Ambiguities in Geomorphic Interpretation: Martian Gullies
Geomorphic analysis has an important shortcoming as a potential indicator of the present three-dimensional distribution and state of subsurface water on Mars, in that its interpretations are often not unique and are therefore controversial. This problem is illustrated here by considering recent geomorphic analyses of martian gullies.
The fluvial appearance of martian gullies (Figure F.1) has been interpreted as evidence of recent and possibly contemporary discharges of liquid water from near-surface aquifers (Malin and Edgett, 2000). This discovery generated considerable interest in the potential use of such features to identify places where liquid water may be present at shallow depth. However, subsequent analyses of the available observational data, and consideration of the environmental challenges posed by the presence of near-surface liquid water, have raised significant doubts about the uniqueness and plausibility of the shallow aquifer hypothesis.
The key characteristics of the gullies are a fluvial-like morphology that incises the local terrain; a shallow depth of origin (~100 to 500 m) on topographic exposures (e.g., simple scarps, mesas, knobs, crater walls, and central peaks); an apparent youthful age (≤107 years) supported by their fresh appearance, superpositional relationship to other assumed transient landforms (e.g., sand dunes), and crater counts; a geographic distribution in both hemispheres that is restricted to latitudes between ~30° and 70°; and a preferential occurrence on poleward-facing slopes (Malin and Edgett, 2000). In addition, the gullies lack any obvious association with areas of past geothermal activity, such as Tharsis and Elysium (Gulick, 2001).
If the gullies were truly formed by the discharge of liquid water from shallow aquifers, this would imply a combination of geothermal heat flow, crustal thermal conductivity, and groundwater freezing temperature sufficient to reduce the local thickness of frozen ground by a factor of ~10 to 100 over the values generally expected to characterize the cryosphere at these latitudes (see Figure 4.1). Of these three variables, the one most likely to exhibit the greatest variability (when considered on a spatial scale of kilometers) is the planet’s geothermal heat flow, which, in localized areas, might easily exceed the estimated global mean by as much as several orders of magnitude (assuming the presence of a local igneous intrusion). Although an enhanced local heat flow might explain the origin of some gullies (Malin and Edgett, 2000; Gulick, 2001), it fails to explain three of their most notable characteristics: their observed latitudinal distribution, their preferential occurrence on poleward-facing slopes, and—most important—their lack of any obvious association with recognized regions of past geothermal activity (like Tharsis and Elysium).
Another explanation for the possible occurrence of near-surface liquid water is the potential presence of potent freezing-point depressing salts, such as CaCl2 and MgCl2, which could lower the freezing point by as much
as 60 K (to ~210 K) (Brass, 1980; Clark and Van Hart, 1981; Knauth and Burt, 2002). But the involvement of brines in the origin of the gullies is difficult to reconcile with the lack of any visible evidence of evaporite deposits associated with these features and with the inability of brines to explain either the latitudinal distribution or the poleward-facing orientation of the gullies.
Mellon and Phillips (2001) have advanced another hypothesis. They argue that the occurrence of shallow aquifers can be explained by the presence of thick mantles of extremely low thermal conductivity regolith, creating a sufficiently large geothermal gradient that the temperature of the local crust is raised above the melting point
within just a few hundred meters of the surface. However, for the mantle to retain its low conductivity, it must remain ice free, a condition that requires that the aquifer be diffusively isolated from the overlying mantle by an essentially impervious barrier. This requirement is imposed by the fact that, given the diffusive characteristics of most reasonable geologic materials, sufficient water vapor will diffuse from the aquifer and condense in the mantle to saturate its pore volume with ice in a geologically short period of time (~103 to 107 years; Clifford, 1995). The resulting increase in the thermal conductivity will be sufficient to cause any near-surface aquifer to freeze. The requirement for diffusive isolation appears inconsistent with the widespread occurrence of gullies in such highly disrupted terrains as the interior walls and central peaks of large craters.
Given the difficulty of reconciling the shallow aquifer hypothesis with both plausible environmental conditions and the need to explain the various enigmatic characteristics of the gullies (e.g., their restriction to mid latitudes to high latitudes and preferential occurrence on poleward-facing slopes), a variety of alternative explanations have been proposed, including erosion by liquid CO2 (Musslewhite et al., 2001) and dry mass wasting (Treiman, 2003). But these alternatives face many of the same environmental challenges confronting the shallow aquifer hypothesis (e.g., Stewart and Nimmo, 2002; Heldmann and Mellon, 2004).
