Impact of seasonal fluctuations of ice velocity on decadal trends observed in Southwest Greenland

By tracking the feature displacement between satellite images spaced approximately one year apart, surface runoff has been shown to have a long-term impact on the average ice flow of a land-terminating sector of Greenland. In this study, we revisit the multi-year trends in ice flow by assessing more carefully the impact of seasonal fluctuation in velocity on the annual mean ice velocity. We find that, depending on the length and period used to measure displacement, seasonal fluctuations do have an impact on observed velocities on up to 15%, and can affect decadal trends. Nevertheless, the magnitude of this fluctuation is small enough to confirm the general slowdown observed during the 2000–2012 period. Between 2012 and 2019, we find significant re-acceleration of low-lying glaciers tongue but velocity trends elsewhere are generally insignificant and not spatially consistent. Finally, we propose a more selective approach to recovering velocity trends using satellite imagery that involves using only measurements where the image pair starting date is before summer, in order to have comparable measurements for every year, sampling a melt season and the following winter.publishedVersio

A large impact crater beneath Hiawatha Glacier in northwest Greenland.

We report the discovery of a large impact crater beneath Hiawatha Glacier in northwest Greenland. From airborne radar surveys, we identify a 31-kilometer-wide, circular bedrock depression beneath up to a kilometer of ice. This depression has an elevated rim that cross-cuts tributary subglacial channels and a subdued central uplift that appears to be actively eroding. From ground investigations of the deglaciated foreland, we identify overprinted structures within Precambrian bedrock along the ice margin that strike tangent to the subglacial rim. Glaciofluvial sediment from the largest river draining the crater contains shocked quartz and other impact-related grains. Geochemical analysis of this sediment indicates that the impactor was a fractionated iron asteroid, which must have been more than a kilometer wide to produce the identified crater. Radiostratigraphy of the ice in the crater shows that the Holocene ice is continuous and conformable, but all deeper and older ice appears to be debris rich or heavily disturbed. The age of this impact crater is presently unknown, but from our geological and geophysical evidence, we conclude that it is unlikely to predate the Pleistocene inception of the Greenland Ice Sheet

