Permafrost, Phillips, Springman & Arenson (eds)
© 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7
High Arctic saline springs as analogues for springs on Mars
J. Heldmann & O. Toon
Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, USA
C. McKay
NASA Ames Research Center, Moffett Field, USA
D. Andersen & W. Pollard
McGill University, Montreal, Canada
ABSTRACT: Geologic evidence for recent liquid water outflows on Mars suggests that these events occurred
under present climatic conditions with mean surface temperatures of 60°C and extensive permafrost. It is well
known that fresh liquid water is not stable under the pressure and temperature conditions of the current Martian
surface. However, aqueous brine solutions would be stable against boiling on the surface of Mars due to the vapour
pressure and freezing point depression of the saline solution. We therefore model the expected lifetimes and flow
distances of liquid water flows for saline solutions. We then examine the behaviour of cold perennial saline springs
and their icings at Expedition Fiord in the Canadian High Arctic. Two groups of perennial springs flow through
thick (⬃600 m) continuous permafrost and produce saline icings up to 2 m thick. The pattern of brine flow and
icing formation provides a potentially valuable analogue for spring activity on Mars.
of perennial springs flowing through continuous permafrost 600 m thick in an area void of volcanic heat
sources. There are two sets of springs on Axel Heiberg
Island which are located 11 km apart. The Gypsum Hill
springs are situated on the northwest side of Expedition
River at 79°2430%N, 90°4305%W and lie 2.5 km downstream from the terminus of the White and Thompson
Glaciers. The Colour Peak springs are located at
79°2248%N, 91°1624%W, approximately 3 km from
the head of Expedition Fiord (Pollard et al. 1999).
These two sets of springs are among the most poleward springs known and are the only known example of
cold springs in thick permafrost on Earth. Therefore
these Arctic springs provide a natural setting in which
1 BACKGROUND
Recent discoveries of geologically recent spring activity on Mars (Malin & Edgett 2000) suggest that these
events occurred under present climatic conditions with
mean surface temperatures of 60°C, pressures below
the triple point of water at the outflow sites, and extensive permafrost several kilometres thick. The Martian
gully features appear to be geologically young based
on the absence of impact craters, lack of highly-eroded
gully features, and the superposition of depositional
aprons on young landforms such as aeolian bedforms
and polygonally-patterned ground (Malin & Edgett
2000). The existence of such fluvial features is a puzzle in two ways: 1) How can water overcome extremely
low temperatures to flow through thick permafrost and
2) How can the water remain stable once it reaches the
surface despite the low surface pressure long enough
to flow across the surface creating the observed
features.
One way to study these various issues surrounding
recent spring activity on Mars is to study similar
springs in the Canadian High Arctic. The polar desert
of the Canadian High Arctic is a prime Martian
analogue with its low mean annual air temperature
(17°C) and the presence of thick continuous permafrost coupled with a potential evaporation that
exceeds the low annual precipitation (Bailey
et al. 1997).
The sites on Axel Heiberg Island (see Figure 1)
located at nearly 80°N latitude in the Canadian High
Arctic are of particular interest because of the presence
Figure 1. Location of Axel Heiberg Island.
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Conditions within the icing were monitored during
the 2002 field season. Typical temperatures of the icing
materials are listed in Table 1.
to study spring activity in polar desert conditions, an
environment strikingly analogous to the polar desert
of Mars.
2.3
Colour Peak
2 THE ARCTIC SPRINGS
2.1
The outlets of the flows at Colour Peak are higher up on
the side of the mountain and therefore have the opportunity to cascade down the hill in well-formed channels. There are several breaks in topography where the
elevation drops much more steeply, and so obviously
the flowing water also follows these trends. There are
alternating regions of open channel flow and flow
under ice and snow along the channel during the winter months.
Conditions within the channels were monitored during the 2002 field season. Typical temperatures of the
flowing water are listed in Table 2.
Overview
The springs at Gypsum Hill and Colour Peak are best
for studying the development of an icing and channel
dynamics, respectively. The Gypsum Hill springs flow
out onto a flood plain such that the icing is preserved
whereas the icing at Colour Peak is lost beneath the sea
ice within Expedition Fiord where it combines with the
underlying ocean water. At Colour Peak the flows are
more discrete and channelised, making them better
candidates for examining the channel dynamics. The
flow system at Stolz is ideal for monitoring flow down
a valley channel as well as the icing development. Each
of these systems will now be discussed in more detail.
