Permafrost, Phillips, Springman & Arenson (eds)
© 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7
Atmosphere/ice energy exchange through a thin debris cover
in Beacon Valley, Antarctica
J. Putkonen, R.S. Sletten, & B. Hallet
Quaternary Research Center and Department of Earth and Space Sciences MS 351310,
University of Washington, Seattle, USA
ABSTRACT: An extensive ice body is buried under less than 20 cm to several meters of debris in Beacon Valley
(Dry Valleys, Antarctica). The thermal behavior of the upper 20 m of permafrost, including the annually freezing
and thawing surface (active layer) in this region of low relative humidity and very low annual precipitation, has
great scientific interest. Distinctly different behavior from wet, arctic or alpine soil is expected due to the extremely
cold and dry environment, essentially ice free soil and very thin active layer. Soil temperatures and thermal
properties in the debris cover are also essential for models of sublimation rates of subsurface ice, and of the
efficiency of vapor transport, magnitude of thermoelastic stresses in soils both on Earth and Mars.
the soil surface down to a depth of 20 m in the ice, soil
thermal conductivity probes, heat flux and moisture
content detectors. Snow depth, surface radiative temperature, thermoelastic strain in the frozen debris and
shallow ice, air temperature, relative humidity and net
radiation are also measured.
1 INTRODUCTION
The growing knowledge and understanding of surface
processes on Mars have sparked an interest in the
Dry Valleys of Antarctica as one of closest natural
terrestrial analog of the Martian environment. Ideas
and hypotheses can be tested there prior to launching
probes into distant planet, and careful analysis of
corresponding periglacial terrestrial environments
(Costard et al., 2002; Mellon, 1997) is likely to aid in
the interpretation of images gathered by Mars Orbital
Imager (Malin and Edgett, 2000).
In this paper we focus on the soil heat transfer and
thermal properties in Beacon Valley, Antarctica to elucidate the important differences between the dry Antarctic
soils and the generally wet and icy Arctic or Alpine
permafrost. We suggest that the soil thermal properties
in Beacon Valley resemble those of Mars, and thus
will provide significant guidance for modeling studies
involving Martian soils.
A collaborative research effort aimed at understanding the genesis and stability of the Beacon Valley
surface and subsurface ice was launched in the
1998/99 Antarctic summer. It included extensive fieldwork, ice coring, and installation of instruments for
continuous monitoring of a diversity of processes
including energy and mass flow in the debris cover
and underlying ice.
3 SURFACE ENERGY BALANCE
Soil surface energy balance reveals the dominant pathways of heat transfer and highlights the minor role that
soil heat flow plays in this dry environment. Due to
the low atmospheric moisture and almost nonexistent
precipitation no ground water could be detected at the
site. Throughout the year the subsurface soil moisture
was below the detection level (about 6%) of the TDR
type soil moisture probes (Vitel Inc).
The general surface energy balance equation is:
Rnet H LE G. Where: Rnet is net radiation at
the surface, H is sensible heat transfer, LE is heat
transfer due to phase change of water at the surface,
and G is ground heat flow.
The lack of soil moisture and surface evaporation
simplifies the general form into: Rnet H G. At the
site we measure net radiation and ground heat flow
and thus can subtract the sensible heat transfer.
Because of the far southern latitude of the field
site, the diurnal solar cycle is strongly influenced by
the season, and therefore the surface energy balance
analysis is separated into two representative sections
(winter and summer) (see figure 1).
Winter (Julian days 97–253: 157 days) is characterized by the strong outflux of radiative energy, rendering
the soil surface essentially colder than the soil below
and air above. This allows a surface inversion to
develop (a condition where air temperature aloft is
warmer than the air closest to soil surface). Thermal
2 INSTRUMENTATION
Two sites were instrumented in the Beacon Valley: mid
valley lat. 77.84°, long. 160.60°, altitude 1350 masl,
and lower valley lat. 77.80°, long. 160.71°, altitude
1000 masl. Slightly different setups consisting of a
selection of following instruments were installed at
each site: thermistors to track soil temperatures from
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Figure 2. At this scale the modeled and measured soil temperatures are indistinguishable (vertical scale deg C), shown
here at four levels in the soil/ice: 0.1, 0.6, 9.6 and 19.6 m
below soil surface. The soil temperatures span one year
(horizontal scale is model time in units of ⬃10 minutes).
Figure 1. The average magnitudes of the main components
of the surface energy budget for summer and winter. Note
the small value of the ground heat flow, G, that attests to the
insulating properties of the dry soil. Rnet is netradiation, and
H is sensible heat flux. The fluxes reported are absolute values; corresponding arrows depict the direction of heat flow.
temperatures with a standard 1-dimensional thermal
conduction model (Putkonen, 1998).
At the field site the soil thickness is 0.3 m. It consists of loose diamicton that is underlain by relatively
clean ice, whose thickness exceeds 20 m, but is not
known. Soil thermal conductivity was measured in
situ by a heated needle probe, and found to be
0.2 Wm1 K1, laboratory measurements yielded
0.3 Wm1 K1. Mass heat capacity is 1000 J kg1 K1
and density 1200–1800 kg m3 (pers. comm.
