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Not necessarilly. Gravity depends on the mass, not the electromagnetic fields. True, Earth's EM field helps keep the atmosphere 'appropriate' for us, but by making it 'layered' and keeping most of the oxygen low down, while 'pushing' hydrogen high. With an artificial atmosphere, we can produce a similar effect even w/o a strong EM core/fields.
yeah, ill elaborate on the forementioned from what i know
smaller the molecule the faster they go around and around, i.e have more kinetic energy. thats why there's so little of hydrogen in the earth's atmosphere. because they can win over the gravity and electromagnetic field of earth. they diffuse into space.
mars, which is relatively smaller than earth, has less gravity and an insignificant electromagnetic field. this will mean a wider range of gases will diffuse into space, heck, maybe even oxygen. i'll need to do some calculations to work them out but can't be bothered now. anyways, strength of gravity is directly proportional to mass, and so is the strength of the electromagnetic field. mars doesn't have enough mass to sustain a fertile atmosphere (thats imho, what made mars devoid of life) and therefore, we'll need somekind of gravity generator to keep the gas molecules tied down to make mars fertile.
on the other hand, we could do what california minor's doing and set up terraforming stations all over the place spewing gases which would make the atmospehre fertile and at the same time, get stripped away by the sun or diffuse themselves. this would be a short term solution, and also a very large waste of resources.
now come to venus, its far too close to the sun. turtle rotation speed. >_< you'd need a year's worth of coffee to stay awake during a venus day. whats more, you know what happens when the planet faces away from the sun. during earth night the temperature falls quite significantly. imagine if this went on for a year. you get winter. living on venus, minus all that hazardous environment would be like living on the poles.
whats more, a year's worth of sun would definitely sizzle up most of the lifeforms we manage to get going, if any, on venus. we'd also have to think about how the bloody hell we're gonna get any type of equipment to survive in a sea of boiling sulphuric acid, which is essentially the atmosphere of venus. the russian venus probe that landed on the surface survived for like couple hours. as in the article, it'd take more than a thousand years to terraform with our tech. few hours aint gonna be enough.
and for titan? yeah, it's just too tiny. too far away. we'd need to make an artificial sun or pull it into the earth's trajectory to have some life on it.