On Jupiter

In Poul Anderson's Three Worlds To Conquer, "Call Me Joe" and "Hunters of the Sky Cave," Jupiter has, under its turbulent atmosphere, a solid surface that suitably adapted bodies can walk around on. I have got the impression that gas giant planets (so named by James Blish: see here) are not really like that, that instead the atmosphere of such a planet increases in density all the way down to a solid core. The Wikipedia article (see the above link) seems to confirm this:

Other than solids in the core and the upper layers of the atmosphere, all matter is above the critical point, where there is no distinction between liquids and gases.^[8] 

"The planet did not even have a 'surface' in the usual sense; instead, the bottom of the atmosphere merged more or less smoothly into a high-pressure sludge, which in turn thickened as it went deeper into solid pressure-ice. At no point on the way down was there any interface between one layer and another, except in the rare areas where a part of the deeper, more 'solid' medium had been thrust far up out of its normal level to form a continent which might last as long as two years or two hundred."

-James Blish, They Shall Have Stars IN Blish, Cities In Flight (London, 1981), 7-129 AT CHAPTER EIGHT, p. 93.