title: Ongoing hydrothermal activities within Enceladus.
abstract: Detection of sodium-salt-rich ice grains emitted from the plume of the Saturnian moon Enceladus suggests that the grains formed as frozen droplets from a liquid water reservoir that is, or has been, in contact with rock. Gravitational field measurements suggest a regional south polar subsurface ocean of about 10 kilometres thickness located beneath an ice crust 30 to 40 kilometres thick. These findings imply rock-water interactions in regions surrounding the core of Enceladus. The resulting chemical 'footprints' are expected to be preserved in the liquid and subsequently transported upwards to the near-surface plume sources, where they eventually would be ejected and could be measured by a spacecraft. Here we report an analysis of silicon-rich, nanometre-sized dust particles (so-called stream particles) that stand out from the water-ice-dominated objects characteristic of Saturn. We interpret these grains as nanometre-sized SiO2 (silica) particles, initially embedded in icy grains emitted from Enceladus' subsurface waters and released by sputter erosion in Saturn's E ring. The composition and the limited size range (2 to 8 nanometres in radius) of stream particles indicate ongoing high-temperature (>90 °C) hydrothermal reactions associated with global-scale geothermal activity that quickly transports hydrothermal products from the ocean floor at a depth of at least 40 kilometres up to the plume of Enceladus.
authors.affiliation: Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado 80303, USA.
authors.forename: Hsiang-Wen
authors.initials: HW
authors.lastname: Hsu
authors.affiliation: 1] Institut für Geowissenschaften, Universität Heidelberg, 69120 Heidelberg, Germany [2] Institut für Raumfahrtsysteme, Universität Stuttgart, 70569 Stuttgart, Germany.
authors.forename: Frank
authors.initials: F
authors.lastname: Postberg
authors.affiliation: Department of Complexity Science and Engineering, University of Tokyo, Kashiwa 277-8561, Japan.
authors.forename: Yasuhito
authors.initials: Y
authors.lastname: Sekine
authors.affiliation: Laboratory of Ocean-Earth Life Evolution Research, JAMSTEC, Yokosuka 237-0061, Japan.
authors.forename: Takazo
authors.initials: T
authors.lastname: Shibuya
authors.affiliation: Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado 80303, USA.
authors.forename: Sascha
authors.initials: S
authors.lastname: Kempf
authors.affiliation: Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado 80303, USA.
authors.forename: Mihály
authors.initials: M
authors.lastname: Horányi
authors.affiliation: 1] Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado 80303, USA [2] Institute for Particle and Nuclear Physics, Wigner RCP, 1121 Budapest, Hungary.
authors.forename: Antal
authors.initials: A
authors.lastname: Juhász
authors.affiliation: European Space Agency, ESAC, E-28691 Madrid, Spain.
authors.forename: Nicolas
authors.initials: N
authors.lastname: Altobelli
authors.affiliation: Research and Development Center for Submarine Resources, JAMSTEC, Yokosuka 237-0061, Japan.
authors.forename: Katsuhiko
authors.initials: K
authors.lastname: Suzuki
authors.affiliation: Research and Development Center for Submarine Resources, JAMSTEC, Yokosuka 237-0061, Japan.
authors.forename: Yuka
authors.initials: Y
authors.lastname: Masaki
authors.affiliation: Graduate School of Environmental Studies, Tohoku University, Sendai 980-8579, Japan.
authors.forename: Tatsu
authors.initials: T
authors.lastname: Kuwatani
authors.affiliation: Department of Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan.
authors.forename: Shogo
authors.initials: S
authors.lastname: Tachibana
authors.affiliation: Graduate School of Environmental Sciences, Nagoya University, Nagoya 464-8601, Japan.
authors.forename: Sin-iti
authors.initials: SI
authors.lastname: Sirono
authors.affiliation: Institut für Raumfahrtsysteme, Universität Stuttgart, 70569 Stuttgart, Germany.
authors.forename: Georg
authors.initials: G
authors.lastname: Moragas-Klostermeyer
authors.affiliation: Institut für Raumfahrtsysteme, Universität Stuttgart, 70569 Stuttgart, Germany.
authors.forename: Ralf
authors.initials: R
authors.lastname: Srama