Maelanie Austin, age 14, of Marshville, North Carolina, for her question:     

What are the planets made of?

The basic ingredients are, of course, atoms of the chemical elements. About 92 different elements were used to build all the materials that occur naturally on and in the planet Earth. Some or all of these same ingredients were used to make the other planets, though the recipes vary.

The starry suns, the planets and their moons are made from a limited list of basic atoms. Various super heavy, radioactive atoms may exist, perhaps for brief moments, in the nuclear furnaces of the stars. A few top heavy atoms exist as radioactive substances in the cool solid crust of the earth. But most of the planet building elements are atoms lighter than uranium. Each becomes solid at its own freezing point and turns to gas at its own boiling point. Aside from temperature, the solid liquid and gaseous states of matter also may be changed by pressures.

It is more or less established that the sun and its planets condensed from the same enormous cloud of cosmic gases. Its mayor ingredient was hydrogen and most of this became concentrated in the sun. Some hydrogen and perhaps 100 or so heavier elements were distributed between the sun and the planets. On the sun, nuclear fusion began converting hydrogen to slightly heavier atoms of helium.

Meantime, the planets orbited at different distances out in the cold and various things happened to change their original gases. For example, the bulk of the earth's ingredients condensed in a solid globe. Several lighter elements formed an atmosphere. But most of its original hydrogen and perhaps other light ingredients escaped into space.. The earth was left with more than its share of heavy elements, such as iron and nickel. It weighs 5.5 times more than an equal amount of water, and is the densest of all the planets.

Venus also kept a lot of dense, heavy ingredients. Its thick suffocating atmosphere is mostly carbon dioxide and more than 20 times heavier than ours. If Mercury has lead, the sun melts it in molten puddles. Each of the large outer planets is somewhat like giant Jupiter.

Jupiter keeps a goodly helping of its hydrogen and other light gases. Moderate helpings of iron, silicon and perhaps other fairly heavy elements may be concentrated in a small central core. But Jupiter's over all density is 1.33. This means that its total assortment of planet making ingredients weighs only slightly more than an equal volume of water.

Intense cold and mighty Jovian gravity do unearthly things to substances near Jupiter's surface. Thick clouds of methane and ammonia whirl through the turbulent atmosphere. Raindrops and snowflakes of liquid and frozen ammonia may shed thick soggy layers on the surface. Under normal earth conditions, this ammonia and methane would be gases.