Thermal properties of lobate ejecta in Syrtis Major, Mars: Implications for the mechanisms of formation

Thermal properties of lobate ejecta in Syrtis Major, Mars:

Implications for the mechanisms of formation

D. Baratoux,1 N. Mangold,2 P. Pinet,1 and F. Costard2

Received 5 July 2004; revised 7 December 2004; accepted 23 February 2005; published 23 April 2005.

[1] This paper reports an analysis of the thermal properties of ejecta layers of single- anddouble-lobe impact craters on Mars. First observations of thermal properties were made atlow resolution from the Phobos’88 mission and did not allow mapping of variations ofthermal properties inside the ejecta layer. The THEMIS instrument on board the MarsOdyssey mission provides new high-resolution thermal mapping of the surface of Mars.From these data we observe a systematic temperature increase at night at the edge of theejecta. We evaluate first the possible influences on the surface temperatures at night ofpostimpact modification processes given the topography of an impact crater and its ejecta.We show that the observed thermal signature is more likely related to a particle sizedistribution inherited immediately after the impact event during the emplacement of ejecta.We propose that the kinetic sieving process, observed in experimental and natural granularflows, like pyroclastic flows, is responsible for the accumulation of larger particles atthe ejecta flow front and thus is responsible for the temperature increase. Despite evidenceof aeolian activity at Syrtis Major, these craters offer an example of preserved surfacephysical properties resulting from geological processes which have occurred since theformation of the volcanic shield of Syrtis Major. Further studies are underway with theMars Express ongoing observations produced by the HRSC and OMEGA instruments inorder to explore the interplay between the surface physical properties and thespectroscopic signatures seen at Syrtis Major.

Citation: Baratoux, D., N. Mangold, P. Pinet, and F. Costard (2005), Thermal properties of lobate ejecta in Syrtis Major, Mars:

Implications for the mechanisms of formation, J. Geophys. Res., 110, E04011, doi:10.1029/2004JE002314.

1. Introduction

[2] Studies by Head and Roth [1976] and Carr et al.[1977] of Viking imagery pointed out that ejecta depositssurrounding a population of Martian craters ranging indiameter from a few kilometers to a few tens of kilometersare different from those around similar-sized craters on theMoon and Mercury. Several models have been proposedand investigated to explain the formation of these fluidizeddeposits. We define fluidization as the process occurring ina mixture of particles suspended by an upward movingliquid or gas such that the friction between the fluid andparticles and the buoyancy force balance the weight of thegrains and the whole mixture behaves like a fluid. Wedistinguish two classes of models in the literature. The firstclass invokes surface flow of fluidized material, the secondclass invokes the interaction between ejecta and the windsgenerated in the atmosphere by the impact.

1.1. Fluidization and Surface Flow

[3] First, models invoking surface flow [Carr et al.,1977] assume that ejecta are granular material mixed witha fraction of gas and liquid responsible for the fluidiza-tion process (see Figure 1). In the case of impactcratering on Mars, the liquid or gas phase content isprovided by the existence of subsurface reservoirs ofvolatiles, more likely composed of liquid water or waterice. Recent experimental studies have shown that thepressure required to produce incipient melting is about0.6 GPa at 263 K and the pressure required to producetotal melting is about 3.7 GPa for the same ambienttemperature [Stewart and Ahrens, 2003]. Consequently,where ground ice is present, impact cratering results inabundant shock-induced melting which could form fluid-ized ejecta morphologies.[4] The terminology used to describe the process usu-

ally refers to the comparison of natural flow on Earthfluidized by the presence of liquid or gas. The surfaceflow of ejecta is thus compared to debris flow or mudflow and it is seen as a mixture of liquid water andparticles [Carr et al., 1977; Costard and Kargel, 1995;Ivanov et al., 1994; Ivanov, 1996a; Ivanov andPogoretsky, 1996]. Liquid water is mixed with the solidparticles and fluidizes the granular flow. At a first order,the mixture behaves as a Bingham flow which has a yieldstrength determining when the flow comes to rest. Actu-

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, E04011, doi:10.1029/2004JE002314, 2005

1Observatoire Midi-Pyrenees, Laboratoire Dynamique Terrestre etPlanetaire, UMR5562, Toulouse, France.

2Interaction et Dynamique des Environnements de Surface, UMR8148,Orsay, France.

Copyright 2005 by the American Geophysical Union.0148-0227/05/2004JE002314

E04011 1 of 14

ally, the rheology of this mixture is more complicated andthe reader is referred to Iverson [1997] for recent con-siderations on the rheology of debris flows. Lobate craterswhose morphologies are similar to Martian craters havebeen obtained by experimental impacts in viscous targets[Greeley et al., 1980]. Alternatively, if the fluidization isthought to mainly result from the presence of gas, the

process is comparable to pyroclastic flows [Wohletz andSheridan, 1983]. Ejecta first form a light cloud collapsingas a density current. The gas escapes due to thepermeability of particles and ejecta deposit along thesurface.[5] This class of models has served as a fundamental

hypothesis for investigations of the distribution of water

Figure 1. Sketch of the different models that have been proposed for the formation of the lobate ejectadeposits on Mars. The first class (A) assumes that ejecta are a granular flow fluidized by a liquid or gasprovided by the melting of subsurface ice [Carr et al., 1977; Wohletz and Sheridan, 1983]. The secondclass (B) invokes the role of impact winds and the generated vortex which can transport to large distancesthe fine fractions of ejecta particles [Barnouin-Jha and Schultz, 1996].

E04011 BARATOUX ET AL.: THERMAL PROPERTIES OF LOBATE EJECTA

2 of 14

E04011

reservoir in the Martian crust assuming that variations in themorphology of the deposit are mainly related to the icedistribution. First statistical studies presented variations ofmorphologies with crater size, latitude, altitude and targetmaterial [Mouginis-Mark, 1979; Barlow and Bradley,1990]. In order to infer variations of water concentrationin ejecta, and thus variations of the amount of water atdepth, geometric parameters as ejecta mobility or sinuositywere studied by statistical approaches [Cave, 1993; Barlow,1994; Costard and Kargel, 1995]. More recently, someattempts have been made to characterize the rheology ofthe flow. The rheology of ejecta has been explored using theconcept of run-out efficiency and comparisons to landslides[Barnouin-Jha and Baloga, 2003]. A physical model hasbeen also proposed to explain the observed sinuosity and toinfer relative variation of effective viscosity of the ejectafrom an analysis of the wavelengths of the sinuous outline[Baratoux et al., 2002].

