Jones Sound is one of three critical waterways in the Canadian Arctic Archipelago that regulate liquid exchange between the Arctic Ocean and northern Atlantic Ocean. However, to date, no high-resolution ocean circulation model exists to study the recent evolution of Jones Sound, meaning that our understanding of circulation within the sound is based either on temporally and spatially sparse oceanographic observations or on extrapolating conditions within Baffin Bay, which has a more dense observational record. To address this, we develop a high-resolution (1/120 degrees, 0.9 km) Jones Sound configuration of the Massachusetts Institute of Technology general circulation model and perform coupled ocean-sea ice-biological productivity simulations between 2003-2016. We find that circulation through Lady Ann Strait, Fram Sound, and Glacier Strait comprises 71 %, 14 %, and 15 % of the volumetric transport into and out of Jones Sound, with tidal flushing enhancing the magnitude of volumetric transport through Fram Sound. Warming Atlantic Water within western Baffin Bay flows into Jones Sound through Lady Ann Strait, becomes well-mixed, and circulates counterclockwise, encroaching on the terminus of most tidewater glaciers that line the eastern periphery of the sound. Furthermore, we find that sustained atmospheric and oceanic warming drives an 11 % reduction in the 2003-2016 mean summertime sea ice area, decreased wintertime sea ice thickness, and delayed onset of sea ice refreeze in the fall (thus lengthening the amount of time during which Jones Sound is ice-free). Tidal flushing through Cardigan Strait is critical in triggering melt-back of sea ice across northern Jones Sound. Lastly, this decline in sea ice increases light availability and, when coupled with warming of the subsurface waters in Jones Sound, facilitates enhanced primary productivity down to similar to 21 m depth. While we note that the modeled warming signal in Baffin Bay is overestimated relative to observations, the results presented here improve our general understanding of how this critical waterway might change under continued polar-amplified global warming and underscores the need for sustained oceanographic observations in this region.

AbstractThe potential habitability of Jupiter's moon Europa has motivated two missions: NASA's Europa Clipper and ESA's JUpiter ICy moons Explorer (JUICE). Both missions are equipped with ice‐penetrating radars which will transmit radio waves into the subsurface, recording reflections from interfaces defined by contrasts in ice shell dielectric properties. Assuming an MgSO4 ocean, we show that salt layers, formed through the freezing of subsurface liquid water reservoirs, can be detected by ice‐penetrating radar instruments on Europa Clipper and JUICE. Furthermore, because these features are thermodynamically stable within the minimally attenuating portion of Europa's ice shell, referred to here as the “pellucid region,” they could produce brighter reflections than deeper liquid water interfaces. We demonstrate how ice‐penetrating radar measurements of salt layer thickness could establish lower bounds on the parameter space of possible initial reservoir thickness and salinity, constrain the origin of reservoirs (ice shell melt vs. ocean injection), and—if sourced through ocean injection—the ocean salinity.

Energy and nutrient sources for life could be delivered to Titan’s subsurface water ocean from both its surface above and its core below. Organic matter forming de novo in Titan’s atmosphere and depositing on the surface may hydrolyze upon descent into the ocean with impact-generated melt pools sinking through the ice, adding to a primordial inventory released by the core during differentiation and/or across geologic time. This raises the possibility that abiotic organic carbon could fuel heterotrophic carbon assimilation into biomass in Titan’s ocean if it is inhabited. Glycine fermentation is one possible metabolism of interest, because mechanisms exist to transport glycine to Titan’s ocean and anaerobic fermentations do not rely on additional strong oxidants which may not be present on Titan. Using bioenergetic modeling, we show that while conditions favorable to glycine fermentation may exist, they are highly dependent on temperature. Additionally, the ability of that metabolism to fuel a global biosphere is limited by the slow delivery of glycine by impact melt pools (<10 nmolal yr ^−1 optimistically, with a >1 mmolal primordial component). A total population of 10 ^14 –10 ^17 cells (a few kilograms of carbon) can be sustained, amounting to less than 1 cell kg ^–1 water when diluted through the entire ocean. Constraining notionally detectable biospheres on Titan will therefore require (i) considering localized environments that may concentrate cells, (ii) better characterizing other candidate metabolisms (e.g., degradation of acetylene or polyaromatic hydrocarbons) for quantitative bioenergetic modeling, and (iii) resolving new mechanisms to deliver organics and oxidants for life.

