The discovery of a plume of water vapour and ice particles emerging from warm fractures ('tiger stripes') in Saturn's small, icy moon Enceladus raised the question of whether the plume emerges from a subsurface liquid source or from the decomposition of ice. Previous compositional analyses of particles injected by the plume into Saturn's diffuse E ring have already indicated the presence of liquid water, but the mechanisms driving the plume emission are still debated. Here we report an analysis of the composition of freshly ejected particles close to the sources. Salt-rich ice particles are found to dominate the total mass flux of ejected solids (more than 99 per cent) but they are depleted in the population escaping into Saturn's E ring. Ice grains containing organic compounds are found to be more abundant in dense parts of the plume. Whereas previous Cassini observations were compatible with a variety of plume formation mechanisms, these data eliminate or severely constrain non-liquid models and strongly imply that a salt-water reservoir with a large evaporating surface provides nearly all of the matter in the plume.

Saturn’s moon Enceladus is erupting a plume of gas and ice grains from its south pole. Linked directly to the moon’s subsurface global ocean, plume material travels through cracks in the icy crust and is ejected into space. The subsurface ocean is believed to be in contact with the rocky core, with ongoing hydrothermal activity present. The Cassini spacecraft’s Ion and Neutral Mass Spectrometer (INMS) detected volatile, gas phase, organic species in the plume and the Cosmic Dust Analyser (CDA) discovered high-mass, complex organic material in a small fraction of ice grains. Here, we present a broader compositional analysis of CDA mass spectra from organic-bearing ice grains. Through analogue experiments, we find spectral characteristics attributable to low-mass organic compounds in the Enceladean ice grains: nitrogen-bearing, oxygen-bearing, and aromatic. By comparison with INMS results, we identify low-mass amines [particularly (di)methylamine and/or ethylamine] and carbonyls (with acetic acid and/or acetaldehyde most suitable) as the best candidates for the N- and O-bearing compounds, respectively. Inferred organic concentrations in individual ice particles vary but may reach tens of mmol levels. The low-mass nitrogen- and oxygen-bearing compounds are dissolved in the ocean, evaporating efficiently at its surface and entering the ice grains via vapour adsorption. The potentially partially water soluble, low-mass aromatic compounds may alternatively enter ice grains via aerosolization. These amines, carbonyls, and aromatic compounds could be ideal precursors for mineral-catalysed Friedel–Crafts hydrothermal synthesis of biologically relevant organic compounds in the warm depths of Enceladus’ ocean.

Interstellar dust (ISD) is the condensed phase of the interstellar medium. In situ data from the Cosmic Dust Analyzer on board the Cassini spacecraft reveal that the Saturnian system is passed by ISD grains from our immediate interstellar neighborhood, the local interstellar cloud. We determine the mass distribution of 36 interstellar grains, their elemental composition, and a lower limit for the ISD flux at Saturn. Mass spectra and grain dynamics suggest the presence of magnesium-rich grains of silicate and oxide composition, partly with iron inclusions. Major rock-forming elements (magnesium, silicon, iron, and calcium) are present in cosmic abundances, with only small grain-to-grain variations, but sulfur and carbon are depleted. The ISD grains in the solar neighborhood appear to be homogenized, likely by repeated processing in the interstellar medium.

Interstellar dust (ISD) from the local interstellar medium (LISM) streams into the solar system from approximately the direction of the constellation Ophiuchus. Prior to the return of the NASA Stardust spacecraft (1) no recognizable samples of this interstellar dust were available for laboratory study. Thus, our understanding of the properties of contemporary ISD has been derived primarily from astronomical observations of the ISM, including optical properties of the ISD and remote spectroscopy of the gas composition (2-4), and from in situ measurements by the dust analyzers on the Cassini, Ulysses and Galileo spacecraft (5-7). The canonical picture of ISD is that it is dominated by ~0.2 µm diameter (8) amorphous silicate grains, with or without carbonaceous mantles. However, the inferred properties of the particles, including size distribution, density and composition are heavily model dependent.

Rationale:Detecting ice grains with impact ionization mass spectrometers in space provides information about the compositions of ice grains and their sources.Depending on the impact speeds of the ice grains onto the metal target of a mass spectrometer, ionization conditions can vary substantially, resulting in changes to the appearance of the resulting mass spectra. Methods:Here we accurately reproduce mass spectra of water ice grains, recorded with the Cosmic Dust Analyzer (CDA) on board the Cassini spacecraft at typical impact speeds ranging between 4 km/s to 21 km/s, with a laboratory analogue experiment. In this Laser-Induced Liquid Beam Ion Desorption (LILBID) approach, a μm-sized liquid water beam is irradiated with a pulsed infrared laser, desorbing charged analyte and solvent aggregates and isolated ions, which are subsequently analyzed in a time-of-flight (TOF) mass spectrometer. Results:We show that our analogue experiment can reproduce impact ionization mass spectra of ice grains obtained over a wide range of impact speeds, aiding the quantitative analyses of mass spectra from space. Conclusions:Spectra libraries created with the LILBID experiment will be a vital tool for inferring the composition of ice grains from mass spectra recorded by both past and future impact ionization mass spectrometers (e.g. the SUrface Dust Analyzer (SUDA) onboard NASA's Europa Clipper Mission or detectors on a future Enceladus Mission).

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Abstract-The Stardust Interstellar Preliminary Examination team analyzed thirteen Al foils from the NASA Stardust interstellar collector tray in order to locate candidate interstellar dust (ISD) grain impacts. Scanning electron microscope (SEM) images reveal that the foils possess abundant impact crater and crater-like features. Elemental analyses of the crater features, with Auger electron spectroscopy, SEM-based energy dispersive X-ray (EDX) spectroscopy, and scanning transmission electron microscope-based EDX spectroscopy, demonstrate that the majority are either the result of impacting debris fragments from the spacecraft solar panels, or intrinsic defects in the foil. The elemental analyses also reveal that four craters contain residues of a definite extraterrestrial origin, either as interplanetary dust particles or ISD particles. These four craters are designated level 2 interstellar candidates, based on the crater shapes indicative of hypervelocity impacts and the residue compositions inconsistent with spacecraft debris.