The Pioneer and Voyager spacecraft made close-up measurements of Saturn's ionosphere and upper atmosphere in the 1970s and 1980s that suggested a chemical interaction between the rings and ...atmosphere. Exploring this interaction provides information on ring composition and the influence on Saturn's atmosphere from infalling material. The Cassini Ion Neutral Mass Spectrometer sampled in situ the region between the D ring and Saturn during the spacecraft's Grand Finale phase. We used these measurements to characterize the atmospheric structure and material influx from the rings. The atmospheric He/H
ratio is 10 to 16%. Volatile compounds from the rings (methane; carbon monoxide and/or molecular nitrogen), as well as larger organic-bearing grains, are flowing inward at a rate of 4800 to 45,000 kilograms per second.
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.
Modeling of the electrical conductivity (EC) of icy moon oceans has previously assumed that chloride, sulfate, and other ions released from rock leaching are the main solutes and carriers of EC. ...Here, we show that accreted volatiles, such as carbon dioxide and ammonia, can add a significant fraction of solutes in bodies whose volatile content was in part supplied from cometary materials. These volatiles can increase the EC of aqueous solutions above 1 S/m. Our salinity and EC estimates can serve as a basis for planning future magnetometer investigations at icy moons and dwarf planets. In particular, oceans expected in some of the Uranian satellites and Neptune's satellite Triton could have EC above 3 S/m as a result of accretion of both carbon dioxide and ammonia, even if rock leaching during water‐rock separation was limited, and if chlorine and sulfur abundances may be at CI carbonaceous chondritic levels.
Plain Language Summary
Searching for deep oceans in icy bodies is a major driver of planetary exploration. Magnetometry is used to detect electric currents in deep oceans generated by a varying magnetic field, from a giant planet or solar wind, as was done at Jupiter's moon Europa during the Galileo mission. Previous studies have assumed that the salinity of deep oceans is determined by the leaching of major elements from accreted rock, which may yield brackish and low electrical conductivity solutions. We show that carbon dioxide, an abundant ice in the outer solar system, could contribute a significant fraction of solutes (carbonate and bicarbonate ions) in ocean worlds, regulated in particular by the presence of ammonia. The latest solar system dynamical models predict that dwarf planets and most icy moons accreted ices rich in carbon dioxide and ammonia. If oceans exist today inside these bodies, the resulting ion concentrations would produce a stronger magnetic signature than previously expected, perhaps detectable by future spacecraft missions.
Key Points
Accreted CO2 and NH3 ices can significantly increase the salinity and electrical conductivity (EC) of deep oceans
Water to rock ratio and ammonia abundance drive the concentrations of carbonate and bicarbonate ions in solution
Predicted EC estimates can be used to plan future ocean search missions at icy moons and dwarf planets
•Gas exsolution-driven cryovolcanism is possible on Pluto and Charon.•Gas-driven cryovolcanism is less likely if a KBO retains an undifferentiated crust.•Hydrothermal gases (H2, CH4, possibly N2 from ...NH3) favor cryovolcanism.
Explosive extrusion of cold material from the interior of icy bodies, or cryovolcanism, has been observed on Enceladus and, perhaps, Europa, Triton, and Ceres. It may explain the observed evidence for a young surface on Charon (Pluto’s surface is masked by frosts). Here, we evaluate prerequisites for cryovolcanism on dwarf planet-class Kuiper belt objects (KBOs). We first review the likely spatial and temporal extent of subsurface liquid, proposed mechanisms to overcome the negative buoyancy of liquid water in ice, and the volatile inventory of KBOs. We then present a new geochemical equilibrium model for volatile exsolution and its ability to drive upward crack propagation. This novel approach bridges geophysics and geochemistry, and extends geochemical modeling to the seldom-explored realm of liquid water at subzero temperatures. We show that carbon monoxide (CO) is a key volatile for gas-driven fluid ascent; whereas CO2 and sulfur gases only play a minor role. N2, CH4, and H2 exsolution may also drive explosive cryovolcanism if hydrothermal activity produces these species in large amounts (a few percent with respect to water). Another important control on crack propagation is the internal structure: a hydrated core makes explosive cryovolcanism easier, but an undifferentiated crust does not. We briefly discuss other controls on ascent such as fluid freezing on crack walls, and outline theoretical advances necessary to better understand cryovolcanic processes. Finally, we make testable predictions for the 2015 New Horizons flyby of the Pluto-Charon system.
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.
Editor’s summaryEuropa, an icy moon of Jupiter, has a subsurface ocean beneath a crust of water ice. Solid carbon dioxide (CO2) has previously been observed on its surface, but the source was ...unknown. Two teams analyzed infrared spectroscopy of Europa from the James Webb Space Telescope to investigate the CO2 source. Trumbo and Brown found that the CO2 is concentrated in a region with geology that indicates transport of material to the surface from within the moon, and they discuss the implications for the composition of Europa’s internal ocean. Villanueva et al. also identified an internal origin of the CO2 and measured its 12C/13C isotope ratio. They searched for plumes of volatile material breaching the surface but found a lower activity than earlier observations. Together, these studies demonstrate that there is a source of carbon within Europa, probably in its ocean. —Keith T. Smith
During Cassini's final, spectacular months, in situ instruments made the first direct measurements of nanoparticles, finding an exceptionally large flow from the rings into Saturn's atmosphere. ...Cassini's Ion and Neutral Mass Spectrometer measured material in three altitude bands and found a global‐integrated flux of 2–20 × 104 kg/s that is dominated by hydrocarbon material <104u. Ranging from clusters of a few molecules to radii of several nanometers, nanoparticles are ubiquitous throughout Saturn's rings but embedded in the regolith of larger particles and not detectable as independent particles using remote observations. The smallest nanoparticles are susceptible to atmosphere drag by Saturn's tenuous exosphere that reaches the inner edge of the D ring. The unsustainable large flux suggests a recent disturbance of Saturn's inner ring material, possibly associated with the clumping that appeared in the D68 ringlet in 2015.
