Recent Aeolian Dune Change on Mars ⁎ M.C

Home , Acheron Fossae, Airy (Martian crater), Arabia Terra, Arkhangelsky (crater), Dawes (Martian crater), Greeley (Martian crater)

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Geomorphology 94 (2008) 247–255 www.elsevier.com/locate/geomorph

Recent aeolian dune change on Mars ⁎ M.C. Bourke a,b, , K.S. Edgett c, B.A. Cantor c

a Planetary Science Institute, Tucson, AZ 85719, USA b School of Geography and the Environment, University of Oxford, Oxford, OX1 3QY, UK c Malin Space Science Systems, PO Box 910148, San Diego, CA 92191-0148, USA Received 26 January 2007; received in revised form 24 May 2007; accepted 24 May 2007 Available online 8 June 2007

Abstract

Previous comparisons of Martian aeolian dunes in satellite images have not detected any change in dune form or position. Here, we show dome dunes in the north polar region that shrank and then disappeared over a period of 3.04 Mars years (5.7 Earth years), while larger, neighboring dunes showed no erosion or movement. The removal of sand from these dunes indicates that not only is the threshold wind speed for saltation exceeded under present conditions on Mars, but that any sand that is available for transport is likely to be moved. Dunes that show no evidence of change could be crusted, indurated, or subject to infrequent episodes of movement. © 2007 Elsevier B.V. All rights reserved.

Keywords: Mars; Aeolian; Dune; Sediment transport

1. Introduction dust is raised by wind somewhere on the planet nearly every Martian day (Cantor, 2003; Cantor et al., 2006; Aeolian bedforms are found at all latitudes on Mars Cantor, 2007). As sand is mobilized at a lower threshold (Thomas, 1982). Their presence requires that sand-sized friction speed than dust (Iverson and White, 1982), and material has been transported by saltation (Cutts and the impact of saltating grains will raise dust at threshold Smith, 1973). Saltation occurs on Mars under the current friction speeds lower than for dust alone (Greeley, 2002), low atmospheric pressures: a dust storm in June 1981 is sand must also be transported by these dust-raising events, believed to have caused the erosion of regolith piles at the provided that sand is available to be moved. Viking1landersite(Moore, 1985), and wind gusts, Many Martian sand dunes exhibit sharp brinks and perhaps during a March 2005 dust storm (Cantor et al., margins (Figs. 1 and 2) and do not appear to be peppered 2006), caused sand grains to be deposited on the 1 m high by small impact craters (Marchenko and Pronin, 1995; deck of the Mars Exploration Rover, Spirit, via saltation Malin and Edgett, 2001). This suggests that they are (Greeley et al., 2006). Daily global observations by the geologically young and may have been mobile at some Mars Global Surveyor (MGS) Mars Orbiter Camera time in the last 100,000 years (Hartmann, 2005; Malin (MOC) over a 4.04 Mars year period (1 Mars year is about et al., 2006). There are no published reports of dune 1.88 Earth years in duration) from March 1999, show that movement. Images of dunes acquired by the Mariner 9 (1972), Viking 1 (1976–1980), Viking 2 (1976–1978), and Mars Global Surveyor (MGS; 1997–2006) orbiters ⁎ Corresponding author. suggest that Martian dunes have not migrated during the E-mail address: [email protected] (M.C. Bourke). past 1–14 Martian years (Edgett and Malin, 2000;

Fig. 1. Location of MGS MOC images on a subframe of Mars Odyssey THEMIS image V12629003. The swath placement and dimensions of each image differs slightly. MOC image numbers 1: FHA-00515 (March 1999), 2: E18-00494 (July 2002), 3: E23-00490 (December 2002), 4: S01-00007 (December 2004). Circles indicate the location of the small dome dunes (i, ii, iii) examined.

Zimbelman, 2000; Malin and Edgett, 2001; Edgett, Several ideas have been put forward as to why 2002; Fenton et al., 2003; Williams, 2004; Schatz et al., Martian sand dunes do not appear to be moving today. 2006). Further, only a small number of images docu- The threshold friction wind speed required to move ment changes that occurred on dune slip-faces—8MOC sediment on Mars is higher than on Earth, and data from pictures studied by Fenton (2006) show new slip-face the Viking and Mars Pathfinder Lander missions indi- avalanches on a dune in Rabe Crater, and 2 MOC cated that these speeds were rarely met (Greeley et al., pictures described by Malin and Edgett (2005) show the 1982; Schofield et al., 1997). Under such conditions, formation of new steep-walled gullies on the slip-face of dunes might be moving so slowly that they are indeed a dune in an unnamed Martian crater. active today, but the rate is immeasurable given the M.C. Bourke et al. / Geomorphology 94 (2008) 247–255 249

