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The great lakes of Europa The Galileo spacecraft revealed a number of 'chaos' regions on Jupiter's moon Europa, where the surface terrain appears to have been disrupted from below. In many places, the surface contains sharp-edged blocks or rafts of ice that have at some point been flipped or rotated. Some characteristics of these regions have been hard to explain, such as the fact that the archetypal Conamara Chaos stands above its surroundings and contains matrix domes. Schmidt et al . apply lessons learned from analogous processes within Earth's subglacial volcanoes and ice shelves to an analysis of archival data that suggests chaos terrain forms above liquid water 'lenses' that are perched only 3 kilometres deep within the ice shell. The data suggest that ice–water interactions and freeze-out give rise to the varied morphology of chaos terrains, implying that more water is involved than has been previously appreciated — for instance, the sunken topography of Thera Macula, a large chaos area, may indicate that Europa is actively resurfacing over a lens comparable in volume to North America's Great Lakes. Europa, the innermost icy satellite of Jupiter, has a tortured young surface 1 , 2 , 3 , 4 and sustains a liquid water ocean 1 , 2 , 3 , 4 , 5 , 6 below an ice shell of highly debated thickness 1 , 2 , 3 , 4 , 5 , 7 , 8 , 9 , 10 . Quasi-circular areas of ice disruption called chaos terrains are unique to Europa, and both their formation and the ice-shell thickness depend on Europa's thermal state 1 , 2 , 3 , 4 , 5 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 . No model so far has been able to explain why features such as Conamara Chaos stand above surrounding terrain and contain matrix domes 10 , 18 . Melt-through of a thin (few-kilometre) shell 3 , 7 , 8 is thermodynamically improbable and cannot raise the ice 10 , 18 . The buoyancy of material rising as either plumes of warm, pure ice called diapirs 1 , 9 , 10 , 11 , 12 , 13 , 14 , 15 or convective cells 16 , 17 in a thick (>10 kilometres) shell is insufficient to produce the observed chaos heights, and no single plume can create matrix domes 10 , 18 . Here we report an analysis of archival data from Europa, guided by processes observed within Earth's subglacial volcanoes and ice shelves. The data suggest that chaos terrains form above liquid water lenses perched within the ice shell as shallow as 3 kilometres. Our results suggest that ice–water interactions and freeze-out give rise to the diverse morphologies and topography of chaos terrains. The sunken topography of Thera Macula indicates that Europa is actively resurfacing over a lens comparable in volume to the Great Lakes in North America.

On Jupiter’s icy moon Europa, enigmatic chaos terrain—where the icy crust is cut by a jumble of ridges and cracks—occurs most commonly at lower latitudes. Simulations of convection in the ocean underlying Europa’s icy crust suggest that ocean dynamics can control an enhanced flow of heat to Europa’s equatorial surface, and hence geological activity. The ice shell of Jupiter’s moon Europa is marked by regions of disrupted ice known as chaos terrains that cover up to 40% of the satellite’s surface, most commonly occurring within 40° of the equator 1 . Concurrence with salt deposits 2 implies a coupling between the geologically active ice shell and the underlying liquid water ocean at lower latitudes. Europa’s ocean dynamics have been assumed to adopt a two-dimensional pattern 3 , 4 , 5 , 6 , 7 , 8 , which channels the moon’s internal heat to higher latitudes. Here we present a numerical model of thermal convection in a thin, rotating spherical shell where small-scale convection instead adopts a three-dimensional structure and is more vigorous at lower latitudes. Global-scale currents are organized into three zonal jets and two equatorial Hadley-like circulation cells. We find that these convective motions transmit Europa’s internal heat towards the surface most effectively in equatorial regions, where they can directly influence the thermo-compositional state and structure of the ice shell. We suggest that such heterogeneous heating promotes the formation of chaos features through increased melting of the ice shell and subsequent deposition of marine ice at low latitudes. We conclude that Europa’s ocean dynamics can modulate the exchange of heat and materials between the surface and interior and explain the observed distribution of chaos terrains.

Antarctica's Getz Ice Shelf has been rapidly thinning in recent years, producing more meltwater than any other ice shelf in the world. The influx of fresh water is known to substantially influence ocean circulation and biological productivity, but relatively little is known about the factors controlling basal melt rate or how basal melt is spatially distributed beneath the ice shelf. Also unknown is the relative importance of subglacial discharge from the grounded ice sheet in contributing to the export of fresh water from the ice shelf cavity. Here we compare the observed spatial distribution of basal melt rate to a new sub-ice-shelf bathymetry map inferred from airborne gravity surveys and to locations of subglacial discharge from the grounded ice sheet. We find that melt rates are high where bathymetric troughs provide a pathway for warm Circumpolar Deep Water to enter the ice shelf cavity and that melting is enhanced where subglacial discharge fresh water flows across the grounding line. This is the first study to address the relative importance of meltwater production of the Getz Ice Shelf from both ocean and subglacial sources.

