HOW COME?: Earth's view of space can't be beat

The sky on a clear night on Earth: thousands of stars visible to the naked eye. Not to mention one or more planets, such as bright Venus, often seen suspended in the western sky, just before sunset. But on an overcast night, or after the sun rises, space seems to disappear, hidden behind a veil of clouds or an atmosphere lit by streaming sunlight.

But what about Earth's sibling planets? Are their days and nights interchangeable, both dominated by the blackness of space and a sky full of stars?

As it turns out, the answer is no. There is only one planet in our solar system, tiny Mercury, that fits the "darkness at noon" stereotype. What sets Mercury apart from Earth and all the others is atmosphere - or rather the lack of one.

Like Earth's Moon, Mercury has virtually no gases to scatter sunlight, so the sky is dark during the daytime. But as the closest planet to the sun, Mercury's daytime sky is dominated by our star, which looms three times as large as it does from Earth, its incredible glare set against the black of space.

Next-door Venus is a different sky story. Unlike Mercury, Venus wears a heavy, crushing atmosphere, decorated with a thick layer of yellow, sulfurous clouds. At night, the stars are perpetually hidden behind the clouds, with no glimpses of space from the planet's 800-degree surface. During the daytime, the sun's face is probably also hidden, its light diffused into a sky of red-orange behind the clouds.

Past Earth lies Mars. The Martian atmosphere is thin, and light that reaches the fourth planet is weaker. But the load of red dust blown aloft from Martian deserts scatters light. So the daytime sky, tinted salmon by the debris, is bright. And at night, as on Earth, the stars come out.

Our solar system's fifth planet is giant, gassy Jupiter. Its enormous hydrogen-helium atmosphere is swathed by bands of ammonia-rich clouds, in shades of peach, brown, and white. If you could descend in daylight through Jupiter's virtually bottomless atmosphere, you might glimpse a dim, pale blue sky behind the billowing clouds. But you would probably have to fly above the cloud tops to see stars at night.

Saturn, planet No. 6, is likewise encircled in ammonia clouds, hiding a faintly blue sky. (Saturn's shiny rings would be visible from high up in its atmosphere.) Uranus is cloaked in thick blue clouds, the color courtesy of a methane haze. Finally, Neptune also lies under its own heavy blanket of blue and white clouds.

So surprisingly, seeing "space" from the surface of a planet isn't all that common in our solar system. In fact, we're privileged on Earth to have both a breathable atmosphere and a night sky full of glittering stars, streaking meteors, and passing comets.

See astronomer William Hartmann's painted vision of the daytime sky on Venus, along with other planetary art, at newsday.com