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Thermal properties of lobate ejecta in Syrtis Major,
Mars : Implications for the mechanisms of formation
David Baratoux, N. Mangold, P. Pinet, F. Costard
To cite this version:
David Baratoux, N. Mangold, P. Pinet, F. Costard. Thermal properties of lobate ejecta in Syrtis Ma-jor, Mars : Implications for the mechanisms of formation. Journal of Geophysical Research. Planets, Wiley-Blackwell, 2005, 1140, pp.E04. �10.1029/2004JE002314�. �hal-00378511�
Thermal properties of lobate ejecta in Syrtis Major, Mars:
Implications for the mechanisms of formation
D. Baratoux,1 N. Mangold,2 P. Pinet,1and 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- and
double-lobe impact craters on Mars. First observations of thermal properties were made at low resolution from the Phobos’88 mission and did not allow mapping of variations of thermal properties inside the ejecta layer. The THEMIS instrument on board the Mars Odyssey 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 the ejecta. We evaluate first the possible influences on the surface temperatures at night of postimpact 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 size distribution inherited immediately after the impact event during the emplacement of ejecta. We propose that the kinetic sieving process, observed in experimental and natural granular flows, like pyroclastic flows, is responsible for the accumulation of larger particles at the ejecta flow front and thus is responsible for the temperature increase. Despite evidence of aeolian activity at Syrtis Major, these craters offer an example of preserved surface physical properties resulting from geological processes which have occurred since the formation of the volcanic shield of Syrtis Major. Further studies are underway with the Mars Express ongoing observations produced by the HRSC and OMEGA instruments in order to explore the interplay between the surface physical properties and the
spectroscopic 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 deposits surrounding a population of Martian craters ranging in diameter from a few kilometers to a few tens of kilometers are different from those around similar-sized craters on the Moon and Mercury. Several models have been proposed and investigated to explain the formation of these fluidized deposits. We define fluidization as the process occurring in a mixture of particles suspended by an upward moving liquid or gas such that the friction between the fluid and particles and the buoyancy force balance the weight of the grains and the whole mixture behaves like a fluid. We distinguish two classes of models in the literature. The first class invokes surface flow of fluidized material, the second class invokes the interaction between ejecta and the winds generated 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 with a fraction of gas and liquid responsible for the fluidiza-tion process (see Figure 1). In the case of impact cratering on Mars, the liquid or gas phase content is provided by the existence of subsurface reservoirs of volatiles, more likely composed of liquid water or water ice. Recent experimental studies have shown that the pressure required to produce incipient melting is about 0.6 GPa at 263 K and the pressure required to produce total melting is about 3.7 GPa for the same ambient temperature [Stewart and Ahrens, 2003]. Consequently, where ground ice is present, impact cratering results in abundant 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 Earth fluidized by the presence of liquid or gas. The surface flow of ejecta is thus compared to debris flow or mud flow and it is seen as a mixture of liquid water and particles [Carr et al., 1977; Costard and Kargel, 1995; Ivanov et al., 1994; Ivanov, 1996a; Ivanov and Pogoretsky, 1996]. Liquid water is mixed with the solid particles and fluidizes the granular flow. At a first order, the mixture behaves as a Bingham flow which has a yield strength determining when the flow comes to rest.
Actu-1
Observatoire Midi-Pyre´ne´es, Laboratoire Dynamique Terrestre et Plane´taire, UMR5562, Toulouse, France.
2
Interaction et Dynamique des Environnements de Surface, UMR8148, Orsay, France.
Copyright 2005 by the American Geophysical Union. 0148-0227/05/2004JE002314
ally, the rheology of this mixture is more complicated and the reader is referred to Iverson [1997] for recent con-siderations on the rheology of debris flows. Lobate craters whose morphologies are similar to Martian craters have been obtained by experimental impacts in viscous targets [Greeley et al., 1980]. Alternatively, if the fluidization is thought to mainly result from the presence of gas, the
process is comparable to pyroclastic flows [Wohletz and Sheridan, 1983]. Ejecta first form a light cloud collapsing as a density current. The gas escapes due to the permeability of particles and ejecta deposit along the surface.
