Observations from the High Resolution Imaging ScienceExperiment (HiRISE): Martian dust devils in Gusevand Russell craters

Circe A. Verba,1,2 Paul E. Geissler,1 Timothy N. Titus,1 and Devin Waller3

Received 24 August 2009; revised 24 March 2010; accepted 14 April 2010; published 1 September 2010.

[1] Two areas targeted for repeated imaging by detailed High Resolution Imaging ScienceExperiment (HiRISE) observations allow us to examine morphological differences andmonitor seasonal variations of Martian dust devil tracks at two quite different locations.Russell crater (53.3°S, 12.9°E) is regularly imaged to study seasonal processes includingdeposition and sublimation of CO2 frost. Gusev crater (14.6°S, 175.4°E) has beenfrequently imaged in support of the Mars Exploration Rover mission. Gusev craterprovides the first opportunity to compare “ground truth” orbital observations of dustdevil tracks to surface observations of active dust plumes. Orbital observations show thatdust devil tracks are rare, forming at a rate <1/110 that of the occurrence of active dustplumes estimated from Spirit’s surface observations. Furthermore, the tracks observedfrom orbit are wider than typical plume diameters observed by Spirit. We conclude thatthe tracks in Gusev are primarily formed by rare, large dust devils. Smaller dust devilsfail to leave tracks that are visible from orbit, perhaps because of limited surfaceexcavation depths. Russell crater displays more frequent, smaller sinuous tracks thanGusev. This may be due to the thin dust cover in Russell, allowing smaller dust devils topenetrate through the bright dust layer and leave conspicuous tracks. The start of thedust devil season and peak activity are delayed in Russell in comparison to Gusev, likelybecause of its more southerly location. Dust devils in both sites travel in directionsconsistent with general circulation model (GCM)‐predicted winds, confirming alaboratory‐derived approach to determining dust devil travel directions based on trackmorphology.

Citation: Verba, C. A., P. E. Geissler, T. N. Titus, and D. Waller (2010), Observations from the High Resolution ImagingScience Experiment (HiRISE): Martian dust devils in Gusev and Russell craters, J. Geophys. Res., 115, E09002,doi:10.1029/2009JE003498.

1. Introduction

[2] Martian dust devils were first discovered in VikingOrbiter (VO) images and later confirmed by Mars GlobalSurveyor (MGS) Mars Orbiter Camera (MOC) images [e.g.,Thomas and Gierasch, 1985; Grant and Schultz, 1987;Edgett and Malin, 2000]. Dust devils are observed overmuch of the surface of Mars, even in the caldera of ArsiaMons [e.g., Balme et al., 2003; Cushing and Titus, 2005;Fisher et al., 2005]. Dust devils act as a mechanism to liftmaterial off the surface and may have a large influence onthe atmospheric dust cycle and global weather patterns [e.g.,Greeley et al., 2003]. Dust devils also play a key role inchanging albedo boundaries and could be responsible forreactivating saltation on defrosting dunes [Geissler, 2005;

Verba, 2009]. The deposition and removal of dust alsoaffect surface robotic operations (for solar‐powered space-craft in particular) and could potentially impact futurehuman explorations [e.g., Verba, 2009].[3] Dust devils are thermally generated, cyclostrophic

vortices that are driven by insolation. Rising warm air fromsolar‐heated surfaces is replaced by colder, dense air sur-rounding the vortex. Particles are lifted by turbulence pro-duced by wind shear and by a suction effect produced by thevertical instability inside the low‐pressure convection core[Sinclair, 1969; Renno et al., 1998; Greeley et al., 2003;Balme and Hagermann, 2006]. Dust devils become visiblewhen they entrain dust particles into the core of the vortex[Greeley et al., 2006]. The net transportation of dustdepends upon variations in wind stress, ambient wind speed,and atmospheric mixing [Newman et al., 2002; Balme andGreeley, 2006].[4] As dust devils remove bright air fall dust from the

surface, the tracks left behind reveal the darker substrate.Martian dust devil tracks display linear, curved, and irreg-ular morphologies that generally vary from 10 m to greater

1U.S. Geological Survey, Flagstaff, Arizona, USA.2NETL, U.S. Department of Energy, Albany, Oregon, USA.3School of Earth and Space Exploration, Arizona State University,

Tempe, Arizona, USA.

Copyright 2010 by the American Geophysical Union.0148‐0227/10/2009JE003498

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, E09002, doi:10.1029/2009JE003498, 2010

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than 200 m in width and can be up to a few kilometers inlength [Edgett and Malin, 2000; Balme et al., 2003; Fisheret al., 2005]. Many tracks cross‐print one another or com-pletely erase previous tracks. The low surface albedo of thenewly exposed substrate enhances convective circulationand results in increased dust devil formation that may createa positive feedback [Verba, 2009; Verba et al., 2009].[5] This paper examines the differences in morphology

and seasonal behavior of dust devil tracks in two diverselocations: Gusev crater, located near the equator at a rela-tively low elevation and the site of concurrent surface ob-servations by the Mars Exploration Rover (MER) Spirit, andRussell crater, located in a more temperate latitude of 53.3°Sand almost 2 km higher in elevation than Gusev. Repeatedimage coverage of these sites includes data from MGS MOCand Mars Reconnaissance Orbiter (MRO) High ResolutionImaging Science Experiment (HiRISE) and Context Camera(CTX). In evaluating the physical morphologies of dustdevil tracks in Gusev and Russell craters, we seek to

understand (1) the differences in track frequency and mor-phology between the two study locations, (2) the differencesin seasonal behavior at the two sites, (3) the physical causesof such morphological and seasonal differences, and (4)how the orbital observations of tracks compare to MERSpirit observations of dust devil plumes in Gusev crater.

