How Much Water Was Delivered From The Asteroid Belt To The Earth After Its Formation? (Planetary Science)

How much water could have been brought to the Earth through asteroid collisions after the formation of earth? Rebecca Martin and Mario Livio now answered this question by using N-body simulations and by comparing the relative impact efficiencies of 3 radially narrow regions of the asteroid belts. Their study recently appeared in Arxiv.

As we all know, the planets in the inner solar system are relatively dry. The precise amount of water in and on the Earth is unknown, but is thought to be between one and ten “oceans”, where one ocean is about 2.5 × 10¯4 M or the mass of water on the Earth’s surface. The question is, how the Earth could have formed with its current amount of water, as it is not possible for Earth to form water by its own. Thus, there have been several suggestions: through adsorption of hydrogen molecules on to silicate grains, delivery of water through meteorites and icy pebbles etc. But, the current leading scenario for the majority of the water delivery is by external pollution from the outer parts of the asteroid belt.

So, by considering this, Martin and Livio now described their n-body simulations in which they modeled three radially narrow regions of the asteroid belt: the ν6 resonance (at about 2.1 au), the 2:1 mean motion resonance with Jupiter (at about 3.3 au) and the chaotic region outside of the asteroid belt. Next, they compared the relative impact
efficiencies between asteroids in these regions and the Earth.

“Our analysis assumed that the asteroid belt contained its primordial mass after the Earth had formed and the giant planets were on their current day orbits. Thus, we have skewed the assumptions to estimate an upper limit to the amount of water that could have been delivered to the Earth.”

They showed that the majority of asteroid collisions with the Earth originate from the ν6 resonance at the inner edge of the asteroid belt. About 2% of asteroids from the resonance collide with the Earth. While, the collision probability from the 2:1 mean motion resonance is about one hundred times smaller and from the chaotic region about a thousand times smaller.

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Figure 1. The outcomes of the n-body simulations in time binned into 0.5 Myr intervals. Note that the points are slightly offset from the centre of the time bin so that they don’t completely overlap. The upper panels have the Earth radius of REarth = 1 R⊕ while the lower panels have Earth radius of REarth = 10 R⊕. The left panels show the simulation of the ν6 resonance in the range a = 2.0-2.1 au. The middle panels show the 2:1 resonance in the range a = 3.3 – 3.35 au. The right panels show the simulation in the region 4 – 4.1 au. The blue points show asteroids that are ejected. The red points show asteroids that hit the Sun. The green points show asteroids that hit the Earth. The magenta points show asteroids that hit Jupiter or Saturn. © Martin and Livio

They also estimated that, if the majority of asteroids in the primordial asteroid belt were moved into the ν6 resonance either through asteroid-asteroid interactions or gas drag, or the Yarkovsky effect, then at most, the asteroid belt could have delivered about eight oceans worth of water. Thus, the delivery of one ocean’s worth from the asteroid belt was certainly possible.

“However, the delivery of 10 oceans worth could have been difficult and if the Earth’s mantle contains such significant amounts of water then the Earth likely formed with a good fraction of it.”

— concluded authors of the study

Reference: Rebecca G. Martin and Mario Livio, “How much water was delivered from the asteroid belt to the Earth after its formation?”, Arxiv, pp. 1-5, 2021.

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