Martian debris-covered glaciers (DCGs) contain large quantities of water ice beneath a protective layer of rock and dust. Properties of the overlying regolith such as density and depth to ice are critical parameters for guiding in situ resource utilization (ISRU) of water and coring missions targeting potential habitats. Englacial debris layers that progressively outcrop at the surface could also provide access to enable shallow sampling of ice age sequences. To assess the capabilities of future drone-based ground-penetrating radar for detecting supraglacial debris thickness and near-surface stratigraphy-properties not resolvable from orbit with the Shallow Radar sounder-we conducted tests over two terrestrial analogs for Martian DCGs. Our platform consists of a DJI Matrice 600 Pro drone and a MALA Geodrone radar operating at a center frequency of 80 MHz. We detected a bulk glacier thickness of up to 28.5 m in Sourdough rock glacier (RG), Alaska, and 48.6 m in Galena Creek RG, Wyoming. We also resolved the supraglacial debris thickness, with a mean thickness of 1.5 m in Sourdough, and in Galena Creek, with a mean thickness of 0.8 m in extensional sections and 1.3 m in compressional sections. Furthermore, we detected layers outcropping at the surface in age sequences within the cirque of Galena Creek RG. We validated the internal reflectors with clutter simulations to discriminate genuine subsurface reflectors from off-nadir surface reflection "clutter." Finally, we provide recommendations for optimal survey design, including solutions to increase the signal-to-noise ratio and reduce clutter.

The last (Wisconsinan) glacial period was punctuated in North America by two glacial maxima, known as the Early and Late Wisconsinan glaciations. In Alaska, these maxima and their subsequent retreats have been the object of dating efforts to reconstruct local climatic events and compare them to global trends. Little is known, however, about the period of milder climate and reduced glacier extents that separated the Early and Late Wisconsinan maxima, likely spanning a significant portion of Marine Isotope Stage 3 (MIS 3). Here we provide a detailed sedimentological analysis for an area at the northern margin of the Chitina River Valley in the Copper River Basin, Alaska. There, a buried proglacial sequence composed of glaciofluvial stream deposits, glaciolacustrine muds, and ice‐proximal subaqueous fan deposits reveals ice‐free conditions at ∼39 ka. Our palaeodepositional reconstructions show that the local glacier, a tributary to one of the largest ancient south‐central Alaskan ice streams, had retreated to near its modern terminus position during early to mid MIS 3, indicative of a stadial–interstadial transition with modern‐like glacier termini. We attribute differences in facies stacking patterns to drastic fluctuations in local base level, driven by the formation and drainage of a regional ice‐dammed lake (Palaeolake Atna). Our results offer new constraints on Late‐Pleistocene glacial fluctuations in south‐central Alaska, including new MIS 3 glacial minima, and provide insights into the relationships between climatic fluctuations, ice extents, and associated landscape evolution in the Copper River Basin.

Long-term historical records of glacier mass change are key to advancing understanding of glaciers' response to climate change and improving predictions of their future. Here, we use historical aerial photographs and new bed topography measurements to provide an 85-year record of glacier change on Kennicott and Root Glaciers in Alaska. At the glacier terminus, little change is observed in the two decades prior to 1957, followed by ongoing and accelerating mass loss with dynamically driven spatial variability. Glacier projections, constrained by these mass loss estimates, predict that Kennicott Glacier will lose 38 ± 14% to 63 ± 18% of its mass by 2100, relative to 2000, and Root Glacier will lose 38 ± 11% to 58 ± 12%, depending on the emissions scenario. These results differ by up to 22% from similar predictions made by projections calibrated from the past two decades of glacier change only. This highlights the importance of long-term glacier mass-loss records that help us better project far-reaching consequences of climate change related to sea level rise, water resources, natural hazards, climate, and culture.

