Estimate of Aeolian Dust Thickness in Arabia Terra, Mars

Home , Adams (Martian crater), Donner (crater)

Géomorphologie : relief, processus, environnement, 2009, n° 1, p. ???

Estimate of aeolian dust thickness in Arabia Terra, Mars: Implications of a thick mantle (>20 m) for hydrogen detection Estimation des épaisseurs de poussières éoliennes dans la région d’Arabia Terra, Mars : conséquences d’une couverture épaisse (> 20 m) sur la détection d’hydrogène

Nicolas Mangold*, Véronique Ansan*, Philippe Masson** and Cédric Vincendon**

Abstract This study describes a method for estimating dust thickness on the surface of Mars using the distribution of small impact craters (< 1 km in diameter), obtained from high resolution Mars Orbiter Camera (MOC) images on the Mars Global Surveyor (MGS) space probe. The distribution of actual impact craters is different from the theoretical distribution of meteoritic impact flux due to the progressive bury- ing of impact craters by dust deposits. The rim height for the largest buried impact crater was found to provide an approximate minimum thickness for dust blanketing the surface. This method was applied to the region of Arabia Terra, found to be covered by a dust mantle with a minimum thickness of 20 meters. The presented results indicate that Arabia Terra was a regional sink for dust during most of the Amazonian epoch (> 3 Ga). Results also suggest that hydrogen, detected by the Neutron Spectrometer within this region in the top meter, is present in the dust and not in the underlying bedrock.

Keywords: Mars, eolian, dust, neutron.

Résumé Notre étude présente une méthode d’estimation de l’épaisseur de poussière à la surface de Mars en utilisant les petits cratères d’impact (< 1 km de diamètre) observés à haute résolution avec l’imagerie de l’instrument Mars Observer Camera de la sonde Mars Global Surveyor. La distribution des petits cratères est différente de celle prédite par le flux météoritique car les cratères sont progressivement recouverts de dépôts éoliens. La hauteur des remparts des plus gros cratères non enfouis permet d’obtenir une estimation approxima- tive de l’épaisseur de ces dépôts éoliens. Cette méthode est appliquée à la région d’Arabia Terra. Les résultats montrent une couverture de poussière d’environ 20 m au minimum. Ceci indique que cette région est une zone de dépôt durant la période Amazonienne (<3 Ga). Notre étude suggère également que l’hydrogène, qui est détecté par le spectromètre Neutron de Mars Odyssey dans cette région, se localise dans les dépôts éoliens et non pas dans le socle sous-jacent.

Mot clés : Mars, éolien, loess, neutron.

Version française abrégée recouvert de vastes zones pendant les périodes glaciaires sur Terre. Ces poussières progressivement indurées forment La planète Mars est le lieu d’une forte activité éolienne en une couverture sédimentaire plus ou moins épaisse selon les milieu périglaciaire. L’activité éolienne la plus fréquente endroits. Des estimations d’épaisseurs métriques ont été ef- correspond à des tempêtes de poussières, un phénomène fectuées à la suite des observations des sondes Viking (Ar- connu depuis les observations astronomiques effectuées de- vidson et al., 1989 ; Greeley et al., 1992). puis la Terre. Ces tempêtes affectent parfois toute la planè- Le dépôt progressif de poussières provoque l’effacement te en modifiant et homogénéisant son albedo. Cette activité des petits cratères d’impact qui criblent la surface de la pla- a pour conséquence la retombée de poussières sur toute la nète. Des surfaces apparemment très anciennes, car cou- planète en plus ou moins grande quantité, et, notamment vertes de cratères importants (>20 km) et nombreux sur les son accumulation dans des régions dépourvues de déflation images à basse résolution, peuvent ne présenter, à haute ré- éolienne efficace. On peut assimiler ce dépôt au loess ayant solution, que quelques petits cratères en raison de ce re-sur-

