Introduction

The stream systems of the McMurdo Dry Valleys of Antarctica represent a relatively simple end member of terrestrial hydrologic systems. Many Dry Valley streams are prominent landscape features, especially in summer when they carry glacial meltwater from the alpine and outlet glaciers to the perennially ice-covered lakes on the valley floors (Fig. 5.1). Observations beginning in 1968 indicate that these channels carry water for 8–12 weeks each year, though some are only wetted in warm, high flow years, and others have been deactivated because of changes to flow routing. In addition to obvious channels incised in the landscape, smaller, less frequent fluvial features may become active in the Dry Valleys, such as small rivulets (shallow, broad gullies that are not wetted annually) carrying snowmelt or meltwater from buried ice down steep valley walls in particularly warm summers. Although these fluvial systems are relatively unique on Earth, the surface of Mars holds evidence of ancient fluvial features that are similar to snowmelt rivulets observed in the Dry Valleys.

In this chapter, we compare the contemporary status and function of streams of the Dry Valleys with those that may have existed on ancient Mars. Our current understanding of martian fluvial processes is limited to what can be inferred by the “leftover” drainages that are readily observed, some of which are quite large.

Abstract

The McMurdo Dry Valleys (MDV), classified as a hyperarid, cold-polar desert, have long been considered an important terrestrial analog for Mars because of their cold and dry climate and their suite of landforms that closely resemble those occurring on the surface of Mars at several different scales, despite significant differences in current atmospheric pressure. The MDV have been subdivided on the basis of summertime measurements of atmospheric temperature, soil moisture, and relative humidity, into three microclimate zones (Marchant and Head,2007): a coastal thaw zone, an inland mixed zone, and a stable upland zone. Minor differences in these climate parameters lead to large differences in the distribution and morphology of features at the macroscale (e.g., slopes and gullies); mesoscale (e.g., polygons, viscous-flow features, and debris-covered glaciers); and microscale (e.g., rock-weathering processes/features, including wind erosion, salt weathering, and surface pitting). Equilibrium landforms form in balance with environmental conditions within fixed microclimate zones. For example, sublimation polygons indicate the presence of extensive near-surface ice in the MDV and identification of similar landforms on Mars appears to provide a basis for detecting the location of current and past shallow ice. The modes of occurrence of the limited and unusual biota in the MDV provide terrestrial laboratories for the study of possible environments for life on Mars. The range of microenvironments in the MDV are hypersensitive to climate variability, and their stability and change provide important indications of climate history and potential stress on the biota.

Introduction

Microbial life on Earth usually requires at least five prerequisites: innoculi, liquid water, and sources of energy, carbon, and nutrients (Rothschild and Manicelli,2001). One of the major advances in the cryospheric sciences during the last decade is the realization that microbial life or innoculi are found in a whole spectrum of environments throughout glacier ice masses of all scales, from the snow cover, through ice surface (or supraglacial) environments, within ice (or englacial) environments through to ice bed (or subglacial) environments (Hodson et al., 2008). A remarkable observation is that apparently viable microbes can be found throughout the whole 4 km of ice column found near the center of the East Antarctic Ice Sheet above subglacial Lake Vostok (Priscu et al., 2008). Hence, glaciers on Earth can now be regarded as biomes or ecotomes, and the question arises whether or not glaciers on other celestial bodies have the potential to act as ecotomes. This chapter begins to provide an answer by first describing how microbial life exists in the cold glaciers of the McMurdo Dry Valleys, and second, by speculating on whether or not there is the chance of life in the glaciers and ice caps of Mars. We make the assumption that potential microbial life on Mars is carbon based and requires the same five prerequisites for microbial life as on Earth (Rothschild and Manicelli, 2001).

The McMurdo Dry Valleys (MDV) comprise a mosaic of habitats at scales ranging from micrometers to the kilometer scale. The varied landscape of the valleys, combined with strong physical and chemical gradients within and across the terrestrial and aquatic habitats, yields an ecosystem dominated by microbes that is both complex and diverse (Gordon et al.,2000; Smith et al., 2006; Mikucki and Priscu, 2007). The cold desert environment is analogous to icy conditions found on other icy worlds. For example, the low organic carbon, cold, arid soils of the MDV are similar to Mars' present-day terrestrial environment and the glaciers and ice-covered lakes of the MDV are comparable to conditions that existed on Mars in the past (Priscu et al., 1998; Wynn-Williams and Edwards, 2000; McKay et al., 2005). If there are extant or extinct life forms on Mars, they likely experience similar physical constraints and environmental challenges as do microbial communities in the MDV. Therefore, the MDV provide a unique earthly setting to gain insight into the diversity, adaptation, and function of life on other icy worlds. Here we describe the ecological processes and conditions that contribute to the microbial diversity observed in the MDV and relate these to potential life on Mars.

The McMurdo Dry Valley ecosystem

The MDV include a variety of unique habitats that are connected physically, chemically, and energetically (Fig. 8.1). Solar radiation and wind are the underlying forces that determine the existence and distribution of biota throughout the valleys (Dana et al., 1998; Nkem et al., 2006).

