I see so many discussions about how the main roadblock to terraforming Mars is its lack of a magnetic field. If Mars had an atmosphere as thick as Earth's but no magnetic field, wouldn't it still take geological time scales for the solar wind to strip away the Martian atmosphere? How many years would we be able to live on Mars before we had to replenish it? And would this thick of an atmosphere sufficiently protect people on the surface from radiation?
Mars has no magnetic field to protect any atmosphere we might add to the planet, but fortunately it would be lost very slowly, over geological time. Fortunately, both protecting and creating a Martian atmosphere can be achieved by parking one giant powerful space station at the Lagrange 1 point which would broadcast a magnetic field which would protect the Martian atmosphere. Surprisingly, there is a little volcanic activity on Mar which releases enough gas to eventually generate a runaway greenhouse effect which would give Mars about a third Earth's atmosphere. https://phys.org/news/2017-03-nasa-magnetic-shield-mars-atmosphere.html
But we can speed that up. Zubrin says a city of a million people would have the economy to manufacture super-greenhouse gases 17,000 times more powerful than CO2. In just 100 years a city of 1 million people would get the following:-
- The Martian CO2 poles warm and melt.
- Mars would then have a CO2 atmosphere about a third the pressure of sea-level air here on Earth. That gives us radiation protection from the sun and cosmic rays.
- It protects all colony domes across the whole of Mars from tinier micrometeorites which would just burn up in the atmosphere. They're just not an issue ever again! (There would of course be a minimum size at which they did still bust through that 0.3 bar atmosphere, and domes would still require urgent repair mechanisms).
- The atmosphere would make every dome on the planet easier to build because it would be less prone to explosive decompression, increasing the size and viability of some of the valley-covering dome habitats.
- It lets us walk on Mars in plain clothes (with an oxygen breather mask of course), meaning you could cycle a bike long distances out in the open with a light breather pack and mask, repair your domes easier, mine resources, and basically live on the surface of an alien world far, far easier.
- It melts water ice, creating oceans and a rain cycle, which will eventually wash out the perchlorates in the regolith.
- This in turn allows plants and trees and farming on the surface and plankton in the oceans. All that about 100 years after the million-strong colony starts releasing super-greenhouse gases. Why move nitrogen from other planets if Mars has enough that 'just' needs baking out of its rocks? Green Mars doesn't immediately require a breathable atmosphere, but a giant space lense could focus sunlight down on the Martian surface concentrated to one point to melt rock and free volatiles.https://www.universetoday.com/9730/zubrin-on-terraforming-mars/
Venus has no Magnetic field and it has a thicker atmosphere than any other terrestrial planet. Which is a direct counterexample to the statement that lack of magnetic field is what makes it impossible to retain an atmosphere, and even if there were a direct causality between the two there are ways to build an artificial magnetosphere.
Mars' shallow gravity well is a much more significant issue, and means that for any plan to terraform Mars there is an inherent tradeoff between temperature and the ability to retain various atmospheric gases because the Maxwell speed distribution of the molecules in the atmosphere can have a component which is above the escape velocity.
CO2 or anything heavier is trivial to hold on to, Oxygen & Nitrogen may need some care over long geological timescales if your upper atmosphere is too hot, while preventing loss of water is hard, and any free hydrogen in the upper atmosphere is going to escape immediately.
Mars does not have the ability to hold an atmosphere breathable for humans, not because of lack of a magnetic field and the solar wind, but because of the lack of mass and the average kinetic energy (and therefore velocity) of for example oxygen, or water vapor. This happens on much faster than geological scales.
off topic, but I believe venus would be a better option, its thick atmosphere would would allow for floating colonies that could avoid much of the heat on the surface. Everything plants need to grow can be mined from the atmosphere, and resistant cyanobacteria could prove useful in lowering the surface temperature, and mining of the acidic salts to get hydrogen for even more water.