* LPGN, UMR6112 CNRS et université de Nantes, 2, rue de la Houssinière, 44322 NANTES. Courriel : [email protected] ** IDES, UMR8148 CNRS et université Paris Sud, Bat 509, 91405 Orsay, France Nicolas Mangold, Véronique Ansan, Philippe Masson et Cédric Vincendon façage progressif. Ce processus peut empêcher une datation re. Or, des lois empiriques permettent de déterminer cette précise de ces terrains ainsi que l’étude de la surface ro- hauteur. Nous en déduisons pour cette image qu’une épais- cheuse proprement dite, entièrement voilée par la couvertu- seur minimale de 26 m recouvre le socle rocheux, expliquant re éolienne. L’exemple de la figure 1 montre une surface très l’oblitération progressive des petits cratères. L’absence de lisse caractéristique d’un recouvrement éolien continu. Les données autour du diamètre d’un kilomètre résulte d’un saut cratères sont frais pour les plus petits (f) et donc très ré- d’échelle fréquent dans les données. Les récentes images à cents, et on observe une dégradation graduelle, qui aug- moyenne résolution acquises par HRSC de Mars Express et mente pour les cratères (ld), puis (sd) jusqu’au cratère (g) THEMIS de Mars Odyssey permettent progressivement de qui est quasiment comblé en totalité. Cependant, nous pou- combler cette lacune. vons tirer parti de cette dégradation et de cet effacement En généralisant cette méthode à toutes les images étu- progressif des petits cratères pour estimer l’épaisseur de la diées, on peut réaliser une carte de distribution de cette couverture éolienne. En effet, les cratères d’impact suivent épaisseur sur toute la zone d’Arabia Terra (fig. 5). Bien que des lois bien établies quant à leur taille (paramètres de l’on observe localement des variations importantes d’une forme bien connus) et à la distribution de leur taille (aug- image à l’autre, la carte d’interpolation des épaisseurs de mentation du nombre de petits cratères suivant une loi de terrain suggère un recouvrement très épais à proximité du puissance), qui permet de donner l’âge d’une surface. Nous cratère Cassini, atteignant 80 m, ainsi qu’un recouvrement renvoyons aux publications de Hartmann (1999) pour une moyen des surfaces de l’ordre de 50 m. Ces épaisseurs res- explication détaillée du calcul de ces distributions et des tent des ordres de grandeur. Des études plus détaillées isochrones. fondées sur les images les plus récentes permettront d’ap- La méthode de datation par les cratères d’impact utilise pliquer cette méthode de manière plus précise. Les résultats des isochrones dont les pentes correspondent à l’augmenta- montrent d’ores et déjà que les terrains représentant des tion du nombre de météores dans l’espace en fonction de la puits de poussières atmosphériques sont très épais, relative- décroissance de leur taille. La distribution en diamètre des ment aux méthodes spectrales qui ne peuvent sonder impacts d’un terrain d’âge homogène suivra un des iso- au-delà de quelques centimètres de profondeur. Cette chrones (fig. 2) qui nous donnera un âge précis à un facteur méthode est applicable sur les planètes où les processus trois près (Hartmann, 1999). Si la distribution des cratères morphologiques s’effectuent à des vitesses de même ordre ne suit pas un isochrone, cela signifie qu’une modification que le criblage par les impacts météoritiques. Enfin, la est survenue postérieurement à la formation de la surface. région d’étude Arabia Terra a été analysée par le spectro- Cette modification a lieu soit à cause de l’érosion des ter- mètre neutron à bord de Mars Odyssey dont les données rains, qui oblitère progressivement des cratères, soit par suggèrent des proportions d’hydrogène non expliquées à ce l’intermédiaire d’un recouvrement sédimentaire ou volca- jour. En effet, des proportions équivalentes de 10-12 % nique qui ennoie les petits cratères. Dans les deux cas, ceux- maximum d’eau (en poids) sont mesurées (Feldman et al., ci sont les premiers à être effacés. On utilise la morphologie 2004), avec un maximum dans les zones équatoriales, et observée sur les images à haute résolution pour distinguer interprétées fréquemment comme la présence d’un socle un processus d’érosion ou au contraire de dépôt. Notre rocheux hydraté. La détection des neutrons se limite à une étude est consacrée à la région d’Arabia fréquemment dé- profondeur d’environ un mètre (par ex. Feldman et al., crite comme étant recouverte d’une couverture éolienne im- 2004). Nos résultats suggèrent donc que cet hydrogène se portante (Zimbelman et Greeley, 1982 ; McEwen et al., trouve dans le dépôt éolien, en relation avec la présence 1988 ; Edgett et Malin, 2000 ; Edgett, 2002). d’eau adsorbée ou de minéraux hydratés enfouis dans la Sur cette région, nous avons utilisé 94 images à haute ré- poussière. solution, et les images de contexte qui leur correspondent, pour déterminer la courbe de distribution des cratères d’im- Introduction pact. L’exemple en figure 2 est typique d’un résultat de la zone d’Arabia soumise à un recouvrement progressif de The surface of Mars is covered by aeolian dust that forms poussières. Les gros cratères, de plus d’1 km de diamètre, thick and widespread deposits that are consolidated over suggèrent que les terrains ont un âge supérieur à 3 Ga ; il time by weathering and diagenesis (Kahn et al., 1992; s’agit de l’âge du socle rocheux dont l’affleurement n’est Christensen et al., 1992). Dust on Mars is transported by pas visible sur l’image haute résolution. En effet, les cra- regional and/or global dust storms. The grain size of Martian tères de plusieurs kilomètres ne sont pas enfouis complète- dust, measured by photometric data, is less than 1 µ diame- ment sous la poussière. Les petits cratères, en revanche, se ter (Kahn et al., 1992). Therefore, dust on Mars is a very regroupent autour d’isochrones de quelques millions d’an- thin material, even thinner than the loess on Earth. At pre- nées seulement, mais surtout, leur distribution ne suit pas un sent, dust accumulates preferentially at the polar ice caps isochrone en totalité, montrant qu’il s’agit d’une surface en and at high elevations where saltation is reduced, likely for- évolution. Nous observons qu’il existe un coude dans la dis- ming mantles up to a few meters thick (Arvidson et al., tribution aux environs de 125 m. Ce changement de pente 1989; Greeley et al., 1992). The geographic distribution of représente les plus gros cratères affectés par le recouvre- the dust mantle for the top decimeter was mapped with ther- ment éolien. Pour ces derniers, l’épaisseur de sédiments éo- mal inertia cartography obtained by the Viking Infrared liens est au moins égale à la hauteur des remparts du cratè- Thermal Mapper (IRTM) and the MGS Thermal Emission