Sít' Tlein (Malaspina Glacier), located in Southeast Alaska, has a complex flow history. This piedmont glacier, the largest in the world of its kind, is fed by three main tributaries that all exhibit similar flow patterns, yet with varying surge cycles. The piedmont lobe is dramatically reshaped by surges that occur at approximately decadal timescales. By combining historical accounts with modern remote sensing data we derive a surge history over the past century. We leverage the Stochastic Matrix Factorization, a novel data analysis and interpolation technique, to process and interpret large datasets of glacier surface velocities. A variant of the Principal Component Analysis allows us to uncover spatial and temporal patterns in ice dynamics. We show that Sít' Tlein displays a wide range of behaviors, spanning quiescence to surge with seasonal to decadal variations of ice flow direction and magnitude. We find that surges dominate the velocity dataset's variance (spanning 1984 to 2021), while seasonal variations represent a much smaller part of the variance. However, despite the regular surge pulses, the glacier lobe is far from equilibrium, and widespread retreat of the glacier is inevitable, even without further climate warming.

S & iacute;t' Tlein, located in the St. Elias Range, which straddles Alaska's Wrangell-St. Elias National Park and Kluane National Park in the Yukon, is the world's largest piedmont glacier. S & iacute;t' Tlein has thinned considerably over 30 years of altimetry, yet its low-elevation piedmont lobe has remained intact in contrast to the glaciers that once filled neighboring Icy and Disenchantment bays. In an effort to forecast changes to S & iacute;t' Tlein over decadal to centennial timescales, we take a data-constrained dynamical modeling approach in which we infer the parameters of a higher-order model of ice flow - the bed elevation, basal traction, and surface mass balance - with a diverse but spatiotemporally sparse set of observations including satellite-derived, time-varying velocity fields; radar-derived bed and surface elevation measurements; and in situ and remotely sensed observations of accumulation and ablation. Nonetheless, such data do not uniquely constrain model behavior, so we adopt an approximate Bayesian approach based on the Laplace approximation and facilitated by low-rank parametric representations to quantify uncertainty in the bed, traction, and mass balance fields alongside the induced uncertainty in model-based predictions of glacier change. We find that S & iacute;t' Tlein is considerably out of balance with contemporary (and presumably future) climate, and we expect its piedmont lobe to largely disappear over the coming centuries. If warming ceases, and surface mass balance remains at 2023 levels, then by 2073 (2173) we forecast a mass loss (expressed in terms of 95 % credible interval) of 323-444 km3 (546-728 km3). If instead surface mass balance continues to change at the same rate as inferred over the historical period, then we forecast a 2073 (2173) mass loss of 383-505 km3 (740-900 km3). In either case, the resulting retreat and subsequent replacement of glacier ice with a marine embayment or lake will yield a significant modification to the regional landscape and ecosystem.

The Shallow Radar (SHARAD) instrument aboard Mars Reconnaissance Orbiter (MRO) has been conducting sounding operations of Mars' near surface for more than 16 years. Results of the SHARAD investigation have been well documented in the literature, with relatively high coverage density in the polar regions leading to some of the earliest scientific discoveries there. Data collection and ensuing surprises of SHARAD's primary and first extended science phases were the impetus for choosing the north polar region for producing a first 3D radar image. This early attempt was successful, producing the first 3D radar image of Mars' north polar layered deposits in Planum Boreum, and shortly thereafter a companion 3D radar image of the south polar layered deposits in Planum Australe. Subsequent work refined the 3D methodology used to produce these images, leading to a significantly higher quality 3D image of the former as well as the first 3D radar image of the debris-covered glaciers in the east-central portion of Deuteronilus Mensae (DM) in the mid-latitudes. Prior experience in terrestrial seismic data processing and analysis methods forms the basis of the 3D methodology used with Martian radar data, and has been indispensable in ongoing efforts to improve this methodology to further clarify the 3D images of the targets. The purposes of this article are to review 1) the SHARAD 3D work history and results, 2) the methodology developed and challenges encountered thus far in producing the SHARAD 3D images, 3) the broader impact on the processing and analysis of SHARAD data, and 4) current efforts and plans for producing follow-on SHARAD 3D images. Similarities and differences between orbital radar and seismic sounding and data processing are sprinkled throughout the article as reminders that the outcomes in this case are very much the product of cross-disciplinary knowledge and experience.