Fig. 2. Subframes of MOC images in Fig. 1. Panels 1–3 show changes in the dome dunes that occurred between March 1999 and December 2004. limited, repeated spacecraft observations. Alternatively, southward toward the informally-named Victoria Crater some or all of the dunes may be crusted or indurated. (Richter et al., 2006). Evidence for crusting or induration Crusts were common in the regolith at the Viking lander of dunes comes from MOC images showing the presence sites (Arvidson et al., 1989); and the Mars Exploration of erosional steps on the windward slopes of barchan Rover, Opportunity, routinely encountered crusted sand- dunes (Bourke, 2004), and the steep walls of avalanche and granule-sized windblown sediment during its trek scars on some dune slip-faces (Malin and Edgett, 2001;

Table 1 Migration rates of small terrestrial dunes in unidirectional wind settings Location Dune type Time interval Movement rate (per year) Reference Pampa de la Joya, Peru Barchan 1955–1958 9–23 m Hastenrath (1967) 1958–1964 17–56 m Pampa de la Joya, Peru Barchan 1955–1958 9–30 m Finkel (1959) Imperial Valley, California, USA Barchan 1941–1956 7–24 m Long and Sharp (1964) 1956–1963 14–40 m Taklimakan Desert, China Dome ‘6’ 1991–1993 75 m Dong et al. (2000) Dome ‘8’ 1991–1993 6 m Namib Sand Sea, Namibia Dome 1976–1999 ~4 m (Bristow and Lancaster, 2004) 250 M.C. Bourke et al. / Geomorphology 94 (2008) 247–255

