41st Lunar and Planetary Science Conference (2010)
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PHYSICAL CONTROLS ON ANTARCTIC DRY VALLEYS PERMAFROST GEOMORPHOLOGY AND SOIL ECOSYSTEM HABITABILITY: COLD-DESERT PROCESSES AND MARS
ASTROBIOLOGICAL IMPLICATIONS. Joseph S. Levy1, Andrew G. Fountain1, James W. Head2,
David R. Marchant3 1Portland State Univ. Dept. of Geol., Portland, OR, USA. 2 Brown Univ. Dept. of
Geol. Sci., Providence, RI, USA. 3Boston Univ. Dept. of Earth Sci. Boston, MA, USA. jlevy@pdx.edu.
Introduction. The polar desert of the
McMurdo Dry Valleys is a matrix of permafrost,
soils, glaciers, streams, and lakes that together
form the southern-most, functioning terrestrial
ecosystem on Earth. The ice-cemented soils, gullies, and paleo-lake deposits of the Antarctic Dry
Valleys may be strongly analogous to icecemented soils, gullies, and paleo-lake deposits
on Mars [1-5]. In these planetary habitats, processes governing hypothetical water and nutrient
cycling are poorly constrained, challenging efforts to efficiently explore and test these locations for signs of past or present biological activity [6-8]. The McMurdo Dry Valleys are one of
the most Mars-like environments on Earth, providing an ideal natural laboratory for defining the
physical and chemical processes that combine to
generate habitable oases that sustain simple ecosystems under cold and dry conditions [9-11].
Here, we report on advances in understanding water and nutrient cycling through Antarctic
permafrost based on investigations conducted
during the 2009-2010 austral summer in Taylor
Valley, Antarctica. Measurements were taken in
conjunction with McMurdo Dry Valleys LongTerm Ecological Research (MCM-LTER) program in order to assess the biological significance of physical permafrost processes. Observations were also linked towards inventorying landscape geomorphological response to differences
in microclimate conditions [11].
Active Layer Processes on Earth and
Mars. The active layer is the upper section of a
permafrost column that seasonally experiences
temperatures above 0˚C [12]. Given the observations suggesting potential active layer conditions
on Mars as recently as 10 Ma [13], the presence
of ice-rich, “ice-age” terrains thought to have
formed as part of the martian Latitude Dependent
Mantle (LDM) over the past ~1-4 Ma [14], and
the presence of water-carved martian gullies in
localized micro-environments within the past ~2
Ma [3, 15], several critical questions regarding
the behavior of ephemerally-wetted, cold-desert
ecosystems can be addressed in terrestrial analog
environments. In particular, 1) How do interactions between liquid water and the ice-cement
table modify the soil habitat—physically, chemically, and morphologically (Fig. 1)? 2) How does
ice-cemented permafrost limit or expand streamand lake-margin hyporheic zone water availability to soil ecosystems (Fig. 1)? 3) How do interactions between brines and the ice-cement table
improve ecosystem access to legacy nutrient reservoirs?
We address these questions through mapping
of the distribution of soil moisture, electrical
conductivity (salinity), and pH as a function of
time, location, and depth in the active layer in
Taylor Valley, and through integration of these
physical parameters with permafrost geomorphology and ecosystem response [16-18].
Defining the effect of shallow permafrost on
near-surface soil moisture is critical for assessing
the potential habitability of astrobiologically important permafrost environments on Mars, where,
like the Dry Valleys, water activity is a primary
determinant of habitability [10]. The documentation of pervasive ice-rich permafrost in the upper
~1 m of martian high-latitude regolith raises
questions regarding the accessibility of past martian active layer moisture to ephemerally present
habitable zones in the martian subsurface. Demonstrating how permafrost depth controls soil
moisture profiles in the Dry Valleys can help ascertain how much moisture would be available to
potential martian active layer habitats present
within craters and other sheltered microenvironments [11], and in turn, can be used to “work
backwards” from martian permafrost geomorphological analysis (below) to determine the degree to which active layer processes have affected recent martian permafrost terrains [5, 14].
In doing so, we can refine planetary protection
protocols for exploring potentially habitable
“special regions” [10].
Exploring Martian Permafrost Landforms. Permafrost-related landforms dominate
the surface at martian high latitudes [5, 14]. Following the approach taken to analyze transects of
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soil properties developed for characterizing Antarctic permafrost habitats and landforms (above),
we present a geo-traverse of martian permafrost
landforms, anchored at the Phoenix landing site
and extending north to the edge of the north-polar
cap, and south to the edge of the Tharsis rise. By
documenting martian permafrost landforms, including high- and low-centered thermal contraction crack polygons [11], sorted or otherwise arranged boulders (including “boulder halos”) [19],
and both massive- and pore-filling ground ice, we
will be able to evaluate the soil and climate conditions prevailing during the formation and modification of these cold-desert landforms.
Acknowledgements. This work is supported
by the Antarctic Organisms and Ecosystems Program in the Antarctic Sciences Division of the
NSF, Award #ANT-0851965.
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Figure 1. The Onyx River, Wright Valley, Antarctica. At image top, changes in surface microtopography
caused by thermal contraction crack polygons strongly influence the distribution of soil moisture (indicated by soil darkening). In contrast, at image bottom, fluvial channel structure more strongly influences
the spatial distribution of soil moisture and biological productivity.