EARTH SURFACE PROCESSES AND LANDFORMS Earth Surf. Process. Landforms 37, 1437–1443 (2012) Copyright © 2012 John Wiley & Sons, Ltd. Published online 31 July 2012 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/esp.3289
The influence of local topography for wind direction on Mars: two examples of dune fields in crater basins Marco Cardinale,1* Goro Komatsu,1 Simone Silvestro2 and Daniela Tirsch3 International Research School of Planetary Sciences, Università G. D’Annunzio, Pescara, Italy 2 SETI Institute – Carl Sagan Center, Mountain View, CA, USA 3 German Aerospace Center (DLR), Institute of Planetary Research, Berlin, Germany
1
Received 31 May 2011; Revised 4 June 2012; Accepted 11 June 2012 *Correspondence to: Marco Cardinale, International Research School of Planetary Sciences, Università G. D’Annunzio, Viale Pindaro 42, 65127 Pescara, Italy. E-mail:
[email protected]
ABSTRACT: In this work, we perform an analysis of large dark dunes within Moreux Crater and Herschel Crater on Mars using High Resolution Imaging Science Experiment (HiRISE) and Context Camera (CTX) data sets. These data allow us to conduct a detailed analysis of dune morphology and slip faces, concluding that the studied dune fields are influenced by topographically-controlled complex wind directions. Our morphological analysis reveals that inside Moreux Crater in particular, the topographic setting dominates the wind flow direction, leading to the development of a sand transport pathway encircling the central peak of the crater. The dune fields in Herschel Crater are also affected by winds controlled by variable topography as suggested by the presence of complex dunes and dune fields. Our analysis indicate that the studied dune systems is not the result of paleo-wind regimes. Furthermore, we perform thermal inertia measurements using thermal emission spectrometer (TES) data, which indicate that the studied dune fields consist of medium sand 250–500 mm in diameter. Copyright © 2012 John Wiley & Sons, Ltd. KEYWORDS: Mars; dunes; aeolian processes; Moreux Crater; Herschel Crater
Introduction Mars is characterized by plains, impact craters and topographic depressions where sand accumulates in dune fields (Thomas et al., 1981; Edgett and Christensen, 1994) that have been observed from early missions such as Mariner 9 and Viking Orbiters (McCauley et al., 1972; Cutts and Smith, 1973; Ward et al., 1985). A variety of large dark dunes are estimated to cover an area of 904 000 km2 on the Martian surface (Hayward et al., 2007), and the morphology of sand dunes has been used as ‘a ground truth’ for atmospheric models (Hayward et al., 2009). Mafic compositions of the dark dune materials reflect mixtures of pyroxene and olivine minerals (Christensen et al., 2000; Mangold et al., 2007), as revealed from the spectral analysis performed by Tirsch et al. (2011). The topography controls the development of dune features (Breed et al., 1979) as recognized in previous investigations of intra-crater dune fields such as Gale Crater (Hobbs et al., 2010) and Proctor Crater (Fenton et al., 2005). These authors showed a possible correlation between the wind regime and the topographic setting that controls the wind flow and, in turn, the morphology of the dunes inside the crater. Crater walls, central peaks and other topographic obstructions can influence the main wind conditions leading to the development of particular dune systems. According to Greeley et al. (2002) and Silvestro et al. (2009), these topographic settings can accelerate wind velocity.
The latest high-resolution images of Mars Reconnaissance Orbiter (MRO) High Resolution Imaging Science Experiment (HiRISE) (McEwen et al., 2007) allow us to study dune fields that were previously unresolved and indeed absent from the Digital Dune Database (Hayward et al., 2007). The goal of our study is to use such high-resolution images for analyzing dark dunes in two different locations on Mars, which are characterized by complex topographic environments: first, we analyzed intra-crater dune fields in Moreux Crater located in the Ismenius Lacus Region (MC05), second, we studied the intra-crater dune fields in Herschel Crater in the Mare Tyrrenium Region (MC22). Finally, thermal inertia (TI) measurements are used to determine the effective grain size of sands in dune fields (Presley and Craddock, 2006, Presley et al., 2009) .The derived range of the grain sizes allows us to differentiate between unconsolidated and movable grains or fixed deposits of immobile grains (Tirsch, 2009).
