The Enceladus and OH Tori at Saturn

Johnson, R. E.; Smith, H. T.; Tucker, O. J.; Liu, M.; Burger, M. H.; Sittler, E. C.; Tokar, R. L.

doi:10.1086/505750

Cited by 119 publications

(151 citation statements)

References 17 publications

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“…According to azimuthally symmetric models, the density at 4.5 R S is less than the density at Enceladus' orbit [Johnson et al, 2006]. Simulations using the azimuthally dependent Smith model concur and also show that the density at 4.0 R S at the equatorial plane varies little between 120°and 180°.…”

Section: Neutral Water Near Enceladus and Azimuthal Distributionmentioning

confidence: 77%

“…The OH observations with HST are sparse, and building sufficient statistics requires summing data from several observation periods. The initial models of OH and its parent, H 2 O, identified a radial and vertical (normal to the equatorial plane) structure to the neutrals but assumed azimuthal symmetry [Richardson et al, 1998;Jurac et al, 2001;Johnson et al, 2006].…”

Section: Introductionmentioning

confidence: 99%

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Cassini INMS observations of neutral molecules in Saturn's E‐ring

et al. 2010

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[1] In 2008, the Cassini ion neutral mass spectrometer (INMS) investigation made in situ measurements of neutral species near Saturn's equatorial plane within 0.5 Saturn radii (R S ) of the orbit of Enceladus. After removing the large background and modeling to interpret instrumental effects, the data provide rough constraints on the neutral distribution and composition. These data show an azimuthal asymmetry in the neutral densities and provide measurements used to compare to simulations of neutral H 2 O emitted from Enceladus. Far from Enceladus, the neutral water densities, at a few times 10 3 molecules/cm 3 , are near the detection limit of INMS. Near Enceladus, but outside of the plumes and north of the equatorial plane, the INMS detects particles within 5000 km of Enceladus, with the density increasing to approximately 10 5 molecules/cm 3 at the equatorial plane. The observations also show CO 2 in the form of its dissociated product, CO. On the basis of the spatial distribution of CO 2 counts, the scale height of the neutral cloud above and below the equatorial plane is less than 7000 km. Far from Enceladus, the concentration of CO 2 with respect to H 2 O increases, a consequence of the predicted decline in H 2 O density. Relatively high counts at 2 amu are infrequently observed. These measurements indicate H 2 released during ice-grain impacts and provide a constraint on the frequency and distribution of small ice grains.

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Section: Neutral Water Near Enceladus and Azimuthal Distributionmentioning

confidence: 77%

Section: Introductionmentioning

confidence: 99%

Cassini INMS observations of neutral molecules in Saturn's E‐ring

et al. 2010

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“…However, the observed OH distribution (Melin et al 2009) requires acceleration and spreading, for which processes identified early on (Jurac & Richardson 2005;Johnson et al 2006) include the release of kinetic energy under photodissociation and collisions with rapid torus ions. Momentum transfer associated with H 2 O-H 2 O collisions ("viscous heating") was first investigated in a fluid model by Farmer (2009) -who actually predicted the observability of the torus with Herschel, but in emission.…”

Section: Excitation and Torus Modelsmentioning

confidence: 99%

Direct detection of the Enceladus water torus withHerschel

Hartogh

Lellouch

Moreno

et al. 2011

A&A

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Cryovolcanic activity near the south pole of Saturn's moon Enceladus produces plumes of H 2 O-dominated gases and ice particles, which escape and populate a torus-shaped cloud. Using submillimeter spectroscopy with Herschel, we report the direct detection of the Enceladus water vapor torus in four rotational lines of water at 557, 987, 1113, and 1670 GHz, and probe its physical conditions and structure. We determine line-of-sight H 2 O column densities of ∼4 × 10 13 cm −2 near the equatorial plane, with a ∼50 000 km vertical scale height. The water torus appears to be rotationally cold (e.g. an excitation temperature of 16 K is measured for the 1113 GHz line) but dynamically excited, with non-Keplerian dispersion velocities of ∼2 km s −1 , and appears to be largely shaped by molecular collisions. From estimates of the influx rates of torus material into Saturn and Titan, we infer that Enceladus' activity is likely to be the ultimate source of water in the upper atmosphere of Saturn, but not in Titan's.

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“…The discrepancy between the observed and calculated densities increases with azimuthal separation and radial distance from Enceladus. A more detailed analysis of calculated densities that includes charge exchange by Johnson et al [2006] and neutral/neutral collisions by Cassidy and Johnson [2010] is required to explain the azimuthal structure.…”

Section: Discussionmentioning

confidence: 99%

“…[6] Several spreading processes have been proposed to explain the broader O and OH distributions reported by Melin et al [2009]: charge exchange, neutral/neutral collisions, and molecular dissociation. For charge exchange, Johnson et al [2006] made a calculation by assuming that the broader OH distribution observed by HST [Richardson et al, 1998] is populated by charge exchange from a narrow H 2 O Enceladus torus due to the plumes. Farmer [2009] and Cassidy and Johnson [2010] suggested that neutral/neutral collisions contribute to the spreading of the O and OH distributions.…”

Section: Introductionmentioning

confidence: 99%

Effect of photo‐dissociation on the spreading of OH and O clouds in Saturn's inner magnetosphere

et al. 2012

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[1] We examine the contribution of photo-dissociation under quiet solar conditions to the global OH and O distributions in Saturn's inner magnetosphere by performing a Monte Carlo simulation. We first calculate the H 2 O distribution generated by H 2 O sources, namely Enceladus' cryo-volcanic plumes, satellite sputtering, and E ring sputtering. We calculate the OH distribution through photo-dissociation reactions using the calculated H 2 O distribution and then calculate the O distribution from the obtained H 2 O and OH distributions. We quantitatively evaluate the role of the energy increment of produced OH and O particles due to photo-dissociation by comparing the resultant distribution of OH and O particles with and without the energy increment. To quantitatively examine the effect of photo-dissociation on the spreading of OH and O clouds, we use the H 2 O model including charge exchange and neutral/neutral collisions based on Cassidy and Johnson (2010), as the initial distribution. For the OH (O) density in the region outside 5 Rs (6 Rs), the density with energy increment is greater than that without energy increment. The contribution of calculated OH density with energy increment to the observation is more than $10%. The OH ratio outside 6 Rs decreases with radial distance from Saturn. On the other hand, the contribution of calculated O density with energy increment to the observation is less than 10% except for around Enceladus.

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The Enceladus and OH Tori at Saturn

Cited by 119 publications

References 17 publications

Cassini INMS observations of neutral molecules in Saturn's E‐ring

Cassini INMS observations of neutral molecules in Saturn's E‐ring

Direct detection of the Enceladus water torus withHerschel

Effect of photo‐dissociation on the spreading of OH and O clouds in Saturn's inner magnetosphere

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