Mathematics – Logic
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.p31a0121s&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #P31A-0121
Mathematics
Logic
5420 Impact Phenomena, Cratering (6022, 8136), 6225 Mars
Scientific paper
Recent observations of Martian ice-related features and theoretical work on the dynamical history of Mars have illuminated how obliquity variations led to the periodic deposition of ice-rich layers, ranging from 10 meters to 1 kilometer thick, on the surface. Because icy layers are thought to be globally distributed and to have formed throughout Martian history, the impact crater record provides a powerful tool to investigate the properties and evolution of these layers. To do so, however, we must first understand how the presence of such layers affects the impact cratering process and the final crater morphology. Furthermore, icy layers have been proposed as a contributing factor to the layered ejecta structures that are found around the majority of Martian impact craters. However, little work has been done on impacts into targets with icy layers. We present the results from a numerical study of impacts into icy layered terrain using the CTH shock physics code with a new material strength and damage model for geologic materials and acoustic fluidization. We find that the layered terrain structure significantly affects the final crater and ejecta blanket morphologies. Impact into a homogeneous basalt target produces the traditional inverted cone-shaped ejecta curtain. Impact into a thin icy layer over a basaltic crust produces a wavy ejecta curtain containing a combination of ballistic and non-ballistic motion. In particular, at the end of crater excavation, the basalt ejecta pushes the icy ejecta, forming a ground-hugging flow near the rim. If the final crater is complex, some of the icy layer flows into the crater during crater collapse. Both these processes act to thin the ejecta just outside the rim and thicken the surrounding ejecta blanket. If the thin icy layer is buried beneath a stronger surface layer, the ejecta motion is even more complicated, but also contains a combination of ballistic and non-ballistic motion. In this case, the upper strong layer pulls away from the underlying icy layer, potentially (depending on the crater size and the thickness of the layers) creating a terraced crater wall. In addition, most of the excavated material is ejected at very high angles, resulting in more material deposition closer to the crater rim. For both simple and complex craters, some of the icy layer is squeezed by the depositing ejecta near the rim and extruded from the crater wall, forming an ice-rich floor deposit. Wave reflections within the surface layer increase the level of brecciation and produce finer sized ejecta compared to a homogeneous surface. The volume of ejected and uplifted material also varies significantly depending on the layering structure. Crater formation in icy layered terrain may contribute to several observed features around Martian craters, including circum-rim moats, thick inner and thin outer ejecta layers, variation in number of secondary craters, and the large volumes of uplifted and ejected material.
Senft Laurel E.
Stewart Sarah T.
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