[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 ejecta
deposits on Mars. The first class (A) assumes that ejecta are a granular flow fluidized by a liquid or gas provided by the melting of subsurface ice [Carr et al., 1977; Wohletz and Sheridan, 1983]. The second class (B) invokes the role of impact winds and the generated vortex which can transport to large distances the fine fractions of ejecta particles [Barnouin-Jha and Schultz, 1996].
reservoir in the Martian crust assuming that variations in the morphology of the deposit are mainly related to the ice distribution. First statistical studies presented variations of morphologies with crater size, latitude, altitude and target material [Mouginis-Mark, 1979; Barlow and Bradley, 1990]. In order to infer variations of water concentration in ejecta, and thus variations of the amount of water at depth, geometric parameters as ejecta mobility or sinuosity were studied by statistical approaches [Cave, 1993; Barlow, 1994; Costard and Kargel, 1995]. More recently, some attempts have been made to characterize the rheology of the flow. The rheology of ejecta has been explored using the concept of run-out efficiency and comparisons to landslides [Barnouin-Jha and Baloga, 2003]. A physical model has been also proposed to explain the observed sinuosity and to infer relative variation of effective viscosity of the ejecta from 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 effect
produced by the impact on the flow of gas when an atmosphere, even as light as on Mars is present. Ejecta trajectories form a curtain that acts as a barrier that forces atmosphere around, causing flow separation at its top edge [Schultz and Gault, 1979; Schultz, 1992]. A ring vortex forms behind this advancing ring vortex that may entrain a significant portion of ejecta. As the flows decay, the coarser ejecta deposit in a contiguous layer while the finer fractions could form the edges of the lobes [Barnouin-Jha and Schultz, 1996]. While the role of the subsurface water is largely favored, a definitive demonstration of the relative importance of these processes is still missing.
[7] The THEMIS-IR instrument, on board the Mars
Odyssey mission, permits the characterization of thermal properties of these ejecta at the one hundred meter scale. From our observation of these data at Syrtis Major, we notice that the edges of the ejecta are warmer than the inner part of the layer. The aim of this paper is to describe this thermal signature and to evaluate different hypotheses which could explain this observation and their implications on the different models of formation.
2. Thermally Distinct Edges of Lobate Ejecta at Night
2.1. Nighttime THEMIS Images and Rock Abundances
[8] The thermal inertia (I) is a surface property defined as
the root square of the product of the thermal conductivity k,
the densityr and the specific heat C:
I¼pffiffiffiffiffiffiffiffirCk: ð1Þ
I is the physical parameter which represents the ability of the subsurface to conduct and store energy away from the surface during the day and to return this heat during the night. The thermal inertia is the main parameter governing the amplitude of temperature variations of a periodically heated surface. The thermal inertia of a region of planetary surface can be generally related to properties such as particles size, degree of induration, abundance of rocks and exposure of the bedrocks [Christensen et al., 2003]. The
thermal conductivity in granular material involves solid conduction, radiative transfer across pores and through grains, and gas conduction across pores. In granular material, the size of pores controls the apparent conductiv-ity. Fine grained loosely packed material typically exhibits a low value of thermal inertia, while higher values are common for rocks and exposed bedrock [Mellon et al., 2000].
[9] For granular material, the dominant grain size d (mm)
is related to the conductivityk by the relationship given by
Presley and Christensen [1997]:
d¼ k CP0:6
X
; ð2Þ
where P is the atmospheric pressure, X depends on the pressure and can be approximated by 0.5 [Irwin et al., 2004] and C is a constant. However, induration of grains creates thermally conductive bridge and makes the thermal inertia higher and comparable to the thermal inertia of rocks. The value of thermal inertia can be also representative of a mixture of rock and fines. In this case, thermal inertia depends on the proportion of fines and rocks. The interpretation of thermal inertia value is never unique and must take in account the context of the region in which they are located.
[10] Surface with higher thermal inertia are warmer at
the end of the night before the sunrise, while they are colder during the afternoon [Christensen et al., 2003]. The nighttime THEMIS images can be thus very useful when looking at geological processes where particle sorting or particle-size control is expected. Qualitatively, it is possible to interpret warmer regions at night as containing more rocks than cooler regions.
