| MadSci Network: Astronomy |
Water is composed of hydrogen, the most abundant element in the Universe, and oxygen, the third most abundant element. There is almost no way that planets, at least in our solar system, could not contain water. Even if somehow a planet managed to avoid having water incorporated in it during formation, over the lifetime of the solar system, comets would deliver water to the planet. (Comets are described often as "dirty snowballs" because they contain large amounts of water, probably mixed with rocks.) The question is not whether a planet contains water, but where and in what form.
Mercury is the planet most likely to have avoided having water incorporated in it during its formation. That's because it formed so close to the Sun that the temperature would have kept water in vapor form and it could have been driven off by the winds of the early Sun. Even so, over the lifetime of the solar system, Mercury has been struck by numerous comets. Thus, there is some evidence to suggest that there is ice near the poles of Mercury in the shadows formed by some craters.
Earth and Venus are remarkably similar in many ways, yet noticably different, too. Venus is the one possibility in the solar system for a planet without water, probably because early in its history it had large oceans much like the Earth does today. However, Venus is closer to the Sun. Thus, it was hotter, and the greenhouse effect on it was stronger. Water vapor is a powerful "greenhouse gas," even more so than carbon dioxide. The result probably was a runaway greenhouse in which the greenhouse effect helped evaporate water from the oceans which made the greenhouse effect stronger which helped evaporate more water from the oceans which .... Once in the atmosphere, the water was broken apart into hydrogen and oxygen by the ultraviolet light of the Sun. Venus' gravity is insufficient to retain the hydrogen, so it would have been lost from Venus forever.
Earth is the only planet known in the Universe (currently!) for which water can exist on the surface in all three phases, solid (ice), liquid, and vapor.
The Earth's Moon, while not considered a planet usually, is a relatively large body in the inner solar system. It may be an example in which water was not present during its formation, because the Moon is thought to have formed from the debris of a massive collision involving the Earth. The energy involved would have baked the Moon dry. Indeed, the relative lack of water in the Moon's rocks is a key piece of evidence supporting this model. However, like Mercury, the Moon may have ice in craters near its poles.
Like Earth and Venus, Mars may have had early oceans. The search for water on Mars is a key focus of many missions to Mars because where there's water there may be life.
Jupiter and Saturn are large balls of gas. Because hydrogen is the most abundant element, most of that gas is hydrogen. However, the three most abundant elements after helium, oxygen, carbon, and nitrogen, must also be present in both planets. The three are probably present as water (H2O), methane (CH4), and ammonia (NH3). Methand and ammonia are seen in Jupiter's atmosphere. Water is not seen, but the temperature of the layers in Jupiter's atmosphere that we can see is below the freezing point of water. Thus, it is thought that lower in Jupiter's atmosphere there are water clouds (much like those in Earth's atmosphere).
Like Jupiter and Saturn, these two planets are dominated by hydrogen compounds like methane, ammonia, and water.
Essentially a large comet, Pluto is thought to be composed largely of water, methane, and ammonia ices.
It is more difficult to say anything about the extrasolar planets. However, on general principles, we would expect them to have significant amounts of water in them, particularly since most of the extrasolar planets known are like Jupiter.
For more information on the planets, see the Nine Planets site.
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