The Case for Extant Life on Mars and Its Possible Detection by the Viking Labeled Release Experiment

Abstract

The 1976 Viking Labeled Release (LR) experiment was positive for extant microbial life on the surface of Mars. Experiments on both Viking landers, 4000 miles apart, yielded similar, repeatable, positive responses. While the authors eventually concluded that the experiment detected martian life, this was and remains a highly controversial conclusion. Many believe that the martian environment is inimical to life and the LR responses were nonbiological, attributed to an as-yet-unidentified oxidant (or oxidants) in the martian soil. Unfortunately, no further metabolic experiments have been conducted on Mars. Instead, follow-on missions have sought to define the martian environment, mostly searching for signs of water. These missions have collected considerable data regarding Mars as a habitat, both past and present. The purpose of this article is to consider recent findings about martian water, methane, and organics that impact the case for extant life on Mars. Further, the biological explanation of the LR and recent nonbiological hypotheses are evaluated. It is concluded that extant life is a strong possibility, that abiotic interpretations of the LR data are not conclusive, and that, even setting our conclusion aside, biology should still be considered as an explanation for the LR experiment. Because of possible contamination of Mars by terrestrial microbes after Viking, we note that the LR data are the only data we will ever have on biologically pristine martian samples. Key Words: Extant life on Mars-Viking Labeled Release experiment-Astrobiology-Extraterrestrial life-Mars. Astrobiology 16, 798-810.

FIG. 1.

LR results from California (“Aiken”) soil under martian conditions in the SN103 flight instrument. Nutrient was added either to untreated soil (active sequence) or to soil that had been preheated for 3 h at 160°C (control sequence). A second nutrient injection was added to the active and control samples approximately 7 and 5 sols, respectively, after first injection. Brief intervals where counts are reduced by approximately half reflect times when only one of the two beta detectors were utilized (single channel counting mode). [Reprinted from Levin, G.V. and Straat, P.A. (1977b) Life on Mars? The Viking Labeled Release Experiment. Biosystems, Vol 9/2–3, pp165–174, September 1977; with permission from Elsevier.]

FIG. 2.

LR response to first and second nutrient injection in VL1 cycle 1 (active) and VL1 cycle 2 (160°C control). [Adapted from Levin and Straat, 1976b]

FIG. 3.

Comparison of Viking LR response with those from several viable terrestrial soil samples. Experiments with terrestrial soils were conducted by the “getter” technique, except for the SN103 data, which were obtained with a flight instrument. All data have been normalized to flight LR instrument counting efficiency for direct comparison. The Viking response (VL1-1) is shown by heavy black squares with frequent time intervals toward the bottom of the figure. [Reprinted from Levin, G.V. Modern myths of Mars. Proc. Instruments, Methods, and Missions for Astrobiology, Proc. of SPIE 6309, 1–15, 2006; with permission from SPIE Proceedings; figure compiled from Levin and Straat, , .]

FIG. 4.

LR response to first and second nutrient injection added to Antarctic soil #664 in the TSM (active). A duplicate run was conducted on soil presterilized at 160°C (control). [Reprinted from Levin, G.V. and Straat, P.A. (1986) A reappraisal of life on Mars. Figure presented at The NASA Mars Conference in conjunction with the American Astronautical Society (AAS) held July 21–23, 1986, Washington, D.C., U.S.A. Published in The NASA Mars Conference, edited by D.B. Reiber, AAS Science and Technology Series, © 1988, v. 71, pp 187–207; with permission from Univelt, Inc.]

FIG. 5.

Complete VL1 cycle 3. A fresh sample was used in this active cycle. Data show times of first, second, and third injections. [Reprinted from Levin, G.V. and Straat, P.A. (1977b) Life on Mars? The Viking Labeled Release Experiment. Biosystems, Vol 9/2–3, pp165–174, September 1977; with permission from Elsevier.]

FIG. 6.

All first injection cycles of VL1. A fresh sample was used for the active sequences of cycles 1 and 3, whereas the sample used for active cycle 4 was stored for approximately 141 sols at 10–26°C prior to use. For cycle 2, a stored portion of the same sample used for cycle 1 was heated for 3 h at 160°C prior to nutrient injection. All data have been corrected for background counts observed prior to nutrient injection. [Reprinted from Levin, G.V. and Straat, P.A. (1979a) Completion of the Viking Labeled Release Experiment on Mars. J Mol Evol 14:167–183, 1979; Journal Molecular Evolution © by Springer-Verlag, 1979; with permission of Springer.]

FIG. 7.

All first injection cycles of VL2. A fresh sample was used for all sequences except cycle 5, which used the same sample as for cycle 4 but was stored for approximately 84 sols before nutrient injection. All soil samples were taken from the same site except the sample for cycle 3, which was taken from under Notch Rock. All data have been corrected for background counts observed prior to nutrient injection. [Reprinted from Levin, G.V. and Straat, P.A. (1979a) Completion of the Viking Labeled Release Experiment on Mars. J Mol Evol 14:167–183, 1979; Journal Molecular Evolution © by Springer-Verlag, 1979; with permission of Springer.]

FIG. 8.

VL2 cycle 2 LR responses to first and second injections. After correcting for background, the cumulative radioactivity evolved in the first peak following first nutrient injection was approximately 15% of that evolved during VL2 cycle 1 in the same time period; little else evolved over the subsequent sols. Following second injection, the peaks represent diurnal movement of gas back and forth from the test cell chamber to the detector chamber as determined by turning off the thermal electric coolers for one sol between Sols 10 and 12, which eliminated the fluctuation. [Reprinted from Levin, G.V. and Straat, P.A. (1977a) Recent Results from the Viking Labeled Release Experiment on Mars. J Geophys Res 82(28):4663–4667. ©1977 by the American Geophysical Union; with permission by Wiley.]

FIG. 9.

Frost at the Viking 2 Lander site. Image acquired May 18, 1979, almost one martian year after the Viking Lander 2 first detected frost at this site. [Image credit: NASA; Viking 2 Lander image P-21873 (other ID 21I093/960) available online at http://nssdc.gsfc.nasa.gov/imgcat/html/object_page/v12_p21873.html.]