Methods We conducted a detailed geomorphological analysis of Moreux Crater and Herschel Crater basins (Figures 1–4). Images have been processed using the United States Geological Survey Integrated Software for Imagers and Spectrometers (ISIS) and
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extracted and used only the nighttime TI data with the values ranging between 24 and 800 J m–2 K–1 s–½. Following Edgett and Christensen (1994), we used nighttime TI because grain size effects are enhanced and variations in slope are reduced. TI maps derived from the TES data show differences between the dark dunes and the surrounding environments. The TI maps of the investigated areas were placed in a GIS.The values were classified using TI ranges of the table adapted from Edgett and Christensen (1994) to obtain unique datasets that fit from 64 to 798 J m–2 K–1 s–½ representing minimum and maximum TI values ranging from silt to pebbles. We used the relationship presented by Edgett and Christensen (1994) for the TI values and the corresponding grain size for silicate mineral. This classification assumes a surface pressure of 65 mbar and a correlation between the nominal dependence of grain size on TI and thermal conductivity as described by Kieffer et al. (1973) and Edgett and Christensen (1991). TES data’s spatial resolution of 3 by 3 km per pixel is not very appropriate for small morphologies. In fact, small dunes might lie within a pixel that contains thermal information from the dunes and the surrounding terrain (mixed pixel). Therefore, our interpretation is more appropriate for dune fields larger than the pixel resolution of our data.
Dune Morphology and Wind Direction Analysis Figure 1. A detailed geomorphological map of the Moreux Crater showing aeolian features overlain over a hill shade Mars Orbiter Laser Altimeter (MOLA) dataset and a Thermal Emission Imaging System (THEMIS) infrared daytime mosaic. The inset at the top represents a MOLA map showing the location of the Moreux Crater.
then integrated into a geographical information system (GIS) project. The high resolution stereo camera (HRSC) (Neukum et al., 2004; Jaumann et al., 2007) image data from Mars Express, with their spatial resolution of 125 m, provide a good coverage of surface features. The context camera (CTX) images (Malin et al., 2007) with a spatial resolution of 6 m/pixel, and the HiRISE images with spatial resolutions of 25 to 32 cm/pixel, allow us to resolve dune slip face orientations. The latter reflects the direction of the net dune migration giving a clue on the wind regime that shaped the dunes. For this reason we carefully mapped the slip faces of the crescentic dunes using HiRISE and CTX data following the work of Fenton et al. (2003). These measurements were plotted on rose diagrams illustrating the net migration direction inferred from dune slip face orientations in a downwind direction. Basic statistical parameters such as mean vector and circular standard deviation are then used to reconstruct the wind regime. Where slip faces are not recognizable we used the trends of other aeolian features such as wind streaks (Thomas et al., 1981) to constrain the wind regime responsible for the accumulation of the studied dune fields. The morphology of the dunes was classified using McKee’s criteria (McKee, 1979). Barchan, barchanoid, linear and transverse dunes are recognized together with star and dome dunes. TI analysis determines the diurnal and seasonal variation of surface temperatures and is related to the thermal conductivity of the surface (Jakosky and Christensen, 1986). Thermal emission spectrometer (TES) data (Christensen et al., 1998, 2001) obtained by the Mars Global Surveyor (MGS) possess diverse information for every location on Mars, but we Copyright © 2012 John Wiley & Sons, Ltd.
