating the vehicle so that it drags sideways through the soil, exposing fresh material. Then the rover backs up for a look at the exposed soil and employs other instruments to probe deeper into the skid mark.

What lies beneath the surface on Mars is anybody’s guess, although there have been some tantalizing hints. Data compiled from previous trips to the planet by the Viking and Pathfinder missions reveal that it is an inhospitable, parched planet blanketed by red dust. But previous probes have shown that the soil looks very different from red dust, leaving more questions than answers.

In the first weeks of exploration the two rovers detected a site with soil rich in hematite, an iron ore typically formed in the presence of water, that is a prime wheel-trenching target.

“By going deeper into the soil we don’t know what to expect,” says Sullivan. “One hypothesis, based on previous detections of a crusted soil, is that minute amounts of water known to be in the atmosphere have interacted with the soil over a very long period to give it cohesion.”

There is a consensus among scientists that the Martian soils consist partly of very fine-grained particles, in the two- to three-micron range, and should behave similar to fine-particle deposits found closer to home. The biggest difference, of course, is that there is no water on Mars.

“On Earth, it is how these very fine particles interact with water that gives them a wide range of characteristics,” Stewart explains. “On Mars they will behave like dry powders in many cases.”

In early February, Stewart and Sullivan were still anxiously waiting for the trenching tests to begin. Software glitches with Spirit, the first rover to touch down in January at the Gusev Crater site, resulted in communications problems that pushed back all tests. The two men acknowledged that their experiments are low on the list of mission priorities but were nonetheless disappointed when a scheduled trenching test using Spirit was postponed.

“They are preparing for some long drives by the rovers, taking trips that can’t be simulated in a research facility, so other scientific endeavors will have to wait,” said Sullivan, who was spending time at JPL during the mission as a member of the science team. In addition to the wheel experiments, Sullivan has an operations role involving the evaluation of data collected by the rovers’ pan-cam and microscopic imager tools.

Although Stewart remained on campus, he followed the mission with rapt attention. “I’m watching the daily briefings by NASA and JPL, and I’m getting tons of mail from those working on the project, although I’m still learning the jargon.”

For Stewart, whose specialty tends to keep him focused on terra firma, participating in interplanetary science is a new experience. “I’ve kept my work pretty much confined to Earth,” he says, “until now.”

Location notwithstanding, there are many aspects of this space mission that parallel civil engineering on Earth. “There are lab tests designed to characterize the Martian soil that we do all of the time,” says Stewart. In geotechnical engineering, he explains, it is common practice to evaluate soil for its stress, strain, and strength characteristics and to perform experiments using that soil to interpret the results. “Rob is working on a soil-structure interaction problem, with the structure being a wheel on Mars,” says Stewart.

Sullivan’s previous research has included studying Martian drifts and dunes, Martian avalanches, along with the geology of asteroids and Europa, a moon orbiting Jupiter. A principal investigator for NASA’s Mars Data Analysis Program since 1998, he served as a scientist in the earlier Mars Pathfinder mission. “I have spent my career in planetary geology, and I like moving parts, so this was a unique opportunity that I could not pass up,” he says. Weeks of anticipation finally paid off for Sullivan and Stewart, when the first rover trenching exercise was completed on February 16, fulfilling their hopes to conduct soil experiments during the mission.

NASA decided that Opportunity, which is in the Meridiani Planum region on the opposite side of the planet from Spirit, would be used for the first trenching tests. Opportunity landed in a crater, with bedrock in close proximity, and has sent back intriguing images of layered and pebbled soil deposits.

The single-wheel rover trenching process actually involves a series of complicated maneuvers, conducted by remote control, before and after each step of single-wheel digging in order to coordinate the investigations of each depression and the pile of soil tailings created when the dirt is moved. Sullivan spent many hours at JPL before the landing, working with rover engineers to perfect the complex sequence of digging and rover maneuvers that result in a unique rover “ballet” that gets the job done.

The efforts were worth it. “The resulting trench exceeded our expectations,” says Sullivan, who was at JPL monitoring the test. Opportunity was situated on a slope in a crater, which required some last minute refinements to the trenching sequence. To Sullivan and Stewart’s delight, the entire science team was as fascinated with the result—a trench 20 centimeters wide, over 50 centimeters long, and 9 centimeters deep—as they were. The science team elected to stay two “sols” (Martian days) to study the floor and walls of the trench with the spectrometers on the rover arm. The composition of materials found at the bottom of the trench differs from the composition at the surface.

 “It was very exciting to get the data back,” says Sullivan, “because the characteristics of the trench—how it all really turns out—depends partly on how cooperative Martian soil is. Thorough preparation is important, but if the soil is ‘hard as a rock’ we wouldn’t be able to dig. Fortunately, things turned out well and the science team was thrilled with the result.” Soon after, Spirit’s science team decided to trench and was equally thrilled with the results, also electing to spend two sols to collect data at their trench site. Final analysis of the soil will take awhile, but Sullivan describes the initial observations as revealing cloddy soil along the walls of both trenches and soil that is somewhat brighter at depth than at the surface. Soil compositions on the floors are different from the surface.

Stewart’s and Sullivan’s work in the laboratory at Cornell really are just beginning. “Depending upon what we learn in the laboratory, what we have uncovered could change the intepretation of data collected thus far by the rovers,” says Sullivan. Results from the trenching tests could put a whole new “spin” on what the MER mission reveals about Mars.