Aha !
The idea that a planetary magnetic field being responsible for retention of atmospheres, is something I have always felt uncomfortable about.
So here's some good news about research which probes into this hypothesis ..
The importance of being magnetized
Quote:
Despite its magnetic field, Earth is losing its atmosphere to space at about the same rate as planets that lack this protective barrier against the solar wind. Scientists now are beginning to question whether magnetic fields really are vital to helping a planet hold on to its atmosphere.
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"My opinion is that the magnetic shield hypothesis is unproven," says Robert Strangeway from UCLA. "There's nothing in the contemporary data to warrant invoking magnetic fields."
Each of the three planets is losing roughly a ton of atmosphere to space every hour. Some of this lost material was originally in the form of water, so this begs the question: how did the planets end up with vastly different quantities of water if they are all "leaking" to space at similar rates?
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They go onto to compare the cases for Mars, Venus and Earth.
Its only my opinion (I have no evidence) but I have always felt the planetary field explanation to have been a 'weak' one.
Great to see scientists questioning the hypothesis, rather than blindly moving on assuming it is so !
Cheers