•Dione O2 and CO2 exosphere of similar density and composition to Rhea found by INMS.•Seasonal and spatial variability suggests winter polar condensation and sublimation.•Gas time evolution suggests ...polar frost diffusion into the surface regolith.•Seasons (equinox exospheric peak) inverse of Saturn ring O2 (solstice peak).•Low O2 densities at Dione/Rhea suggest a sputter-resistant surface lag layer.
A Dione O2 and CO2 exosphere of similar composition and density to Rhea's is confirmed by Cassini spacecraft Ion Neutral Mass Spectrometer (INMS) flyby data. INMS results from three Dione and two Rhea flybys show exospheric spatial and temporal variability indicative of seasonal exospheres, modulated by winter polar gas adsorption and desorption at the equinoxes. Cassini Plasma Spectrometer (CAPS) pickup ion fluxes also show exospheric structure and evolution at Rhea consistent with INMS, after taking into consideration the anticipated charge exchange, electron impact, and photo-ionization rates. Data-model comparisons show the exospheric evolution to be consistent with polar frost diffusion into the surface regolith, which limits surface exposure and loss of the winter frost cap by sputtering. Implied O2 source rates of ∼45(7)×1021s−1 at Dione(Rhea) are ∼50(300) times less than expected from known O2 radiolysis yields from ion-irradiated pure water ice measured in the laboratory, ruling out secondary sputtering as a major exospheric contributor, and implying a nanometer scale surface refractory lag layer consisting of concentrated carbonaceous impurities. We estimate ∼30:1(2:1) relative O2:CO2 source rates at Dione(Rhea), consistent with a stoichiometric bulk composition below the lag layer of 0.01(0.13) C atoms per H2O molecule, deriving from endogenic constituents, implanted micrometeoritic organics, and (in particular at Dione) exogenous H2O delivery by E-ring grains. Impact deposition, gardening and vaporization may thereby control the global O2 source rates by fresh H2O ice exposure to surface radiolysis and trapped oxidant ejection.
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-as suggested most recently by the discovery of salts in E-ring particles derived from the plume-or warm ice that is heated, causing dissociation of clathrate hydrates. Here we report that ammonia is present in the plume, along with various organic compounds, deuterium and, very probably, (40)Ar. The presence of ammonia provides strong evidence for the existence of at least some liquid water, given that temperatures in excess of 180K have been measured near the fractures from which the jets emanate. 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.
•Europa plumes may feed a global exosphere with complex structure and dynamics•Water and organics may be enhanced on the dayside during polar plume activity•Prior plumes or chemically enriched ...terrain may yield detectable exospheric signs•Model gives species density criteria for plume or enriched terrain detection
A Europa plume source, if present, may produce a global exosphere with complex spatial structure and temporal variability in its density and composition. To investigate this interaction we have integrated a water plume source containing multiple organic and nitrile species into a Europan Monte Carlo exosphere model, considering the effect of Europa's gravity in returning plume ejecta to the surface, and the subsequent spreading of adsorbed and exospheric material by thermal desorption and re-sputtering across the entire body. We consider sputtered, radiolytic and potential plume sources, together with surface adsorption, regolith diffusion, polar cold trapping, and re-sputtering of adsorbed materials, and examine the spatial distribution and temporal evolution of the exospheric density and composition. These models provide a predictive basis for telescopic observations (e.g. HST, JWST) and planned missions to the Jovian system by NASA and ESA. We apply spacecraft trajectories to our model to explore possible exospheric compositions which may be encountered along proposed flybys of Europa to inform the spatial and temporal relationship of spacecraft measurements to surface and plume source compositions. For the present preliminary study, we have considered four cases: Case A: an equatorial flyby through a sputtered only exosphere (no plumes), Case B: a flyby over a localized sputtered ‘macula’ terrain enriched in non-ice species, Case C: a south polar plume with an Enceladus-like composition, equatorial flyby, and Case D: a south polar plume, flyby directly through the plume.