Abstract. Sít' Tlein in Alaska's St. Elias Range (briefly known as Malaspina Glacier) is the world's largest piedmont glacier and has thinned considerably over 30 years of altimetry, yet it's low-elevation piedmont lobe has remained intact in contrast to the glaciers that once filled neighboring Icy and Disenchantment bays. In an effort to forecast changes to Síit' Tlein over decadal to centennial time scales, we take a data-constrained dynamical modelling approach, in which we constrain the parameters of a higher order model of ice flow – the bed elevation, basal traction, and surface mass balance – with a diverse but spatio-temporally sparse set of observations including satellite-derived time-varying velocity fields, radar-derived bed and surface elevation measurements, and in situ and remotely sensed observations of accumulation and ablation. Nonetheless, such data do not uniquely constrain model behavior, so we adopt an approximate Bayesian approach based on the Laplace approximation and facilitated by low-rank parametric representations to quantify uncertainty in the bed, traction, and mass balance fields alongside the induced uncertainty in model-based predictions of glacier change. We find that Sít' Tlein is considerably out of balance with contemporary (and presumably future) climate, and we expect its piedmont lobe to largely disappear over the coming 150 years. We forecast a total mass loss at Sít' Tlein of between 500 and 1000 km3 of ice, a range that represents not only uncertainty in model inputs, but also in future warming scenarios. The resulting retreat and subsequent replacement of glacier ice with a marine embayment or lake will yield a significant modification to the regional landscape and ecosystem.

Ruth Glacier is situated in the Central Alaska Range, with the Don Sheldon Amphitheater comprising much of its broad accumulation area, directly adjacent to North America's tallest mountain, Denali. From there it funnels through the ‘Great Gorge,’ flanked by steep valley walls reaching over 1500 m. We combine airborne and ground-based radar measurements of ice thickness with satellite-derived surface velocities to constrain ice flux above and below the gorge, and employ a mass conservation approach to estimate the glacier's thickness within the gorge. We measure ice thickness in the amphitheater to reach 950 m, and estimate centerline thickness in the gorge to range from 610 to 960 m. Our estimates are up to two times greater than those suggested by global models, and allow us to confirm that the Great Gorge rivals Hells Canyon as the deepest gorge in North America. We found that the geometry of the gorge prevents radar measurements of ice thickness there since returns from the subglacial valley walls would precede and potentially occlude nadir bed returns. The same may be true of other unmapped mountain glaciers; however, thickness may be determined using appropriately located flux gates where radar sounding is feasible, combined with mass conservation methods.

Since November 2006, the Shallow Radar (SHARAD) aboard the U.S. National Aeronautics and Space Administration's (NASA's) Mars Reconnaissance Orbiter (MRO) has been conducting subsurface sounding operations from orbit around Mars. This extended campaign has provided tens of thousands of radar profiles of Mars shallow subsurface, with coverage density in some regions having become sufficient for performing three-dimensional (3D) imaging. Adapting methods and tools used to produce, analyze, and interpret terrestrial seismograms, we have produced and studied fully imaged 3D radargrams in Mars' polar and mid-latitudes regions. In this report, we provide some background on the SHARAD instrument, summarize the methods and tools used in creating 3D radargrams from SHARAD data, and present example views from the latest 3D radargram in the north polar region known as Planum Boreum (PB).

The availability of remote sensing imagery at high spatiotemporal resolutions presents the opportunity to monitor the surface motion of rock glaciers, a key constraint for characterizing the dynamics of their evolution. In this paper, we investigate four North American rock glaciers by automatically measuring their horizontal surface displacement using photogrammetric data acquired with crewed and uncrewed aircraft along with orbital spacecraft over monitoring periods of up to eight years. We estimate vertical surface changes on these rock glaciers with photogrammetrically generated digital elevation models (DEM) and digitized topographic maps. Uncertainty analysis shows that the imagery with the highest resolution and most precise positioning have the best performance when used with the automated change detection algorithm. This investigation produces gridded velocity fields over the entire surface area of each study site, from which we estimate the age of rock glacier formation using along-flow velocity integration. Though the age estimates vary, the ice within the modern extent of these landforms began flowing between 3000 and 7000 years before present, postdating the last glacial maximum. Surface elevation change maps indicate present-day thinning at the lower latitude/higher elevation sites in Wyoming, while the higher latitude/lower elevation sites in Alaska exhibit relatively stable surface elevations.

