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LUNAR VOLATILE DEPLETION DUE TO INCOMPLETE ACCRETION WITHIN AN IMPACT-GENERATED DISK.

ABSTRACT: The Moon may have formed from an Earth-orbiting disk of vapor and melt produced by a giant impact.1 The Moon and Earth's mantles have similar compositions. However, it is unclear why lunar samples are more depleted in volatile elements than terrestrial mantle rocks2-3, given that an evaporative escape mechanism4 appears inconsistent with expected disk conditions.5 Dynamical models6-7 suggest that the Moon initially accreted from the outermost disk, but later acquired up to 60% of its mass from melt originating from the inner disk. Here we combine dynamical, thermal and chemical models to show that volatile depletion in the Moon can be explained by preferential accretion of volatile-rich melt in the inner disk to the Earth, rather than to the growing Moon. Melt in the inner disk is initially hot and volatile-poor, but volatiles condense as the disk cools. In our simulations, the delivery of inner disk melt to the Moon effectively ceases when gravitational interactions cause the Moon's orbit to expand away from the disk, and this termination of lunar accretion occurs prior to condensation of potassium and more volatile elements. Thus, the portion of the Moon derived from the inner disk is expected to be volatile depleted. We suggest that this mechanism may explain part or all of the Moon's volatile depletion, depending on the degree of mixing within the lunar interior.

SUBMITTER: Canup RM

PROVIDER: S-EPMC6662721 | biostudies-literature | 2015

REPOSITORIES: biostudies-literature

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LUNAR VOLATILE DEPLETION DUE TO INCOMPLETE ACCRETION WITHIN AN IMPACT-GENERATED DISK.

Canup Robin M RM Visscher Channon C Salmon Julien J Fegley Bruce B

Nature geoscience 20151109

The Moon may have formed from an Earth-orbiting disk of vapor and melt produced by a giant impact.1 The Moon and Earth's mantles have similar compositions. However, it is unclear why lunar samples are more depleted in volatile elements than terrestrial mantle rocks2-3, given that an evaporative escape mechanism4 appears inconsistent with expected disk conditions.5 Dynamical models6-7 suggest that the Moon initially accreted from the outermos ...[more]

PMID: 31360221

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Project description:Understanding how spacecraft alter planetary environments can offer important insights into key physical processes, as well as being critical to planning mission operations and observations. In this context, it is important to recognize that almost any powered lunar landing will be an active volatile release experiment, due to the release of exhaust gases during descent. This presents both an opportunity to study the interaction of volatiles with the lunar surface, and a need to predict how non-indigenous gases are dispersed, and how long they persist in the lunar environment. This work investigates these questions through numerical simulations of the transport of water vapor during a nominal lunar landing and for two lunar days afterwards. Simulation results indicate that the water vapor component of spacecraft exhaust is globally redistributed, with a significant amount reaching permanently shadowed regions (cold traps) near the closest pole, where temperatures are sufficiently low that volatiles may remain stable over geological timescales. Exospheric evolution and surface deposition patterns are highly sensitive to desorption activation energy, providing a means to constrain this critical parameter through landed or orbital measurements during future missions. Contamination of cold traps by exhaust gases is likely to scale with exhaust mass and proximity of the landing site to the poles. Exhaust propagation is perhaps the most widespread and long-lived impact of spacecraft operations on a nominally airless solar system body, and should be a key consideration in mission planning and in interpreting measurements made by landed lunar missions, particularly at near-polar regions.Plain language summaryThere has been increasing interest lately in learning more about the origin and distribution of water on the Moon. However, whenever a spacecraft descends to land on the lunar surface, it releases water vapor and other gases into the lunar environment, complicating the situation. In this work, we use computer simulations to understand what happens to the water released by a spacecraft during a typical landing. The simulated landing creates a temporary, very thin atmosphere all around the Moon. The behavior of this atmosphere depends on how strongly water sticks to the lunar surface, such that comparing simulations to measurements of water in the lunar environment during and after future lunar landings could help us figure out the "stickiness" of the lunar surface - something that we don't yet accurately know, but is important to understanding the past, present and future distribution of water on the Moon. Our simulations also show that some spacecraft-delivered water travels to regions near the poles that are cold enough to trap water for very long periods of time. If the spacecraft is heavier, or lands closer to the poles, its influence on the lunar surface and atmosphere may be more significant.

Dataset Information

LUNAR VOLATILE DEPLETION DUE TO INCOMPLETE ACCRETION WITHIN AN IMPACT-GENERATED DISK.

Publications

LUNAR VOLATILE DEPLETION DUE TO INCOMPLETE ACCRETION WITHIN AN IMPACT-GENERATED DISK.

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