18 Géomorphologie : relief, processus, environnement, 2009, n° 1, p. ?? Estimate of aeolian dust thickness in Arabia Terra, Mars

Spectrometer (TES) (Christensen, 1986; Jakosky et al., latively rich deposits of water-equivalent hydrogen (>20%), 2000). Low thermal inertia regions interpreted as dust depo- surround the poles. The third reservoir, of which Arabia sits were found in areas such as Tharsis Montes, Amazonis Terra is a part, is an equatorial/mid latitude reservoir with a Planitia, Elysium Mons, and Arabia Terra. range of water-equivalent hydrogen reaching locally 12%. Dust often buries small impact craters and as a result Since free water should not exist at the Martian surface in modifies their distribution. For example, the image in figu- equatorial latitudes, questions remain regarding whether re 1 displays very few craters. Among these craters only the water is present in minerals of the bedrock, in minerals smallest are fresh and recent (f), whereas (ld) indicates a contained in aeolian deposits, or as adsorbed water origina- low degradation, (sd) a strong degradation, and (g) a ghost ting from atmospheric deposition (e.g. Feldman et al., crater, i.e. a crater completely filled which is almost not 2005). Therefore, understanding aeolian material thickness identifiable. The different stages represent the progressive is helpful for determining hydrogen sources. infill of craters by dust, with the smallest craters being the An understanding of the main sinks of Martian dust is also quickest buried. an important parameter for climate models and for unders- The study presented here aims to determine the modifica- tanding the role of dust in the atmospheric circulation. Dust tion of small impact crater distributions using Mars Observer thickness is currently only estimated from thermal infrared Camera (MOC) images in order to provide mapping of dust data in the top ten centimeters of the regolith. Therefore, mantle thickness. The method described in this manuscript large regions determined to be ‘dusty’ using thermal IR data was applied to the region of Arabia Terra where the observed can result from the recent deposition of dust deposits less dust mantle has been well documented since the Viking mis- than 1 m thick, whereas other regions with similar inertia sions and later Mars Global Surveyor (e.g. Zimbelman and may be a sink for dust for billions of years with thickness Greeley, 1982; McEwen et al., 1988; Edgett and Malin, over 100 m. In the results that follow, we will show that Ara- 2000; Edgett, 2002). In Arabia Terra, it seems likely that bia Terra is likely a main sink for dust on a geological scale, other external phenomena did not influence the aeolian depo- and plays a role in global estimates of total dust volume. sits during recent geological times, as no evidence of recent volcanic or hydrologic activity is superimposed on the area Method for dust thickness estimate (Greeley and Guest, 1987). using crater distribution The study area of Arabia Terra extends over 6.5 mil- lion km2 (0° to 30°N in latitude and 300° to 360°W in lon- Distinct Martian chronological models based on impact gitude, fig. 2 and fig. 3) in the Martian highlands, with ele- crater distribution, which were obtained from Viking and vations ranging from -2 to 4 km. Arabia Terra was mainly Mariner 9 imagery, were proposed by several authors inclu- shaped by thermal inertia below 250 tiu (tiu refers to the ding Soderblom et al. (1974), Neukum and Wise (1976), thermal inertia unit: J m-2 K-1s-1/2). Soils with values < 250 tiu are generally interpreted as being composed of dust or indurated dust in the top ten centimeters (e.g. Putzig et al., 2005). In this study, dust thickness was estimated and mapped from crater distributions observed in MOC and Vi- king images for 94 sites in Arabia Terra. Arabia Terra is also an interesting region when one consi- ders hydrogen concentrations as revealed by the Neutron Spectrometer onboard Mars Odyssey (e.g. Feldman et al., 2004). These data are obtained by the orbital detection of neutrons ejected by atomic interactions between cosmic rays and hydrogen atoms from the Martian surface. Neutron data provide input for a map of water-equivalent hydrogen concentrations within approximately one meter down the surface. Three major reservoirs can be readily distinguished (Feldman et al., 2004). Two of the reservoirs, which have re-

