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Keywords:

  • aeolian activity;
  • Endeavour crater;
  • dune movement;
  • Opportunity rover;
  • mesoscale atmospheric modeling

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction and Study Area
  4. 2. Method and Data
  5. 3. Results
  6. 4. Discussion
  7. 5. Summary and Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

[1] Although aeolian landforms are pervasive on Mars, evidence for contemporary activity has been limited. The next major campaign for the Mars Exploration Rover “Opportunity” is the investigation of the ∼20 km diameter Endeavor crater, ∼6 km to southeast of the rover's position as of December 2010. We present evidence from orbital imagery that eight aeolian bed forms (∼14,000 m2) in Endeavor crater have been active within the past decade (2001–2009), at a spatial scale that should be directly observable by Opportunity from the crater rim. Two dunes appear to show translational migration (∼10–20 m), but all dunes indicate erosion to be the dominant process with up to 100% sediment removal. Thermophysical properties of these dunes are consistent with very fine to fine sand sizes, the particle sizes most easily moved by the Martian atmosphere. The dunes that show the most surface change have a rippled appearance without well-defined slip faces. Based on their morphology (elliptical shape), we classify them as dome dunes. Mesoscale atmospheric modeling is employed to provide insight into the atmospheric forcing of this aeolian system. The major wind regimes from modeling are consistent with observations of wind streaks, sand streamers, ripples, and slip faces of regional dune fields although modeled wind speeds are insufficient to move sand. The translation and erosion of these dunes constitutes the largest contemporary movement of sand-sized sediment reported on Mars to date and demonstrates that Endeavor crater has been subject to wind profiles exceeding the threshold velocity at the surface (daily/seasonally and/or episodically) in the recent past.

1. Introduction and Study Area

  1. Top of page
  2. Abstract
  3. 1. Introduction and Study Area
  4. 2. Method and Data
  5. 3. Results
  6. 4. Discussion
  7. 5. Summary and Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

[2] Despite a dynamic atmosphere and plentiful sediment supply, orbital detection of dune movement on Mars has been elusive. For more than a century, telescopic observations of Mars showed temporal albedo variations associated with regions such as (what is now known as) Meridiani Planum [Flammarion, 1892]. Spacecraft data from Mariner 9 and Viking suggested aeolian deposits are common and dynamic on Mars, with wind-related albedo changes occurring during these missions [e.g., Sagan et al., 1973; Greeley et al., 1992]. Geissler [2005] used three decades of orbital data to document changes in albedo, and suggested that more than a third of the Martian surface had brightened or darkened by roughly 10% in that time by wind-related processes. Albedo changes have been attributed to the aeolian deposition and raising of the light-toned dust particles [e.g., Sagan et al., 1973; Geissler, 2005; Cantor, 2007]. The calculated saltation threshold friction speeds (verified in wind tunnel experiments under Martian atmospheric conditions) are much greater for dust than for sand-size particles because of the cohesive effects of interparticle forces [Greeley et al., 1980]. Winds that are strong enough to directly mobilize single dust particles should also be of sufficient magnitude to initiate saltation of most sand sizes. Yet somewhat paradoxically, although lofting of dust from the Martian surface is amply documented, the orbital detection of sand dune modification has been lacking [Edgett and Malin, 2000; Malin and Edgett, 2001; Bridges et al., 2007]. The Mars Global Surveyor's (MGS) Mars Orbiter Camera (MOC) experiment was used to look for dune migration that may have occurred between the time of the Viking and MGS missions [Edgett and Malin, 2000; Malin and Edgett, 2001]. No dune migration was observed over that 14 Mars year time span [Edgett and Malin, 2000; Malin and Edgett, 2001].

[3] However, Bourke et al. [2008, 2009a] observed the gradual disappearance of two small (∼1000 m2) dome dunes and ∼85% deflation of a third over a 5 year time span (1999–2004) in MOC images. That study reported no bed form migration. More recently, orbital images from the Mars Reconnaissance Orbiter (MRO) High Resolution Imaging Science Experiment (HiRISE) [McEwen et al., 2007] have identified meter-scale modification of ripples and dune edges in less than a Martian season [Silvestro et al., 2010a, 2010b]. These events demonstrate that the threshold wind speed for sand entrainment was exceeded under current conditions in these locations.

[4] Evidence for aeolian activity, both ancient and contemporary, has been detected many times in surface-based observations. The combination of abundant sand-sized particles and erosional surfaces (e.g., ventifact flutes, wind streaks, etc.) at the Viking and Pathfinder landing sites suggested that abrasion due to repeated saltation has been commonplace [Bridges et al., 1999; Greeley et al., 2002], at least relative to geologic timescales. The Mars Exploration Rover (MER) Spirit has encountered a number of aeolian features and phenomena at Gusev crater, including: ventifacts, ripples, wind streaks, the El Dorado ripple field, and active dust devils [Greeley et al., 2006, 2008; Sullivan et al., 2008]. Evidence for geomorphically effective wind events occurred during the 2007 global dust storm, with Spirit observing rover track modification and ripple migration [Sullivan et al., 2008]. In addition to observed ripple modifications with associated sand movement, sand-sized grains were found on the 66 cm tall rover deck [Greeley et al., 2006], supporting the notion of contemporary sand saltation.

[5] In the last decade, Meridiani Planum has become one of the most intensely studied regions on Mars. The orbital detection of crystalline hematite associated with regional-scale layered deposits [Christensen et al., 2001] contributed to Meridiani being selected as the landing site for the Opportunity rover [Golombek et al., 2003; Squyres et al., 2004]. At the Opportunity landing site, ripples (of mixed particle sizes) and occasional wind streaks (dark and light) are pervasive [Sullivan et al., 2005; Fergason et al., 2006a]. The last major ripple migration episode along the Opportunity traverse is estimated from crater statistics to have occurred between ∼50 ka and ∼200 ka [Golombek et al., 2010], and is thought to be related to the atmospheric conditions during periods of higher axial obliquity [Arvidson et al., 2011]. Evidence for modern aeolian activity at the rover landing site was found in the form of the deposition and erosion of basaltic sand observed by Opportunity in a dark streak at Victoria Crater [Geissler et al., 2008]. However, modeled near-surface mean winds for the landing site at Ls ∼ 320° [Rafkin and Michaels, 2003] were below the minimum threshold for saltating sand on Mars [cf. Greeley et al., 1980], and no wind-related bed form change has been detected in orbital observations.

[6] The apparent disagreement between surface and orbital observations at Meridiani Planum, and the limited orbital detection of motion in general on Mars raise some pertinent questions concerning aeolian processes on Mars. (1) Why is bed form change not more commonly observed from orbit? (2) Why do some bed forms show changes from orbit but not others? (3) What factors are controlling bed form movement?

[7] The next major campaign planned for Opportunity will be an investigation of Endeavor crater (2.3°S, 5.3°W; Figure 1 and Figure S1), which reaches deeper into the stratigraphy and geologic history of this region than any of the other craters previously visited by the rover. Endeavor crater is a degraded primary crater with a diameter of ∼20 km. This crater predates the deposition of the sulfate-bearing layered deposits and is Noachian in age [Wray et al., 2009; Arvidson et al., 2011]. Crater rim segments are in various preservation states: rim segments to the north, west, and east have been rounded and degraded, whereas rims to the northwest and south have been completely removed or buried. Endeavor's floor shows heightened spectral contrast for basalt, hematite, and possibly hydrated sulfates as compared with the surrounding plains [Chojnacki et al., 2010a]. This enhancement was attributed to the intracrater dust coverage being lower than outside the crater (i.e., more frequent dust cleaning events), rather than to a greater mineral abundance. As of December 2010, Opportunity is ∼6 km from the western rim of Endeavor crater. The rim of Endeavor has been found to include phyllosilicate minerals [Wray et al., 2009], a mineralogy not yet examined in situ on Mars and a driving motivator for Opportunity's exploration there.