Elysium Planitia includes several outflow channels that were likely carved by aqueous erosion and subsequently infilled by younger lava flows, making Elysium Planitia the youngest volcanic terrain on Mars. Studying this region is critical for constraining the recent hydrological and thermal evolution of the planet. Here, we investigate the lava flow areas, thicknesses, and volumes in Elysium Planitia using Context (CTX) camera images in combination with SHAllow RADar (SHARAD) sounder data. Compiling 1,777 reflectors over an area of 9,126,790 km2 allows us to reconstruct the subsurface landscape evolution over time. Our findings show that Elysium Planitia is composed of material from about 40 episodes of effusive volcanic activity. We report volumes for individual eruptions of 4,000 +/- 1,600 km3 infilling Athabasca Valles, 12,200 +/- 2,500 km3 in Marte Vallis, and 16,000 +/- 4,000 km3 in Rahway Valles for the major flow units and volumes as small as 100 +/- 50 km3 in Cerberus Plains. The surface morphologies and inferred dielectric properties of lobe interfaces suggests that the regions consists of basaltic lava. The region also experienced multiple aqueous flooding events. Although, we found evidence of past lava-water interactions, present-day ground-ice (if present) is likely limited to local patches. Further, the pre-eruption landscape reveals that the aqueously carved Marte Vallis is more areal extensive, but shallower than previously suggested, with a likely paleo-flow direction from northwest to southeast. The channel is most likely sourced from a segment in the northwestern portion of Cerberus Fossae, and is now buried by multiple Late Amazonian lavas with the same lava flow direction. Elysium Planitia on Mars has a fascinating history of water and lava flows that shaped its landscape. It is the youngest volcanic terrain on the planet, and studying it helps us to better understand Mars' past as well as recent hydrological and volcanic history. We examined this region by using spacecraft images and radar data to constrain areas, thicknesses, and volumes. An area almost as large as Europe was investigated. The study revealed the products of more than 40 volcanic events, with one of the largest flows infilling Athabasca Valles with a volume of 4,000 km3. The surface appearance and material properties suggest that Elysium Planitia is composed of basalt, the most common type of lava on Earth. The area also experienced several large floods of water, and there is evidence that lava and water interacted in the past. However, while there could be ice in the ground today, it likely occurs in small patches. The study also provides new insights into the Marte Vallis outflow channel. It seems to be larger, but not as deep as previously thought, with water flowing from northwest to southeast and fed from a fissure in the northwest. Marte Vallis was later covered by several lava layers. We performed detailed surface and subsurface mapping of the entire Elysium Planitia region to constrain lava areas, thicknesses, and volumesElysium Planitia is composed of the products of about 40 effusive eruptions including large flood lava flows and lava shieldsResults indicate that there is no singular direction in dike propagation

Hebrus Valles is an outflow channel system in the plain-forming terrains of southeastern Utopia Planitia, Mars. These terrains may have formed through a combination of liquid water and volcanic processes, yet their nature, subsurface structure, and composition remain unclear. We investigate these terrains by mapping subsurface reflectors across 540 Shallow Radar (SHARAD) profiles and applying two complementary loss tangent inversion techniques. We find moderate loss tangent values across some subregions of Granicus Valles and Hyblaeus Fossae (tan δ = 0.0162 ± 0.0004 and tan δ = 0.019 ± 0.002, respectively), suggesting the presence of basaltic lava flows. We interpret non-detections in the other flows in Granicus Valles to be due to the presence of radar-lossy materials formed through aqueous processes, which supports the hypothesized occurrence of lahars in this region. A small area near Hebrus Valles exhibits subsurface reflectors with low to moderate loss tangents (tan δ = 0.010 ± 0.003), suggesting the presence of pristine lava flows or sedimentary materials capped by lava flows. We also find a widespread occurrence of very low-loss tangent materials near Hyblaeus Dorsa (tan δ = 0.0045 ± 0.0002), which may represent a lobe of the Medusae Fossae Formation or similar high-porosity materials buried underneath a lava flow. Together, these findings suggest that volcanic activity played a central role in the formation of terrains across the broader Hebrus Valles region.

