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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.

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