Fig. 1 – MOC image R1100390 (26.5°E, 27.6°N) in Arabia Terra with examples of more or less degraded craters. Crater (f) is fresh, crater (ld) displays low degradation, crater (sd) is strongly degraded, and crater (g) is a ghost crater almost fully obliterated. North is up. Fig. 1 – Image MOC R1100390 (26,5°E, 27,6°N) montrant la région d’Arabia Terra avec quelques exemples de cratères plus ou moins dégradés. Le cratère (f) est frais, le cratère (ld) montre une faible dégradation, le cratère (sd) montre une forte dégradation, et (g) est un cratère presque totalement recouvert, souvent nommé « cratère fantôme ». Le Nord est en haut.

Géomorphologie : relief, processus, environnement, 2009, n° 1, p. ?? 19 Nicolas Mangold, Véronique Ansan, Philippe Masson et Cédric Vincendon

Fig. 2 – Location of the studied area and an example of a crater count. a: Location of the study region in Arabia Terra on a MGS MOC Wide Angle photo mosaic map of Mars with cylindrical projection (NASA/JPL/MSSS); b: Example of crater counts in the Arabia Terra region on a 3 m per pixel MOC image (M1000717, 23.22°N, 308.24°W, NASA/JPL/MSSS) and Viking images. Counts are plotted in an incremental logarithmic diagram containing Hartmann isochrons (1999) (dashed lines) revised by Ivanov (2001) with uncertainty in the range of a factor of two (Hartmann and Neukum, 2001). This diagram contains the distribution curve for saturated surfaces (Hartmann, 1984) (black solid curve) with an uncertainty about plus or minus a factor of two (Hartmann and Gaskell, 1997) (gray area around black solid curve). The site has a crater distribution shape and surface morphology representative of Arabia Terra. The counted surface is of 30 km2. Error regarding the number of crater counts is represented by bars surrounding the plots. Craters in the 125-177m bin to the 250-350 m bin follow isochrons, but craters smaller than 125 m show a flat distribution, a turndown typical of modification by aeolian processes. D indicates the diameter of this turndown which is used as the critical size for craters not obliterated by dust mantle. The depletion of craters becomes more and more important for small crater diameters due to their quicker obliteration; c: Part of the counted area on the MOC image M1000717 displays a smooth surface with poor cratering typical of aeolian mantle. Fig. 2 – Localisation de la zone d’étude et exemple de comptage de cratères. a : localisation de la zone d’étude d’Arabia Terra sur la mosaïque grand angle MOC ; b : exemple d’un comptage de cratère dans Arabia Terra sur une image MOC (M1000717, 23.22N, 308.24W, NASA/JPL/MSSS) et des images Viking. Les comptages sont représentés sur le diagramme des isochrones de Hartmann (1999) (lignes pointillées) révisées par Ivanov (2001) avec une incertitude d’un facteur deux (Hartmann and Neukum, 2001). Ce diagramme contient aussi la distribution des surfaces saturées en cratères (courbe pleine). La surface comptée est de 30 km2. Les barres d’erreur du comptage de cra- tères figurent autour des points de mesures et sont calculées par écart-type (racine carré de la densité de cratère divisée par la surface). Les résultats montrent que les cratères entre les intervalles de 125-177 m et 250-350 m suivent une isochrone, tandis que la distribution des cratères inférieurs à 125 m s’en écarte. Ce changement de pente est typique d’une modification par des processus éoliens. Le diamètre critique D indiqué par la flèche correspond à la première classe de diamètre de cratères dont la densité n’a pas été affectée par le recouvrement éolien, et donc celle qui servira pour l’estimation de son épaisseur. En effet, les cratères inférieurs à cette taille critique ont été touchés par le recouvrement éolien : la courbe marque un déficit de ces cratères, sinon la courbe continuerait de suivre un isochrone; ce déficit crois- sant pour les petits cratères signifie que les petits cratères ont été oblitérés d’autant plus qu’ils sont petits ; c : zoom sur l’image MOC utili- sée en b) montrant peu de cratères en surface et une surface lisse typique d’un recouvrement éolien fin et continu.