Traitement et Analyse des Données du Radar MARSIS/Mars Express

This manuscript describes the data processing and analysis of the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS). The data processing consists mainly to compensate the ionospheric distorsion. This correction provides as beneficial by-product to estimate the total electron content of the martian ionosphere. Using this results, we study in detail the martian ionosphere. We show that the remnant magnetic fied changes the spatial distribution of electrons in the ionosphere by accelerated the solar wind particles along magnetic field lines.The corrected radargrams provide the opportunity to study in detail the martian polar deposits. After the development of numerical tools to select the subsurface interfaces, we make a volume balance of the martian polar deposits. We show that the volume of the martian ice sheets is equivalent to a global water layer thickness of about 20 m.In a last part, we study the martian surface reflectivity. To do that, we extract the surface echo amplitude from each MARSIS pulse, and then, after calibrate it, we construct a global map of radar reflectivity (radar albedo). We describe the reflectivity map, firstly in a global point of view and secondly, more regionally around Medusae Fossae and the south residual cap of Mars. We show that the reflectivity decreases with the latitude, this constatation is probably linked to the presence of permafrost in the shallow subsurface. Near equator, we observe that the Medusae Fossae Formation corresponds to a dielectric constant of 2,4+/-0.5, which is carateristic of a porous and/or ice rich terrain. In the same region, we show that the dielectric constant of Elysium and Amazonis Planitia is equal to 7+/-1 and we observe a interface at about 140+/-20 m under the surface, our main conclusion is that these results are characterics of lava floods. Finally, the study of the reflectivity of south residual of Mars by a multi-layers model of reflectivity, shows that the CO2 layer covering this region has a thickness of about 11+/-1.5 m.Ce mémoire décrit le traitement et l'analyse des données du Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS). Le traitement de ces données a principalement consisté à compenser la distorsion ionosphérique. Cette correction a permis de réaliser une mesure indirecte du contenu électronique de l'ionosphère. Grâce à ces mesures, nous avons pu étudier en détail l'ionosphère martienne. Nous avons ainsi montré que le champ magnétique rémanent modifiait la distribution des électrons de l'ionosphère en précipitant les particules du vent solaire le long des lignes de champ radiales.Les radargrammes corrigés nous ont permis d'étudier en détail les calottes martiennes. Nous faisons le bilan du volume des calottes polaires de Mars en utilisant des outils numériques développés pour le pointage des interfaces. Nous montrons ainsi que le volume des calottes correspondrait à une couche d'eau d'environ 20 m d'épaisseur répartie sur toute la planète.Nous étudions enfin la réflectivité de la surface martienne. Pour cela, nous avons extrait l'amplitude de l'écho de surface de chaque pulse MARSIS, puis, après avoir calibré ces mesures, nous avons créé une carte globale de l'albédo radar. Nous nous sommes attachés à décrire cette carte de réflectivité, d'abord de manière globale, puis plus localement autour de Medusae Fossae et de la calotte résiduelle sud. Nous montrons que la réflectivité décroît avec la latitude, cette constatation est surement liée à la présence d'un pergélisol lorsqu'on remonte vers les hautes latitudes. Près de l'équateur, nous observons que les formations de Medusae Fossae possèdent une constante diélectrique de 2,4+/-0.5 ce qui est caractéristique d'un terrain poreux et/ou riche en glace. Dans cette même région, nous montrons que, dans les plaines d'Elysium et Amazonis, la constante diélectrique est égale à 7\pm1 et nous observons une interface dans la plaine d'Amazonis à environ 140+/-20 m, notre conclusion est que ces résultats sont caractéristiques d'écoulements de lave. L'étude de la calotte résiduelle sud de Mars, à l'aide d'un modèle de réflectivité multi-couches, nous permet d'estimer l'épaisseur de CO2 qui couvre cette région à 11+/-1.5 m