Table 2 Mars dune sites repeatedly monitored for movement by MGS MOC Location Lat. Lon. First image Last image Interval (Mars years) Picture no. Date Ls Picture no. Date Ls Unnamed crater 1.7°N 351.7°W Viking 709A42 27-May-1978 92° S19-01867 24-Jun-2006 71° 13.9 Near Briault 8.8°S 271.1°W Viking 755A19 12-Jul-1978 113° R14-01173 10-Feb-2004 348° 13.7 Unnamed crater 20.1°N 280.7°W SP1-23008 10-Apr-1998 308° S18-02048 25-May-2006 57° 4.3 Kaiser crater 47.1°S 341.3°W AB1-10004 21-Jan-1998 260° S12-02197 23-Nov-2005 329° 4.2 North polar 76.4°N 264.8°W FHA-00515 10-Mar-1999 108° S22-00197 4-Sep-2006 102° 4.0 Kaiser crater 46.9°S 340.9°W M02-04432 28-Jun-1999 162° S02-01242 31-Jan-2006 153° 4.0 Herschel crater 14.3°S 231.8°W SP2-36507 14-Jun-1998 345° S12-02169 22-Nov-2005 329° 4.0 North polar 81.1°N 188.0°W M00-00545 5-Apr-1999 120° S21-02221 21-Aug-2006 96° 3.9 Gale crater 4.7°S 222.7°W M03-01521 9-Jul-1999 168° S19-00049 1-Jun-2006 60° 3.7 North polar 76.8°N 105.4°W M02-03150 22-Jun-1999 159° S13-02421 25-Dec-2005 346° 3.5 Dawes crater 9.5°S 323.1°W M09-03471 14-Nov-1999 244° S12-02647 28-Nov-2005 331° 3.2 North polar 79.1°N 251.2°W M14-00429 6-Apr-2000 331° S17-02242 27-Apr-2006 45° 3.2 Unnamed crater 4.1°S 348.0°W M12-01178 11-Feb-2000 300° S14-02432 23-Jan-2006 1° 3.2 Herschel crater 15.1°S 232.3°W M02-02819 10-Jun-1999 158° S06-01757 18-May-2005 214° 3.2 Herschel crater 14.4°S 231.6°W FHA-01381 23-Mar-1999 114° S04-00076 2-Mar-2005 169° 3.2 Unnamed crater 19.9°N 280.6°W M14-00676 10-Apr-2000 334° S14-01565 15-Jan-2006 357° 3.1 Proctor crater 47.5°S 329.8°W M03-03088 16-Jul-1999 172° S05-00330 6-Apr-2005 189° 3.0 Trouvelot crater 16.3°N 13.5°W SP2-53203 3-Sep-1998 24° R17-02141 25-May-2004 38° 3.0 Green crater 53.0°S 8.8°W E01-02009 2-Feb-2001 123° S23-00204 4-Oct-2006 116° 3.0 Unnamed crater 7.3°N 353.4°W M21-00827 13-Nov-2000 76° S20-01013 16-Jul-2006 80° 3.0 Unnamed crater 10.9°N 2.9°W M07-01793 10-Sep-1999 204° S05-01727 27-Apr-2005 201° 3.0 Proctor crater 47.7°S 330.0°W M10-01334 11-Dec-1999 261° S08-03035 28-Jul-2005 258° 3.0 Proctor crater 47.5°S 329.7°W M02-02711 20-Jun-1999 157° S02-01021 25-Jan-2005 150° 3.0 North polar 76.7°N 239.4°W M02-04193 27-Jun-1999 161° S03-00289 5-Feb-2005 156° 3.0 North polar 84.8°N 26.7°W M02-00783 8-Jun-1999 151° S02-00302 7-Jan-2005 141° 3.0 North polar 75.9°N 84.3°W E02-00183 3-Mar-2001 125° S23-00440 8-Oct-2006 118° 3.0 Kaiser crater 46.8°S 340.8°W M02-04432 28-Jun-1999 162° S02-01242 31-Jan-2005 153° 3.0 Unnamed crater 10.8°N 351.0°W SP1-26004 24-Apr-1998 317° R10-05174 30-Oct-2003 290° 2.9 Unnamed crater 6.3°N 345.9°W M11-01425 10-Jan-2000 280° S08-00832 8-Jul-2005 245° 2.9 North polar 82.7°N 45.0°W E01-01325 19-Feb-2001 120° S17-00850 19-Apr-2006 41° 2.8 North polar 76.5°N 264.7°W M02-03026 21-Jun-1999 158° R19-01239 17-Jul-2004 62° 2.7 North polar 74.9°N 50.9°W M23-01695 26-Jan-2001 108° S13-00388 30-Dec-2005 349° 2.7 North polar 76.6°N 268.4°W E01-01881 25-Feb-2001 122° S13-02134 22-Dec-2005 345° 2.6 Unnamed crater 8.4°N 55.2°W E02-00082 2-Mar-2001 124° S13-02634 27-Dec-2005 347° 2.6 Unnamed crater 8.5°N 15.8°W E05-03231 29-Jun-2001 186° S16-01017 12-Mar-2006 24° 2.6 Unnamed crater 11.0°N 14.5°W E05-01993 21-Jun-2001 182° S13-02485 25-Dec-2005 346° 2.5 North polar 76.7°N 256.1°W E14-01084 16-Mar-2002 343° S23-00067 2-Oct-2006 115° 2.4 Rabe crater 43.9°S 325.6°W M17-01061 28-Jul-2000 28° S01-00886 20-Dec-2004 132° 2.3 Unnamed crater 8.9°N 1.2°W E09-02066 26-Oct-2001 260° S14-01054 10-Jan-2006 354° 2.3 North polar 76.2°N 264.7°W E16-00714 11-May-2002 11° S21-00757 13-Aug-2006 92° 2.2 Unnamed crater 2.4°N 9.5°W E02-00161 2-Mar-2001 124° S05-01341 23-Apr-2005 198° 2.2 Unnamed crater 50.5°S 327.6°W M10-03033 26-Dec-1999 270° R12-01731 16-Dec-2003 317° 2.1 Rabe crater 43.7°S 325.5°W E03-01352 15-Apr-2001 146° S05-00564 10-Apr-2005 191° 2.1 Unnamed crater 50.4 S 327.5°W M10-03033 26-Dec-1999 270° R12-01731 16-Dec-2003 317° 2.1 North polar 76.7°N 250.2°W E15-00308 11-Apr-2002 356° S16-00971 11-Mar-2006 24° 2.1 Unnamed crater 4.1°S 347.9°W M13-00493 9-Mar-2000 315° R12-02868 24-Dec-2003 322° 2.0 Bunge crater 33.8°S 48.9°W M22-00197 3-Dec-2000 84° R20-01421 27-Aug-2004 79° 2.0 Unnamed crater 5.4°N 351.6°W M07-05541 28-Sep-1999 215° R06-01811 28-Jun-2003 212° 2.0 North polar 76.9°N 270.4°W M15-01266 19-May-2000 354° R14-01655 15-Feb-2004 350° 2.0 North polar 81.2°N 198.3°W E01-02121 27-Feb-2001 123° R23-01360 29-Nov-2004 122° 2.0 North polar 77.4°N 113.2°W M18-00471 8-Aug-2000 33° R17-02331 27-May-2004 39° 2.0 Unnamed crater 5.4°N 351.6°W M07-05541 28-Sep-1999 215° R06-01811 28-Jun-2003 212° 2.0 Unnamed crater 10.9°N 348.0°W M07-06117 30-Sep-1999 216° R03-01420 29-Mar-2003 159° 1.8 North polar 76.9°N 118.9°W E02-00645 8-Mar-2001 127° R18-00708 9-Jun-2004 45° 1.8 North polar 77.6°N 49.6°W E02-00538 7-Mar-2001 127° R16-01704 21-Apr-2004 23° 1.7 Unnamed crater 17.9°N 17.1°W M11-00431 3-Jan-2000 276° S04-00875 19-Mar-2005 178° 1.7 M.C. Bourke et al. / Geomorphology 94 (2008) 247–255 251

