If such a buried clathrate deposit was suddenly exposed by a landslide, the result would be a blast of liquid water -- squirted outward by the pressure of the vaporizing CO2 -- which could pour down the slope for some distance before vaporizing in the thin Martian air.

[Since a rise in buried clathrate's temperature -- produced by geothermal warmth or a change in Mars' long-term climate -- could cause it to erupt in the same way, this is actually a variant on the "soda-water fountain" theory I described in Part 1 of my report -- except that the eruption of soda water would be released by "uncapping the bottle" through a landslide, rather than by through a temperature rise.]

The obvious problem here is that, for the water part of the released clathrate to turn into liquid water rather than remaining a shower of ice fragments, the eruption would have to occur where the ground temperature was above 0 deg C. -- and the gullies are all in much colder areas.

A possible way out of this problem, however, would be for the water part of the clathrate to be highly salty. As I mentioned in Part 1, some brines made out salts that are thought to be very common in Mars' soil -- such as magnesium sulfate and calcium sulfate -- have melting points dozens of degrees below that of pure water.

And such "briny clathrates" might exist in solid form in these cold regions, but still be warm enough to turn into a short-lived torrent of liquid brine when a landslide released the pressure on them.

There is also, perhaps, one more long-shot possibility. Carbon dioxide can't exist in liquid form under the 1-bar pressure of Earth's atmosphere (thus, frozen CO2 is known as "dry ice" because it sublimates directly from solid into gas).

However, liquid CO2 can exist if the pressure is more than 5 bars -- and so there's a real possibility that large amounts of it may also exist underground in some regions of Mars.

If a landslide released such a deposit, it just might remain liquid long enough to pour down a slope in the same manner as liquid water before boiling into gas.

This, however, is unlikely -- Tanaka points out that such high-pressure liquid CO2 would explode into gas almost immediately on exposure to Mars' near-vacuum, and he thinks that - unlike released liquid water - it would have almost no time to pour down a slope in still-liquid form. Carr, however, disagrees, and that this theory cannot be completely ruled out.)

Another problem is that for CO2 to be under enough pressure to exist in liquid form, it must be much more deeply buried than a water ice-CO2 clathrate -- under at least 30 meters of Martian soil and rock -- and landslides big enough to suddenly unearth such a deposit would have to be a lot larger.

However, it's possible that a smaller landslide could make the covering layer of rock and soil over such a deposit shallow enough that the underlying pressure of the liquid CO2 could cause it to burst out into the open.

More discussion will probably be necessary to decide whether there's any chance at all that this theory is feasible. Indeed, obviously a lot more investigation will be necessary before we can decide which of the alternative models being suggested is correct.

We simply do not yet know enough about Mars' subsurface, or its long-term climate changes, to begin to decide. And if liquid water (briny or otherwise) does exist even part of the time near Mars' surface, the implications for the possibility of life on Mars are obviously enormous.

In the last part of this series, I'll discuss the various ways in which future spacecraft may investigate this new Martian surprise, and allow us to determine its real nature.

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