1.2. Atmospheric Processes

[6] The second class of model invokes the strong effectproduced by the impact on the flow of gas when anatmosphere, even as light as on Mars is present. Ejectatrajectories form a curtain that acts as a barrier that forcesatmosphere around, causing flow separation at its top edge[Schultz and Gault, 1979; Schultz, 1992]. A ring vortexforms behind this advancing ring vortex that may entrain asignificant portion of ejecta. As the flows decay, the coarserejecta deposit in a contiguous layer while the finer fractionscould form the edges of the lobes [Barnouin-Jha andSchultz, 1996]. While the role of the subsurface water islargely favored, a definitive demonstration of the relativeimportance of these processes is still missing.[7] The THEMIS-IR instrument, on board the Mars

Odyssey mission, permits the characterization of thermalproperties of these ejecta at the one hundred meter scale.From our observation of these data at Syrtis Major, wenotice that the edges of the ejecta are warmer than the innerpart of the layer. The aim of this paper is to describe thisthermal signature and to evaluate different hypotheseswhich could explain this observation and their implicationson the different models of formation.

2. Thermally Distinct Edges of LobateEjecta at Night

2.1. Nighttime THEMIS Images and Rock Abundances

[8] The thermal inertia (I) is a surface property defined asthe root square of the product of the thermal conductivity k,the density r and the specific heat C:

I ¼ffiffiffiffiffiffiffiffirCk

p: ð1Þ

I is the physical parameter which represents the ability ofthe subsurface to conduct and store energy away from thesurface during the day and to return this heat during thenight. The thermal inertia is the main parameter governingthe amplitude of temperature variations of a periodicallyheated surface. The thermal inertia of a region of planetarysurface can be generally related to properties such asparticles size, degree of induration, abundance of rocks andexposure of the bedrocks [Christensen et al., 2003]. The

thermal conductivity in granular material involves solidconduction, radiative transfer across pores and throughgrains, and gas conduction across pores. In granularmaterial, the size of pores controls the apparent conductiv-ity. Fine grained loosely packed material typically exhibits alow value of thermal inertia, while higher values arecommon for rocks and exposed bedrock [Mellon et al.,2000].[9] For granular material, the dominant grain size d (mm)

is related to the conductivity k by the relationship given byPresley and Christensen [1997]:

d ¼ kCP0:6

� �X

; ð2Þ

where P is the atmospheric pressure, X depends on thepressure and can be approximated by 0.5 [Irwin et al., 2004]and C is a constant. However, induration of grains createsthermally conductive bridge and makes the thermal inertiahigher and comparable to the thermal inertia of rocks. Thevalue of thermal inertia can be also representative of amixture of rock and fines. In this case, thermal inertiadepends on the proportion of fines and rocks. Theinterpretation of thermal inertia value is never unique andmust take in account the context of the region in which theyare located.[10] Surface with higher thermal inertia are warmer at

the end of the night before the sunrise, while they arecolder during the afternoon [Christensen et al., 2003]. Thenighttime THEMIS images can be thus very useful whenlooking at geological processes where particle sorting orparticle-size control is expected. Qualitatively, it is possibleto interpret warmer regions at night as containing morerocks than cooler regions.

2.2. Thermal Properties of Lobate Ejectaat Syrtis Major

[11] Syrtis Major is an old volcanic center where tens oflobate craters are observed [Costard, 1989; Barlow andBradley, 1990]. The region of Syrtis Major is part of thethird unit defined by Mellon et al. [2000], referring to themoderate thermal inertia and low-albedo regions. Duricrustand indurated soil material have been observed at all thelanding sites and may represent this unit [Mellon et al.,2000].[12] Despite the occurrence of albedo changes monitored

by telescopic observation over the last decades, Syrtis Majoris one of the permanently darkest regions of Mars, in clearrelation with its volcanic morphology [e.g., Schaber, 1982]and belongs to the surface type I of basaltic compositionidentified by the TES instrument [Bandfield et al., 2000;Hamilton et al., 2001]. Its regional mafic mineralogy,involves a mixture of high- and low-calcium pyroxenes,with a possible minor amount of olivine [e.g., Pinet andChevrel, 1990; Erard et al., 1990; Mustard et al., 1993; Bellet al., 1997]. Spectral variations across Syrtis Major havebeen interpreted as being related to a mixing of volcanicbedrock and debris, dust and soil, with a range of possibleaerial and nonlinear intimate mixings invoked, including thepossible occurrence of sand-sized particles and/or silt-sizedmaterial [Mustard et al., 1993; Harloff and Arnold, 2002;Poulet et al., 2003]. Indeed, telescopic studies had already


3 of 14

E04011

pointed out the peculiar regional photometric behavior ofSyrtis Major which could be related to the presence of bothduricrust-type material and coarse sand-sized particles [DeGrenier and Pinet, 1995; Pinet and Rosemberg, 2001].Syrtis Major may be covered by an indurated soil resistantto erosion but there is also some geomorphic evidence ofthe presence of an irregular mantling by fine particles fromthe analysis of the populations of impact craters, smallercraters being buried by the few tens of meters of indurateddeposit in some places, this indurated mantling being bothconsistent with the paucity of dunes or other aeolianfeatures and the medium values of thermal inertia [Edgettand Malin, 2000; Poulet et al., 2003; Hiesinger and Head,2004].[13] First detailed observations of thermal properties of

ejecta have been made in 1993 from the thermoskan dataset of the Phobos’88 mission [Betts and Murray, 1993].In this study, the authors mentioned the existence ofthermally distinct ejecta blankets and argued that thethermal properties of these ejecta in the equatorial regionmust be due to primary ejecta formation process insteadof secondary modification processes. They associatedthe thermal anomalies to variations of target properties.However, the temperature appeared constant inside theejecta layer at the resolution of the instrument. THEMIS

has detected distinctive temperature patterns around numer-ous impact craters [Christensen et al., 2003]. Generally,ejecta which have the non lobate morphology displaycoarser material than the terrain around, whatever the sizeof the crater. Christensen et al. [2003] mentioned that flowejecta craters can exhibit several distinct thermal inertiaboundaries between different lobes. They suggested that thedifferences could be related to differences in the processesof ejecta emplacement or modification.[14] We processed with the ISIS software 50 nighttime

infrared THEMIS images of the region of Syrtis Major(see Figure 2 for the map of the 54 craters analyzed).Qualitatively, about all of the lobate craters observed onthe available THEMIS nighttime images display thermallydistinct edges of ejecta. The edges of ejecta appear alwayswarmer at night than the remaining of the ejecta layer. Inorder to estimate the strength of this effect, we compute thebrightness temperature by fitting the observed radiance ofthe band 9 (12.56 mm) with a blackbody curve for eachcrater on three different units (Figure 3). A dust opacity of 0and a surface emissivity of 1 is assumed for this estimation.The first unit is the edge of ejecta which is defined by theregion of 4–5 pixels large where the radiance level isclearly above the values in the inner part of the layer. Thesecond unit is chosen to average the radiance level inside