Table 2 (continued) Location Lat. Lon. First image Last image Interval (Mars years) Picture no. Date Ls Picture no. Date Ls Syrtis major 6.7°N 292.1°W E22-00491 10-Nov-2002 93° S14-02408 23-Jan-2006 1° 1.7 North polar 77.7°N 110.1°W E01-01924 26-Feb-2001 123° R17-00457 6-May-2004 29° 1.7 Unnamed crater 9.9°N 352.0°W M11-03934 28-Jan-2000 291° R04-00624 10-Apr-2003 166° 1.7 Unnamed crater 5.5°N 351.5°W R04-01220 17-Apr-2003 170° S17-01316 16-Apr-2006 40° 1.6 Unnamed crater 13.2°N 19.9°W E02-02645 29-Mar-2001 138° R11-04604 30-Nov-2003 308° 1.5 Unnamed crater 41.5°S 322.6°W M07-02254 12-Sep-1999 205° E14-02054 29-Mar-2002 350° 1.4 Unnamed crater 9.1°N 347.9°W E02-01236 15-Mar-2001 131° R10-01558 9-Oct-2003 276° 1.4 Unnamed crater 8.9°N 1.2°W R10-02850 17-Oct-2003 281° S16-00422 5-Mar-2006 21° 1.3 Baltisk crater 42.5°S 54.9°W R15-01756 19-Mar-2004 7° S21-01494 26-Aug-2006 98° 1.3 North polar 75.6°N 82.5°W E14-00303 9-Mar-2002 339° R17-02318 27-May-2004 39° 1.2 Arkhangelsky 41.2°S 25.0°W R14-01899 17-Feb-2004 352° S18-00861 11-May-2006 51° 1.2 Herschel crater 14.9°S 232.2°W R15-02358 27-Mar-2004 11° S19-00161 2-Jun-2006 61° 1.1 Unnamed crater 8.9°N 1.2°W R12-02015 18-Dec-2003 318° S14-01750 17-Jan-2006 358° 1.1 North polar 76.6°N 258.6°W R13-04376 28-Jan-2004 341° S14-02956 28-Jan-2006 3° 1.1 North polar 76.5°N 262.3°W R13-04116 26-Jan-2004 340° S14-02713 26-Jan-2006 2° 1.1 North polar 76.9°N 256.1°W E17-00953 15-Jun-2002 27° R18-01671 20-Jun-2004 50° 1.1 Trouvelot crater 15.5°N 13.4°W E05-01291 14-Jun-2001 178° R05-01684 20-May-2003 188° 1.0 Proctor crater 48.0°S 330.2°W E01-00094 2-Feb-2001 111° R01-00116 3-Jan-2003 118° 1.0 Proctor crater 47.4°S 329.5°W M23-01221 18-Jan-2001 105° E22-01263 23-Nov-2002 99° 1.0 Proctor crater 47.5°S 329.2°W M23-00763 11-Jan-2001 102° E22-00433 9-Nov-2002 93° 1.0 North polar 80.2°N 242.8°W R14-01861 17-Feb-2004 351° S14-00805 7-Jan-2006 353° 1.0 North polar 84.8°N 28.7°W E02-02610 29-Mar-2001 138° R02-00982 18-Feb-2003 140° 1.0 North polar 79.8°N 201.7°W R13-04654 30-Jan-2004 104° S22-00003 1-Sep-2006 101° 1.0 North polar 79.0°N 145.3°W R16-01411 17-Apr-2004 21° S15-01350 13-Feb-2006 11° 1.0 Airy crater 5.3°S 0.0°W M04-00914 14-Aug-1999 188° E05-00326 4-Jun-2001 173° 1.0 The table is a list of 81 sites that were observed over time intervals of ≥1 Martian year. It includes all sites that were specifically monitored for movement because they contain small barchan or dome dunes considered most likely to move a distance measurable in images of spatial resolutions of 1.5–6.0 m/pixel over durations ≥1 Mars year. Note, the table does not include every dune field that was repeatedly imaged by MGS.

Schatz et al., 2006). The cementing agent is unknown but 2006 (Table 2). Some sites were previously imaged by could, in some locations, be ice or geochemical the Mariner 9 spacecraft (1972) and the two Viking precipitates (Bourke, 2004, 2005; Schatz et al., 2006). orbiters (1976–1980). Only a few Viking images are of On Earth, sand dune movement can be monitored sufficient spatial resolution that they are suitable for using aerial photographs at scales similar to MOC narrow comparison with images acquired more than a Mars angle images (1.5–12 m/pixel) taken over intervals of 2– decade later by MGS (see Zimbelman, 2000; Fig. 40 in 10 year. Table 1 shows examples, from Earth, of small Malin and Edgett, 2001; Fig. 6 in Schatz et al., 2006). aeolian dune migration rates as measured on annual to Examples comparing MOC images of dunes acquired at decadal time scales. Given these movement rates on different times during that mission are also presented Earth, the occurrence of almost daily dust-raising events elsewhere (Edgett and Malin, 2000; Malin and Edgett, on Mars, and the appearance of sand grains on the Spirit 2001; Edgett, 2002; Schatz et al., 2006). rover's deck after a dust storm, we therefore expect to be The inverse relationship between dune size and able to detect movement of Martian dunes in MOC migration rate, known from Earth (e.g., Bagnold, 1941; images over time intervals of 2–10 Mars years. Repeat Long and Sharp, 1964; Hesp and Hastings, 1998), imaging enables detection of dune movement by suggests that smaller bedforms will be more likely to comparison of image data. Here we report on the show change. We therefore focused our investigation on results of a satellite image targeting survey that the smallest dunes (landforms typically covering detected change in three small dome dunes on Mars. b300 m2) to look for evidence of movement, such as the encroachment and engulfment of a slip-face upon a 2. Methodology boulder or adjustments in dune planform and outline. We found that change occurred at only one of the sites Eighty-one dune sites were repeatedly imaged by monitored. The MOC was targeted four times in the MOC between mid-September 1997 and mid-October study area between March 1999 and December 2004 252 M.C. Bourke et al. / Geomorphology 94 (2008) 247–255

