The statistical mechanics of solar wind hydroxylation at the Moon, within lunar magnetic anomalies, and at Phobos

Farrell, W. M.; Crider, D. H.; Esposito, Vincent J.; McLain, J. L.; Zimmerman, M. I.

doi:10.1002/2016je005168

Cited by 47 publications

(73 citation statements)

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“…However, we obtained both a latitudinal and diurnal distributions of hydroxyl more consistent with Li and Milliken (). We attribute this result the ratio of U / T and not solely to local surface temperature (Farrell et al, ). It is not clear what physical processes would lead to uniform hydroxylation at all latitudes.…”

Section: Discussionmentioning

confidence: 70%

“…The snapshot of the OH concentration after 18 lunations shown in Figure a is consistent with the Li and Milliken () globally averaged map constructed from the M 3 observations, for example, their Figure 1a. Farrell et al () used this same distribution and the preexponential factor to the diffusion coefficient of D 0 = 10 −12 m 2 /s with equation but estimated a dynamic mass fraction that was less than 0.3 ppm at latitudes above 85° for timescales much less than a lunation. Therefore, Farrell et al showed that if 90% of the diffusing H atoms had an activation energy of 0.5 eV and 10% had an activation energy of 0.7 eV, the amount retained would be >30 ppm.…”

Section: Resultsmentioning

confidence: 99%

“…The observational data were to be adequately reproduced with an activation energy distribution of f ( U 0 = 0.5 eV, U W = 0.078–0.1 eV). This distribution is mitigated both by the change in U / T with latitude and throughout the lunar day (e.g., Farrell et al, ) and the magnitude of the SW flux. The diurnal change in surface temperature is an important driver at low latitudes, and the effective activation energy and SW flux are important at high latitudes.…”

Section: Resultsmentioning

confidence: 99%

“…The Gaussian distribution has the form of

F ((), U) = \frac{n}{U_{W} \sqrt{π}} exp ([], \frac{- {((), I - U_{0})}^{2}}{U_{W}^{2}})

where U 0 is the energy of the peak of the distribution and U W is the width about the peak. Over timescales much less than the diffusive timescale, a quasi‐steady state density of implanted H atoms can be obtained using the continuity equation:

\frac{dn}{dt} = \frac{(⟨⟩, D ((),, U, T) n)}{h^{2}} - \frac{n_{SW} v_{SW} cos ((), Z)}{h} \sim 0

where n SW and v SW are the solar wind density and velocity respectively and Z is the solar zenith angle (Farrell et al, ). In equation the diffusive flux is defined in terms of a mean value because of the use of the Gaussian distribution

⟨j⟩ = ⟨\frac{Dn}{h}⟩ = D_{0} h^{- 1} \int exp ((), - U / T) F ((), U) italicdU

.…”

Section: Methodsmentioning

confidence: 99%

“…Similar approaches have been used in the studies of hydrogen diffusion in silica glass (Devine, ; Shelby & Keeton, ). Using equation , Farrell et al () define an analytical expression of the implanted density as a function of solar zenith angle assuming dynamic equilibrium on timescales much less than a lunation:

italicnv \sim n_{SW} v_{SW} cos ((), Z)

where

v = D_{0} h^{- 1} exp ((), U_{0} / T) exp ((), - ((), U_{W}^{2}) / 4 T^{2})

is defined as the diffusive velocity, or in terms of a diffusive lifetime

τ_{2} = h / v = D^{- 1} h^{2} exp ((), U_{0} / T) exp ((), - ((), U_{W}^{2}) / 4 T^{2})

. A comparison of expressions of τ 2 and τ 1 or D eff and D shows that the diffusion rate is decreased by the factor of

exp ((), - ((), U_{W}^{2}) / 4 T^{2})

when considering a distribution of activation energies.…”

Section: Methodsmentioning

confidence: 99%

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Solar Wind Implantation Into the Lunar Regolith: Monte Carlo Simulations of H Retention in a Surface With Defects and the H₂ Exosphere

et al. 2019

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The solar wind implants protons into the top 20–30 nm of lunar regolith grains, and the implanted hydrogen will diffuse out of the regolith but also interact with oxygen in the regolith oxides. We apply a statistical approach to estimate the diffusion of hydrogen in the regolith hindered by forming temporary bonds with regolith oxygen atoms. A Monte Carlo simulation was used to track the temporal evolution of bound OH surface content and the H2 exosphere. The model results are consistent with the interpretation of the Chandrayaan‐1 M3 observations of infrared absorption spectra by surface hydroxyls as discussed in Li and Milliken (2017, https://doi.org/10.1126/sciadv.1701471). The model reproduced the latitudinal concentration of OH by using a Gaussian energy distribution of f(U0 = 0.5 eV, UW = 0.078–0.1 eV) to characterize the activation energy barrier to the diffusion of hydrogen in space weathered regolith. In addition, the model results of the exospheric content of H2 are consistent with observations by the Lyman Alpha Mapping Project on the Lunar Reconnaissance Orbiter. Therefore, we provide support for hydroxyl formation by chemically trapped solar wind protons.

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Section: Discussionmentioning

confidence: 70%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

“…The Gaussian distribution has the form of