Geomorphology 121 (2010) 1–14 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph Extraterrestrial dunes: An introduction to the special issue on planetary dune systems Mary C. Bourke a,b,⁎, Nick Lancaster c, Lori K. Fenton d, Eric J.R. Parteli e, James R. Zimbelman f, Jani Radebaugh g a Planetary Science Institute, 1700 E Ft Lowell, #106, Tucson AZ, USA b School of Geography and the Environment, University of Oxford, South Parks Road, Oxford OX1 3QY, United Kingdom c Desert Research Institute, Reno, NV 89512, USA d Carl Sagan Center, NASA Ames Research Center, MS 245-3, Moffett Field, CA 94035, USA e Departamento de Física, Universidade Federal do Ceará, 60455-900 Fortaleza, Ceará, Brazil f Center for Earth and Planetary Studies, MRC 315, National Air and Space Museum, Smithsonian Institution, Washington, D.C. 20013-7012, USA g Department of Geological Sciences, Brigham Young University, Provo, UT 84602, USA article info abstract Article history: Aeolian dune fields have been described on Earth, Mars, Venus and Titan. The plethora of data returned from Received 20 March 2010 recent planetary missions has enabled a new era in planetary geomorphic studies. Much of our Accepted 9 April 2010 understanding of planetary dune systems comes from the application of Earth analogs, wind tunnel Available online 29 April 2010 experiments and modeling studies. Despite the range of atmospheric pressures, composition and gravity, many of the dune forms on extraterrestrial surfaces are similar to those on Earth, although some have Keywords: notable differences in bedform scale and composition. As an introduction to the special issue on planetary Aeolian Dune dune systems this paper summarizes the current state of knowledge of planetary dune studies and highlights Mars outstanding questions that require further investigation. Venus © 2010 Elsevier B.V. All rights reserved. Titan Earth 1. Introduction stimulated the production of a dune atlas for Earth dune systems (Lancaster, 2008). Aeolian sand dune systems are known to occur on four bodies in Planetary geomorphic studies are conducted using remote sensing our solar system (Earth, Mars, Venus and Titan). Despite the range of data returned from missions. Whereas the majority of these data are atmospheric pressures, composition and gravity (Table 1) many of the obtained from space-borne platforms, landers and rovers also return dune forms appear similar albeit with notable difference in dune scale images captured on the surface. Much of our understanding of and composition on some bodies. planetary dune systems comes from the application of Earth analogs. The high volume of data returned recently from planetary surfaces In addition, important findings have resulted from wind tunnel has triggered a new era in planetary geomorphic studies. Conse- experiments and modeling studies that employ the different quently researchers are currently in the exploration and discovery atmospheric and gravity parameters pertinent to each of the bodies. phase of extra terrestrial dune systems. Similar to the approach of Each new mission utilizes instruments with higher spatial and Earth scientists during the late 19th and early 20th centuries (e.g., the spectral resolution. These produce a stream of new data that French studies of the Sahara, Rolland, 1881; Aufrere, 1932), significant sometimes deliver unexpected results (e.g., the discovery of a strong effort is currently dedicated to describing dune and dune field sulfate signature in one the North Polar Ergs, on Mars, Langevin et al., attributes and to the production of maps of global distribution. Much 2005). Earth analogs become increasingly useful as discoveries of the of the planetary dune mapping is still ongoing. Progress is limited on similarities and differences in dune systems on the different bodies bodies such as Titan by incomplete data coverage whereas on others continue to emerge. In some cases these analogs find renewed significant progress has been made (e.g., the work of Hayward et al., application on Earth. For example, Schatz et al. (2006) use the 2007 on Mars). Such initiatives for planetary mapping have morphology of oil-soaked barchans in Saudi Arabia (Kerr and Nigra, 1952) to suggest that dune cementation/induration may cause the unusual elliptical shape of some barchans in the North Polar region on ⁎ Corresponding author. PSI, 1700 E Ft Lowell, #106, Tucson, AZ 85719, USA. Tel.: +1 520 6226300. Mars. In turn, cohesion is invoked to explain the formation of linear E-mail address: [email protected] (M.C. Bourke). dunes in China and on Titan (Rubin and Hesp, 2009). Planetary 0169-555X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2010.04.007 2 M.C. Bourke et al. / Geomorphology 121 (2010) 1–14 Table 1 although dunes composed of gypsum, carbonate, and volcaniclastic Properties of planetary environments. Data synthesized from Greeley and Iversen sand as well as clay pellets also occur. (1985) and Lodders and Fegley (1998). Investigations on Earth have used field surveys