[8] The next major campaign planned for Opportunity will be an investigation of Endeavor crater (2.3°S, 5.3°W; Figure 1 and Figure S1), which reaches deeper into the stratigraphy and geologic history of this region than any of the other craters previously visited by the rover. Endeavor crater is a degraded primary crater with a diameter of ∼20 km. This crater predates the deposition of the sulfate-bearing layered deposits and is Noachian in age [Wray et al., 2009; Arvidson et al., 2011]. Crater rim segments are in various preservation states: rim segments to the north, west, and east have been rounded and degraded, whereas rims to the northwest and south have been completely removed or buried. Endeavor's floor shows heightened spectral contrast for basalt, hematite, and possibly hydrated sulfates as compared with the surrounding plains [Chojnacki et al., 2010a]. This enhancement was attributed to the intracrater dust coverage being lower than outside the crater (i.e., more frequent dust cleaning events), rather than to a greater mineral abundance. As of December 2010, Opportunity is ∼6 km from the western rim of Endeavor crater. The rim of Endeavor has been found to include phyllosilicate minerals [Wray et al., 2009], a mineralogy not yet examined in situ on Mars and a driving motivator for Opportunity's exploration there.

image

Figure 1. (a) A CTX visible-wavelength image mosaic showing Endeavor crater and the location of the Opportunity rover as of December 2010. (b) CTX image P17_007849_1793_XN_00S005W showing the crater basin and the aeolian sand dunes forming two distinct groups to the east and west (black arrows). For the well-formed barchans in the west, white arrows are oriented perpendicular to dune slip faces (downwind). For the degraded dunes and bed forms in the east, white lines are oriented perpendicular to transverse dune orientation. (c) A closer view of the eastern dunes showing the bed forms that were observed to change from 2001–2003 to 2008–2009. Black arrows indicate inferred primary wind direction, and white arrows show areas of dune morphological modification of larger dunes. North is toward the top in all images unless otherwise indicated. For a version with fewer annotations see Figure S1.

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[9] In this paper, we present evidence for contemporary dune deflation, modification, and translation in Endeavor Crater. First we provide a summary of the methods used to characterize Martian dunes and likely atmospheric conditions with associated spacecraft data and mesoscale atmospheric modeling. Next we present an overview of Endeavor crater and the characterization of aeolian activity there using a suite of remote sensing data sets and mesoscale atmospheric modeling. We then discuss the factors operating to create or reveal such an apparently high level of local aeolian activity and the implications of dune change on Mars. We conclude with a brief summary of our findings.

2. Method and Data

  1. Top of page
  2. Abstract
  3. 1. Introduction and Study Area
  4. 2. Method and Data
  5. 3. Results
  6. 4. Discussion
  7. 5. Summary and Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

2.1. Visible Wavelength Images

[10] This study utilized visible-wavelength images from several spacecraft to search for surface changes. Narrow-angle images acquired between 2001 and 2003 from the MGS MOC instrument [Malin et al., 1992] provide visible wavelength data at ∼3 m/pixel. Additionally, we utilized images acquired by MRO during the period 2007 through 2009 with both the Context Camera (CTX) [Malin et al., 2007] at 5–6 m/pixel and the HiRISE instrument at up to 25 cm/pixel. Images of Endeavor crater from all three instruments were examined for photogeologic evidence of aeolian bed forms. We utilized MOC images acquired in 2001 and 2003 to compare with MRO images taken in 2008, a time period of 3.3 Mars years (6.3 Earth years) and 2.3 Mars years (4.3 Earth years), respectively (see Table 1 for image identification and resolution). We examined other MRO images in addition to those listed in Table 1 for evidence of subsequent or intermediate surface change. Images were processed using U.S. Geological Survey (USGS) Integrated Software for Imagers and Spectrometers (ISIS) software [Gaddis et al., 1997] for radiometric calibration, and spatially registered to each other using ArcGIS and ENVI software, allowing dune morphometry to be measured.

Table 1. Morphometric Values Before and After Dune/Bed Form Removal and/or Transporta
 Image ID, DOA, and LsbLength (m)Width (m)Area (m2)Area Loss (%)Distance Moved (m)Time Change Earth (years)Time Change Mars (years)Area Removal Rate Earth (m2/yr)Area Removal Rate Mars (m2/yr)Transport Rate Earth (m/yr)Transport Rate Mars (m/yr)
  • a

    The dates of image of acquisitions (DOA) are given. Image identification and resolution are as follows: C1, P17_007849_1793_XN_00S005W at 5.37 m/pixel; C2, B11_013954_1780_XN_02S005W at 5.37 m/pixel; H1, PSP_007849_1775 at 0.25 m/pixel; M1, E1101328 at 3.61 m/pixel; and M2, R1203949 at 3.59 m/pixel, where C, CTX; H, HiRISE; and M, MOC. Values for dunes iii, vii, and viii are based on two pairs of observations, M1-H1 and C1-C2. Here na, not applicable.

  • b

    Date format is month/day/year.

Dune iM1, 12/8/2001, Ls 287°58241,071        
 H1, 3/30/2008, Ls 52°nana0100na6.33.3170325nana
Dune iiM1, 12/8/2001, Ls 287°63291,630        
 H1, 3/30/2008, Ls 52°nana0100na6.33.3259494nana
Dune iiiM1, 12/8/2001, Ls 287°71432,094        
 H1, 3/30/2008, Ls 52°47301,15945na6.33.3148283nana
 C1, 3/30/2008, Ls 52°45311,090        
 C2, 7/18/2009, Ls 306°412982724na1.30.7202376nana
Dune ivM2, 12/30/2003, Ls 325°62391,929        
 C1, 3/30/2008, Ls 52°48341,25935na4.32.3156291nana
Dune vM2, 12/30/2003, Ls 325°109454,322        
 C1, 3/30/2008, Ls 52°69502,72837204.32.33716934.78.7
Dune viM2, 12/30/2003, Ls 325°131848,666        
 C1, 3/30/2008, Ls 52°88745,78633104.32.36701,2522.34.3
Dune viiM1, 12/8/2001, Ls 287°97764,718        
 H1, 3/30/2008, Ls 52°75603,10334na6.33.3256489nana
 C1, 3/30/2008, Ls 52°71623,185        
 C2, 7/18/2009, Ls 306°57511,42255na1.30.71,3562,519nana
Dune viiiM1, 12/8/2001, Ls 287°129967,313        
 H1, 3/30/2008, Ls 52°117826,6669na6.33.3103196nana
 C1, 3/30/2008, Ls 52°122837,010        
 C2, 7/18/2009, Ls 306°110816,01914na1.30.77621,416nana
Total   14,059        

[11] Complications in comparing data from the three imagers arise from differences in atmospheric conditions, in viewing geometries, and, most significantly, in spatial resolution. These differences have been mitigated through the superior resolution and high signal-to-noise ratio of HiRISE data and the coregistration of small features (e.g., craters). Additionally, we have applied an empirically derived contrast enhancement to the MRO images to match the dynamic range of pixel brightness in the MOC images. This method for modifying the contrast takes the (MOC) image near-minimum-maximum pixel values and ties them to the (MRO) near-minimum-maximum pixels values linearly. One example of the result of this method is shown in Figures 2a and 2b where we have also downsampled the HiRISE image to match the original MOC resolution.

image

Figure 2. Visible-wavelength images showing the deflation of dunes i–iii and vii. Blue polygons represent the extent of dunes in 2001, and yellow polygons represent the extent in 2008. See Table 1 for all image numbers and resolutions. (a) MOC image and (b) a HiRISE image downsampled to MOC resolution with approximately the same contrast enhancement of the scene. A closer view in the color HiRISE image illustrating that the texture of the eastern dune is no longer visible where dune i was previously located (inset with a ∼80 m field of view). (c) MOC shows dune ii in 2001. (d) HiRISE shows the deflation for dune ii and the ∼50% reduction in size of dune iii in 2008. Closer examination shows differences in surface texture from dune ii to dune iii and hints at a southward erosional direction (inset with a ∼80 m field of view). (e) MOC image and (f) HiRISE shows a ∼35% reduction in area for dune vii. Closer examination shows differences in surface texture from the previous extent (inset with a ∼80 m field of view).

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[12] As a simple assessment of the accuracy of our area measurements for the different image combinations, we made area measurements of several (presumably unchanging) impact craters of approximately the same scale as the dunes that were the subject of this study (1000–8000 m2). We estimate minimum threshold values for detectability of 6%, 4%, and 4% change in area (i.e., the amount of dune area that would have to change in order to be detected by comparing image pairs) for the MOC-HiRISE, MOC-CTX, and CTX-CTX combinations, respectively.

