supercritical fluid

Supercritical fluid (not the same thing as superfluid) is a state of matter (as is "solid", "liquid", "gas") which requires a certain temperature and pressure (in many cases, substantial), the minimum amount of the two (termed the critical point, which is different than the triple point) depending upon the substance. The state has some characteristics of a liquid and some of a gas:

• It can dissolve materials (as can a liquid).
• It can effuse through solids (pass through porous material that would hold back a liquid, as can a gas).
• It is generally miscible, i.e., substances put together form a solution rather than remain separate: like water and alcohol rather than like water and oil.

The supercritical fluid state is of interest to planetary science, especially within solar system bodies that have the necessary conditions: the gravity of Saturn and Jupiter give them the internal pressure to produce supercritical hydrogen through much of their interior; the combined mass of their supercritical-fluid interiors is sufficient that most of the matter in the solar system other than that within the Sun is in this state. Uranus and Neptune also many have internal supercritical fluid, and Venus's high-pressure atmosphere might.