Auroral hiss, electron beams and standing Alfvén wave currents near Saturn's moon Enceladus

On 16 October 2001, the Galileo spacecraft made a close flyby of Jupiter's moon Io. During the flyby the plasma wave instrument detected an electric field emission with spectral characteristics very similar to a type of whistler‐mode noise called “auroral hiss” that is commonly observed in Earth's auroral region. This paper gives a detailed analysis of the “auroral hiss” observed near Io. The frequency‐time spectrum of the emission has a sharp high‐frequency cutoff near the electron cyclotron frequency and a V‐shaped low‐frequency cutoff. On a frequency‐time spectrogram these cutoffs give the emission a characteristic funnel shape that is very similar to the spectrum of terrestrial auroral hiss. Strong magnetic field perturbations occurred near the vertex of the funnel indicating the presence of a field‐aligned current. To explain the origin of the emission, a brief review is given of whistler‐mode wave propagation and the unipolar inductor model of Io's interaction with the magnetosphere of Jupiter. Assuming propagation near the whistler‐mode resonance cone, ray‐tracing analyses show that the radiation originates from a source very close to the surface of Io. The source is located in the same region where field‐aligned currents are believed to originate in the ionosphere of Io. Since terrestrial auroral hiss is known to be produced by beams of low‐energy auroral electrons, these observations suggest that the auroral hiss at Io is generated by an electron beam that is part of the field‐aligned current system induced by the interaction of Io with the rapidly rotating magnetosphere of Jupiter.

Ultraviolet observation of Enceladus' plume in transit across Saturn, compared to Europa

The Jovian regular satellite system mainly consists of four Galilean satellites that have similar masses and are trapped in mutual mean motion resonances except for the outer satellite, Callisto. On the other hand, the Saturnian regular satellite system has only one big icy body, Titan, and a population of much smaller icy moons. We have investigated the origin of these major differences between the Jovian and Saturnian satellite systems by semi-analytically simulating the growth and orbital migration of proto-satellites in an accreting proto-satellite disk. We set up two different disk evolution/structure models that correspond to Jovian and Saturnian systems, by building upon previously developed models of an actively-supplied proto-satellite disk, the formation of gas giants, and observations of young stars. Our simulations extend previous models by including the (1) different termination timescales of gas infall onto the proto-satellite disk and (2) different evolution of a cavity in the disk, between the Jovian and Saturnian systems. We have performed Monte Carlo simulations and show that in the case of the Jovian systems, four to five similar-mass satellites are likely to remain trapped in mean motion resonances. This orbital configuration is formed by type I migration, temporal stopping of the migration near the disk inner edge, and quick truncation of gas infall caused by Jupiter opening a gap in the Solar nebula. The Saturnian systems tend to end up with one dominant body in the outer regions caused by the slower decay of gas infall associated with global depletion of the Solar nebula. The total mass and compositional zoning of the predicted Jovian and Saturnian satellite systems are consistent with the observed satellite systems.

The moons of giant planets can create a phenomenon called an “auroral footprint,” which is due to an interaction between the moon and the planet's magnetosphere. This phenomenon, which has been observed for Jupiter's moons Io, Europa, and Ganymede as well as Saturn's moon Enceladus, is usually seen as a single spot in the planet's aurora. However, Io has an auroral footprint that is known to be made up of at least three spots. Scientists wondered if a multispot auroral footprint is common or unique to Io.

The interplanetary space probe Cassini/Huygens reached Saturn in July 2004 after seven years of cruise phase. The Cosmic Dust Analyzer (CDA) measures the interplanetary, interstellar and planetary dust in our solar system since 1999 and provided unique discoveries. In 1999, CDA detected interstellar dust in the inner solar system followed by the detection of electrical charges of interplanetary dust grains during the cruise phase between Earth and Jupiter. The instrument determined the composition of interplanetary dust and the nanometre sized dust streams originating from Jupiter's moon Io. During the approach to Saturn in 2004, similar streams of submicron grains with speeds in the order of 100 km/s were detected from Saturn's inner and outer ring system and are released to the interplanetary magnetic field. Since 2004 CDA measured more than one million dust impacts characterizing the dust environment of Saturn. The instrument is one of three experiments which discovered the active ice geysers located at the south pole of Saturn's moon Enceladus in 2005. Later, a detailed compositional analysis of the water ice grains in Saturn's E ring system lead to the discovery of large reservoirs of liquid water (oceans) below the icy crust of Enceladus. Finally, the determination of the dust- magnetosphere interaction and the discovery of the extended E ring (at least twice as large as predicted) allowed the definition of a dynamical dust model of Saturn's E ring describing the observed properties. This paper summarizes the discoveries of a ten year story of success based on reliable measurements with the most advanced dust detector flown in space until today. This paper focuses on cruise results and findings achieved at Saturn with a focus on flux and density measurements.