2.2. Thermal Inertia

[13] Thermal inertia is a key surface property controlling diurnal temperature variations of the near surface and provides insight into the physical properties (e.g., grain size, degree of induration) of the Martian surface. Thermal inertia maps were derived from Mars Odyssey's Thermal Emission Imaging System (THEMIS) nighttime infrared data [Christensen et al., 2004; Fergason et al., 2006b] using the thermal model of Putzig and Mellon [2007], as implemented in the “jENVI” software suite (http://arsia.gg.utk.edu/∼utmars/jenvi/). This method uses THEMIS Band 9 (12.57 μm) nighttime brightness temperature to derive best fit thermal inertia values by interpolation within a seven-dimensional lookup table using: season, time of sol (Mars day), latitude, thermal inertia, albedo, elevation, and visible dust opacity [Putzig and Mellon, 2007]. THEMIS thermal inertia values were converted to effective particle sizes via the relationships determined experimentally by Piqueux and Christensen [2009]. The effective particle size describes the upper centimeters of an unconsolidated surface and is a function of mixing, cementation, porosity, and subsurface layering [Presley and Christensen, 1997]. Although thermal inertia is largely controlled by particle size, several factors can complicate the relationship. Ambiguities in the interpretation can arise from particle size mixing, grain cementation, subsurface layering, and atmospheric effects [Fergason et al., 2006b; Putzig and Mellon, 2007; Piqueux and Christensen, 2009]. However, some of these issues are mitigated when dealing with well-sorted sedimentary deposits (i.e., dunes and aeolian bed forms) [Fergason et al., 2006a, 2006b; Putzig and Mellon, 2007]. The latter point is related to the fine component properties, rather than those of the rock components, in controlling the bulk nighttime thermal inertia within the field of view [Christensen, 1986]. Additionally, bed forms that are thicker than several diurnal skin depths reduce the chance of subsurface vertical heterogeneities or layering effects (e.g., sand over bedrock) [Putzig and Mellon, 2007; Piqueux and Christensen, 2009].

2.3. Topography

[14] To quantitatively investigate surface morphology and estimate the underlying slopes digital elevation models (DEMs) were used. DEMs created from Mars Express High Resolution Stereo Camera (HRSC) data [Neukum et al., 2004] were acquired from the HRSC data explorer at http://hrscview.fu-berlin.de and then reprojected and reformatted using ISIS software.

2.4. Mesoscale Atmospheric Modeling

[15] Mesoscale climate modeling can provide insight into wind direction, speed, and its ability to move sediment on a planetary surface. The mesoscale Mars atmospheric model used in this work was the Mars Regional Atmospheric Modeling System (MRAMS) [Rafkin et al., 2001]. This model is a regional (versus global) three-dimensional, nonhydrostatic atmospheric model, enabling relatively high spatial resolution (grid spacing tens of kilometers or less) over timescales of sols at a targeted location. The output from a NASA Ames Mars General Circulation Model (MGCM) [Haberle et al., 1993] simulation is used for initial states and time-dependent boundary conditions for MRAMS runs. In this work, a series of five nested computational grids (each with a successively smaller total area and horizontal grid spacing) is used to achieve a grid spacing of ∼2.5 km over Endeavor crater and its surroundings. In these simulations, approximately 10 × 10 horizontal model grid points are within Endeavor crater.

[16] Seasonal MRAMS simulations were conducted for Ls ∼ 30°, 120°, 210°, and 300°, each ∼4 sols in duration (first sol is spin-up), to characterize basic seasonal differences in the aeolian environment at Endeavor crater. MRAMS and the MGCM both used latitudinally and seasonally varying atmospheric dust loadings based on MGS Thermal Emission Spectrometer (TES) dust opacity observations [e.g., Smith, 2004]. MRAMS bases its terrain and surface characteristics on 1/64 deg/pixel (degree per pixel) gridded MGS Mars Orbiter Laser Altimeter (MOLA) topography [Smith et al., 2001], 1/8 deg/pixel gridded MGS TES albedo, and 1/20 deg/pixel gridded MGS TES-based nighttime thermal inertia [Putzig and Mellon, 2007]. The model-resolved topography of the crater is generally muted compared to what is known from higher-resolution data (see section 3.6.). This issue is unavoidable, given the finite resolution of the computational grid and a numerical stability requirement that the model topography not contain structure at scales less than two times the horizontal grid spacing. Instantaneous snapshots of the model state were output every 1/3 Mars hour (simulated).

[17] Previous attempts to identify correlations between mesoscale atmospheric modeling and Mars aeolian systems [e.g., Fenton et al., 2005; Hayward et al., 2009] have met with mixed degrees of success. This study is different, however, in that it targets a dune system that is known to have changed over a relatively short period of time. The atmospheric modeling aspect of this work aims not only to determine whether sediment-mobilizing winds are occurring, but also to gain insight into when these winds occur, and into what causes these winds. This approach often involves identifying and characterizing the wind regimes for the area of interest, Endeavor crater. Wind regimes we define as distinct periods (>30 Mars minutes in duration) when strong and directionally coherent near-surface mean winds occur at nearly the same time of sol for one or more seasons. The mix of wind regimes present may vary seasonally. Additionally, the modeling work aims to achieve a measure of model “validation” (i.e., gain confidence that MRAMS can satisfactorily predict winds relevant to aeolian surface processes), and provide information to help interpret future observations of Endeavor crater aeolian features and processes.

3. Results

  1. Top of page
  2. Abstract
  3. 1. Introduction and Study Area
  4. 2. Method and Data
  5. 3. Results
  6. 4. Discussion
  7. 5. Summary and Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

3.1. Recent Aeolian Activity in Endeavor Crater

[18] Endeavor crater (Figure 1a and Figure S1a) has three types of aeolian morphologies: two main populations of dune bed forms (Figure 1b and see Figure S1b for a less-annotated version) and transverse aeolian ridges (TARs). Dune fields are superposed on the following units (in stratigraphic order, lowest to highest): a light-toned etched unit (described by Hynek et al. [2002]), an immobile midtoned mantling unit (related to the hematite-bearing plains intercrater unit [Christensen et al., 2001; Hynek et al., 2002]), and TARs in a relatively sediment-starved landscape [Chojnacki et al., 2010a]. Barchans and barchanoid dunes are found in the western portions of the crater (Figure 1b and Figure S1b). In contrast the eastern half of the crater is populated by a greater diversity of dune morphologies, including bed forms that may be classified as transverse dunes, sand sheets, and small dome dunes (Figures 1c, 2, and 3, respectively). The consistently darker tone (lower DN values) on the down-Sun side of these modest bed forms suggests muted topography (shallower slopes) compared to the western barchan dunes. High-resolution images reveal a rippled, “crosshatched” morphology with 2–5 m wavelengths (e.g., Figures 2b, 2d, and 2f). Although these eastern dunes lack prominent slip faces, we suggest that they qualify as “dunes” as defined by Bagnold [1941]: mounds of windblown sand-sized sediment with a central higher-standing rise. We classify the bed forms in Figures 2, 3c, and 3d as dome dunes; that is, relatively small dunes, circular to elliptical in plan view, and without prominent slip faces [cf. McKee, 1979]. We use the more general term bed forms when describing the collective group of eastern aeolian morphologies (Figure 1c and Figure S1c).

image

Figure 3. Deflation, modification, and transport of Endeavor crater dunes and bed forms. Blue polygons represent the extent of dunes in 2001 and 2003, and yellow polygons represent the extent of bed forms in 2008. Modifications (white arrows) of larger dunes (south of dunes ii and iii) are shown in (a) the 2001 MOC and (b) the 2008 HiRISE image. (c) A 2003 MOC image showing the easternmost dunes iv and v and their subsequent reduction in size over 2.3 Mars years in (d) the CTX image. Close examination of dune v suggests southward migration ∼20 m in addition to sediment loss (inset with a ∼150 m field of view).

