Biological contamination of Mars II. Cold and aridity as constraints on the survival of terrestial microorganisms in simulated Martian environments

Abstract

It has been postulated that the accidental introduction of terrestial microorganisms to other planets during the course of space exploration may impede or bias the detection of organic matter and possible indigenous organisms, and thereby obscure subsequent studies of extraterrestial life. To assess the likelihood of biological contamination of Mars, we have applied the principle of natural selection on a laboratory scale. Terrestial microorganisms were collected from a variety of environments, including regions of high alkalinity, low mean daily temperature, and low annual rainfall. The air-dried soils were then subjected in the presence of a dessicant to a stimulated Martian environment involving 12-hour freeze-thaw cycles from about −60°C to about +20°C; atmospheres of 95% nitrogen, 5% carbon dioxide, and low moisture content; ≤0.1 atm pressure; and a total ultraviolet dose at 2537A˚of 10 9 erg cm −2. Survivors were scored on supplemented agar. Preliminary results indicate a wide variety of survivors including obligate and facultative anaerobic spore-formers and non-spore-forming facultative anaerobic bacteria. Diurnal freezing and thawing were continued for 6 months. There was no significant loss of viability after the first freeze-thaw cycle. An extensive survey of the literature shows that survival of terrestial microorganisms under individual simulated Martian conditions has been known for decades. Survival varies greatly with such parameters as the rate of freezing and thawing, the physics and chemistry of the medium, and the individual species studied. A primary deleterious effect of low temperatures is the unavailability of liquid water. The present investigation shows the absence of pronounced synergistic effects inhibiting survival. The probable existence of organic matter and moisture on Mars, at least in restricted locales and times, makes it possible that terrestial microorganisms can also reproduce on Mars. The demonstration that all samples of terrestial soil tested contain a population of microorganisms which survive in simulated Martian environments strongly underscores the need for scrupulous sterilization of all spacecraft intended for Mars landings.