Figure 2. Map of single and double lobate cratersanalyzed from THEMIS nighttime images. Craters arerepresented by a circle on the MOLA shaded relief.

Figure 3. Definition of the different units for temperaturemeasurement: (1) edges of the ejecta, (2) ejecta layer, (3) rimof the crater.


4 of 14

E04011

the layer of ejecta. The third unit is the warmer part of eachcrater, which is the rim of the crater. Craters rims arecomposed of large boulders, and exposure of bedrocks.Thus it is not surprising that this unit has the highesttemperatures. The third unit is used to know whetherthe edge temperature is closer to the layer of ejecta orcloser to the rim temperature. We generally encountered noambiguity for the definition of these three units for themeasurement on each crater.[15] It is usually not possible to compare nighttime

temperatures between images acquired at both differentlocal times and different seasons. One approach is to reducethe average differences in temperature by normalizing theradiance values or temperatures. This approximation is used,for example, by Pelkey et al. [2003] and has the effect towarm cooler images. Our approach is to investigate theinfluence of the season and local time on our measurementsof temperature difference. We plot temperature differences(rim and ejecta edge, ejecta layer and ejecta edge) versus the

local time and the solar longitude (Figures 4 and 5). TheTHEMIS images set used in this study have been acquiredfor solar longitudes that range from 330� to 60�, with a localtime ranging from 3.3 hours to 4.4 hours. For this range ofvalues, no trend can be observed and we conclude that oncan make a direct comparison of temperature differencewithout taking into account differences in local times andsolar longitudes for this set of data.[16] Our results are presented on a histogram which

represents the number of craters which displays a giventemperature difference with bins of 0.5 degree (Figure 6).The edges of the ejecta are in average 4–5 degreeswarmer that the whole layer at night, with some valuesup to 8 degrees. The average thermal inertia on SyrtisMajor is about 250 J m�2 K�1 s�1/2 (Figure 7). FromMellon et al. [2002] it can be estimated that five degreestemperature differences at night could imply an increaseof thermal inertia from 250 J m�2 K�1 s�1/2 to about350 J m�2 K�1 s�1/2. Using equation (2) with X = 0.5, itis possible to write a relationship at constant pressure

Figure 4. Temperature differences versus local time ofacquisition. (a) Temperature differences between the rimand the edge of the ejecta. Formal errors are estimated usingsDT =

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffisTrimð Þ2 þ sTejecta

� �2q, where DT is the temperature

difference, sTrim is the brightness temperature error for therim, and sTejecta is the brightness temperature error for theejecta layer. All errors for this figure and Figure 5 arecalculated the same way. (b) Temperature differencesbetween the edge of the ejecta and the ejecta layer.

Figure 5. Temperature differences versus solar longitudeat the time of acquisition. (a) Temperature differencesbetween the rim and the edge of the ejecta. (b) Temperaturedifferences between the edge of the ejecta and the ejectalayer.


5 of 14

E04011

between a the ratio of two values of thermal inertia and theratio of the corresponding dominant sizes of particles:

d1

d2¼ I1

I2

� 2

: ð3Þ

Given a typical value of 250 J m�2 K�1 s�1/2 for SyrtisMajor area, we find that an increase of 100 J m�2 K�1 s�1/2

can be explained by particles larger by a factor 2 at the edgeof the ejecta. It has been already mentioned that theinterpretation of thermal inertia is not unique, and differentmixtures of even larger particles with a fine fraction maydisplay the same thermal inertia. However, this first orderestimation demonstrates that the increase of temperature atthe edge of the ejecta is high enough to invoke a significantincrease in particle sizes or rocks abundance.

3. Thermally Distinct Edges of Ejecta:Evaluating Different Hypotheses

[17] We evaluate different hypotheses to explain theobservation of warmer edge. The first set of hypothesesconcern processes which are not related to the impact event.These hypotheses include the thermal effects producedby the topography and the dust erosion or depositionpatterns. The following hypotheses are directly related tothe emplacement of ejecta. They include the interactionbetween the ejecta and atmosphere and the kinetic sievingprocess which occurs during the surface flow of granularmaterial. Our approach includes besides the thermal mea-surements at Syrtis Major a combined analysis of MOLA,THEMIS and MOC data for one crater with fluidized ejecta

Figure 6. Histogram of brightness temperature differences at night between the edge of the lobate ejectaand the ejecta layer. Each bar represents the number of crater displaying a given temperature differencewithin a bin of 0.5 degree. Brightness temperature differences range from 0.5 to 8 degrees, with anaverage value around 4–5 degrees.

Figure 7. Thermal inertia of the Syrtis Major area takenfrom the thermal inertia map of Mars estimated from theTES instrument [Mellon et al., 2002].


6 of 14

E04011

at Syrtis Major. We include here some comparison withlobate craters in other areas on Mars. Topographic datahave been used as the highest resolution possible by theextraction of individual MOLA points from the Web sitehttp://image.univ-lyon1.fr (this Web site is currentlydown but may be revived) [Delacourt et al., 2003]. Theobservations made for this crater are presented in thefollowing paragraph.

