Elsevier

Icarus

Volume 196, Issue 2, August 2008, Pages 565-577
Icarus

Stability of mid-latitude snowpacks on Mars

https://doi.org/10.1016/j.icarus.2008.03.017Get rights and content

Abstract

Christensen [2003. Nature 422, 45–48] suggested that runoff from melting snowpacks on martian slopes might be responsible for carving gullies. He also suggested that snowpacks currently exist on Mars, for example on the walls of Dao Valles (approximately 33° S). Such snowpacks were presumably formed during the last obliquity cycle, which occurred about 70,000 years ago. In this paper we investigate a specific scenario under conditions we believe are favorable for snowpack melting. We model the rate at which a snowpack located at 33° S on a poleward-facing slope sublimates and melts on Mars, as well as the temperature profile within the snowpack. Our model includes the energy and mass balance of a snowpack experiencing diurnal variations in insolation. Our results indicate that a dirty snowpack would quickly sublimate and melt under current martian climate conditions. For example a 1 m thick dusty snowpack of moderate density (550 kg/m3) and albedo (0.39) would sublimate in less than two seasons, producing a small amount of meltwater runoff. Similarly, a cleaner snowpack (albedo 0.53) would disappear in less than 9 seasons. These results suggest that the putative snowpack almost certainly could not have survived for 70,000 years. For most of the parameter settings snowpack interior temperatures at this latitude and slope do reach the melting point. Under most conditions melting occurs when the snowpack is less than 10 cm thick. The modeled snowpack will not melt if it is covered by a 1 cm dust lag. In general, these findings raise interesting possibilities regarding gully formation, but perhaps mostly during a past climate regime when snowfall was expected to have occurred. If there currently are exposed snowpacks on martian mid-latitude slopes, then these ice sheets cannot last long. Hence they might be time variable features on Mars and should be searched for.

Introduction

Gully-like features have been noted on Mars since the Mars Orbiting Camera (MOC) onboard the Mars Global Surveyor (MGS) arrived in 1997 (Malin and Edgett, 2000). Gullies typically are about 1 km long (Heldmann and Mellon, 2004). Many of the gully-like features appear to be geologically young (within the last few million years) since most are not cratered and many are superimposed on surfaces which themselves are geologically young.

Several models have been proposed for the formation of gully-like features (hereafter just referred to as gullies). Some researchers (Musselwhite et al., 2001) propose that a liquid subsurface CO2 reservoir may be responsible for carving the gullies. There are numerous problems with the CO2 flow hypothesis, however, including the fact that the flow-channel morphologies are inconsistent with the physics of CO2 flow (Heldmann and Mellon, 2004). Other researchers (Mellon and Phillips, 2001) have argued that subsurface aquifers with impermeable boundaries may be released by fractures in the ice plug, causing a cascade of liquid that could have incised some gullies. Heldmann (Heldmann et al., 2007, Heldmann and Mellon, 2004) studied hundreds of gullies and determined that most of the gullies were best explained by the characteristics of subsurface fluid sources rather than surface snowmelt, ground ice melt or wind.

Following the work of previous researchers (Clow, 1987, Farmer, 1976) Christensen (2003) suggested that gullies formed when exposed dusty snowpacks slowly melt. The meltwater becomes runoff and incises gullies underneath the melting snowpack. Specifically, he hypothesized the following:

  • 1.

    Water is transported from the martian poles to mid-latitudes in the form of snow during periods of high obliquity.

  • 2.

    Melting occurs at mid-latitude during low obliquity producing liquid water that is stable beneath snow.

  • 3.

    Meltwater causes surface erosion and the formation of small gullies.

  • 4.

    Gullies incised into the substrate are observed where snow has been removed.

  • 5.

    Patches of snow remain today where they are protected from sublimation by a layer of desiccated dust/sediment.

  • 6.

    Melting could be occurring at present in favorable locations in these snowpacks.

Christensen noted images of a curious mantling in the Dao Valles region (∼33° S by 93° E) as evidence (Christensen, 2003). He suggested that this mantling (Fig. 1) is a remnant snowpack/ice-cemented soil. He pointed out that several gullies can be seen protruding from this mantling, indicating that they therefore could be the result of the snowpack melting.