(Fig. 1). Dune change was observed by author K.S. the sample dunes are 20 m long and 20 to 24 m wide. Edgett as the MOC data were received and was briefly The third dune is larger at 50 m long and 70 m wide noted by Malin et al. (2007). (Table 3). These dimensions are smaller than the average The site where change occurred (Fig. 1) is located dome dune size of 150 m measured from Viking-era data south of the north polar ice cap near 76.4°N, 265.0°W. It on Mars (Cwick and Campos-Marquetti, 1984). is situated in the upwind section of a larger barchanoid dune field that, downwind, merges to form closely 3. Results spaced coalesced dunes. Barchan dune length in the immediate location is ∼275 m and average height is The three dome dunes (i, ii, iii in Fig. 2)arevisible ∼40 m. These are typical dimensions of barchan dunes in the 1999 image. However, dune i was not present in in the north polar region (Bourke et al., 2004). the subsequent images, suggesting that sediment was The width and length of dome dunes were measured removed from that site some time between March from the images and the height estimated (see Bourke 1999 and July 2002. Dune ii was subject to significant et al., 2006, for methods). The planform shape of the sediment loss between March 1999 and July 2002 dunes varied from roughly circular (dune i) to ellipsoid (∼80% volume) and was gone by December 2004. (dunes ii and iii). To account for this, we used two Dune iii showed a small decrease in length between July mathematical approximations to calculate dune volume. 1999 and December 2004 (∼15% volume loss). Dune For dune i, we calculated volume for an oblate dome iii did not display clear evidence of movement – just shape (Monolithic Dome Institute, 2001), and for dunes sediment loss – and the windward slope of the ii and iii we calculated volume for an ellipsoid shape succeeding dune does not show any evidence of (Filip, 2006). It should be noted that these mathematical sediment aggradation at the available image resolution. approximations have yet to be tested in the field. Two of Sediment movement from the dunes is between 80 and

Table 3 Change in dune morphometry over time on Mars Dune morphometry March 10 1999 July 9 2002 December 10 2002 December 1 2004 Dune i Length (m) 19 nv a nv – Width (m) 20 nv nv – Height (m) 1 b nv nv – Volume (m3) 210 nv nv – Bulk volume moved (tones) c 462 Rate of movement (Mars time) 0.18 m3/sol 120 m3/year Rate of movement (Earth time) 0.17 m3/day 62 m3/year Dune ii Length (m) 19 – 7nv Width (m) 24 – 12 nv Height (m) 1 b – 1 b nv Volume (m3) 240 – 44 nv Bulk volume moved (tones)c 528 Rate of movement (Mars time) 0.12 m3/sol 80 m3/year Rate of movement (Earth time) 0.11 m3/day 40 m3/year Dune iii Length (m) 53 – 45 44 Width (m) 70 – 71 72 Height (m) 2 b – 21 21 Volume (m3) 3885 – 3345 3318 Bulk volume moved (tones) c 7299 Rate of movement (Mars time) 0.28 m3/sol 187 m3/year Rate of movement (Earth time) 0.27 m3/day 99 m3/year a nv: dune not detected at this image resolution. Dash symbol indicates dune study site not covered in image swath. b Dune height was not measured directly, but estimated using similar dune dimensions on Earth. c We assume the bulk density of sand to be 2200 kg m− 3 (Bristow and Lancaster, 2004). M.C. Bourke et al. / Geomorphology 94 (2008) 247–255 253