We present Bedmap2, a new suite of gridded products describing surface elevation, ice-thickness and the seafloor and subglacial bed elevation of the Antarctic south of 60° S. We derived these products using data from a variety of sources, including many substantial surveys completed since the original Bedmap compilation (Bedmap1) in 2001. In particular, the Bedmap2 ice thickness grid is made from 25 million measurements, over two orders of magnitude more than were used in Bedmap1. In most parts of Antarctica the subglacial landscape is visible in much greater detail than was previously available and the improved data-coverage has in many areas revealed the full scale of mountain ranges, valleys, basins and troughs, only fragments of which were previously indicated in local surveys. The derived statistics for Bedmap2 show that the volume of ice contained in the Antarctic ice sheet (27 million km3) and its potential contribution to sea-level rise (58 m) are similar to those of Bedmap1, but the mean thickness of the ice sheet is 4.6% greater, the mean depth of the bed beneath the grounded ice sheet is 72 m lower and the area of ice sheet grounded on bed below sea level is increased by 10%. The Bedmap2 compilation highlights several areas beneath the ice sheet where the bed elevation is substantially lower than the deepest bed indicated by Bedmap1. These products, along with grids of data coverage and uncertainty, provide new opportunities for detailed modelling of the past and future evolution of the Antarctic ice sheets.

Totten Glacier has the largest thinning rate in East Antarctica. A derivation of the sea floor bathymetry reveals entrances to the ice cavity beneath the glacier that could allow deep warm water to enter and enhance basal melting. Totten Glacier, the primary outlet of the Aurora Subglacial Basin, has the largest thinning rate in East Antarctica 1 , 2 . Thinning may be driven by enhanced basal melting due to ocean processes 3 , modulated by polynya activity 4 , 5 . Warm modified Circumpolar Deep Water, which has been linked to glacier retreat in West Antarctica 6 , has been observed in summer and winter on the nearby continental shelf beneath 400 to 500 m of cool Antarctic Surface Water 7 , 8 . Here we derive the bathymetry of the sea floor in the region from gravity 9 and magnetics 10 data as well as ice-thickness measurements 11 . We identify entrances to the ice-shelf cavity below depths of 400 to 500 m that could allow intrusions of warm water if the vertical structure of inflow is similar to nearby observations. Radar sounding reveals a previously unknown inland trough that connects the main ice-shelf cavity to the ocean. If thinning trends continue, a larger water body over the trough could potentially allow more warm water into the cavity, which may, eventually, lead to destabilization of the low-lying region between Totten Glacier and the similarly deep glacier flowing into the Reynolds Trough. We estimate that at least 3.5 m of eustatic sea level potential drains through Totten Glacier, so coastal processes in this area could have global consequences.

Thwaites Glacier is one of the largest, most rapidly changing glaciers on Earth, and its landward-sloping bed reaches the interior of the marine West Antarctic Ice Sheet, which impounds enough ice to yield meters of sea-level rise. Marine ice sheets with landward-sloping beds have a potentially unstable configuration in which acceleration can initiate or modulate grounding-line retreat and ice loss. Subglacial water has been observed and theorized to accelerate the flow of overlying ice dependent on whether it is hydrologically distributed or concentrated. However, the subglacial water systems of Thwaites Glacier and their control on ice flow have not been characterized by geophysical analysis. The only practical means of observing these water systems is airborne ice-penetrating radar, but existing radar analysis approaches cannot discriminate between their dynamically critical states. We use the angular distribution of energy in radar bed echoes to characterize both the extent and hydrologic state of subglacial water systems across Thwaites Glacier. We validate this approach with radar imaging, showing that substantial water volumes are ponding in a system of distributed canals upstream of a bedrock ridge that is breached and bordered by a system of concentrated channels. The transition between these systems occurs with increasing surface slope, melt-water flux, and basal shear stress. This indicates a feedback between the subglacial water system and overlying ice dynamics, which raises the possibility that subglacial water could trigger or facilitate a grounding-line retreat in Thwaites Glacier capable of spreading into the interior of the West Antarctic Ice Sheet.

Accurate quantification of Antarctic ice-sheet mass balance and its contribution to global sea-level rise remains challenging. Gravity Recovery and Climate Experiment data spanning the period April 2002 to January 2009 confirm earlier estimates of ice loss for Antarctica and indicate that East Antarctica started losing mass in about 2006. Accurate quantification of Antarctic ice-sheet mass balance and its contribution to global sea-level rise remains challenging, because in situ measurements over both space and time are sparse. Satellite remote-sensing data of ice elevations and ice motion show significant ice loss in the range of −31 to −196 Gt yr −1 in West Antarctica in recent years 1 , 2 , 3 , 4 , whereas East Antarctica seems to remain in balance or slightly gain mass 1 , 2 , 4 , with estimated rates of mass change in the range of −4 to 22 Gt yr −1 . The Gravity Recovery and Climate Experiment 5 (GRACE) offers the opportunity of quantifying polar ice-sheet mass balance from a different perspective 6 , 7 . Here we use an extended record of GRACE data spanning the period April 2002 to January 2009 to quantify the rates of Antarctic ice loss. In agreement with an independent earlier assessment 4 , we estimate a total loss of 190±77 Gt yr −1 , with 132±26 Gt yr −1 coming from West Antarctica. However, in contrast with previous GRACE estimates, our data suggest that East Antarctica is losing mass, mostly in coastal regions, at a rate of −57±52 Gt yr −1 , apparently caused by increased ice loss since the year 2006.

