In one of these (currently favored by Dr. Jeffrey Kargel of the U.S. Geological Survey in Flagstaff, Arizona), the gullies are indeed due to water runoff, but from something even more spectacular in its implications than subsurface springs -- namely, the intermittent accumulation and melting of thick layers of surface ice on Mars.

This would be due to the same "obliquity cycle" which I used in the previous report to explain the "intermittent soda fountain" theory of the runoff gullies, but operating in a different way.

As I said in my previous report, Mars' "obliquity" -- the tilt of its spin axis -- is known to slowly increase and decrease between about 15 degrees and 35 degrees over a 124,000-year cycle (unlike Earth's tilt, which slowly rocks through a range of only 4 degrees thanks to the stabilizing tuggings of our large Moon).

Indeed, recent studies suggest that occasionally -- at intervals of a few tens of millions of years -- Mars' obliquity may swing from 0 degrees all the way up to 60 degrees. (Thus Mars sometimes joins Uranus and Pluto as the only planets that "lie on their side".) At present -- by sheer chance -- Mars is about halfway through one of its obliquity cycles, at a tilt of about 25 degrees.

Kargel's theory makes use of what happens to Mars' climate during these cycles. He agrees with many other planetologists that the planet has large subsurface deposits of water ice near its surface in some regions fairly near the poles (not to mention the water-ice polar caps themselves).

In his model, as Mars' axial tilt increases, the warmth of each polar region's summer naturally increases -- and this causes much of its water ice to "sublimate" (evaporate directly from ice into vapor), greatly increasing Mars' atmospheric humidity.

But much of this increasing atmospheric water vapor then freezes back onto Mars' surface as snow or frost each winter -- and while most of this winter ice cover naturally evaporates back into the air each summer, there are some patches of Mars' middle and high latitudes that remain cold enough on a year-round basis that the new surface ice steadily accumulates there (the so-called "cold traps").

To quote Kargel: "It is likely that favorable sites would be dependent on latitude, time within the obliquity cycle, elevation, slope, slope aspect [heading], and proximity to local reservoirs of ice, e.g., frozen crater lakes." Meanwhile, despite the planet's increasing tilt, Mars' equatorial regions remain warm enough that almost no such patches of accumulating ice can build up there.

These patches of accumulating surface ice may, in some regions, get very thick indeed as thousands of high-obliquity summers and winters pass: "25,000 years of accumulation at 1 centimeter per year would amass to 250 meters, and local accumulations of kilometers of ice are possible."

As Mars' obliquity -- and thus the warmth of its polar summers -- increase still more, however, the second stage of the process starts: some of the accumulated patches of surface ice become as much as 20 deg C warmer.

And the increased warmth of the polar regions has another important effect (recognized by almost all Mars researchers): a great deal of carbon dioxide stored underground -- either as frozen dry ice, a "clathrate" mixture of CO2 with water ice, gaseous CO2 that has been "adsorbed" by Mars' soil (that is, chemically stuck to tis grains), and perhaps even underground liquid CO2 under pressure -- evaporates and is released into the air.

Mars' somewhat cooling equatorial soil absorbs part of this new CO2 -- but it is well outweighed by the thawing polar CO2, and so Mars' air pressure significantly increases.

This still leaves its air pressure only a tiny fraction of Earth's (at most, about 4%) -- but it produces enough of an added CO2 greenhouse effect to warm Mars by another 10 deg C. And it also raises Mars' air pressure enough that liquid water can exist on its surface (indeed, it's only a little too low for liquid water to briefly exist there right now).

The result of these two factors -- as you might expect -- is that many of the accumulated thick patches of water ice on Mars' surface are able to melt, producing the runoff gullies we see today.

But then -- as the next stage of the obliquity cycle occurs and Mars' axial tilt starts to shrink again -- these high-latitude areas cool down again, and the extra carbon dioxide is once again absorbed back into Mars' soil and subsurface, lowering its air pressure again to its current tiny level (or even lower, during those period when Mars' obliquity is less than its current 25 degrees).

So any liquid water near the surface refreezes -- and then, as the air pressure drops lower, the ice "sublimates" back into vapor and then refreezes only at or near the polar caps. So Mars returns to its current state, devoid of all surface liquid water -- but ready to be reawakened when the obliquity next increases.

Once again, there are several reasons why this theory may not work -- in particular, it's possible that the balance of meteorological conditions during the changing obliquity cycle may never allow thick enough local patches of surface ice to evelop. And there are several problems already apparent.

First, why are 90% of the runoff gullies seen in the south, and only 10% in the north? Kargel thinks this may be due to the fact that the much smoother terrain of the northern plains contains fewer shadowy slopes that serve as cold traps for thick surface ice to build up.

It's harder to explain, though, why a very large majority of the runoff gullies are seen on slopes that face poleward; Kargel thinks that the illumination conditions of Mars' slopes may be able to cause most of the ice patches to build up on such slopes, although he admits that more calculations are needed.

Finally, there's the problem that Malin and Edgett think that some of the runoff gullies are very recent -- perhaps only a few dozen years old, much too young to be explained by Mars' last period of high obliquity.

Kargel, however, has some doubts about their interpretation: "To me, the evidence would allow these features to be tens of thousands of years old (or even a million years, judging from cratering evidence alone)."

Malin and Edgett's main piece of evidence for several runoff areas being only a few years old is that they are distinctly darker in color than the immediately surrounding surface -- and since most areas on Mars become lighter in color as even a thin layer of wind-blown dust accumulates there, the implication is that the surface material in the runoff area has not even been undisturbed yet for the few years necessary for such a thin dust layer to build up.

At first, this looks rather convincing. But Kargel and Dr. Richard Zurek have propsoed an alternative theory, based on the fact that -- just as Martian winds during any period tend to dump dust on some parts of the planet -- they tend to sweep it away from other ("deflated") regions.

Thus, these dark runoff areas may actually be fairly old, while the surface immediately surrounding them still has a thick layer of dust left on it from an earlier period in Mars' climate cycle when its wind patterns were different and the winds were actually dumping dust onto the same area -- a dust layer that has not yet been completely removed by the current deflationary winds, but which was covered over by the runoff when it occurred.

In short -- once again -- we simply don't know yet. And there is still a third set of theories, based on the possibility that the "fluid" that caused the runoff was not water -- and may not even have been liquid! In the next part of this report, I'll discuss that possibility.

Authors Note: I'd like to thank Dr. Kargel, who went above and beyond the call of duty in explaining his fairly complex theory to me.