Introduction

The McMurdo Dry Valleys are the largest and one of the most southernly exposed terrestrial antarctic environments (Ugolini and Bockheim,2008) and have been a prominent analog environment for speculations about surface processes (Mahaney et al., 2001; Dickenson and Rosen, 2003; Marchant and Head, 2007) and potential biology (McKay, 1997; Wynn-Williams and Edwards, 2000) on Mars. The extremes in cold and aridity, the paucity of visually conspicuous life forms, and the undisturbed conditions of the McMurdo Dry Valleys make this region an obvious candidate for such comparisons. Recent discoveries of evidence demonstrating past and perhaps present availability of liquid water on the martian surface detected by the Mars Global Surveyor (Malin and Edgett, 2000; Baker, 2001) and the Spirit and Opportunity rovers (Squires et al., 2004a; Haskin et al., 2005) have extended the foundation of these comparisons beyond similarities in climate to surface geomorphology, geochemistry, and mineralogy (Chevrier et al., 2006; Marchant and Head, 2007; Amundson et al., 2008).

Water is the primary limitation to geochemical weathering and biological activity in the McMurdo Dry Valleys of Antarctica and other cold desert ecosystems where availability and movement of liquid water is limited by low temperatures (Kennedy, 1993; Convey et al., 2003; Barrett et al., 2008). This limitation of liquid water results in slow weathering and highly constrained biological activity contributing to relatively stable geochemical conditions in surface environments. Thus, in the McMurdo Dry Valleys, the legacy of paleo-aquatic environments is preserved in contemporary patterns of soil geochemistry.

Abstract

The McMurdo Dry Valleys detailed in previous chapters represent one environment for life thought to have existed on Mars among many. This chapter illustrates other potential habitats and their significance: (1) high-altitude lakes subjected to rapid climate change in the Andes provide analogy to the Noachian/Hesperian transition on Mars; (2) Río Tinto, Spain, where conditions are reminiscent of Meridiani Planum, unravels an underground anaerobic chemoautotroph biosphere that could resemble a modern refuge for life on Mars; (3) the High Arctic hosts gullies analogous to those observed on Mars, whose fresh deposits could provide access to traces of past and/or present underground oases; it is also in this polar environment that the Haughton-Mars Project helps answer long-standing questions, revisiting classical assumptions, and sometimes reshaping our thinking on many issues in planetary science and astrobiology, in particular in relation to Mars; (4) the search for microbial life in the arid soils of the Atacama desert and its robotic detection characterize what role aridity plays in the distribution of life and how to search for evidence of rare and scattered biosignatures.

Introduction

Because of its geology and climate evolution, Mars is likely to have developed a diversity of potential habitats for life over time. The main ingredients for habitability (i.e., water, energy, and nutrients) were present early, as demonstrated by the Spirit and Opportunity rovers at Gusev crater and Meridiani Planum (Knoll et al., 2005; Des Marais et al.,2005, 2008).

Introduction

The antarctic cryptoendolithic microbial ecosystem lives under sandstone surfaces in the dry valley region (Friedmann and Ocampo,1976; Friedmann, 1977). It is relatively simple, consisting of cyanobacterial or algal primary producers, fungal consumers, and bacterial decomposers. It lacks animals and, possibly, also archaea. With rock temperatures rising above 0 °C only for a few weeks in the austral summer to allow photosynthetic productivity, this ecosystem is permanently poised on the edge of existence.

Before we talk about these specific rock-inhabiting organisms, it is useful to be familiar with all lithophytic life forms. Epilithic organisms live on rocks. Endolithic organisms grow inside rocks, with three subcategories that denote the mode of entry or the presence or absence of a protective surface crust (Golubic et al., 1981). Euendolithic algae and cyanobacteria actively bore into limestone in the intertidal zone and, occasionally, in deserts (Friedmann et al., 1993a; Garty, 1999). Chasmoendolithic organisms occupy weathering cracks and fissures in a variety of rocks. Cryptoendolithic organisms colonize pre-existing pore spaces in translucent rocks, most commonly sandstones (Friedmann and Ocampo-Friedmann, 1984; Bell, 1993, Nienow et al., 2002; Omelon et al., 2006). The colonized zone, in this case, is covered by a silicified surface crust.

The dry valleys of East Antarctica are at first glance a barren landscape. This was certainly Robert Falcon Scott's impression when he was the first to visit the dry valleys in 1903. As his expedition marched down what is now called Taylor Valley, he commented in his journal “we have seen no living thing, not even a moss or lichen” and “It is certainly the valley of the dead; even the great glaciers which once pushed through it have withered away” (Scott, 1905). A party from Scott's second expedition, led by senior geologist Griffith Taylor, also visited the valleys in 1911 (Taylor, 1922). Another 45 years elapsed before other visitors came to the valleys when Operation High Jump established logistics bases at nearby McMurdo Station and Scott Base in 1956. These bases provided relatively easy access to the valleys by tracked vehicles and helicopters across the McMurdo Sound to these previously hard-to-get-to areas. Afterwards, the New Zealand national program carried out all kinds of natural science research in the valleys, largely based out of the busy Lake Vanda station which supported three manned over-winter investigations (Harrowfield, 1999). Early biological work in the dry valleys was also carried out by the U.S. program in the 1960s by now well-known ecologists Gene Likens, Charles Goldman, and John Hobbie who founded long-term monitoring programs at Hubbard Brook, Lake Tahoe, and Toolik Lake in Alaska (respectively).