An instrument developed by the United States to measure the rate at which metals and organics corrode when exposed to the Martian environment is set for launch on Nov. 16 aboard two small landing stations on the Russian Mars '96 spacecraft.
The instrument, called the Mars Oxidant Experiment (MOx), was built at NASA's Jet Propulsion Laboratory, Pasadena, CA, and is part of the expanding U.S.-Russian cooperation effort in space exploration.
Integration and final testing of the experiment on the Russian landers, also referred to as "small autonomous stations," was completed in late October at the Lavochkin Research and Production Association in Moscow, where the landers were designed and assembled, said Mark Herring, manager of the experiment at JPL. Two of the MOx instruments will fly on the mission, one on each of the two landers.
"This was an engineering milestone for the U.S. experiment, culminating a development effort which started in 1992," Herring said. "In the course of integration on the landers, the U.S. team was required to take numerous trips to Helsinki and Moscow during the past year. We've gained valuable experience in the participation on an international mission."
The goal of the Mars '96 mission is to investigate the evolution of the Martian atmosphere, surface and interior. The mission, which will be launched by a Russian Proton launch vehicle from Baikonur Cosmodrome in Kazakhstan, is designed to acquire, using a variety of instruments, wide-scale, comprehensive measurements of the physical and chemical processes that occur on Mars today and those which took place in the past.
The Mars Oxidant Experiment was developed 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 hope MOx will 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," said Dr. Christopher McKay, project scientist at NASA's Ames Research Center, Mountain View, CA.
"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 added. "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, 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 will operate autonomously, Herring said, according to a sequence that is programmed into its internal "read-only memory." While the mission is designed for a one-year lifetime, the operating life of MOx will be limited by its battery power source. Depending on the actual conditions on the surface of Mars, the operating time will be between 80 and 160 days.
"The instrument's sensor head is located on a petal of each of the two Russian small stations 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," Herring said. "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," he explained. "These membranes will be broken upon deployment, exposing the active films. The reference sites will remain permanently sealed. The sensor films have been selected to provide a broad range of chemical reactions. Each film type is duplicated in the air and soil cells."
Each of the 96 sensor sites is illuminated by two light- emitting diodes (LEDs), one operating at a wavelength of 590 nanometers and the other at 870 nanometers. The reflected signal will be measured by a silicon photodiode detector array. The sensor sites are coupled to the LEDs and the detector array through fiber optics.
Data will be stored in the instrument's own internal computer memory and read out in response to a request from the small station's data system, Herring said. Command software is designed so that data acquisition takes priority over measurements. "If a measurement is in process when telemetry is requested, it will be halted and then continued after the telemetry session is complete," he said.
Another feature of the experiment's data transmission sequencing is its ability to transmit data three times in order to reduce the data loss associated with various communications links. During the mission, the experiment team will distribute calibration data and mission data sets in which data from the instruments are merged with pertinent mission information.
Also onboard the spacecraft, attached to the MOx electronics case, is a CD-ROM, entitled "Visions of Mars," produced by The Planetary Society, Pasadena, CA, which is analogous to the records carried into space in 1977 by the twin Voyager spacecraft. The Mars '96 CD-ROM contains a collection of science fiction stories, sounds and artwork which chronicle humanity's fascination with Mars over four centuries of human history, 10 alphabets, 17 languages and 26 nations. The collection covers the earliest references to Mars in science fiction to present day stories about the red planet. A label pointing to the location of the CD-ROM is mounted on the outside of the spacecraft and includes a microdot of 100,000 names of Planetary Society members and instructions on how to use the CD-ROM.
If launched on time, Mars '96 will reach the orbit of Mars in mid-September 1997, at about the same time as Mars Global Surveyor, and land the two small stations and two penetrators on the surface of the planet. On-time launches of both spacecraft will enable Mars Global Surveyor to assist in relaying data from the Russian small stations once its primary mapping mission begins in March 1998.
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