Mathematics – Logic
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..515k&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
1 Introduction In the northern lowland regions of Mars such as Chryse Planitia and Utopia Planitia there exist several wel-preserved fresh-looking craters having distinct fluidized ejecta morphologies. They are mostly composed of two contrasting types of ejecta, One is massive and forms clear topography concentrated near the rim. The other is less massive and faintly spreading in the outer region. When they are clearly recognized, it is classified as Double Layered Ejecta crater(DLE) and they are named Inner Lobe and Outer Lobe,respectively. But sometimes Outer Lobe is faintly recognisable only in the albedo pattern or completely lacking. This suggests that the material forming the Outer Lobe is gas or liquid dominant. To understand the nature of the Outer Lobe material is a key in the exploration of the subsurface environments in the northern lowlands of Mars, where large amount of volatiles are supposed to be stored. Here we present several examples of images supporting this nature as well as experimental images simulating the formation process of Outer Lobe in the laboratory. 2 THEMIS IR Night images THEMIS-IR Night-time images have been used to characterize the nature of the ejecta materials (1,2,3). Because of the contrast of thermal inertia, night time IR image is a powerful tool to identify the morphology of the Outer Lobe even when the associated topography is so faint. Fig. 1 shows such example. The Inner Lobe is recognized as topographic high in the Day-time image. In the Night-time image, this part is characterized by bright color. The remarkable point is the splash pattern morphology. The edge of the Inner Lobe is round shaped while that of the Outer Lobe is sharp angular shaped and irregular. The Outer Lobe is composed of many slender flow units whose length seems quite heterogeneous. Sometimes the extent of the flow units exhibits anisotropic pattern while the Inner Lobe and the cavity are circular. This suggests the flow forming the Outer Lobe has much higher speed than that of the Inner Lobe. Fig.2 is another examples. The left image represents the topography (MOLA grid data) and the right image shows radiance at night time (THEMIS-IR Night-time mosaic). The Outer Lobe recognized in IR-image extends well further than that of MOLA. This mean the Outer Lobe doesn't have a significant amount of mass. The surface pattern of the Outer Lobe resembles the splashing morphology when a liquid drop collides with solid surface.Fig.3 shows three examples of craters of similar size in the same Night time- IR image. They are located in the same geological unit and under similar surface erosion. The topographical features of the rim look similar between them, which means the formation age isn't so different. But A has clear outer lobe and C is completely lacking.One probable interpretation of this difference is that the material forming the Outer Lobe disappears in relatively short time scale. 3 Laboratory experiments simulating the splash pattern To simulate formation of the splashing pattern, we have conducted simple experiments. A small droplet of laponite gel is fallen from a height of 10-60 cm on the solid plate. The impact velocity is about 1.4-3.4 m/sec. Splashing process is observed by high speed video camera. Several kinds of laponite gel having different water contents are prepared. Laponite gel is known to have a yield strength and have strongly non-linear rheology. Fig.4 and 5 show typical examples of the final shape. When a droplet collides with the plate a sheet-like flow is oberved to spread on the surface. This flow has a topographically high edge and finally it forms a rampart. This part seems to be stronger than the inner part probably because of preexisting heterogeneity. The final morphology and flowing behavior strongly depend on the surface condition of the plate, particularly surface roughness. When the water content of the gel is high, splashing pattern is observed. In the high speed video images, several small droplets are observed to be formed and jump out from the surface of the flowing part. The splashed parts are mostly water with less content of solid gel. The following formation process is suggested: when the droplet collides with the surface, the compression promotes selective segregation of liquid abundant part from the main body. When this part gets higher momentum it jumps out to form the splashing. At moment the laboratory experiments can not completely simulate the pattern. This is probably because the difference in the colliding velocity. But heterogeneity in fluid content is a key factor in the formation of splash pattern. This would be a significant implication for the formation of the Outer Lobe. References 1).Christensen et al.(2003),Science,300,2056-2061 2).Baratoux D., N.Mangold, P.Pinet, F.Costard (2005),J. Geophys.Res.,110,E04011,doi:10.1029/2004JE002314. 3).Mouginis-Mark, et al(2004), J. Geophys. Res., 109, E08006, doi:10.1029/2003JE002147.
Kawaguchi Cota
Kurita Kei
Suzuki Ayako
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