HOW COME? How the planets get their colors

Why does the planet Mars look red, while Venus looks white? a reader asksScan the sky some night, and you may see one object that appears to glow with a red-orange light. If so, you've located one of our next-door-neighbor planets, Mars, basking in the light of the sun. The Red Planet's color is evidence that Mars was, once upon a time, a warmer, wetter, more Earthlike place.

But Mars is now a desert world, covered with red-tinged rocks and sand dunes. Why red? When NASA's Viking space probes landed on Mars in 1976 and tested the planet's soil, they found that 19 percent of it was ferric oxide -- rust, just like the rust on a bike left out in the rain.

On Mars, as on Earth, iron is part of the minerals in rocks. When iron combines with oxygen (found in water and in atmospheric gases, including carbon dioxide), rust can form. Mars has some water locked in ice, and a thin carbon dioxide atmosphere. But scientists think that long ago Mars had a thicker atmosphere and that rivers of liquid water ran on the then-warmer planet.

Most of Mars is now covered with red-tinted sand (which swirls into the air during dust storms). So when sunlight strikes Mars, its rusty surface reflects back red-orange light.

But we never see the surface of our other next-door-neighbor, Venus. Hidden under a thick covering of sulfuric-acid clouds, the 800-degree F. surface is rocky and studded with volcanoes. The yellow-white clouds prevent all but a little sunlight from reaching the ground. But the clouds do reflect 75 percent of the sunlight that strikes them, making Venus shine with a brilliant white light in the night sky.

But solar-system colors aren't limited to red and white. One of the most strikingly colored worlds in our sun's neighborhood is Neptune. Look at icy Neptune through a telescope, and you'll see a planet of the brightest blue, set against the jet black of space.

Neptune's color comes from its methane atmosphere and high clouds. About 80 percent of Neptune's atmosphere is hydrogen gas; almost 19 percent is helium. The remaining 1.5 percent or so is methane.

In Neptune's frigid upper atmosphere, at temperatures as low as -360 degrees F., methane condenses into ice crystals, creating high cirrus clouds. When white sunlight, with its hidden rainbow of colors, enters the atmosphere, the red end of its spectrum is absorbed by methane. But blue light is reflected back into space by the icy clouds, creating the color some call Neptune Blue.