2.2. Thermal Properties of Lobate Ejecta
at Syrtis Major
[11] Syrtis Major is an old volcanic center where tens of
lobate craters are observed [Costard, 1989; Barlow and Bradley, 1990]. The region of Syrtis Major is part of the third unit defined by Mellon et al. [2000], referring to the moderate thermal inertia and low-albedo regions. Duricrust and indurated soil material have been observed at all the landing 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 Major is one of the permanently darkest regions of Mars, in clear relation with its volcanic morphology [e.g., Schaber, 1982] and belongs to the surface type I of basaltic composition identified 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 and Chevrel, 1990; Erard et al., 1990; Mustard et al., 1993; Bell et al., 1997]. Spectral variations across Syrtis Major have been interpreted as being related to a mixing of volcanic bedrock and debris, dust and soil, with a range of possible aerial and nonlinear intimate mixings invoked, including the possible occurrence of sand-sized particles and/or silt-sized material [Mustard et al., 1993; Harloff and Arnold, 2002; Poulet et al., 2003]. Indeed, telescopic studies had already
pointed out the peculiar regional photometric behavior of Syrtis Major which could be related to the presence of both duricrust-type material and coarse sand-sized particles [De Grenier and Pinet, 1995; Pinet and Rosemberg, 2001]. Syrtis Major may be covered by an indurated soil resistant to erosion but there is also some geomorphic evidence of the presence of an irregular mantling by fine particles from the analysis of the populations of impact craters, smaller craters being buried by the few tens of meters of indurated deposit in some places, this indurated mantling being both consistent with the paucity of dunes or other aeolian features and the medium values of thermal inertia [Edgett and 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 data set of the Phobos’88 mission [Betts and Murray, 1993]. In this study, the authors mentioned the existence of thermally distinct ejecta blankets and argued that the thermal properties of these ejecta in the equatorial region must be due to primary ejecta formation process instead of secondary modification processes. They associated the thermal anomalies to variations of target properties. However, the temperature appeared constant inside the ejecta 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 display coarser material than the terrain around, whatever the size of the crater. Christensen et al. [2003] mentioned that flow ejecta craters can exhibit several distinct thermal inertia boundaries between different lobes. They suggested that the differences could be related to differences in the processes of 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 on the available THEMIS nighttime images display thermally distinct edges of ejecta. The edges of ejecta appear always warmer at night than the remaining of the ejecta layer. In order to estimate the strength of this effect, we compute the brightness temperature by fitting the observed radiance of
the band 9 (12.56 mm) with a blackbody curve for each
crater on three different units (Figure 3). A dust opacity of 0 and a surface emissivity of 1 is assumed for this estimation. The first unit is the edge of ejecta which is defined by the region of 4 – 5 pixels large where the radiance level is clearly above the values in the inner part of the layer. The second unit is chosen to average the radiance level inside
Figure 2. Map of single and double lobate craters
analyzed from THEMIS nighttime images. Craters are represented by a circle on the MOLA shaded relief.
Figure 3. Definition of the different units for temperature
measurement: (1) edges of the ejecta, (2) ejecta layer, (3) rim of the crater.
the layer of ejecta. The third unit is the warmer part of each crater, which is the rim of the crater. Craters rims are composed of large boulders, and exposure of bedrocks. Thus it is not surprising that this unit has the highest temperatures. The third unit is used to know whether the edge temperature is closer to the layer of ejecta or closer to the rim temperature. We generally encountered no ambiguity for the definition of these three units for the measurement on each crater.
[15] It is usually not possible to compare nighttime
temperatures between images acquired at both different local times and different seasons. One approach is to reduce the average differences in temperature by normalizing the radiance values or temperatures. This approximation is used, for example, by Pelkey et al. [2003] and has the effect to warm cooler images. Our approach is to investigate the influence of the season and local time on our measurements of 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). The THEMIS images set used in this study have been acquired for solar longitudes that range from 330 to 60, with a local time ranging from 3.3 hours to 4.4 hours. For this range of values, no trend can be observed and we conclude that on can make a direct comparison of temperature difference without taking into account differences in local times and solar longitudes for this set of data.
