Michael Chromczak; age 14, of Utica, New York, for his question:

Why is Earth the only planet with water?

Is it true that Earth is the most watery world in the Solar System. However, gaseous and frozen water exists on almost all the other planets, though nobody knows exactly how much. So far as we know, the other planets have no oceans of liquid water. However, recent close up photographs of Mars suggest that it might have been deluged with rains in the distant past.

We know that liquid water cannot exist on Mercury. The nearby sun would boil it to gaseous vapor and the gravity of the small planet is too weak to hold an atmosphere. If water ever existed on Mercury, its gaseous molecules long since escaped and got lost in space. Recent space probes indicate that no, liquid water can exist on the scorching surface of Venus. But many astronomers suspect that water vapor, perhaps in large quantities, exists in the thick, cloudy Venusian atmosphere.

On Mars, huge white polar caps come and go with the alternating seasons. As they retreat, they leave a darker tinge on the landscape, suggesting moisture from crystals of frost or light sprinklings of snow. The gravity of this small, light weight planet can hold only a very thin atmosphere. But space probes detected a percentage of water vapor in the thin Martian air. Modern Mars has no oceans, streams or lakes. But extensive surface erosion suggests that perhaps rains deluged down during the planet's infancy.

Recently we got some startling new information that upsets almost all of our previous notions about giant Jupiter. It emits more radiation than it gets from the sun and may be more like a toy star than a large planet. Previously we assumed that its water quota would be in the form of solid ice or frozen flakes. Until Jupiter's temperature is up dated, we cannot say whether liquid water can exist there.

So far as we know, none of the other planets has anything like the amount of liquid water we have on Earth. Since they are the majority, perhaps we should ask how come the earth has so much more than its share. The answer seems to be a complex set of circumstances involving the earth's density, temperature and past history.

The bare and arid surface of our infant planet suffocated under a huge dense atmosphere. Much of its hydrogen and other light gases escaped. Eventually dense clouds formed moisture and shed rain  ¬deluges of rain that filled the vast ocean basins. In most regions, temperatures settled between the freezing and boiling points of water. The global temperature conspired with the gravity of our dense little planet to hold onto this abundant supply of life giving water. Things are different on the other planets.