March 3l, l977

       While preparing Viking Lander 2 for the fast approaching Martian winter, flight controllers at Jet Propulsion Laboratory are working on new problem: the lander's sampler arm stopped while trying to pick up soil for the final biology experiment.

       Instead of picking up soil, the sampler shut down about midway through its sequence. The collector head is pointing directly at camera l.

       Engineers are diagnosing the problem. When they complete that work, they will stow the boom until Martian spring, when they will try to reactivate it. Meanwhile, all soil sampling activities have been suspended.

       A final planned Lander 2 biology experiment has been cancelled. During that experiment the soils were to have been incubated at much colder temperatures than had been used in earlier experiments. Heaters were turned off during the soil acquisition sequence that failed. They are still off, and nutrient in two experiments has frozen.

       However, the heaters will be turned on again March 3l, and about April 2l, scientists will start sequence to do two experiments on the soil already in the instruments from previous soil tests. Gas exchange will heat the soil several times and analyze resulting gas changes in further attempt to understand the chemistry of the Martian soil.

       Labeled release will inject nutrient onto larger than normal (approximately 2.2 cc instead of 0.5 cc) amount of soil. The objective is to try to understand if it is the quantity of soil tested that limits the size of the strong responses seen in earlier cycles.

       Meanwhile, temperatures at Utopia Planitia, where Viking Lander 2 touched down last Sept. 3, are nearing the frost point of carbon dioxide, the lowest experienced on Mars. The critical temperature is -l23 degrees Celsius (-l90 Fahrenheit). That compares with the relatively balmy daily highs of -25 to -30 Fahrenheit recorded during the primary mission.

       Viking Lander 2's science duties are being reduced in hopes of surviving the bitter winter.

       From April 8 through l4 series of commands will be sent to Viking lander 2 to collect reduced scientific information. The survival mission will begin April l7. All remaining power will run heaters in an attempt to maintain survivable temperatures inside the lander.

       The meteorology station and the seismic instrument will continue to gather data through the winter. The cameras will take occasional pictures, looking for growth of carbon dioxide frost on the surface and on the spacecraft. The inorganic soil analysis instrument will run periodic tests, too, on the chance that some windblown dust will enter the open funnel. The organic-analysis experiment will be turned off April 5. All other experiments will be suspended until Martian spring.

       As long as sunlight shines strongly on the Martian surface and no frost forms, temperatures remain above the frost point of carbon dioxide. But when an overnight layer of ice forms, it reflects much of the early-morning sunlight back to space; surface (and lander) temperatures then fall rapidly.

       High-altitude water-ice clouds and atmospheric dust help screen out sunlight, accelerating the plunge below safe operating levels.

       Unlike snow on Earth, which is frozen water vapor (a minor constituent of our atmosphere), snow on Mars is mostly frozen carbon dioxide, about 95 per cent of the Martian atmosphere. Water freezes at Earth's surface at 0 degrees Celsius (32 degrees Fahrenheit); carbon dioxide freezes at the Martian surface at -l90 F. That is colder than the spacecraft was designed to survive. Scientists predict winter will begin sometime after mid-April and last until about mid-October. By the end of June, they say, frost should stay on the ground throughout the day. layer of frost may even coat parts of the lander itself.

       Viking Lander 2 is located about 48 degrees north latitude, roughly equivalent to the United States-Canada border. Scientists believe the Martian north polar ice cap may spread that far south.

       The severe winter cold is expected to have little effect on Viking Lander l. It is in the Chryse basin within the Martian tropics -- about the same latitude as Honolulu, on Earth.

       Viking is managed for NASA by Langley Research Center, Hampton, Va. G. Calvin Broome is mission director for the Viking Extended Mission, scheduled to continue through May l978.