If you tried to land on Jupiter, your spaceship would fall through increasingly dense layers of atmosphere, and you'd be crushed by the pressure before ever reaching solid ground. In fact, there may not be any solid ground to reach. Scientists can't look deep into Jupiter's atmosphere, so they don't know for sure, but if there is solid ground beneath the swirling clouds of hydrogen gas, it definitely isn't like any landform on Earth.
The Formation of Jupiter
Jupiter probably has a dense core because of the way that it came into being. According the accretion theory, planets form as rocks, gases and chunks of ice collect under the force of gravity. The heavier elements naturally sink toward the center of the planet while the lighter elements, which in Jupiter's case are mostly hydrogen and helium gas, remain in the outer layers of the atmosphere. As the planet grows in size, its upper atmosphere collects more and more of these lighter gases from space while the core, which is under intense pressure, also grows.
A Swiftly Spinning Planet
Jupiter's core must be small in relation to its overall size. Astrophysicists infer this from two facts. The first is that the shape of the planet is oblate -- it bulges at the equator and is flattened at the poles. If it had a large solid core, it would be more spherical. The second fact is the planet's high rate of rotation -- it spins once on it axis in just 9.8 Earth-hours, which is more characteristic of a body composed of churning fluids than one that is solid. Gravitation comparisons between Jupiter and Earth suggest that the core has a mass about 15 times that of Earth.
A Supersized Magnet Generator
Scientists generally agree that Jupiter's core has a large concentration of rocky materials, but they don't agree that it is solid. They estimate its temperature to be about 24,000 degrees Celsius (43,000 degrees Fahrenheit), which is hotter than the surface of the sun. Under the intense pressures that exist at the boundaries of Jupiter's core, hydrogen gas, which is the main component of the atmosphere, coalesces into semisolid substance called metallic hydrogen. This boundary is the closest an Earth explorer would come to finding a landform on Jupiter. As this ocean of metallic hydrogen spins, it creates a magnetic field that is 20 times stronger than Earth's.
Jupiter's Moons, at Least, Are Solid
Anyone traveling to Jupiter looking for solid ground would do best to land on one of the planet's more than 60 moons. The largest of these, Ganymede, is the largest moon in the solar system. It is larger than Mercury, almost as large as Mars and features a cratered, icy surface and a magnetic field but no atmosphere. Io, the third-largest moon, is a sulfurous world dominated by volcanoes, while Europa, the fourth largest, may have subterranean oceans containing twice as much water as exists on Earth.
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