492-7(1) - Vol 447 | 14 June 2007 | doi:10.1038/nature05873...

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LETTERS Evidence for an ancient martian ocean in the topography of deformed shorelines J. Taylor Perron 1 { , Jerry X. Mitrovica 2 , Michael Manga 1 , Isamu Matsuyama 3 & Mark A. Richards 1 A suite of observations suggests that the northern plains of Mars, which cover nearly one third of the planet’s surface, may once have contained an ocean 1–7 . Perhaps the most provocative evidence for an ancient ocean is a set of surface features that ring the plains for thousands of kilometres and that have been interpreted as a series of palaeoshorelines of different age 1,7 . It has been shown, however, that topographic profiles along the putative shorelines contain long-wavelength trends with amplitudes of up to several kilo- metres 4,5,8 , and these trends have been taken as an argument against the martian shoreline (and ocean) hypothesis 8 . Here we show that the long-wavelength topography of the shorelines is consistent with deformation caused by true polar wander—a change in the orientation of a planet with respect to its rotation pole—and that the inferred pole path has the geometry expected for a true polar wander event that postdates the formation of the massive Tharsis volcanic rise. Parker et al. 1,3 and Clifford and Parker 7 used an array of geologic and topographic features to identify several possible palaeoshorelines near the margins of the northern plains of Mars. Two of these can be traced without major interruptions for thousands of kilometres (Fig. 1): the Arabia shoreline (contact 1 of refs 1 and 3), which coin- cides roughly with the dichotomy in crustal thickness between the northern and southern hemisphere, and the Deuteronilus shoreline (contact 2 of refs 1 and 3), which is inferred to be younger because it is encircled by the Arabia shoreline and is less degraded. However, subsequent studies have challenged the notion that these features are shorelines 9 . The most basic objection stems from the observation that topographic profiles along the contacts do not follow surfaces of equal gravitational potential (that is, sea level) 4,5,8 , as the margins of a standing body of water should. In particular, the contacts contain significant long-wavelength trends, with amplitudes of , 2.5 km for Arabia and , 0.7 km for Deuteronilus (Fig. 1). The antiquity of the putative shorelines (at least 2 billion years (Gyr), the age of most of the northern plains surface 10 ) allows for the possibility that they have been subject to long-wavelength, large- amplitude deformation since their formation. On Earth, true polar wander—TPW, the reorientation of the planet relative to its rotation pole—has been implicated in very-long-wavelength sea-level varia- tions with timescales exceeding 10 6 –10 8 years (refs 11, 12). In the following, we investigate whether TPW also provides a plausible explanation for the observed deformation of the Arabia and Deuteronilus contacts on Mars. That is, we seek a TPW path that is consistent with the displacements of these contacts away from an
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This note was uploaded on 02/05/2012 for the course ES 492 taught by Professor Staff during the Summer '11 term at S. Alabama.

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492-7(1) - Vol 447 | 14 June 2007 | doi:10.1038/nature05873...

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