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O. J. Tucker

Publications and source records attributed to O. J. Tucker.

4 recordsLinked to original sources
arXiv2026

Solar Wind Proton Implantation and Hydrogen Diffusion: Model Comparisons to M3 Observations

The Moon Mineralogy Mapper (M3) observed a widespread 3-micron absorption feature on the lunar surface, commonly attributed to surficial OH and/or H2O. The feature varies with latitude, local time, and lunar phase, suggesting that at least part of the optically active reservoir is dynamic. We compare phase- and local-time-resolved M3 abundance estimates with a global solar wind proton implantation, hydrogen diffusion, and H2 exosphere model. The model tracks implanted H in the upper grain rim, thermally activated retention and loss, recombinative H2 release, and phase-driven source variations associated with the solar wind, magnetosheath, and magnetotail. We compute the surface abundance using a weighted activation-energy-bin representation, in which the activation-energy distribution is discretized into fixed probability-weighted bins rather than sampled stochastically by Monte Carlo particles. This implementation reduces particle sampling noise in low-flux regions. A refined grid of Gaussian effective activation-energy distributions shows that the low- and mid-latitude M3 trends are best matched by distributions centered near Ec = 0.520 eV with width Ew = 0.090 eV. The preferred case gives a combined low- and mid-latitude RMSE of approximately 27 ppm and a small mean bias of only a few ppm. These fitted parameters should be interpreted as an effective response of the M3-sensitive retained H/OH reservoir, not as a unique mineralogical activation energy. The model reproduces the dominant local-time and phase-dependent abundance structure, including the reduced retained abundance near local noon, while model-data differences near plasma-transition intervals and high-latitude behavior may reflect additional sampling, saturation, or surface-plasma processes.

astro-ph.EP↗
arXiv2021

The influence of upper boundary conditions on molecular kinetic atmospheric escape simulations

Molecular kinetic simulations are typically used to accurately describe the tenuous regions of the upper atmospheres on planetary bodies. These simulations track the motion of particles representing real atmospheric atoms and/or molecules subject to collisions, the object's gravity, and external influences. Because particles can end up in very large ballistic orbits, upper boundary conditions (UBC) are typically used to limit the domain size thereby reducing the time for the atmosphere to reach steady-state. In the absence of a clear altitude at which all molecules are removed, such as a Hill sphere, an often used condition is to choose an altitude at which collisions become infrequent so that particles on escape trajectories are removed. The remainder are then either specularly reflected back into the simulation domain or their ballistic trajectories are calculated analytically or explicitly tracked so they eventually re-enter the domain. Here we examine the effect of the choice of the UBC on the escape rate and the structure of the atmosphere near the nominal exobase in the convenient and frequently used 1D spherically symmetric approximation. Using Callisto as the example body, we show that the commonly used specular reflection UBC can lead to significant uncertainties when simulating a species with a lifetime comparable to or longer than a dynamical time scale, such as an overestimation of escape rates and an inflated exosphere. Therefore, although specular reflection is convenient, the molecular lifetimes and body's dynamical time scales need to be considered even when implementing the convenient 1D spherically symmetric simulations in order to accurately estimate the escape rate and the density and temperature structure in the transition regime.

astro-ph.EP↗
arXiv2013

Hybrid fluid/kinetic modeling of Pluto's escaping atmosphere

Predicting the rate of escape and thermal structure of Pluto's upper atmosphere in preparation for the New Horizons Spacecraft encounter in 2015 is important for planning and interpreting the expected measurements. Having a moderate Jeans parameter Pluto's atmosphere does not fit the classic definition of Jeans escape for light species escaping from the terrestrial planets, nor does it fit the hydrodynamic outflow from comets and certain exoplanets. It has been proposed for some time that Pluto lies in the region of slow hydrodynamic escape. Using a hybrid fluid/molecular-kinetic model, we previously demonstrated the typical implementation of this model fails to correctly describe the appropriate temperature structure for the upper atmosphere for solar minimum conditions. Here we use a time-dependent solver to allow us to extend those simulations to higher heating rates and we examine fluid models in which Jeans-like escape expressions are used for the upper boundary conditions. We compare these to hybrid simulations of the atmosphere under heating conditions roughly representative of solar minimum and mean conditions as these bracket conditions expected during the New Horizon encounter. Although we find escape rates comparable to those previously estimated by the slow hydrodynamic escape model, and roughly consistent with energy limited escape, our model produces a much more extended atmosphere with higher temperatures roughly consistent with recent observations of CO. Such an extended atmosphere will be affected by Charon and will affect Pluto's interaction with the solar wind at the New Horizon encounter. Since we have previously shown that such models can be scaled, these results have implications for modeling exoplanet atmospheres for which the energy limited escape approximation is often used.

astro-ph.EP↗