187 m3/Martianyear,or40and99m3/Earth year will occur during storms. Dust storms are extremely (Table 3). For comparison, two small dome dunes in the common in the Martian north polar region, particularly Taklimakan Desert had a loss of between 4 and 38 m3/ in the frost-free periods of summer. Fig. 3 presents the year (Dong et al., 2000). Similarly, the disappearance of record of dust storms, both local and regional in size that the 551 m3 dome dune in the Namib Desert (C.S. passed over the study area from all directions between Bristow, University of London, and N. Lancaster, March 1999 and January 2006. An example of one of Desert Research Institute, personal communication, these dust storms is illustrated in Fig. 4 as it moved 2007) suggests the movement of 183 m3/year of through the study area on 5 April 2001, during the sediment. Therefore, the rates for Mars as estimated in period when dune i was observed to have disappeared. this study are comparable with those measured for This storm, one of many that passed through the region Earth. Dune migration is only a small part of sand that summer, moved to the ENE at a speed of ∼19 m/s. transport in many areas on Earth and may represent as That the dunes did not migrate, but were eroded, little as 1% of the total potential sand transport (Bristow suggests that they were not in equilibrium with their and Lancaster, 2004). If this is also the case for Mars, setting. Dome dune morphology is not always as effec- then it suggests that sand transport may be an active and tive as barchan morphology for trapping sediment, par- vigorous part of the sediment transport regime in areas ticularly in locations of high velocity winds (McKee, of sand availability on Mars. 1966; Bristow and Lancaster, 2004). Alternatively, their erosion may indicate that sand supply is not sufficient to 4. Discussion maintain dune form in this location. The large barchan and barchanoid dunes adjacent These sediment transport rates, averaged over a to the sample site displayed no measurable change Martian year, are conservative, as the dunes in the north during the survey. Dunes of different size require polar region of Mars are covered by H2O and CO2 frosts during much of the autumn, winter, and spring seasons. Dunes on Mars are typically the first surfaces to frost in the autumn and among the first to show evidence of defrosting during the late-winter/early-spring, but the frost may persist on dunes long after surrounding ter- rains have defrosted (Malin and Edgett, 2000). We used MOC images and MGS Thermal Emission Spectrometer (TES) thermal and albedo bolometer data to estimate when the study area was covered by frost during the period of our observations. We used TES bolometer surface temperature retrievals that were accurate to within 10% or better. Because of excessive dust storm activity and polar night conditions, neither MOC wide angle camera images or TES bolometer data were very useful during early northern fall to estimate when the Fig. 3. A plot of the dust storm activity in the study region, between – – dunes were frosted over again. However, a MOC narrow 71.4 81.4° N., 249.8 279.8° W., during portions of the four Mars years spanning from 9 March 1999 (L ∼107.3°, Year 1) to 21 January angle camera image, M04-02215, taken on 20 August s 2006 (Ls ∼360°, Year 4). Ls is the aerocentric (Mars-centric) longitude 1999, shows that the dunes were frosted over by Ls of the Sun, where Ls 0° is the start of northern hemisphere spring and 191.6° (Malin and Edgett, 2001). See Fig. 3 caption for southern hemisphere autumn. Mars is tilted on its axis approximately 25°, thus the planet has seasons. The seasons begin and end at L 0°, explanation of Ls. TES bolometer data did suggest that s 90°, 180°, and 270°. Ls is commonly used by the Mars science CO2 frost may return by Ls ∼196.1° (noted in 1999) and ∼ community to express a given time of the Martian year. The storms Ls 203.8° in the second Mars year. The data show were observed in 7.5 km/pixel daily global images acquired by the that Martian north polar dunes are frosted over in early MGS MOC. No distinction in storm size (local or regional) was made. ∼ autumn (Ls 191.6°) and become defrosted in late spring The data were binned using a bin size of 5° of Ls. The black vertical lines indicate when each of the MOC images shown in Fig. 1 was taken (Ls ∼82°). In other words, seasonal frost likely inhibits sand saltation and dune movement for approximately (the mission subphase is used as the label). The gray shaded regions show when the study site is typically covered with seasonal frost. 70% of the Martian year at this northern latitude (Fig. 3). During late autumn and much of winter, the study region was not As on Earth, the majority of saltation events and any visible to the MOC owing to development of thick cloud cover and resultant changes in dune position and/or morphology winter darkness. 254 M.C. Bourke et al. / Geomorphology 94 (2008) 247–255