Advocates suggest that there is one explanation for the origin of the gullies that satisfies the most serious environmental and observational constraints. The martian obliquity is known to be chaotic on a timescale of ~107 years, varying from ~0° to 60° (Touma and Wisdom, 1993; Laskar and Robutel, 1993; Laskar et al., 2004). For obliquities ≤45°, the peak insolation on poleward-facing slopes at mid latitudes to high latitudes can yield summer-time surface temperatures that easily exceed the melting point for continuous periods that range from hours to many months (Toon et al., 1980; Pathare and Paige, 1998; Costard et al., 2002). Under these conditions, large amounts of water ice are expected to sublime and melt from the summer polar ice cap, increasing the atmospheric vapor pressure of H2O sufficiently to allow liquid water to flow readily across the surface. This input of vapor could also amplify the extent of polar warming by creating a transient and localized water vapor greenhouse (Pathare and Paige, 1998). Under such conditions, formerly stable near-surface ice deposits (whether in the form of snow or shallow subsurface ice) could conceivably melt and produce sufficient runoff to form the gullies (Paige, 2002; Costard et al., 2002; Stewart and Nimmo, 2002; Christensen, 2003). Indeed, it has been argued that the local condensation and melting of atmospheric H2O may be contributing to the development of the gullies even today (Hecht, 2002), a mechanism perhaps greatly enhanced at times of high obliquity. However, the high obliquity model is not without its critics. For example, Mellon and Phillips (2001) have argued that at high obliquity the atmospheric vapor pressure of H2O is so low that ground ice will sublime from the regolith before any liquid melt is produced. In addition, Treiman (2003) and Heldmann and Mellon (2004) have noted that while most gullies do show a general poleward orientation, there are many exceptions that appear inconsistent with a high-obliquity origin.
The preceding analysis is by no means exhaustive, nor does it conclusively refute the viability of any suggested mechanism for the origin of the gullies. But it does demonstrate the enormous technical difficulties and interpretive uncertainties associated with understanding these landforms—uncertainties that for now preclude the unambiguous interpretation of not only the gullies, but also most other potential geomorphic indicators of the current distribution of subsurface H2O as well (e.g., rampart craters, softened terrain, and patterned ground).
Brass, G.W. 1980. Stability of brines on Mars. Icarus 42: 20-28.
Christensen, P.R. 2003. Formation of recent martian gullies through melting of extensive water-rich snow deposits. Nature 422: 45-48.
Clark, B.C., and D.C. Van Hart. 1981. The salts of Mars. Icarus 45: 370-378.
Clifford, S.M. 1995. Mars: The response of an ice-rich crust to burial by a volatile poor mantle. Pp. 261-262 in Lunar and Planetary Science Conference XXVI, March 1995, Houston, Tex.
Costard, F., F. Forget, N. Mangold, and J.P. Peulvast. 2002. Formation of recent Martian debris flows by melting of near-surface ground ice at high obliquity. Science 295: 110-113.
Gulick, V.C. 2001. Some ground water considerations regarding the formation of small Martian gullies. Abstract No. 2193. 32nd Annual Lunar and Planetary Science Conference, Houston, Tex.
Hecht, M.H. 2002. Metastability of liquid water on Mars. Icarus 156(2): 373-386.
Heldmann, J.L., and M.T. Mellon. 2004. Observations of martian gullies and constraints on potential formation mechanisms. Icarus 168: 285-304.
Knauth, L.P., and D.M. Burt. Eutectic brines on Mars: Origin and possible relation to young seepage features. Icarus 158: 267-271.
Laskar, J., and P. Robutel. 1993. The chaotic obliquity of the planets. Nature 361: 608-612.
Laskar, J., A.C.M. Correia, M. Gastineau, F. Joutel, B. Levrard, and P. Robutel. 2004. Long term evolution and chaotic diffusion of the insolation quantities of Mars. Icarus 170: 343-364.
Malin, M.C., and K.S. Edgett. 2000. Evidence for recent groundwater seepage and surface runoff on Mars. Science 288: 2330-2335.
Mellon, M.T., and R.J. Phillips. 2001. Recent gullies on Mars and the source of liquid water. J. Geophys. Res. 106: 23165-23180.
Musslewhite, D.S., T.D. Swindle, and J.I. Lunine. 2001. Liquid CO2 breakout and the formation of recent small gullies on Mars. Geophys. Res. Lett. 28: 1283-1286.
Paige, D.A. 2002. Near surface liquid water on Mars. Abstract No. 2049. 33rd Annual Lunar and Planetary Science Conference, March 11-15, 2002, Houston, Tex.
Pathare, A.V., and D.A. Paige. 1998. Recent liquid water in the polar regions of Mars. P. 31 in First International Conference on Mars Polar Science and Exploration. LPI Contribution No. 953 Lunar and Planetary Institute, Houston, Tex.
Stewart, S.T., and F. Nimmo. 2002. Surface runoff features on Mars: Testing the carbon dioxide formation hypothesis. Geophys. Res. (Planets) 107 (E9): 7-1. CiteID 5069, DOI: 10.1029/2000JE001465.
Toon, O.B., J.B. Pollack, W. Ward, J.A. Burns, and K. Bilski. 1980. The astronomical theory of climatic change on Mars. Icarus 44: 552-607.
Touma, J., and J. Wisdom. 1993. The chaotic obliquity of Mars. Science 259: 1294-1296.
Treiman, A.H. 2003. Geologic settings of Martian gullies: Implications for their origins. Geophys. Res. (Planets) 108: GDS 12-1. CiteID 8031, DOI: 10.1029/2002JE001900.