Detection of negative pickup ions at Saturn's moon Dione

We investigate a possible negative ion feature observed by the Cassini Plasma Spectrometer (CAPS) during a flyby of Saturn's moon Dione that occurred on April 7, 2010. By examining possible particle trajectories, we find that the observed particles are consistent with negative pick‐up ions originating near the moon's surface. We find that the mass of the negative pick‐up ions is in the range of 15 – 25 u and tentatively identify this species as O‐, likely resulting from ionization and subsequent pick‐up from Dione's O2‐CO2 exosphere. Our estimates show that the negative ion density is ~3 x 10‐3 cm‐3. This is comparable to, but slightly smaller than, that previously reported for the density of O2+ pick‐up ions for the same flyby, indicating that negative pick‐up ions may represent a major loss channel for Dione's exosphere

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

Macromolecular organic compounds from the depths of Enceladus

Abstract Saturn’s moon Enceladus harbours a global water ocean¹, which lies under an ice crust and above a rocky core². Through warm cracks in the crust³ a cryo-volcanic plume ejects ice grains and vapour into space⁴–⁷ that contain materials originating from the ocean⁸,⁹. Hydrothermal activity is suspected to occur deep inside the porous core¹⁰–¹², powered by tidal dissipation¹³. So far, only simple organic compounds with molecular masses mostly below 50 atomic mass units have been observed in plume material⁶,¹⁴,¹⁵. Here we report observations of emitted ice grains containing concentrated and complex macromolecular organic material with molecular masses above 200 atomic mass units. The data constrain the macromolecular structure of organics detected in the ice grains and suggest the presence of a thin organic-rich film on top of the oceanic water table, where organic nucleation cores generated by the bursting of bubbles allow the probing of Enceladus’ organic inventory in enhanced concentrations

Exoplanetary Biosignatures for Astrobiology

Since life evolved on our planet there have been subtle interplays between biology and Earth System Components (atmosphere-lithosphere-ocean-interior). Life, for example, can impact weathering rates which, in turn, influence climate stabilizing feedback cycles on Earth. Photosynthesis is ultimately responsible for our oxygen-rich atmosphere, which favours the formation of the protective ozone layer. The recent rise of exoplanetary science has led to a re-examination of such feedbacks and their main drivers under different planetary conditions. In this work we present a brief overview of potential biosignatures (indicators of life) and review knowledge of the main processes, which influence them in an exoplanetary context. Biosignature methods can be broadly split into two areas, namely “in-situ” and “remote”. Criteria employed to detect biosignatures are diverse and include fossil morphology, isotope ratios, patterns in the chemical constituents of cells, degree of chirality, shifts from thermal or redox equilibrium, and changes in the abundance of atmospheric species. For the purposes of this review, our main focus lies upon gas-phase species present in Earth-like atmospheres, which could be detected remotely by spectroscopy. We summarize current knowledge based on the modern (and early) Earth and the Solar System then review atmospheric model studies for Earth-like planets, which predict climate, photochemistry and potential spectral signals of biosignature species