Is there a giant impact basin beneath the Antarctic ice?

At present there are only two reliable means of surveying variations in the Earth’s gravitational field: at the surface using gravimeters and from space, by processing measurements the height of the ocean surface from radar measurements or by accurately measuring the variation in distance between two satellite travelling in tandem over the Earth’s surface. The last is used by the Gravity Recovery and Climate Experiment (GRACE) designed by NASA and the German Space Agency. It is the only realistic means of usefully precise gravity surveys over Antarctica. A truly multinational team (von Frese, R.R.B. et al. 2009. GRACE gravity evidence for an impact basin in Wilkes Land, Antarctica. Geochemistry,Geophysics, Geosystems, v. 10, Q02014, doi:10.1029/2008GC002149 – on-line journal) has discovered a prominent positive free-air gravity anomaly over a roughly 500-km diameter subglacial basin in Wilkes Land. A basin filled with low-density ice would normally give a negative gravitational ‘signature’, so the positive anomaly suggests either unusually dense crustal rocks beneath it, or that the mantle is unusually close to the surface; i.e. the crust is thin. The authors suggest that the central anomaly is surrounded by roughly concentric circular features, and that it is a hitherto unsuspected impact structure, three time larger than the Chicxulub structure (also mapped by gravity data off the Yucatan Peninsula of Mexico) that caused an upward bulge of the mantle. To my eye, the hypothesis only becomes convincing when concentric circles are drawn around the undoubted major anomaly, and the evidence for them is scant compared with the similarly detected structures of Mars and the Moon. What intrigues the authors is the position of the anomaly on a Permian continental reconstruction, It is at the antipode of the Siberian Traps flood basalt province, implicated strongly in the end-Permian mass extinction: the most devastating known. This harks back to speculation that the undoubted Chicxulub structure and caused the mantle to melt beneath its antipode to form the Deccan Traps…

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