The flyby measurements of the Cassini spacecraft at Saturn's moon Rhea reveal a tenuous oxygen (O₂)-carbon dioxide (CO₂) atmosphere. The atmosphere appears to be sustained by chemical decomposition ...of the surface water ice under irradiation from Saturn's magnetospheric plasma. This in situ detection of an oxidizing atmosphere is consistent with remote observations of other icy bodies, such as Jupiter's moons Europa and Ganymede, and suggestive of a reservoir of radiolytic O₂ locked within Rhea's ice. The presence of CO₂ suggests radiolysis reactions between surface oxidants and organics or sputtering and/or outgassing of CO₂ endogenic to Rhea's ice. Observations of outflowing positive and negative ions give evidence for pickup ionization as a major atmospheric loss mechanism.
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 7 April 2010. By examining possible particle ...trajectories, we find that the observed particles are consistent with negative pickup ions originating near the moon's surface. We find that the mass of the negative pickup ions is in the range of 15–25 u and tentatively identify this species as O−, likely resulting from ionization and subsequent pickup from Dione's O2‐CO2 exosphere. Our estimates show that the negative ion density is ~3 × 10−3 cm−3. This is comparable to, but slightly smaller than, that previously reported for the density of O2+ pickup ions for the same flyby, indicating that negative pickup ions may represent a major loss channel for Dione's exosphere.
Key Points
Analysis of data from the Cassini Plasma Spectrometer confirms negative pickup ions originating from Saturn's moon Dione
The negative pickup ions are consistent with O− originating from Dione's exosphere
Density estimates indicate that negative pickup ions may represent a significant loss channel for Dione's exosphere
We have characterized the porosity of vapor-deposited amorphous solid water (ice) films deposited at 30-40 K using several complementary techniques such as quartz crystal microgravimetry, UV-visible ...interferometry, and infrared reflectance spectrometry in tandem with methane adsorption. The results, inferred from the gas adsorption isotherms, reveal the existence of microporosity in all vapor-deposited films condensed from both diffuse and collimated water vapor sources. Films deposited from a diffuse source show a step in the isotherms and much less adsorption at low pressures than films deposited from a collimated source with the difference increasing with film thickness. Ice films deposited from a collimated vapor source at 77 degrees incidence are mesoporous, in addition to having micropores. Remarkably, mesoporosity is retained upon warming to temperatures as high as 140 K where the ice crystallized. The binding energy distribution for methane adsorption in the micropores of ice films deposited from a collimated source peaks at approximately 0.083 eV for deposition at normal incidence and at approximately 0.077 eV for deposition at >45 degrees incidence. For microporous ice, the intensity of the infrared bands due to methane molecules on dangling OH bonds on pore surfaces increases linearly with methane uptake, up to saturation adsorption. This shows that the multilayer condensation of methane does not occur inside the micropores. Rather, filling of the core volume results from coating the pore walls with the first layer of methane, indicating pore widths below a few molecular diameters. For ice deposited at 77 degrees incidence, the increase in intensity of the dangling bond absorptions modified by methane adsorption departs from linearity at large uptakes.
We report on the measurements of the Cassini Ion Neutral Mass Spectrometer (INMS) of the density and structure of Enceladus' south polar plume during the E3 and E5 flybys on 12 March and 9 October ...2008. Using a Monte Carlo simulation, we analyze the dependence of the INMS gas inlet transmittance on spacecraft pointing and the effect on the measurements at E3. We apply a finite element analysis to correct for water physisorption in the inlet and obtain a maximum plume density almost twice that suggested by the raw INMS data. The results indicate uniform spreading of the plume vapor from the source with a source rate of at least 100 kg/s. We also analyze the detection of ice grains by the INMS and find that, in contrast to the plume's vapor component, the grains are concentrated within the plume jets seen in Cassini imaging, supporting the suggestion that the jets are composed of fine‐grained ice.
•We provide INMS density data for six Enceladus encounters.•Ice grains affect INMS measurements and their uncertainty.•Mass-dependent thermal velocity modifies jet composition.
During six encounters ...between 2008 and 2013, the Cassini Ion and Neutral Mass Spectrometer (INMS) made in situ measurements deep within the Enceladus plumes. Throughout each encounter, those measurements contained density variations that reflected the nature of the source, particularly of the high-velocity jets. Since the dominant constituent of the vapor, H2O, interacted with the walls of the INMS inlet, we track changes in the external vapor density by using more-volatile species that responded promptly to those changes. However, the most-abundant volatiles, at 28u and 44u, behaved differently from each other in the plume. At least a portion of their differences may be attributed to mass-dependent thermal velocity that affects Mach number in the high-velocity jets. Variations between volatiles place an emphasis on modeling as a means to construct overall plume density from the volatile densities and to investigate the velocity, gas temperature, and location of the jets. Ice grains, entering the INMS aperture add complexity and uncertainty to the physical interpretation of the data because the grains modified the INMS measurements. A comparison of data from the last three encounters, E14, E17, and E18, are consistent with the VIMS observation of variability in jet production and a slower, more diffuse gas flux from the four sulci or tiger stripes. We provide and describe the INMS data, its processing, and its uncertainty.
