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Name: Ian
Status: student
Grade: 9-12
Country: United Kingdom
Date: Spring 2013

How did Earth end up with much water while other planets do not have any (or much)? I imagine when the solar system was first being formed, there was lots of debris circling the sun, and eventually the debris formed planets - why were not all the water molecules spread evenly to all the planets?


There are many factors that have an effect on the distribution of substances among planets.

Since the formation of the star causes the remaining gases to spin (revolve) around the star, then this will mean that the distribution of substances will depend on the effects of two main and opposing forces: gravitational and centripetal. In our planetary system, this balance of forces resulted in rocky planets close to the Sun and gas planets farther out.

When the planets eventually form, the mass of the planet (its gravity) and temperature will determine whether the water that happened to form with the planet remains with the planet over time. Mars may have had water in the early part of its history, but its lower mass (compared to Earth) may have resulted in that water escaping into space. Likewise the high temperatures of Mercury (and its low mass) may have resulted in the water escaping.

The formation of ice-comets also redistributed the water. These comets - which were more abundant in the early history of a solar system - would have brought water to other planets, and, depending on the distribution and orbits of the planets, may have preferentially brought water to inner planets.

So while it should be true that the formation of water should be uniform - in the beginning, many factors would affect the eventual distribution of the water.

Greg (Roberto Gregorius) Canisius College

Dear Ian,

Very good question. I suspect that all the inner planets, and in fact all the planets received comet and asteroid impacts proportionate to their sizes. However on planets like Venus (too hot) and Mars and Mercury (too small) the amount of water delivered was too small and what water did come was vaporized into space. This probably happened with the primordial Earth as well, but the water supply here eventually increased so that the atmosphere could accept more as time passed.

Sincerely David H. Levy

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