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[19] Modeling and spectroscopy give some indication of a primary wind direction and propensity for moving sediment. Previously published results from the MRAMS described a generally easterly (toward the west) wind flow at ∼10 m/s in the vicinity of Endeavor crater during afternoon at Ls ∼ 320° [Rafkin and Michaels, 2003]. These modeled wind vectors are consistent with the north-south orientations of ripples on the smaller dome dunes (insets in Figure 2) and dark streaks associated with some of the western barchans (Figure 4a), although not with the western group dunes slip face orientations (toward the south-to-southeast). MGS TES-derived dust cover index (DCI) [Ruff and Christensen, 2002] values of 0.96 to 0.98 in the southern interior of Endeavor crater indicate minimal to no surface dust at the time of data acquisition, consistent with a surface that has been scoured by aeolian processes or has not undergone dust deposition.

image

Figure 4. HiRISE images (a) PSP_005779_1775 and (b) PSP_007849_1775 of a degrading barchan dune on the eastern edge of the western dune group. These two images, taken on 20 October 2007 (Ls ∼ 334°) and 30 March 2008 (Ls ∼ 51°), are separated by one Martian season (0.24 Mars year). Sand streamers appear both to the southeast and west-southwest in Figure 4a. However, the distribution appears to be more concentrated to the southeast, and the dark-toned material, inferred to be sand, present in Figure 4a appears to have been largely removed in the western portion of Figure 4b. Also, note the degraded state of the slip face of this former barchan dune (gray arrow). Phase angle is ∼15° greater for Figure 4b.

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[20] Our analysis of Endeavor crater orbital images from the past ∼2–3 Mars years (∼4–6 Earth years) shows several significant styles of dune and bed form erosion, modification, and translation (Figures 2 and 3). The eight dunes that show surface change are labeled i–viii in Figure 1c and Figure S1c. Eight eastern dunes appear in the 2001 and 2003 MOC images; however, in CTX and HiRISE images taken in 2008, two of the dunes (i and ii) have disappeared entirely (e.g., Figures 2a2d and see Figure S2 for a animated time step sequence of dunes ii–iii and viii), whereas six others (iii–viii) have decreased in area (e.g., Figures 2c2f, 3c, and 3d, Figure S2, and Table 1). Dunes iv–vi are only imaged in the lower-resolution MOC-CTX pair of images, making area estimates more difficult than for the other dunes, which are covered by the MOC-HiRISE pair of images.

3.2. Dune Deflation

[21] There are two potential explanations for the disappearance of these dunes (dunes i–viii): (1) aeolian removal and/or deflation of dune sediment or (2) dust deposition with consequent obscuration via an increase in albedo, effectively “blending” the dunes into the lighter-toned background plains. The latter explanation is less credible because the locations where the dark-toned features were originally present now lack (as of 2008) the crosshatched, rippled appearance seen in the other dark-toned dunes that are still present (see insets in Figures 2b, 2d, and 2f). Although the MOC images from 2001 to 2003 and CTX images are of insufficient resolution to distinguish this texture in the now-missing features, it is reasonable to assume that all the dark-toned features in this vicinity originally had the same rippled surface texture. If a mantle of bright dust were responsible for the disappearance of the dark-toned features the rippled texture would have been preserved, but it is not observed. Moreover, substantial dust mantlings are relatively uncommon on the surface of Meridiani Planum, as observed by Opportunity [Herkenhoff et al., 2006]. Thus, we infer that these eight dunes were partially or completely deflated within the 2.3–3.3 Mars years (4.3–6.3 Earth years) or less, between the images.

[22] The majority of dune or bed form change is found in the eastern group, although there are sand streamers or dark streaks emanating from one western dune that suggest periods of winds capable of saltating this material (Figure 4 and Figure S3, showing change in dominance of west-southwest and southeast orientations). For dunes captured in the 2001–2003 MOC and 2008 MRO combinations, dune area reduction ranged from ∼20% to 100% (Table 1). An additional CTX observation in 2009 supports the trend of deflation with dunes iii and vii reduced in area ∼24% and ∼55%, respectively, when compared with CTX in 2008 (Table 1). For these eastern dunes we estimate that the total surface area from which sediment has been removed is greater than ∼14,000 m2 (Table 1). In comparison to dune fields previously visited by MER, the total area of Endeavor dunes removed is roughly one quarter the area of the total Victoria crater dune field (55,000 m2), or about the same area as Gusev crater's El Dorado ripple field (13,000 m2). Dune coverage area lost per unit time varies widely from dune to dune (Table 1), but the mean for all dunes is 750 m2 per Mars year (400 m2 per Earth year). These rates are averages assuming removal at a steady rate throughout the time between image acquisitions. The assumption of steady state may be incorrect, as some dune(s) may have been active more intermittently (see section 4.2.).

3.3. Dune Morphology Modification

[23] The dunes presented in Figure 2 show no evidence for downwind or southward migration. However, several larger dunes or bed forms ∼500 m to the southeast (the inferred downwind direction) of dunes ii–iii do show surface modification (Figure 1c and Figure S1c, white arrows). Minor changes, presumably due to aeolian modification, are evident on the windward side of what appears to be a deflated barchan dune and a minor limb of a transverse dune (Figures 3a and 3b, white arrows) and other locations (Figure 1c and Figure S1c, white arrows). From this evidence, we suggest two likely outcomes for the sand involved in the dune changes between 2001 and 2009. The sand removed from the northernmost dome dunes may have been dispersed downwind too diffusely to be detected in HiRISE imagery, but some of this sand may have been absorbed and incorporated into the larger dunes to the south, including those that show shape modification (Figures 3a and 3b). Some combination of the two scenarios is also possible.

3.4. Dune Migration

[24] In addition to dune deflation and the modification of dune morphology, several dunes noticeably translate from 2003 to 2008. Dune v, in addition to losing ∼40% of its area, shows a ∼20 m southeastward change of its centroid, defined as the location equidistant along the long and short axis (Figures 3c and 3d; blue to yellow polygon). This evidence is interpreted as southeastward dune migration (see section 4.1.). Likewise, dune vi also appears to have migrated ∼10 m from its position in earlier images (Table 1). To our knowledge, this change is the first documentation of dune migration on Mars, although smaller ripples have been observed to migrate at landing sites [Sullivan et al., 2008] and in HiRISE observations [Silvestro et al., 2010a]. These estimates of dune translation are based on the available MOC-CTX pair of observations, and thus precision is limited. If migration continues, the dune centroid positions could be better constrained using future high-resolution observations.

3.5. Thermal Inertia and Topography

[25] A THEMIS thermal inertia map of Endeavour crater (Figure 5) exhibits values from 80 to 350 J m−2 K−1 s−1/2 (hereafter referred to as “tiu,” or thermal inertia unit). The western barchan sand dunes exhibit a thermal inertia of 150–240 tiu, whereas the eastern dunes have a thermal inertia range of 110–190 tiu. For unconsolidated sediments, these thermal inertia values are consistent with particle sizes of 100–600 μm (very fine to coarse sand sizes) and 50–200 μm sediment (very fine to fine sand sizes), respectively [Piqueux and Christensen, 2009]. The elevated thermal values could be the result of differences in dune thickness or sediment induration, instead of or in addition to differences in grain size.

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Figure 5. CTX image colorized with THEMIS thermal inertia (I22535006) of the Endeavor crater basin and dune fields. Dunes that show change in morphology are highlighted in white. The western dune fields are found to have thermal inertia values consistent with fine to medium sand sizes, while eastern dunes that show change have thermal inertia values suggestive of very fine to fine sand sizes.

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[26] A CTX visible-wavelength image colorized with HRSC elevation is shown in Figure 6, illustrating the local topography of the Endeavour crater basin. Topographic profiles aligned parallel to the inferred primary wind direction indicate hummocky terrain dropping off southward across the eastern dunes and bed forms.

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Figure 6. CTX image colorized with HRSC elevation (75 m/pixel H2064_0000_DT4) of Endeavor crater dunes. The eastern dunes and bed forms are situated at the top of a ∼200 m tall hill that drops off southward and downwind across the dune field. Four topographic profiles (in units of meters) with the same vertical spacing are given across dunes that show change (highlighted in white). Profiles are parallel to and lead downwind (inferred direction) of the dunes that show change (shown by black ellipse in the profiles).

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3.6. Mesoscale Atmospheric Modeling Results

[27] The MRAMS simulations performed for this work do not have the spatial resolution (due to computational limitations usual for such modeling) needed to properly resolve boundary layer turbulence, although wind gusts generated by such turbulence are suspected of being very important to aeolian processes on Mars [e.g., Fenton and Michaels, 2010]. The MRAMS output fields analyzed here are effectively the spatial mean (within each ∼2.5 km × 2.5 km grid cell) of the actual variability within those fields (a range that exists due to turbulence). One of those fields is the aerodynamic surface shear stress (τ = ρatmu*2; with units of mN m−2 or mPa). Shear stress is an important quantity for estimating aeolian saltation potential because it includes the effects of both the local atmospheric density (ρatm; varies significantly between night and day on Mars) and friction speed (u*; also varies significantly, especially when turbulence is present). The threshold aerodynamic surface shear stress (τt) needed to initiate saltation of sand-sized particles was also calculated for every MRAMS grid point at all output times using the semiempirical expressions of Newman et al. [2002, equation (5)], the local atmospheric density at the surface, and a particle mass density of 2800 kg m−3 (similar to weathered basalt).