2. Background and Location

[6] Gusev crater’s ColumbiaHills region (14.6°S, 175.4°E)is an ideal location to study dust devil track morphology as itis the landing site of the MER Spirit and has a considerableamount of orbital image coverage in support of the MERmission. Spirit has crossed over many dust devil tracks andobserved 533 active dust devils during the first monitoreddust devil season over the period LS 173.2°–339.5° fromMarch–December 2005 (sols 421–691), with peak activity atLS ∼250° [Greeley et al., 2006]. This is consistent with theresults of many other studies that show that the peak occur-rence of equatorial dust devils is during southern spring and

Figure 1. Illustration of dust devil track changes in Russell crater (53.3°S, 12.9°E) [Verba, 2009; Verbaand Geissler, 2008]. (a–b) Dunes are covered in CO2 frost early in the spring season; sinuous trackmorphology is evident as the surface begins to defrost. (c–d) Increase of dust devil activity with decreaseof surface albedo. (e–f) Tracks are covered as dust is deposited and the dune ridges begin to frost over.(Map projected from Reduced Data Records obtained from NASA/LPL/University of Arizona.)

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summer [Malin and Edgett, 2001; Balme et al., 2003; Fisheret al., 2005; Balme and Greeley, 2006].[7] Russell crater (53.3°S, 12.9°E) is located in the

southern midlatitude region and is influenced by seasonalperiods of deposition and sublimation of CO2 frost that havebeen frequently monitored by HiRISE imaging. Russellcrater is 140 km in diameter and had several earlier MOCimages acquired for seasonal monitoring of gullies [Reissand Jaumann, 2002]. In the northeast corner of the craterfloor is a 30 km dune field decorated by dust devil tracksthat extend through the troughs and crests of these dunes.[8] The Russell mega‐dune is part of a complex dune field

that is composed of two major tiers: an upper layer oftransverse dunes, with a lateral dune rim that has gullychannels on the southwest slope, and a lower layer com-prised of linear and reverse dunes. The transverse duneswere formed by southeasterly winds and influenced by anorthwesterly wind that produced a semilinear morphology.This dune type was identified by the presence of avalanchefalls and slope streaks that appear during the winter seasonon the northwestern slip face. The lower linear dunes have asinuous morphology with no determinable slip face. Theouter edge of the mega‐dune has star and barchanoid/bar-chan dunes controlled by a multidirectional wind regime.[9] The mega‐dune height determined from a HiRISE

digital elevation model (DEM) is ∼560 m with respect to thecrater floor, and the lower tier of dunes measures ∼200 mhigh. Reiss and Jaumann [2002] found the gully slopes to be8° based on Mars Orbiter Laser Altimeter‐track ap13426;HiRISE data show the slopes to be about 10.5° and thetransverse dunes to be ∼398 m in height with a slope angleof 17.13° (S. Matson, personal communication, 2009).

3. Approach

3.1. Image Analysis

[10] HiRISE images with 25 cm/pixel spatial resolutionwere the focus of this study (Earth years 2006–2008),

although MOC (narrow angle up to ∼1.5 m/pixel resolution,wide angle up to 240 m/pixel resolution) and CTX (up to 6 m/pixel resolution) images were also examined to providecontext and clarify results. The combination of MOC andHiRISE imagery provides more complete coverage of eachcrater and a longer time frame to more accurately determinethe beginning and end of the dust devil seasons.[11] To interpret the dust devil activity in the images,

quantitative morphological data were derived through imageanalysis. The images were first map‐projected using theU.S. Geological Survey Integrated Software for Imagersand Spectrometers system [Gaddis et al., 1997] to compareregions that overlapped (Figures 1 and 2) [Verba, 2009;Verba and Geissler, 2008; Verba et al., 2009]. The processedHiRISE images were then transferred into EnvironmentalSystems Research Institute’s ArcGIS ArcMap software. Thedirection of travel of dust devils was determined fromoverlapping scallop patterns, based on the laboratory ex-periments of Greeley et al. [2004]. The trailing edge of a dustdevil erases the track as it moves forward and leaves behind ascallop‐circular appearance as a result of picking up dust.Based on this approach, tracks were mapped for their for-ward direction for comparison to wind models. The trackswithin the shape file were divided into three categories: newdirection‐determined tracks, duplicate tracks found in theprevious image, and new tracks without determined direction(an example is shown in the next section, Figure 3). Direction‐determined tracks were then smoothed to remove loopsand azimuths were exported into Rozeta to create a rosediagram.[12] Once the tracks were mapped in ArcMap, measure-

ments were made of their total spatial density, new trackactivity, length, width, and sinuosity. The sinuosity is cal-culated using the number of line segments (n) that make upthe line together with the lengths of the line segments (d)and the cumulative length of the line of all line segments (L)as D = log(n) + log (d/L).

Figure 2. Changes in Gusev crater (14.6°S, 175.4°E) [Verba, 2009; Verba and Geissler, 2008]: (a) slowincrease in dust devil activity, (b) large increase of activity, (c) 2007 dust storm erases tracks, (d) Increaseof dust devil activity after dust storm, and (e and f) no erasure or dust deposition until the end of theseason. (Browse images from NASA/LPL/University of Arizona.)