[16] Our results are presented on a histogram which
represents the number of craters which displays a given temperature difference with bins of 0.5 degree (Figure 6). The edges of the ejecta are in average 4 – 5 degrees warmer that the whole layer at night, with some values up to 8 degrees. The average thermal inertia on Syrtis
Major is about 250 J m2 K1 s1/2 (Figure 7). From
Mellon et al. [2002] it can be estimated that five degrees temperature differences at night could imply an increase
of thermal inertia from 250 J m2 K1 s1/2 to about
350 J m2 K1 s1/2. Using equation (2) with X = 0.5, it
is possible to write a relationship at constant pressure
Figure 4. Temperature differences versus local time of
acquisition. (a) Temperature differences between the rim and the edge of the ejecta. Formal errors are estimated using
sDT = ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi sTrim ð Þ2þ s Tejecta 2 q
, where DT is the temperature
difference, sTrim is the brightness temperature error for the
rim, and sTejecta is the brightness temperature error for the
ejecta layer. All errors for this figure and Figure 5 are calculated the same way. (b) Temperature differences between the edge of the ejecta and the ejecta layer.
Figure 5. Temperature differences versus solar longitude
at the time of acquisition. (a) Temperature differences between the rim and the edge of the ejecta. (b) Temperature differences between the edge of the ejecta and the ejecta layer.
between a the ratio of two values of thermal inertia and the ratio of the corresponding dominant sizes of particles:
d1 d2 ¼ I1 I2 2 : ð3Þ
Given a typical value of 250 J m2 K1 s1/2 for Syrtis
Major area, we find that an increase of 100 J m2K1s1/2
can be explained by particles larger by a factor 2 at the edge of the ejecta. It has been already mentioned that the interpretation of thermal inertia is not unique, and different mixtures of even larger particles with a fine fraction may display the same thermal inertia. However, this first order estimation demonstrates that the increase of temperature at the edge of the ejecta is high enough to invoke a significant increase in particle sizes or rocks abundance.
3. Thermally Distinct Edges of Ejecta: Evaluating Different Hypotheses
[17] We evaluate different hypotheses to explain the
observation of warmer edge. The first set of hypotheses concern processes which are not related to the impact event. These hypotheses include the thermal effects produced by the topography and the dust erosion or deposition patterns. The following hypotheses are directly related to the emplacement of ejecta. They include the interaction between the ejecta and atmosphere and the kinetic sieving process which occurs during the surface flow of granular material. 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 ejecta
and the ejecta layer. Each bar represents the number of crater displaying a given temperature difference within a bin of 0.5 degree. Brightness temperature differences range from 0.5 to 8 degrees, with an average value around 4 – 5 degrees.
Figure 7. Thermal inertia of the Syrtis Major area taken
from the thermal inertia map of Mars estimated from the TES instrument [Mellon et al., 2002].
at Syrtis Major. We include here some comparison with lobate craters in other areas on Mars. Topographic data have been used as the highest resolution possible by the extraction of individual MOLA points from the Web site http://image.univ-lyon1.fr (this Web site is currently down but may be revived) [Delacourt et al., 2003]. The observations made for this crater are presented in the following paragraph.
