Here's why:

* Composition: Jupiter's atmosphere is mostly hydrogen and helium, the lightest elements. However, the immense pressure from the planet's gravity compresses these gases into a dense, fluid state.

* Depth: Jupiter's atmosphere extends hundreds of kilometers deep, far deeper than Earth's.

* Pressure: The atmospheric pressure at Jupiter's cloud tops is about 1 atmosphere (the same as at sea level on Earth). But as you go deeper, the pressure increases dramatically. At the planet's core, it's estimated to be millions of times greater than Earth's atmospheric pressure.

In essence, Jupiter's atmosphere is so heavy that it's practically a liquid ocean of compressed gas.