QUOTE (djellison @ Nov 15 2007, 02:12 PM)
And of course, the fact that water can exist, at some times, in some places - doesn't mean it does. It is a transient thing and would boil away quite easily - thus it would have to be replenished in some way.
Good thread. I remember I got asked about this one by NPR when the Mars Gullies story first came out. 'Isnt
Mars too cold for liquid water?' Well, yes, I answered, and Earth is too cold for liquid rock - doesnt mean it
Another data point - when I was at the Mars wind tunnel at NASA Ames doing wave generation experiments,
I noticed they have a big jar of water in the chamber just inside the window to the control room (wind tunnel is open
circuit - inside a large room that gets pumped down). As the pressure drops, the water starts to bump and boil,
but then stops, while still liquid. (i.e. it boils until the evaporative cooling brings the temperature well below
that at which the saturation vapor pressure equals ambient). We had a big tray of water on which we were
hoping to generate waves - we could see on the video link that it bumped once or twice, I guess with bubbles
of dissolved air coming out. Then we turned the airflow on in the hope of making waves at 10mbar or so, and saw
the water glaze over - the enhancement in evaporative cooling by the airflow was enough to freeze it.
Pure water has 6mb vapor pressure at 0 C (and 20mb at 20C - I remember it as 20:20)
When I lived in Arizona, this issue of the metastability of water on Earth's surface was rather evident - spill
water in the kitchen, no problem, it'll dry up by itself in 3 minutes. At the DPS conference in Monterey (2003?)
I raised the question in connection with Titan (known to have 50% or so relative humidity, so where are the
oceans?) - I pointed out that Earth is 60% covered in water, and yet we can hang out laundry to dry. Clearly
this wouldnt work unless the relative humidity were much lower than 100% (because of circulation to higher
altitudes, which dries the air..)
Anyway, it remains a subtle issue. I'd urge people read Mike Hecht's work on the topic (he has a rather
fresh perspective). And I think Titan (where ethane takes the role of salt, in lowering the saturation vapor
pressure of the volatile component in a solution) will be very instructive in comparisons with Mars.
So - Doug's words 'transient' and 'replenished' are key - if the system is out of equilibrium, then lots of
things are possible. And I guess I am learning over the years that disequlibrium isnt that hard to generate...