3.1. Observations From MOLA, THEMIS,and MOC Imagery

[18] The crater, chosen as an example (Figures 8 and 9), islocated at 18.45�N and 73.05�E. This crater presents adouble-lobe morphology. The inner lobe is globally warmerand is thermally uniform. The inner lobe is composed byejecta with lower velocity which have experienced less

fragmentation and could be generally coarser. The outerlobe presents a regular and systematic thermally distinctedge of ejecta. The thermal anomaly extends 3–4 pixelsinward on the THEMIS, which corresponds to 300–400 m.One MOC image, available at the southern part, overlaps theTHEMIS-IR image. The thermally distinct area is not relatedto any systematic change of albedo at the MOC scale. Theslopes over the thermally distinct edge has been estimatedfrom 3 MOLA points and ranges from 4 to 6 degrees.[19] The systematic study presented here focuses on

Syrtis Major. However, we argue here that the thermallydistinct edge of single lobe and double lobe morphologiesis also developed in other areas. Some examples havebeen selected in volcanic plains around Valles Marineris(Figure 10). The thermally distinct edge is present. Thisregion displays higher thermal inertia than Syrtis Major

Figure 8. Example of lobate ejecta at Syrtis Major. (a) THEMIS nighttime image. (b) MOC image ofthe front of the ejecta layer which overlaps the THEMIS image.


7 of 14

E04011

(200–300 J m�2 K�1 s�1/2) and air-fall dust should notcontrol the thermal signal. The presence of the thermallydistinct edge confirms that the rock-size distributionobserved is more likely due to the ejecta emplacementprocess. On the other hand, we found some craters locatedin a bright and low-inertia region (the northern plainsaround Olympus Mons, part of the unit A from [Mellon etal., 2002]) which do not have the clear thermal signaturethey display everywhere else. On these examples, theinward face of rims are barely seen in the thermal data,and ejecta lobes and boundaries of ejecta unit cannot bedetected. These observations suggest that in these regions,the ejecta layers, including the edges, could be covered by athick dust mantling. In this case, it would not be possible toinvestigate the grain size distribution inherited from theimpact. However, a statistical and global analysis of theoccurrence of a warmer edge needs to be undertaken todemonstrate this, but this analysis on thousands of lobatecraters is beyond the scope of this paper.

3.2. Influence of the Topography on the Coolingof Surfaces at Night

[20] The topography at the local scale can influence thecooling of surfaces at night (Figure 11). A perfectly flat

surface cools more efficiently than a surface which cannotradiate through the atmosphere over the full hemisphere.Indeed, in the perfectly flat case, the surface exchanges heatwith the atmosphere only. When an inclined plane ispresent, heat is exchanged between the inclined plane, theatmosphere and the ground around. Since the radiationcoming from the surface of the ground is higher than theradiation coming from the atmosphere, the inclined planecools less efficiently at night than its flat counterpart. Thisthermal effect, which has not been quantified so far, couldaccount for some of the warm slopes observed at the end ofthe night. Thus the inclined forward part of the distal ejectaridge could be expected to be warmer. This effect could beinvoked for explaining the thermal anomaly seen at theedges of the ejecta.[21] However, the warmer part often extends inward,

where the slopes are lower and where the surface canradiate over the full hemisphere (Figures 8 and 9). Thescales to which this effect could be observed have not beenquantified yet, and Christensen et al. [2003] concludedthat the higher slope temperature is due to a concentrationof coarse material on slopes relative to the surrounding flat-lying surface. From this observation, we conclude that onecan rule out this hypothesis.

3.3. Influence of the Topography on the Mantlingof Particulate Material

[22] The presence of granular material can be a result ofdebris accumulation at the foot of hillslope. In this case,the friction angle limits the presence of debris aprons tohillslopes reaching 25� or more. This effect is not relatedto the impact event itself but depends on the slopes of theresulting topography. Indeed, the edge of the ejecta repre-sents the steepest part of the ejecta unit. Slopes of 4�–6� arereported from the MOLA data. Such gentle slopes have noreason to present special segregation of granular material attheir foot. Given the distance between MOLA measure-ments (300 m) and spot sizes (130 m), these slopes may beunderestimated and transport of granular material may occurif higher slopes are present at shorter wavelengths. However,the thermal feature extends over a few hundreds ofmeters andhas a size comparable to the distance between individualMOLA points. A higher average slope angle over thedistance of a few hundred meters would be required toexplain the extent of the thermal anomaly. These values arethus too low to produce the formation of such large debrisaprons of granular material.[23] The region of Syrtis Major has intermediate thermal

inertia values. MOC images show irregular dust or sandmantling and a dunes field is present in Nili Paterae. It couldbe thus argued that the thermal anomaly seen on the edge ofejecta results of different thickness of mantling, or evenabsence of dust mantling. We thus need to describe theinteraction of windblown particles and the topography ofthe edge of the ejecta. Given the slope observed at theMOLA scale (4�–6�) the topographical feature correspondsto the case of the smoothly contoured ridge described byGreeley and Iversen [1985]. The streamline pattern has beenrepresented over the edge of the ejecta (Figure 12) using thetopography of the edge as measured by MOLA on theexample given in Figures 8 and 9. The two-dimensionalsimplification is particularly relevant in the description of

Figure 9. (a) Topographic map from MOLA and(b) topographic profile fromMOLA of the crater of Figure 8.


8 of 14

E04011

streamline pattern around the edge of the ejecta and we thusdo not consider the complexity of the three-dimensionstreamline pattern around lobes. The convergence of thestreamlines at the ridge top implies an increase in windspeed and thus higher surface stresses. These processescould prevent dust accumulation on the edge of the ejecta

and in many places on Mars as already emphasized byChristensen et al. [2003]. Furthermore, separation of the airflow will occur in the leeward side if the hill is sufficientlysteep. The surface shear stress beneath the vortex resultingfrom flow separation can be very high and can be respon-sible for the drift of granular material. Given the difference

Figure 10. Examples of two lobate craters displaying the thermally distinct edge in the region of VallesMarineris (the crater on the left is 17.609�S, 277.812�E; the crater on the right is 19.219�S, 275.375�E).

Figure 11. Effect of the topography at the local scale on the cooling of surfaces at night. (a) In the flatcase, the surface radiates over the full hemisphere and receives radiation from the atmosphere only.(b) The inclined surface exchanges heat with the atmosphere, for one part, and with the ground. Since thethermal radiation coming from the ground seen by the slope is higher than the one which would havecome from the atmosphere, the inclined plane cools less efficiently than the flat surface and warmertemperatures could be observed at the end of the night.