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[13] These characteristics were measured for tracks in allavailable HiRISE images in both Gusev (Table 1) andRussell (Table 2) craters in order to determine the temporalbehavior of dust devils throughout the season. The tracklengths were remeasured using CTX images with an averageareal coverage of ∼12,400 km2 (Table 3), as HiRISE dataprovide a restricted range of track lengths due to limitedareal coverage of ∼130 km2 and width of under 6 km, lessthan the average track length. Many of the tracks run acrossthe narrow HiRISE footprint and are much longer thaninferred from HiRISE data. Some tracks extend from craters

rims on the edge of the low albedo feature in Gusev crater.CTX measurements provide a better upper limit to the truelengths, with resolution (∼6 m/pixel) sufficient to accuratelydelineate the tracks.

3.2. Global Climate Model

[14] The collection of wind vectors used for this projectwas based on predictions from the NASA Ames generalcirculation model (GCM) [Haberle et al., 1993] obtainedfrom the Mars Global Digital Dune Database (MGD3)[Hayward et al., 2007]. GCM small magnitude winds,<10 m/s, were originally excluded from MGD3 but werecombined with the MGD3 data set to complete the set ofwind speeds in the range 1–30 m/s. No data were availablefor winds >30 m/s; however, it is uncommon for dust devilsto form in winds with velocities greater than 30 m/s based onterrestrial experience [Sinclair, 1969] and Martian observa-tions [Stanzel et al., 2008] of 205 dust devils detected by theMars Express High‐Resolution Stereo Camera. Output fromthe GCM includes detailed attributes of shear stress, windvelocity, magnitude, and direction to define the modeledatmospheric behavior. The height of the reported windvelocity varies with pressure and is typically between 3 and8 m above the surface. The wind data were imported intoArcMap to compare with the HiRISE scalloped‐measureddirections of dust devil motion around Columbia Hills andthe Russell crater mega‐dune field. The GCM output wasseparated into ideal solar heating times (1300–1500 localmean time) and seasons (solar longitude 170°–40°) whenpeak dust devil activity occurs. These model predictions ofwind orientations and strengths (1–30 m/s) in Gusev andRussell craters were used to determine the direction of windsexpected to influence dust devil motion to track azimuths(Figure 4). However, GCM wind vectors reflect only generaltrends and neglect topographic influences; mesoscale modelsof the local atmospheric circulations are better suited todetailed comparisons.

4. Results

4.1. Frequency and Morphology

[15] Fifteen MGS MOC orbital frames were acquiredof Gusev crater (Table 4) between 2001 and mid‐2006,3 months before the beginning of MRO HiRISE observa-tions. Although MOC coverage was sparse, the images span5 Earth years (∼2.5 Martian years) and allow monitoring ofseasonal behavior and track erasure prior to the arrival ofMRO. These observations show that the tracks do notcompletely fade between active seasons and have a hightrack density per spatial area (per image) of ∼50% of old andnew tracks until the 2005 dust storm. Between the dust stormand MOC image S1200095 (LS = 316°), dust devil trackshave a low areal coverage of 1%–2%. Later images docu-ment tracks gradually fading until HiRISE imagePSP_003689_1650 (LS = 139°) when dust devil activityspikes.[16] The HiRISE 2006–2008 dust devil season in the

Columbia Hills region of Gusev crater began with a fewremnant tracks from the previous season (seen in earlierMOC images). There was an increase in activity until the2007 dust storm, which displaced and redeposited dust. Thedust devil activity briefly resumed after the global dust

Figure 3. An example in progress of the directional map-ping procedure in Russell crater done in ArcMap based ondust devil track scallops in HiRISE PSP_002548_1255.The white colored arrows in PSP_002548_1255 are newtracks and red represents previously mapped tracks.

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storm and then slowly decreased until the end of the season.The tracks gradually faded (presumably by dust air fall) butdid not completely disappear into the winter months [Verbaand Geissler, 2008; Greeley et al., 2008].[17] Russell crater was imaged by 10 MOC orbital frames

(Table 5) with the majority of the sampling taking placeduring late winter and early spring. Most of the images showa frost covered surface with no tracks. The images that showtracks are consistent with the HiRISE season, but the densityis much lower. There are no distinguishable tracks from2005 that remain visible in the later HiRISE data.[18] The 2006–2008 dust devil season in Russell crater as

observed in HiRISE data started with a complete absence oftracks due to frost coverage on the dunes. Dust devil activityslowly increased as the frost sublimated away and the sur-face warmed up. The tracks were initially concentrated inthe frost‐free zones on the northwestern slipfaces. Dust devilactivity increased until the 2007 dust storm modified thesurface with dust deposition and wind streaks that coveredthe tracks. After the dust storm, dust devil activity restartedand increased until the tracks reached peak density, then

slowly decreased in activity until the region was covered indust that was deposited by winter winds and frost. All signsof these tracks were erased after the frost sublimated thesubsequent season.[19] The orientations of mapped tracks and the directions