Moreux Crater The first study site is the Moreux Crater (inset at the top of Figure 1) (41.4 N, 44.2 E) located in the Ismenius Lacus Region (MC05). The Moreux Crater is a 135-km diameter impact basin situated at the edge of the southern upland plateau, and its southern portion represents the dichotomy boundary, dislocating the regional scarp (Marchant et al., 2006). The crater floor lies 3 km below the rim and the central peak is more than 2 km in height. Diverse types of large dark dunes, that cover an area of 530 km2, are accumulated around the eroded central peak in the Moreux Crater and evolve under the influence of a circular pattern of multi-directional airflow controlled by the topographic setting (Figure 1). Northwestwards of the central peak, Figure 2a shows a 10-km2-dune field which consists of barchanoid ridges (average crest length of 800 m) and barchan dunes (subset of Figure 2a). The latter has clearly recognizable slip faces orientated towards the southwest. Moving south, two main wind directions, from the northeast and from the northwest, are inferred from dune slip face orientations (Figure 2b). The convergence of these flows leads to the development of the star dunes shown in the subset of Figure 2b. Along the western side of the peak, a 28-km-long sand corridor is visible, which is mainly composed of barchan dunes. The barchan slip face orientations and sand streaks are clearly recognizable and are orientated towards the south, indicating main wind flows from the north as illustrated in Figure 2c. The barchans forming this sand transport-path link the diverse dune fields, similar to the situation described in other areas of Mars (Silvestro et al., 2010a). Towards the south a small number of irregular dune forms are present, which seem to reflect the convergence of several wind directions (Figure 2d). Southeast of the central peak, the dune field consists of barchan and transverse dunes whose slip faces show nearly opposite orientations (Figure 2e) in which the bimodal nature of the wind regime is clearly recognizable. These two wind flows seem to be channeled by the depression between the inner rim of the crater and the central peak. The dune field Earth Surf. Process. Landforms, Vol. 37, 1437–1443 (2012)
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Figure 2. (a) Complex dune system formed by barchanoid ridges and barchan dunes with the inferred wind directions (black arrows) shown in the context camera (CTX) image P03_002098_2220_XI_42315W. Details of the dune system in the subset showing a group of spaced barchans. (b) CTX image P03_002098_2220_XI showing dunes with multiple wind orientations. The black arrows represent bimodal wind flows measured by slip face orientations. Details of the dune system in the subset showing star dunes. (c) A close-up of a sand corridor showing barchan dunes from the CTX image P03_002098_2220_XI_42N315W. (d) A close-up of irregular dune forms in High Resolution Imaging Science Experiment (HiRISE) image PSP_001735_2220. (e) Dune field to the west side of the central peak in the Moreux Crater in which the black arrows represent two opposite wind flows measured by slip face orientations. CTX image P05_002810_2220_XI_42N314W. (f) Dunes characterized by bimodal winds are shown in CTX image P05_002810_2220_XI_42N314W, the white arrows represent the general wind directions measured by slip face orientations.
shown in Figure 2f like the area shown in Figure 2e, it is characterized by a complex dune arrangement with dune slip faces being oriented toward the east and west, suggesting the presence of at least two wind flows. Sand streaks departing from singular barchan horns are also visible on the western margin of the dune field. In Figure 3 the slip face orientations for the crescentic dunes in the Moreux Crater is plotted on rose diagrams; generally the orientation of the slip face reflect dominant winds from the north (rose diagram in inset of Figure 3) that are then diverted by the complex topographic setting of the crater. The high value for the standard deviation (m = 146 690 43 430 ) reflects such wind complexity. The derived wind direction is not in agreement with the global climate model (GCM) results which predict regional winds from southwest as illustrated in the inset map in Figure 3. Copyright © 2012 John Wiley & Sons, Ltd.
