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If an ocean like the one below has been on Mars, where is the water now? Any explanations?

Spatula, the temperature on Venus' surface is ~465C which will melt lead. This is above the critical temperature of water (374C), where no matter what the pressure it is at it does not form a liquid. So there will be no water to dissolve CO2 and do the other things you mention. The water in the atmosphere, will photo-disassociate and the hydrogen will be lost. I believe I've read that NASA space probes have detected hydrogen being lost from Venus. Anyone have a link?

Something to consider is that when most of the atmosphere becomes water by composition, how heat will be distributed in such an atmosphere.

Antius, when the sun was only 70% as hot as it is now, Venus was thought to have a hot wet greenhouse. The temperature then was enough to evaporate the seas. The carbonates were baked out of rocks releasing CO2 and Venus has turned into the dry greenhouse it now is as the hydrogen was lost.

I think I made a good case for the probability that Venus never had appreciable amounts of water. Stellar evolution models and the lack of atmospheric oxygen would preclude significant water in Venus's past. It was always a furnace.

Now if we add water (which is a VERY powerful greenhouse gas) we make the temperature problems worse. If we could somehow turn the CO2 atmosphere into oxygen then Venus will then radiate a lot of its heat and we might be able to get it to a stable state. But for that to happen, you have to have liquid water at the same place as the phosphorus, iodine, iron & other trace elements. I don't see this happening unless the planet is made much cooler, presumably with some sort of space mirror or the like.

Water is a very strong greenhouse gas, but unlike CO2 it undergoes phases. Hot water vapor is not as dense as CO2, and rises quickly to the top of the atmosphere, taking heat away from the surface. Upon radiating this heat into space, it precipitates to the surface as liquid or solid depending on the atmospheric temperatures.

The water itself traps a lot of infrared radiation due to its polar hydrogen bonds, but because of its phases it tends to be more of an anti-greenhouse gas in planetary settings.

For more anti-greenhouse effects, see Mars. Mars is so cold that CO2 experiences phases on its surface, and because CO2 can freeze and sublimate, it decreases the planet's temperature further. Or perhaps Titan, where Methane, another greenhouse gas, experiences phases and cools the moon off. Or perhaps Pluto, where the cryogenic temperatures are low enough for Nitrogen to have phases and act in a similar fashion.

Venus is a lot hotter than Earth (1.913 times the Earth's insolation), so its oceans will always be in danger of ending up in the run away wet greenhouse that it suffered when Venus was getting 1.33 times Earth's current insolation. This suggests to me that we will ALWAYS want to have a mirror reducing the amount of heat it gets from the sun, or be adding more floating mirrors, manually sequestering CO2, moving Venus farther from the sun, etc. It just sounds like more work than for Mars.

A good concern. With increasing levels of light comes increasing reflectivity. As a material gains heat energy, its specific heat increases, causing it to have trouble gaining further energy. Instead it becomes reflective. A lot of this ties into black-body radiation - as it gets brighter, an object will act more and more like a white body. Venus's natural temperature without the greenhouse effect should not be twice as high as Earth's. Closer to the edge of livability, but well within it.

Floating mirrors are the objects of sci-fi, and will probably remain that way, concerning terraforming. If we screw up, and the planet is still a bit too hot, supplementing the atmosphere with reflective particles and anti-greenhouse gases would push it into safe territory.

Atomoid references the 2004 July / August issue of the Planetary Report which discusses how needed a magnetic field is to keep an atmosphere. Some of the main points...

- For a 1 atm pressure atmosphere you need more gas (because the lower gravity does not compress it as much). This thicker atm. actually gives more radiation protection with out a magnetic field than Earth does with it.

- The article says that 2 m of water was lost over 4 billion years. (This is less than the 14 to 34 meters quoted above.) A thicker atm. won't erode faster than the current one, so if we give Mars a new atm. it would last a very long time. e.g. billions of years.

- The article suggests that the water has not been lost but is frozen under the surface. It then talks about McKay / Zubrins terriforming idea.

This is exactly what I've been saying. A thick atmosphere will not have the high escape rate that a thin one has under the same gravity, due to the real gas law. What Mars lost over billions of years could take trillions if it had the atmospheric density required to create 1 bar of pressure. An Earth-like atmosphere with a distinct ionosphere layer will be more than enough to protect from the Sun's high frequency radiation. This occurs at Earth's poles, where the magnetosphere is weak or nonexistant. It occurs when Earth's magnetosphere enters a dormant phase for hundreds of thousands of years. Not having a dynamo is not a problem.

Now, not having geological activity except for the occasional fumarole or hot spring could indeed be a problem for Mars in the long term. We'll have to see.

Most of the mass of the atmosphere could come from the moon, as could 90% of the mass of the water needed to produce an active hydrosphere.

A lunar atmosphere consisting of 90% pure oxygen and perhaps 10% water vapour at 1/3rd bar, would produce the same O2 concentration in human blood as Earth's atmosphere.  Only trace amounts of nitrogen would actually be needed.

Don't forget that under 1/6th earth gravity, it would take an atmosphere twice as dense as Earth's to create 1/3rd pressure at the Moon's surface.

If, at any point, we have to rely on industrial refining to get something, such as chemically extracting components from the moon's crust, we might as well give up. This would be an impossibly expensive step to conduct on a planetary scale. Getting stuff like Oxygen without refining would be insurmountable.

The best thing I can think of is genetically engineering a very sturdy type of bacteria to pull it out of the minerals in the crust.

I'm not saying it's impossible to terraform the Moon. I'm saying that for the cost, it's not worth it.

What's the best way to communicate from Mars to Earth and vice versa?
Communication amongst people on Mars?

Nitrogen is more abundant on Venus than on Earth.

Not proportionately - Venus atmosphere is ~96.5% CO2

Do you believe thermodynamic loss ...

http://cseligman.com/text/planets/retention.htm

... is a factor at all in atmospheric retention?

Venus is another case and example.
And it do not have any magnetic field.

But it has a 90 bar atmosphere!  And even so, most of the lighter elements have been lost (hydrogen, helium, nitrogen, oxygen).  Now only CO2 is left. 

Why didn't the ionosphere stop the lighter gas molecules being lost?

Forget ecology;
All the supergreenhouse gasses,
extreme pollution is just a small step in terraformation.