2.2
2.4
Stolz
Stolz presents an excellent opportunity to study both
the channel flow and the icing of a perennial saline
spring in the High Arctic. The flow emanates from a
large salt diapir at the head of a valley. Water enters the
valley into a large (order of 10 meter diameter),
extremely deep pool of saline water where salt immediately begins to precipitate out of solution. This salt is
seen as crystals along the edge of the pool (perhaps
hydrohalite). The water eventually continues to flow
down the valley floor in a well-developed channel.
Along the way, large (order of 10 s of meters) high
walls of salt are precipitated out of solution and the
water then flows over these structures to cascade down
to the bottom of the valley. Throughout the valley there
is again alternating areas of open channel flow and flow
under ice during the winter months. Snow covering the
Gypsum Hill
The flow from the Gypsum Hill springs results in an
icing during the winter months reaching a size of
approximately 300 m wide by 700 m long which extends
into the floodplain at the base of the hill. The thickness
of the icing is variable, but based on observations
of cracks within the ice the icing is at least 1 m thick.
The advance of the icing is a dynamic process.
Liquid water from the spring outlet flows out over the
top of the icing in certain locations and at the edges of
the flow is wicked away by the snow lying on top of the
icing, helping the water front move forward. This
slushy salt water material extends down for ⬃10 cm
and continues to advance away from the springs outlet.
The water flows as a thin film over the top of the preexisting icing and then freezes, forming the layers
observed in the ice itself. Several tongues of this wet
water movement are observed, and evidence of previous such flows is evident in the topography of the icing,
even as covered with the thin layer of snow. The water
flows until it is diverted along another path, generally as
the icing freezes and hence becomes at a higher elevation – then the water will preferentially flow within the
surrounding lower-lying regions.
Liquid water also flows beneath the surface, however. Water is often found running in channels under
the insulating cover of ice within the icing. Also, at the
edge of the icing, liquid water tends to be wicked out
within the snow cover and a slushy snow and water
mixture is found beneath the snow cover at a maximum lateral distance of ⬃1.5 m away from the apparent icing edge which is visible at the surface.
Table 1. Gypsum Hill temperature measurements.
Location
Description
Temperature
1
2
3
4
Spring outlet
Slushy tongue on icing
Tongue of ice within icing
End of icing
Water 6.6°C
Slush 9.5°C
Ice 24°C
Ice 32°C
Table 2. Colour Peak temperature measurements.
Location
Description
Temperature
1
0.5 m down from
channel outlet
Within main channel
Within main channel
Pool of liquid water
within icing
Water 2.4°C
2
3
4
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Water 0.3°C
Water 2.7°C
Water 12°C
is dependent on the mole fraction of solute present in
the liquid based on the freezing point depression curve.
The presence of more salt in the solution increases the
freezing point depression correspondingly.
The rate of cooling during this first phase is
dependent upon the amount of energy lost due to
evaporation and the corresponding drop in temperature of the remaining liquid water to maintain an
energy balance. Temperature-dependent evaporation
rates for Mars are derived from Moore et al. (1995)
who used an atmospheric diffusive boundary layer
theory to calculate the water vapour flux entering the
atmosphere from a water surface. The time for cooling
to the freezing point can be calculated using any starting initial outlet temperature and any freezing point
depression.
Once the liquid reaches the freezing point, ice
begins to form. The amount of ice formed is governed
by a balance of energy among evaporation, cooling,
and ice formation terms as well as the relationship
between temperature and mole fraction of solute on
the freezing point depression curve. For each timestep
and hence each correspondingly cooler water temperature, the amount of ice formed is determined by a convergence of the energy balance and freezing point
depression equations. As ice is formed, the remaining
liquid solution becomes more concentrated and hence
ice forms at a new lower temperature due to the
increased freezing point depression. During each iteration a new stream radius, surface area, and mass of
liquid are calculated based on the amount of water lost
from the system due to the formation of ice. This
process continues until the solution reaches the eutectic point. Once the solution has reached the eutectic
point then salt begins to precipitate out of the solution
and the salt pan begins to form.
ice in some regions was most likely blown into the valley by wind from a recent storm. The valley itself
extends approximately 0.5–1.0 km in length. The last
example of liquid flow in an open channel during April
is seen at the end of the valley and then the water flows
under the snow and ice until it is again visible on the
surface at the icing itself (order of 0.5 km away).