Herschel Crater The second study site is the Herschel Crater (inset at the top of Figure 4) (14.4 S, 129.4 E) located in the Mare Tyrrenium Region (MC22). This crater is a 300-km-diameter impact basin situated in the cratered southern highland, 100 km east of the volcanic terrain of Hesperia Planum (Greeley and Guest, 1987). The basin rim stands 15 km above the floor, with the southern rim modified by diverse impact craters > 10 km in diameter. Diverse types of dark dunes are deposited in the Herschel Crater (Figure 4). The dune field in Figure 5a resides within a 23-km-diameter crater in the central zone of the Herschel Crater and covers an area of 15 km² (Cardinale et al., 2010). Similar intra-crater dune fields on Mars have been described as single complex dunes (Silvestro et al., in press). This type of complex dune is formed by the coalescing of several barchans Earth Surf. Process. Landforms, Vol. 37, 1437–1443 (2012)
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Figure 3. Wind rose diagrams of all the studied large dark dunes, overlain on their respective areas in the Moreux Crater. Boxed numbers indicate the mean and the standard deviation wind director. The rose diagram in the inset represents the distribution and the orientation of all the crescentic dunes on the floor of the crater. The inset at the bottom represents the location map of the Moreux Crater with the wind directions (the white arrows) predicted by the global climate model (GCM) simulations.
and barchanoid ridges influenced by winds from different directions that converge inside this crater. A complex pattern is visible in the southeast margin (Figure 5b) as a result of the complex wind regime characterizing this area. The dune field shown in Figure 5c is accumulated around the central peak of a 45-km-diameter crater at the southwest margin of the Herschel Crater, and it covers an area of 23 km2. The diverse dunes in this crater (barchan, barchanoid ridges and star dunes) (Figures 5d and 5e) also reflect winds from different directions influenced by the topographic setting. The slip face orientations of these two dune fields have a high dispersion as suggested by the high values of standard deviations (m = 186 100 68 880 for Figure 5a and m = 177 450 39 330 for Figure 5c). We suggest that this variability in the orientation of the slip faces is related to the complex wind regime that is strongly topographically controlled. Conversely, outside these craters, the other dune fields show a more normal distribution of slip faces as illustrated in Figure 6, suggesting a less complex wind regime compared to the two earlier-mentioned smaller craters. Here the slip face orientations generally indicate sand migration from the north, being more in agreement with GCM wind predictions (Figure 6).
Thermal Inertia (TI) In the Moreux Crater, the dune field illustrated in Figure 7a shows TI values of approximately 322 J m–2 K–1 s–½. According Copyright © 2012 John Wiley & Sons, Ltd.
Figure 4. A detailed geomorphological map of the Herschel Crater over a merged MOLA map and a THEMIS infrared daytime mosaic. The studied dune fields are marked with black boxes. The inset at the top represents a MOLA map showing the location of the Herschel Crater.
to the table of Figure 7, this value falls within the range of medium sand. High TI values, about 400 J m–2 K–1 s–½, corresponding to coarse sand, are found all around the south side of the perimeter. This may be from the mixture of lower TI signature from the dune field material with the higher TI values of the solid crater floor; some of the high values spots on the left of Figure 7a are artifacts due to the TES data processing. In the Herschel Crater we analyzed two intra-crater dune fields and we obtained the TI values as illustrated in Figures 7b and 7c. The TI values of the intra-crater dune field shown in Figure 7b report a clear value boundary between the deposit and the crater floor of 242 J m–2 K–1 s–½ corresponding to medium sand, meanwhile in the south side of the intra-crater dune field an increase of TI values is observed. The bulk of intra-crater dune field illustrated in Figure 7c shows TI values around 281 J m–2 K–1 s–½ corresponding to medium sand (see the table in Figure 7). We noticed sharp boundaries of TI values corresponding to the sand deposits and the crater floor. An increase of TI is noticeable all around the eroded central peak where Figure 7c displays high TI values indicating consolidated surfaces. The analysis we conducted on the TI values of the dune sediments indicates that the predominant range is 240–330 J m–2 K–1 s–½. These values correspond to the grain size range of medium sand, following the estimation by Edgett and Christensen (1991).
