J--My understanding is that the planet Venus is about the same size as the planet Earth. Why is one planet of the same size, able to hold down more atmosphere than the other?

M-Earth could hold slightly more atmosphere than Venus of a given composition. It's geo and atmospheric chemistry not physics that dictates much of the difference not gravity. Venus and Earth have similar carbon inventories. Most of Venus' is in its atmosphere while Earth's is tied up in rock. Venus has no water due to its high temperature. On Earth water converts CO2 (the prime componant of Venus and Mars' atmosphere) to carbonic acid which forms carbonate minerals when neutralized.

J--Could you traslate that to number of atmospheres?

M-Divide by 14.696

J-- Is it bouyancy or something else that keeps the hovercrafts 'up in the air' so to speak? How high is the pressure under the hovercraft?

M-I think hovercraft use jets of air not bouyancy. Balloons use bouyancy.

J--How can life exist in the deepest parts of the oceans?

M-They use gills not lungs. Gills don't inflate and deflate against pressure.

J--Naturally 'strange matter' and black holes are different. :-)
My idea on this accout is that when the Earth was forming, it was composed of elements that were mostly in a gassious state. Perhaps some of these gasses were trapped inside the planet when the surface cooled and solidified. If that was the case, then the solid surface would be experiencing pressures on both the inner and outer sides. If the pressure on the outer surface is reduced then the inner pressure would excert more force on the solid surface, even if this pressure was not enough to increase the size of the planet, it might be enough to cause shifting in the weaker parts of the mantle.

M-Most planet formation models are based on Earth being built from solid planetisimals similar to asteroids and comets. Gas Giants formed similarly but at a point where abundant volatiles were available to be trapped by the growing planets gravity as it grew in size.

Regards,
Mark