[28] Figure 7a shows the model topographic contours on a portion of grid 5 (∼20% of the full area), an area which includes Endeavor crater as well as Opportunity's traverse, overlaid on a mosaic of CTX imagery colored with HRSC topography. An area of 4 × 4 model grid points, where dune activity is known from photogeologic evidence, is analyzed in detail. This target area was selected to receive more detailed attention because of its relevance to the observed dune modifications discussed above (see Figure 7a, black box). Polar coordinate scatterplots were generated of τ/τt versus wind direction (Figure 8) within that area, valid at each of the four seasons that were simulated (Ls ∼ 30°, 120°, 210°, and 300°). Two additional sets of similar plots (Figure S4), illustrating the dependence of τ/τt and wind direction on local mean solar time (LMST), were also utilized for analysis. Values of τ/τt greater than or equal to one would indicate that the spatial mean winds at the associated model grid point(s) are likely capable of initiating the saltation of basaltic sand.

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Figure 7. (a) CTX visible-wavelength image mosaic of Endeavor crater and its vicinity, overlain by color-shaded HRSC elevation (100 m/pixel H1183_0000_DT4) and the MOLA-based elevation contours used by MRAMS. (b–d) CTX visible-wavelength image mosaic of the Endeavor crater vicinity with examples of flow regimes: wind regime 1 (Figure 7b), MRAMS wind vectors and magnitude 1.3 m above ground level (AGL) (interval 0.5, purple < 6.5, red > 8.5 m/s) at Ls ∼ 300 and 0950 LMST; wind regime 2 (Figure 7c), MRAMS wind vectors and magnitude 1.3 m AGL (interval 0.5, purple < 1, red > 3.5 m/s) at Ls ∼ 120 and 1650 LMST; and wind regime 3 (Figure 7d), MRAMS wind vectors and magnitude at 1.3 m AGL (interval 0.3, light purple < 1.5, dark red > 4.5 m/s) at Ls ∼ 210 and 1750 LMST. The magenta circle in the northwest quadrant of the images represents the location of the Opportunity rover in December of 2010.

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Figure 8. MRAMS polar coordinate scatterplot for four representative seasons (Ls ∼30°, 120°, 210°, and 300°). Each of the 20 model grid points within the outlined area in Figure 7 are plotted at each of the 72 model output times within in a single sol. Points with LMST between approximately 12 and 16 Mars hours are plotted in gray to indicate that their magnitude and direction are contaminated (to varying degrees) by the occurrence within the model results of poorly resolved/realistic dry convective structures at those times. Inspection of the other (lower-resolution) model grids strongly suggests that τ and direction during this time interval should not have a significantly different character than that in the late morning. Directions are given using the meteorological convention (i.e., the direction the flow is from). Fraction of threshold aerodynamic surface shear stress (τ/τt) versus wind direction (degrees clockwise from north) is plotted, with a grid ring interval of 0.25, illustrating the flow directions with the greatest/least magnitudes.

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[29] The regional-scale atmospheric model (MRAMS) results are only broadly similar to the much coarser resolution GCM results (used as distant boundary conditions for the MRAMS runs discussed here), with both models suggesting that regional-scale winds should evolve seasonally, with a significant easterly wind component occurring often. The MRAMS results suggest that the study area within Endeavor crater is subject to three flow regimes (two major, one minor) that vary seasonally. The wind regime with the highest modeled winds (WR1) occurs for ∼2–3 Mars hours during the midmorning at all seasons, with the direction and strength varying with season: from the east (with the strongest overall magnitudes) during the warmer seasons (e.g., Ls ∼ 210°, 300°; Figures 4a, 7b, and 8 and Figures S3 and S4), and from the southeast to south at during the southern fall and winter (e.g., Ls ∼ 30°, 120°; Figure 8 and Figure S4). The strength and direction of this wind regime is generated by regional forces associated with both the atmospheric diurnal tides and the regional-scale topography. The wind regime with the second highest modeled winds (WR2) occurs for ∼1 Mars hour in the early evening, and is characterized by winds primarily from the northwest. It is strongest in southern spring (e.g., Ls ∼ 210°; Figures 3b, 4b, 7c, and 8 and Figure S4) and nearly absent during southern winter (e.g., Ls ∼ 120°). This northwest flow is initially generated by regional forces, but its magnitude is significantly enhanced through interaction with Endeavor crater. Furthermore, this enhancement only appears to occur in the northern half of the crater, with weak winds elsewhere within the crater. The third flow regime (WR3) is a minor one that is only significantly present during the southern winter (e.g., Ls ∼ 120°; Figures 7d and 8 and Figure S4). It is characterized by flow from the southeast to south-southeast into the southern third of Endeavor crater, and is a result of local flow modification due to the presence of the crater.

[30] At all seasons, the modeled τ/τt has magnitudes below 0.5, and more commonly less than 0.25. Although the MRAMS-modeled τ/τt values are less than one (indicating winds nominally incapable of mobilizing sediment), MRAMS does not take into account possible turbulent gusts superposed on the mean wind speed. To achieve the needed increase in τ/τt of a factor of two to four, corresponding gust strength of equation image to equation image times the mean wind speed would be required. Such enhancements are plausible according to Fenton and Michaels [2010]. In discussing correlations between MRAMS wind vector orientations and the orientations of aeolian surface changes, we make the assumption that this level of gustiness is achieved. Therefore we suggest, in a relative sense, the results indicate that the southern spring and summer seasons (e.g., Ls ∼ 210°, 300°; Figure 8 and Figure S4) are more likely to support aeolian activity within Endeavor crater than the other two seasons. According to the model results, winds would move, if of sufficient magnitude, aeolian features predominantly westward or southeastward across the basin with some minor reworking northwestward (Figure 7). The results also suggest that southern autumn (Ls ∼ 30°) is the least likely time of year for significant aeolian activity within the crater. The modeling also indicates that some sol-to-sol variability occurs, but does not significantly change the general results presented above.

4. Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction and Study Area
  4. 2. Method and Data
  5. 3. Results
  6. 4. Discussion
  7. 5. Summary and Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

4.1. Comparison With Other Martian and Terrestrial Examples

[31] Our results have documented a high degree of dune activity in Endeavor crater during the relatively short time span of ∼2–3 Mars years (∼4–6 Earth years). We suggest saltation, rather than creep or suspension, as the primary mechanism to be responsible for the erosion, modification, and migration of these dunes. A surface of coarse sand particles, which is less prone to saltation than to surface creep, is inconsistent with estimates from the thermal inertia data. Likewise, removal by suspension would require subsand sizes that would correspond with lower thermal inertia surfaces than estimated from THEMIS data (Figure 5).

[32] This evidence for widespread dune deflation implies local winds that are sufficient to initiate saltation, as observed elsewhere on Mars [Greeley et al., 2006; Bourke et al., 2008; Geissler et al., 2008; Sullivan et al., 2008; Silvestro et al., 2010a, 2010b]. Indeed, Endeavor crater dune area removal rates are an order of magnitude larger than polar dunes that show surface change (see below). This sediment movement in Endeavor crater represents the most significant contemporary sand-sized aeolian sediment transport on Mars detected to date. This movement indicates that Endeavor crater experienced significant near-surface wind speeds, although the temporal nature of those winds (e.g., daily winds? episodic wind events?) (see section 4.2) is poorly constrained. Regardless of the time frame for movement, documentation of relatively large scale (>1000 m2) bed form activity may have been inevitable due to spacecraft imaging campaigns with progressively greater spatial and temporal resolution, as well as areal coverage [Sullivan et al., 2008].