Malaspina Glacier, located on the coast of southern Alaska, is the world's largest piedmont glacier. A narrow ice-cored foreland zone undergoing rapid thermokarst erosion separates the glacier from the relatively warm waters of the Gulf of Alaska. Glacier-wide thinning rates for Malaspina are greater than 1 m/yr, and previous geophysical investigations indicated that bed elevation exceeds 300 m below sea level in some places. These observations together give rise to the question of glacial stability. To address this question, glacier evolution models are dependent upon detailed observations of Malaspina's subglacial topography. Here, we map 2,000 line-km of the glacier's bed using airborne radar sounding data collected by NASA's Operation IceBridge. When compared to gridded radar measurements, we find that glaciological models overestimate Malaspina's volume by more than 30%. While we report a mean bed elevation 100 m greater than previous models, we find that Malaspina inhabits a broad basin largely grounded below sea level. Several subglacial channels dissect the glacier's bed: the most prominent of these channels extends at least 35 km up-glacier from the terminus toward the throat of Seward Glacier. Provided continued foreland erosion, an ice-ocean connection may promote rapid retreat along these overdeepened subglacial channels, with a global sea-level rise potential of 1.4 mm.

We present first results from a new 3D radargram produced from 3399 Mars Reconnaissance Orbiter (MRO) Shallow Radar observations of the north polar region of Mars. While incorporating an additional 5 yr of observations relative to the prior 3D radargram, we employed surface-clutter simulations to improve the coregistration of the input data and thereby enhance the effective vertical resolution of features. Combining those improvements with the geometric corrections and an increase in signal-to-noise ratio afforded by the 3D imaging process, this data product provides new details about the interior of Planum Boreum, the Martian north polar cap. We assess the overall characteristics and compare portions of the new 3D radargram to results from prior studies that used either the prior 3D radargram or sets of 2D radargrams from individual MRO orbits. We find that the new 3D radargram has recovered essentially all of the vertical resolution inherent to the input data, and the increased coverage density has substantially reduced artifacts while enabling much greater detail in the imaging of subsurface layering and structures. These improvements extend throughout the 3D radargram, from the basal units to the shallowest subsurface layering in Planum Boreum, and out into the surrounding plains. Subsurface features such as a buried chasma, other layering structures and unconformities, and trough-bounding surfaces that offset shallow layering are now visible in unprecedented detail. A thorough analysis of this new 3D radargram and its implications for the geologic and climate history of Planum Boreum will extend over many years.

Climate change-induced glacier retreat can have substantial localized impacts that often go unnoticed in the sparsely populated regions where they occur. Here we predict that retreat of Grand Plateau Glacier, in southern Alaska, USA, will reroute the outlet of a major river with consequences for human activity in this remote region. The glacier terminus separates Alsek Lake and the present Alsek River outlet from Grand Plateau Lake. In response to thinning and retreat of that terminus, both lakes have more than doubled in size since 1958. Laser altimetry shows that terminus thinning continued at rates of up to 10 m/yr from 2017 to 2020. Radar soundings show that the bed of the thinning glacier terminus extends to >400 m below sea level, and that the two lakes will become conjoined within at most a few decades as the terminus further retreats. We predict that Alsek River will then abandon its present Dry Bay outlet channel in favor of the much steeper outlet of Grand Plateau Lake, 28 km to the southeast. Anadromous fish and associated predators in the lower Alsek will need to adapt to this change. Traditional and modern human activities centered on Dry Bay include commercial fishing, subsistence and sport hunting and fishing, and the finishing point for a world-renowned wilderness rafting expedition. Under present management guidelines, those activities cannot be relocated to the predicted future outlet, which sits within the federally designated wilderness of Glacier Bay National Park. (c) 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).