9 of 14

E04011

in slopes inward and outward, we represented the streamlinepattern with the two opposite wind directions, which dem-onstrates that a nonaxisymmetric thermal edge anomalymay be expected in the case of a dominant wind orientation.[24] However, some observations may indicate that the

rock-size distribution inherited from impact cratering hasnot been completely overprinted by recent dust depositionand wind transport of particulate material. The thermalanomaly is present and similar, regardless the full rangeof variation of wind directions relative to the orientation ofthe flow front over the perimeter of the ejecta layer.Furthermore, the presence of the inner lobe with uniformwarmer temperature indicates that dust mantling is notefficient in the area to mask the rock-size distributioninherited from the impact event.[25] It has to be recalled that correlations between thermal

properties and topography have been already mentioned byPelkey and Jakosky [2002] and Pelkey et al. [2003].However, such correlations have been observed betweenelevations and nighttime temperatures but not betweenslopes and nighttime temperatures. Pelkey et al. [2003]interpreted this correlation as variations of the thicknessof granular material falling from the plateau in the region ofMelas Chasma.[26] In order to establish whether or not the slopes at

Syrtis Major can be seen as the primary factor of temper-ature variations at night, we need to compare and estimatethe possible correlation between slopes and radiance values.Slopes must be registered to thermal measurement andestimated with a similar resolution. THEMIS images havea resolution of about 100 m. Individual MOLA pointshaving a spacing of 300 m along tracks provide with thebest resolution of topographic data at these latitudes. Threerepresentative THEMIS IR nighttime images, having bothlobate ejecta and other topographic features, have beenregistered to individual MOLA profiles. The local shift in

longitude and latitude of each THEMIS image has beenestimated using a least squares method and about 30–50 tiepoints selected manually between the THEMIS imageand the MOLA DEM in the same cylindrical projection.Residuals indicate a registration better than 1 km for eachimage. The local slopes have been calculated for eachMOLA point in the direction of the profile, so the slopesare actually minimum local slope values. Radiance has beenestimated at the location of each individual MOLA point byinterpolating and averaging radiance values with a cell of300 m of diameter centered on the corresponding latitudeand longitude. The average and standard deviation ofradiance values estimated for 20 classes of slope valuesbetween 0� and 20� show no correlation between slope andradiance (Figure 13). The standard deviation for each classindicates a larger dispersion of radiance values for theslopes steeper than 10�, implying as observed that somesteep slopes display higher values of radiance (as the inwardfacing slopes of crater rims). However, this trend occurs forslopes steeper than 10� which are steeper than the measuredslope at the ejecta edge.[27] We thus conclude that the thermal properties of

ejecta at Syrtis Major are more controlled by rock-sizedistributions inherited immediately after the impact eventthan air-fall dust and mantling processes. The thermalanomaly at the edge of ejecta at Syrtis Major will be anindicator of the rock-size distribution produced by ejectaformation and emplacement. We will thus now investigatethe ejecta emplacement processes and their implications forthe rock-size distribution.

3.4. Impact Processes and Particle Segregation

3.4.1. Atmospheric Effect[28] Atmospheric effects on ejecta emplacement can

result in size segregation of particles [Schultz and Gault,1979]. In this case, the change in ejecta morphology reflectsthe combined effect of the deceleration of smaller ejectathan a critical size, and the entrainment of these ejectawithin atmospheric vortices created as the outward movingcurtain of ejecta displaces the atmosphere [Schultz, 1992].In some experiments, at the laboratory scale, size segrega-tion occurred when a bimodal size particle distribution ischosen for the target. In these experiments a circular rampart(no lobes or no sinuosity is observed), close to the rim (lessthan one crater radius), of contiguous ejecta is observed.This rampart has a higher concentration of large particlesand is interpreted as the result of ballistically emplacedcoarse ejecta. The effect of the advancing curtain on theemplacement of the ejecta is described by Barnouin-Jhaand Schultz [1996]: ‘‘A ring vortex forms behind thisadvancing curtain. Airflow impinging on the curtain andwithin the vortex decelerates and entrains sufficiently finegrained ejecta. As the flow decays, winds in the vortex mayinitially scour ejecta and target surfaces, with the coarsegrained ejecta deposited in contiguous rampart, and finerfractions in flow lobes.’’ The ejecta initially emplacedballistically, are moved forward by the scouring vortex(loaded with the finer fraction) to from the contiguousrampart which is thus a mixture of coarse and fine material.The sinuous (the outer one) rampart is made with the finesthat are left in the vortex (O. S. Barnouin-Jha, personalcommunication).

Figure 12. Streamline pattern due to the interaction of thewind and the topography of a typical edge of ejecta adaptedfrom Greeley and Iversen [1985]. Winds are represented inthe two opposite directions. Separation will occur on theleeward side if the hill is sufficiently steep.


10 of 14

E04011

[29] Barnouin-Jha and Schultz [1996] demonstrated thatatmospheric vortices created by an impact can entrain ejectaup to 5 mm in size at large distances. The size is estimatedfrom a balance between the drag and gravitational forces(terminal velocity analysis). For a dusty flow moving at thesurface, this value can be underestimated since saltationcould result in the effective transport of larger particles.

Then, this hypothesis has been further investigated bycomparing the number of lobes predicted and observed onMars and Venus by Barnouin-Jha [1998]: ‘‘In laboratoryexperiments the number of sinuous features at the edge ofthe continuous ejecta ramparts is consistent with the theo-retical expectations for the origin of waves created in acurtain-driven vortex ring.’’ Thus, according to this model,the lobes observed at about or more that one crater radiuswould be composed of the fine fraction. The observation ofwarmer edge from THEMIS-IR images is thus not consis-tent with the prediction of this model. Size segregation ofthe fines in the vortex could also occur when the particlestransported in the vortex deposits as a gravity flow (orturbidity current). This effect would only produce sizesegregation of the fines, and can be considered as a secondorder effect when considering the size segregation of theejecta. We thus see this explanation as very unlikely.3.4.2. Kinetic Sieving Process[30] Following the first class of models, called surface

