Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p11a1251s&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P11A-1251
Physics
5420 Impact Phenomena, Cratering (6022, 8136), 6225 Mars
Scientific paper
Martian impact ejecta are famous for their morphologies suggesting ejecta would be formed by radial ground- hugging flows in the late stage of the impact. The atmosphere [Schultz, 1992] and/or the subsurface volatiles [Carr et al., 1977] have been suggested as causes of fluidization. Examining the process to generate and emplace the radial flow would allow us to understand the nature of the entrained fluid. Double Layered Ejecta (DLE), one of the major subclasses of martian ejecta, have many unique features. Most enigmatic is the presence of two distinct layers of ejecta: a thick inner lobe and a thin outer lobe. The striking differences between the two lobes suggest that two different processes occur independently during ejecta emplacement, a case implausible by a single ballistic trajectory. In this study, based on the hypothesis that an impact- induced vortex ring modifies surface materials in the late stage of the impact to produce the outer lobe, the volumes of displaced particles by the vortex ring were measured in laboratory experiments to compare the volumes of the outer lobes. We utilized the experimental situation of a vortex ring impacting on a particle layer. Two dimensionless numbers based on particle size (Shields' and Reynolds number) in lab fall within the same ranges as those on Mars [Suzuki et al., 2007], it is thus possible to compare the relationships between volumes and Γ, a parameter expressing the strength of a vortex ring. We fit the dependence of Vdisplaced on Gamma using a power law Vdisplaced = a Γb and we found b = 1.25 ± 0.17. The volumes of the outer lobes were measured, selecting 7 fresh craters larger than 5km in diameter in the survey area (0N-60N, 90E-150E). In the case of impact cratering, the vortex strength Γ can be scaled with the crater diameter [Barnouin-Jha and Schultz, 1998] as Γ ∝ D⅔. Using also a power law Vouter = c Γd, we obtained d = 1.42 ± 0.24 for the volume of outer lobes of DLE. As the power indices of Γ fall in the same range in lab and on Mars, we conclude that atmospheric vortex ring is a plausible phenomenon explaining the outer lobe of the DLE and we propose a scenario for the formation of these craters.
Baratoux David
Kurita Kazuyoshi
Suzuki Akihiro
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