Paul VanDeren, age 10, of Coventry, Rhode Island, for his question:

How does Mars get its ice?

In 1973, the brave little space probe Mariner Nine sent back close up information from the planet Mars. Telescopes already had shown that snowy white Martian polar caps come and go with the changing seasons. Already we knew that the Martian climate is cold enough to freeze water into snow and solid ice. But Mars seemed to be very short of water. Surely there was not enough spare moisture to form those enormous polar caps.

Mariner Nine found that Mars is as cold as expected, much colder. than freezing. It also sent pictures of Martian geography, with canyons and gullies that seem to be worn down by running water. Maybe there once was plenty of water on Mars to make big thick ice caps. But nowadays there is very little. For this reason, it is thought that the ice caps may be very thin layers of ordinary ice, perhaps with some carbon dioxide frozen solid in the chilly Martian air.