The icing exists on the floodplain at the foot of the
channelised valley. The outline of the icing is clearly
visible as it is not obscured by snow as at Gypsum Hill.
The icing is approximately 85 m by 100 m in size.
Towards the end of the icing where the icing is furthest
from the source there is abundant pooling of liquid
water which has a temperature of 26°C. A liquid
film several centimetres thick sits on top of the icing,
with small, individual flakes or crystals of ice within
and on top of the water layer. Towards the bottom of
the icing where the icing is closest to the outlet source
there are several frozen, sinuous channels which lead
into the icing itself.
3 MODELLING
By studying the saline spring systems on Axel Heiberg
Island, the mechanics of icing and channel development
can be thoroughly explored. Data collected at these
sites are used to constrain numerical models being
developed to simulate the flow and icing formation
processes in the Arctic, and these models are then
extrapolated to understand similar flows on Mars with
increased confidence.
A computational model of the springs using an
energy balance method is being developed which takes
into account several different facets of the springs system. The chemistry of the system will be explored as
different salts come out of solution at varying temperatures and concentrations.
The competition between evaporation and freezing
of the water once it leaves the spring must also be
understood and modelled. Therefore a hydrologic
model of the system will be developed such that all
equations used can be changed to Martian conditions
(i.e. atmospheric pressure, gravity, etc). In this way the
model can be used to accurately describe the Arctic
springs and then can be extrapolated to a Martian
spring system.
Models of water flowing over the surface of both
Earth and Mars are developed by dividing the flow
into three phases. These phases include cooling down
to the freezing point, the mutual coexistence of ice
and liquid below the freezing point, and then the precipitation of salt from the solution at and below the
eutectic point. First, the water leaving the ground via
the spring must cool down from the exit temperature
to the freezing point. The freezing point of the solution
4 RESULTS
The presence of the cold perennial saline springs in the
High Canadian Arctic demonstrate that liquid water
can exist even as air and ground temperatures are well
below the freezing point.
Liquid water is capable of flowing through hundreds of meters of permafrost (Andersen et al. 2002)
and hence could potentially flow underground on Mars
as well in similar permafrost conditions. The Arctic
springs likewise show that the liquid water can
persist and flow long enough to create well-developed
channels before evaporating and/or completely freezing over.
Observations of the icings at Gypsum Hill, Colour
Peak, and Stolz reveal a high degree of internal plumbing within the icing which allows the brine solution to
be transported hundreds of meters away from the
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laterally away from the visible surficial extent of the
icing. Likewise, within the spring channels water often
flows underneath a protective snow and/or ice cover
which provides insulation against heat loss such that
the water cools more slowly and can consequently
travel further before freezing.
Computer modelling of these flows suggests that liquid water could persist for significant periods of time on
Mars as well. Figures 2 and 3 show cooling times for
liquid water on the present day martian surface. In
Figure 2, liquid water cools from a starting temperature of 6°C (the nominal outlet temperature of the
Arctic springs) to the freezing point (10°C assuming
a mole fraction of NaCl solute of 0.1). In Figure 3, the
liquid water cools and ice forms as the solution cools to
the NaCl eutectic point (21°C). Therefore the presence of the Arctic springs has helped to prove the feasibility of liquid water flows on the surface of Mars.
Figure 2. Curve showing the cooling time of a briny solution from 279K to freezing point.
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1–16.
Figure 3. Curve showing the cooling time of a briny solution down to the eutectic point.
spring outlet. Numerous active and abandoned channels exist within the icing as water moves both within
as well as on top of the icing in overflow events.
At the icing edges the brine solution is often wicked
out by the surrounding snow to extend the icing cover.
A slushy snow and water mixture can be found beneath
an undisturbed snowcover extending up to a meter
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