12 March 2004

Calling All Martians!

The Viking Mission Labeled Release Experiment and the Search for Martian Life: Part I

Reginald Smith

With the recent discoveries of new evidence of large bodies of water on Mars, the obvious question in everyone's mind is this: Was there in the past, or is there now, life on Mars?

The question is as pressing now as when we first realized Mars was a heavenly body, when Schiaparelli first saw his "canals" on the surface of Mars, when H. G. Wells wrote "War of the Worlds," and, most importantly for this series of articles, when the Viking landers touched down on the harsh Martian landscape in 1976.

The trio of Viking experiments that searched for evidence of life are among the most famous in the history of human interplanetary exploration. Their ambiguous results are still debated to this day. Given recent events and the extremely interesting nature of this debate, it is fitting that we revisit this spectacular mission and its results.

This will be the first in a series of articles. Rather than merely report on the results of the Viking missions, I will invite the entire SAS community to join me in a reanalysis of the actual data from the most important of the three experiments designed to find life, the Labeled Release Experiment.

In this first article, I will explain the Labeled Release experiment, its key science and purpose, and where to retrieve the data from the Viking Missions.

I will follow the steps I outlined in a past E-Bulletin about how amateur scientists can analyze interplanetary space probe data (you can find this article here ).

I want to strongly emphasize from the outset that these data are not beyond the reach of the average person. I will try to explain necessary concepts as I go along. The only thing that is necessary to analyze this data is a willingness to learn and one package of complex scientific analysis software: Microsoft Excel.

The Viking Mission

On August 20, 1975, and September 9, 1975, Viking 1 and Viking 2 were respectively launched to the Red Planet. Viking 1's lander touched down on July 20, 1976 in Chryse Planitia, and Viking 2 touched down on September 3, 1976 in Utopia Planitia ( see these locations on Mars ).

The Viking orbiters remained in orbit around Mars to relay the data sent from the landers back to Earth. The Viking landers made a variety of measurements from many different instruments, but the one that will concern us here involves the search for life. In total, there were three experiments in the Viking's biological payload (1).

1. Pyrolytic release experiment

This experiment was designed to test for the fixation of carbon dioxide (CO₂ ) and/or carbon monoxide (CO) into organic compounds. Both gases are present in the martian atmosphere, especially CO₂, which comprises about 95 percent of martian air. A sample of Martian soil collected by Viking's robotic arm was exposed to a CO₂ :CO mixture having a ratio of 95:5. After incubation for five days, the sample was heated to about 625 degrees Celsius, flushed with helium to remove any trapped CO or CO₂ , and then heated again to detect organic compounds that might have been created by microbes in the soil.

2. Gas exchange experiment

In this experiment, a sample of Martian soil was incubated with water and CO₂ to look for the production of gases associated with metabolism on Earth, especially oxygen and nitrogen.

3. Labeled release experiment

This experiment was designed to test for the metabolism of simple organic compounds into CO₂ by putative martian microorganisms. The Labeled Release (hereafter LR) experiment searched for heterotrophic organisms, those that metabolize simple carbon compounds. On Earth, organisms like animals and fungi that consume and metabolize nutrients are heterotrophic. Organisms like plants and photosynthetic bacteria that manufacture their own nutrients from sunlight, CO₂ and water are autotropihic.

An aqueous solution containing many simple carbon compounds was injected into a martian soil sample. The catch is that these carbon compounds have been "labeled" by substituting the carbon isotope carbon-14 (14 C) for their carbon atoms. 14 C is the same radioactive isotope used in dating human remains. 14 C is a beta emitter. Beta radiation consists of electrons emitted when neutrons become protons.

Both Viking landers conducted all three experiments. The results of the pyrolytic and gas exchange experiment did not indicate the presence of life. However, one must keep in mind that each of the three experiments looked for different elements of metabolism known to Earth organisms. Therefore, the results of these two experiments (which I will describe more in detail later) should not conclusively rule out martian life. There is more to discuss about the results of the labeled release experiment.

The Labeled Release (LR) Experiment Setup

References 2 and 3 discuss the design of the labeled release experiment in detail. Briefly, the Viking probe robotic arm retrieved samples from the surrounding soil and placed them into an onboard incubator, where they were sheltered from the martian environment.

Three measurements were made in the LR experiment, two of temperature and one of the beta radiation emitted by 14 C.

The temperature of the head space over the sample and the temperature of the beta detector were measured so that any temperature related error could be eliminated from the final analysis.

During the first 24 hours of the experiment, the beta detector monitored the background radiation of the incubator so it could be removed from future readings. Most of the background radiation came from the radioactive power sources in the landers.

Next, the soil sample received its first nutrient injection. For the first two hours, radiation measurements were made every four minutes. After the first two hours, measurements were made every sixteen minutes. Temperature measurements were also made every sixteen minutes.