of travel inferred from scallops were compared to modelpredictions of wind directions and strengths based onglobal circulation simulations. The tracks at both study sitesfollow the general wind trends predicted by the GCM(speeds 1–30 m/s) as the winds change throughout the sea-son. The dominant track direction in Russell crater is alignedwith the northwesterly prevailing wind as predicted by theGCM. However, there are dust devil tracks influenced bysoutheasterly winds starting at LS = 17° that move up thegully slope to the top tier of dunes that do not correspond topredicted winds. The tracks in Gusev crater also match thepredicted GCM peak daytime winds from the northwest.There are also tracks that trend northeast that may indicatesecondary crater winds or changing seasonal winter winds.[20] Russell crater tracks are dominantly smaller, nar-

rower, and more sinuous than at Gusev crater, with an

Table 1. Gusev Crater HiRISE Data

Image LsLengthAverage

WidthAverage

SinuosityAverage

%Coverage

NewTracks

TotalTracks Notes

PSP_001513_1655 139.138 2011.90 42.33 1.19 2.37% 73 73 Thin veneer of dust; old tracksPSP_001777_1650 149.472 2033.80 44.39 1.06 3.09% 23 43 No fading, tracks extend over Columbia HillsPSP_002133_1650 163.993 1260.40 30.86 1.12 2.36% 0 11 Difficult to tell if there’s new tracksPSP_003689_1650 235.467 2320 65.1 1.07 34.45% 126 127 Immediate increase of tracks NW/SW trendingPSP_003834_1650 242.61 2627 63.1 1.06 32.52% 49 176 Dark albedo feature increases around Columbia HillsPSP_003900_1650 245.87 2415 88.4 1.05 29.31% 23 85 Very few distinguishable tracks (lack of dust)PSP_004256_1650 263.459 2257.4 83.3 1.04 21.30% 33 68 DD coverage decreasingPSP_005034_1650 300.891 4656.54 61.68 1.02 6.77% 9 20 Dust storm hits; wind streaks and lack of dust in

NE Columbia Hills region and net dust depositionSE/E Columbia Hills

PSP_005245_1650 310.588 4116.58 49.59 1.14 16.01% 53 62 Dust devil activity resumes SE directionPSP_005456_1650 320.039 2726.44 51.84 1.13 18.13% 108 161 NE/E tracks; overprintingPSP_006524_1650 4.04 2598.6 40.1 1.16 18.62% 97 142 Minor new tracks,PSP_006735_1650 12.048 3006.91 61.32 1.16 16.70% 76 160 SE/E tracks; overprinting and decreased activityPSP_008963_1650 89.85 2419.63 52.1 1.18 8.00% 21 116 Short tracks; region dust mantledPSP_009174_1650 97.14 2419.63 52.1 1.19 7.67% 0 99 No new tracks; slight fading of previous tracks

Table 2. Russell Crater HiRISE Data

Image LsWidthAverage

LengthAverage

SinuosityAverage

%Coverage

NewTracks

TotalTracks Notes

PSP_001440_1255 136.335° No data Frost covered; no tracksPSP_001981_1255 157.704° Frost; 2 looping tracks on upper dunes slipfacePSP_002337_1255 172.628° 21.25 325.2 1.45 2.35% 100 100 CO2 sublimation; sinuous track morphology

around the frostPSP_002482_1255 178.915° 41.02 600.7 1.40 6.81% 186 286 Slipface frost‐free; some linear tracksPSP_002548_1255 181.817° 35.89 680.7 1.33 8.54% 16 302 Slightly faded tracks; no newPSP_002904_1255 197.894° 42.29 658.4 1.18 4.43% 116 142 Completely new tracks; bright wind streaks on

gully alcovesPSP_003326_1255 217.794° 57.82 1288.2 1.28 34.75% 325 467 Long and linear tracksPSP_004038_1255 252.689° 50.93 1095.8 1.31 51.15% 610 1077 Large agglomerations on both sets of dunesPSP_004249_1255 263.109° 50.51 868.0 1.21 48.68% 189 1266 High density; uphill preferencePSP_005238_1255 310.265° 31.88 687.0 1.27 30.42% 72 1201 Dust storm erased previous tracks; density decreasedPSP_005383_1255 316.793° 37.9 746.0 1.30 51.99% 2160 2232 Technical peak; new tracks over entire mega‐dunePSP_005528_1255 323.202° 39.81 929.0 1.27 51.62% ∼1000 1862 Wider tracks; overprintingPSP_006873_1255 17.18° 29.64 588.0 1.24 9.62% 198 454 Erasure of majority of tracks; minor activityPSP_007018_1255 22.5° 29.99 752.0 1.27 5.16% 100 100 Minor image overlap; segmented tracksPSP_007229_1255 30.113° 26.2 810.3 1.20 7.95% 300 382 New tracks; segmented tracksPSP_007519_1255 40.368° No data Minor tracks; density ∼5%PSP_009879_1255 122° No tracks; frostPSP_010090_1255 130° No tracks; frost

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average width of 38 m (HiRISE data) and lengths rangingfrom <1 km to a rare distance of 13 km with an average of∼2.6 km (CTX data) and a mean sinuosity of ∼1.3. Russellcrater tracks show an anticorrelation between dust deviltrack length and sinuosity; the longer the track, the lower the

sinuosity. No other linear correlations were found. Gusevcrater tracks are less sinuous (∼1.08) and have an averagewidth of 56 m (HiRISE) and lengths ranging <1–16 km withan average of ∼4.69 km (CTX data).[21] Figure 5 shows the size/frequency distributions of the

2006–2008 dust devil track season, derived from measure-ments of 640 tracks in Gusev crater and 1486 tracks inRussell crater. Gusev crater commonly had larger dustdevils (track widths 40–60 m), whereas Russell crater hadmore numerous, smaller dust devils (widths of 30–40 m).The track widths in Gusev crater are notably larger than thediameters of the dust devil plumes observed by the MERrover Spirit (typically 10–20 m [Greeley et al., 2006]).