3.1. Observations From MOLA, THEMIS,
and MOC Imagery
[18] The crater, chosen as an example (Figures 8 and 9), is
located at 18.45N and 73.05E. This crater presents a double-lobe morphology. The inner lobe is globally warmer and is thermally uniform. The inner lobe is composed by ejecta with lower velocity which have experienced less
fragmentation and could be generally coarser. The outer lobe presents a regular and systematic thermally distinct edge of ejecta. The thermal anomaly extends 3 – 4 pixels inward on the THEMIS, which corresponds to 300 – 400 m. One MOC image, available at the southern part, overlaps the THEMIS-IR image. The thermally distinct area is not related to any systematic change of albedo at the MOC scale. The slopes over the thermally distinct edge has been estimated from 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 thermally distinct edge of single lobe and double lobe morphologies is also developed in other areas. Some examples have been selected in volcanic plains around Valles Marineris (Figure 10). The thermally distinct edge is present. This region displays higher thermal inertia than Syrtis Major
Figure 8. Example of lobate ejecta at Syrtis Major. (a) THEMIS nighttime image. (b) MOC image of
(200 – 300 J m2 K1 s1/2) and air-fall dust should not control the thermal signal. The presence of the thermally distinct edge confirms that the rock-size distribution observed is more likely due to the ejecta emplacement process. On the other hand, we found some craters located in a bright and low-inertia region (the northern plains around Olympus Mons, part of the unit A from [Mellon et al., 2002]) which do not have the clear thermal signature they display everywhere else. On these examples, the inward face of rims are barely seen in the thermal data, and ejecta lobes and boundaries of ejecta unit cannot be detected. These observations suggest that in these regions, the ejecta layers, including the edges, could be covered by a thick dust mantling. In this case, it would not be possible to investigate the grain size distribution inherited from the impact. However, a statistical and global analysis of the occurrence of a warmer edge needs to be undertaken to demonstrate this, but this analysis on thousands of lobate craters is beyond the scope of this paper.
3.2. Influence of the Topography on the Cooling
of Surfaces at Night
[20] The topography at the local scale can influence the
cooling of surfaces at night (Figure 11). A perfectly flat
surface cools more efficiently than a surface which cannot radiate through the atmosphere over the full hemisphere. Indeed, in the perfectly flat case, the surface exchanges heat with the atmosphere only. When an inclined plane is present, heat is exchanged between the inclined plane, the atmosphere and the ground around. Since the radiation coming from the surface of the ground is higher than the radiation coming from the atmosphere, the inclined plane cools less efficiently at night than its flat counterpart. This thermal effect, which has not been quantified so far, could account for some of the warm slopes observed at the end of the night. Thus the inclined forward part of the distal ejecta ridge could be expected to be warmer. This effect could be invoked for explaining the thermal anomaly seen at the edges of the ejecta.
[21] However, the warmer part often extends inward,
where the slopes are lower and where the surface can radiate over the full hemisphere (Figures 8 and 9). The scales to which this effect could be observed have not been quantified yet, and Christensen et al. [2003] concluded that the higher slope temperature is due to a concentration of coarse material on slopes relative to the surrounding flat-lying surface. From this observation, we conclude that one can rule out this hypothesis.
3.3. Influence of the Topography on the Mantling
of Particulate Material
[22] The presence of granular material can be a result of
debris accumulation at the foot of hillslope. In this case, the friction angle limits the presence of debris aprons to hillslopes reaching 25 or more. This effect is not related to the impact event itself but depends on the slopes of the resulting topography. Indeed, the edge of the ejecta repre-sents the steepest part of the ejecta unit. Slopes of 4 – 6 are reported from the MOLA data. Such gentle slopes have no reason to present special segregation of granular material at their foot. Given the distance between MOLA measure-ments (300 m) and spot sizes (130 m), these slopes may be underestimated and transport of granular material may occur if higher slopes are present at shorter wavelengths. However, the thermal feature extends over a few hundreds of meters and has a size comparable to the distance between individual MOLA points. A higher average slope angle over the distance of a few hundred meters would be required to explain the extent of the thermal anomaly. These values are thus too low to produce the formation of such large debris aprons of granular material.
[23] The region of Syrtis Major has intermediate thermal
inertia values. MOC images show irregular dust or sand mantling and a dunes field is present in Nili Paterae. It could be thus argued that the thermal anomaly seen on the edge of ejecta results of different thickness of mantling, or even absence of dust mantling. We thus need to describe the interaction of windblown particles and the topography of the edge of the ejecta. Given the slope observed at the MOLA scale (4 – 6) the topographical feature corresponds to the case of the smoothly contoured ridge described by Greeley and Iversen [1985]. The streamline pattern has been represented over the edge of the ejecta (Figure 12) using the topography of the edge as measured by MOLA on the example given in Figures 8 and 9. The two-dimensional simplification is particularly relevant in the description of
Figure 9. (a) Topographic map from MOLA and
streamline pattern around the edge of the ejecta and we thus do not consider the complexity of the three-dimension streamline pattern around lobes. The convergence of the streamlines at the ridge top implies an increase in wind speed and thus higher surface stresses. These processes could prevent dust accumulation on the edge of the ejecta
and in many places on Mars as already emphasized by Christensen et al. [2003]. Furthermore, separation of the air flow will occur in the leeward side if the hill is sufficiently steep. The surface shear stress beneath the vortex resulting from 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 Valles
Marineris (the crater on the left is 17.609S, 277.812E; the crater on the right is 19.219S, 275.375E).