Several icy moons of Jupiter and Saturn, including Europa and Enceladus, host liquid water oceans buried beneath their icy crusts. Geological features of the ice crusts as well as large-scale variations of the ice thickness are often attributed to endogenic processes within the ice. However, the ice shell is also coupled to the rocky interior via the convective ocean which controls heat and material exchanges. The amount of tidal heating in Europa's silicate mantle is highly uncertain, and heterogeneous. We investigate the effect of heterogeneous tidal heating in the silicate mantle on rotating thermal convection in the ocean and its consequences on ice shell thickness. Using global direct numerical simulations, we show that, under the assumption of no salinity or ocean-ice shell feedbacks, convection largely transposes the latitudinal variations of tidal heating from the seafloor to the ice, leading to a higher oceanic heat flux in polar regions. Longitudinal variations are efficiently transferred when boundary-driven thermal winds develop, but are reduced in the presence of strong zonal flows and may vanish in planetary regimes. We investigate the impact on ice shell thickness using the conductive equilibrium model of Nimmo et al. (2007) (doi: 10.1016/j.icarus.2007.04.021). We find that if spatially homogeneous radiogenic heating is dominant in the silicate mantle, the ocean’s contribution to ice shell thickness variations is negligible compared to tidal heating within the ice. If tidal heating is instead dominant in the mantle, the situation is reversed and the ocean controls the pole-to-equator thickness contrast, as well as possible longitudinal variations.

The Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) is a dual-frequency ice-penetrating radar (9 and 60 MHz) onboard the Europa Clipper mission. REASON is designed to probe Europa from exosphere to subsurface ocean, contributing the third dimension to observations of this enigmatic world. The hypotheses REASON will test are that (1) the ice shell of Europa hosts liquid water, (2) the ice shell overlies an ocean and is subject to tidal flexing, and (3) the exosphere, near-surface, ice shell, and ocean participate in material exchange essential to the habitability of this moon. REASON will investigate processes governing this material exchange by characterizing the distribution of putative non-ice material (e.g., brines, salts) in the subsurface, searching for an ice–ocean interface, characterizing the ice shell’s global structure, and constraining the amplitude of Europa’s radial tidal deformations. REASON will accomplish these science objectives using a combination of radar measurement techniques including altimetry , reflectometry , sounding , interferometry , plasma characterization , and ranging . Building on a rich heritage from Earth, the moon, and Mars, REASON will be the first ice-penetrating radar to explore the outer solar system. Because these radars are untested for the icy worlds in the outer solar system, a novel approach to measurement quality assessment was developed to represent uncertainties in key properties of Europa that affect REASON performance and ensure robustness across a range of plausible parameters suggested for the icy moon. REASON will shed light on a never-before-seen dimension of Europa and – in concert with other instruments on Europa Clipper – help to investigate whether Europa is a habitable world.