Fig. 4. A time-series of subframes of MOC red wide angle daily global images in polar stereographic projection at a resolution ∼7.5 km/pixel. These images, acquired 5 April 2001 (Ls =141.1°), were taken roughly 2 h apart (local Mars time noted in upper right corner). The study area is indicated by the white box. different temporal scales to adjust. On Earth, it can be the saltating grains would have further mobilized more as little as 10–100 years for crescentic dunes (i.e., dust (Greeley, 2002). Mars may have been a much dustier they are governed by annual or seasonal wind place in the recent past, perhaps at a time before the first patterns) (Havholm and Kocurek, 1988; Lancaster, telescopes were turned Mars-ward and the surface fea- 1988). Compound crescentic dunes may take 100– tures began to be known to observers here on Earth. 1000 years (Stokes et al., 1997), and the larger complex linear dunes may take 1000 to 10,000 years Acknowledgements (Wilson, 1972; Lancaster, 1988). The large dunes on Mars may also require several millennia to display This work was supported, in part, by NASA grant evidence of change. Alternatively, the sediment in the NNG04GJ92G MDAP and, in part, by NASA/Caltech/ larger dunes may be unavailable for transport at the Jet Propulsion Laboratory contract 959060. The authors present time. Nevertheless, the change observed in the thank the two anonymous reviewers for helpful com- small dome dunes indicates that not all dunes on Mars ments. PSI contribution # 414. are effectively stabilized and immobile. References 5. Conclusion Arvidson, R.E., Gooding, J.L., Moore, H.J., 1989. The Martian surface We have shown that three small dome dunes on Mars as imaged, sampled, and analyzed by the Viking landers. Rev. exhibited change during the 9 Earth-year MGS mission. Geophys. 27, 39–60. The estimated rates of sediment transport in this study of Bagnold, R.A., 1941. The Physics of Blown Sand and Desert Dunes. Methuen, London. 265 pp. Martian dunes are comparable with those measured for Bourke, M.C., 2004. Niveo-aeolian and denivation deposits on Earth. Higher friction velocities are required to initiate Mars. Eos, Transactions, Fall Meeting Suppl. 85 (46) (Abstract saltation on Mars because of the lower atmospheric P21B-01). density (White, 1979; Greeley et al., 1980). This sug- Bourke, M.C., 2005. Alluvial fans on dunes in Kaiser Crater suggest gests a lower frequency of transport events relative to niveo-aeolian and denivation processes on Mars. Lunar and Planetary Science Conference, XXXVI, Houston, Tx, Abs. #2373. Earth. However, the longer saltation path length coupled Bourke, M.C., Balme, M., Zimbelman, J.R., 2004. A comparative with the lower frictional resistance should result in very analysis of barchan dunes in the intra-crater dune fields and the rapid dune migration rates on Mars (Zimbelman, 2000). north polar sand sea. Lunar and Planetary Science Conference Our data indicate that rapid aeolian transport of sand XXXV, Houston, Tx, Abs. #1453. does indeed occur on Mars. Bourke, M.C., Balme, M., Bayer, R., Williams, K.K., Zimbelman, J.R., 2006. A comparison of methods used to estimate the height Our results also show that the majority of dunes on of sand dunes on Mars. Geomorphology 81 (3–4), 440–452. Mars are immobile at the present time, but their geo- Bristow, C.S., Lancaster, N., 2004. Movement of a small slipfaceless logically youthful appearance suggests that there was a dome dune in the Namib Sand Sea, Namibia. Geomorphology 59 period of time in the not-too-distant past when they were (1–4), 189–196. active. This may have occurred during climate changes Cantor, B.A., 2003. MGS-MOC observations of martian dust storm activity. Sixth International Conference on Mars, Pasadena, CA, triggered by variations in the eccentricity, obliquity and Abs. 3166. precession that are as recent as 100,000 years (Laskar Cantor, B.A., 2007. MOC observations of the 2001 Mars planet- et al., 2002). When these sands were mobile, the impact of encircling dust storm. Icarus 186, 60–96. M.C. Bourke et al. / Geomorphology 94 (2008) 247–255 255