Plain Language Summary
For 40 years, calculations based on remote observations indicated that Saturn's magnetic field carries ions and charged particles from the rings to the midlatitudes of Saturn. In Cassini's last few months of life, direct, in situ measurements found that 10 tons/s of molecules and particles smaller than two nanometers are streaming along the plane of the rings into Saturn's atmosphere by another process: atmospheric drag. Saturn's extended atmosphere reaches the inner edge of Saturn's rings and extracts neutral particles less than one thousandth the thickness of a human hair by slowing them down until they fall into Saturn. Surprisingly, the flux is a hundred times larger than past predictions, and at least half of the material is hydrocarbon, which comprises less than 5% of the water ice‐dominated rings. Cassini's data also show that the influx varies at least a factor of 4 and may be linked to clumps that appeared in 2015 on D68, the ringlet on the inner edge of the rings. These newly discovered particles and processes alter the evolutionary landscape of the rings and provide an exciting, rich field for future research aimed at understanding the origin and history of the rings.
Key Points
Atmospheric drag, an interaction newly applied to Saturn and its rings, extracts >104 kg/s of neutral nanoparticles from Saturn's rings
Molecules and particles <2 nm in radius are preferentially transported by atmospheric drag
Neutral, nanometer‐sized hydrocarbon material is currently the largest mass loss from Saturn's rings
Jupiter’s icy moon, Europa, harbors a subsurface liquid water ocean; the prospect of this ocean being habitable motivates further exploration of the moon with the upcoming NASA Europa Clipper ...mission. Key among the mission goals is a comprehensive assessment of the moon’s composition, which is essential for assessing Europa’s habitability. Through powerful remote sensing and
in situ
investigations, the Europa Clipper mission will explore the composition of Europa’s surface and subsurface, its tenuous atmosphere, and the local space environment surrounding the moon. Clues on the interior composition of Europa will be gathered through these assessments, especially in regions that may expose subsurface materials, including compelling geologic landforms or locations indicative of recent or current activity such as potential plumes. The planned reconnaissance of the icy world will constrain models that simulate the ongoing external and internal processes that act to alter its composition. This paper presents the composition-themed goals for the Europa Clipper mission, the synergistic, composition-focused investigations that will be conducted, and how the anticipated scientific return will advance our understanding of the origin, evolution, and current state of Europa.
During Cassini's Grand Finale, the spacecraft flew between Saturn's upper atmosphere and its innermost ring, the D ring. Throughout these final orbits, Cassini encountered material flowing from the D ...ring into Saturn's atmosphere. Here, we present a compositional analysis of this material using data collected by Cassini's Ion Neutral Mass Spectrometer. We confirm the identification of CH4, CO2, CO, N2, H2O, NH3, and organics in the ring material, and provide upper limits for organic compounds of interest. The H/C, O/C, N/C, and S/C ratios of ring material are constrained using three different kinds of model spectra: automated fits, hand fits of inorganics + hydrocarbons, and hand fits of inorganics + organics with priority given to O-, N-, and S-bearing organics. Additionally, we compare data from the final plunge to earlier orbits, and find that ring material can be divided into gas and dust constituents, with CO2 at the volatility boundary between the two phases. At increasing distance above the equatorial plane, the gas/dust molar ratio increases.
•INMS measured complex material from the D ring during the Cassini Grand Finale.•This material is ~20% water and ~35% non-methane organics by mass.•Constraints on elemental ratios of the ring material are presented.•Ring material is latitudinally fractionated according to gas and dust components.
James Webb Space Telescope's NIRSpec infrared imaging spectrometer observed the outer solar system dwarf planets Eris and Makemake in reflected sunlight at wavelengths spanning 1 through 5 μm. Both ...objects have high albedo surfaces that are rich in methane ice, with a texture that permits long optical path lengths through the ice for solar photons. There is evidence for N2 ice absorption around 4.2 μm on Eris, though not on Makemake. No CO ice absorption is seen at 4.67 μm on either body. For the first time, absorption bands of two heavy isotopologues of methane are observed at 2.615 μm (13CH4), 4.33 μm (12CH3D), and 4.57 μm (12CH3D). These bands enable us to measure D/H ratios of (2.5 ± 0.5) × 10−4 and (2.9 ± 0.6) × 10−4, along with 13C/12C ratios of 0.012 ± 0.002 and 0.010 ± 0.003 in the surface methane ices of Eris and Makemake, respectively. The measured D/H ratios are much lower than that of presumably primordial methane in comet 67P/Churyumov-Gerasimenko, but they are similar to D/H ratios in water in many comets and larger outer solar system objects. This similarity suggests that the hydrogen atoms in methane on Eris and Makemake originated from water, indicative of geochemical processes in past or even ongoing hot environments in their deep interiors. The 13C/12C ratios are consistent with commonly observed solar system values, suggesting no substantial enrichment in 13C as could happen if the methane currently on their surfaces was the residue of a much larger inventory that had mostly been lost to space. Possible explanations include geologically recent outgassing from the interiors as well as processes that cycle the surface methane inventory to keep the uppermost surfaces refreshed.
•New JWST NIRSpec infrared spectral observations of Eris and Makemake.•Absorption bands of 13CH4 and CH3D used to measure isotopic ratios in CH4.•Similarity to D/H in water suggests source in interior geochemistry.
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