4.2. Seasonal Variations

[22] Measurements of dust devil track density as a func-tion of season throughout one Martian year (2006–2008) ofHiRISE observations show that the dust devil season inGusev crater is longer, as indicated by the presence offreshly formed tracks from LS = 149° to beyond 12°, thanthat of Russell crater tracks (which extends from LS = 172°–40°). The orbital observations also indicate that the dustdevil season in Gusev crater is longer than the season inwhich dust devils have so far been spotted by MER Spiritobservations (LS = 173.2°–339.5°) [Greeley et al., 2006].Peak dust devil frequencies occur sooner at Gusev (Spirit:LS = 250°; orbital data: LS = 245°) than at Russell crater(LS = 316°).[23] Changes in the morphological characteristics of the

tracks as a function of season are shown in Figures 6–11.Also shown for reference in Figures 6–11 are surface‐to‐atmosphere temperature difference measurements made bythe Thermal Emission Spectrometer (TES) over Earth years

Table 3. Context Camera Data

Ls

AverageLength(km)

MinimumLength(km)

MaximumLength(km)

CTX: Gusev craterP01_001513_1654 139.14 4.12 0.936 12.23P02_001777_1653 149.47 5.19 0.603 14.1P03_002344_1654 172.93 2.79 0.445 5.3P06_003333_1652 218.48 3.93 1.24 7.64P07_003689_1650 235.48 5.1 1.6 12.51P07_003900_1653 245.89 5.85 0.49 12.89P11_005245_1653 310.61 4.13 1.71 6.92P11_005456_1652 320.06 5.16 0.88 16.17P14_006524_1654 4.05 5.52 1.44 14.53P15_006735_1653 12.05 4.7 1.74 9.91P19_008528_1656 74.99 5.12 1.73 8.6

CTX: Russell craterP03_002337_1253 172.63 0.884 0.229 2.61P04_002482_1253 178.92 1.57 0.389 5.7P04_002548_1254 181.82 2.8 0.821 7.46P05_002904_1253 197.9 0.94 0.44 1.59P06_003326_1255 217.8 4.01 0.882 12.08P08_004038_1254 252.7 2.77 0.71 6.09P08_004249_1252 263.13 5.26 1.77 13.18P11_005238_1252 310.28 3.49 0.97 7.87P11_005383_1253 316.81 4.38 1.18 8.88P12_005528_1252 323.22 3.69 0.85 9.97P15_006873_1254 17.19 1.76 0.36 4.05P15_007018_1253 22.5 1.71 0.474 4.35P16_007229_1252 30.11 1.73 0.82 4.2P17_007519_1255 40.37 1.85 0.63 3.97

Figure 4. GCM predictions of average daytime wind orientations and strengths (1–30 m/s) in (a) Gusev,each band represented by 5 m/s, and (b) Russell craters, each band represented by 10 m/s. (c) Peak windsduring dust devil active period in Gusev crater and (d) Russell crater at LMT: 1300–1500 and Ls = 170°–40°. The measured azimuth orientations of the dust devil tracks in (e) Gusev and (f) Russell craters matchthe general trends of the GCM modeled peak winds.

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1999–2001. More relevant MRO MCS measurements,acquired at the same time as the HiRISE observations, arenot yet available for this study, so the 1999–2001 TESmeasurements are shown as a general guide to the temper-ature variations expected during a typical Martian year (i.e.,a year with no global dust storms). Surface and air tem-peratures were extracted from the TES “vanilla” databasethat contains both surface temperatures and atmospherictemperature profiles. The TES atmospheric temperatureprofiles were derived using algorithms described by Smith etal. [2000] and Smith [2004], represent a general atmospherictemperature profile that is fairly representative of each craterlocation relative to pressure versus height [Conrath et al.,2000]. Atmospheric temperatures were taken at pressuresof 6.1, 4.75, 2.24, 1.06, 0.5, and 0.11 mbar and subtractedfrom the corresponding surface temperatures to represent thetemperature contrast as a function of season. The dips inthese curves presumably show periods when the atmospherewas briefly warmer than usual.[24] Figure 6 shows the dust devil track frequency at the

Gusev crater study site as a function of solar longitude (Ls).Vertical bars indicate the number of newly formed dustdevil tracks seen in each HiRISE observation, countedseparately from the tracks of the previous season or tracksthat are fading, divided by the number of sols since theprevious observation, all divided by the area of overlapbetween observations (typically >40 km2 at Gusev). Thefrequency of new track formation reached a peak of nearly0.1 track/km2/sol in the HiRISE observation closest toperihelion (acquired at Ls 245°). The sizes of the dust devilsresponsible for the tracks, expressed as the mean measured

track widths (Figure 7), also reached a peak near perihelion.The frequency of track formation dropped dramaticallyduring the 2007 global dust storm, the midpoint of which ismarked with an arrow in Figures 6–11. The track lengths(Figure 8) reached a maximum just after the global duststorm, perhaps indicating strong surface winds associatedwith the storm. Note that the width and length measure-ments are for all tracks present in an image and could be thesame as the previous image if there were no additionaltracks.[25] It should be pointed out that the frequency of track