Hartmann et al. (1981), and Neukum and Hiller (1981). In classified the diameter of each crater. Obtained data were this study, we use distribution curves plotted with a loga- then plotted with a Hartmann logarithmic incremental histo- rithmic incremental diagram, as established in the last Hart- gram which showed the number of impact craters per km2 mann’s chronological model (Hartmann, 1999), and revised versus their diameters (fig. 2b). Diameter precision was de- by Ivanov (2001). Because cumulative plots tend to hide termined to be approximately one pixel for each impact cra- subtle variations in the distribution (Hartmann and Neukum, ter. Below six pixels in diameter it became difficult to iden- 2001), we use an incremental. Impact crater distributions tify impact craters, thus explaining an impact crater deficit were obtained with statistical counts of visible impact craters for craters with diameters less than or equal to 6 pixels (e.g., on MOC and Viking images. We developed a computer pro- a crater diameter of 18 m on a MOC image of 3m per pixel, gram to help to determine crater counts. We plotted three or a crater diameter of 1.5 km on a Viking image of 250 m points on the rim of each visible crater, and calculated and per pixel). Due to this limitation, craters less than or equal to

20 Géomorphologie : relief, processus, environnement, 2009, n° 1, p. ?? Estimate of aeolian dust thickness in Arabia Terra, Mars

Fig. 3 – Thermal inertia and hydrogen map in the studied region of Arabia Terra. a: A map of thermal inertia in the Arabia Terra region as obtained from the Putzig et al. (2005) dataset. Dark blue regions display low inertia typical of dust mantle; b: Map of the same area of Mars with the water equivalent abundance of hydrogen in weight percent, as published by Feldman et al. (2004) using the Neu- tron Spectrometer onboard Mars Odyssey. Fig. 3 – Inertie thermique et distribution de l’hydrogène dans la région d’étude d’Arabia Terra. a : carte de l’inertie thermique d’Ara- bia Terra d’après Putzig et al. (2005). Les régions en bleu sombre correspondent à de faibles inerties typiques de recouvrement éolien ; b : carte de la même zone de la Mars montrant la teneur d’hydro- gène en proportion massique équivalent d’eau, comme publiée dans Feldman et al. (2004) en utilisant le Spectromètre Neutron de Mars Odyssey.