Figure 11. Effect of the topography at the local scale on the cooling of surfaces at night. (a) In the flat
case, 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 the thermal radiation coming from the ground seen by the slope is higher than the one which would have come from the atmosphere, the inclined plane cools less efficiently than the flat surface and warmer temperatures could be observed at the end of the night.
in slopes inward and outward, we represented the streamline pattern with the two opposite wind directions, which dem-onstrates that a nonaxisymmetric thermal edge anomaly may 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 has not been completely overprinted by recent dust deposition and wind transport of particulate material. The thermal anomaly is present and similar, regardless the full range of variation of wind directions relative to the orientation of the flow front over the perimeter of the ejecta layer. Furthermore, the presence of the inner lobe with uniform warmer temperature indicates that dust mantling is not efficient in the area to mask the rock-size distribution inherited from the impact event.
[25] It has to be recalled that correlations between thermal
properties and topography have been already mentioned by Pelkey and Jakosky [2002] and Pelkey et al. [2003]. However, such correlations have been observed between elevations and nighttime temperatures but not between slopes and nighttime temperatures. Pelkey et al. [2003] interpreted this correlation as variations of the thickness of granular material falling from the plateau in the region of Melas 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 estimate the possible correlation between slopes and radiance values. Slopes must be registered to thermal measurement and estimated with a similar resolution. THEMIS images have a resolution of about 100 m. Individual MOLA points having a spacing of 300 m along tracks provide with the best resolution of topographic data at these latitudes. Three representative THEMIS IR nighttime images, having both lobate ejecta and other topographic features, have been registered to individual MOLA profiles. The local shift in
longitude and latitude of each THEMIS image has been estimated using a least squares method and about 30 – 50 tie points selected manually between the THEMIS image and the MOLA DEM in the same cylindrical projection. Residuals indicate a registration better than 1 km for each image. The local slopes have been calculated for each MOLA point in the direction of the profile, so the slopes are actually minimum local slope values. Radiance has been estimated at the location of each individual MOLA point by interpolating and averaging radiance values with a cell of 300 m of diameter centered on the corresponding latitude and longitude. The average and standard deviation of radiance values estimated for 20 classes of slope values between 0 and 20 show no correlation between slope and radiance (Figure 13). The standard deviation for each class indicates a larger dispersion of radiance values for the slopes steeper than 10, implying as observed that some steep slopes display higher values of radiance (as the inward facing slopes of crater rims). However, this trend occurs for slopes steeper than 10 which are steeper than the measured slope at the ejecta edge.
[27] We thus conclude that the thermal properties of
ejecta at Syrtis Major are more controlled by rock-size distributions inherited immediately after the impact event than air-fall dust and mantling processes. The thermal anomaly at the edge of ejecta at Syrtis Major will be an indicator of the rock-size distribution produced by ejecta formation and emplacement. We will thus now investigate the ejecta emplacement processes and their implications for the 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 reflects the combined effect of the deceleration of smaller ejecta than a critical size, and the entrainment of these ejecta within atmospheric vortices created as the outward moving curtain of ejecta displaces the atmosphere [Schultz, 1992]. In some experiments, at the laboratory scale, size segrega-tion occurred when a bimodal size particle distribusegrega-tion is chosen for the target. In these experiments a circular rampart (no lobes or no sinuosity is observed), close to the rim (less than one crater radius), of contiguous ejecta is observed. This rampart has a higher concentration of large particles and is interpreted as the result of ballistically emplaced coarse ejecta. The effect of the advancing curtain on the emplacement of the ejecta is described by Barnouin-Jha and Schultz [1996]: ‘‘A ring vortex forms behind this advancing curtain. Airflow impinging on the curtain and within the vortex decelerates and entrains sufficiently fine grained ejecta. As the flow decays, winds in the vortex may initially scour ejecta and target surfaces, with the coarse grained ejecta deposited in contiguous rampart, and finer fractions in flow lobes.’’ The ejecta initially emplaced ballistically, are moved forward by the scouring vortex (loaded with the finer fraction) to from the contiguous rampart which is thus a mixture of coarse and fine material. The sinuous (the outer one) rampart is made with the fines that are left in the vortex (O. S. Barnouin-Jha, personal communication).
