Andy Karaba, age 10, of Phoenix, Ariz., for his question:

HOW COULD THE OCEANS STAY ON THE HOT EARTH?

This story takes us back, back to when the planet earth was very young. It is not easy to trace back its early history, but scientists suspect that at first it was cold, very cold. Then dramatic changes caused it to become hot, hot enough to melt most of its materials. Naturally, if there was water on this hot surface, it turned to steam. So the watery oceans had to wait until things grew cooler.

Earth scientists estimate that our planet has had about 4 ½ billion birthdays and survived through a lot of astonishing changes. It is easier to grasp the stupendous events if we shrink the long story to the length of just one year. For the first eight months of our make believe year, things were so uncomfortable that no living thing could survive here.

The make believe month of January started out very cold. Then heavier materials began to sink to the middle of the newly formed planet. Some of these were hot radioactive materials, and pressure from heavy rocks added more heat. By the end of January the surface of the young planet was perhaps hot enough to melt some of its rocks. But gradually its heat escaped and things grew cooler.

During the February of our make believe year, the surface grew cool enough to form a solid crust. Thick, heavy rain clouds hung in the sky. But if they shed rain, it evaporated on the warm ground and the vapor rose to form more clouds. However, the ground was getting cooler.

Scientists are not sure when the surface grew cool enough to keep its rainfall. Maybe it was in early March, April or late May. In our make believe year this would be between 2 and 3 billion years ago. After the long, hot, dry ages, quite suddenly the earth became a very different place.

The thick, heavy atmosphere contained countless tons of moisture just waiting to deluge the ground. And as soon as the surface was cool enough, down it came and stayed. For a while the world must have been flooded. But the water streamed down the slopes and gathered in the hollows. When at last the skies cleared, the earth’s wide oceans were filled to the brim.

Those early seas were fresh rainwater, pouring down from the clouds. Later, the deluges became showers. The runoff started streaming down slopes on the dry land dissolving salty chemicals from the rocks. The streams still steal chemicals from the land and dump them into the oceans, which is how the sea became salty and why it gets saltier year by year.