6 pixels were not taken into account in crater count curves. The large difference in resolution between MOC and Viking images also created a gap for craters approximately 500 m in diameter (fig. 2b), despite the fact that high resolution Vi- king (typically 40-50 m per pixel) images were used when available. In future studies, visible THEMIS (Thermal Emission Infrared Spectrometer) images (19 m per pixel) and HRSC (High Resolution Stereo Camera) images (12 m per pixel) will significantly contribute to fill this gap. Here we focus only on MOC imagery and Viking imagery. Hartmann (1999) proposed theoretical distribution curves or ‘isochrons’ for Martian terrain of specified ages not modified by surface processes. A plot of a crater density with bins of increasing diameter size can provide the age of the surface. For a fresh surface, not influenced by erosion, deposition, or crater degradation, crater density should follow isochrons corresponding to the age of the formation of the terrain regardless of the range of the chosen crater diameter. The interpretation of these curves is more complex when Rim height (H) is proposed as an appropriate measure- surface processes occur, as is the case for most of the surfa- ment for the minimum dust thickness (T) needed to ce of Mars. When surface modifications do occur, the age is completely cover and obliterate the impact crater, assuming referred to as the ‘crater retention age’, since the determined that the crater was not previously eroded and that aeolian age can correspond to a degradation process rather than to a erosion did not occur in the past (fig. 4). The relationship formation age. When influenced by aeolian processes, the (1) only provides heights for diameters below 7 km. All smallest impact craters are the first to be buried. Therefore, crater sizes obtained during this study were below 7 km, the curve does not follow the isochrons for all crater diame- enabling us to use this single relationship. The method pro- ters, and shows the turn down of small craters, which plot cross isochrons toward younger ages. As a consequence, the diameter of the largest completely buried craters can be obtained by comparing the distribution of measured craters and the theoretical Hartmann isochrons (revised by Ivanov, 2001). Buried craters are also usually buried by dust, but not by enough dust to be obliterated, whereas craters from the diameter bin below that critical value display a small depletion, identified by the departure of the plot from the isochron (see the example in next section). The critical diameter allowed us to estimate the avera- ge of the minimum aeolian dust thickness covering the Fig. 4 – Dust thickness (T) overlaying a crater of diameter (D). The rim height (H) gives an estimate of the minimum dust thickness nee- surface around the crater using the geometric relationship ded to completely cover and obliterate a crater. between the impact crater diameter (D) and the rim et al Fig. 4 – Epaisseur de poussière T recouvrant un cratère de dia- height (H) (Garvin ., 2002), as described in relation- mètre D. L’épaisseur du rempart H fournit une estimation minimale de ship (1) below : l’épaisseur de poussière nécessaire pour remplir et oblitérer le cratè- H=0.07D0.52 (for D < 7km) (1) re de la surface.

