Where does the Solar System keep its water? Beyond Mars, the trend seems to favor more and more water content the farther out we go. Thus Jupiter, which is considered depleted in water, is eclipsed in these terms by Saturn, though that planet has less water than other volatiles. Move on to Uranus and Neptune and you get into serious water enrichment. “The farther out you go in the solar system, the more water you find,” says Bruce Fegley (Washington University, St. Louis).

Fegley’s work, discussed at the Chicago meeting of the American Chemical Society last March, points to a compelling theory about the outer planets: From Jupiter to Neptune, these are worlds whose atmospheres are ‘primary,’ drawn directly from the solar nebula as the planets of our Solar System were forming. Just as the Sun is rich in hydrogen and helium, Jupiter likewise shows large amounts of hydrogen and helium, though less carbon, nitrogen and oxygen than the other gas giants.

Observations of methane, hydrogen and carbon monoxide help us calculate the water vapor abundance on a given world. By the time you get to Uranus and Neptune, it becomes clear from such studies that the water content is considerable. And what of Earth and the other inner planets? Here’s Fegley’s take:

“On the other hand, the terrestrial planets Venus, Earth and Mars have secondary atmospheres formed afterwards by outgassing — heating up the solid material that was accreted and then releasing the volatile compounds from it. That then formed the earliest atmosphere.”

Where all this ties into exoplanet studies is that Fegley thinks his methods can help us study the likely atmospheric composition of Earth-like planets in other solar systems. This Washington University news release quotes the scientist again on what we might find out there:

“Because the composition of the galaxy is relatively uniform, most stars are like the sun — hydrogen-rich with about the same abundances of rocky elements — we can predict what these planetary atmospheres would be like. I think that the atmospheres of extrasolar Earth-like planets would be more like Mars or Venus than the Earth.”

And here’s why: We get abundant oxygen on Earth because of photosynthesis, without which we would be engulfed in nitrogen, carbon dioxide and water vapor. All of which makes spectroscopic studies of exoplanet atmospheres — something we’ll be doing on Earth-size planets in the not distant future — a project of no little significance. For if we know what we ought to find there, and if we find things like oxygen and a reduced gas like methane or nitrous oxide in unusual quantities instead, we’ll have a bio-signature that should launch a thousand dissertations.