flow, we propose that size segregation of grains couldaccount for the thermal signature we observe. Indeed, sizesegregation of particles is commonly observed on experi-mental flow [Savage and Lun, 1988; Savage, 1989, p. 241].Different mechanisms have been proposed to explain thesize segregation of particles in granular mixtures [e.g.,Makse et al., 1997]. Studying size degregation in granularflow, Bagnold [1954] has introduced the concept of disper-sive pressure generated by interparticle collisions. Bagnoldshowed that this pressure depends, among other factors, onthe grain size. Hence, when particles are sheared together,the larger grains should drift to the zone of least shear strain(the free surface at the top of the flow) and the smaller grainsto the region of greatest shear (i.e., the base). However, theBagnold’s mechanism has been criticized because the anal-ysis was based on particles of constant size and the theorywas developed for neutrally buoyant particles [Naylor,1980]. Moreover, recent revisiting of the 1954 suspensionexperiments of Bagnold have shown that his results appearto be dictated by the design of the experimental facility[Hunt et al., 2002]. Another mechanism, named the kineticsieving mechanism, was proposed by Middleton [1970] andis supported by observations made for experimental flows[Pouliquen and Vallance, 1999]. In this segregation process(Figure 14), large particles rise rapidly to the free surface.The process operates as follows. Gravitational attractioncauses all particles to percolate downward through thegranular medium whenever a sufficiently large void openbeneath them. Small particles percolate thus more often thatlarge ones because they more often encounter voids largeenough to go through. Percolation operates preferentially forsmaller particles and only downward. The smaller particlesaccumulate at the base of the flow, while large particlesaccumulate at the free surface. Then, the velocity gradientexisting in the flow implies that large particles will go fasterthan the smaller ones closer to the bed. Large particlesgradually collect at the front of the flow as it moves.[31] Deposits of many natural granular flows, like debris

flow, pyroclastic flows, and debris avalanches have accu-mulations of large particles at their perimeters and digitatemargins. The accumulations of coarse debris at the front ofthe flows have been reported for example for the debrisflows after the 1991 Pinatubo eruptions [Pouliquen and

Figure 13. Average and standard deviation of radiancevalues estimated for three THEMIS images at Syrtis Majorfor 20 classes of slope values between 0� and 20�. Theresults show no correlation for none of the images. Thedispersion of radiance values increases for steeper slopes.


11 of 14

E04011

Vallance, 1999], for a pyroclastic flow after the 1980 MountSt. Helens eruption [Pouliquen and Vallance, 1999] and forthe Blackhawk landslide in California [Yarnold, 1993].Large particles in these natural flows segregate to the freesurface and then migrate to flow margins in the same wayas outlined here [Savage, 1989, p. 241]. This process,observed both for relatively slow experimental flows andpyroclastic flows, operates for a wide range of velocity.[32] The thermal anomaly seen on the THEMIS-IR

images related to a higher abundance of rocks at the edgeof the ejecta could be explained by the kinetic sievingprocess operating during the emplacement of ejecta. Theabundance of rocks is difficult to assess from the observedincrease in thermal inertia of about 100 J m�2 K�1 s�1/2.The front is expected to be still a mixture of fine and largeparticles with an increased proportion of large particles incomparison with the inner part of the layer. The moderateincrease in thermal inertia is thought to be consistentwith moderate variations in lithology produced by kineticsieving. It should be mentioned here that the inner lobe ofthe crater in Figures 8 and 9 does not display the temper-ature increase at the edge of the ejecta. The shorter length ofthe flow may reduce the sieving efficiency. These ejecta arecoming from the more external part of the excavation zonewhich have experienced lower shock pressure, and thus lessfragmentation. A different size distribution of particles,which is observed from the thermal signal, may also explainwhy the kinetic sieving has not operated in this case.[33] The mechanism of kinetic sieving operates both in

pyroclastic flow or in granular flow fluidized by thepresence of a liquid or gas phase [Pouliquen and Vallance,1999]. The observation of such a sorting of particles is notdiagnostic to distinguish between the role of a liquid or gasphase. However, the presence of liquid inhibits size segre-gation. First, the buoyant effect reduces the percolation ofparticles downward, and, second, the viscosity of the fluidretards the percolation. Both of these effects inhibit the riseof particles to the free surface and the accumulation ofcoarse particles at the front. For lobate ejecta where theprimitive size distribution of particles is observed, theabundances of rock at the front could be an indicator ofthe amount of water during the flow.[34] Furthermore, the process of size segregation induces

instabilities and thus produces digitate deposits which hasbeen reported in experimental granular flow by Pouliquenand Vallance [1999]. So, we suggest here that the kineticsieving process may be responsible for the sinuous pattern of

lobate ejecta. Actually, the kinetic sieving process canoperate with or without the presence of water, but thepresence of water reduces the efficiency of the size segrega-tion by increasing the time needed for a small particle to falldown through the viscous fluid. Conversely to the idea thathigher volatile concentration may have been involved in theformation of more sinuous ejecta deposit [Kargel, 1986;Barlow and Bradley, 1990], the ejecta morphologies havingthe less sinuous outlines may have experienced a moreefficient sorting of particles and thus implies a lower volatileconcentration.

4. Conclusion

[35] This study detailed the thermal characteristic of lobateejecta at Syrtis Major from the nighttime THEMIS-IRimages. The edges of the ejecta appear warmer than the innerpart. Four hypotheses which could potentially explain thisobservation have been investigated: a thermal effect of thetopography, dust erosion or deposition pattern, atmospheresize segregation process and kinetic sieving process duringthe flow. The analysis of MOLA topography and MOCimagery indicates that the observed thermal properties morelikely result from the inherited size segregation occurringduring the ejecta emplacement phase. The kinetic process,which segregated larger particles at the front, has acted asan efficient grain sorting mechanism. Despite evidence ofaeolian activity at SyrtisMajor, these craters offer an exampleof preserved surface physical properties resulting fromgeological processes which have occurred since the forma-tion of the volcanic shield of Syrtis Major. Further studiesare planned with the Mars Express ongoing observations.Indeed, multiangular HRSC data give a unique opportunityof mapping variations of size and optical properties alongthe ejecta layer through photometric inversion of Hapkeparameters [Cord et al., 2003; Pinet et al., 2004]. This wouldconfirm the presence of larger particles at the flow front andwould indicate the presence and distribution or absence ofa dust mantling. These results dealing with the surfacephysical properties will be then combined with the OMEGAimaging spectroscopy observations in order to quantify thecontributions, respectively arising from the surface state andfrom the mineralogic composition, in the unusually deepabsorption features observed at Syrtis Major.

