Recent missions have renewed interests in the atmosphere of intermediate size objects. A subset o... more Recent missions have renewed interests in the atmosphere of intermediate size objects. A subset of these objects with significant atmospheres may have considerable molecular escape occurring. We show that escape occurs in a kinetic manner and not as an organized outflow as previously modeled on Pluto and similar sized objects. The method introduced couples a fluid model of the lower atmosphere and a kinetic model of the upper atmosphere to determine escape rate and show the transition of atmospheric structure from dense to tenuous regimes.
Introduction: Infrared spectra of the Moon's surface obtained by the Moon Mineralogy Mapper (M 3)... more Introduction: Infrared spectra of the Moon's surface obtained by the Moon Mineralogy Mapper (M 3) provide evidence for a global OH veneer. M 3 observations indicate the surface concentration varies with latitude, time of day, surface composition and when shielded in the Earth's magnetotail [1]. Recently, we modeled M 3 surface densities presented in Li and Milliken (2017) [1] using a Monte Carlo simulation. The model linked the M 3 observations of hydrogen within the surface to LAMP observations of the H 2 exosphere. The model highlighted the effect hindered H diffusion due to the formation of metastable OH on the surface degassing rates [2]. Here we used this model including the effect of shielding by Earth's magnetosphere on both the H surface content and H 2 exosphere for qualitative comparisons to M 3 observations [3]. Background: Throughout most of the Moon's 29.5 day orbit its surface is exposed to the unperturbed solar wind which is primarily composed of protons with a density of n sw = 5 × 10 6 H + /m 3 at flow speeds of v sw = 400 km/s. However, for 6-8 days of the orbit when the Moon traverses Earth's magnetosheath and magnetotail its surface is partially shielded from the solar wind yet still exposed to the terrestrial plasma environment. ARTEMIS (Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun) measurements indicate the proton flux incident on the Moon's surface on average can be reduced by approximately an order of magnitude during this time period [4]. Li et al. (2018) [3] reported on an asymmetric distribution of the hydroxyl content in the M 3 that may be due to the shielding effect, shown in Figure 1. We note that M 3 does not distinguish between hydroxyl and water. However, our previous modeling efforts in Tucker et al. (2018) [2] support the absorption feature being due to surficial OH.
<p>The JUpiter ICy moons Explorer (JUICE) of the European Space Agency will... more <p>The JUpiter ICy moons Explorer (JUICE) of the European Space Agency will investigate Jupiter and its icy moons Europa, Ganymede, and Callisto, with the aim to better understand the origin and evolution of our Solar System and the emergence of habitable worlds around gas giants. The Particle Environment Package (PEP) on JUICE is designed to measure neutrals and ions and electrons at thermal, suprathermal, and radiation belt energies (eV to MeV). </p><p>In the vicinity of Callisto, PEP will characterize the plasma environment, the outer parts of Callisto's atmosphere and ionosphere and their interaction with Jupiter's dynamic magnetosphere. About 20 Callisto flybys with closest approaches between 200 km and 5000 km altitude are<br>planned over the course of the JUICE mission. In this presentation, we review the state of knowledge regarding Callisto's ambient environment and magnetospheric interaction with recent modeling efforts for Callisto's atmosphere and ionosphere to identify science opportunities for the PEP observations and to optimize scientific insight gained from the foreseen JUICE flybys. These considerations inform science operation planning of PEP and JUICE and they will guide future model development for the atmosphere and ionosphere of Callisto and their interactions with the plasma environment.</p>
A combined fluid/kinetic model is developed to calculate thermally driven escape of N 2 from Plut... more A combined fluid/kinetic model is developed to calculate thermally driven escape of N 2 from Pluto’s atmosphere for two solar heating conditions: no heating above 1450 km and solar minimum heating conditions. In the combined model, one-dimensional fluid equations are applied for the dense part of the atmosphere, while the exobase region is described by a kinetic model and calculated by the direct simulation Monte Carlo method. Fluid and kinetic parts of the model are iteratively solved in order to maintain constant total mass and energy fluxes through the simulation region. Although the atmosphere was found to be highly extended, with an exobase altitude at ∼6000 km at solar minimum, the outflow remained subsonic and the escape rate was within a factor of two of the Jeans rate for the exobase temperatures determined. This picture is drastically different from recent predictions obtained solely using a fluid model which, in itself, requires assumptions about atmospheric density, flow...