Data obtained by the Cassini spacecraft show that the plume of ice particles at the south pole of Saturn's moon Enceladus is four times brighter when the moon is farthest away from the planet than when it is closest. See Letter p.182 Between 2005 and 2012 the Cassini spacecraft's Visual and Infrared Mapping Spectrometer (VIMS) obtained 252 images of the plume of water vapour and ice particles emitting from near the south pole of Saturn's moon Enceladus. These images have been analysed with a view to establishing the nature of the geological forces driving the plume. The authors show that as Enceladus moves along its elliptical orbit, the brightness of the plume peaks — and larger amounts of material join the plume — when the moon is furthest from Saturn. This is consistent with a model in which tidal forces have an important role in controlling plume activity, perhaps by changing the width of the conduits between the surface and various underground reservoirs supplying the fissures through which the plume emerges.

Satellite observations have revealed electrostatic solitary structures in the Earth's auroral region. These structures have positive electrostatic potentials and move along the ambient magnetic field. In this paper we performed one‐dimensional electrostatic particle simulations of electrostatic solitary waves (ESW) in plasma composed of three electron components: cold, hot, and beam electrons. First, the nearly monochromatic electrostatic acoustic waves are excited. When the amplitude of the electron acoustic (EA) waves is sufficiently large, part of hot and beam electrons are trapped by the electron acoustic waves. These waves coalesce each other during their nonlinear evolution, and at last the solitary structures with travel speed related to the beam velocity are formed at the quasi‐equilibrium stage. These structures have positive potential signatures, and they seem to be stable. Electron density cavities for cold electron component are always accompanied with these structures. In addition, the corresponding electric fields have a bipolar structure, which has also been observed in the Earth's auroral region recently. The conditions for existence of such solitary structures are investigated through our simulations, and the comparisons between our simulated results and satellite observations in the Earth's auroral regions are also discussed.

The icy moons of the outer Solar System display evidence of subsurface liquid water and, therefore, potential habitability for life. Flybys of Saturn's moon Enceladus by the Cassini spacecraft have provided measurements of material from plumes that suggest hydrothermal activity and the presence of organic matter. Jupiter's moon Europa may have similar plumes and is the target for the forthcoming Europa Clipper mission that carries a high mass resolution and high sensitivity mass spectrometer, called the MAss Spectrometer for Planetary EXploration (MASPEX), with the capability for providing detailed characterization of any organic materials encountered. We have performed a series of experiments using pyrolysis-gas chromatography-mass spectrometry to characterize the mass spectrometric fingerprints of microbial life. A range of extremophile Archaea and Bacteria have been analyzed and the laboratory data converted to MASPEX-type signals. Molecular characteristics of protein, carbohydrate, and lipid structures were detected, and the characteristic fragmentation patterns corresponding to these different biological structures were identified. Protein pyrolysis fragments included phenols, nitrogen heterocycles, and cyclic dipeptides. Oxygen heterocycles, such as furans, were detected from carbohydrates. Our data reveal how mass spectrometry on Europa Clipper can aid in the identification of the presence of life, by looking for characteristic bacterial fingerprints that are similar to those from simple Earthly organisms.

[1] The Cassini Ultraviolet Imaging Spectrograph (UVIS) observed an occultation of the Sun by the water vapor plume at the south polar region of Saturn's moon Enceladus. The Extreme Ultraviolet (EUV) spectrum is dominated by the spectral signature of H2O gas, with a nominal line-of-sight column density of 0.90 ± 0.23 × 1016 cm−2 (upper limit of 1.0 × 1016 cm−2). The upper limit for N2 is 5 × 1013 cm−2, or <0.5% in the plume; the lack of N2 has significant implications for models of the geochemistry in Enceladus' interior. The inferred rate of water vapor injection into Saturn's magnetosphere is ∼200 kg/s. The calculated values of H2O flux from three occultations observed by UVIS have a standard deviation of 30 kg/s (15%), providing no evidence for substantial short-term variability. Collimated gas jets are detected in the plume with Mach numbers of 5–8, implying vertical gas velocities that exceed 1000 m/sec. Observations at higher altitudes with the Cassini Ion Neutral Mass Spectrometer indicate correlated structure in the plume. Our results support the subsurface liquid model, with gas escaping and being accelerated through nozzle-like channels to the surface, and are consistent with recent particle composition results from the Cassini Cosmic Dust Analyzer.

The Cassini spacecraft has made 20 close flybys of the icy moon Enceladus between its arrival at Saturn in 2004 and 2012. Of those 20, strong whistler mode emissions (often called auroral hiss) were clearly observed on seven encounters. The propagation paths of these emissions are determined by the background magnetic field, which allows their source regions to be studied using simple ray‐tracing codes. In this paper, we trace the auroral hiss observations from Cassini's trajectories to possible source locations near Enceladus. We find that all of the detected emissions could be generated by field‐aligned electron beams in one of two regions around the moon: upstream of the Saturnward terminator and downstream of the anti‐Saturnward terminator. These results suggest that electron beam acceleration near the solid body is a quasi‐time‐stationary feature of the plasma interaction and that the auroral hiss generated by these beams may be used to remotely study plasma processes in regions separated from the spacecraft.