The MAss Spectrometer for Planetary EXploration (MASPEX) is a high-mass-resolution, high-sensitivity, multi-bounce time-of-flight mass spectrometer (MBTOF) capable of measuring minor species with ...abundances of sub-parts-per-million in Europa’s sputter-produced and radiolytically modified exosphere and in its oceanic plumes. The goal of the MASPEX-Europa investigation is to determine, through in-situ measurement of the exosphere and plume composition, whether the conditions for habitability exist or have existed on Europa. As conventionally defined, based on our knowledge of Earth life, the three fundamental conditions for habitability are: (1) the presence of liquid water; (2) the presence of organic compounds and the biogenic elements CHNOPS; and (3) a source of energy available for metabolic processes, which for Europa will most probably be chemosynthetic rather than photosynthetic. Condition (1) is already established by previous indirect (magnetic field) measurements, while MASPEX will contribute directly to the evaluation of condition (2) through highly specific compositional measurements in the Europan exosphere and plumes. The composition measurements will also contribute to the test of condition (3) through disequilibrium states of chemical reactions. Thus, the primary goal of MASPEX for Europa Clipper is to assess the habitability of Europa and specifically of its interior ocean. MASPEX has been developed successfully, and its calibration has demonstrated that it meets its specified requirements for sensitivity, dynamic range, and mass resolution. This paper reports the development of the MASPEX scientific investigation, the instrument, its performance, and calibration.
O2, H2, and H2O2 radiolysis from water ice is pervasive on icy astrophysical bodies, but the lack of a self‐consistent, quantitative model of the yields of these water products versus irradiation ...projectile species and energy has been an obstacle to estimating the radiolytic oxidant sources to the surfaces and exospheres of these objects. A major challenge is the wide variation of O2 radiolysis yields between laboratory experiments, ranging over 4 orders of magnitude from 5 × 10−7 to 5 × 10−3 molecules/eV for different particles and energies. We revisit decades of laboratory data to solve this long‐standing puzzle, finding an inverse projectile range dependence in the O2 yields, due to preferential O2 formation from an ~30 Å thick oxygenated surface layer. Highly penetrating projectile ions and electrons with ranges ≳30 Å are therefore less efficient at producing O2 than slow/heavy ions and low‐energy electrons (≲ 400 eV) which deposit most energy near the surface. Unlike O2, the H2O2 yields from penetrating projectiles fall within a comparatively narrow range of (0.1–6) × 10−3 molecules/eV and do not depend on range, suggesting that H2O2 forms deep in the ice uniformly along the projectile track, e.g., by reactions of OH radicals. We develop an analytical model for O2, H2, and H2O2 yields from pure water ice for electrons and singly charged ions of any mass and energy and apply the model to estimate possible O2 source rates on several icy satellites. The yields are upper limits for icy bodies on which surface impurities may be present.
Plain Language Summary
Ice‐covered comets and moons in the outer solar system are irradiated by charged particles from space, which break apart the surface water molecules to form hydrogen peroxide, as well as molecular hydrogen and oxygen which are ejected into the atmospheres of these objects. Here we develop a comprehensive model of the production of peroxide, hydrogen, and oxygen, versus the mass and energy of the charged particles, and the temperature of the ice. The model predicts self consistently the production rates of all three molecules as measured in laboratory experiments and enables quantitative predictions of the supply of these molecules to the surfaces and atmospheres of icy objects in space.
Key Points
We give predicted yields for O2, H2, and H2O2 radiolysis from ice for electrons and ions at any energy, (ion) mass, and ice temperature
Lab experiments reveal an inverse range dependence in radiolytic O2 yields from ice with highly penetrating projectiles
The model's predictions of icy satellite O2, H2 sources, and H2O2 column densities can be compared to data to elucidate surface composition
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