Curses, lost this post in limbo. Let's try again...

I agree that I "seem" to have got that point wrong, but what I was trying to do was wrap a lot of description of geysers and geothermal systems into a short space.

First, let me credit Ralph Lorenz of ASU. He wrote an excellent paper on how Geysers work, comparing water geysers on Earth with nitrogen geysers on Titan. I planned to look at hypothetical liquid CO2 geysers on Mars, but have still not found the time for it.

Anyway, Lorenz showed that for a geyser to work in spectacular fashion, the geothermal gradient in the rock around the "plumbing" must be such that the whole water column is at or near the boiling point simultaneously. That way, when boiling starts at any part of the column, water is displaced and pressure drops. As you note, lower pressur leads to lower boilng point and hence you get more boiling, without adding any heat to the system. More boiling leads to more liquid expulsiuon and further drop of pressure. This leads to runaway boiling and expulsion of the whole water column as a geyser.

Essentially, the geothermal gradient must match the pressure-boiling curve for that liquid. This requires quite high geothermal gradients and makes geysers rare. Given the lower expected geotherm on Mars, they should be rarer still.

Mike Carr's numbers are clearly incorrect. He doesn't even take account of the different density of Mars, let alone anything else. As a minimum, you would assume the same materials as Earth, but somehow forming a less dense planet. That leaves you less material to generate heat from.

Beyond that, we can analyse WHY Mars has a lower density, and make reasonable assumptions about radiogenic nucleides in the planet.

THEN we add what we know about Mars' history - a smaller planet cools more efficiently, even without plate tectonics. The nett result is a lower geothermal gradient.

OK, that aside, let's look a hypothetical geothermal system on Mars:-

If it is sealed from the surface, it can exist at high pressure. You can get all sorts of subsystems with a liquid convection leg, a vapour convection leg, and possible 2-phase convection with vapour ascent then condensation and liquid descent. These sorts of convective systems are efficient and have low geothermal gradients WITHIN the system, leading to hot temperatures near the surface and high geotherms between the convective system and the surface.

If it is NOT sealed from the surface, the top of the liquid zone is essentially at Mars atmospheric pressure. Liquid will boil readily at low temperatures (only just above zero C, for pressures at or above Mars' average - i.e. at low altitudes and low lattitudes). Boiling liquid will escape as vapour (possibly forming ice towers as it leaves), but over time liquid will be lost from the system.

The water level will drop and due to the small difference between the boiling and freezing points the walls of the plumbing will freeze rapidly (especially given the cooling due to loss of latent heat to steam), stopping any water influx. The liquid level will thus decay rapidly with time until the system is dry and frozen.

Frozen water has a very high thermal conductivity (about 6 times that of rock and water), so heat is efficiently transported through the walls of the plumbing. This acts to prevent re-establishment of the geothermal system.

The nett effect of an open gothermal system on Mars is to boil-off the water and reduce reservoir pressure until it freezes and dies. This is what I meant to say, I think...

Some interesting things happen if you start with a closed system and breach it to draw-down pressure. If you drop the confining pressure, explosive boiling will occur in regions that were previously stable, but hot, liquids. An example in New Zealand is the "Craters of the Moon", where a nearby geothermal power plant tapped reservoir pressure and initiated explosive blowout of the shallow reservoir in a new geyser and maar field.

And as for what happens in the piping of geothermal power stations - all sorts of minerals get deposited, ranging from gold to radioactive (natural) minerals....