Lunar and Planetary Science XXXIII (2002) 1307.pdf

POSSIBLE DUST DEVILS TRACKS DETECTED IN TENERE’ DESERT (NIGER): AN ANALOGUE TO MARS. A. P. Rossi, International Research School of Planetary Sciences, Universita’ d’Annunzio, Viale Pindaro 42, 65127 Pescara, Italy, ([email protected]).

Introduction: Features resembling dust devils tracks have been observed in Tenere’ Desert (Niger), few kilometers NW of Adrar Madet (Figure 1), about 300 km far from Agadez. These linear features were detected on ASTER imagery, in visible and near infrared bands. SWIR and TIR bands don’t show any clear evidence of trails. The initially observed area is centered at about 18◦450 N, 10◦300 E. The area containing dark tracks is at least 60 km x 100 km wide. ASTER frames (three contiguous in the same path) were acquired on May 26th 2001, at around 10:00 a.m. One of these frames (covering Adrar Madet) is showed in Figure 2. A slightly larger region affected by tracks is likely. The Tenere’ desert experienced great enviromental and geomorphological changes during Quaternary. Three main humid phases have been described in East Niger [1]: 40.000- 20.000 y B.P., 14.000-7500 y B. P. and 4500-3000 y B.P. The last period is linked to a widespread soil formation in large parts of present Tenere’. Dust devils are formed by vortices carrying fine-grained Figure 1: Adrar Madet massif: dark tracks are mainly located material. They have been observed on Mars during various at its Western side (Shuttle photo). martian missions [e.g. 2,3], with different details. The pro- cesses responsible for their formation is common also on Earth [4,5] and laboratory simulations have been carried out to study their behaviour [6]; theoretical calculations have been per- formed by Renno´ et al. [7,8]. Dark tracks could be formed either by removal of surficial dust [3] or deposition of dust itself. Altough the occurrence of dust devils on Earth is rather well documented, the study of tracks possibly associated with them could be important to understand their behaviour and significance in sediment transportation both on Mars and Earth. Description: The tracks are located in the western portion of Tenere’ desert. Adrar Madet is a 20 km long massif sur- rounded by recent sand sheets and dunes and Pleistocene linear dunes [1]. The entire massif constitutes a large scale obstacle structure, with eolian deposits distributed in a droplet shape around the Adrar. This evidence, together with the mean align- ment direction of linear dunes in the area is consistent with a predominant NE-SW wind direction. The surface is mainly constituted by sand sheets, with locally coarser surfaces [1]. The tracks appear as stripes with a lower albedo respect to the surrounding areas. The highest contrast between tracks and undisturbed surface is observed in smooth plain areas. The features are clearly visible in Band 1 (Green, 0.52- Figure 2: ASTER frame (RGB 321) containing dust devils 0.60 µm), lightly in Band 2 (Red) and appear almost invisible tracks. North is up. The region covered in Fig. 3. is outlined in Band 3 (Near Infrared). in red. The area apparently affected by this process is at least 60 km x 100 km wide. The desert surface nearby (several tens of km far) seems free of tracks. The tracks show a wide range of thicknesses and lenghts. The thickness is variable from few tens up to few hundreds Lunar and Planetary Science XXXIII (2002) 1307.pdf

DUST DEVILS TRACKS IN TENERE’: A. P. Rossi

dm) sand sheets as it appears from satellite images. Under these sand sheets a dark palaeosoil with a maximum thick- ness of one meter is present. These soil horizon appear to be widespread in Tenere’, East of Air Mountains. It have been attributed to the last humid phase in Niger. Sand sheets are mainly unconsolidated, not weathered and mobile [1]. The occurrence of these features over a large area close to Adrar Madet and the lack of them in the surroundings suggests a certan action of the masssif on the local or regional winds in the process of track forming. Air Mountains could also play a role. The tracks presented in this work appear strikingly similar to martian ones [e.g. 3] There are strong analogies with some Figure 3: Appearance of the trails, located on a smooth plain of the linear features observed in Hellas Basin by Ormo et al. (Green band, B1). [9] too. Averaging the three visible and near infrared bands a com- parison with MOC panchromatic imagery is possible, despite of meters. The lower thickness limit is probably lower, but the great difference in spatial resolution. The spatial scale the spatial resolution of available imagery is not enough to of these features is comparable with dark dust devils tracks appreciate it. The maximum observed lenght exceeds 10 km, on Mars. A direct analogy in their forming process could be but the average lenght is around 2-5 km. hypothesized. The orientation seems not linked with local wind direction Conclusions: Dust devils tracks-like features have been inferred from linear dune orientation. The tracks show pre- found in Niger Tenere’ desert. It appears that these low albedo ferred NW and NNW directions, but different orientations are tracks are caused by removal of a cm scale sand sheet underlain also observed. They often appear criss-crossing. It appears by darker palaeosoils [1]. The observed features could be a that dunes are not conditioning much the tracks: in many cases terrestrial analogue to martian ones. The study of these forms streaks cross dunes without interruption, as noticed on Mars on Earth might provide an insight on dust devils formation and [3]. morphogenetic importance on both planets. Possibly few active dust devils are visible in the images. The formation of dust devils tracks might be a common They appear as bright blurred areas in the track covered field. process on Earth, like dust devils themselves. The limiting They are sometimes located roughly at the tip of some tracks. factor for their detection is constituted by the particular condi- Devils themselves often appear not very distinct even on MOC tions needed for their recognition: the presence of a relatively images, with much higher spatial resolution. Nevertheless the thin loose deposit overlaining material with a different albedo. spatial resolution of the observed images is too low in order to These exceptional conditions are met in W Tenere’ desert, determine the nature of bright blurred spots. giving the possibility to study dust devils tracks in detail. SW of Adrar Madet clear wind streaks are also visible; Observation of multitemporal images is planned. It is also they are likely produced by Adrar Madet ostacle effect. planned an expedition to perform ground truthing and collect Discussion: The formation of dust devils is linked to par- information on surficial characteristics of the desert in that ticular atmospheric conditions. The formation of visible tracks area, to study grain size, mineralogy and surface features of is also related to peculiar surface conditions, such as the in- the eolian sediments in Adrar Madet area. ferred presence of unconsolidated deposits on a darker (or References: [1] Felix-Henningsen, P. (2000) Catena, 41, brighter) substrate [3]. 43-60. [2] Thomas, P. and Gierasch, P. J. (1985) Science, 230, Thus, even if geologically the preservation potential of 175-177. [3] Edgett, K. S., Malin, M. C. (2000) LPSC XXXI, such features should be extremely low, the occurrence of the 1073. [4] Metzger, S. M. and Lancaster, N. LPSC XXVII. [5] process responsible for track formation could be common dur- Metzger, S. M. (2001) LPSC XXXII, 2157. [6] Greeley, R. et ing the year, given proper boundary conditions. al. (2000) LPSC XXXI, 1088. [7] Renno,´ N. O. et al. (2000) The presented tracks could be interpreted as erosive. This JGR, 105, E1, 1859-1865. [8] Renno,´ N. O. et al. (1998) J. fact is supported by pedological studies [1]: wide areas in Atmos. Sci., 55 3244-3252. [9] Ormo, J. et al. (2001) LPSC Western Tenere’ are covered by large and thin (few cm to few XXXII, 1517.