ALBUQUERQUE, N. M. — As the Russian Mars ’96 mission, scheduled for launch this month from the Baikonur Cosmodrome in Kazahkstan, hurtles toward the Red Planet, it will be carrying a key science instrument based on chemical sensor technology developed at Sandia National Laboratories.
With Mars ’96, Russia’s Space Research Institute has put together a small but science-rich planetary expedition. If it launches as scheduled on November 16, Mars ’96 will go into orbit around earth’s nearest planetary neighbor next September, when it will deploy two small landers and two surface penetrators.
The Mars ’96 mission has a distinctly international flavor – its suite of science instruments includes contributions from several countries. The US instrument, built at NASA’s Jet Propulsion Laboratory (JPL) with fundamental contributions from a team of Sandia scientists, is called the Mars Oxidant Experiment, or MOx.
Sandia engineers also made key contributions to NASA’s Mars Pathfinder mission, scheduled to launch from the Kennedy Space Center in December. Mars Pathfinder, whose science mission includes collecting Martian surface and atmospheric data, is also viewed by NASA as a major engineering flight. The mission is designed to demonstrate a low-cost approach to bringing down a lander and diminutive (22-pound) rover to the Martian surface.
Sandia engineers developed an innovative airbag assembly for Pathfinder. The airbag is designed to deploy after the Pathfinder lander has been slowed by parachute during its descent through the Martian atmosphere. That airbag array provides the final margin of protection for the craft and its little robotic rover, named Sojourner.
The airbag system was conceptualized and designed at Sandia and a one-third scale model prototype was built and tested at Sandia. Following R&D work at Sandia, NASA’s Jet Propulsion Laboratory awarded the prime contract for the airbag assembly to ILC Dover, which built the operational model.
Fiber-optic technology ideally suited for instrument
When Russian Mars ’96 mission planners approached JPL in 1992 and offered it a chance to contribute a science package to the upcoming Mars flight, JPL scientists remembered a sensor technology they had discussed with Sandia researchers a few months earlier. The subject of that discussion was micromirror fiber-optic-based chemical sensor technology. The NASA scientists realized this was the ideal technology for a compact, sophisticated instrument capable of characterizing oxidation reactions first observed during the US Viking soft landing mission 20 years ago.
The MOx instrument, explains Mike Butler, a Sandia physicist and a key designer of the device along with fellow Sandia scientist Tony Ricco, works by measuring changes in reflectivity that occur as a result of chemical reactions between various thin film coatings in the MOx instrument and the Martian soil or atmosphere.
“Basically, what we’re doing,” says Butler, “is shining light on a thin film of material and then looking at the amount of light that’s reflected back. We’re doing this by shining light into an optical fiber. The fiber carries the light down to the sensor surface where the reaction occurs. The reflected light is carried back through a parallel optical fiber to the detector.”
Upon landing and deployment, MOx will operate autonomously, following a sequence programmed into its internal memory. The instrument’s three-inch sensor head is located on a petal of each of the two Russian landers and is comprised of eight sensor cell assemblies, four of which are designed to contact the soil and four that will be exposed to the atmosphere. Within each cell assembly there are six active sensing sites and six reference sites, for a total of 96 sites.
The active sites are protected by thin membranes of silicon nitride, which protect the sensor films from premature oxidation. These membranes will be broken upon deployment, exposing the active film coatings. The reference sites will remain permanently sealed.
The specially tailored thin film coatings, including both metals and organics, react chemically with specific chemicals, for example ozone, hydrogen peroxide, or some solid oxidant that might be present. The resulting chemical reaction changes the reflectivity of the coating in a characteristic and measurable way. The sensor film coatings have been selected to provide a broad range of chemical reactions. Each film type is duplicated in the air and soil cells. By observing the change in reflectivity for a given coating, scientists can determine what chemicals were involved in the reaction.
Viking found strong oxidation reactions
The 1970s-era US Viking landers, which carried three instruments designed to look for signs of life, measured some strong oxidation reactions in Martian soil, but found no signs of organic material. During several years following the Viking landings, scientists developed a number of alternative explanations, none of them without drawbacks, for the oxidation reactions and the lack of observed organics.
The lack of organic materials, Butler says, was particularly troublesome. Meteorites, for example, are well known to harbor organic materials, and Mars is known to have been subjected to countless meteorite strikes over its history, yet no organics were found. Why? Again: Solar ultraviolet radiation acting on atmospheric carbon dioxide (of which Mars has a relative abundance) should produce organic materials. Where are they? Is there, scientists wondered, a chemical process on Mars that essentially “eats” organics? Viking posed lots of good questions, but at least in this context, few answers. Clearly, more data on Martian reaction processes were needed.
“If we plan to search for the organic remnants of early life on Mars with future missions,” says NASA scientist Christopher McKay, “then we have to understand the processes that are destroying these organics on the surface so that we know how deep we have to dig to reach unoxidized material. Viking, for instance, dug under a rock as deep as 11 centimeters but found only oxidized sand.”
While MOx promises to provide excellent data to augment Viking’s findings, Butler says, the challenge of building the instrument within the constraints laid down by the Russian mission planners was daunting. The entire instrument, Butler says, had to fit within the very small excess volume in the Russian lander, tucked in almost as an afterthought. The instrument was not to exceed 850 grams, nor consume more than 25 to 50 milliwatts of lander power, and that only for very short periods. Because the lander will not come down “soft,” but will be slam into Mars cushioned by an airbag assembly similar to Pathfinder’s, the MOx instrument had to be able to sustain landing shocks of 250 G’s, and function in an environment characterized by daily temperature variations of approximately 100 degrees Celsius. The instrument also had to be essentially autonomous: it had to provide its own central processor, command set, and memory.
“In my own opinion,” says Butler, “with the size, weight, power, time, and dollar constraints that were imposed it would be very difficult to do this through any approach other than a micromirror, fiber-optic-based system.”
Butler says the concept for the instrument has been refined considerably since it was first conceived.
“The original concept was Sandia’s,” Butler says, “but it has evolved considerably through the project at JPL. We’ve contributed to that evolutionary process.”
As continuing members of the MOx science team for the mission, Butler and Tony Ricco not only initiated the instrument concept, but also were involved with JPL from the project’s inception in refining the instrument design and developing specifications for the array of chemically sensitive coatings. In fact, three of the coatings to be flown to Mars (hydrogenated carbon films that emulate material expected to be deposited by meteorites on the Martian surface ) were deposited at Sandia, two by Sandia chemist Rick Buss and one (based on Buckminsterfullerene or “Buckyballs”) by Ricco.
Ultimately, Butler says, MOx, like the Viking experiments, may pose as many new questions as answer old ones. Still, he adds, the experiment will advance the state of knowledge.
“What we’re doing,” he says, “is flying a different experiment [from Viking] with the expectation that it will give us different information and allow us to see things a little better.”