and experiments, Table from Zimbelman (2008). in combination with aerial and satellite images, to identify basic types Earth Mars Venus Titan of dune and the processes that form them; and to trace the origins of Gravity (m/s2) 9.81 3.73 8.88 1.36 the sediments (for reviews see Pye and Tsoar, 1990; Lancaster, 1995; Atmospheric pressure (mb) 1013 7 90,000 1600 Livingstone et al., 2007; Thomas and Wiggs, 2008). Such studies Atmospheric composition (%) N2,O2 CO2,N2 CO2,N2 N2,CH4 provide a baseline for comparative analyses of dunes on other (77, 21) (95, 2.7) (96, 3.5) (90, 10) Surface temperature (°C) 22 −23 480 −200 planetary bodies. A key aspect of the formation and development of Earth dunes and sand seas is the role of Quaternary changes in climate and sea level, which give rise to changes in sediment supply, availability and mobility and result in the formation of different generations of dunes, as well as episodic accumulation of large dunes (Kocurek and Lancaster, 1999a). geomorphology improves our understanding of extraterrestrial forms and processes and also moves forward our understanding of dune 2.2. Dunes on Mars systems on Earth. Alongside the more traditional geomorphic approaches, numerical Globally dune fields on Mars are estimated to cover an area of modeling of dunes and dune patterns has advanced significantly in 904,000 km2 (Hayward et al., 2007, 2008; Fenton and Hayward, 2010- recent years and provides valuable insight into fundamental proper- this issue). Similar to Earth, the global distribution of Martian dunes is ties of dune morphology and sediment transport (Livingstone et al., related to wind patterns and they likely lie proximal to the sediment 2007). Numerical simulations have proven to be extremely helpful in source regions (Thomas, 1982; Ward and Doyle, 1983; Greeley et al., determining wind systems, predicting the timescale of dune migra- 1992b; Thomas and Gierasch, 1995; Fenton, 2005). tion, and testing hypotheses on the formation of planetary dune fields. Sand dunes are found in a series of generally unconfined sand seas At the same time, the possibility of using models to make indirect that surround the North Polar ice cap; on the floors of impact craters, estimates of the sediment properties of planetary dunes is motivating in the inter-basin plains of the southern hemisphere, and in low continued research on improvement of the numerical calculations for latitude topographic traps such as troughs and channels. Individual the equations of sediment transport (Andreotti and Claudin, 2007; dunesonMarshavebeenidentified as barchan, barchanoid, Parteli et al., 2007b). transverse, star, linear and dome (Cutts, 1973; Breed, 1977; Breed et The first international workshop on planetary dune systems was al., 1979; Tsoar et al., 1979; Thomas et al., 1984; Cwick and Campos- held in Alamorgordo, New Mexico in 2008 (Titus et al., 2008). Forty- Marquetti, 1984; Edgett and Christensen, 1994; Lee and Thomas, five Earth and planetary scientists and their students traveled from 1995)(Fig. 1). Unusually-shaped dunes which currently defy existing eight countries to deliver a series of oral and poster presentations and classification systems are also a significant proportion of dunes on to participate in discussions on the current state of knowledge. Mars (comprising ~30% of mid-low latitude dune fields, Hayward et Through constructive and vigorous discussions, some areas for future al., 2007). These morphologies are likely the result of variable wind research initiatives were identified. The workshop resulted in the regimes and modification by volatiles (Schatz et al., 2006; Parteli and publication of 22 scientific papers, 8 of which follow in this special Herrmann, 2007a). A survey of barchan morphometry found that they issue of Geomorphology (Feldman et al., 2008; Balme et al., 2008; are, on average, larger than barchans in many dune fields on Earth, Bourke et al., 2009; Horgan et al., 2009; Lorenz and Radebaugh, 2009; with stoss slope lengths of 215 m and dune widths of ~400 m (Bourke Necsoiu et al., 2009; Bourke and Goudie, 2009; Yizhaq et al., 2009; et al., 2004a). Recently the production of digital elevation models Bourke, 2010; Bristow et al., 2010a; Bristow et al., 2010b; Scheidt et from HiRISE stereo images allows 3-D perspective views (Fig. 2) and a al., in press; Bishop, in press; Diniega et al., 2010-this issue; Fenton more accurate estimates of dune height, volume and morphometric and Hayward, 2010-this issue; Gillies et al., 2010-this issue; profiles of dunes than had been previously possible (e.g., Bourke et al., Radebaugh et al., 2010-this issue; Rodriguez et al., 2010-this issue; 2006; Zimbelman, 2010-this issue). Silvestro et al., 2010-this issue; Szynkiewicz et al., 2010-this issue; The majority of sand dunes on Mars are composed of unweathered Zimbelman, 2010-this issue). basaltic (Paige et al., 1994; Herkenhoff and Vasavada, 1999)or This paper summarizes the current state of knowledge of planetary andesitic (Bandfield, 2002) fragments.