Cassini Observes the Active South Pole of Enceladus

Porco, C. C.; Helfenstein, P.; Thomas, P. C.; Ingersoll, A. P.; Wisdom, Jack; West, Robert A.; Neukum, G.; Denk, T.; Wagner, Roland; Roatsch, Thomas; Kieffer, S. W.; Turtle, E. P.; McEwen, A. S.; Johnson, T. V.; Rathbun, J. A.; Veverka, J.; Wilson, Daren; Perry, Jason; Spitale, J. N.; Brahic, A.; Burns, J. A.; DelGenio, Anthony D.; Dones, L.; Murray, C. D.; Squyres, S. W.

doi:10.1126/science.1123013

Cited by 1,039 publications

(1,059 citation statements)

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“…Measurements made by Voyagers 1 and 2 found that energetic plasma is ubiquitous within Saturn's magnetosphere [ Krimigis et al , 1982, 1983]. Later, early in the Cassini mission, it was found that Enceladus ejects plumes of water group molecules into Saturn's magnetosphere [e.g., Dougherty et al , 2006; Porco et al , 2006]. A small fraction of these are ionized into a plasma, and this can greatly influence the dynamics that drive the magnetosphere.…”

Section: Introductionmentioning

confidence: 99%

Internally driven large‐scale changes in the size of Saturn's magnetosphere

et al. 2015

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Saturn's magnetic field acts as an obstacle to solar wind flow, deflecting plasma around the planet and forming a cavity known as the magnetosphere. The magnetopause defines the boundary between the planetary and solar dominated regimes, and so is strongly influenced by the variable nature of pressure sources both outside and within. Following from Pilkington et al. (2014), crossings of the magnetopause are identified using 7 years of magnetic field and particle data from the Cassini spacecraft and providing unprecedented spatial coverage of the magnetopause boundary. These observations reveal a dynamical interaction where, in addition to the external influence of the solar wind dynamic pressure, internal drivers, and hot plasma dynamics in particular can take almost complete control of the system's dayside shape and size, essentially defying the solar wind conditions. The magnetopause can move by up to 10–15 planetary radii at constant solar wind dynamic pressure, corresponding to relatively “plasma‐loaded” or “plasma‐depleted” states, defined in terms of the internal suprathermal plasma pressure.

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Section: Introductionmentioning

confidence: 99%

Internally driven large‐scale changes in the size of Saturn's magnetosphere

et al. 2015

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“…The spatial extent can be explained by ∼200-km high plumes of water vapor on the anti-Jovian hemisphere near Europa's south pole (14), presumably originating from effusive or explosive cryovolcanism similar to the plumes on the Saturnian moon Enceladus (e.g., ref. 15). Such cryovolcanic plumes might be fed by subsurface liquid water environments and might thus open a new window to probe these potentially habitable environments (16).…”

mentioning

confidence: 99%

Orbital apocenter is not a sufficient condition for HST/STIS detection of Europa’s water vapor aurora

Roth

Retherford

Saur

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

113

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Significance Images of Europa’s UV aurora taken by the Hubble Space Telescope in December 2012 have revealed local hydrogen and oxygen emissions in intensity ratios that identify the source as electron impact excitation of water molecules. The existence of water vapor plumes as a source for the detected localized water vapor and the possible accessibility of subsurface liquid water reservoirs at these locations have important implications for the exploration of Europa’s potentially habitable environments. The observations reported here tested whether orbital position near the apocenter is an essential requirement for plume activity. Only an upper limit on the amount of water vapor was obtained. Orbital position is therefore not a sufficient condition for detecting plumes and they may be episodic events.

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“…Tidal heating is responsible for the volcanic activity on Io (Peale et al, 1979) and probably the geysers of Enceladus (Hansen et al, 2006;Porco et al, 2006;Hurford et al, 2007). Tidal theory has a long and established body of work; however, tidal processes remain poorly understood.…”

Section: Tidal Heatingmentioning

confidence: 99%

Tidal Venuses: Triggering a Climate Catastrophe via Tidal Heating

et al. 2013

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Traditionally, stellar radiation has been the only heat source considered capable of determining global climate on long timescales. Here, we show that terrestrial exoplanets orbiting low-mass stars may be tidally heated at highenough levels to induce a runaway greenhouse for a long-enough duration for all the hydrogen to escape. Without hydrogen, the planet no longer has water and cannot support life. We call these planets ''Tidal Venuses'' and the phenomenon a ''tidal greenhouse.'' Tidal effects also circularize the orbit, which decreases tidal heating. Hence, some planets may form with large eccentricity, with its accompanying large tidal heating, and lose their water, but eventually settle into nearly circular orbits (i.e., with negligible tidal heating) in the habitable zone (HZ). However, these planets are not habitable, as past tidal heating desiccated them, and hence should not be ranked highly for detailed follow-up observations aimed at detecting biosignatures. We simulated the evolution of hypothetical planetary systems in a quasi-continuous parameter distribution and found that we could constrain the history of the system by statistical arguments. Planets orbiting stars with masses < 0.3 M Sun may be in danger of desiccation via tidal heating. We have applied these concepts to Gl 667C c, a *4.5 M Earth planet orbiting a 0.3 M Sun star at 0.12 AU. We found that it probably did not lose its water via tidal heating, as orbital stability is unlikely for the high eccentricities required for the tidal greenhouse. As the inner edge of the HZ is defined by the onset of a runaway or moist greenhouse powered by radiation, our results represent a fundamental revision to the HZ for noncircular orbits. In the appendices we review (a) the moist and runaway greenhouses, (b) hydrogen escape, (c) stellar mass-radius and mass-luminosity relations, (d) terrestrial planet mass-radius relations, and (e) linear tidal theories.

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Cassini Observes the Active South Pole of Enceladus

Cited by 1,039 publications

References 31 publications

Internally driven large‐scale changes in the size of Saturn's magnetosphere

Internally driven large‐scale changes in the size of Saturn's magnetosphere

Orbital apocenter is not a sufficient condition for HST/STIS detection of Europa’s water vapor aurora

Tidal Venuses: Triggering a Climate Catastrophe via Tidal Heating

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