Jupiter’s largest moon, Ganymede, is the main target of the upcoming ESA mission JUpiter ICy moons Explorer (JUICE), which is planned to launch in 2023. One of the top priorities of the JUICE mission is investigation of past and/or recent cryovolcanic and tectonic activity and the exchange processes with the subsurface and possibly with the ocean (Grasset et al., 2013). Following that objective, the science team has defined ‘potential cryovolcanic regions’ as a category of high interest for observation by JUICE (Stephan et al. 2021). Hence, for preparation of the scientific return of the mission, it is important to study in detail the regions that are considered to be good candidates for past or present activity. Areas on Ganymede imaged by Voyager that showed the presence of dark terrain were speculated to represent a heavily cratered surface modified by cryovolcanism (e.g., Murchie et al., 1989); however, this was disputed based on the higher-resolution images of the Galileo mission (e.g., Pappalardo et al., 2004). Light material observed by the Voyager instruments was similarly suggested to represent dark terrain resurfaced by cryovolcanic flows (e.g., Parmentier et al., 1982). Later, Galileo high-resolution data showed the significant role of tectonism in the formation of these areas, while the role of cryovolcanism remained inconclusive due to the limited resolution of the available data and secondary processes that complicate their interpretation, such as fracturing, mass wasting, etc (e.g., Patterson et al., 2010). Currently, small, isolated depressions called ‘paterae’, are the best candidate regions for cryovolcanic activity on Ganymede and suggested to be potential caldera-like cryovolcanic source vents (Fig. 1;2)(e.g., Lucchita, 1980; Spaun et al. 2001). Collins et al. (2013) characterize the paterae as ‘flat-floored depressions surrounded by inward-scalloped walls, breached on one side and typically associated with light subdued materials’ while they interpret their nature as ‘possible cryovolcanic source vents for extrusion of clean icy material to form light material units’. The small size of paterae (20 x 70 km at most) is consistent with a cryovolvanic origin that operates on a local scale.The high-resolution JUICE camera, JANUS, in combination with other remote sensing instruments, is expected to resolve many of the mysteries concerning cryovolcanism on Ganymede and the origin of the moon’s varied geologic features. The known paterae (Fig. 1) are located in smooth and bright terrains, where extensional tectonism and volcanism are speculated to have operated concurrently (Pappalardo et al. 2004). The ‘potential cryovolcanic regions’ identified by the JUICE team includes 19 out of 30 paterae mapped by Collins et al. (2013) using Voyager and Galileo images. In this study, we provide a thorough view of all 19 paterae regions and a constructional comparison of their characteristics in order to constrain the morphology of the paterae and their surroundings, in preparation of the JUICE mission and its science return.Figure 1. Locations (red circles and ellipsoids) of 19 paterae identified as ‘potential cryovolcanic regions’ that are of high interest for JUICE observations (Stephan et al. 2021). Basemap: Galileo Solid State Imaging (SSI)/Voyager Imaging Science Subsystem (ISS) mosaic.Figure 2. Major Ganymede Paterae as observed from the Voyager and Galileo instruments: a. Hammamat (1 in Fig. 1); b. Musa (4); c. Rum (2); d. Yaroun (5); e. Hamra (6); f. Natrun (2). Data from: Galileo Solid State Imaging (SSI)/Voyager Imaging Science Subsystem (ISS).References: Grasset et al. (2013), PSS, 78, 1-21; Stephan et al. (2021), PSS in press; Murchie et al. (1989). Icarus, 81, 271-297; Pappalardo, R., et al. (2004), Cambridge, UK: Cambridge University Press; Parmentier, E., et al. (1982). Nature, 295, 290-293; Patterson, G., et al. (2010). Icarus, 207, 845-867; Lucchita, (1980). Icarus, 44, 481-501; Spaun et al. (2001), LPSC, 32; Collins, et al. (2013), USGS Scientific Investigations Map 3237, 4.Acknowledgments: This work was conducted at the California Institute of Technology (Caltech) under contract with NASA. K.K. was supported by the Czech Science Foundation through project No. 19-10809S. ©2021 California Institute of Technology. Government sponsorship acknowledged.

Titan has an organic-rich atmosphere and surface with a subsurface liquid water ocean that may represent a habitable environment. In this work, we determined the amount of organic material that can be delivered from Titan's surface to its ocean through impact cratering. We assumed that Titan's craters produce impact melt deposits composed of liquid water that can founder in its lower-density ice crust and estimated the amount of organic molecules that could be incorporated into these melt lenses. We used known yields for HCN and Titan haze hydrolysis to determine the amount of glycine produced in the melt lenses and found a range of possible flux rates of glycine from the surface to the subsurface ocean. These ranged from 0 to 1011 mol/Gyr for HCN hydrolysis and from 0 to 1014 mol/Gyr for haze hydrolysis. These fluxes suggest an upper limit for biomass productivity of ∼103 kgC/year from a glycine fermentation metabolism. This upper limit is significantly less than recent estimates of the hypothetical biomass production supported by Enceladus's subsurface ocean. Unless biologically available compounds can be sourced from Titan's interior, or be delivered from the surface by other mechanisms, our calculations suggest that even the most organic-rich ocean world in the Solar System may not be able to support a large biosphere.

The magnetospheric systems of ice giants, as the ideal and the unique template of a typical class of exoplanets, have not been sufficiently studied in the past decade. The complexity of these asymmetric and extremely dynamic magnetospheres provides us a great chance to systematically investigate the general mechanism of driving the magnetospheres of such common exoplanets in the Universe, and the key factors of influencing the global and local magnetospheric structures of this type of planets. In this paper, we discuss the science return of probing magnetospheric systems of ice giants for the future missions, throughout different magnetospheric regions, across from the interaction with upstream solar wind to the downstream region of the magnetotail. We emphasize the importance of detecting the magnetospheric systems of ice giants in the next decades, which enables us to deeply understand the space enviroNMent and habitability of not only the ice giants themselves but also the analogous exoplanets which are widely distributed in the Universe.

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.

This chapter reviews the way the six key questions about planetary systems, from their origins to the way they work and their habitability, identified in Chapter 1 (Blanc et al., 2021), can be addressed by means of solar system exploration, and how one can find partial answers to these six questions by flying to the different provinces to the solar system: terrestrial planets, giant planets, small bodies, and up to its interface with the local interstellar medium. It derives from this analysis a synthetic description of the most important space observations to be performed at the different solar system objects by future planetary exploration missions. These “observation requirements” illustrate the diversity of measurement techniques to be used as well as the diversity of destinations where these observations must be made. They constitute the base for the identification of the future planetary missions we need to fly by 2061, which are described in Chapter 4.