Discussion and Conclusions Our morphological analysis of dark dune systems in the Moreux and Herschel Craters suggests that the dark dunes were deposited under a multi-directional wind regime. This is Earth Surf. Process. Landforms, Vol. 37, 1437–1443 (2012)
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Figure 5. (a) Complex dune system occurring in a 23-km-diameter crater nestled within the Herschel Crater (location is shown in Figure 4). CTX image P01_001396_1655_XI_14S231W. (b) Subset of CTX image P01_001396_1655_XI_14S231W showing star dunes. (c) Complex dune system in a 45-km-diameter crater nestled within the Herschel Crater (location is shown in Figure 4).CTX images P15_006974_1639_XN_16S231W, P15_006974_1639_XN_16S231W and THEMIS image V07249003RDR. (d) Subset of HiRISE image PSP_007185_1635 showing star dunes. (e) Subset of HiRISE image PSP_004350_1625 showing star dunes.
Figure 6. Wind rose diagrams of all the studied large dark dunes, overlain on their respective areas in the Herschel Crater. The dune fields are represented by mapped units. Boxed numbers indicate the mean and the standard deviation wind director. The white arrows indicate the azimuth of the wind shear stresses derived from GCM. This figure is available in colour online at wileyonlinelibrary.com/journal/espl
confirmed by the presence of long sand corridors encircling the central peak in the Moreux Crater, and by the presence of complex dunes in the Herschel Crater. In both the examples described, the slip face analysis has revealed that dunes were accumulated in the crater by prevailing wind flows from the north and that such winds are then diverted by the complex topography of the two craters (i.e. crater rims and central peaks). Different wind conditions, Copyright © 2012 John Wiley & Sons, Ltd.
potentially related to different phases of the Martian climatic history, are not necessary to explain the dark dune morphologies described in this work. This interpretation is supported by the recent observation of dark dune movement from different areas on Mars (Silvestro et al., 2010b; Chojnacki et al., 2011; Hansen et al., 2011; Silvestro et al., 2011) which indicate that the dark dunes are currently in equilibrium with the current atmospheric conditions. As already noted by other workers (Hayward et al., 2007), the comparison between GCM predictions and slip face orientations is controversial. Mesoscale wind models would be probably better in agreement with dune morphology in topographically complex areas as the ones described in this work (Silvestro et al., in press). The TI analysis suggests that medium sand grain size is the most common (250–500 mm) in the studied dune fields. According to Edgett and Christensen (1991), the grain sizes covering the medium to coarse sand range can be movable and deposited as dunes. According to Tirsch (2009), the TI value separating unconsolidated, movable dune grains and consolidated, immovable dune grains is about 400 J m–2 K–1 s–½ (corresponding to a grain size of about 900 ìm). However, in the studied areas the intra-crater dune fields show only TI values lower than 400 J m–2 K–1 s–½, corresponding to the range of unconsolidated deposits (Tirsch, 2009). TI values corresponding to indurated deposits are not recognized in our study. This means that the analyzed dune surfaces are likely unconsolidated and indeed potentially movable. This is consistent with their pristinelooking morphologies. Acknowledgements—The authors would like to thank Rosalyn Hayward, Robert Craddock and Mark Bishop, the guest editor of the special issue for their helpful comments and suggestions, which greatly improved the manuscript. S. Silvestro is supported by a grant from the NASA Mars Data Analysis program (NNH09ZDA001N). Earth Surf. Process. Landforms, Vol. 37, 1437–1443 (2012)
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Figure 7. (a) Intra-crater dune field in the Moreux Crater, represented by gridded TES thermal inertia (TI) data overlain over CTX image P03_002098_2220_XI_42315W. High value spots on the west side are artifacts from the TES data processing. (b) Intra-crater dune field in the Herschel Crater, represented by gridded TES TI data overlain over CTX image P01_001396_1655_XI_14S231W. (c) Intra-crater dune field Herschel Crater, represented by gridded TES TI data overlain over CTX image P15_006974_1639_XN_16S231W, P15_006974_1639_XN_16S231W and THEMIS visible image V07249003RDR. Table showing a comparison of grain sizes with TI values. Adapted from Edgett and Christensen (1994).
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