The existence of a dense lunar ionosphere has been controversial for decades. Positive ions produced from the lunar surface and exosphere are inferred to have densities that are less than or similar to 10(6) - 10(7) m(-3) near the surface and smaller at higher altitudes, yet electron densities derived from radio occultation measurements occasionally exceed these values by orders of magnitude. For example, about 4% of the single-spacecraft radio occultation measurements from Kaguya/SELENE were consistent with peak electron densities of similar to 3 x 10(8) m(-3). Space plasmas should be neutral on macroscopic scales, so this represents a substantial discrepancy. Aditional observations of electron densities in the lunar ionosphere are critical to resolving this longstanding paradox. Here we theoretically assess whether radio occultation observations using two-way coherent S-band radio signals from the Lunar Reconnaissance Orbiter (LRO) spacecraft could provide useful measurements of electron densities in the lunar ionosphere. We predict the uncertainty in a single LRO radio occultation measurement of electron density to be similar to 3 x 10(8) m(-3), comparable to occasional observations by Kaguya/SELENE of a dense lunar ionosphere. Thus an individual profile from LRO is unlikely to reliably detect the lunar ionosphere; however, averages of multiple (similar to 10) LRO profiles acquired under similar geophysical and viewing conditions should be able to make reliable detections. An observing rate of six ingress occultations per day (similar to 2000 per year) could be achieved with minimal impact on current LRO operations. This rate compares favorably with the 378 observations reported from the single-spacecraft experiment on Kaguya/SELENE between November 2007 and June 2009. The large number of observations possible for LRO would be sufficient to permit wide-ranging investigations of spatial and temporal variations in the poorly understood lunar ionosphere. These findings strengthen efforts to conduct such observations with LRO. (C) 2021 COSPAR. Published by Elsevier B.V. All rights reserved.

Ganymede's surface is subject to constant bombardment by Jovian magnetospheric and Ganymede's ionospheric ions. These populations sputter the surface and contribute to the replenishment of the moon's exosphere. Thus far, estimates for sputtering on the moon's surface have included only the contribution from Jovian ions. In this work, we have used our recent model of Ganymede's ionosphere Carnielli et al., 2019 to evaluate the contribution of ionospheric ions for the first time. In addition, we have made new estimates for the contribution from Jovian ions, including both thermal and energetic components. For Jovian ions, we find a total sputtering rate of 2.2 x 10(27) s(-1), typically an order of magnitude higher compared to previous estimates. For ionospheric ions, produced through photo- and electron-impact ionization, we find values in the range 2.7 x 10(26)-5.2 x 10(27) s(-1) when the moon is located above the Jovian plasma sheet. Hence, Ganymede's ionospheric ions provide a contribution of at least 10% to the sputtering rate, and under certain conditions they dominate the process. This finding indicates that the ionospheric population is an important source to consider in the context of exospheric models.

The origin of the Moon's ionosphere has been explored using Chandrayaan-1 radio occultation (RO) measurements and a photochemical model. The electron density near the Moon's surface, obtained on 31 July 2009 (approximate to 300cm(-3)), is compared with results from a model which includes production and recombination of 16 ions, solar wind proton charge exchange, and the electron impact ionization. The model calculations suggest that in the absence of transport, inert ions, namely Ar+, Ne+, and He+, dominate lunar ionosphere (density approximate to 5 x 10(4)cm(-3)). Interaction with solar wind, however, leads to their complete removal (approximate to 2-3cm(-3)). Assuming the Moon's exosphere to have CO2, H2O, O, OH, H-2, CH4, and CO molecules in addition to the inert gases, the model calculations suggest that the lunar ionosphere is dominated by molecular ions, namely H2O+, CO , and H3O+, with near-surface density approximate to 250cm(-3). We surmise that lunar ionosphere can be molecular in nature.

Low energy secondary ions ejected by the solar wind are an important component of tenuous exospheres surrounding airless bodies, since these ions carry information on the planetary surface composition. In this work we examine the dependence of secondary-ion abundance, as a function of energy and mass, on surface composition. The surface compositions of two Apollo soils (10084 and 62231) and a synthetic Corning glass lunar simulant were measured with X-ray photoelectron spectroscopy and correlated with the spectra of secondary-ions ejected from the same soils by 4 key He ions. XPS spectra for lunar soils show that the surface compositions are similar to the bulk, but enriched in Fe and 0, while depleted in Mg and Ca. 4 keV He irradiation on the lunar soils and a glass simulant preferentially removes 0 and Si, enriching the surface in Al, Ti, Mg, and Ca. Secondary-ion species ejected from the Apollo soils by 4 keV He include: Na+, Mg+, Al+, Si+, Ca+, Ca++, Ti+, Fe+, and molecular species: NaO+, MgO+ and SiO+. Secondary ion energy distributions for lunar soil 10084 and 62231 rise rapidly, reach a maxima at similar to 5 eV for molecular ions and Na+, similar to 7.5 eV for Fe+, and similar to 10 eV for Mg+, Al+, Si+, Ca+ and Ti+, then decrease slowly with energy. We present species-dependent relative conversion factors for the derivation of atomic surface composition from secondary-ion count rates for 4 keV He irradiation of lunar soils 10084 and 62231, as well as the Corning glass lunar simulant. (c) 2014 Elsevier Inc. All rights reserved.

With a Saturnian magnetopause average stand-off distance of about 21 planetary radii, Titan spends most of its time inside the rotating magnetosphere of its parent planet. However, when Saturn's magnetosphere is compressed due to high solar wind dynamic pressure, Titan can cross Saturn's magnetopause in the subsolar region of its orbit and therefore to interact with the shocked solar wind plasma in Saturn's magnetosheath. This situation has been observed during the T32 flyby of the Cassini spacecraft on 13 June 2007. Until a few minutes before closest approach, Titan had been located inside the Saturnian magnetosphere. During the flyby, Titan encountered a sudden change in the direction and magnitude of the ambient magnetic field. The density of the ambient plasma also increased dramatically during the pass. Thus, the moon's exosphere and ionosphere were exposed to a sudden change in the upstream plasma conditions. The resulting reconfiguration of Titan's plasma tail has been studied in real-time by using a three-dimensional, multi-species hybrid simulation model. The hybrid approximation treats the electrons of the plasma as a massless, charge-neutralizing fluid, while ion dynamics are described by a kinetic approach. In the simulations, the magnetopause crossing is modeled by a sudden change of the upstream magnetic field vector as well as a modification of the upstream plasma composition. We present real-time simulation results, illustrating how Titan's induced magnetotail is reconfigured due to magnetic reconnection. The simulations allow to determine a characteristic time scale for the erosion of the original magnetic draping pattern that commences after Titan has crossed Saturn's magnetopause. Besides, the influence of the plasma composition in the magnetosheath on the reconfiguration process is discussed in detail. The question of whether the magnetopause crossing is likely to yield a detachment of Titan's exospheric tail from the satellite is investigated as well.