Europa's surface composition and sputter‐produced ionosphere

Johnson, R. E.; Killen, R. M.; Waite, J. H.; Lewis, William S.

doi:10.1029/98gl02565

Cited by 62 publications

(50 citation statements)

References 41 publications

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“…One may be tempted to conclude that the impact of energetic ions from the magnetosphere of Jupiter is needed to sputter water and other molecules from the surface to create a neutral atmosphere [cf. Cooper et al, 2001;Johnson and Leblanc, 2001;Johnson et al, 1998] to be accompanied by photoionization to create an ionosphere. However, we feel that it is dangerous to speculate much beyond the facts, namely that detectable ionospheres were only observed when the ramside was sunlit.…”

Section: Discussionmentioning

confidence: 99%

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Ionosphere of Callisto from Galileo radio occultation observations

et al. 2002

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[1] An ionosphere has been detected at Callisto by the Galileo spacecraft, using the radio occultation technique. There were four usable occultations by Callisto, providing eight observation opportunities, all equatorial and near the terminator (entry and exit observations). Detectable electron densities were obtained from six of the eight opportunities. It was found that a detectable ionosphere was only present at the observed location when the trailing hemisphere of Callisto, which is the one that is impacted by the corotating plasma of Jupiter's magnetosphere, was illuminated by the Sun. Two of these observations yielded well-defined electron density profiles, having peak densities of 15,300 and 17,400 cm À3 at altitudes of 27.2 and 47.6 km and topside plasma scale heights of 29.6 and 49.0 km. Four different methods, based on both photoionization and electron impact ionization, were used to obtain estimates of the corresponding neutral densities at the surface. The various assumptions inherent in these methods required using a variety of parameters, (cross sections, rate constants, etc.) all with their associated uncertainties. It was rather surprising and reassuring to find that all of the methods used to estimate the surface neutral density gave very similar results in each of the eight cases.The estimated values fall between 1 and 3 Â 10 10 cm À3 , leading to an estimate for the column density of from 3 to 4 Â 10 16 cm À2 .

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Section: Discussionmentioning

confidence: 99%

“…First, by analogy with Europa, it is reasonable to assume that the neutral atmosphere consists primarily of O 2 formed from dissociation of H 2 O sputtered from Callisto's surface [Hall et al, 1995;Johnson et al, 1998]. …”

Section: Discussionmentioning

confidence: 99%

Ionosphere of Callisto from Galileo radio occultation observations

et al. 2002

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“…Graphitecompounds could be created through radiation bombardment and surface chemistry [Lifshitz et al, 1990;McCord et al, 1998], which could produce C − 2 . Sputtering experiments indeed predict the efficient production of the C2H − x anions from hydrocarbon compounds [Johnson et al, 1992], and these are suggested as candidate sputter products at Europa [Johnson et al, 1998]. Carbon chain anions have also possibly been observed at Comet Halley [Cordiner & Charnley, 2014] and exist elsewhere in Saturn's magnetosphere, amongst carbon-rich compounds in Titan's ionosphere [Desai et al, 2017a].…”

Section: Origin Of the Negative Ionsmentioning

confidence: 99%

Cassini CAPS Identification of Pickup Ion Compositions at Rhea

Desai

Taylor

Regoli

et al. 2018

Geophysical Research Letters

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Saturn's largest icy moon, Rhea, hosts a tenuous surface‐sputtered exosphere composed primarily of molecular oxygen and carbon dioxide. In this Letter, we examine Cassini Plasma Spectrometer velocity space distributions near Rhea and confirm that Cassini detected nongyrotropic fluxes of outflowing CO2+ during both the R1 and R1.5 encounters. Accounting for this nongyrotropy, we show that these possess comparable along‐track densities of ∼2 × 10−3 cm−3. Negatively charged pickup ions, also detected during R1, are surprisingly shown as consistent with mass 26 ± 3 u which we suggest are carbon‐based compounds, such as CN−, C2H−, C2−, or HCO−, sputtered from carbonaceous material on the moon's surface. The negative ions are calculated to possess along‐track densities of ∼5 × 10−4 cm−3 and are suggested to derive from exogenic compounds, a finding consistent with the existence of Rhea's dynamic CO2 exosphere and surprisingly low O2 sputtering yields. These pickup ions provide important context for understanding the exospheric and surface ice composition of Rhea and of other icy moons which exhibit similar characteristics.

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Section: Introductionmentioning

confidence: 99%

“…This is important as radiolysis is a dominant surface alteration process on outer solar system bodies including those in the Kuiper belt and the Oort cloud. Having an accurate model can also be helpful for predictions, since Europa's atmosphere is expected to contain other interesting, possibly organic, molecules sputtered from the surface (Johnson et al, 1998).The plasma interaction with the surface is the principal source of 02, and the plasma interaction with the atmosphere is a principal loss process, and therefore a large atmosphere does not accumulate ( Johnson et al, 1982). Ip (1996) modeled the atmosphere using the sputtering rates estimated by Shi et a1.…”

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confidence: 99%