Traitement et analyse des données du radar MARSIS/Mars Express

Ce mémoire décrit le traitement et l'analyse des données du Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS). Le traitement de ces données a principalement consisté à compenser la distorsion ionosphérique. Cette correction a permis de réaliser une mesure indirecte du contenu électronique de l'ionosphère. Grâce à ces mesures, nous avons pu étudier en détail l'ionosphère martienne. Nous avons ainsi montré que le champ magnétique rémanent modifiait la distribution des électrons de l'ionosphère en précipitant les particules du vent solaire le long des lignes de champ radiales. Les radargrammes corrigés nous ont permis d'étudier en détail les calottes martiennes. Nous faisons le bilan du volume des calottes polaires de Mars en utilisant des outils numériques développés pour le pointage des interfaces. Nous montrons ainsi que le volume des calottes correspondrait à une couche d'eau d'environ 20 m d'épaisseur répartie sur toute la planète. Nous étudions enfin la réflectivité de la surface martienne. Pour cela, nous avons extrait l'amplitude de l'écho de surface de chaque pulse MARSIS, puis, après avoir calibré ces mesures, nous avons créé une carte globale de l'albédo radar. Nous nous sommes attachés à décrire cette carte de réflectivité, d'abord de manière globale, puis plus localement autour de Medusae Fossae et de la calotte résiduelle sud. Nous montrons que la réflectivité décroît avec la latitude, cette constatation est surement liée à la présence d'un pergélisol lorsqu'on remonte vers les hautes latitudes. Près de l'équateur, nous observons que les formations de Medusae Fossae possèdent une constante diélectrique de 2,4+/-0.5 ce qui est caractéristique d'un terrain poreux et/ou riche en glace. Dans cette même région, nous montrons que, dans les plaines d'Elysium et Amazonis, la constante diélectrique est égale à 7\pm1 et nous observons une interface dans la plaine d'Amazonis à environ 140+/-20 m, notre conclusion est que ces résultats sont caractéristiques d'écoulements de lave. L'étude de la calotte résiduelle sud de Mars, à l'aide d'un modèle de réflectivité multi-couches, nous permet d'estimer l'épaisseur de CO2 qui couvre cette région à 11+/-1.5 m.This manuscript describes the data processing and analysis of the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS). The data processing consists mainly to compensate the ionospheric distorsion. This correction provides as beneficial by-product to estimate the total electron content of the martian ionosphere. Using this results, we study in detail the martian ionosphere. We show that the remnant magnetic fied changes the spatial distribution of electrons in the ionosphere by accelerated the solar wind particles along magnetic field lines. The corrected radargrams provide the opportunity to study in detail the martian polar deposits. After the development of numerical tools to select the subsurface interfaces, we make a volume balance of the martian polar deposits. We show that the volume of the martian ice sheets is equivalent to a global water layer thickness of about 20 m. In a last part, we study the martian surface reflectivity. To do that, we extract the surface echo amplitude from each MARSIS pulse, and then, after calibrate it, we construct a global map of radar reflectivity (radar albedo). We describe the reflectivity map, firstly in a global point of view and secondly, more regionally around Medusae Fossae and the south residual cap of Mars. We show that the reflectivity decreases with the latitude, this constatation is probably linked to the presence of permafrost in the shallow subsurface. Near equator, we observe that the Medusae Fossae Formation corresponds to a dielectric constant of 2,4+/-0.5, which is carateristic of a porous and/or ice rich terrain. In the same region, we show that the dielectric constant of Elysium and Amazonis Planitia is equal to 7+/-1 and we observe a interface at about 140+/-20 m under the surface, our main conclusion is that these results are characterics of lava floods. Finally, the study of the reflectivity of south residual of Mars by a multi-layers model of reflectivity, shows that the CO2 layer covering this region has a thickness of about 11+/-1.5 m.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

Satellite-Derived Annual Glacier Surface Flow Velocity Products for the European Alps, 2015–2021

International audienceDocumenting glacier surface flow velocity from a longer-term perspective is highly relevant to evaluate the past and current state of glaciers worldwide. For this purpose, satellite data are widely used to obtain region-wide coverage of glacier velocity data. Well-established image correlation methods allow for the automated measurement of glacier surface displacements from satellite data (optical and radar) acquired at different dates. Although computationally expensive, image correlation is nowadays relatively simple to implement and allows two-dimensional displacement measurements. Here, we present a data set of annual glacier surface flow velocity maps at the European Alps scale, covering the period 2015–2021 at a 50 m × 50 m resolution. This data set has been quantified by applying the normalized cross-correlation approach on Sentinel-2 optical data. Parameters of the cross-correlation method (e.g., window size, sampling resolution) have been optimized, and the results have been validated by comparing them with in situ data on monitored glaciers showing an RMSE of 10 m/yr. These data can be used to evaluate glacier dynamics and its spatial and temporal evolution (e.g., quantify mass fluxes or calving) or can be used as an input for model calibration/validation or for the early detection of regional hazards associated with glacier destabilization

Optimization of Cross Correlation Algorithm for Annual Mapping of Alpine Glacier Flow Velocities; Application to Sentinel-2

International audienceNowadays, satellite observations cover most of the Earth’s surface in a repetitive manner. This information is crucial for documenting variability and environmental changes such as glacier surface velocity. With this in mind, digital image processing has been developed and improved over the past decades. The processing challenges are now related to optimizing parameters that account for the high variability of natural processes, as well as filtering and aggregating the results to provide useful products to end-users. Based on the normalized cross correlation (NCC) method applied to Sentinel-2 optical satellite observations up to 400 days apart, we present a series of tests to derive optimal parameter values for the quantification of alpine glacier ice velocity that we have applied to the Mont-Blanc massif where in situ measurements are available. We found that a search distance adapted to the temporal baseline, a 16×16 pixel window size, and a 5×5 pixels sampling provide an appropriate combination of parameters to process Sentinel-2 with the NCC method when applied to small alpine glaciers. Combining several spatial and temporal filters applied to a large set of more than 18000 displacement maps obtained between 2015 and 2021, then aggregating these filtered maps using statistical or linear regressions into annual maps, yields near-complete maps of the test region with a root mean square error (RMSE) reduced to about 10 m.yr −1 compared to in situ measurements