[36] Acknowledgments. This work was supported by the ProgrammeNational de Planetologie, by the CNES (French Space Agency), and by the

Figure 14. Kinetic sieving process in granular flow. Percolation operated downward preferentially forsmall particles. Large particles at the free surface have a higher velocity and thus accumulateprogressively at the flow front.


12 of 14

E04011

European Community’s Improving Human Potential Program under con-tract RTN2-2001-00414, MAGE. Two reviews by O. Barnouin-Jha and V.Hamilton have greatly improved the quality of the manuscript. Weacknowledge important comments of S. Ruff considering the use ofTHEMIS data for areas presenting steep slopes. M. Monnereau providedhelpful suggestions concerning our observations.

ReferencesBagnold, R. A. (1954), Experiments on a gravity-free dispersion of largesolid spheres in a Newtonian fluid under shear, Proc. R. Soc. London,Ser. A, 225, 49–63.

Bandfield, J. L., V. E. Hamilton, and P. R. Christensen (2000), A globalview of Martian surface compositions from MGS-TES, Science,287(5458), 1626–1630.

Baratoux, D., C. Delacourt, and P. Allemand (2002), An instability mecha-nism in the formation of the Martian lobate craters and the implicationsfor the rheology of ejecta, Geophys. Res. Lett., 29(8), 1210, doi:10.1029/2001GL013779.

Barlow, N. G. (1994), Sinuosity of Martian rampart ejecta deposits,J. Geophys. Res., 99(E5), 10,927–10,935.

Barlow, N. G., and T. L. Bradley (1990), Martian impact craters: Correla-tions of ejecta and interior morphologies with diameter, latitude andterrain, Icarus, 87, 156–179.

Barnouin-Jha, O. S. (1998), Lobateness of impact ejecta deposits fromatmospheric interactions, J. Geophys. Res., 103(E11), 25,739–25,756.

Barnouin-Jha, O., and S. Baloga (2003), Comparing run-out efficiency offluidized ejecta on Mars with terrestrial and Martian mass movements,Lunar Planet. Sci., XXXIV, abstract 1599.

Barnouin-Jha, O. S., and P. H. Schultz (1996), Ejecta entrainment byimpact-generated ring vortices: Theory and experiments, J. Geophys.Res., 101(E9), 21,099–21,115.

Bell, J. F., III, M. J. Wolff, P. B. James, R. T. Clancy, S. W. Lee, and L. J.Martin (1997), Mars surface mineralogy from Hubble Space Telescopeimaging during 1994–1995: Observations, calibration, and initial results,J. Geophys. Res., 102(E4), 9109–9124.

Betts, B., and B. Murray (1993), Thermally distinct ejecta blankets fromMartian craters, J. Geophys. Res., 98(E6), 11,043–11,059.

Carr, M. H., L. Crumpler, J. Cutts, R. Greeley, J. Guest, and H. Masursky(1977), Martian impact craters and emplacement of ejecta by surfaceflow, J. Geophys. Res., 82, 4055–4065.

Cave, J. A. (1993), Ice in the northern lowlands and southern highlands ofMars and its enrichment beneath the Elysium lavas, J. Geophys. Res.,98(E6), 11,079–11,097.

Christensen, B. M., et al. (2003), Morphology and composition of thesurface of Mars: Mars Odyssey THEMIS results, Science, 300, 2056–2060, doi:10.1126/science.1080885.

Cord, A., P. Pinet, Y. Daydou, and S. Chevrel (2003), Planetary regolithsurface analogs: Optimized determination of Hapke parameters usingmulti-angular spectro-imaging laboratory data, Icarus, 165(2), 414–427, doi:10.1016/S0019-1035(03)00204-5.

Costard, F. (1989), The spatial distribution of volatiles in the Martianhydrolithosphere, Earth Moon Planets, 45, 265–290.

Costard, F., and J. Kargel (1995), Outwash plains and themokarst on Mars,Icarus, 114, 93–112.

De Grenier, M., and P. Pinet (1995), Near-opposition Martian limb-darkening: Quantification and implication for visible-near-infraredbidirectional reflectance studies, Icarus, 115, 354–368.

Delacourt, C., D. Baratoux, N. Gros, and P. Allemand (2003), OnlineMars DEM derived from MOLA profiles, Eos Trans. AGU, 84(52),583.

Edgett, K. S., and M. C. Malin (2000), New views of Mars eolian activity,materials, and surface properties: Three vignettes from the MarsGlobal Surveyor Mars Orbiter Camera, J. Geophys. Res., 105(E1),1623–1650.

Erard, S., J.-P. Bibring, Y. Langevin, M. Combes, S. Hurtrez, C. Sotin, J. W.Head, and J. F. Mustard (1990), Determination of spectral units in theSyrtis Major-Isidis Planitia region from Phobos/ISM observations, LunarPlanet. Sci., XXI, 327–328.

Greeley, R., and J. Iversen (1985), Wind as a Geolgical Process,Cambridge Planet. Sci. Ser., vol. 4, Cambridge Univ. Press, New York.

Greeley, R. J., J. Fink, D. E. Gault, D. B. Snyder, J. Guest, and P. Schultz(1980), Impact experiments in viscous targets: Laboratory experiments,Lunar Planet. Sci., XI, 2075–2097.

Hamilton, V. E., M. B. Wyatt, Y. Harry, J. McSween, and P. R. Christensen(2001), Analysis of terrestrial and Martian volcanic compositions usingthermal emission spectroscopy: 2. Application to Martian surface spectrafrom the Mars Global Surveyor Thermal Emission Spectrometer, J. Geo-phys. Res., 106(7), 14,733–14,746.

Harloff, J., and G. Arnold (2002), The near-infrared continuum slopeof Martian dark region reflectance spectra, Earth Moon Planets, 88,223–245.

Head, J., and R. Roth (1976), Mars pedestal craters escarpments: Evidencefor ejecta-related emplacement, in Symposium of Planetary CrateringMechanics, pp. 50–52, Lunar and Planet. Inst., Houston, Tex.

Hiesinger, H., and J. W. Head III (2004), The Syrtis Major volcanicprovince, Mars: Synthesis from Mars Global Surveyor data, J. Geophys.Res., 109, E01004, doi:10.1029/2003JE002143.

Hunt, M. L., R. Zenit, C. S. Campbell, and C. E. Brennen (2002), Revisitingthe 1954 suspension experiments of R. A. Bagnold, J. Fluid Mech., 452,1–24.

