How quickly would Mars loose its atmosphere?

[Menninger]

Besides low gravity and high doses of radiation, one of the more intractable difficulties in terraforming Mars is that if we build its atmosphere back up, then presumably it would lose it again via the same mechanisms it lost it the first time (ie solar wind, warmth, and low gravity).

However, this may or may not occur quickly enough to really matter to prospective terraformers. Given that it takes considerable atmospheric pressure to allow for the existence of liquid water, and as the general view seems to be that Mars had liquid water during its Noachian and Hesperian eons, it seems logical to suppose that it takes millions of years for Mar's atmosphere to become mostly lost. So imports of gases that mimic the rate outgassed by volcanoes during those eons are sufficient to keep Mar's atmosphere dense indefinitely.

Am I missing something here?

 

[Simon Bridge]

Outgassing rate depends on the atmosphere, it's temperature and pressure and so on.
But it certainly was a very long time for the original Martian atmosphere to be lost.
Getting Mars a new atmosphere in the first place would be the trick. It may take so long as to not matter to the terraformers...

Once you had one, and you could presumably use the mase means to replenish it, you would only need to match the outgassing rate ... which should be less than the early vulcanism rate.

OTOH: if you are resigned to having a non-breathable atmosphere anyway, you could give Mars a greenhouse atmosphere by allowing industrial air pollution perhaps? At least it could warm the place up.

There are a lot of discussions about Mars atmosphere in these forums - have a look.

 

[Menninger]

An alternative explanation could be that Mars lost its atmosphere slowly back then only because it had a magnetic field that partially shielded it from the solar wind. Since Mars no longer has such a magnetic field, it follows that nowadays it could loose its atmosphere relatively quickly. Unfortunately, I can't find a direct answer from a reputable source as to how quickly Mars would loose a rebuilt atmosphere. Maybe we will have such an answer after the MAVEN probe arrives and does its science.

Now a new computer model suggests Mars's magnetic field may have been slowly weakened by four especially large impacts and then snuffed out completely by a fifth and final blow.

That impact created the 2,000-mile-wide (3,300-kilometer-wide) Utopia crater, which dates back roughly 4.1 billion years, said study team member James Roberts of the Johns Hopkins Applied Physics Lab in Maryland.

"It's possible that the four earlier impacts set everything up, and the Utopia crater was the straw that broke camel's back."

http://news.nationalgeographic.com/news/2009/05/090511-mars-asteroid.html

Venus doesn't have a strong magnetic field, meaning that having a molten core doesn't guarantee one.

Regarding being to able to build an atmosphere, there is more than one theoretically possible way you could do it quickly, but using present technology, nuking the ice caps ought do the trick. It might take quite a few nukes though.

Although I would imagine that using super greenhouse gases to sublimate the ice caps would be very time consuming, regardless of their utility in building up the atmosphere, with enough industrial production they should, as you say, allow us to make Mars as hot as we want it to be.

 

[Mordred]

This article in section 2.2.1 covers a bit on Mars CO2 levels
the article also contains some good info on local planets vs habitability.