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Spakman and Hall2 provide a crustal structure and the interactions References compelling account of the formation of between the migrating plates, the 1. Milosm, J. Tectonophyics 338, 167–178 (2001). the Banda arc system through subduction subducting slab and the mantle. ❐ 2. Spakman, W. & Hall, R. Nature Geosci. 3, 562–566 (2010). of a single slab. The extreme curvature of 3. Hall, R. J. Asian Earth Sci. 20, 353–434 (2002). 4. Cardwell, R. & Isacks, B. J. Geophys. Res. the surface volcanoes and the spectacular Mike Sandiford is in the School of Earth Sciences, 83, 2825–2838 (1978). shape of the deformed slab can be University of Melbourne, 3010, Australia. 5. McCaffrey, R. & Abers, G. A. Geology 19, 288–295 (1996). explained as a result of the pre-existing e-mail: [email protected] 6. Sandiford, M. Geophys. J. Int. 174, 659–671 (2008).

PlaneTary Science on the trail of the dancing devil

The surface of bears the scars of form almost anywhere, but most commonly exposing coarser-grained material numerous giant dust devils — vortices they form in barren desert regions. They below. The coarse-grained substrate of warm air that swirl across the planet’s develop when air near the surface is heated has a lower albedo compared with the barren surface. During their fleeting and rises. More surface air rushes in to fill surface dust and appears darker. The existence, the dust devils dance and the gap left by the rising air. As it passes therefore creates a dark trail weave through the craters, climbing crater over and around uneven topography, the in its wake. Although dust devils are walls and leaving dark trails in their wake. air may begin to rotate. The vertical motion abundant on Earth, observations of their The Viking orbiters first identified intensifies the spinning, drawing in more darkened tracks are rare. Because of dust devils in the 1970s and 1980s. They air and forming a vortex. The vortex can their rarity, we do not know precisely how are generally much larger than their whip up any loose dust and debris from terrestrial dust-devil tracks form. equivalents on Earth. Although their size the surface, creating a visible funnel. Dust Dennis Reiss at the Westfälische and power can pose a significant threat devils provide an important mechanism Wilhelms-Universität, Münster, and to our visiting technology, they have also for transporting dust from the surface into colleagues carried out the firstin situ proved to be strangely useful. In March the atmosphere. analysis of rare darkened dust-devil 2005, the martian rover saw her The surface of Mars is littered with tracks observed in the Turpan Desert, power levels jump. A passing dust devil abundant dark dust-devil tracks. Analysis north-western China (Geophys. Res. Lett. is thought to have cleaned Spirit’s solar of martian imagery has provided some doi:10.1029/2010GL044016; 2010). panels by removing a layer of dirt that insight into how the tracks may form. Using high-resolution photographs, they had accumulated, giving her a new lease It is thought that the dust devil erodes measured the size of the grains in the dark of life. the surface, removing the fine-grained tracks compared with the surrounding Dust devils are whirlwinds, formed by fraction of surface dust, or causing it surface. The dark tracks were composed of vertical columns of rotating air. They can to infiltrate deeper into the substrate, very coarse-grained material that had been stripped of any finer grain sizes by the dust devil, creating a dark, low-albedo surface. Their analyses confirmed that dark dust- devil tracks on Earth can form in the same way as those on Mars. The Turpan Desert analyses reveal that a layer of dust only ~2 μm thick needs to be removed to create a dark dust-devil track, implying that it is not the thickness of the removed layer that creates the dark track. The researchers suggest that the formation of dark tracks is dependent on the albedo contrast that is formed. If the grain size of the dust that is removed from the surface is significantly smaller than the grain size of the underlying material, a ) strong albedo contrast is formed and a dark

ONA track appears. Thus, the lack of contrast iz R between the grain size of the surface dust and the underlying surface, for example, in a desert where fine sand rests on yet more fine sand, could help to explain why dark dust-devil tracks are observed so rarely SE, MRO, LPL (U. A SE, MRO, i on Earth.

NASA, HiR NASA, aMy WHiTcHUrcH

520 nature geoscience | VOL 3 | AUGUST 2010 | www.nature.com/naturegeoscience

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