formation shown in Figure 6 is vastly smaller than the fre-quency of dust devil plumes observed at the surface bySpirit. Greeley et al. [2006] extrapolated a frequency of∼50 active dust devils/km2/sol in Gusev crater from Nav-cam observations (over 271 sols from March to December2005 during the hours 0930–1630 local solar time). Theactive season monitored by Spirit (over 137 sols fromFebruary 2007 to June 2007) extrapolates to a frequency of11 dust devils/km2/sol [Greeley et al., 2010], whereasorbital data detected only 0.0011–0.103 dust devil tracks/km2/sol. This implies that the number of dust devil tracksobserved by HiRISE is between 1/500 [Greeley et al.,2006] and 1/110 [Greeley et al., 2010], the number ofdust‐filled vortices observed by Spirit. These differencesare further discussed in section 5.1.[26] The frequency of new track formation at Russell

crater (Figure 9) reached a peak of nearly 1.0 track/km2/sol,an order of magnitude greater than in Gusev. Surprisingly,the peak rate of track formation occurred after the 2007 duststorm at Ls ∼ 316°. Both the widths (Figure 10) and lengths

Table 4. MOC Data at Gusev Crater

MOC: Gusev Crater Date Ls Notes

E0300012 Apr 2001 139 Lack of dust; track areal coverage ∼5%E1601962 May 2002 18.77 ∼50%–60% lower Columbia Hills region dark albedo (lack of dust)R0200357 Feb 2003 134.1 ∼50%–60% SE dominant directionR0802402 Aug 2003 249.2 Upper Columbia Hills region; ∼50% (SE/NE)R1301467 Jan 2004 331 Small image; ∼30%R2001024 Aug 2004 75.9 SW tracks ∼50% & fadingS0200972 Jan 2005 149.2 Long NW tracks to central Columbia HillsS0800891 Jul 2005 245.6 Lower albedo, wind streaks; very faded tracks; 2005 dust stormS0802718 Jul 2005 256 Minor distinct tracks to SE. NW wind‐blown materialS0902527 Aug 2005 275 Low albedo around entire Columbia Hills; SE distinct tracksS1200095 Nov 2005 316 Distorted; no tracksS1800976 May 2006 51.8 ∼2% SW tracks transverse from small cratersS2000248 Jul 2006 74.7 Very light tracks (fading?) ∼1%S2000860 Jul 2006 78 DistortedS2100323 Aug 2006 89.2 Close up of Columbia Hills, very light tracks (fading?) <1%

Table 5. MOC Data at Russell Crater

MOC: Russell Crater Date Ls Notes

E0201493 1 Mar 2001 132 No tracks; bad imageE0300976 1 Apr 2001 143.9 No tracksE0400835 1 May 2001 160 No tracks; frost, CO2 sublimation, dark slope streaksE0200881 1 Feb 2003 138.9 No tracks, high albedo feature on dune (illumination or frost)R0200996 1 Feb 2003 139.9 No tracksR0601007 1 Jun 2003 205 ∼5% high albedo feature on the south facing slopes (frost in summer!)

no track formation preferenceR1301912 1 Jan 2004 332 ∼15% distinguished scallops prefer crests/upper dark dunesR1801512 1 Jun 2004 49 ∼5%–10% NW slip faceS0300120 1 Feb 2005 154.2 No tracksS0702672 1 Jun 2005 236 Can see image but distorted, minor tracks

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(Figure 11) of the tracks rose early in the season, reachedmaximums at Ls ∼ 220° and then gradually declined. TheTES data for 1999–2001 plotted in Figures 9–11 show thatin Russell crater there was a wintertime thermal inversionwhen the surface temperature was colder than the atmo-spheric temperature, consistent with observations of surfacefrost in other years. During this season, when the surface

temperature was expected to be lower than the temperatureof the atmosphere, there were no HiRISE detections oftracks or other indications of dust devil activity (Figure 9).Tracks remained completely absent until the next dust devilseason the following spring.

5. Interpretation

5.1. HiRISE and Spirit Comparison

[27] There are several key differences between HiRISEobservations of dust devil tracks in Gusev crater and Spiritobservations of active dust devil plumes described byGreeley et al. [2006]. The average track widths measured by

Figure 5. Size‐frequency distributions for the entire dustdevil track season (2006–2008) in Gusev and Russell craters.The graph shows measurements of 640 cases in Gusev craterand 1486 tracks in Russell crater.

Figure 6. Profile of the 2006–2008 frequency of dust deviltracks in Gusev crater as a function of solar longitude.Shown for reference are plots of the surface‐to‐atmospheretemperature contrast as a function of atmospheric pressureand season for 1999–2001. Temperature differences areplotted at atmospheric pressures of 6.1, 4.75, 2.24, 1.06,0.5, and 0.11 mbar. The arrow represents the timing of the2007 dust storm (LS = ∼270°–310°) that suppressed dustdevil activity. The dust devil track frequency ranged from0.0011 to 0.103 tracks/km2/sol, reaching a peak near perihe-lion (Ls 251°).

Figure 7. Profile of dust devil track widths in Gusev crateras a function of solar longitude. Peak widths were reachednear Martian perihelion. The arrow represents the timingof the 2007 dust storm (LS = ∼270°–310°) that suppresseddust devil activity.