[33] Thorough image analysis campaigns looking for dune change have been ongoing, with comparisons being made between Viking-acquired images to MOC, MOC to MOC (for a review see Bourke et al. [2008]), and most recently MRO to MRO [Bridges et al., 2007], yet have been limited in success. Endeavor crater, Nili Patera (where ripple migration has been observed for the first time from orbit [Silvestro et al., 2010b]), and the small dome dunes seen to be modified in the north polar region [Bourke et al., 2008], are the only such identifications thus far. An intriguing common thread between these three known anomalous sites is that the features with observed aeolian modification were much smaller in size (lesser sediment volume) than the more typical “larger” dunes across Mars that have yielded nearly no evidence of contemporary aeolian modification. This correlation supports the possibility that the contemporary aeolian sediment transport processes on Mars may be of a magnitude and/or intermittency which allows low-volume bed forms to change detectably over a few Mars years (or less), but may take Mars decades to produce detectable changes on larger bed forms.

[34] The average migration and removal rates we have deduced for the dunes in Endeavor crater provide an opportunity for comparison with terrestrial rates. The migration rates for the two Martian dunes were ∼4–9 m per Mars year (∼2–5 m per Earth year; Table 1), which are generally lower rates than terrestrial studies of small dune (typically barchans) movement [Finkel, 1959; Pye and Tsoar, 1990; Jimenez et al., 1999]. However, Antarctic hyperarid polar dunes, thought to be analogous to at least some Martian dunes, travel at an average rate of 1.5 m per Earth year [Bourke et al., 2009b]. Similarly, terrestrial studies of dome dune migration rates vary from 4 to 8 m per Earth year [Dong et al., 2000; Bristow and Lancaster, 2004], which is similar to our estimates of 2–5 m per Earth year for the Endeavor crater dome dunes.

[35] The estimated areal removal fluxes vary by an order of magnitude from dune to dune, but the higher-resolution MOC-HiRISE comparisons suggest ∼350 m2 per Mars year (∼190 m2 per Earth year). For comparison, we have also calculated areal removal rates for the deflating polar dome dunes (dunes i–iii) identified by Bourke et al. [2008]. We derive areal change rates for these dunes of ∼180 m2 per Mars year (100 m2 per Earth year), which is smaller than but comparable to the removal rates in Endeavor crater. The unusual parameter of areal removal rate is necessitated for the Mars dunes because no data are available on the thickness of the dunes (see section 4.3.). This parameter is not what is typically used in terrestrial studies, but Dong et al. [2000] do report several instances of small crescent and dome dunes deflating in the Taklimakan Desert, China. Dune morphometric data from these authors suggests rates of ∼85 and 115 m2 per Earth year for the collective study (including small crescent dunes) and dome dunes, respectively. Reports of terrestrial dunes experiencing 100% deflation are uncommon. However, one small (∼45 m diameter) dome dune in the Namib Sand Sea [Bristow and Lancaster, 2004] was reported to have dissipated 100% within 3 years (1999–2002) of that study (C. S. Bristow, personal communication, 2010). This report results in a poorly constrained, areal removal flux of ∼1000 m2 per Mars year (∼550 m2 per Earth year), clearly a much higher rate than has been observed on Mars to date.

4.2. Timing: Episodic and/or Gradual Aeolian Activity

[36] Both episodic and gradual aeolian events are suspected to have taken part in the aeolian activity at Endeavor crater. Surface observations during operations at Mars Exploration Rover landing sites led Sullivan et al. [2008] to conclude (1) strong wind events, usually associated with regional or global dust storms, occur with some randomness; (2) the strongest events are also the rarest; and (3) these strong wind events contribute the most to surface change. A planet-encircling dust storm occurred in July–August 2007 and disrupted MER surface operations at both landing sites. Several local changes associated with this storm were observed from the surface and from orbit. During that period, atmospheric opacity (tau) reached ∼5.0 at Victoria crater, and eventually Opportunity's solar array performance dropped to record lows [Geissler et al., 2010; Vaughan et al., 2010]. Commonly associated with these sometimes sudden increases in dust opacity were episodic strong wind events, which stripped sediment from MER solar panels [Geissler et al., 2010; Vaughan et al., 2010]. The spatial extent of these exceptional wind events, and thus their potential effect on Endeavor crater bed forms, is unknown, in part because all of these dust-cleaning events occurred during long hiatuses between overlapping Endeavor crater imaging. During the course of the mission, several local changes associated with this storm were observed from the surface and from orbit [Geissler et al., 2010]. Opportunity observed its rover tracks both gradually fade (brighten) due to dust deposition and be erased due to aeolian deflation, including some that were less than 1 Mars year old [Geissler et al., 2010]. In contrast to the slow rate of dust deposition, episodic transport and deposition of basaltic sand by surface winds may produce rapid erasure, as suggested by the disappearance of the rover tracks and dark streaks around Victoria crater during the 2007 dust storm [Geissler et al., 2008, 2010]. It is thus conceivable that much of the surface change observed herein is due to one major episode of high-magnitude winds associated with the 2007 planet-encircling dust storm. Because it is known only that changes occurred sometime between the acquisition of the two images showing dune transport and removal, these time spans for change and the removal and migration rates shown in Table 1 are temporal averages that assume continuous activity, as no intermediate images of suitable quality are available.

[37] Alternatively, some evidence exists that dune bed form surface change is more gradual with changes occurring on an interseasonal or even intraseasonal basis. At the Opportunity landing site, landing rockets fired prior to the rover dropping during the landing sequence made bright scour marks easily resolvable in subsequent MOC images. The bright marks are likely created by the removal of sand and hematite-rich concretions by the rocket blast [Geissler et al., 2010]. The blast marks were observed to gradually fade in several orbital images, presumably due to seasonal wind-driven deflation, in less than a Martian year [Geissler et al., 2010]. Contemporary “reversing” bright wind streaks (each feature having two streak orientations, northwest and southeast) that are associated with small craters are also common in this portion of Meridiani Planum [Sullivan et al., 2005; Jerolmack et al., 2006]. There is evidence from repeated visible-wavelength data that these dust streaks appear at predicable and repeatable times of the Martian year (see section 4.4) [Jerolmack et al., 2006]. This evidence suggests some element of seasonal aeolian behavior controls the emplacement and erosion of dust, producing these features. Bourke et al. [2008, 2009a] documented polar domes that showed gradual deflation every 1–2 Earth years and some changes occurring over a single Martian season. Other than the 2001–2007 time step, the frequent MRO imagery coverage of Endeavor crater supports a gradual erasure of dune iii and possibly neighboring bed forms. Future observations will reveal whether or not areal removal and/or migration rates are relatively continuous in time.

4.3. Factors Contributing to Surface Change

[38] Coordinated orbital and rover observations at Meridiani and Gusev crater made significant progress in addressing question 1, why bed form change is not more commonly observed from orbit [Geissler et al., 2008; Sullivan et al., 2008]. These studies have provided some insight for observations of other, less (remotely) characterized aeolian features on Mars and their potential for contemporary activity. For example, the temporal change of the Victoria crater dark streaks are easily resolved at MOC-NA and HiRISE spatial resolutions, but required MER-B observations to determine that they were due to the saltation of dark sediment (versus the removal of a bright mantling material overlying a dark substrate) [Geissler et al., 2008]. In contrast to the Victoria crater dark streaks the El Dorado ripple migration (∼2 cm) [Sullivan et al., 2008] would not have been discovered solely from orbital observations even with repeated HiRISE imaging. The recent discovery of dune mobilization in Endeavor crater and ripples migrating ∼1.7 m in Nili Patera over one Martian season [Silvestro et al., 2010b] was accomplished with the 25 cm/pixel resolution of HiRISE and the multiple of overlapping images.

[39] The reasons some bed forms change and others do not as observed from orbit (question 2) and the factors controlling detectable movement (question 3) may be a combination of effects associated with: particle sizes, degree of sediment induration, areal extent and/or volume, position relative to associated topography, bed form morphology, and smaller-scale atmospheric flow structure. The thermophysical properties of the local sediment may give us some indication why some dunes show a greater propensity for change than others (Figure 5). The THEMIS-derived mean grain size value of ∼120 μm (50–200 μm) for the eastern dunes and bed forms encompasses the particle size (∼115 μm) most easily mobilized by the Martian atmosphere [Greeley et al., 1980]. Particles are expected to be composed of basaltic grains (low-calcium pyroxene), rather than lower-density dust aggregates based on MRO's Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) spectroscopic observations [Chojnacki et al., 2010a]. This particle size range for basaltic grains, at the estimated Meridiani Planum atmospheric pressure (∼6.7 mb), would require an friction threshold velocity (u*t) of ∼2 m/s [Greeley et al., 1980] or surface wind velocity of ∼17.5 m/s (at ∼1 m height and an aerodynamic roughness value of 3 cm [Sullivan et al., 2005]) for entrainment. The thermal inertia of these dunes and bed forms also supports the notion that on these dunes, like the El Dorado ripples [Sullivan et al., 2008], saltation of hard sand-sized particles is winning its competition against surface induration.