There are numerous unresolved issues with the above hypotheses. First, are the mantled deposits in Fig. 1 actually dusty snowpacks? If so then they must have survived for a very long time, since according to Christensen (2003) they were presumably laid down when the obliquity was substantially higher than at present, probably more than 70,000 years BP (Fig. 2). Second, can melting occur below the surface of mid-latitude snowpacks, and if so can it do so under current conditions and in enough volume to create runoff and subsequent erosion? Hypothesis #2 (above) asserts that melting occurs at mid-latitude and that the liquid water would be stable beneath the snow. There is an important difference, however, between liquid water being produced and liquid water being produced in sufficient quantities to achieve runoff and erosion (Clow, 1987, Paterson, 1994). For water to achieve runoff capability there needs to be a refrozen (impermeable) snow boundary along the base (which may even inhibit erosion). The presence of such a layer has been modeled by Clow but his results suggest that runoff was achieved only under a very narrow set of circumstances: runoff was achieved only if the snowpack was thin (∼23 cm) and very dusty (1000 ppmw) at 7 mbar ambient pressure (Clow, 1987). In other situations the liquid water may be briefly stable but never achieve runoff capability before it is refrozen in the basal layer of the snowpack. Clow (1987) found that if the snowpack was too thin (less than ∼23 cm) or too thick (greater than ∼35 cm) then runoff was not produced. If the snowpack was too thin the meltwater was used entirely in creating the refrozen basal snow layer and no runoff was produced. If the snowpack was too thick there was not enough internal heating to produce meltwater, since the heat is more efficiently wicked away from warm spots in thicker snowpacks.

In addition, according to Clow (1987), reliable and widespread runoff and melting would need substantially higher atmospheric pressure than currently exist on Mars. It is now doubtful whether these higher atmospheric pressures ever existed, since recent research suggests that the permanent polar caps (a CO2 reservoir) would themselves probably only contribute 0.36 mb of additional CO2 if they completely sublimated (Byrne and Ingersoll, 2003).

More generally, however, Clow's investigation made several simplifications that need to be addressed. Clow (1987) did not explicitly include mass loss in his model (so his snowpack never disappeared). He also did not explicitly treat the sensible heat flux, latent heat flux and atmospheric heat flux terms in the surface energy balance but instead parameterized the sum of these terms to be sinusoidally varying around a mean value (derived from mid-latitude values). He did not apply his model to different latitudes and slopes/aspects, but rather to mid-latitudes on level ground. Lastly, his radiative transfer model did not allow for inhomogeneous dust mixtures within the snowpack. Christensen (2003) relies heavily on Clow's calculations and it is necessary to examine the applicability of Clow's work to slopes.

Note that in this study we are specifically modeling snow, and not bubble-free ice (so-called “blue” or “black” ice). The subsurface melting possibilities of bubble-free ice are intriguing, but the likelihood of bubble-free ice on martian slopes is unknown, and it seems more likely that ice or snow on martian would contain both bubbles/pore spaces as well as dust.

In this paper we examine the question of whether a 1 m thick snowpack located in the southern mid-latitudes and on a slope such as in Fig. 1 could melt under current Mars climate conditions. We also investigate whether a 1 m thick snowpack formed at high obliquity on the same slope could survive to the present day and whether such a snowpack could melt and/or provide runoff (which could form a gully). We focus on conditions exemplified by Dao Valles gullies where apparently 1–10 m of mantle deposit exists (Christensen, 2003). We use a model similar to the model of Clow (1987) but with applicability to slopes and with atmospheric terms developed in much greater detail than in Clow's model, as well as a more sophisticated radiative transfer component.

Section snippets

Description of our model

The full model solves heat, radiation and mass transfer equations in the ice. Below we first discuss the heat transfer equation within the snow. We then describe the boundary conditions on the heat balance equation. Following that we describe the mass balance equation. Finally we discuss the radiative transfer equations. Table 1 summarizes the parameter values we have chosen, and the ranges of variation we considered in sensitivity tests.

The model is iterated over time until the remaining

Numerical model results

In this section we present the results of our numerical model. We first examine the base case, where we have run the model using parameters initialized with values that are the most reasonable (justified in Section 2). We then consider sensitivity to the various parameters by varying the uncertain parameters between reasonable limits.

Results for sensitivity tests

Table 2 lists the parameters that were varied in order to test the sensitivity of the model. The sensitivity tests were done for a 1 m thick snowpack. The model snow lifetime was most sensitive to slope inclination, followed by dust content of snowpack, obliquity and eccentricity of the orbit and density of the firn.

From Table 2 it appears that the snowpack melting and runoff amounts are also sensitive to various model parameters. However, upon further analysis, our model findings suggest that

Discussion and conclusion

Given the snow model we have used, it is not possible that a 1 m thick, dirty snowpack persisted at 33° S and 21° slope with 135° aspect for longer than two seasons. We conclude that the mantled deposit in Fig. 1 is not likely to be an exposed snowfield since it would have a very short lifetime. Melting temperatures are reached when the snowpack was very thin (5.9 cm), however, and a small amount of meltwater runoff was produced (∼1.4 l/m2). Note that this slope geometry is specific to the

Acknowledgments

The authors wish to thank two anonymous reviewers for their helpful comments. K.E.W. also wishes to acknowledge financial support from a NASA GSRP fellowship, and to thank Gary Clow, Mark Williams and W. Tad Pfeffer for helpful discussions.

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