The Interaction of the Atmosphere of Enceladus with Saturn's Plasma

Sampling Accelerated Micron Scale Ice Particles with a Quadrupole Ion Trap Mass Spectrometer

The Enceladus plume is a target of astrobiological interest in planetary science since it may carry signs of extraterrestrial life entrapped in ice grains formed from the subsurface ocean of this moon of Saturn. Fly-by mission concepts have been proposed to perform close investigations of the plume, including detailed in situ measurements of chemical composition with a new generation of mass spectrometer instrumentation. Such a scenario involves high-velocity collisions (typically around 5 km/s or higher) of the instrument with the encountered ice grains. Postimpact processes may include molecular fragmentation, impact ionization, and various subsequent chemical reactions that could alter the original material prior to analysis. In order to simulate Enceladus plume fly through conditions, we are developing an ice grain accelerator and have coupled it to the quadrupole ion trap mass spectrometer (QITMS) developed for flight applications. Our experimental setup enables the creation and acceleration of ice particles with well-defined size, charge, and velocity, which are subsequently directed into the QITMS, where they impact the surface of the mass analyzer and the analysis of postimpact, volatilized molecules takes place. In this work, we performed mass spectral analysis of ice grains of ca. 1.3 μm in diameter, accelerated and impacted at velocities up to 1000 m/s, with an upgrade of the accelerator in progress that will enable velocities up to 5000 m/s. We report the first observations of ice grain impacts measured by the QITMS, which were recorded as brief increases in the abundance of water molecules detected within the instrument.

Enceladus Plume Structure and Time Variability: Comparison of Cassini Observations

During three low-altitude (99, 66, 66 km) flybys through the Enceladus plume in 2010 and 2011, Cassini's ion neutral mass spectrometer (INMS) made its first high spatial resolution measurements of the plume's gas density and distribution, detecting in situ the individual gas jets within the broad plume. Since those flybys, more detailed Imaging Science Subsystem (ISS) imaging observations of the plume's icy component have been reported, which constrain the locations and orientations of the numerous gas/grain jets. In the present study, we used these ISS imaging results, together with ultraviolet imaging spectrograph stellar and solar occultation measurements and modeling of the three-dimensional structure of the vapor cloud, to constrain the magnitudes, velocities, and time variability of the plume gas sources from the INMS data. Our results confirm a mixture of both low and high Mach gas emission from Enceladus' surface tiger stripes, with gas accelerated as fast as Mach 10 before escaping the surface. The vapor source fluxes and jet intensities/densities vary dramatically and stochastically, up to a factor 10, both spatially along the tiger stripes and over time between flyby observations. This complex spatial variability and dynamics may result from time-variable tidal stress fields interacting with subsurface fissure geometry and tortuosity beyond detectability, including changing gas pathways to the surface, and fluid flow and boiling in response evolving lithostatic stress conditions. The total plume gas source has 30% uncertainty depending on the contributions assumed for adiabatic and nonadiabatic gas expansion/acceleration to the high Mach emission. The overall vapor plume source rate exhibits stochastic time variability up to a factor ∼5 between observations, reflecting that found in the individual gas sources/jets. Key Words: Cassini at Saturn-Geysers-Enceladus-Gas dynamics-Icy satellites. Astrobiology 17, 926-940.

Cassini observations of ionospheric plasma in Saturn's magnetotail lobes

Studies of Saturn's magnetosphere with the Cassini mission have established the importance of Enceladus as the dominant mass source for Saturn's magnetosphere. It is well known that the ionosphere is an important mass source at Earth during periods of intense geomagnetic activity, but lesser attention has been dedicated to study the ionospheric mass source at Saturn. In this paper we describe a case study of data from Saturn's magnetotail, when Cassini was located at ≃ 2200 h Saturn local time at 36 R_S from Saturn. During several entries into the magnetotail lobe, tailward flowing cold electrons and a cold ion beam were observed directly adjacent to the plasma sheet and extending deeper into the lobe. The electrons and ions appear to be dispersed, dropping to lower energies with time. The composition of both the plasma sheet and lobe ions show very low fluxes (sometimes zero within measurement error) of water group ions. The magnetic field has a swept-forward configuration which is atypical for this region, and the total magnetic field strength is larger than expected at this distance from the planet. Ultraviolet auroral observations show a dawn brightening, and upstream heliospheric models suggest that the magnetosphere is being compressed by a region of high solar wind ram pressure. We interpret this event as the observation of ionospheric outflow in Saturn's magnetotail. We estimate a number flux between (2.95 ± 0.43) × 10⁹ and (1.43 ± 0.21) × 10¹⁰ cm^-2 s^-1, 1 or about 2 orders of magnitude larger than suggested by steady state MHD models, with a mass source between 1.4 ×10² and 1.1 ×10³ kg/s. After considering several configurations for the active atmospheric regions, we consider as most probable the main auroral oval, with associated mass source between 49.7 ±13.4 and 239.8 ±64.8 kg/s for an average auroral oval, and 10 ±4 and 49 ±23 kg/s for the specific auroral oval morphology found during this event. It is not clear how much of this mass is trapped within the magnetosphere and how much is lost to the solar wind.