Introduction: Recently, the near-infrared observations of the OH veneer on the lunar surface by t... more Introduction: Recently, the near-infrared observations of the OH veneer on the lunar surface by the Moon Mineralogy Mapper (M) have been refined to constrain the OH content to 500 – 750 parts per million (ppm) [1]. The observations indicate diurnal variations in OH up to 200 ppm possibly linked to warmer surface temperatures at low latitude. We examine the M observations using a statistical mechanics approach to model the diffusion of implanted H in the lunar regolith [2, 3]. We present results from Monte Carlo simulations of the diffusion of implanted solar wind H atoms and the subsequently derived H and H2 exospheres. Hydrogen Retention: The hydrogen retention model is taken from Farrell et al. (2015, 2017) [2, 3]. The solar wind (SW) flow is composed of density, nsw = 5 × 10 H/m, with velocity vsw = 400 km/s. The source rate is defined with nswvswcos(Z) where Z is the solar zenith angle. Farrell et al. (2015) [2] demonstrated that the outgassing of hydrogen atoms implanted by the...
A formula is derived for the rate of thermal atmospheric escape, valid, and asymptotically exact,... more A formula is derived for the rate of thermal atmospheric escape, valid, and asymptotically exact, at low Knudsen number.
Predicting the rate of escape and thermal structure of Pluto's upper atmosphere in preparatio... more 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 conditi...
Molecular kinetic simulations are typically used to accurately describe the tenuous regions of th... more 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 choic...
Recent missions have renewed interests in the atmosphere of intermediate size objects. A subset o... more Recent missions have renewed interests in the atmosphere of intermediate size objects. A subset of these objects with significant atmospheres may have considerable molecular escape occurring. We show that escape occurs in a kinetic manner and not as an organized outflow as previously modeled on Pluto and similar sized objects. The method introduced couples a fluid model of the lower atmosphere and a kinetic model of the upper atmosphere to determine escape rate and show the transition of atmospheric structure from dense to tenuous regimes.
Introduction: Infrared spectra of the Moon's surface obtained by the Moon Mineralogy Mapper (M 3)... more Introduction: Infrared spectra of the Moon's surface obtained by the Moon Mineralogy Mapper (M 3) provide evidence for a global OH veneer. M 3 observations indicate the surface concentration varies with latitude, time of day, surface composition and when shielded in the Earth's magnetotail [1]. Recently, we modeled M 3 surface densities presented in Li and Milliken (2017) [1] using a Monte Carlo simulation. The model linked the M 3 observations of hydrogen within the surface to LAMP observations of the H 2 exosphere. The model highlighted the effect hindered H diffusion due to the formation of metastable OH on the surface degassing rates [2]. Here we used this model including the effect of shielding by Earth's magnetosphere on both the H surface content and H 2 exosphere for qualitative comparisons to M 3 observations [3]. Background: Throughout most of the Moon's 29.5 day orbit its surface is exposed to the unperturbed solar wind which is primarily composed of protons with a density of n sw = 5 × 10 6 H + /m 3 at flow speeds of v sw = 400 km/s. However, for 6-8 days of the orbit when the Moon traverses Earth's magnetosheath and magnetotail its surface is partially shielded from the solar wind yet still exposed to the terrestrial plasma environment. ARTEMIS (Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun) measurements indicate the proton flux incident on the Moon's surface on average can be reduced by approximately an order of magnitude during this time period [4]. Li et al. (2018) [3] reported on an asymmetric distribution of the hydroxyl content in the M 3 that may be due to the shielding effect, shown in Figure 1. We note that M 3 does not distinguish between hydroxyl and water. However, our previous modeling efforts in Tucker et al. (2018) [2] support the absorption feature being due to surficial OH.
