The evidence for this "wet and cold" scenario on Mars derives mainly from lower erosion compared to even the driest places on Earth. But most befuddling in this picture is perhaps that remote sensing cannot find some of the mineral signatures of water, like surface carbonates, which accompany terrestrial water.

A team of geologists from the Universidad Complutense and and the Universidad Rey Juan Carlos, Madrid, Spain, is studying other ways for liquid water to flow in cold places. The team includes Roberto Oyarzun, Cristobal Viedma, Alvaro M�rquez and Javier Lillo. Their proposal is that under unusual conditions, liquid water can persist all the way down to the frigid temperature of -40 F. The key to how ice might not turn to water vapor, but instead persist as liquid water, could be whether it is trapped in tiny pores.

The team has focused their image analysis on a suspected mudflow basin in the southern hemisphere, called the Gorgonum crater (37.4 S, 168.0 W). The researchers have recently compared the orbital pictures from Mars with known mudflows seen on Earth, such as in Chile (see banner image example, in the Atacama desert of northern Chile).

As the scientists write about the ongoing controversy about water on Mars: "The debate on the existence of water on Mars has lasted for many years. Contrary to what might have been foreseen, the arrival of high-resolution images from the Mars Global Surveyor has merely stirred the debate further." One particular point of contention is whether erosion is from mudflows or just debris, particularly unstable rock flows resembling volcanic avalanches.

Astrobiology Magazine had the opportunity to interview the science team in Madrid, and discuss under what cold conditions, liquid water might account for what has been photographed from orbit so far while circling the red planet.

Astrobiology Magazine (AM): Your recent paper in a European journal states that non-equilibrium water can be liquid as low as -40 F. (Terra Nova: 15, 243, August, 2003; Blackwell Science; European Union of Geosciences).

What are the best-case conditions for liquid water to achieve such a low freezing point, such as high salts, brine content, and volume considerations in porous media?

Oyarzun-Viedma-M�rquez-Lillo (OVML): The best conditions for achieving such low melting temperatures are a combination of both pore size in the sediment, and a depression of the melting temperature by dissolved salts.

In the case of liquids, the hysteresis freezing temperature (as opposed to the thermodynamic freezing temperature), is defined as the limit of metastability of the liquid phase during freezing. Liquids can be supercooled below the thermodynamic freezing transition because of the presence of a kinetic barrier to crystallization.

This phenomenon is particularly important in small, completely confined spaces where kinetic energy is substantially reduced. In other words, the freezing temperature can be severely depressed in confined spaces such as porous media, for example, clastic sediments. If dissolved salts are present, a depression of the freezing point of water is further enhanced. The latter is controlled by the concentration and nature of salts in solution.

AM: Do you still consider the best evidence for run-off to be the imagery and geometry of terrain? Are there other proposals that account for these effects visually?

OVML: We believe that the images show, beyond dispute, water run-off.

Regarding other options, Hoffman (2000; Icarus, 146, 326-342) proposed that many of these geomorphic features could have been originated by flows generated by the collapse of unstable rocks masses near to cliffs, in a way very much resembling volcanic avalanches. In fact the analogy goes further, because Hoffman suggested that these flows would be akin to terrestrial gas supported pyroclastic flows such as ignimbrites or surges, only that a freezing temperatures, with CO2 as the main gaseous phase.

AM: How does one rule out melting carbon dioxide ice, or is that not a consideration because it sublimes directly to vapor at martian low pressures?

OVML: Carbon dioxide (CO2) sublimes directly to the gaseous phase. A CO2 (gas) supported flow would not have the capacity of eroding the Martian surface as observed in the imagery. For example, the existence of V-shaped channels in Gorgonum Crater, cannot be explained unless water was involved in the process.

A pyroclastic flow (a 'hot' Earth analogue to the suggested CO2 Martian flows) does not create topography while it moves, but accommodates to the existent one.

AM: How does one rule out or compare water runoff to rock-collapses, volcanic flows, or wind?

OVML: Rock-collapses are rejected because the observed morphological features strongly suggests flows with considerable fluidity, moving large runout distances.

Volcanic flows can be ruled out because the source area (the alcove) does not have volcanic constructs.

We can also rule out the involvement of winds because meter-scale boulders at the Pathfinder landing Site show C-axis imbrication, which implies deposition in situ from a dense, high energy transport system.

AM: The suggested lower-end on sediment content is around 25% correct, to define a mudflow? What kind of velocities are typical, such as whether these are rapid or very slow?

OVML: Mudflows are a type of gravity mass flows for which the whole sediment concentration by volume is 50 - 90% (normally about 80% in mass). The suggested lower-end of 25% is to discriminate among [highly viscous] or natural Newtonian flows (such as water two-phase floods) and non-Newtonian flows (like mudflows).

The average peak velocity of mudflows on Earth is around 20-30 meters per second (72-108 km per hour). Exceptionally, peak velocity may reach 60 meters per second (about 200 km per hour).

AM: Would it be correct to say that a likely condition in your model is also liquid water trapped underneath a surface ice layer, and how does such a flow generate a tapering runoff channel?

OVML: An inspection of crater image shows that the series of deeply entrenched channels and debris aprons occur only in the northern half of Gorgonum crater. We suggest that this phenomenon might be related to the regional slope, which decreases in altitude to the south.

If groundwater exists within a specific regional stratigraphic horizon, this should leak along the northern face of basins and craters following the regional slope. This phenomenon is also observed in the northern faces of canyons in the so-called Gorgonum Chaos, some 100 km to the west of the crater.

AM: So is Gorgonum Crater a case of a fluidized bed?

OVML: We are aware of the many thermodynamic considerations ruling out the existence of liquid water on the surface of Mars . For example, the extremely low pressures in the region of Gorgonum are below that of the water triple point. Low pressures in the region of Gorgonum (4.5-5.5 milli-bar; Nasa Ames Mars General Circulation Model, 2002) are below that of the solid-water-gas transition, or water triple point (6.1 milli-bar), so any ice would evaporate or sublime rather than flow as a liquid.

Thus, although the highest temperatures in this zone may reach up to 290 K, pressure constraints only allow a phase change from solid to gas. However, in the Gorgonum case we would not be dealing with a typical case of on-surface, free-water, but with water confined to a porous material (i.e. a mudflow).

As early as the 1960s it was recognized that classic thermodynamics was of limited use for many soil-water interactions. Modern studies have shown that liquid water can be found in soils and other porous media at temperatures well below the bulk melting temperatures. Although soils do freeze (e.g. permafrosts in cold regions), the process is not necessarily complete, and a good example is provided by the so-called taliks, i.e. localized unfrozen layers located underneath or within masses of permafrost.

Based on the physics of nonequilibrium we have discussed a plausible model to account for the integrity of liquid water on the Martian surface (despite Martian pressure and temperature, or P-T, conditions), providing that this water is hosted by a porous material. The medium can be provided by a moving fluidized sediment (mudflow-debrisflow).

This finding opens new insights into the debate on the existence of liquid water on the Martian surface.

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