Irwin, R. P., III, T. R. Watters, A. D. Howard, and J. R. Zimbelman (2004),Sedimentary resurfacing and fretted terrain development along the crustaldichotomy boundary, Aeolis Mensae, Mars, J. Geophys. Res., 109,E09011, doi:10.1029/2004JE002248.

Ivanov, B. (1996), Spread of ejecta from impact craters and the possibilityof estimating the volatile content of the Martian crust, Sol. Syst. Res., 30,43–58.

Ivanov, B., and A. Pogoretsky (1996), Bingham parameters for fluidisedejecta spreading on Mars and Martian volatiles, in Lunar Planet. Sci.,XXVII, 587–588.

Ivanov, B., B. Murray, and A. Yen (1994), Dynamics of fluidized ejectablanket on Mars, Lunar Planet. Sci., XXV, 599–600.

Iverson, R. M. (1997), The physics of debris flows, Rev. Geophys., 35(3),245–296.

Kargel, J. S. (1986), Morphologic variations of Martian rampart cratersand their dependencies and implications, Lunar Planet. Sci., XVII,410–411.

Makse, H. A., S. Havlin, P. R. King, and E. H. Stanley (1997), Spontaneousstratification in granular mixtures, Nature, 386, 379–382.

Mellon, M. T., B. Jakosky, B. M. Kieffer, and B. M. Christensen (2000),High-resolution thermal inertia mapping from the Mars GlobalSurveyor Thermal Emission Spectrometer, Icarus, 148, 437–455.

Mellon, M. T., K. A. Kretke, M. D. Smith, and S. M. Pelkey (2002),A global map of thermal inertia from Mars Global Surveyor mappingmission, Lunar Planet. Sci., XXXIII, abstract 1416.

Middleton, G. V. (1970), Experimental studies related to problems ofFlysh sedimentation, in Flysh Sedimentology in North America, editedby Lajoie, Geol. Assoc. Can. Spec. Pap., 7, 253–272.

Mouginis-Mark, P. (1979), Martian fluidized crater morphology: Variationswith crater size, latitude, altitude and target material, J. Geophys. Res.,84, 8011–8022.

Mustard, J. F., S. Erard, J.-P. Bibring, J. W. Head, S. Hurtrez, Y. Langevin,C. M. Pieters, and C. Sotin (1993), The surface of Syrtis Major: Compo-sition of the volcanic substrate and mixing with altered dust and soil,J. Geophys. Res., 98(E2), 3387–3400.

Naylor, M. A. (1980), The origin of inverse grading in muddy debris flowdeposits—A review, J. Sediment. Petrol., 50(4), 1111–1116.

Pelkey, S. M., and B. M. Jakosky (2002), Surficial geological surveys ofGale Crater and Melas Chasma, Mars: Integration of remote sensing data,Icarus, 160, 228–257.

Pelkey, S. M., B. M. Jakosky, and P. R. Christensen (2003), Surficialproperties in Melas Chasma, Mars, from Mars Odyssey THEMIS data,Icarus, 165, 68–69.

Pinet, P., and S. Chevrel (1990), Spectral identification of geologicalunits on the surface of Mars related to the presence of silicates fromEarth-based near-infrared telescopic charge-coupled device imaging,J. Geophys. Res., 95, 14,435–14,446.

Pinet, P., and C. Rosemberg (2001), Regional photometry and spectralalbedo of the eastern hemisphere of Mars in the 0.7 - 1 micron domain,Lunar Planet. Sci., XXXII, abstract 1640.

Pinet, P. C., D. Baratoux, A. Jehl, Y. Daydou, A. Cord, S. Chevrel, A. Inada,G. Neukum, and the HRSC Team (2004), Orbital imaging photometryand surface geological processes at Mars, paper presented at InternationalMars Conference, Ital. Space Agency, Ischia Island, Italy.

Poulet, F., N. Mangold, and S. Erard (2003), A new view of dark Martianregions from geomorphic and spectroscopic analysis of Syrtis Major,Astron. Astrophys., 412(19–23), doi:10.1051/0004-6361:20031661.

Pouliquen, O., and J. W. Vallance (1999), Segregation induced instabilitiesof granular fonts, Chaos, 9(3), 621–629.

Presley, M., and P. R. Christensen (1997), Thermal conductivity mea-surements of particulate materials, J. Geophys. Res., 102(E3), 6535–6549.

Savage, S. (1989), Flow of granular materials, in Theoretical and AppliedMechanics, edited by P. Germain, M. Piau, D. Callerie, pp. 241–266,Elsevier, New York.

Savage, S., and C. Lun (1988), Particle size segregation in inclinedchute flow of dry cohesionless granular solids, J. Fluid Mech., 189,311–335.


13 of 14

E04011

Schaber, G. G. (1982), Syrtis Major: A low relief volcanic schield,J. Geophys. Res., 87, 9852–9866.

Schultz, P. (1992), Atmospheric effects on ejecta emplacement, J. Geophys.Res., 97, 13,257–13,302.

Schultz, P. H., and D. E. Gault (1979), Atmospheric effects on Martianejecta emplacement, J. Geophys. Res., 84(B13), 7669–7687.

Stewart, S. T., and T. J. Ahrens (2003), Shock Hugoniot of H2O ice,Geophys. Res. Lett., 30(6), 1332, doi:10.1029/2002GL016789.

Wohletz, K., and M. Sheridan (1983), Martian rampart crater ejecta:Experiments and analysis of melt-water interaction, Icarus, 56, 15–37.

Yarnold, J. (1993), Rock-avalanche characteristics in dry climates andthe effect of flow into lakes: Insight from mid-tertiary sedimentarybreccias near Artillery Peak, Arizona, Geol. Soc. Am. Bull., 105,345–360.

��D. Baratoux and P. Pinet, Observatoire Midi-Pyrenees, Laboratoire

Dynamique Terrestre et Planetaire, UMR5562, 14 Avenue Edouard Belin,F-31400 Toulouse, France. ([email protected])F. Costard and N. Mangold, Interaction et Dynamique des Environne-

ments de Surface, UMR8148, Orsay, France.


14 of 14

E04011

Documents

Thermal properties of lobate ejecta in Syrtis Major, Mars: Implications for the mechanisms of formation