Sometime in the early 2030s, a washing machine-sized robot could be carefully descending toward the icy crust of Jupiter’s moon Europa. Armed with cameras, a spectrometer, a microscope, and a scoop, the vehicle would be lowered from a UFO-like sky crane similar to the one that delivered the Curiosity rover to Mars. As the robot nears the frozen ground, its autonomous navigation system may have to take evasive action. “Maybe the surface is nice and flat and smooth,” says Curt Niebur, a program scientist at the National Aeronautics and Space Administration (NASA) headquarters in Washington, DC. “But maybe it’s covered in penitentes, which are literally six-foot-tall ice spikes.” Saturn’s moon Enceladus, shown here via a mosaic of images collected by the Cassini spacecraft in 2005, is one of many moons with oceans that could harbor signs of life. Image courtesy of NASA/JPL/Space Science Institute. The probe’s mission is a familiar one: to find signs of life beyond Earth. But its target for this investigation, a moon’s ocean, has only recently gained popularity. Europa is thought to have a vast liquid water ocean beneath its frozen crust, a potentially perfect place to find extraterrestrial organisms. The Jovian moon is merely one of many similar locations. In recent decades, exploratory spacecraft have revealed that our solar system is chock full of icy ocean worlds. Along with Europa, there are Saturn’s moons, the geyser-spewing Enceladus, and the methane-filled Titan. Then there's Neptune’s cryovolcanic Triton and the distant dwarf planet Pluto, just to name a few. “You throw a stone, and you find another ocean world,” says planetary scientist Francis Nimmo of the University of California, Santa Cruz. “They’re all over the place.” The moons of Jupiter, Saturn, Uranus, and Neptune are built largely from frozen water, which becomes hard a rock at the …

Over the last three years, the Cassini spacecraft has been in a series of high inclination orbits, allowing investigation and measurements of Saturnian auroral phenomena. During this time, the Radio and Plasma Wave Science (RPWS) Investigation on Cassini detected low frequency whistler mode emissions propagating upward along the auroral field lines, much like terrestrial auroral hiss. Comparisons of RPWS data with Cassini Plasma Spectrometer (CAPS) plasma measurements during a high‐latitude pass on 17 October 2008, show that intense upward moving electron beams with energies of a few hundred eV were associated with auroral hiss emissions. In this paper we show that these beams produce large growth rates for whistler‐mode waves propagating along the resonance cone, similar to the generation of auroral hiss at Earth.

Since Cassini spacecraft images revealed plumes of water vapour and ice particles erupting from Saturn's moon Enceladus in 2006, the search for the water source has been on. Possibilities include liquid water under the ice shell and ice that is subject to heating. The chemical composition of the jets can give clues as to their source. The 9 October 2008 Cassini fly-by provides a data set ideal for the purpose — mass spectrometry with the best signal-to-noise ratio so far obtained, sufficient to allow the identification of chemicals present in trace amounts. Waite et al. use these data to reveal the presence of ammonia in the plume, strong evidence for the existence of at least some liquid water. The overall composition of the plume suggests that it arises from both a liquid reservoir (or ice derived from one) and from the degassing of ice containing volatile materials. Jets of water ice from surface fractures near the south pole of Saturn's icy moon Enceladus produce a plume of gas and particles. The source of the jets may be a liquid water region under the ice shell. Here, ammonia is reported to be present in the plume, providing strong evidence for the existence of at least some liquid water. Jets of water ice from surface fractures near the south pole1 of Saturn’s icy moon Enceladus produce a plume of gas and particles2,3,4,5. The source of the jets may be a liquid water region under the ice shell—as suggested most recently by the discovery of salts in E-ring particles derived from the plume6—or warm ice that is heated, causing dissociation of clathrate hydrates7. Here we report that ammonia is present in the plume, along with various organic compounds, deuterium and, very probably, 40Ar. The presence of ammonia provides strong evidence for the existence of at least some liquid water, given that temperatures in excess of 180 K have been measured near the fractures from which the jets emanate8. We conclude, from the overall composition of the material, that the plume derives from both a liquid reservoir (or from ice that in recent geological time has been in contact with such a reservoir) as well as from degassing, volatile-charged ice.

Waves ripple surface of Saturn's moon

Three satellites of Jupiter, seven satellites of Saturn, and five satellites of Uranus show spectroscopic evidence of H 2 O ice on their surfaces, although other details of their surfaces are highly diverse. The icy surfaces contain contaminants of unknown composition in varying degrees of concentration, resulting in coloration and large differences in albedo. In addition to H 2 O, Europa has frozen SO 2, and Ganymede has O2 in the surface; in both of these cases external causes are implicated in the deposition or formation of these trace components. Similarly O3 is found in the surface ices of Ganymede, Dione, and Rhea, probably induced by the effects of magnetospheric particles. Variations in ice exposure across the surfaces of the satellites are measured from the spectroscopic signatures. While H 2 O ice occurs on the surfaces of many satellites, the range of bulk densities of these bodies shows that its contribution to their overall compositions is highly variable from one object to another.KeywordsSpectral ReflectanceHubble Space TelescopeGalilean SatelliteLarge SatelliteOuter Solar SystemThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.