Jupiter's atmosphere is the thickest in the solar system, and it is possible that the gas giant is nothing but atmosphere all the way down. According to Space.com, Jupiter lacks a solid surface, and its atmosphere is composed mainly of hydrogen and helium.
The hydrogen in Jupiter's atmosphere exists in highly exotic states at deep levels of the atmosphere. According to Space.com, at depth, the Jovian atmosphere becomes an ocean of liquid metallic hydrogen. Astronomers have arbitrarily designated a region of Jupiter's atmosphere as its "surface." This is the point at which the gas pressure is equal to 1 bar, or the air pressure felt on Earth at sea level.