Observations from Cassini have identified nanometer-sized silica grains in Saturn’s E-ring although their origin is unclear. Tidal deformation within Enceladus’ silicate core has been predicted to generate hot hydrothermal fluids that rise from the core-ocean boundary and traverse the subsurface ocean. This raises the possibility that the particles observed by Cassini could have been produced by hydrothermal alteration and ejected via the south polar plumes. Here, we use an analytical model to quantify potential for particle entrainment in Enceladus’ ocean. We use scaling relations to characterize ocean convection and define a parameter space that enables particle entrainment. We find that both the core-ocean heat fluxes and the transport timescale necessary to drive oceanic convection and entrain particles of the observed sizes are consistent with observations and predictions from existing thermal models. We conclude that hydrothermal alteration at Enceladus’ seafloor could indeed be the source of silica particles in Saturn’s E-ring.

The Galileo mission to Jupiter revealed that Europa is an ocean world. The Galileo magnetometer experiment in particular provided strong evidence for a salty subsurface ocean beneath the ice shell, likely in contact with the rocky core. Within the ice shell and ocean, a number of tectonic and geodynamic processes may operate today or have operated at some point in the past, including solid ice convection, diapirism, subsumption, and interstitial lake formation. The science objectives of the Europa Clipper mission include the characterization of Europa’s interior; confirmation of the presence of a subsurface ocean; identification of constraints on the depth to this ocean, and on its salinity and thickness; and determination of processes of material exchange between the surface, ice shell, and ocean. Three broad categories of investigation are planned to interrogate different aspects of the subsurface structure and properties of the ice shell and ocean: magnetic induction, subsurface radar sounding, and tidal deformation. These investigations are supplemented by several auxiliary measurements. Alone, each of these investigations will reveal unique information. Together, the synergy between these investigations will expose the secrets of the Europan interior in unprecedented detail, an essential step in evaluating the habitability of this ocean world.

The habitability of Europa is a property within a system, which is driven by a multitude of physical and chemical processes and is defined by many interdependent parameters, so that its full characterization requires collaborative investigation. To explore Europa as an integrated system to yield a complete picture of its habitability, the Europa Clipper mission has three primary science objectives: (1) characterize the ice shell and ocean including their heterogeneity, properties, and the nature of surface-ice-ocean exchange; (2) characterize Europa's composition including any non-ice materials on the surface and in the atmosphere, and any carbon-containing compounds; and (3) characterize Europa's geology including surface features and localities of high science interest. The mission will also address several cross-cutting science topics including the search for any current or recent activity in the form of thermal anomalies and plumes, performing geodetic and radiation measurements, and assessing high-resolution, co-located observations at select sites to provide reconnaissance for a potential future landed mission. Synthesizing the mission's science measurements, as well as incorporating remote observations by Earth-based observatories, the James Webb Space Telescope, and other space-based resources, to constrain Europa's habitability, is a complex task and is guided by the mission's Habitability Assessment Board (HAB).

In the outer solar system, a growing number of giant planet satellites are now known to be abodes for global oceans hidden below an outer layer of ice. These planetary oceans are a natural laboratory for studying physical oceanographic processes in settings that challenge traditional assumptions made for Earth's oceans. While some driving mechanisms are common to both systems, such as buoyancy-driven flows and tides, others, such as libration, precession, and electromagnetic pumping, are likely more significant for moons in orbit around a host planet. Here, we review these mechanisms and how they may operate across the solar system, including their implications for ice-ocean interactions. Future studies should continue to advance our understanding of each of these processes as well as how they may act together in concert. This interplay also has strong implications for habitability as well as testing oceanic hypotheses with future missions.

The circulation in Europa's ocean determines the degree of thermal, mechanical and chemical coupling between the ice shell and the silicate mantle. Using global direct numerical simulations, we investigate the effect of heterogeneous tidal heating in the silicate mantle on rotating thermal convection in the ocean and its consequences on ice shell thickness. Under the assumption of no salinity or ocean-ice shell feedbacks, we show that convection largely transposes the latitudinal variations of tidal heating from the seafloor to the ice, leading to a higher oceanic heat flux in polar regions. Longitudinal variations are efficiently transferred when boundary-driven thermal winds develop, but are reduced in the presence of strong zonal flows and may vanish in planetary regimes. If spatially homogeneous radiogenic heating is dominant in the silicate mantle, the ocean's contribution to ice shell thickness variations is negligible compared to tidal heating within the ice. If tidal heating is instead dominant in the mantle, the situation is reversed and the ocean controls the pole-to-equator thickness contrast, as well as possible longitudinal variations.

