Mathematics – Logic
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..640s&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Mathematics
Logic
Scientific paper
Introduction: Valles Marineris is known as a place of numerous and well preserved landslides on Mars. In comparison with terrestrial landslides, martian landslides are distinctive in their size and morphology. As a consequence of the topography of the canyon, the averaged drop height of these landslides is about 6.5 km and the averaged volume is about 102~4 km3[1], which is 2~3 orders of magnitude larger than terrestrial ones, at the exception of marine landslides[2]. As for the morphology, clear levees with longitudinal lineations are typical features of martian landslides, whereas surfaces of the terrestrial mass movements are dominated by a rather chaotic topography with, in some cases, the occurrence of transverse ridges. The characteristics of the deposits should reflect the dynamics of the emplacement and the subsurface material properties. In particular, there is a longstanding debate about the relation between the long run-out length and the existence of subsurface volatiles (water ice, clathrates, ground water) [1,3,4,5,6,7]. The motivation of our research is the fact that material properties are expected to be deduced from the morphology of the deposits and the knowledge of the flow dynamics. Then, the characteristics of subsurface materials partially collapsed as mass movements could be documented as a function of time, considering the age of each landslide. In this study, we focus on the longitudinal grooves which are found on the surface of landslide deposits at Valles Marineris (Fig.1). This pattern is a typical feature in the martian landslides[3], and extremely rarely observed in the terrestrial mass movements. The origin is not well clarified, but it seems strong relation with the flow style or physical property of transported materials. With the objective to determine the condition of formation of the lineations, the geometric characteristics (volume, surface, thickness, run-out length) of lineated and non-lineated landslides are compared. Then, the difference in flow dynamics are discussed based on the physics of granular flow. Topographic measurements are derived from HRSC and MOLA data. HRSC, MOC, THEMIS-VIS are used for the morphological observations. Result and Discussions: Not all the landslides have longitudinal lineation as shown in Fig.1. Almost half numbers of landslides observed in Valles Marineris show irregularly shaped knobby surface as shown in Fig.2. We classified all the landslides in this area as lineated type and knobby type, including a few exceptions. We compare two types of landslide by measured morphological parameters, such as volume, averaged thickness, area, maximum run-out (Lmax), and maximum drop height (Hmax). When we compare by volume and Hmax/Lmax plot, where Hmax/Lmax corresponds to the apparent coefficient of friction[2], a clear difference is recognized. Landslides of the lineated type show a negative correlation of Hmax/Lmax with volume, a similar trend to terrestrial dry landslides. On the other hand, the plots of knobby type concentrate in a small area at larger volumes without any clear correlation of two parameters. From the comparison at the plot of averaged thickness and total volume of deposit, lineated type show systematically thinner geometry than knobby type, bounded around 200 to 250 m. From the comparison of averaged thickness and root square of deposit area (Fig.5), a roughly linear correlation is reported for the lineated type. The square root of deposit area means the index length of horizontal shape of deposit. Usually the deposit of landslide at Valles Marineris spread out at flat broad valley floor showing semi-radialy spreaded shape. Thus it can be invoked as index length independent with actual horizontal deposit shape. The linear correlation of thickness and square of the deposit area for the lineated type implies that deposits shapes are homothetic, and keep similarity. This observation is striking given the large range of volumes reported. In contrast, a constant square of deposit area for thicknesses ranging from 100 to 600 meters is reported. The different geometric characteristics of the two deposit types evidenced in these three plot diagrams are summarized as follows; Lineated type landslide have thinner deposit and similarity in final deposit shape; Hmax/Lmax shows similar negative trend as the terrestrial dry landslides. Knobby type landslide has thick deposit with almost constant deposit area. Hmax/Lmax does not show any clear correlation with volume. From these morphological characteristics, the flow style of lineated type is thought to be much more developed in order to reach the similarity shape at any deposit volumes, with probably enough fragmentation during the collapse. On the other hand, knobby type is thought to correspond to un-developed flow without enough spreading. As a consequence, large volume does not produce large areas for the deposit, but rather correspond to ticker morphologies. References [1] Quantin C. P. et al. (2004) Planetary and Space Sci., 52, 1011-1022. [2] Legros F. (2002) Engineering Geology, 63, 301- 331. [3] Luccitta B. K. (1979) JGR, 84, 8097-8113. [4] Harrison K. P. and Grimm R. E. (2003) Icarus, 163, 347-362. [5] Bulmer M.H. and Zimmerman B.A. (2005) GRL, 32, doi:10.1029/2004GL022021. [6] Soukhovitskaya V. and Manga M. (2006) Icarus, 180, 348-352. [7] Lucas A. and Mangeney A. (2007) GRL, 34, doi:10.1029/2007GL029835.
Baratoux David
Kurita Kazuyoshi
Pinet Patrick
Sato Haruo
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