Could asteroids have supplied enough water to fill Earth’s oceans?

Three-quarters of the asteroids orbiting the Sun are of the carbon-rich C-type, whose significant freight of hydrated minerals has a similar ratio of deuterium to hydrogen as the water in Earth’s oceans. Asteroids of all types were far more abundant when Earth’s oceans formed around 4.6 billion years ago. Unsurprisingly, asteroids are a leading contender for the source of Earth’s water.

But how many of those ancient asteroids could have found themselves in orbits that sent them crashing into Earth? To find out, Rebecca Martin and Mario Livio of the University of Nevada, Las Vegas, conducted a series of numerical simulations, each with 10 000 test particles (asteroids). Saturn, Jupiter, Earth, and the Sun were included as sources of gravitation.

The likelihood of an asteroid hitting Earth is low. To generate sufficient statistics—even with 10 000 test particles—Martin and Livio focused on three subpopulations most likely to acquire increases in eccentricity or inclination that would lead to a collision with Earth. Those subpopulations are asteroids in the ν₆ orbital resonance with Saturn, in the 2:1 orbital resonance with Jupiter, and in a now much depleted zone of chaotic orbits beyond Jupiter.

When Martin and Livio ran their simulations, they discovered that only asteroids in the ν₆ resonance were likely to have hit Earth in sufficient numbers to fill its oceans. They also estimated that the maximum amount of water that the ν₆ asteroids could have delivered is eight times as much as the amount of water currently covering Earth’s surface.

Is it a plausible vindication of the asteroid theory of Earth’s water? Not quite. Whereas the amount of water in Earth’s oceans is straightforward to determine, the amount of water in Earth’s mantle is more uncertain. The mobility of Earth’s tectonic plates suggests the mantle is softened by the presence of copious water (see “Water in Earth’s mantle” by Marc Hirschmann and David Kohlstedt, Physics Today, March 2012, page 40 ). Some estimates place the total as high as 10 times the amount in oceans.

Last year Laurette Piani of the Center for Petrographic and Geochemical Research in France and her colleagues published their analysis of the composition of 13 enstatite chondrite (EC) meteorites. Unlike carbonaceous chondrite meteorites, whose parents are the C-type asteroids, EC meteorites have an isotopic composition similar to that of terrestrial rocks. Although the EC meteorites in Piani’s sample lacked water, their hydrogen content was surprisingly high.

The asteroid parents of EC meteorites were likely the principal building blocks of early Earth. If the asteroids’ high H content was converted to water, Earth’s entire inventory of water could be accounted for by the C-type and EC-type asteroids. (R. G. Martin, M. Livio, Mon. Not. R. Astron. Soc. Lett. 506, L6, 2021 .)