Mars is a planet commonly brought up when talking about terraforming. Even if we were to manage to get past all economic barriers, and somehow form a thick atmosphere, would the lack of a magnetic field on Mars stop any attempt to terraform it? Would the atmosphere be destroyed by solar wind?
Without a magnetic field any changes we do are seemingly temporary. As we make atmosphere it will be torn away by double solar winds.
Double solar winds are the worst, occurring about 15% of the time this occurs when a faster solar wave catches a slower and rolls into one bigger wave.
And these happen frequently, very frequently!
First Edberg and his colleagues identified 41 doubled solar particle waves and solar powerful particle waves from what are called coronal mass ejections from 2007 to 2008 detected by the Advanced Composition Explorer (ACE) spacecraft that monitors the space weather near Earth. Then they identified 36 of the same events hitting Mars in Mars Express data.
The thin atmosphere left on Mars is due to the small amounts protected at each pole by what remains of the magnetic fields, (which still exist at the poles).
Now I hear you asking, why does Venus have a thick atmosphere but Mars does not?
Mars is much smaller than earth and Venus and it is theorized (not definite) that its liquid outer core cooled and solidified, this had two effects on Mars.
Firstly it meant the large volcanoes became extinct, with no liquid core there was nothing left for them to eject. The volcanoes were responsible for the production of a large amount of the gasses released into the atmosphere, much like they were on Earth millions of years ago. Without these, there was nothing replenishing the atmosphere.
Secondly it meant that the magnetic field slowly died off at all places but the poles, which still retain a small magnetic field. This meant that these double solar waves we mentioned before are now ripping whats left of the atmosphere off of mars.
For the sake of Venus, Venus no longer has a magnetic field, despite popular belief, though the solar winds have torn away all of the lighter hydrogen and oxygen layers. It is unknown to me at this time why the solar winds are unable to tear away the remaining gasses though I think this might be due to them being larger and heavier, such as the carbon dioxide and sulfuric acid layers that make up Venus atmosphere today.
So you could theoretically terra-form Mars by producing atmosphere quicker than its torn away, like the volcanoes tried to do, but that will be expensive and highly resource intensive and not really worth it compared to just sticking a dome there and filling that with air.
There was once talk of liquidating the core with a thermo-nuclear explosion but again, were talking very expensive and difficult.
If one could get the core moving again, presuming it has stopped and solidified, then the magnetic field should replenish itself, and an atmosphere would stay put when generated, but the risks and costs are insanely high compared to finding a planet that already has a magnetic field.
The lack of magnetic field wouldn't stop our attempts, but it would reverse them all over time due to these double solar waves.
The lack of field also wouldn't stop radiation, so any life, such as trees, would die very quickly when exposed to that un-absorbed level.
So yes, terra-forming Mars is theoretically possible, but practically improbable. You are more likely to find bio domes on atmosphere-less planets until we find a simple way to regenerate the magnetic fields.
Using liberally "The Case For Mars" second edition. Here's a few facts that help out in the discussion.
- There is substantial Carbon Dioxide at the poles, which if all of it melted, would thicken the atmosphere considerably.
- The theory is current that a 4 degree centigrade rise in temperature in the South Pole (Sustained) would trigger a run away greenhouse effect, more drastic than is predicted by Earth global warming scenarios.
- We don't really know how much gas Mars loses every day, but there is definitely some lost.
- The absence of a magnetic field is a significant contributor to the loss of atmosphere on Mars.
- The atmospheric levels change rapidly as a result of gas being switched between poles during season changes. That level is 30%. I believe that accounts for the 30% @RhysW quoted in his answer, but it comes back on a regular basis.
Bottom line, some level of terraforming on Mars is entirely possible. We would likely have to replenish the atmosphere periodically, perhaps once every thousand or tens of thousands of years. MAVEN will answer many of the unanswered questions (Specifically, what is the rate of loss of atmosphere on Mars)
I would say, probably "no".
Mars has now a thin atmosphere because of the magnetic field, and because of its mass (1/10 the Earth one). The gravity of the planet depends on its mass, and it is the gravity which keeps the atmosphere there.
Therefore, if you want to have an earth-like atmosphere, you should have the magnetic field of the Earth, and its gravity. Otherwise you will progressively lose atmosphere. This means that the atmosphere needs to be artificially maintained, which is again a matter of costs (assuming that we have the technology to generate it...).
A global terraforming would cost incredible amount of energy, work and money, but I think it's possible by starting with some giant domes that acts like green houses for the vegetation and then connecting them with additional green house tunnels and then, slowly expanding them. It terraforms the surface, but without the defense of domes, it would be useless, though.
Adding enormous amount of oxygen, nitrogen, ozone, carbon-dioxide and other components would help making pressure and composition of air to non-lethal - vegetation would help making this method easier by releasing oxygen they create. Then, the domes may be removed.
It's just my theory, and might be totally useless, so feel free to vote down. However, I think it's a possible way of terraforming.