High Arctic Saline Springs As Analogues for Springs on Mars

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 (ϳ600 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.

1 BACKGROUND of perennial springs flowing through continuous permafrost 600 m thick in an area void of volcanic heat Recent discoveries of geologically recent spring activ- sources. There are two sets of springs on Axel Heiberg ity on Mars (Malin & Edgett 2000) suggest that these Island which are located 11 km apart. The Gypsum Hill events occurred under present climatic conditions with springs are situated on the northwest side of Expedition mean surface temperatures of 60°C, pressures below River at 79°2430N, 90°4305W and lie 2.5 km down- the triple point of water at the outflow sites, and exten- stream from the terminus of the White and Thompson sive permafrost several kilometres thick. The Martian Glaciers. The Colour Peak springs are located at gully features appear to be geologically young based 79°2248N, 91°1624W, approximately 3 km from on the absence of impact craters, lack of highly-eroded the head of Expedition Fiord (Pollard et al. 1999). gully features, and the superposition of depositional These two sets of springs are among the most pole- aprons on young landforms such as aeolian bedforms ward springs known and are the only known example of and polygonally-patterned ground (Malin & Edgett cold springs in thick permafrost on Earth. Therefore 2000). The existence of such fluvial features is a puz- these Arctic springs provide a natural setting in which zle 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.

373 to study spring activity in polar desert conditions, an Conditions within the icing were monitored during environment strikingly analogous to the polar desert the 2002 field season. Typical temperatures of the icing of Mars. materials are listed in Table 1.

2.3 Colour Peak 2 THE ARCTIC SPRINGS The outlets of the flows at Colour Peak are higher up on 2.1 Overview the side of the mountain and therefore have the opportunity to cascade down the hill in well-formed chan- The springs at Gypsum Hill and Colour Peak are best nels. There are several breaks in topography where the for studying the development of an icing and channel elevation drops much more steeply, and so obviously dynamics, respectively. The Gypsum Hill springs flow the flowing water also follows these trends. There are out onto a flood plain such that the icing is preserved alternating regions of open channel flow and flow whereas the icing at Colour Peak is lost beneath the sea under ice and snow along the channel during the win- ice within Expedition Fiord where it combines with the ter months. underlying ocean water. At Colour Peak the flows are Conditions within the channels were monitored dur- more discrete and channelised, making them better ing the 2002 field season. Typical temperatures of the candidates for examining the channel dynamics. The flowing water are listed in Table 2. flow system at Stolz is ideal for monitoring flow down a valley channel as well as the icing development. Each 2.4 Stolz of these systems will now be discussed in more detail. Stolz presents an excellent opportunity to study both 2.2 Gypsum Hill the channel flow and the icing of a perennial saline spring in the High Arctic. The flow emanates from a The flow from the Gypsum Hill springs results in an large salt diapir at the head of a valley. Water enters the icing during the winter months reaching a size of valley into a large (order of 10 meter diameter), approximately 300 m wide by 700 m long which extends extremely deep pool of saline water where salt imme- into the floodplain at the base of the hill. The thickness diately begins to precipitate out of solution. This salt is of the icing is variable, but based on observations seen as crystals along the edge of the pool (perhaps of cracks within the ice the icing is at least 1 m thick. hydrohalite). The water eventually continues to flow The advance of the icing is a dynamic process. down the valley floor in a well-developed channel. Liquid water from the spring outlet flows out over the Along the way, large (order of 10 s of meters) high top of the icing in certain locations and at the edges of walls of salt are precipitated out of solution and the the flow is wicked away by the snow lying on top of the water then flows over these structures to cascade down icing, helping the water front move forward. This to the bottom of the valley. Throughout the valley there slushy salt water material extends down for ϳ10 cm is again alternating areas of open channel flow and flow and continues to advance away from the springs outlet. under ice during the winter months. Snow covering the The water flows as a thin film over the top of the pre- existing icing and then freezes, forming the layers Table 1. Gypsum Hill temperature measurements. observed in the ice itself. Several tongues of this wet Location Description Temperature water movement are observed, and evidence of previ- ous such flows is evident in the topography of the icing, 1 Spring outlet Water 6.6°C 2 Slushy tongue on icing Slush 9.5°C even as covered with the thin layer of snow. The water flows until it is diverted along another path, generally as 3 Tongue of ice within icing Ice 24°C 4 End of icing Ice 32°C the icing freezes and hence becomes at a higher elevation – then the water will preferentially flow within the surrounding lower-lying regions. Table 2. Colour Peak temperature measurements. Liquid water also flows beneath the surface, how- Location Description Temperature ever. Water is often found running in channels under the insulating cover of ice within the icing. Also, at the 1 0.5 m down from Water 2.4°C edge of the icing, liquid water tends to be wicked out channel outlet within the snow cover and a slushy snow and water 2 Within main channel Water 0.3°C mixture is found beneath the snow cover at a maxi- 3 Within main channel Water 2.7°C mum lateral distance of ϳ1.5 m away from the appar- 4 Pool of liquid water Water 12°C ent icing edge which is visible at the surface. within icing

