Ferrate(VI) is a very strong oxidizing agent and will readily oxidize many organic molecules in aqueous solution [19
]. High oxidation power selectivity and by-production of non-toxic Fe(III) make potassium ferrate(VI) an environmentally friendly oxidant for removal of industrial contaminants from natural waters [22
]. We confirmed this degradative activity of ferrate using 2,4,6-trinitrotoluene (TNT), though a mixture of dodecane and hexadecane, as might be found within microbial membranes, was not degraded by ferrate(VI) (Figures and ).
Fe(VI) is known to be highly active in water as a biocide. For example, the kinetics of inactivation of a bacteriophage by potassium ferrate was studied with the F-specific RNA-coliphage Q beta. Inactivation in phosphate buffer (pH 6, 7, and 8) containing ferrate was effective and could be described by Hom's model [23
]. The effectiveness of ferrate(VI) in wastewater treatment has been studied, and it exhibits a strong bactericidal effect on both coliforms and total bacteria [24
]. However, the conditions used in those prior experiments with ferrate(VI) do not at all mimic the conditions that might be encountered on the surface or in the subsurface of much of Mars, where there is thought to be essentially no available water [15
], there is a highly oxidized atmosphere of mostly CO2
, and average temperatures are very low (typically -60 to -80°C, but sometimes reaching temperatures near 0°C in areas of sunlight). These Mars-like conditions were used in the experiments reported here.
Certainly these conditions did not exactly duplicate the Martian atmosphere, which would have required use of a vacuum chamber. The pressure of CO2 used was much higher than on Mars. However, since CO2 is unreactive, the higher pressure should not have significantly affected the results. It is of course possible that at very low CO2 pressures endospores might be more sensitive to their environmental conditions than at one atmosphere. This experiment will require a more sophisticated apparatus than is presently available to us, though we hope to obtain such equipment soon in order to further examine this question.
It is likely that liquid water exists at least transiently on the surface of some locations on Mars [16
]. Because of the high reactivity of ferrate with water, especially under alkaline conditions, it is likely that these locations on Mars contain iron mostly in the non-toxic Fe(III)-oxyhydroxide state. Also, endospores were not killed by transient exposure to ferrate in the presence of water at room temperature. Thus, the transient occurrence of liquid water in the soils of Mars is insufficient reason to reject the general conclusion that if ferrate is present in the soils of Mars, it probably is not
a sporocidal agent. In locations where other forms of iron dominate, spores would be even more stable. However, additional experiments looking at spore survival that involve additions of small amounts of water (e.g., 1% w/w) to a Mars surrogate, ferrate-enriched soil, with brief fluxes of temperature between -80°C and about +5°C would be useful to further address this question.
Though UV light is a potent biocide and is present at high levels on the Mars surface, beneath the surface or under objects such as rocks, even the natural high flux of ultraviolet radiation on Mars will likely be greatly attenuated. We tested this hypothesis, confirming that spores are protected from the UV flux, even in the presence of a strong soil oxidant (permanganate), by a few mm of soil cover. The UV light, provided by a mercury vapor lamp, was not filtered to exactly duplicate its passage through the atmosphere of Mars, though this might be accomplished with the appropriate apparatus (e.g., an irradiated vacuum chamber). The intervening gas was normal air at about 21% oxygen, 0.03% CO2
, and 78% N2
. This clearly did not filter enough of the lethal wavelengths (e.g., 254 nm) to significantly affect the strongly sporocidal quality of the light. On Mars, however, the thinner atmosphere might allow better passage of other shorter lethal wavelengths (e.g., around 200 nm) that might be filtered to some extent under the conditions used here. However, the likely effect of the difference in light wavelength filtration on Mars versus
the experiments described here would be the ultimate soil cover depth needed to protect spores from UV damage, and this difference is likely not to be very large (less than an order of magnitude). Also, some endospores are considerably more UV-resistant than those we employed [25
], and substitution of such spores likely would have changed the results (survival vs. depth) to some degree. The important observation is the demonstrated protective effect of soil materials.
Earthly life forms can produce dormancy structures that are highly resistant to adverse environmental conditions, including the presence of chemical oxidants and ionizing radiation, conditions that are thought to occur on Mars. Bacterial endospores are a prime example of resistant dormancy structures, and recent research indicates that such dormant forms of bacteria can survive for even millions of years, under appropriate conditions. Cano and Borucki [26
] revived a bacterial spore from the abdominal contents of extinct bees preserved for 25 to 40 million years in buried Dominican amber and cultured and identified it. Greenblatt, et al. [27
] reported successful culture of bacteria from Israeli (Lebanese lode) amber dated at 120 million years old. Vreeland, et al. [28
] reported the isolation and growth of a previously unrecognized spore-forming bacterium (a Bacillus
species, designated 2-9-3) from a brine inclusion within a 250 million-year-old salt crystal from the Permian Salado Formation. Due to the difficulty in absolutely excluding modern contamination as sources of these isolates, these reports have been controversial. However, to date these claims have not been invalidated [29
]. Thus, even studies of earthly systems indicate that under environmental conditions possibly less life preserving than those encountered on Mars, dormant life forms may have survived on Earth for very long periods, even of geological time scales. The most important factor for such survival appears to be desiccation, a defining characteristic of amber, earthly brines, and much of the Mars environment. Though there apparently is water in the soil of the polar regions of Mars (Mars Odyssey mission observations), it is thought to be present as ice and probably is not biologically available.
Little work has been done in the study of microbial survival in the extraterrestrial environment. Horneck, et al. [30
] did observe survival of a significant fraction of Bacillus subtilis
spores after 11 months of exposure to the space environment on board the European Retrievable Carrier (EURECA). They exposed spores of Bacillus subtilis
to space for about two weeks in the BIOPAN facility of the European Space Agency onboard the Russian Earth-orbiting FOTON satellite. The spores were exposed under a variety of conditions: in dry layers without any protecting agent; mixed with clay, red sandstone, Martian analogue soil, or meteorite powder; or in "artificial meteorites" (cubes filled with clay and spores in naturally occurring concentrations). Unprotected spores in layers open to space or behind a quartz window were completely or nearly completely inactivated, and similarly low survival was obtained behind a thin layer of clay acting as an optical filter. Survival was increased by five or more orders of magnitude when the spores in the dry layer were directly mixed with powder of clay, rock, or meteorites. Up to 100% survival was reached in soil mixtures with spores comparable to the natural soil to spore ratio. These data confirm the deleterious effects of extraterrestrial solar UV radiation but also confirm that thin layers of clay, rock, or meteorite can successfully shield spores from UV damage if they are in direct contact with the spores.
Mileikowsky, et al. [31
] considered the probability of natural transfer of viable microbes from Mars to Earth in meteoroids ejected from Mars during the period 4 Ga BP to the present and during the 700 Ma from Ga 4.5 to 3.8. They used NASA's HZETRN transport code to calculate probabilities, examining many variables potentially affecting microbe survival. The species modeled used the environmentally robust bacteria Bacillus subtilis
(spore-former) and Deinococcus radiodurans
(highly radiation resistant). Their modeling suggested that if microbes existed or exist on Mars, viable transfer to Earth is not only possible but also highly probable, due to these microbes' impressive resistance to the hazards of space transfer and to the dense traffic of Martian meteorites that have fallen to Earth since the dawn of our planetary system.