Re: Minimum density and gravity for inhabitable world

- Relatively small but cold can retain or even collect volatiles for
billion years (eg.Titan)

- Small and warm ussually not: the speed of some gas molecules excede
escape velocity so in a long time they loose any atmospere (eg. our
Moon that actualy is not small at all, is one of the largest bodies in
the solar system excluding mayor planets and the sun)

So to terraform a planet/satellite as you said you have 2 ways:

- heat a small cold body, using a composite solar mirror done by
multiple solar sails around some lagragian point (the same of the
solar shield proposed for Earth and Venus, but that acts as a
concentrating mirror). It could be interesting to see what will happen
and how much atmosphere will be generated boiling a little Europa,
Ganymede or Callisto

- add volatiles to a warm and small body by comet bombing or other.
This is the "classical" terraforming proposal for Mars and for our
Moon (but too close to Earth to do risky manouvres).

If a tick solar shield was possible, even Mercury could be a very good
candidate in the second category considering that is relatively dense,
massive with an high gravity.

Venus is big and very warm but still possible to do something:
http://groups.google.it/group/rec.arts.sf.science/browse_frm/thread/e80a864b3f746eda/1f8bbe144a012699

In any case you need to replace the depleted volatiles if you want
this to last more that some million years... :-)

Of course chemical composition of the athmospere and radiation without
decent shielding and magnetical field is another story.

Maybe a tick atmosphere and an artificial magnetic field would
sufficie to protect, say, Europa by the Jupiter radiations, solar
flares and cosmic rays....But I'm just trying to guessing.

The lack of continents is not an issue, compared to the huge problems
above. We are yet doing floating buildings and floating cities on the
Earth, now.

If you are able to do a terraform you are certainly able to do some
floating island or even floating continents taking the materials from
the bottom of the ocean.

.