Rapid Communication Conceptual model for the origin of the Olympus Mons cliffs, Mars: An essential inﬂuence of water? Fabio Vittorio De Blasio a,b,n a Department of Earth Sciences, Sapienza University of Rome, Italy b Department of Earth Sciences and Geo-technologies, University of Milan Bicocca, Italy article info Article history: Received 15 July 2011 Received in revised form 6 April 2012 Accepted 10 April 2012 Available online 4 May 2012 Keywords: Mars Olympus Mons Landslides Aureole Lava ﬂow Oceanus Borealis abstract With a height of 21 km above the mean Martian altitude and a diameter of 600 km, the Olympus Mons of Mars is the highest and one of the largest volcanoes in the Solar System. It is a distinctive shield volcano, formed by stacked sequences of low-viscosity magma. Whereas the central part of the Olympus Mons exhibits slope angles of less than 1–51, the periphery of the ediﬁce terminates with steep cliffs sloping 12–151 up to 281. Another remarkable feature is the aureole, a chain of crown-like deposits surrounding the ediﬁce of Olympus Mons from an average distance of 400 km. The aureole deposits, which lack any obvious analogue on the Earth, have been variously interpreted as volcanic products, pyroclastic or ash ﬂows, slow deep-seated deformation, or catastrophic landslides. Numerical simulations and a comparative study of similar volcanic structures on Earth suggest that a volcanic ediﬁce with the characteristics of Olympus Mons cannot be formed without the presence of water at the base. Because of the low cooling rate of lava in sub-aerial conditions, the superposition of purely subaerial lava ﬂows would contribute with gentle slope to the topography. In contrast, the presence of a medium like water increases the convective heat exchange rate by nearly three orders of magnitude, thus stopping the lava ﬂow and causing a slope increase at the borders of the ediﬁce, which subsequently collapses. A model for the evolution of the Olympus Mons is consequently suggested in analogy with the Canary and the Hawaii island on Earth. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction Olympus Mons is an enormous shield volcano about 600 km across standing above the plains of Amazonis Planitia (Fig. 1, top). A remarkable feature of the Olympus Mons (OM hereafter) is the aureole, a complex deposit of gigantic sub-circular halos (several Mkm 3 ) surrounding the main ediﬁce from distances in the range of 200–700 km. The aureole deposits have been variously attrib- uted to diverse processes such as lava ﬂows (McCauley et al., 1972), subglacial volcanic deposits (Hodges and Moore, 1979), ash ﬂows (Morris, 1981), deep-seated deformations (Francis and Wadge, 1983), catastrophic landslides (Lopes et al., 1980,1982; McGovern et al., 2004; Harrison and Grimm, 2003), slow land- slides lubricated by ice (Tanaka, 1985), ancient subaqueous land- slides (Baker et al., 2000; De Blasio, 2011a). The ediﬁce of OM terminates along the plains of Amazon Planitia with different possible height proﬁles. Along some sections, like B, C-north or D, E-west of Fig. 1, it features relatively steep cliffs some 8–9 km high and dipping 10–151. Other sections exhibit gentler slopes and absence of cliffs (sections A and C-south in Fig. 1), while in a mixed geometry like in section C, a 4 km cliff is followed by a moderate slope. The southern edge presents much steeper cliffs, up to 251 (section E). The part of the OM ediﬁce surrounded by cliffs is morphologically similar, albeit at a much greater scale, to the volcanic seamounts on Earth like the Canaries and the Hawaii (Morgan et al., 2003). Similar is also probably the composition of the main ediﬁce, consisting of superimposed sequences of basaltic lava ﬂows. Remarkably, both the Canaries and the Hawaii exhibit a constellation of landslide deposits, extending to a much greater overall area than those of the islands themselves. This is in marked contrast with shield volcanoes and ﬂood basalt ﬁelds erupting on land, which do not show major failures (e.g. Walker, 2000). There has been much interest in the last decades for the possible presence of an ancient ocean (Oceanus Borealis assumed of Noachian to Hesperian age) in the northern basins (Parker et al., 1993), which in some of the suggested interpretations would affect the OM area. The recent detection of phyllosilicates in the Northern Plains (Carter et al., 2010) and the analysis of dielectric properties of soil in the same units (Mouginot et al., 2012) reinforce the scenario of abundant water in the past. Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/pss Planetary and Space Science 0032-0633/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pss.2012.04.005 n Correspondence address: Department of Earth Sciences, Sapienza University of Rome, Piazzale Aldo Moro 5, 00185, Rome, Italy E-mail address: fvblasio@geologi.uio.no Planetary and Space Science 69 (2012) 105–110