Ice velocity and thickness of the world's glaciers

International audienceThe effect of climate change on water resources and sea-level rise is largely determined by the size of the ice reservoirs around the world and the ice thickness distribution, which remains uncertain. Here, we present a comprehensive high-resolution mapping of ice motion for 98% of the world's total glacier area during the period 2017-2018. We use this mapping of glacier flow to generate an estimate of global ice volume that reconciles ice thickness distribution with glacier dynamics and surface topography. The results suggest that the world's glaciers have a potential contribution to sea-level rise of 257 ± 85 mm, which is 20% less than previously estimated. At low latitudes, our findings highlight notable changes in freshwater resources, with 37% more ice in the Himalayas and 27% less ice in the tropical Andes of South America, affecting water availability for local populations. This mapping of glacier flow and thickness redefines our understanding of global ice-volume distribution and has implications for the prediction of glacier evolution around the world, since accurate representations of glacier geometry and dynamics are of prime importance to glacier modelling

Un atlas mondial pour caractériser la réponse des glaciers au changement climatique

Cet article présente notre capacité actuelle à élaborer, à partir de données satellitaires, des produits relatifs aux glaciers à l'échelle globale afin de documenter leur réponse au changement climatique. Notre atlas cartographie, d'une part, les vitesses d'écoulement en surface pour caractériser la dynamique glaciaire et quantifier la répartition des épaisseurs de glace et, d'autre part, les changements d'altitude de la surface à l'échelle pluriannuelle pour quantifier les variations de volume des glaciers des 20 dernières années. Ces produits, issus de chaînes de traitement en grande partie automatisées, sont mis à disposition de la communauté par le biais du pôle national de données Theia. This article presents our current ability to derive global-scale glacier products from satellite data to document their response to climate change. We map, on the one hand, surface flow velocities to characterize glacier dynamics and quantify ice thickness distribution and, on the other hand, surface elevation changes to quantify glacier volume variations over the last 20 years. These products, derived from largely automated processing chains, are made available to the community through the Theia national data center

Surface Instability Mapping in Alpine Paraglacial Environments Using Sentinel-1 DInSAR Techniques

International audienceCurrent climate warming leads to widespread glacier shrinkage in high alpine terrains and associated changes in surface dynamics of deglacierized environments. In consequence, slope instabilities increasingly develop along retreating glaciers through debuttressing effects or degrading permafrost conditions. In the context of associated hazards to the local environment and infrastructure, a thorough analysis of slope instabilities is highly relevant. Affected regions are mostly inaccessible and cover large areas, therefore remote sensing techniques such as differential interferometric synthetic aperture radar (DInSAR) are valuable tools to monitor surface movements and assess their evolution. We apply standard and advanced DInSAR methods using Sentinel-1 SAR data from 2015 until late 2021 to map and classify slope instabilities in three glacierized regions in the European Alps. The final products include an inventory per region, with a total of 815 mapped slope instabilities, of which 38% move 30 cm/yr. An additional assessment of four landslides occurring along shrinking glaciers shows time series with recent accelerations in 2018/19. Validation of Sentinel-1 derived slope movement products is performed by comparison with shorter wavelength TerraSAR-X and optical Sentinel-2 derived data using offset tracking. Results clearly show the suitability of Sentinel-1 DInSAR methods to detect a range of slope movements in high alpine terrain, yet also highlight the limitations. We therefore recommend a combination of advanced Sentinel-1 DInSAR and Sentinel-2 offset tracking methods to develop a comprehensive inventory of alpine slope motion