Figure 8. Profile of dust devil track lengths in Gusev crateras a function of solar longitude. The arrow represents thetiming of the 2007 dust storm (LS = ∼270°–310°) that influ-enced dust devil activity. The track lengths were largest nearthe end of the dust storm, perhaps because of strong windsassociated with the storm.

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HiRISE (sampling 640 cases) are larger than the averagediameter of the dust devil vortices measured by Spirit(sampling 498 cases). HiRISE track measurements showthat dust devils leave behind tracks 40–60 m across, with amean of 56 m, whereas Spirit sees plumes that are generallybetween 10 and 20 m in diameter. CTX observations showeven larger dust devil tracks in the plains west of ColumbiaHills beyond the HiRISE study area and the region observedby Spirit. We also found that the number of dust devil tracksper square kilometer per sol is less than 1/500 the number of

dust devils estimated by Spirit. HiRISE observations of theGusev study site recorded a total of 640 dust devil tracksover the entire Martian year 2006–2008. For comparison,Spirit detected 533 active dust devils in a similar area(∼40 km2) during the 2005 dust devil season but withobservations lasting only a few minutes per sol. Spiritobservations for the period 2006–2008 show a level ofactivity similar to the previous season until the 2007 duststorm (after which all activity observed by Spirit ceased),observing 101 dust devils from LS = 181°–267° and sug-gesting a track to plume frequency ratio of 1/110 [Walleret al., 2009; Greeley et al., 2010].[28] We hypothesize that the tracks in Gusev are made by

large dust devils that represent a small subset of the dustdevil population at Gusev. This hypothesis is consistent withprevious studies that concluded that many Martian dustdevils leave no tracks [Balme et al., 2003] and that tracksmay not be a good proxy measure of all dust devil activity,particularly for smaller vortices [Fisher et al., 2005]. Lorenz[2009] fitted the population of dust plume widths measuredin Gusev by Greeley et al. [2006] to a minus 2 power lawand found that the smaller dust devils are much morenumerous than larger dust devils. The track widths shown inFigure 5 differ from the theoretical distribution of plumesizes, with relatively few small (<20 m) or very large tracks(>90 m) compared to intermediate‐sized tracks. The smallerdust devils evidently fail to leave visible tracks, perhapsbecause they do not excavate deeply enough to penetratethrough the coating of bright dust on the surface (less than amillimeter thick, as shown by Spirit surface observationssuch as those reported by Herkenhoff et al. [2008]). We notethat the smaller dust devils are so much more frequent thateven if they were able to lift only 1% of the dust carried bytheir larger counterparts, their combined effects wouldoverwhelm those of the vastly outnumbered larger dustdevils. Hence, we expect that most of the erosion in Gusevcrater is done by small dust devils that leave no tracks.

Figure 9. Profile of dust devil track frequency in Russellcrater as a function of solar longitude. Shown for referenceare plots of the surface‐to‐atmosphere temperature contrastas a function of atmospheric pressure and season for 1999–2001. Temperature differences are plotted at atmosphericpressures of 4.75, 3.7, 2.24, 1.06, 0.5, and 0.11 mbar.The arrow represents the timing of the 2007 dust storm(LS = ∼270°–310°) that suppressed dust devil activity.The frequency of dust devil tracks ranges from 0.04 to0.95 tracks/km2/sol.

Figure 10. Profile of dust devil track lengths in Russellcrater as a function of solar longitude. The arrow representsthe timing of the 2007 dust storm (LS = ∼270°–310°).

Figure 11. Profile of dust devil track widths in Russellcrater as a function of solar longitude. The arrow repre-sents the timing of the 2007 dust storm (LS = ∼270°–310°) that decreased dust devil size.

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[29] Orbital observations of the seasonal variations of dustdevils in Gusev crater are consistent with the surfaceobservations. Greeley et al. [2006] determined that the 2005Spirit dust devil season extended from southern middlespring to late summer with the peak activity (highest numberof dust devils) occurring near perihelion. The largest dustdevils formed between LS 173.2°–339.5° with a peak at LS246.75°, which matches well with the peak activity. HiRISEobservations of tracks show that rare dust devil activity takesplace earlier (Ls 139°–149°) and later (Ls 12°–90°) than theactive season. The 2006–2008 HiRISE season peak of trackfrequency (LS 245°) corresponds to peak track widths, with asecond peak at LS 4° as activity increased after the globaldust storm. Observations from Spirit’s Navigational Camera(Navcam) showed that the 2007 dust storm significantlyaltered dust devil production in Gusev. There was an almostimmediate halt in dust devils imaged by MER Spirit’scameras as the atmospheric opacity (measured from Spirit)increased above 1 on sol 1240 [Waller et al., 2009].[30] The average horizontal speed of a dust devil deter-

mined with HiRISE imagery based on the average durationobserved by Spirit (170 s) together with the average lengthof the tracks was found to be ∼30 m/s. This speed is muchlarger than the mean horizontal speed estimated by Spirit(∼11.2 m/s; Greeley et al. [2006]), indicating either that thedurations of the largest dust devils were longer than averageor that much of the dust devils’ motion was directed towardthe rover and underestimated. If we assume an averageambient wind speed of 15 m/s as predicted by the GCM andour average measured length (4.69 km), we predict anaverage duration of 312 s or longer if the dust devils travelat less than the ambient wind speed.[31] HiRISE observations of overprinting tracks show no

signs of bright margins that should be expected if sedimentwere redeposited in the immediate vicinity of dust devils.This result is consistent with surface observations [Greeleyet al., 2006] that the dust devils in Gusev crater lack near‐surface dust or sand skirts. The fine (dust) particles areassumed to be lifted into the atmosphere and removed fromthe region.

