It's actually not clear. Jupiter is made almost entirely of gas. Most scientists think it may have a small, rocky core, but we do not know if this is true. If that core does exist, it would be possible to land on it, but any probe would have to venture through the extremely turbulent and hazardous atmosphere that is thousands of miles thick. The atmosphere is very stormy and consists almost entirely of hydrogen and helium. For example the great red spot on Jupiter is a massive storm about 3x the size of Earth.