In fact, Carr and Brown University's James Head think that during the Noachian era, a good deal of the water running through the valley networks did not refreeze at the surface near their far ends, but instead trickled down through Mars' meteor-shattered upper surface and then joined a near-surface underground liquid water table -- eventually pouring out again on the great slopes that separate Mars' southern highlands from the lowland plains that cover most of its north, and completely filling those plains with an ocean of frozen ice. (That ocean's bottom would ALSO have been melted by early Mars' internal geothermal warmth, to form a thin layer of liquid water which may have weathered the basalt of the northern lowlands to form the different rocks detected by Mars Global Surveyor there.)

Only after Noachian Mars' dense atmosphere was finally blasted into space by those giant meteor impacts, and stripped away more gradually by the solar wind, did Mars' surface freeze completely -- and then, after its air pressure dropped to its current tiny level, even that big northern ice sheet sublimated away into vapor and then refroze at the poles or in the underground permafrost layer.

In the case of both theories, we're talking about an intermittent, "pulsing" flow of water through the channels of the valley networks, rather than a continuous one. If Segura and Colaprete are right, the networks were carved by sudden, high-volume flows of warm water down the channels, lasting just a few years to centuries apiece -- flash floods like those that carve desert arroyos, but occurring at intervals of millions of years rather than just a few months.

If Carr's view is right, they were carved by gentler but longer-running flows of snowmelt, lasting thousands of years at a time and then repeating again about every hundred thousand years later on the average. In either theory, the valley networks would have spent millennia-long periods completely frozen and dry, until the next flow of water down their existing channels further deepened and widened them.

And these "intermittent-flow" theories of the networks also explain -- just as well as a continuous flow -- such phenomena as the "sediment fan" identified a few weeks ago in Mars Global Surveyor's photos of a channel that opens into a small crater near Holden crater (which has itself been suspected for some time of having been a water-filled lake in Noachian times).

This fan -- which was unmistakably laid down as a series of overlapping layers, varying somewhat in shape and flow pattern from each other -- provides the final "smoking gun" proof that the valley networks must have had water flowing through them for a fairly long total amount of time during the Noachian, rather than being carved by brief and violent one-time floods lasting just a few weeks or months. But it could have been made just as well by repeated intermittent flows of water through the channel as by a continuous nonstop flow, and perhaps even better.

(The widely spread-out form of the fan suggests that the water was flowing into an already-existing lake filling the crater and thus gradually laying down a sediment delta -- but, again, the surface of the crater lake could have been permanently frozen, with the channel itself completely frozen except during the brief periods when it thawed out enough for water to resume flowing into the lake.)

But if Carr is right, then we're back to the original problem: how do we explain Mars' apparent shortage of water-weathered minerals -- especially at the liquid base of his hypothetical northern frozen ocean?

Phil Christensen tells "SpaceDaily" that some recent articles on the subject have to some extent misrepresented his views -- he still thinks it quite possible that Noachian Mars did have significant amounts of liquid water, provided that almost all that water was very cold at the near-freezing point. Such cold water is enormously retarded in its ability to turn silicate rocks into carbonates -- for instance, they're almost totally absent on the floor of the North Sea -- and it's also greatly slowed in its ability to weather rocks into clays.

(Moreover, while IR spectra seem to rule out large amounts of clays on Mars, they may not rule out fairly large amounts of "zeolites" -- claylike minerals that are formed more easily than clays by water weathering, made out of volcanic glass of the type found in both solidified lava and volcanic ash.)

According to Christensen, near-frozen liquid water could have flowed across the southern highlands only occasionally and briefly -- such as Carr and Colaprete both theorize -- carving the valley networks there while leaving the basalt rocks there almost totally unweathered. It could then have flowed into the northern ocean, whose melted bottom layer would have weathered the basalt rocks there but been too cold to produce carbonates or any large amount of clays.

However, the sprinkling of ancient olivine outcrops across Mars -- a few of which are found in the northern lowlands -- is harder to explain by this view. Olivine is such an unstable mineral that hundreds of millions of years of exposure to liquid water, even if ice-cold, should have broken it down there.

But Christensen thinks it possible that subterranean flows of olivine formed out of magma (underground lava) in the northern lowlands and had only their outer portions weathered into other minerals by contact with the northern ocean -- and that those softer minerals have been eroded away during the billions of years since the ocean disappeared, exposing the small number of olivine outcrops on the northern surface.

In short, we're still very far from knowing whether Mars did have significant amounts of liquid water on its surface during the Noachian. What we DO know now with reasonable certainty is that such water could not possibly have been any warmer than near-freezing. Noachian Mars may have been "cold and damp", but we can now rule out the view of some hopeful scientists that it must have been "warm and wet".