<p>The JUpiter ICy moons Explorer (JUICE) of the European Space Agency will... more <p>The JUpiter ICy moons Explorer (JUICE) of the European Space Agency will investigate Jupiter and its icy moons Europa, Ganymede, and Callisto, with the aim to better understand the origin and evolution of our Solar System and the emergence of habitable worlds around gas giants. The Particle Environment Package (PEP) on JUICE is designed to measure neutrals and ions and electrons at thermal, suprathermal, and radiation belt energies (eV to MeV). </p><p>In the vicinity of Callisto, PEP will characterize the plasma environment, the outer parts of Callisto's atmosphere and ionosphere and their interaction with Jupiter's dynamic magnetosphere. About 20 Callisto flybys with closest approaches between 200 km and 5000 km altitude are<br>planned over the course of the JUICE mission. In this presentation, we review the state of knowledge regarding Callisto's ambient environment and magnetospheric interaction with recent modeling efforts for Callisto's atmosphere and ionosphere to identify science opportunities for the PEP observations and to optimize scientific insight gained from the foreseen JUICE flybys. These considerations inform science operation planning of PEP and JUICE and they will guide future model development for the atmosphere and ionosphere of Callisto and their interactions with the plasma environment.</p>
A combined fluid/kinetic model is developed to calculate thermally driven escape of N 2 from Plut... more A combined fluid/kinetic model is developed to calculate thermally driven escape of N 2 from Pluto’s atmosphere for two solar heating conditions: no heating above 1450 km and solar minimum heating conditions. In the combined model, one-dimensional fluid equations are applied for the dense part of the atmosphere, while the exobase region is described by a kinetic model and calculated by the direct simulation Monte Carlo method. Fluid and kinetic parts of the model are iteratively solved in order to maintain constant total mass and energy fluxes through the simulation region. Although the atmosphere was found to be highly extended, with an exobase altitude at ∼6000 km at solar minimum, the outflow remained subsonic and the escape rate was within a factor of two of the Jeans rate for the exobase temperatures determined. This picture is drastically different from recent predictions obtained solely using a fluid model which, in itself, requires assumptions about atmospheric density, flow...
Introduction: Recently, the near-infrared observations of the OH veneer on the lunar surface by t... more Introduction: Recently, the near-infrared observations of the OH veneer on the lunar surface by the Moon Mineralogy Mapper (M) have been refined to constrain the OH content to 500 – 750 parts per million (ppm) [1]. The observations indicate diurnal variations in OH up to 200 ppm possibly linked to warmer surface temperatures at low latitude. We examine the M observations using a statistical mechanics approach to model the diffusion of implanted H in the lunar regolith [2, 3]. We present results from Monte Carlo simulations of the diffusion of implanted solar wind H atoms and the subsequently derived H and H2 exospheres. Hydrogen Retention: The hydrogen retention model is taken from Farrell et al. (2015, 2017) [2, 3]. The solar wind (SW) flow is composed of density, nsw = 5 × 10 H/m, with velocity vsw = 400 km/s. The source rate is defined with nswvswcos(Z) where Z is the solar zenith angle. Farrell et al. (2015) [2] demonstrated that the outgassing of hydrogen atoms implanted by the...
A formula is derived for the rate of thermal atmospheric escape, valid, and asymptotically exact,... more A formula is derived for the rate of thermal atmospheric escape, valid, and asymptotically exact, at low Knudsen number.
Predicting the rate of escape and thermal structure of Pluto's upper atmosphere in preparatio... more 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 conditi...
Molecular kinetic simulations are typically used to accurately describe the tenuous regions of th... more 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 choic...
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Papers by OJ Tucker