Robert Ballard, age 8, of Huntington Beach, Calif., for his question:

Why does Mars have north and south polar caps?

We need a telescope to see the polar caps on Aars. Hut we can see only one of them at a time. They form in winter, and while one pole has winter, the other is having summer. This is because the axis of Mars is tilted. As it swings around its yearly orbit, first one pole, then the other pole bows toward the sun. Then the opposite pole is tipped away from the sun    and the weather grows cold enough to freeze a crusty white polar cap.

On Mars, the seasons are twice as long as ours. Also, at the Martian South Pole the winter lasts a little longer than it does at the North Pole.

This gives the south polar cap time to grow larger. But after about one earth year, summer returns and the big frosty cap begins to melt and disappear. When this happens, winter arrives at the North Pole and a frosty white crust forms the north polar cap. After a year or so, the north polar cap melts away and the south polar cap forms again. Early telescopic observations revealed that Mars has permanent ice caps at both poles; they're visible even with a small telescope. We now know that they're composed of water ice and solid carbon dioxide ("dry ice"). The ice caps exhibit a layered structure with alternating layers of ice with varying concentrations of dark dust. In the northern summer the carbon dioxide completely sublimes, leaving a residual layer of water ice.