[40] Dune area removal rates appear relatively similar across the study area and between imaging epochs with the exception of dunes v–vii, where measurements were done with lower-resolution MOC-CTX data. Volumetric sediment fluxes are difficult to estimate in Endeavor crater because no appropriate imagery currently exists that can be used to construct a DEM with sufficient resolution. However, we can make crude estimates of dome dune volumes, mass, and associated flux by looking at different dune heights. To obtain an upper limit for dune height, we note that these dome dunes have no slip faces and thus possess slopes below the angle of repose (<33°). This lack of slip faces can be used to put upper limits on dune heights. For example, dune iii at ∼39 m wide could be ∼9 m tall (assuming slopes of 25°) but not possess a slip face. This dune height is likely an overestimate, as terrestrial dome dunes have relatively modest slope angles when compared with other dune morphologies [Dong et al., 2000; Bristow and Lancaster, 2004], so we will also consider a height of 2 m. Volume estimates from the 2008 dune iii measurement are estimated at ∼900 m3 and 16,900 m3 (Monolithic Dome Institute, Spherical dome formulas, 2001; available at http://www.monolithic.com/stories/advanced-dome-calculator), when using heights of 2 m and 9 m, respectively. These estimates illustrate dunes that are shorter are more susceptible to deflation and surface change, because they contain less sediment.

[41] Bed forms move proportional to the wind velocity and inversely to their size [Bagnold, 1941; Finkel, 1959; Sauermann et al., 2000; Hesse, 2009]. Endeavor crater dunes show no clear inverse correlation between areal size and removal rate, as a proxy for transport rate (i.e., the smallest dunes do not appear to be removed faster than larger dunes in Table 1). However, this lack of an inverse correlation is likely an artifact of poor temporal resolution and we suspect the smallest dunes (<2000 m2) were deflated in a shorter time span than larger ones.

[42] Topography and degree of bed form exposure are additional factors influencing dune sediment transport rates and dune morphology [Taylor et al., 1987; Pye and Tsoar, 1990]. All dunes (except vi) that show removal or reduction in size are located at the inferred upwind perimeter of the dune field (Figure 1c and Figure S1c), making them most exposed to atmospheric flow and prone to deflation. Profiles across the eastern dune field (Figure 6) parallel with what we have defined as the primary wind/transport direction indicate that many mobile dunes are superimposed on a surface that slopes downward to the southeast (with 200 m of relief). The majority of dunes that show erosion and transport are located at the summits of small hills or on gently southeastward sloping (1°–7°) surfaces (Figure 6). Local topographic highs, as with the stoss slope leading up to the crest of an individual dune, are locations of increased wind velocity due to streamline convergence [Taylor et al., 1987; Pye and Tsoar, 1990]. Saltation path lengths are longer where wind vectors align with the downhill direction, enhancing the wind's capacity for transporting sediment [Tsoar et al., 1996; Pye and Tsoar, 1990].

[43] As described above (section 3.1) these bed forms are classified as simple dome dunes [cf. McKee, 1979]. Dome dunes are not as efficient at trapping sand as barchans and are known to be highly susceptible to wind erosion [Bagnold, 1941; McKee, 1979; Bourke et al., 2008, 2009a]. Dome dunes may develop into barchan dunes given sufficient sand supply and wind velocity [McKee, 1979; Bourke and Goudie, 2009] or develop from larger barchan limbs as part of a calving event [Bourke, 2010]: thus an evolutionary relationship between dome and barchan morphologies can exist. We suggest the changing dome dunes in Endeavor crater are an example of progressive degradation of barchan dunes leading to present-day dome dunes and the loss of slip faces. A survey of dune field morphology supports the idea of disequilibrium. Dune height monotonically decreases from the westernmost barchan dunes toward the east, as inferred by using the technique of Bourke et al. [2006], in which the horizontal lengths of slip faces (where present) provide a proxy for dune heights.

[44] All of the barchan dunes in western Endeavor crater are classified as fat barchans, based on the ratio of length of windward slope to horn-to-horn width [Long and Sharp, 1964]. Dunes in this subclass are common on Earth, but less so on Mars [Bourke and Goudie, 2009]. Models suggest fat barchans form in the presence of varying wind directions (up to ∼40°) [Reffet et al., 2009], in locations that are topographically influenced and/or on the margins of larger dune fields [Bourke, 2010]. There is evidence for all of these factors in the western dune field of Endeavor crater. Many dunes of the western group have maintained a crescent shape, but have greatly degraded slip faces (e.g., Figure 4 and Figure S3). In general, the presence of barchan dune morphologies commonly has been suggested to imply a relatively unidirectional wind regime and limited sand supply [Bagnold, 1941]. The sediment supply north of these dunes, presumably upwind, certainly is sparse according to our photogeologic and thermophysical analyses (Figures 1b, 1c, 4a, and 5). The dune field, primarily the eastern group, shows evidence for a trend in dune degradation (from north to south) due to wind energy stripping material away (erosion) from the northernmost dunes and depositing sand southward as part of larger bed forms.

[45] The remaining dunes exhibiting recent movement (e.g., dune iii) have similar dune morphologies to those eroding in the north polar region [Bourke et al., 2008, 2009a]. Figure 9 compares the meter-scale dune morphology of the polar dome dune iii from Bourke et al. [2008] to the dune in Endeavor crater that we have labeled dune iii, presented at the same scale. Low-relief dome dunes with meter-scale rippled surface textures without major slip faces predominate in both cases. We suggest that, based on evidence for bed form movement from both polar and equatorial dome dunes, these small dome dunes are the best morphological candidates, with the available spatial and temporal resolutions of current spacecraft instruments, for discrete bed form surface change on the time scale of orbital missions.

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Figure 9. A comparison between (a) the polar dome dune iii as described by Bourke et al. [2008] and (b) the Endeavor crater dune iii at the same resolution, in HiRISE PSP_009295_2565 and PSP_007849_1775, respectively (inset with a ∼90 m field of view). Note the similar morphologies of the different dunes that have both been areally deflated in the past decade. Arrows show inferred wind direction. North is toward the top.

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4.4. Aeolian Morphology Versus Modeled Winds

[46] MRAMS predicts three modeled wind regimes (see section 3.6; Figures 7 and 8), none of which has sufficient strength to move sediment, but each of which correlates with some of the observed aeolian morphology orientations (Figure 10). The disconnect between the modeled wind strength and evidence from spacecraft data for aeolian activity suggests episodic or gusty winds not resolved by MRAMS are responsible for surface change. This scenario would be consistent with terrestrial bed form activity, as sediment moves only rarely during unusually intense events [Conradsen et al., 1984]. However, the seasonality of MRAMS-predicted wind regime is consistent with the timing of surface changes observed in orbital images. Figure 10 summarizes the multiple aeolian morphologies of the region (dune slip face orientation, megaripple fields, sand streamers, dust streaks), which all align to three observed wind regimes: easterly, northwesterly, and southeasterly [Sullivan et al., 2005; Jerolmack et al., 2006; Geissler et al., 2008, 2010; Arvidson et al., 2011; Chojnacki et al., 2010b].

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Figure 10. Table relating morphological evidence of aeolian activity to location, wind direction, time, figure, and MRAMS wind regimes. Abbreviations are as follows: WR, wind regime and SH, southern hemisphere. Here 1, this study; 2, Arvidson et al. [2011]; 3, Sullivan et al. [2005]; 4, Jerolmack et al. [2006]; 5, Geissler et al. [2008]; 6, Geissler et al. [2010].