374 ice in some regions was most likely blown into the val- is dependent on the mole fraction of solute present in ley by wind from a recent storm. The valley itself the liquid based on the freezing point depression curve. extends approximately 0.5–1.0 km in length. The last The presence of more salt in the solution increases the example of liquid flow in an open channel during April freezing point depression correspondingly. is seen at the end of the valley and then the water flows The rate of cooling during this first phase is under the snow and ice until it is again visible on the dependent upon the amount of energy lost due to surface at the icing itself (order of 0.5 km away). evaporation and the corresponding drop in tempera- The icing exists on the floodplain at the foot of the ture of the remaining liquid water to maintain an channelised valley. The outline of the icing is clearly energy balance. Temperature-dependent evaporation visible as it is not obscured by snow as at Gypsum Hill. rates for Mars are derived from Moore et al. (1995) The icing is approximately 85 m by 100 m in size. who used an atmospheric diffusive boundary layer Towards the end of the icing where the icing is furthest theory to calculate the water vapour flux entering the from the source there is abundant pooling of liquid atmosphere from a water surface. The time for cooling water which has a temperature of 26°C. A liquid to the freezing point can be calculated using any start- film several centimetres thick sits on top of the icing, ing initial outlet temperature and any freezing point with small, individual flakes or crystals of ice within depression. and on top of the water layer. Towards the bottom of Once the liquid reaches the freezing point, ice the icing where the icing is closest to the outlet source begins to form. The amount of ice formed is governed there are several frozen, sinuous channels which lead by a balance of energy among evaporation, cooling, into the icing itself. 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 3 MODELLING and hence each correspondingly cooler water temperature, the amount of ice formed is determined by a con- By studying the saline spring systems on Axel Heiberg vergence of the energy balance and freezing point Island, the mechanics of icing and channel development depression equations. As ice is formed, the remaining can be thoroughly explored. Data collected at these liquid solution becomes more concentrated and hence sites are used to constrain numerical models being ice forms at a new lower temperature due to the developed to simulate the flow and icing formation increased freezing point depression. During each iter- processes in the Arctic, and these models are then ation a new stream radius, surface area, and mass of extrapolated to understand similar flows on Mars with liquid are calculated based on the amount of water lost increased confidence. from the system due to the formation of ice. This A computational model of the springs using an process continues until the solution reaches the eutec- energy balance method is being developed which takes tic point. Once the solution has reached the eutectic into account several different facets of the springs sys- point then salt begins to precipitate out of the solution tem. The chemistry of the system will be explored as and the salt pan begins to form. different salts come out of solution at varying temperatures and concentrations. The competition between evaporation and freezing 4 RESULTS of the water once it leaves the spring must also be understood and modelled. Therefore a hydrologic The presence of the cold perennial saline springs in the model of the system will be developed such that all High Canadian Arctic demonstrate that liquid water equations used can be changed to Martian conditions can exist even as air and ground temperatures are well (i.e. atmospheric pressure, gravity, etc). In this way the below the freezing point. model can be used to accurately describe the Arctic Liquid water is capable of flowing through hun- springs and then can be extrapolated to a Martian dreds of meters of permafrost (Andersen et al. 2002) spring system. and hence could potentially flow underground on Mars Models of water flowing over the surface of both as well in similar permafrost conditions. The Arctic Earth and Mars are developed by dividing the flow springs likewise show that the liquid water can into three phases. These phases include cooling down persist and flow long enough to create well-developed to the freezing point, the mutual coexistence of ice channels before evaporating and/or completely freez- and liquid below the freezing point, and then the pre- ing over. cipitation of salt from the solution at and below the Observations of the icings at Gypsum Hill, Colour eutectic point. First, the water leaving the ground via Peak, and Stolz reveal a high degree of internal plumb- the spring must cool down from the exit temperature ing within the icing which allows the brine solution to to the freezing point. The freezing point of the solution be transported hundreds of meters away from the

375 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 Figure 2. Curve showing the cooling time of a briny solu- a mole fraction of NaCl solute of 0.1). In Figure 3, the tion from 279K to freezing point. 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 feasi- bility of liquid water flows on the surface of Mars.

REFERENCES

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