The sample was monitored for about seven to eight sols (a sol is a martian day or 24.6 Earth hours), and then a second nutrient injection was made. The sample was typically monitored for at least another six sols. Nine LR experiments were performed, four in Viking 1 and five in Viking 2. Three of these experiments were controls in which the sample was exposed to high temperature before nutrient injection to try to sterilize any microorganisms. This would separate false indications of metabolism caused by chemical phenomena.

Obtaining the Viking LR data

You can obtain the Viking data in two ways. First through NASA's Planetary Data System (http://pds.jpl.nasa.gov ). Go to Data Services and choose "Data Search" from the menu bar. Then search for the Viking mission and the Labeled Release experiment with the beta detector as the instrument. Alternatively, you can go straight to the data archive at http://wufs.wustl.edu/geodata/vl1_vl2-m-lr-2-edr-v1/vl_9010/ (3). Just download everything in the data folder and peruse the other folders for useful information explaining the mission.

Format of the Viking LR data

The Viking data is organized by experiment and by measurement type. There are different files for each experiment and each measurement. The file names have the following format:

•  The first three characters are "vl1": and "vl2," corresponding to Viking Lander 1 or 2, respectively.

•  The next two characters are "c," which stands for the experiment cycle, followed by the cycle number. Recall that there are nine total cycles, four on Viking 1 and five on Viking 2.

•  The last two characters identify the type of measurement. The files ending in "rc" are the beta radiation counts. The files ending in "dt" are the detector temperatures and those ending in "ht" are the head end temperatures.

•  The files ending in ".lbl" explain the format and meaning of each data field. The ".tab" files contain the actual data. The .tab files can easily be imported into Microsoft Excel by selecting "File," "Open," and checking comma delimiting.

What's Next?

What did Viking find on Mars? What do the data tell us? What questions are raised by the results?

These questions will be addressed in the next installment of this article, in which we will begin analyzing the data and interpreting its results.

Until then I encourage you to look over the data and visits web sites about the Viking project to get as much information as possible. If you are really ambitious, visit an academic library at a college or university and find the papers below for a deeper look into the Viking program mission. These papers will give you considerably more technical details than we can include in these E-Bulletin articles.

Main References

1.  Klein, Harold et. al. "The Viking Mission search for life on Mars." Nature, 262 July 1, 1976 24-27.

2. Klein, Harold et. al. "The Viking Biological Investigation: Preliminary Results." Science, 194 October 1, 1976 99-105.

3. "volinfo.txt" file, Viking Data directory http://wufs.wustl.edu/geodata/vl1_vl2-m-lr-2-edr-v1/vl_9010/

Other References

We'll refer to some of these in future installments.

Clark, L. V., D. S. Crouch, and R. D. Grossart, Summary of Primary Mission Surface Sampler Operations, Viking '75 Project Document VFT-019, 1977.

Hubbard, J. S., J. P. Hardy, and N. H. Horowitz, Photocatalytic production of organic compounds from CO and H2O in a simulated martian atmosphere, Proc. Nat. Acad. Sci., U.S. 68, 574. 1971.

Klein, H. P., Automated life-detection experiments for the Viking Mission to Mars, Origins of Life, 5, 431-441, 1974.

Klein, H. P., General constraints on the Viking biology investigation, Origins of Life, 7, 273-279, 1976.

Levin, G. V., Detection of metabolically produced labeled gas: The Viking Mars lander, Icarus, 16, 153-166, 1972.

Levin, G. V., and P. A. Straat, A search for a nonbiological explanation of the Viking labeled release life detection experiment, Icarus, 45, 494-516, 1981.

Levin, G. V., and P. A. Straat, Completion of the Viking labeled release experiment on Mars, J. Mol. Evol., 14, 167-183, 1979a.

Levin, G. V., and P. A. Straat, Laboratory simulations of the labeled release flight data: Kinetics following second nutrient injection and the nature of the gaseous end product, J. Mol. Evol., 14, 185-197, 1979b.

Levin, G. V., and P. A. Straat, Life on Mars? The Viking labeled release experiment, Biosystems, 9, 165-174, 1977a.

Levin, G. V., and P. A. Straat, Recent results from the Viking labeled release experiment on Mars, J. Geophys. Res., 82, 4663-4667, 1977b.

Levin, G. V., and P. A. Straat, Labeled Release - An experiment in radiorespirometry, Origins of Life, 7, 293-311, 1976a.

Levin, G. V., and P. A. Straat, Viking labeled release biology experiment: Interim results, Science, 194, 1322-1329, 1976b.

Moore, H. J., R. E. Hutton, G. D. Clow, and C. R. Spitzer, Physical properties of the surface materials at the Viking landing sites on Mars, USGS Prof. Paper 1389, 1987.

Pike, D. D., P. M. Coulson, D. S. Crouch, and J. P. Yarbrough, Summary of Extended Mission Surface Sampler Operations, Viking '75 Project Document VFT-023, 1978.

Yoder, C. F. and E. M. Standish, Martian precession and rotation from Viking Lander range data, J. Geophys. Res., 102, 4065-4080, 1997.