5.2. Morphological Differences Between Gusev andRussell Craters

[32] Our observations have shown that dust devil tracks inRussell crater are more numerous but shorter, more sinuous,and narrower than those in Gusev crater. Several factorsmight contribute to these morphological differences. Onepossibility is that the dust cover is thinner in Russell crater,allowing smaller dust devils to excavate through and leaveconspicuous tracks. Other factors include the physical set-tings and local topography of the two study sites. Themaximum height of a dust devil is limited by the thicknessof the convective boundary layer, so we should expect largerdust devils at lower latitudes and elevations (Gusev) than athigher latitudes and elevations (Russell) [e.g., Verba, 2009;Balme and Greeley, 2006; Renno et al., 1998; Sinclair,1969]. On the other hand, the temperature contrast betweenthe surface and the lower atmosphere ought to be greater athigher elevations because the atmosphere is cooler. Thisshould enhance atmospheric instability and the potential fordust devil activity at Russell crater, which is situated almost2000 m higher in elevation than Gusev.

[33] Local winds also play a role in track morphology,with lighter winds producing shorter, more sinuous tracks.Topographic features, such as the mega‐dune field in Russellcrater that reaches heights of up to 560 m above the craterfloor, enhance convective circulation due to surface tem-perature heterogeneity and near‐surface nonadiabatic heatingof air parcels moving upslope [Souza et al., 2000]. Thesegiant dunes also present obstacles to the airflow and hinderthe motion of the dust devils, limiting their track lengths andcausing measurably greater sinuosity than in Gusev crater.In contrast, the Columbia hills stand a mere 60 m abovethe relatively flat floor of Gusev, where the straight tracksof dust devils frequently cross the hills with little variation.

5.3. Seasonal Differences Between Gusev and RussellCraters

[34] Our results indicate a longer dust devil season inGusev crater than in Russell. However, there was littleactivity in Gusev for much of the season, with most of theactivity confined to the period between LS = 235° and 12°(total 137°). At Russell crater, the active season extendedfrom LS = 172° to 40° (total 228°) and the tracks were farmore frequent. We also found that peak dust devil fre-quencies occur sooner at Gusev (Spirit: LS ∼250°; trackdata: LS = 245°) than at Russell crater (LS = 316°).[35] These seasonal differences can be partly explained by

the locations of the two study sites. Mars is at perihelionduring southern summer, resulting in more direct sunlight(increased insolation) over Russell crater. Gusev crater iswithin the tropical zone, so it gets sunlight sooner andtherefore displays earlier dust devil activity. The latitudedifference delays the timing of dust devil onset and peak inRussell because of its more southerly location and the frost/ice cover on the dunes. No dust devil activity is expected orobserved to take place in Russell crater during the winterand early spring seasons when the surface is frost coveredand colder than the atmosphere.[36] The length of the dust devil season and the timing of

peak activity are also partly controlled by seasonal winds,which compete with dust devils to cool the surface byconduction rather than convection. Orbital observations ofdust devil tracks, together with concurrent measurements ofsurface and atmospheric temperatures, can help unravelthese competing effects.

6. Conclusions

[37] Our study of dust devil tracks in Gusev and Russellcraters has identified several differences in frequency,morphology, and seasonal behavior between two diversestudy sites and identified some surprising differencesbetween orbital observations of tracks and surface ob-servations of active plumes. The key results of this study aresummarized below.[38] 1. Comparison of MRO HiRISE observations of dust

devil tracks and ground‐based MER Spirit observations ofactive dust devils in Gusev crater shows that only a smallfraction (range of 1/500 to 1/110) of dust devils producetracks that are visible from orbit. The track widths are alsolarger than the average diameters of the dust devil vorticesmeasured by Spirit. These observations suggest that the

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majority of tracks are left behind by larger dust devils thatare only a small subset of the total dust devil activity.[39] 2. Orbital HiRISE data (2006–2008) show that the

dust devil season in Gusev is considerably longer thanpreviously observed by Spirit, albeit at a reduced levelof activity. The peaks in dust devil activity observed byboth Spirit and HiRISE coincide with the peak occurrenceof the largest dust devils, as inferred from track widthmeasurements.[40] 3. Dust devil tracks at Russell crater are more

numerous, narrower, and more sinuous than at Gusev. Thesemorphological differences could be explained by a thinnerdust cover at Russell, allowing smaller and more numerousdust devils to penetrate the dust layer and leave detectabletracks.[41] 4. The seasonal peak in dust devil activity occurs first

at Gusev (LS ∼ 250°) and later at Russell (LS ∼ 316°), due tothe difference in latitude between the study sites. No tracksare found in HiRISE observations during periods of thermalinversion at Russell crater when the atmosphere is stableagainst convection and dust devil activity.[42] 5. In general, track directions inferred from scallops

follow the trends of winds predicted by global circulationmodels but show exceptions due to secondary winds andwinds diverted by the local topography.

[43] Acknowledgments. The authors wish to thank Trent Hare forGIS advice, Colleen Watling for processing MOC images, Sarah Matsonfor providing a digital elevation model of Russell crater for this study, RoseHayward for MGD3, and the HiRISE team. We also thank Matt Balme for aconstructive and helpful review of an earlier draft of this manuscript.

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