The atmosphere of Jupiter is 90 percent hydrogen. The remaining 10 percent is almost completely made up of helium, though there are small traces of other gases inside.
These gases pile on top of one another, forming layers that extend downward. Because there is no solid ground, the surface of Jupiter is defined as the point where the atmospheric pressure is equal to that of Earth. At this point, the pull of gravity is almost two and a half times stronger than it is on our planet.
Trying to stand on that surface would be impossible, since it is simply another layer of gases. Spacecraft and astronauts would only sink into the mire. A probe or spacecraft traveling farther toward the center of the planet would continue to find only thick clouds until it reached the core.