Sediments Water Snow Glaciers and Permafrost on Mars

The carbonate minerals and magnetites in close association with possible microfossils in ALH84001 triggered intense debate concerning the possibility of life and water on Mars. The primary requirement for active microbial life on Earth is liquid water and a source of energy and organics, although some microbes can remain viable for long time periods in a frozen or desiccated state without water. Viking, Global Surveyor, and Odyssey images and data have provided dramatic evidence for water and ice on present-day Mars. The absence of water on Mars can no longer be thought to preclude the possibility of Martian microbial life or microfossils. Viking, Pathfinder, Global Surveyor, and Odyssey data have established that water was very abundant on ancient Mars and presently exists in the polar ice caps and permafrost of Mars. The Mars Global Surveyor Mars Orbiter Camera (MOC) images provide evidence that large quantities of liquid water were present on ancient Mars when the ALH84001 microfossils would have been formed. Viking images also provided dramatic evidence of water snow on Mars at the Lander site (48° N, 226° W) over several days in May of 1979 (Fig. 2). This is obviously snow and not frost as it is absent on vertical surfaces and collects in small depressions. The Viking detector temperature was - 8.10°C, and thus it is impossible for this to be carbon dioxide (CO2) frost. The snow is on rocks in full sunlight, as indicated by the sharp spacecraft shadows. Solar heating of the soil and rocks on Mars should produce localized melting to permit liquid water to seep into the soil grain interstices and form thin water films within the permafrost. These films might be capable of supporting indigenous microbial life and the positive Viking Labeled Release data [16,17] should be carefully reexamined.

Zuber et al. [18] analyzed elevation measurements from Mars Orbiter Laser Altimeter (MOLA) on Global Surveyor in 1998. These data show the volume of water ice in the Mars North Polar Cap is 1.2 million km3, about half the size of the Greenland Ice Cap. The Mars North Polar Cap in summer was 1,200-km across and 3.8-km thick with a shape showing it

Figure 3. (a) MOC image of double rimed polygons in Mars permafrost and (b) double-rimmed polygons in permafrost of Kolyma lowlands of northeast Siberia. (Photos courtesy MSFC and NASA).

to be primarily composed of water ice. This is almost the same thickness as the Central Antarctic Ice Sheet at Vostok. Large areas of the ice sheet are extremely smooth—varying only a few feet over many miles, and other areas exhibit pinnacles and are cut by deep fracture crevasses and moulins (crevasses cut by flowing water) plunging to 1 km depth. The margins of the Mars polar cap are surrounded by giant conical mounds that are possibly pingos formed of ice and rocks tens of kilometers in diameter and over 1 km in height. There are also double-rimmed polygonal patterns in the permafrost similar to those known in patterned ground of the permafrost of Siberia, Alaska, and Antarctica. MOC, aboard the Mars Global Surveyor, also shows large amounts of water ice frozen in the Mars permafrost. MOC images show double-rimed polygons in the permafrost of Mars (Fig. 3 (a)) [20-24]. Images of similar double-walled polygons in permafrost were taken during the International Expedition Beringia by R. Hoover to the Kolyma lowlands of northeast Siberia (Fig. 3 (b)) and as also explained later in this volume by R. Paepe and Van Overloop, in Taylor Valley (Antarctica) in comparison with the Mars Moc image.

The polar ice caps of Earth and Mars are paleomicrobiological traps capable of cryopre-serving ancient microfossils, organic remains, and intact microbial cells in dead and viable states [19,25]. Prior meteorite impact events may have ejected crustal rocks and debris from sediments, volcanic deposits, glaciers, and permafrost into the atmosphere and onto the polar ice caps. Consequently, if microbial life presently exists or has existed on Mars in the geological past, the traces, remains, and biosignatures of this microbial life should be cryo-preserved in a frozen state in the permafrost and polar ice caps. Marsic et al. [26-28] have described the DNA amplification and gene cloning of ribosomal RNA extracted from anaerobic psychrophiles of the Fox Tunnel of Alaska and from a living 40,000-yr-old Pleistocene moss cultured from samples collected in the Kolyma lowlands of northeastern Siberia. On Earth, the permafrost, ice wedges, glaciers, and polar ice sheets preserve organic chemicals, molecular biomarkers, intact cells, and viable ancient microorganisms, fungi, and mosses for several hundreds of thousands to millions of years, and analogous processes may operate on Mars.

Abyzov et al. [29,30] detected viable Pleistocene microorganisms distributed throughout the Central Antarctic Ice sheet at Vostok, and Gilichinsky et al. [31,32] found ancient viable microorganisms cryopreserved in permafrost. The long-term preservation of microbes in ice is of astrobiological significance since many species of microorganisms can remain viable for geological periods of time frozen in glaciers, permafrost, and the polar ice caps of Earth. If life ever existed on these bodies, it might have remained viable, cryopre-served in the polar ice cap of Mars; the ice crusts of Europa, Ganymede, or Callisto; or the water ice of comets. Hoover et al. [33] discussed terrestrial ice-diatoms, snow algae, and cyanobacteria as analogs for the types of microbiota that might be capable of surviving in the ice of comets. Diatoms are the most prolific eukaryotic life forms of the terrestrial cryosphere. They are abundant in permafrost, glaciers, polar ice, and at the ice/ocean interfaces. Although diatoms are primarily photosynthetic organisms, some species live in the total darkness of deep-sea sediments and grow epiphytically on larger life forms in the hydrothermal vent ecosystem. Diatoms also employ heterotrophic nutritional modes to survive in the absolute darkness of the deep abyss or during the long polar winters.

ALH84001 demonstrated that impact ejection phenomena can result in the transfer of crustal material from one planetary body to another. [34-36] Viable microorganisms exist in Antarctic ice, and complex microbial ecosystems thrive in deep crustal rocks and marine sediments. Rocks and ice ejected from Earth by large impact ejection events could transport terrestrial organic chemicals and microbiota into space, where they might contaminate comets, planets, or the parent bodies of carbonaceous chondrites. The Mars meteorites confirm that impact ejection can transport crustal rocks from Mars to Earth and that the terrestrial ecosystem is not closed. Carbonaceous chondrites can no longer be considered pristine because their parent bodies must have been contaminated by countless interactions and collisions with debris encountered during the past 4.6 Ga. Meteorites have obviously been extensively contaminated during their lifespan and exhibit dramatic heterogeneity the centimeter, millimeter, micrometer, and nanometer size scales. Any materials present on the meteorite when it entered the Earth's atmosphere should be considered indigenous and only postarrival contaminants should be considered contamination. Furthermore, evidence for chemical and mineral biomarkers and microfossils that may be found in situ in meteorites should not be dismissed as contaminants solely because they may be similar to terrestrial microorganisms or biochemicals. The extreme hardiness of a wide variety of microbial ex-tremophiles has clearly shown that the possibility of trans-planetary cross contamination of microbiota can no longer be totally excluded. Indeed, the ability of some microbes to survive phenomenal shocks, high pressure, hard vacuum, and deep-space temperatures for geological epochs combined with the presence on Earth of meteorites from Mars and the Moon indicate that microbial life may be far more widely distributed throughout the cosmos than previously thought possible.

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