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[47] The formation of the plains ripples [Sullivan et al., 2005; Golombek et al., 2010; Arvidson et al., 2011], and ripples superimposed on dome dunes (Figure 2, insets) are both consistent with winds from the east or west. Sand streamers (Figure 4 and Figure S3) indicate strong contemporary winds from both the east and northwest. Endeavor crater dune slip face orientations (Figure 1b and Figure S1b) suggest a wind direction from the northwest (Figure 4). Indeed, all five craters similar in size (5–20 km diameter) to and within ∼100 km of Endeavor crater possess similar intracrater concentrations of dark-toned sediment forming dune fields. All of these intracrater dune fields have dune slip faces consistent with a wind direction at the time of formation from the northwest. Observed dune modifications within Endeavor crater (Figures 24) indicate the presence of a contemporary wind regime, also from the northwest, that is able to mobilize sediment. Contemporary “reversing” bright wind streaks (each feature having two streak orientations, northwest and southeast) that are associated with small craters are also common in this portion of Meridiani Planum [Sullivan et al., 2005; Jerolmack et al., 2006]. Additionally, ubiquitous crater-associated dark wind streaks, formed from south-southeasterly winds, are found on the plains adjacent to Endeavor crater, including those associated with Victoria crater [Geissler et al., 2008].

[48] Seasonal dust streaks were mapped out from 1999 to 2005 using 54 MOC images by Jerolmack et al. [2006] to infer two major wind regimes associated with their formation (dust streak azimuths of ∼130° and ∼320°). The dust streak azimuths were found to correspond with the seasons immediately preceding the MRAMS modeled WR1 (southeast, at Ls ∼ 120°) and WR2 (northwest, at Ls ∼ 210°), (Figures 8 and 10). The season when the Victoria crater dark streak typically emerges has been found to be ∼Ls 90° (with some variability) [Geissler et al., 2010] and is broadly correlated with GCM and MRAMS predicted patterns (Figure 10).

[49] However, as previously discussed (section 3.6) MRAMS does not explicitly predict flow magnitudes that are greater than the threshold required for the initiation of sand saltation. Prior modeling work has suggested that Meridiani Planum is not a windy place by Martian standards [e.g., Rafkin and Michaels, 2003]. Wind magnitudes predicted in other mesoscale atmospheric modeling work commonly are too low to move sediment [Fenton et al., 2005; Hayward et al., 2009]. This result may be due to insufficient model spatial and temporal resolution, such that convective turbulent gusts, likely important in aeolian processes on Mars [e.g., Michaels and Fenton, 2010; Fenton and Michaels, 2010], are not resolved. Alternatively (or additionally), energy from decaying northern autumn and winter baroclinic storms from southern Chryse Planitia (to the northwest) may episodically enhance the northwesterly flow so that it is more effective at mobilizing sediment than the easterly regime. Further atmospheric modeling work (particularly at higher resolutions) is needed to improve our understanding of the contemporary aeolian sediment transport processes at work in and near Endeavor crater. To conclude, modeled (mean) winds are insufficient to initiate saltation, but less frequent turbulent winds superimposed on these mean winds would provide sufficient shear stress to entrain sediment.

4.5. Possible Future Observations by Opportunity

[50] Endeavor crater has been the future destination for Opportunity since leaving Victoria crater in 2008. Its rim has become increasingly apparent in panoramic images of the southern horizon (Figure 11). Upon Opportunity's anticipated arrival at the northwestern rim of Endeavor, possibly as early as 2011, the rover will be ∼6 km from the portion of the crater floor where we have identified eroding dunes and bed forms. The average lateral extent (perpendicular to the line of sight from the northwest rim) of dunes that have experienced 100% deflation is ∼60 m. Opportunity's Pancam instrument [Squyres et al., 2003; Bell et al., 2003] has a resolution of 0.28 mrad/pixel [Bell et al., 2006] meaning that dunes of this size would span ∼18 Pancam pixels (assuming 100% deflation) as viewed from the northwest rim. If Opportunity spends as much time at Endeavor crater as it spent at Victoria crater (>1 Mars year), the rover could detect the removal of additional dunes if more occur at the scale and rates we have observed in recent orbital data. Additionally, the MRAMS modeling work suggests that certain seasons (and even times of sol) would be optimal for documenting dune change in and near the crater (see section 3.6, Figure 8 and Figure S4). Documentation of aeolian bed form movement from the surface in coordination with orbital observations would help clarify the apparent disconnect between the two observational scales that has been prevalent until now. If Opportunity is still healthy upon reaching the rim of Endeavor crater, an attractive long-term intracrater drive target might be to the vicinity of these changing dunes, allowing this active erosional process to be viewed in situ (or nearly so) for the first time on Mars.

image

Figure 11. Opportunity's southward view (top, bottom, and middle Pancam mosaic insets) of Endeavor crater rim segments visible on the horizon, in comparison to HRSC orbital data (south is toward the top). HRSC data has been reprojected into distance (vertical coordinate) and azimuth (horizontal coordinate) from the location where the Pancam mosaic was taken. The azimuth coordinate is rescaled to match that of the Pancam resolution such that the same features in both ground and orbital data align vertically across the image. Pancam mosaics have been vertically stretched to exaggerate the horizon and were taken near Concepcion crater (Figure 1a) in early 2010. Image credit: NASA/JPL/Cornell University (Pancam), ESA/DLR/FU Berlin (HRSC), and processing courtesy of James Canvin and http://www.nivnac.co.uk/mer/.

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5. Summary and Conclusions

  1. Top of page
  2. Abstract
  3. 1. Introduction and Study Area
  4. 2. Method and Data
  5. 3. Results
  6. 4. Discussion
  7. 5. Summary and Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

[51] Eight Endeavor crater dunes with a combined surface area of ∼14,000 m2 show activity over the past decade. These changes constitute the most extensive contemporary activity of sand-sized sediment observed to date. These dunes primarily show evidence for deflation, up to 100% in some cases, but two dunes have translated as part of bed form migration. The active dunes in Endeavor crater are dome dunes, the same morphologic class as the only other group of dunes known to be active on Mars, identified in the north polar region by Bourke et al. [2008]. The fact that these two groups share the same morphologic type despite the vastly different surface environmental conditions (e.g., temperature, carbon dioxide frost cycle and water ice) implies that small dome dunes may be the best candidates for seeking out decadal-timescale surface dune change from remote sensing. The dune activity in Endeavor crater shows that it has experienced relatively strong atmospheric flow, which could have been relatively continuous or episodic. Mesoscale atmospheric modeling suggests that wind energy and possible sand transport within the crater may peak during the southern spring and summer seasons. The sand from these dunes has either been thinly dissipated and/or incorporated into larger dunes approximately hundreds of meters downwind that also show evidence for modification. Future orbital observations of this dune field with greater temporal resolution may be used to document surface evolution and sand transport rates. Anticipated MER operations at the rim of Endeavor crater may provide a chance to document large-scale aeolian modification of the surface as it occurs.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction and Study Area
  4. 2. Method and Data
  5. 3. Results
  6. 4. Discussion
  7. 5. Summary and Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

[52] We would like to thank Jennifer Piatek for use of her jENVI routines, Nathaniel Putzig for use of his thermal inertia lookup table, Sylvain Piqueux for his particle size spreadsheet, James Canvin for Figure 11, faculty at the University of Tennessee for their constructive suggestions, and Paul Geissler and an anonymous reviewer for their helpful comments and suggestions that greatly improved the manuscript. Also, we thank the many people responsible for the success of the MER, MGS, MO, MEX, and MRO missions and the relevant instrument teams for their wonderful data. Partial support (M.C. and J.E.M.) for this research was provided by the MER Participating Scientist Program provided by NASA contracts through Cornell and the Jet Propulsion Laboratory and by the THEMIS Participating Scientist Program provided by Arizona State University.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction and Study Area
  4. 2. Method and Data
  5. 3. Results
  6. 4. Discussion
  7. 5. Summary and Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction and Study Area
  4. 2. Method and Data
  5. 3. Results
  6. 4. Discussion
  7. 5. Summary and Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

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FilenameFormatSizeDescription
jgre2845-sup-0001-readme.txtplain text document8Kreadme.txt
jgre2845-sup-0002-fs01.tifTIFF image3977KFigure S1. A less-annotated version of Figure 1.
jgre2845-sup-0003-fs02.gifGIF image10806KFigure S2. An animated time-step sequence of dunes ii–iii and viii between 2001 and 2008.
jgre2845-sup-0004-fs03.gifGIF image1896KFigure S3. An animated time-step sequence of a degrading barchan dune on the eastern edge of the western dune group.
jgre2845-sup-0005-fs04.tifTIFF image12180KFigure S4. MRAMS polar coordinate scatter plot for four representative seasons.
jgre2845-sup-0006-t01.txtplain text document2KTab-delimited Table 1.

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