Géomorphologie : relief, processus, environnement, 2009, n° 1, p. ?? 21 Nicolas Mangold, Véronique Ansan, Philippe Masson et Cédric Vincendon vides a measurement for the minimum dust infill in the thod gives for this example an estimated thickness of plains. The infill inside the craters can be much higher due roughly 26 m for this location. to the lack of erosion. The exact thickness cannot be esti- We estimated a minimum dust thickness using the largest mated by the method and requires a more detailed procedure buried crater diameter for all 94 MOC images (fig. 5a). Many as proposed by Forsberg-Taylor et al. (2004). The described images (34 from the 94) did not provide a real minimum be- method is also subject to a series of approximations. Error cause the largest craters at the MOC scale did not reach a bars, usually obtained from the root square of the crater den- point for which the depletion of small craters begins, at the sity divided by the area (e.g. Hartmann et al., 1981), were difference of the 177 m for figure 1b, due to the resolution used for crater counts, and did not modify the critical dia- gap. Therefore, for these 34 images the effective value of the meter obtained. The use of the critical diameter depends minimum thicknesses is underestimated. In order to address upon the bin ranges used for crater counts, which decrease this issue, crater distribution curves were extrapolated using by factors of √2 (e.g. from the bin (352 m – 500 m), to the a linear regression between MOC and Viking counts when bin (250 m – 352 m). Thus, a difference of √2 in the value necessary. Among the 94 sites, 60 distribution curves provi- of this diameter would result from a doubt regarding the bin ded reasonable linear regression to obtain their diameters. diameter of this critical thickness. As an example, the diffe- We use these 60, better constrained curves, to provide a map rence between a critical diameter taken at 500 m, or from the of minimum thickness on Arabia Terra by interpolating the bin below at 352 m, corresponds to a final difference in 60 data points (fig. 5b). Using this method, minimum dust thickness of 20% using the relationship (1).Therefore, this thickness varied from 10 to 60 m. In more than half of the method should be used as a rough estimate, with error of studied areas, the thickness was found to be 20 to 40 m. The approximately 20 of the value of the individual thickness thickest dust accumulation is located in the Cassini crater extracted. The number of thickness values extracted from where it reached a height of approximately 60 m (dashed individual images, however, was large enough to provide a circle in figure 5b). We observed an increasing gradient of good statistical sampling in the studied region and allowed dust thickness from the edge of the studied region into Cas- us to establish a regional thickness, as well as to distinguish sini area and slight regional variations. Aeolian mantle seems regional variations. present all over the entire study area, in contrast to previous studies, which identified an Arabia mantle only in the nor- Application to Arabia Terra region thern and western sides (Greeley and Guest, 1987; Schultz and Lutz, 1988) (dashed line, fig. 5a). The study area in Arabia Terra exhibits smooth morpho- logical surface characteristics typical of an aeolian dust Implications mantle (fig. 1 and fig. 2c). Figure 2b shows results for crater counts on a site located at approximately 23° N and Crater counts on Viking images in different parts of Ara- 308° W, with a close-up of the image in figure 2c. This site bia Terra suggest that its mantle is one of the oldest is representative of Arabia Terra surface morphologies, as geological units on Mars, dating to the Noachian to Hespe- well as for the shape of its crater distribution. The distribu- rian epochs (around 3 Gy) (Greeley and Guest, 1987). Those tion for large craters counted on Viking images (> 1 km in previous studies interpreted the age of the Arabia mantle diameter) does not fit with the Hartmann isochrons. A without evaluating the difference between buried formations major surficial process seems to modify the distribution and surficial ones because Mariner and Viking data did not curve for large craters, which was described and interpreted have sufficient resolution to study small craters (< 1 km in as hydrologic and/or volcanic activity in previous works diameter). Schultz and Lutz (1988) estimated that the mant- (e.g. Hartmann, 1973). This process may have stopped or le is up to 1 km thick. However, their data were determined declined approximately 3 to 1 Gy ago. Craters from 125 to using Viking scale images where only large structures are 500 m in the MOC image M1000717 follow isochrons pret- observed, and where surficial deposits are not significant. ty well over three bin sizes, indicating that this crater size Our study, which is based on high resolution MOC images has not been modified for ≈1 Gy. In contrast, a depletion of analysis, shows that an Amazonian dust mantle, tens of small craters occurs between 22 m and 177 m (fig. 2b) im- meter thick, covers the older geologic formations previous- plying a process of obliteration that modifies the distribu- ly described in Arabia Terra. This dust deposit is estimated tion of small impact craters. This depletion of small craters to be 1 Gy old at a maximum (plus or minus a factor of 2). was interpreted to result from obliteration by aeolian depo- Dust accumulation was still active 1 My ago, and is possi- sits observed at the surface, since no evidence of aeolian bly still active at present time, which explains the low erosion (such as yardangs) or volcanic and hydrologic acti- measured thermal inertia (Christensen, 1986). vity exists in all of the studied deposits in the last 3 Gy. De- Regional variations in dust thickness apparently exist. For termined ages suggest a nearly continuous process between instance, dust accumulation between Syrtis Major, on the about 1 My and 1 Gy (Amazonian epoch), confirming this eastern side of the study area, and in the center of Arabia interpretation. The largest craters completely buried by dust Terra increases westward. Such a finding may be consistent are in the diameter range of 125 m to 177 m wide, as indi- with the observation of more aeolian erosion to the east, as cated by the arrow in figure 2b. Taking the mean of the dia- revealed by wind streaks in the Syrtis Major region. The meter bin at 151 m, and using the relation (Eq. 1), this me- same gradient has been observed between the center of the