Radar sounding is a powerful geophysical approach for characterizing the subsurface conditions of terrestrial and planetary ice masses at local to global scales. As a result, a wide array of orbital, airborne, ground-based, and in situ instruments, platforms and data analysis approaches for radioglaciology have been developed, applied or proposed. Terrestrially, airborne radar sounding has been used in glaciology to observe ice thickness, basal topography and englacial layers for five decades. More recently, radar sounding data have also been exploited to estimate the extent and configuration of subglacial water, the geometry of subglacial bedforms and the subglacial and englacial thermal states of ice sheets. Planetary radar sounders have observed, or are planned to observe, the subsurfaces and near-surfaces of Mars, Earth's Moon, comets and the icy moons of Jupiter. In this review paper, and the thematic issue of the Annals of Glaciology on ‘Five decades of radioglaciology’ to which it belongs, we present recent advances in the fields of radar systems, missions, signal processing, data analysis, modeling and scientific interpretation. Our review presents progress in these fields since the last radio-glaciological Annals of Glaciology issue of 2014, the context of their history and future prospects.

Geophysical and geological data reveal increased ice-sheet variability and surface meltwater—possibly analogous to future conditions—offshore of the Aurora subglacial basin of East Antarctica during warm climate intervals of the past 50 million years. The history of an ice sheet The East Antarctic Ice Sheet contains the water equivalent of approximately 50 metres of sea-level rise. Understanding the past behaviour of this mass of ice is therefore of much interest in times of a rapidly warming climate. Sean Gulick, Amelia Shevenell and colleagues show that the Aurora subglacial basin of East Antarctica had marine-terminating glaciers by the early to middle Eocene epoch. Over millions of years, the amount of ice in the area repeatedly waxed and waned, but then stabilized by the cooler late Miocene and even through the warmth of the Pliocene. The study's geochemical and radar data suggest that the combination of warmer temperatures and increased surface meltwater—conditions that will possibly occur again in the future—influenced past ice sheet retreats. Antarctica’s continental-scale ice sheets have evolved over the past 50 million years 1 , 2 , 3 , 4 . However, the dearth of ice-proximal geological records 5 , 6 , 7 , 8 limits our understanding of past East Antarctic Ice Sheet (EAIS) behaviour and thus our ability to evaluate its response to ongoing environmental change. The EAIS is marine-terminating and grounded below sea level within the Aurora subglacial basin, indicating that this catchment, which drains ice to the Sabrina Coast, may be sensitive to climate perturbations 9 , 10 , 11 . Here we show, using marine geological and geophysical data from the continental shelf seaward of the Aurora subglacial basin, that marine-terminating glaciers existed at the Sabrina Coast by the early to middle Eocene epoch. This finding implies the existence of substantial ice volume in the Aurora subglacial basin before continental-scale ice sheets were established about 34 million years ago 1 , 2 , 3 , 4 . Subsequently, ice advanced across and retreated from the Sabrina Coast continental shelf at least 11 times during the Oligocene and Miocene epochs. Tunnel valleys 12 associated with half of these glaciations indicate that a surface-meltwater-rich sub-polar glacial system existed under climate conditions similar to those anticipated with continued anthropogenic warming 10 , 11 . Cooling since the late Miocene 13 resulted in an expanded polar EAIS and a limited glacial response to Pliocene warmth in the Aurora subglacial basin catchment 14 , 15 , 16 . Geological records from the Sabrina Coast shelf indicate that, in addition to ocean temperature, atmospheric temperature and surface-derived meltwater influenced East Antarctic ice mass balance under warmer-than-present climate conditions. Our results imply a dynamic EAIS response with continued anthropogenic warming and suggest that the EAIS contribution to future global sea-level projections 10 , 11 , 15 , 17 may be under-estimated.

Fluid flow through fractured media is typically governed by the distribution of fracture apertures, which are in turn governed by stress. Consequently, understanding subsurface stress is critical for understanding and predicting subsurface fluid flow. Although laboratory‐scale studies have established a sensitive relationship between effective stress and bulk electrical conductivity in crystalline rock, that relationship has not been extensively leveraged to monitor stress evolution at the field scale using electrical or electromagnetic geophysical monitoring approaches. In this paper we demonstrate the use time‐lapse 3‐dimensional (4D) electrical resistivity tomography to image perturbations in the stress field generated by pressurized borehole packers deployed during shear‐stimulation attempts in a 1.25 km deep metamorphic crystalline rock formation. Plain Language Summary Time‐lapse electrical geophysical sensing is used to image 3D changes in rock stress generated by an isolated and pressurized interval of a borehole in a deep, dense, fractured rock formation. Key Points Remotely monitoring stress is challenging but important for relating geomechanical behavior to flow pathways during energy production Bulk electrical conductivity is sensitive to stress in crystalline rock Time‐lapse electrical resistivity tomography can be used to remotely monitor 3D changes in effective stress