The operation of the solar dynamo, with all of its remarkable spatio-temporal ordering, remains an outstanding problem of modern solar physics. A number of mechanisms that might plausibly contribute to its operation have been proposed, but the relative role played by each remains unclear. This uncertainty stems from continuing questions concerning the speed and structure of deep-seated convective flows. Those flows are in-turn thought to sustain both the Sun's turbulent EMF and the large-scale flows of differential rotation and meridional circulation suspected of influencing the dynamo's organization and timing. Continued progress in this area is complicated by (i) inconsistencies between helioseismic measurements of convective and meridional flow made with different techniques and instruments, and (ii) a lack of high-latitude data for convection, differential rotation, and meridional flow. We suggest that the path forward to resolving these difficulties is twofold. First, the acquisition of long-term helioseismic and emissivity measurements obtained from a polar vantage point is vital to complete our picture of the Sun's outer convection zone. Second, sustained and expanded investment in theory-oriented and combined theory/observational research initiatives will be crucial to fully exploit these new observations and to resolve inconsistencies between existing measurements.

An internally generated magnetic field once existed on the Moon. This field reached high intensities (∼10–100 μ T, perhaps intermittently) from ∼4.3 to 3.6 Gyr ago and then weakened to ≲5 μ T before dissipating by ∼1.9–0.8 Gyr ago. While the Moon’s metallic core could have generated a magnetic field via a dynamo powered by vigorous convection, models of a core dynamo often fail to explain the observed characteristics of the lunar magnetic field. In particular, the core alone may not contain sufficient thermal, chemical, or radiogenic energy to sustain the high-intensity fields for >100 Myr. A recent study by Scheinberg et al. suggested that a dynamo hosted in electrically conductive, molten silicates in a basal magma ocean (BMO) may have produced a strong early field. However, that study did not fully explore the BMO’s coupled evolution with the core. Here we show that a coupled BMO–core dynamo driven primarily by inner core growth can explain the timing and staged decline of the lunar magnetic field. We compute the thermochemical evolution of the lunar core with a 1D parameterized model tied to extant simulations of mantle evolution and BMO solidification. Our models are most sensitive to four parameters: the abundances of sulfur and potassium in the core, the core’s thermal conductivity, and the present-day heat flow across the core–mantle boundary. Our models best match the Moon’s magnetic history if the bulk core contains ∼6.5–8.5 wt% sulfur, in agreement with seismic structure models.

Enceladus is a dynamic icy moon of Saturn and a leading target for future planetary missions focused on the search for life beyond Earth. For such missions, instruments that can provide geophysical and geochemical context for ice shell and ocean processes are critical to evaluate whether conditions are suitable for life and biosignature detection. Radar sounding is a powerful geophysical technique to probe the thermophysical and chemical properties of icy moons, like Enceladus, and to investigate the subsurface context for the exchange of material and energy between their subsurface oceans, ice shells, and plumes. To inform the scientific potential and instrument performance demands of such a radar-sounding investigation of Enceladus' ice shell, we adapt and extend previous radar attenuation analysis done for Europa to the configuration and conditions of Enceladus. We also discuss how attenuation (both as an obstacle for the detection of ice shell reflectors and as a signal itself) can help constrain the thermal, physical, and chemical configuration of Enceladus' ice shell and reveal the processes governing the moon's ocean/shell/plume system.

Accreted ice retains and preserves traces of the ocean from which it formed. In this work, we study two classes of accreted ice found on Earth-frazil ice, which forms through crystallization within a supercooled water column, and congelation ice, which forms through directional freezing at an existing interface-and discuss where each might be found in the ice shells of ocean worlds. We focus our study on terrestrial ice formed in low temperature gradient environments (e.g., beneath ice shelves), consistent with conditions expected at the ice-ocean interfaces of Europa and Enceladus, and we highlight the juxtaposition of compositional trends in relation to ice formed in higher temperature gradient environments (e.g., at the ocean surface). Observations from Antarctic sub-ice-shelf congelation ice and marine ice show that the purity of frazil ice can be nearly two orders of magnitude higher than congelation ice formed in the same low temperature gradient environment (∼0.1% vs. ∼10% of the ocean salinity). In addition, where congelation ice can maintain a planar ice-water interface on a microstructural scale, the efficiency of salt rejection is enhanced (∼1% of the ocean salinity) and lattice soluble impurities such as chloride are preferentially incorporated. We conclude that an ice shell that forms by gradual thickening as its interior cools would be composed of congelation ice, whereas frazil ice will accumulate where the ice shell thins on local (rifts and basal fractures) or regional (latitudinal gradients) scales through the operation of an "ice pump."