Figure 12. Streamline pattern due to the interaction of the
wind and the topography of a typical edge of ejecta adapted from Greeley and Iversen [1985]. Winds are represented in the two opposite directions. Separation will occur on the leeward side if the hill is sufficiently steep.
[29] Barnouin-Jha and Schultz [1996] demonstrated that
atmospheric vortices created by an impact can entrain ejecta up to 5 mm in size at large distances. The size is estimated from a balance between the drag and gravitational forces (terminal velocity analysis). For a dusty flow moving at the surface, this value can be underestimated since saltation could result in the effective transport of larger particles.
Then, this hypothesis has been further investigated by comparing the number of lobes predicted and observed on Mars and Venus by Barnouin-Jha [1998]: ‘‘In laboratory experiments the number of sinuous features at the edge of the continuous ejecta ramparts is consistent with the theo-retical expectations for the origin of waves created in a curtain-driven vortex ring.’’ Thus, according to this model, the lobes observed at about or more that one crater radius would be composed of the fine fraction. The observation of warmer edge from THEMIS-IR images is thus not consis-tent with the prediction of this model. Size segregation of the fines in the vortex could also occur when the particles transported in the vortex deposits as a gravity flow (or turbidity current). This effect would only produce size segregation of the fines, and can be considered as a second order effect when considering the size segregation of the ejecta. 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 could account for the thermal signature we observe. Indeed, size segregation of particles is commonly observed on experi-mental flow [Savage and Lun, 1988; Savage, 1989, p. 241]. Different mechanisms have been proposed to explain the size segregation of particles in granular mixtures [e.g., Makse et al., 1997]. Studying size degregation in granular flow, Bagnold [1954] has introduced the concept of disper-sive pressure generated by interparticle collisions. Bagnold showed that this pressure depends, among other factors, on the 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 grains to the region of greatest shear (i.e., the base). However, the Bagnold’s mechanism has been criticized because the anal-ysis was based on particles of constant size and the theory was developed for neutrally buoyant particles [Naylor, 1980]. Moreover, recent revisiting of the 1954 suspension experiments of Bagnold have shown that his results appear to be dictated by the design of the experimental facility [Hunt et al., 2002]. Another mechanism, named the kinetic sieving mechanism, was proposed by Middleton [1970] and is 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 attraction causes all particles to percolate downward through the granular medium whenever a sufficiently large void open beneath them. Small particles percolate thus more often that large ones because they more often encounter voids large enough to go through. Percolation operates preferentially for smaller particles and only downward. The smaller particles accumulate at the base of the flow, while large particles accumulate at the free surface. Then, the velocity gradient existing in the flow implies that large particles will go faster than the smaller ones closer to the bed. Large particles gradually 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 digitate margins. The accumulations of coarse debris at the front of the flows have been reported for example for the debris flows after the 1991 Pinatubo eruptions [Pouliquen and
Figure 13. Average and standard deviation of radiance
values estimated for three THEMIS images at Syrtis Major for 20 classes of slope values between 0 and 20. The results show no correlation for none of the images. The dispersion of radiance values increases for steeper slopes.
Vallance, 1999], for a pyroclastic flow after the 1980 Mount St. Helens eruption [Pouliquen and Vallance, 1999] and for the Blackhawk landslide in California [Yarnold, 1993]. Large particles in these natural flows segregate to the free surface and then migrate to flow margins in the same way as outlined here [Savage, 1989, p. 241]. This process, observed both for relatively slow experimental flows and pyroclastic 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 edge of the ejecta could be explained by the kinetic sieving process operating during the emplacement of ejecta. The abundance of rocks is difficult to assess from the observed
increase in thermal inertia of about 100 J m2 K1 s1/2.
