There is enough CO2 ice on the north and South Pole, should you melt it into a gas, to more than double the thickness of the atmosphere. But this would also triple the distance into space the atmosphere would extend. Gravity becomes weaker at the inverse square of the distance meaning a lot of the atmosphere will simply drift off like a comets tail behind Mars.

The problem of terraforming mars has always been the lack of mass… colliding large amount of astroids or small moons from outer Jupiter and Saturn into Mars to increase its mass is not feasible. And would just serve to destroy the planet making it unapproachable for hundreds of years.

Comets of water ice could be set into an decaying elliptical orbit to add mass by melting in the upper atmosphere a little bit at a time without any planetary destruction if a sufficient supply could be found.

A alternative is to substitute the atmosphere with a heavier molecule that will provide atmospheric pressure and heat. A heavier molecule like chlorofluorocarbon, or other complex molecules that resist breaking down under UV radiation.
At twice the distance from the sun, photosynthesis is unlikely to be productive, so living on the surface is not realistic. With a thicker artificial atmosphere you won’t need to have the bulky space suits, just a way to breathe, and the heat generated by the air pressure will provide enough warmth to be manageable. Liquid H402 (“water”….there’s no such thing as H2O outside a laboratory) Will be a problem, being very buoyant as it is here on earth, and will float above heavier air molecules (like oil on top of water) into outer space just like CO2 and nitrogen and oxygen.
Every solution has its unintended consequences.

I see no long-term alternative to human habitation outside of pressurized structures on the surface and pressurized containers under ground to prevent poisonous gases leaking into the habitable caves and tunnels. (robotics is the future of most habitable construction and maintenance).
Even living in the ice caps themselves, where the poisonous gases of CO2 can be managed, will have other challenges to overcome… It would at least solve the problem of plentiful water, nitrogen, and a mostly dust free environment from the resources of tunneling into the ice. (The cold is an issue requiring a large energy source no matter where you live on Mars)

As for Venus, better to focus on “Bio Geo engineering” of a buoyant carbon dioxide eating plankton that is suspended in the upper atmosphere to “reduce” (eat) the atmosphere of Venus into Methane, water and oxygen to eventually infect and survive on the surface making calcium carbonate to condense the current atmosphere onto the surface as a solid. (That’s what happened to earth that formed its lime stone mountains and fossil fuels)

The same could be done for Saturn and possibly Neptune and Uranus as well. Saturn would be reduced to a Waterworld with only 10% more gravity than earth… it would probably still be too hot, but the rings surrounding the huge water planet would be beautiful! All that’s required is the introduction of life without a natural enemy and it will reproduce until it consumes all of the CO2 in the atmosphere. Eventually it will choke on its own waste byproduct (Poisonous oxygen) and the reaction will slow to a crawl. At this point the planet is ready for higher life forms.

I just wish I could dump most of Venus’ atmosphere on Mars in one go. Let the Sun blow that away.

I can show you the Martian Koradteebar* chart that PROVES it.

Believe the SCIENCE!!

* Barsoomian for Hockey Stick

Don C: You need to reread my last paragraph in this post. Mars still has gobs of water. There will be many other much more important tasks for the first, second, and even hundredth settlers to tackle first.

The first case of bottled Europan water, slightly carbonated, should sell for enough to cover the U.S. debt in several thousand years.

And don’t you deny it.