Inefficient volatile loss from the Moon-forming disk: reconciling the giant impact hypothesis and a wet Moon. (arXiv:1812.10502v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Nakajima_M/0/1/0/all/0/1">Miki Nakajima</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Stevenson_D/0/1/0/all/0/1">David J. Stevenson</a>
The Earth’s Moon is thought to have formed from a circumterrestrial disk
generated by a giant impact between the proto-Earth and an impactor
approximately 4.5 billion years ago. Since the impact was energetic, the disk
would have been hot and partially vaporized. This formation process is thought
to be responsible for the geochemical observation that the Moon is depleted in
volatiles. This model predicts that the Moon should be significantly depleted
in water as well, but this appears to contradict some of the recently measured
lunar water abundances and D/H ratios that suggest that the Moon is more
water-rich than previously thought. Alternatively, the Moon could have retained
its water if the upper of the disk were dominated by heavier species because
hydrogen would have had to diffuse out from the heavy-element rich disk, and
therefore the escape rate would have been limited by this slow diffusion
process (diffusion-limited escape). To identify which escape the disk would
have experienced and to quantify volatiles loss from the disk, we compute the
thermal structure of the Moon-forming disk considering various bulk water
abundances and mid-plane disk temperatures. Our calculations show that the
upper parts of the Moon-forming disk are dominated by heavy atoms or molecules
and hydrogen is a minor species. This indicates that hydrogen escape would have
been diffusion-limited, and therefore the amount of lost water and hydrogen
would have been small compared to the initial abundance assumed. This result
indicates that the giant impact hypothesis can be consistent with the
water-rich Moon. Furthermore, since the hydrogen wind would have been weak, the
other volatiles would not have escaped either. Thus, the observed volatile
depletion of the Moon requires another mechanism.
The Earth’s Moon is thought to have formed from a circumterrestrial disk
generated by a giant impact between the proto-Earth and an impactor
approximately 4.5 billion years ago. Since the impact was energetic, the disk
would have been hot and partially vaporized. This formation process is thought
to be responsible for the geochemical observation that the Moon is depleted in
volatiles. This model predicts that the Moon should be significantly depleted
in water as well, but this appears to contradict some of the recently measured
lunar water abundances and D/H ratios that suggest that the Moon is more
water-rich than previously thought. Alternatively, the Moon could have retained
its water if the upper of the disk were dominated by heavier species because
hydrogen would have had to diffuse out from the heavy-element rich disk, and
therefore the escape rate would have been limited by this slow diffusion
process (diffusion-limited escape). To identify which escape the disk would
have experienced and to quantify volatiles loss from the disk, we compute the
thermal structure of the Moon-forming disk considering various bulk water
abundances and mid-plane disk temperatures. Our calculations show that the
upper parts of the Moon-forming disk are dominated by heavy atoms or molecules
and hydrogen is a minor species. This indicates that hydrogen escape would have
been diffusion-limited, and therefore the amount of lost water and hydrogen
would have been small compared to the initial abundance assumed. This result
indicates that the giant impact hypothesis can be consistent with the
water-rich Moon. Furthermore, since the hydrogen wind would have been weak, the
other volatiles would not have escaped either. Thus, the observed volatile
depletion of the Moon requires another mechanism.
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