On the protonation of water

Dissociative photoionization onsets of water and water dimer, measured by Imaging Photoelectron Photoion Coincidence (iPEPICO) Spectroscopy, are used in a floating thermochemical cycle to determine the proton affinity of water with unprecedented accuracy, as confirmed by state-of-the-artab initioquantum-chemical calculations.

Io's Tortured Interior

Magnetic measurements made by the Galileo spacecraft reveal an ocean of magma under Io's frozen surface.

Dynamics of whistler spheromaks in magnetized plasmas

Recent laboratory experiments [Stenzel et al., Phys. Rev. Lett. 96, 095004 (2006)10.1103/PhysRevLett.96.095004] have demonstrated interesting phenomena of propagating nonlinear whistler structures (spheromaks) and stationary field-reversed configurations, whose magnetic fields exceed the ambient magnetic field strength. Our objective here is to present simulation studies for these nonlinear whistler structures based on the three-dimensional nonlinear electron magnetohydrodynamic equations. The robustness and longevity of the propagating whistler spheromaks found in the experiments are confirmed numerically. Varying the toroidal field of the spheromak in the initial conditions, we find that the polarity and the amplitude of the toroidal field determine the propagation direction and speed of the spheromak. Our simulation results are in excellent agreement with those observed in the laboratory experiments.

Cassini encounters Enceladus: background and the discovery of a south polar hot spot

The Cassini spacecraft completed three close flybys of Saturn's enigmatic moon Enceladus between February and July 2005. On the third and closest flyby, on 14 July 2005, multiple Cassini instruments detected evidence for ongoing endogenic activity in a region centered on Enceladus' south pole. The polar region is the source of a plume of gas and dust, which probably emanates from prominent warm troughs seen on the surface. Cassini's Composite Infrared Spectrometer (CIRS) detected 3 to 7 gigawatts of thermal emission from the south polar troughs at temperatures up to 145 kelvin or higher, making Enceladus only the third known solid planetary body-after Earth and Io-that is sufficiently geologically active for its internal heat to be detected by remote sensing. If the plume is generated by the sublimation of water ice and if the sublimation source is visible to CIRS, then sublimation temperatures of at least 180 kelvin are required.

Enceladus: cosmic gymnast, volatile miniworld

The exploration of Saturn by the Cassini/Huygens mission has yielded a rich collection of data about the planet and its rings and moons, in particular its small satellite Enceladus and giant satellite Titan. Once believed too small to be active, Enceladus has been found to be one of the most geologically dynamic objects in the solar system. Among the surprises are a watery, gaseous plume; a south polar hot spot; and a surface marked by deep canyons and thick flows.

Enceladus' Varying Imprint on the Magnetosphere of Saturn

The bombardment of Saturn's moon Enceladus by >20-kiloelectron volt magnetospheric particles causes particle flux depletions in regions magnetically connected to its orbit. Irrespective of magnetospheric activity, proton depletions are persistent, whereas electron depletions are quickly erased by magnetospheric processes. Observations of these signatures by Cassini's Magnetospheric Imaging Instrument allow remote monitoring of Enceladus' gas and dust environments. This reveals substantial outgassing variability at the moon and suggests increased dust concentrations at its Lagrange points. The characteristics of the particle depletions additionally provide key radial diffusion coefficients for energetic electrons and an independent measure of the inner magnetosphere's rotation velocity.

Does Enceladus Govern Magnetospheric Dynamics at Saturn?

Instruments on the Cassini spacecraft reveal that a heat source within Saturn's moon Enceladus powers a great plume of water ice particles and dust grains, a geyser that jets outward from the south polar regions and most likely serves as the dominant source of Saturn's E ring. The interaction of flowing magnetospheric plasma with the plume modifies the particle and field environment of Enceladus. The structure of Saturn's magnetosphere, the extended region of space threaded by magnetic-field lines linked to the planet, is shaped by the ion source at Enceladus, and magnetospheric dynamics may be affected by the rate at which fresh ions are created.