M.Mellon 4/26/2002). The ice thermal conductivity is
2.36 Wm1 K1 volumetric heat capacity is 1.77 106 J m3 K1 (Yen, 1981).
Standard 1-dimensional thermal conduction model
with 2 layers (surface – 0.3 m soil, 0.3–50 m ice) was
used to evaluate the internal consistency of the measurements and assumptions. The thermal model is
driven by observed soil surface temperatures at
0.02 m depth and constant ice temperature at 50 m
depth. The 50 m temperature is allowed to vary in the
model along with thermal diffusivities to find the best
fit between the observed and modeled temperatures.
The fit is determined for modeled and observed
hourly temperatures at 0.1, 0.6, 9.6, and 19.6 m.
For best fit the thermal diffusivity of the ice in the
model is larger than expected, ice apparent 1.18 ice
theoretical. However, the difference is fairly small and
may be explained by the relatively high debris content
(up to 10%) of the ice. The soil thermal diffusivity
is also larger than measured in situ, soil apparent 1.79 soil measured, or 0.4 Wm1 K1. The soil
thermal capacity was measured only in the laboratory
and is assumed to be fairly constant in the field,
energy is supplied from the air through sensible heat
transfer to the soil surface and from soil through conduction. The low thermal conductivity of the surficial
soil is highlighted in the inefficiency of the soil thermal
flow compared to the sensible heat flux from the air,
which is 7 times larger.
During Polar day (Julian days 253–365 and Julian
days 1–97: 208 days) the direction of heat flow is
reversed and subsurface is gaining energy from soil
surface through thermal conduction, and the air is
heated through sensible heat flux from the soil surface. Again the sensible heat flux is much stronger,
about 21 times the soil heat flux (Figure 1).
Mean annual soil heat flux is close to zero
(0.1 Wm2), which is expected when the thermal
system (ice/soil/atmosphere) is in long term thermal
equilibrium. However, the mean annual net radiation
is 24 Wm2, which suggests that there is a net advection of sensible heat from the area, that shows up as a
positive mean annual sensible heat flux. (Note, that
the fluxes in Figure 1 are shown for different lengths
of time and thus can not be subtracted without weighing in the time).
4 SOIL/ICE THERMAL ANALYSIS
The soil/ice thermal analysis is based on 1) in situ
measurements of the soil thermal properties. 2) continuous measurements of soil/ice temperatures and
soil heat flow over one year. 3) modeling of soil/ice
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conductivity of the dry mineral soil acts as an effective
insulator that limits the penetration of the heat wave in
the summer and hence, the buried ice is very insensitive
to the short lived annual temperature extremes and
essentially integrates the long term mean soil surface
temperature that averages 24°C.
The Beacon Valley soil thermal conductivity is
2–9 times larger than the conductivity that has been
suggested for Martian soil (0.045 Wm1 K1) (Mellon
and Phillips, 2001), but 6–26 times smaller than the
wet/icy arctic soil (Putkonen, 1998), and thus reinforces the notion that Antarctica is the closest terrestrial analog for Martian environments.
because no moisture or ice lenses were observed in
the soil. The relative difference in modeled and measured thermal conductivity is large, but the soil thermal conductivity is still small, and the absolute
difference is only about 0.2 Wm1 K1, and may
reflect the measurement uncertainty of the needle
probe. The larger value in the range (0.4 Wm1 K1)
reported here compares favorably to a model based
thermal conductivity (0.6 Wm1 K1) from Linnaeus
Terrace in the Asgard Range, Antarctic Dry Valleys
(McKay et al., 1998).
The mean annual air temperature in the field site
is 23.9°C, and the mean annual relative humidity
is 47%.
The modeled and observed temperatures showed no
signs of phase change of soil ice or water, in contrast
with most arctic or alpine soils in which latent heat
plays an important role, and a prominent feature of the
thermal regime is the zero degree curtain in the fall.
The active layer depth (thickness of the surficial
soil layer that reaches temperatures above 0°C) varies
directly in response to soil surface heating, because
there is no moisture in the soil to increase the effective
heat capacity. Maximum active layer depth (about
0.1 m) may be reached within one day starting from
zero depth and it may freeze completely within hours.
No relation is found between the current climate
induced active layer depth and the thickness of the
near surface ice-free layer.
REFERENCES
Costard, F., Forget, F., Mangold, N., and Peulvast, J.P., 2002:
Formation of Recent Martian Debris Flows by Melting
of Near-Surface Ground Ice at High Obliquity. Science,
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Malin, M.C., and Edgett, K.S., 2000: Evidence for Recent
Groundwater Seepage and Surface Runoff on Mars.
Science, 288: 2330–2335.
McKay, C.P., Mellon, M.T. and Friedmann, E.I., 1998: Soil
temperatures and stability of ice-cemented ground in the
McMurdo Dry Valleys, Antarctica. Antarctic Science,
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Mellon, M.T., 1997: Small-scale polygonal features on Mars:
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Yen, Y., 1981: Review of Thermal Properties of Snow, Ice
and Sea Ice. Cold Regions Research and Engineering
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5 CONCLUSIONS
Modeled and measured soil thermal conductivity in the
dry debris cover was found to be 0.1–0.4 Wm1 K1.
Such a low thermal conductivity promotes a steep
thermal gradient, which was measured to be up
to 200 Km1 near soil surface. The low thermal
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