The front is expected to be still a mixture of fine and large particles with an increased proportion of large particles in comparison with the inner part of the layer. The moderate increase in thermal inertia is thought to be consistent with moderate variations in lithology produced by kinetic sieving. It should be mentioned here that the inner lobe of the crater in Figures 8 and 9 does not display the temper-ature increase at the edge of the ejecta. The shorter length of the flow may reduce the sieving efficiency. These ejecta are coming from the more external part of the excavation zone which have experienced lower shock pressure, and thus less fragmentation. A different size distribution of particles, which is observed from the thermal signal, may also explain why 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 the presence of a liquid or gas phase [Pouliquen and Vallance, 1999]. The observation of such a sorting of particles is not diagnostic to distinguish between the role of a liquid or gas phase. However, the presence of liquid inhibits size segre-gation. First, the buoyant effect reduces the percolation of particles downward, and, second, the viscosity of the fluid retards the percolation. Both of these effects inhibit the rise of particles to the free surface and the accumulation of coarse particles at the front. For lobate ejecta where the primitive size distribution of particles is observed, the abundances of rock at the front could be an indicator of the amount of water during the flow.
[34] Furthermore, the process of size segregation induces
instabilities and thus produces digitate deposits which has been reported in experimental granular flow by Pouliquen and Vallance [1999]. So, we suggest here that the kinetic sieving process may be responsible for the sinuous pattern of
lobate ejecta. Actually, the kinetic sieving process can operate with or without the presence of water, but the presence of water reduces the efficiency of the size segrega-tion by increasing the time needed for a small particle to fall down through the viscous fluid. Conversely to the idea that higher volatile concentration may have been involved in the formation of more sinuous ejecta deposit [Kargel, 1986; Barlow and Bradley, 1990], the ejecta morphologies having the less sinuous outlines may have experienced a more efficient sorting of particles and thus implies a lower volatile concentration.
4. Conclusion
[35] This study detailed the thermal characteristic of lobate
ejecta at Syrtis Major from the nighttime THEMIS-IR images. The edges of the ejecta appear warmer than the inner part. Four hypotheses which could potentially explain this observation have been investigated: a thermal effect of the topography, dust erosion or deposition pattern, atmosphere size segregation process and kinetic sieving process during the flow. The analysis of MOLA topography and MOC imagery indicates that the observed thermal properties more likely result from the inherited size segregation occurring during the ejecta emplacement phase. The kinetic process, which segregated larger particles at the front, has acted as an efficient grain sorting mechanism. Despite evidence of aeolian activity at Syrtis Major, these craters offer an example of preserved surface physical properties resulting from geological processes which have occurred since the forma-tion of the volcanic shield of Syrtis Major. Further studies are planned with the Mars Express ongoing observations. Indeed, multiangular HRSC data give a unique opportunity of mapping variations of size and optical properties along the ejecta layer through photometric inversion of Hapke parameters [Cord et al., 2003; Pinet et al., 2004]. This would confirm the presence of larger particles at the flow front and would indicate the presence and distribution or absence of a dust mantling. These results dealing with the surface physical properties will be then combined with the OMEGA imaging spectroscopy observations in order to quantify the contributions, respectively arising from the surface state and from the mineralogic composition, in the unusually deep absorption features observed at Syrtis Major.
[36] Acknowledgments. This work was supported by the Programme National de Plane´tologie, by the CNES (French Space Agency), and by the
Figure 14. Kinetic sieving process in granular flow. Percolation operated downward preferentially for
small particles. Large particles at the free surface have a higher velocity and thus accumulate progressively at the flow front.
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. We acknowledge important comments of S. Ruff considering the use of THEMIS data for areas presenting steep slopes. M. Monnereau provided helpful suggestions concerning our observations.
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![Figure 12. Streamline pattern due to the interaction of the wind and the topography of a typical edge of ejecta adapted from Greeley and Iversen [1985]](https://thumb-eu.123doks.com/thumbv2/123doknet/14758414.583739/11.888.74.441.99.416/figure-streamline-pattern-interaction-topography-typical-greeley-iversen.webp)