Cantor, B., Kanak, A.K.M., Edgett, K.S., 2006. Mars Orbiter Camera Laskar, J., Levrad, B., Mustard, J.F., 2002. Orbital forcing of the observations of martian dust devils and their tracks (September Martian polar layered deposits. Nature 419, 375–377. 1997 to January 2006) and evaluation of theoretical vortex mod- Long, J.T., Sharp, R.P., 1964. Barchan-dune movement in Imperial els. Journal of Geophysical Research (Planets) 111 (E12002). Valley, California. Geological Society of America Bulletin 75, doi:10.1029/2006JE002700. 149–156. Cutts, J.A., Smith, R.S.U., 1973. Eolian deposits and dunes on Mars. Malin, M.C., Edgett, K., 2000. Frosting and defrosting of Martian Journal of Geophysical Research 78, 4139–4154. polar dunes. Lunar and Planetary Science Conference XXXI, Cwick, G.J., Campos-Marquetti, P., 1984. An analysis of dome-shaped Houston, TX, Abs. 1056. dunes in a portion of the north polar region of Mars. Department of Malin, M.C., Edgett, K.S., 2001. The Mars global surveyor mars orbiter Geography & Geology Professional Paper, 15. Indiana State camera: interplanetary cruise through primary mission. Journal of University, Terre Haute, pp. 31–44. Geophysical Research (Planets) 106 (E10), 23,429–23,570. Dong, Z., Wang, X., Chen, G., 2000. Monitoring sand dune advance in M.C. Malin, K.S. Edgett. 2005. 8 Years at Mars #1: New Dune Gullies. the Taklimakan Desert. Geomorphology 35 (3–4), 219–231. NASA/JPL Planetary Photojournal, http://photojournal.jpl.nasa. Edgett, K.S., 2002. Low albedo surfaces and eolian sediment: Mars gov/, catalog number PIA04290. Orbiter Camera views of Western Arabia Terra craters and wind Malin, M.C., Edgett, K.S., Posiolova, L.V., McColley, S.M., Dobrea, streaks. Journal of Geophysical Research 107 (E6). doi:10.1029/ E.Z.N., 2006. Present-day impact cratering rate and contemporary 2001JE001587 (Planets). gully activity on Mars. Science 314 (5805), 1573–1577. Edgett, K.S., Malin, M.C., 2000. New views of Mars eolian activity, Malin, M., Bell, J.F., Cantor, B.A., Caplinger, M.A., Calvin, W.M., materials, and surface properties: three vignettes from the Mars et al., 2007. The context camera investigation onboard the Mars global surveyor Mars orbital camera. Journal of Geophysical reconnaissance orbiter. Journal of Geophysical Research (Planets) Research (Planets) 105 (E1), 1623–1650. 112 (E05S04). doi:10.1029/ 2006JE002808. Fenton, L.K., 2006. Dune migration and slip face advancement in the Marchenko, A.G., Pronin, A.A., 1995. Study of relations between Rabe Crater dune field, Mars. Geophysical Research Letters 33 small impact craters and dunes on Mars. 22nd Russian- (L20201). doi:10.1029/2006GL027133. American Microsymposium on Planetology. Vernadsky Institute, Fenton, L.K., Bandfield, J.L., Ward, A.W., 2003. Aeolian processes in Moscow. Proctor Crater on Mars: sedimentary history as analyzed from McKee, E.D., 1966. Structures of dunes at White Sands National multiple data sets. Journal of Geophysical Research (Planets) 108 Monument, New Mexico. Sedimentology 7, 1–60. (E12). doi:10.1029/2002JE002015. Monolithic Dome Institute, 2001. Spherical dome formulas. Available Filip, S.D., 2006. ABE volume calculator page. Agricultural and at http://www.monolithic.com/plan_design/calcs/index.html#12. Biological Engineering, Mississippi State University, available at Moore, H.J., 1985. The martian dust storm of sol 1742. Journal of http://grapevine.abe.msstate.edu/~fto/tools/vol/index.html. Geophysical Research (Planets) 90, 163–174. Finkel, H.J., 1959. The barchans of southern Peru. Journal of Geology Richter, L., Grzesik, A., Krause, C., MER Athena Science Team, 2006. 67, 614–647. Soil crusts observed and investigated at the MER landing sites. Greeley, R., 2002. Saltation impact as a means for raising dust on European Geosciences Union, Abs. 05489. Mars. Planetary and Space Science 50 (2), 151–155. Schatz, V., Tsoar, H., Edgett, K.S., Parteli, E.J.R., Herrmann, H., 2006. Greeley, R., Leach, R., White, B.R., Iverson, J.D., Pollack, J.B., 1980. Evidence for indurated sand dunes in the Martian north polar Threshold windspeeds for sand on Mars: wind tunnel simulations. region. Journal of Geophysical Research (Planets) 111 (E4). Geophysical Research Letters 7 (2), 121–124. doi:10.1029/2005JE002514. Greeley, R., Leach, R.N., Williams, S.H., White, B.R., Pollack, J.B., Schofield, J.T., Barnes, J.R., Crisp, D., Haberle, R.M., Larsen, S., et al., 1982. Rate of wind abrasion on Mars. Journal of Geophys- et al., 1997. The Mars Pathfinder atmospheric structure investiga- ical Research (Planets) 87 (B12), 10,009–10,024. tion/meteorology (ASI/MET) experiment. Science 278, 1752–1758. Greeley, R., Arvidson, R.E., Barlett, P.W., Blaney, D., Cabrol, N.A., Stokes, S., Kocurek, G., Pye, K., Winspear, N.R., 1997. New et al., 2006. Gusev crater: wind-related features and processes evidence for the timing of aeolian sand supply to the Algodones observed by the Mars Exploration Rover Spirit. Journal of Geo- dunefield and East Mesa area, southeastern California, USA. physical Research (Planets) 111 (2). Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1–4), Hartmann, W.K., 2005. Martian cratering 8: isochron refinement and 63–75. the chronology of Mars. Icarus 174, 294–320. Thomas, P.C., 1982. Present wind activity on Mars: relation to large Hastenrath, S.L., 1967. The barchans of the Arequipa region, southern latitudinally zoned sediment deposits. Journal of Geophysical Peru. Zeitschrift fur Geomorphologie 11, 300–311. Research 87, 999–10,008. Havholm, K.G., Kocurek, G., 1988. A preliminary study of the White, B.R., 1979. Soil transport by winds on Mars. Journal of dynamics of a modern draa, Algodones, southeastern California. Geophysical Research (Planets) 84, 4643–4651. Sedimentology 35 (4), 649–669. Williams, K.K., 2004. The search for dune movement on Mars. Eos, Hesp, P.A., Hastings, K., 1998. Width, height and slope relationships Transactions, Fall Meeting Suppl. 31, 0985. and aerodynamic maintenance of barchans. Geomorphology 22 Wilson, I.G., 1972. Aeolian bedforms — their development and (2), 193–204. origins. Sedimentology 19, 173–210. Iverson, J.D., White, B.R., 1982. Saltation thresholds on Earth, Mars Zimbelman, J.R., 2000. Non-active dunes in the Acheron Fossae and Venus. Sedimentology 29, 111–119. region of Mars between the Viking and Mars Global Surveyor eras. Lancaster, N., 1988. Controls of eolian dune size and spacing. Geology Geophysical Research Letters 27 (7), 1069–1072. 16, 972–975.