22 Géomorphologie : relief, processus, environnement, 2009, n° 1, p. ?? Estimate of aeolian dust thickness in Arabia Terra, Mars

mum abundance of hydrogen is located close to the equator. Due to edge effects, we cannot know the dust mantle thickness in this location. Nevertheless, due to the one meter limit of penetration of the neutron spectroscopy, our results show that the detected hydrogen is likely contained inside the >20 m thick dust mantle for most of the Arabia Terra region. Therefore, the presence of the hydrogen detected in hydrated minerals within the bedrock (such as hydrated sulfates or clays) is unlikely. Based on our dust mantle thickness map, the hydrogen should be present ei- ther as hydrated minerals present in dust, or as adsorbed water deposited with dust during last excursions of Mars’ rotational axis at high obliquity (Feldman et al., 2005). The first hypothesis, hydrated minerals in dust, is not consistent with near infrared spectral data, which does not reveal any hydration in the surface of these dust deposits (e.g. Poulet et al., 2007). Nevertheless, this does not rule out possible hydrated minerals buried beneath the first centimeter of de- siccated ground that is not accessible by near infrared imagery. One critical issue regarding the second hypothesis, that is the presence of hydrogen as water in dust, is the high hydrogen content south of the equator, south of the low inertia region (fig. 3b). If hydrogen is present in the dust, this could mean that a dust mantle is also present south of the equator, contrary to thermal inertia values. However, a more indurate dust mantle, as seen from thermal inertia va- Fig. 5 – Maps of dust thickness throughout Arabia Terra. a: The lues >200 tiu might be possible, since this region may be an distribution of minimum dust thickness in meters estimated locally at older part of the dust deposits studied in our work. For fu- the 94 studied sites in the region of Arabia Terra. The dashed line ture work, this region south of Arabia may be a good test shows the maximum southern extent of the Arabia mantle of Schultz region for this hypothesis, as well as for further investiga- and Lutz (1988); b: Interpolation map of the estimated dust thick- tions using the method developed in this study coupled with ness using the 60 best crater plots. The interpolation is based on a local smoothing technique with polynomial regression (IDL based). THEMIS or HRSC images. The dashed circle is the location of the Cassini crater. The strong increase or decrease at the edges of the studied area (especially in Conclusion the northern and the southern parts) is an edge effect of the pro- gram (no measurements as seen from dots reported on the map). This study presents a method for obtaining estimates of Fig. 5 – Cartes de l’épaisseur de poussière dans Arabia Terra. dust thickness over the Martian surface using the distribu- a : distribution de l’épaisseur minimum de poussière en mètres esti- tion of small impact craters (<1 km) observed in high mée localement en 94 endroits de la région d’Arabia Terra. La ligne resolution Mars Orbiter Camera (MOC) images. Using the pointillée indique la délimitation antérieure de la zone de couverture éolienne établie par Schultz et Lutz (1988) ; b : interpolation des method, the distribution of small craters was found to be épaisseurs estimées en utilisant les 60 données les plus fiables. L’in- different from the theoretical distribution due to the meteo- terpolation est basée sur une technique de lissage par régression ritic impact flux since many craters are progressively polynomiale (dans le logiciel IDL). Le cercle pointillé est le cratère buried by dust deposits. The rim height of the largest buried d’impact Cassini. Les fortes valeurs en bordures sont des effets de impact crater was determined to approximate the minimum bord dus au programme (aucun point de mesures). dust thickness blanketing the surface. The method was applied to the region of Arabia Terra, which appears to be study area and the two regions of Acidalia Planitia and Terra covered by a dust mantle with a minimum thickness of 20 Meridiani, to the west and the southwest, respectively, of the meters. The results show that Arabia Terra was a regional study area. Such variations are not visible in the thermal sink for dust during most of the Amazonian epoch. The inertia map, which is not surprising since it can not account results also show that the hydrogen present in Neutron for thickness > 1m. Spectrometer data should belong to the dust and not the Estimates for Arabia dust thicknesses are interesting in underlying bedrock. Further improvements in the mapping regards to Neutron Spectrometer data interpretation. Re- of aeolian dust can be accomplished in the future with addi- gions with water equivalent hydrogen abundance higher tional crater counts and new data from THEMIS imagery than 8% are all contained in regions of dust mantle > 20 m and Mars Express high resolution images. Regional maps thick. We observed differences in maximum hydrogen of dust thickness will improve our knowledge of the Mar- concentrations and dust mantle: the regional maximum of tian surface and aeolian dynamics controlled by climatic dust is located close to the Cassini crater, whereas the maxi- changes during its evolution.

Géomorphologie : relief, processus, environnement, 2009, n° 1, p. ?? 23 Nicolas Mangold, Véronique Ansan, Philippe Masson et Cédric Vincendon

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