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Soil Chemistry Experiment Likely To Fly To Mars Again


Jet Propulsion Laboratory UNIVERSE
Pasadena, California - Vol. 26, No. 24 - November 27, 1996

Soil Chemistry Experiment Likely To Fly To Mars Again


An instrument similar to the U.S. instrument flown on the doomed Mars '96 mission-known as the Mars Oxidant Experiment (MOx)-is likely to fly on a future mission to Mars because of its scientific value in helping scientists understand the nature of oxidation reactions and soil chemistry on the surface of the red planet.

Mars '96, carrying instruments from 12 countries including the United States, France and Germany, was lost a day after its launch on Nov. 16, when the Proton launch vehicle's fourth-stage booster failed to catapult the spacecraft out of Earth's gravity and on its way to Mars.

During the last eight months, a team of engineers at JPL had made numerous trips to Moscow to complete the integration of the two identical MOx instruments, each mounted on a petal of two small Russian landers that were to be deployed on the surface of Mars in September 1997. Despite the tragic setback of Mars '96, collaborative efforts with the Russians proved to be an invaluable learning experience for the U.S. team.

"Final integration was an engineering milestone for the U.S. experiment, culminating a development effort which started in 1992," said Mark Herring, project manager at JPL. "In the course of our travels, we gained a lot of experience participating in this collaborative effort as part of an international mission."

MOx, built at JPL as part of the expanding U.S.-Russian cooperative effort in space exploration, was designed to measure the rate at which metals and organics corrode when exposed to the Martian environment. The $8 million instrument was carried into space from Baikonur Cosmodrome, Kazakhstan, Russia, at 11:48 p.m. Moscow time (12:48 p.m. Pacific time) on Nov. 16, but the spacecraft failed to leave Earth's orbit and plunged into the southeastern Pacific Ocean on Nov. 17, roughly between Easter Island and Chile.

The goal of the Mars '96 mission was to investigate the evolution of the Martian atmosphere, surface and interior. The mission was to acquire, using a variety of instruments, wide-scale, comprehensive measurements of the physical and chemical processes that occur on Mars today and those that took place in the past.

Loss of the mission leaves a gap in the science information that was to be gathered during NASA's 10-year-long program of robotic exploration. Among others, scientists working on the MOx experiment, however, are optimistic that an experiment very similar to MOx will fly to Mars in the next century.

"The importance of the surface chemistry on Mars to our understanding of the Martian environment, including the search for life, makes MOx a leading contender for flight on a future mission," Herring noted.

The experiment was designed to further investigate the presence of a strong oxidizing agent in the Martian soil, which was inferred from the results of the biology experiments onboard the Viking landers in the mid- 1970s.

"We had hoped MOx would be able to tell us more about the surprisingly reactive properties of the Martian soil first detected by the Viking biology experiments and tell us if this reactivity is the cause of the complete absence of organics in the surface soil on Mars," added Dr. Christopher McKay, project scientist at NASA's Ames Research Center in Mountain View, Calif., who plans to propose that a MOx-like instrument be built for a future mission to Mars.

"If we plan to search for the organic remnants of early life on Mars with future missions, 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," he said. "Viking, for instance, dug under a rock as deep as 11 centimeters (4 inches) but found only oxidized sand."

MOx uses chemical sensor technology originally developed at the Sandia National Laboratories in Albuquerque, N.M. The instrument measures the oxidizing power of the Martian soil and atmosphere using a detector that monitors the change in reflectivity of a thin chemical film that is exposed to the Martian environment. The instrument, which weighs only 1.3 kilograms (3 pounds), relies on its own power source, a set of batteries, to carry out the measurements.

Upon landing and deployment, MOx would have operated autonomously, Herring said, according to a sequence that would have been programmed into its internal "read-only memory." While the mission was designed for a one-year lifetime, the operating life of MOx was limited by its battery power source. Depending on the actual conditions on the surface of Mars, the operating time would have been between 80 and 160 days.

"The instrument's sensor head was located on a petal of each of the two Russian small stations and was comprised of eight sensor cell assemblies, four of which were designed to contact the soil and four that would have been exposed to the atmosphere," Herring said. "Within each cell assembly there were six active sensing sites and six reference sites, for a total of 96 sites.

"The active sites were protected by thin membranes of silicon nitride, which would have protected the sensor films from premature oxidation," he explained. "These membranes would break upon deployment, exposing the active films. The reference sites would remain permanently sealed. The sensor films were selected to provide a broad range of chemical reactions. Each film type was duplicated in the air and soil cells."

Each of the 96 sensor sites was illuminated by two light-emitting diodes (LEDs), one operating at a wavelength of 590 nanometers and the other at 870 nanometers. The reflected signal would have been measured by a silicon photodiode detector array. The sensor sites were coupled to the LEDs and the detector array through fiber optics.

Data from the experiment promised to shed new light on a range of chemical activity occurring in the Martian environment and tell scientists whether oxidizing agents were present in the atmosphere. The Martian atmosphere is 95.3 percent carbon dioxide; a few percent nitrogen and argon; and trace amounts of oxygen, carbon monoxide, water and ozone. In addition, specific minerals in the Martian surface material may have been identified if their pattern of reactivity was sufficiently distinct.

Loss of the mission and new strategies for ensuring that a MOx-like instrument flies on a Mars mission in 2001 or thereafter will likely be addressed in the next several months.

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