PLANETARY SCIENCE Tectonic overturn on Enceladus

摘要

The ener gy budg et of Enceladus, Saturn's second nearest major satellite, holds a number of perplexing enigmas. This ice moon of only about 500 km in diameter orbits Saturn in the frozen outer reaches of the Solar System, yet it is volcanically and tectonically active, with giant geysers of water vapour and ice erupting into space through fissures in its surface. This inten se geological activity is focused only at the bod y's south pole, whereas other regions of the surface have been either tectonically inactive for eons or little affected by tectonic resurfacing at all. But whether the physical mechanisms that at present resurface the south polar region are the same as those that resurfaced other parts of Enceladus long ago is not known. Writing in Nature Geoscience,O'Neill and Nimmo' explain Enceladus 's regionally focused volcanic and tectonic activity, as well as its peculiar geological history, by simulating onvective overturn in a regime that altern ates between long periods in which the icy cru st is immobile and short ones when it can regionally deform and partially overturnin response to a subsurface convection cell.