Pedagoguery

Earth has abundant water, but it is actually a bit of a puzzle how it all got here. Current models of the formation of the solar system place the forming earth well within the “snow line”. This is the distance in the solar nebula at which water would vaporize. In addition, any water in the planetesimals that formed Earth would probably vaporize because the collisions would have heated them up past their melting point. So, how did Earth get its water?

An early candidate for the source of the water were comets. Comets formed well outside the snow line, and are an abundant source of water. They have long, elliptical orbits that could conceivably cross Earth's, so they appeared like good candidates. However, there is a problem. Spectroscopic studies of the water in bright comets such as Halley, Hyakutake, and Hale-Bopp determined that the deuterium to hydrogen ratio was much too high, about twice as high as found in water on Earth. Why is this a problem? Deuterium is a heavy isotope of water, with one proton and one neutron. Over time, gasses naturally escape any planet's atmosphere. Lighter elements tend to find it easier to escape. So, over time, more regular hydrogen would be lost than deuterium. So, if comets were the source of Earth's water, the deuterium/hydrogen ratio on Earth should be at least as high as that in comets, but definitely not lower. A different answer was needed.

Meanwhile, further information about the asteroid belt was being discovered. Detailed density data about Ceres, the largest asteroid, suggested that it has a large water-ice mantle, potentially more fresh water than all of Earth's lakes, rivers, ponds, and streams combined. Additionally, spectroscopic studies of other asteroids, in particular 65 Cybele and 24 Themis, indicate that they have some surface ice. This ice would probably sublimate and be replenished from interior ice. In addition, carbonaceous meteorites typically have hydrated minerals inside. Studies of the the deuterium/hydrogen ratio of those minerals are a good match to the Earth's ratio. This is not definitive, since the chemical reactions that formed the hydrated minerals could have altered the ratio, but it is quite suggestive. Perhaps Earth got its water from the asteroids.

A new model of solar system formation perhaps points a way for this to happen. In past issues, I have described how the four outer planets formed closer in than they currently are. However, that does not answer some questions about the inner solar system. For instance, simple models would predict that Mars would be 10 times more massive than it actually is. More recent models, however, point to an interesting scenario. Tidal interactions between Jupiter and the solar nebula would cause it to spiral inwards. This is evidenced by the large number of “hot Jupiter” type exoplanets we see. It could have gotten as close as 1.5 astronomical units (A.U.) from the Sun. This would have caused a separation between the inner solar disk and the outer disk, stunting Mars' growth. It would also have scattered many of the small bodies that formed in the 2 to 4 A.U. range outwards. What would have stopped Jupiter from getting even closer to the Sun? When it became gravitationally linked to Saturn in a 3:2 orbital resonance, it would have sent both planets migrating outwards. During this outward migration, they would have scattered the small bodies inward, including many that had significant water. Some of these bodies would have formed the asteroid belt we see today, and some would have gone further into the inner solar system, hitting the young inner planets, delivering water to each one. The fate of that water depended on the body it hit. For instance, Mercury is too small and too close to the Sun to retain any of that water. Venus suffered a runaway greenhouse effect that caused all of its water to boil off. Most of the water on Mars is probably in underground ice. And Earth has oceans.

Next time, a look at the kinds of stars where we have found planets.