Deeply hidden beneath the kilometer-thick ice of Antarctica are rocky landscapes with mountains, valleys and deeply cut former river courses. This is shown by a study that used satellite data and ice flow calculations to map the ground beneath the ice with a level of accuracy never before achieved. In addition to providing insights into one of the least mapped planetary surfaces in the inner solar system, the results may help better predict the future dynamics of the Antarctic ice sheet.
The Antarctic ice sheet covers an area of more than 14 million square kilometers and hides the underlying geological structures. But these not only reveal a lot about the history of the continent, but also influence how the ice sheet behaves today and in the future. Because depending on how the landscape is shaped, the ice above it moves differently, especially as it begins to melt. The resulting tensions can, in turn, cause some parts to break off more quickly and thus unexpectedly cause further sea level rise, or to be kept inland for longer.
Yet we know less about the Antarctic topography beneath the ice than we know about the surface of Mars. Previous geophysical investigations have usually not been carried out systematically across the entire region, but sometimes leave gaps of 100 kilometers. “In order to predict future sea level with as little uncertainty as possible, a resolution on a kilometer scale is required,” explains a team led by Helen Ockenden from the University of Grenoble in France.
Insights into the past and future
Ockenden and her colleagues have now combined high-resolution satellite data with measurements of ice thickness and physical calculations of ice flow to create the most accurate map yet of the landforms beneath the Antarctic ice sheet. With a spatial resolution of two to 30 kilometers, the results reveal extensive landscapes of mountains, valleys, plains, basins, rivers and lakes. Many of these features were probably formed at a time when Antarctica was still ice-free and liquid water was making its way.
“Perhaps the most surprising thing is that so many details of the ground topography are ultimately visible in the shape of the ice surface high above,” says co-author Robert Bingham from the University of Edinburgh. “The changes on the surface are extremely subtle: When ice three kilometers thick flows over a subglacial gorge perhaps 100 meters deep, the height of the ice surface usually drops by only a few meters, a change that is hardly noticeable when walking on the ice surface itself.” However, these differences are visible in the satellite data and, in combination with physical calculations, allow conclusions to be drawn about the structure of the soil.
Guide for future investigations
Geophysicist Duncan Young of the University of Texas at Austin writes in a commentary accompanying the study, also published in the journal Science, that the new findings help better understand the history of the world’s largest ice sheet and predict changes in the volume of the cryosphere. “However, this map is not the final word on the geography of Antarctica,” says Young. The research team often had to rely on assumptions about the interactions between ice and subsurface, which in reality are probably much more complex than in the models.
In addition, the resolution was limited by the fact that the methods used could only capture structures that were at least as large as the thickness of the ice cover at the respective location. For example, a gorge can only be identified under a two-kilometer-thick layer of ice if it is also at least two kilometers long.
In order to record the nature of the subglacial subsurface more reliably and precisely, further investigations are necessary. But the current study can also help with this: “Our landscape classification and our topographic map serve as important guidelines for more targeted studies of the subglacial landscape of Antarctica,” write Ockenden and her team. “They provide information about where future detailed geophysical surveys should be conducted and what extent and resolution are required to capture the fine details necessary for ice flow modeling.”
Source: Helen Ockenden (Université Grenoble-Alpes, Saint-Martin d’Hères, France) et al., Science, doi: 10.1126/science.ady2532