Lunar exospheric argon modeling

Grava, C.; Chaufray, Jean‐Yves; Retherford, Kurt D.; Gladstone, G. Randall; Greathouse, T. K.; Crider, D. H.; Hodges, R. R.; Bayless, Amanda J.; Cook, J. C.; Stern, S. A.

doi:10.1016/j.icarus.2014.09.029

Cited by 31 publications

(31 citation statements)

References 69 publications

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“…So, it should not be interpreted as a precise estimate of the true energy, especially given the aforementioned details that are all approximated into this single parameter. For example, Grava et al () used a different prefactor and an extra free parameter, so their fitted value of Q = 27 kJ mol −1 results in a curve with the sunrise peak around 275°, comparable to our Q = 30 kJ mol −1 . We focused on fitting the observed sunrise peak time at 270°, so find an effectively lower energy.…”

Section: Resultssupporting

confidence: 81%

“…The seasonal variation acts in the opposite way to the trend seen in 1973. Therefore, if there are seasonal cold traps that explain the LADEE argon data and they were active during the LACE measurements, then the loss rate required to match the drop measured by LACE needs to be significantly larger than had been anticipated by Grava et al (). It is also noteworthy that the later LACE measurements fall well below the minimum measured by LADEE at the same location.…”

Section: Resultsmentioning

confidence: 95%

“…Those that impact the surface may be neutralized and “recycled” back into the exosphere. Following Butler () and Grava et al (), such processes can be combined to give a single solar radiation destruction timescale, τ . The probability of loss during a flight of time t is then

P (t) = 1 - e^{- t / τ} .

…”

Section: Modelmentioning

confidence: 99%

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Evidence for a Localized Source of the Argon in the Lunar Exosphere

et al. 2017

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We perform the first tests of various proposed explanations for observed features of the Moon's argon exosphere, including models of the following: spatially varying surface interactions; a source that reflects the lunar near‐surface potassium distribution; and temporally varying cold trap areas. Measurements from the Lunar Atmosphere and Dust Environment Explorer (LADEE) and the Lunar Atmosphere Composition Experiment (LACE) are used to test whether these models can reproduce the data. The spatially varying surface interactions hypothesized in previous work cannot reproduce the persistent argon enhancement observed over the western maria. They also fail to match the observed local time of the near‐sunrise peak in argon density, which is the same for the highland and mare regions and is well reproduced by simple surface interactions with a ubiquitous desorption energy of 28 kJ mol−1. A localized source can explain the observations, with a trade‐off between an unexpectedly localized source or an unexpectedly brief lifetime of argon atoms in the exosphere. To match the observations, a point‐like source requires source and loss rates of ∼1.9 × 1021 atoms s−1. A more diffuse source, weighted by the near‐surface potassium, requires much higher rates of ∼1.1 × 1022 atoms s−1, corresponding to a mean lifetime of just 1.4 lunar days. We do not address the mechanism for producing a localized source, but demonstrate that this appears to be the only model that can reproduce the observations. Large, seasonally varying cold traps could explain the long‐term fluctuation in the global argon density observed by LADEE, but not that by LACE.

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Section: Resultssupporting

confidence: 81%

Section: Resultsmentioning

confidence: 95%

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Evidence for a Localized Source of the Argon in the Lunar Exosphere

et al. 2017

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“…LADEE also confirmed the presence of argon at the equator and the Lyman-Alpha Mapping Project (LAMP, Gladstone et al 2010) placed limits on Ar at the poles (Hodges and Mahaffy 2016;Grava et al 2015;). These in situ observations, when coupled with global data on the structure of the lunar exosphere including local time dependence and vertical scale heights, are essential for determining production rates and polar cold trapping efficiencies.…”

Section: The Moonmentioning

confidence: 84%

Imaging Plasma Density Structures in the Soft X-Rays Generated by Solar Wind Charge Exchange with Neutrals

et al. 2018

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Both heliophysics and planetary physics seek to understand the complex nature of the solar wind's interaction with solar system obstacles like Earth's magnetosphere, the ionospheres of Venus and Mars, and comets. Studies with this objective are frequently conducted with the help of single or multipoint in situ electromagnetic field and particle observations, guided by the predictions of both local and global numerical simulations, and placed in con- text by observations from far and extreme ultraviolet (FUV, EUV), hard X-ray, and energetic neutral atom imagers (ENA). Each proposed interaction mechanism (e.g., steady or transient magnetic reconnection, local or global magnetic reconnection, ion pick-up, or the KelvinHelmholtz instability) generates diagnostic plasma density structures. The significance of each mechanism to the overall interaction (as measured in terms of atmospheric/ionospheric loss at comets, Venus, and Mars or global magnetospheric/ionospheric convection at Earth) remains to be determined but can be evaluated on the basis of how often the density signatures that it generates are observed as a function of solar wind conditions. This paper reviews efforts to image the diagnostic plasma density structures in the soft (low energy, 0.1-2.0 keV) X-rays produced when high charge state solar wind ions exchange electrons with the exospheric neutrals surrounding solar system obstacles. The introduction notes that theory, local, and global simulations predict the characteristics of plasma boundaries such the bow shock and magnetopause (including location, density gradient, and motion) and regions such as the magnetosheath (including density and width) as a function of location, solar wind conditions, and the particular mechanism operating. In situ measurements confirm the existence of time-and spatial-dependent plasma density structures like the bow shock, magnetosheath, and magnetopause/ionopause at Venus, Mars, comets, and the Earth. However, in situ measurements rarely suffice to determine the global extent of these density structures or their global variation as a function of solar wind conditions, except in the form of empirical studies based on observations from many different times and solar wind conditions. Remote sensing observations provide global information about auroral ovals (FUV and hard X-ray), the terrestrial plasmasphere (EUV), and the terrestrial ring current (ENA). ENA instruments with low energy thresholds (∼ 1 keV) have recently been used to obtain important information concerning the magnetosheaths of Venus, Mars, and the Earth. Recent technological developments make these magnetosheaths valuable potential targets for high-cadence wide-field-of-view soft X-ray imagers.Section 2 describes proposed dayside interaction mechanisms, including reconnection, the Kelvin-Helmholtz instability, and other processes in greater detail with an emphasis on the plasma density structures that they generate. It focuses upon the questions that remain as yet unanswered, such as the significanc...

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“…Thus exospheric models that would examine the structure of the exosphere in the vicinity of cold traps should otherwise account for surface temperature in permanently shadowed regions. Historically, cold trapping over such regions has been treated probabilistically in such models by randomly removing a certain fraction of particles that impact within prescribed latitude bands (e.g., Butler, 1997;Grava et al, 2015).…”

Section: Model Approximations and Over-simplificationsmentioning

confidence: 99%