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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.04.019 Byline: Satoru Yamamoto (a), Olivier S. Barnouin-Jha (b), Takashi Toriumi (c), Seiji Sugita (d), Takafumi Matsui (e) Keywords: Impact processes; Cratering; Collisional physics Abstract: The present paper describes observations of crater growth up to the time of transient crater formation and presents a new empirical model for transient crater growth as a function of time. Polycarbonate projectiles were impacted vertically into soda-lime glass sphere targets using a single-stage light-gas gun. Using a new technique with a laser sheet illuminating the target [Barnouin-Jha, O.S., Yamamoto, S., Toriumi, T., Sugita, S., Matsui, T., 2007. Non-intrusive measurements of the crater growth. Icarus, 188, 506-521], we measured the temporal change in diameter of crater cavities (diameter growth). The rate of increase in diameter at early times follows a power law relation, but the data at later times (before the end of transient crater formation) deviates from the power law relation. In addition, the power law exponent at early times and the degree of deviation from a power law at later times depend on the target. In order to interpret these features, we proposed to modify Maxwell's Z-model under the assumption that the strength of the excavation flow field decreases exponentially with time. We also derived a diameter growth model as: d(t)a[1-exp(-[beta]t)].sup.[gamma], where d(t) is the apparent diameter of the crater cavity at time t after impact, and [beta] and [gamma] are constants. We demonstrated that the diameter growth model could represent well the experimental data for various targets with different target material properties, such as porosity or angle of repose. We also investigated the diameter growth for a dry sand target, which has been used to formulate previous scaling relations. The obtained results showed that the dry sand target has larger degree of deviation from a power law, indicating that the target material properties of the dry sand target have a significant effect on diameter growth, especially at later times. This may suggest that the previously reported scaling relations should be reexamined in order to account for the late-stage behavior with the effect of target material properties. Author Affiliation: (a) Center for Global Environmental Research, National Institute for Environmental Studies, Tsukuba 305-8506, Japan (b) The Johns Hopkins University, Applied Physics Laboratory, Laurel, MD 20723, USA (c) Institute of Physical Education, Keio University, Yokohama, 223-8521, Japan (d) Graduate School of Frontier Sciences, University of Tokyo, Chiba, 277-8561, Japan (e) Planetary Exploration Research Center, Chiba Institute of Technology, Chiba, 275-0016, Japan Article History: Received 6 October 2008; Revised 2 April 2009; Accepted 6 April 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2007.01.009 Byline: Olivier S. Barnouin-Jha (a)(b), Satoru Yamamoto (b), Takashi Toriumi (b), Seiji Sugita (b), Takafumi Matsui (b) Keywords: Impact processes; Cratering Abstract: An experimental technique to measure crater growth is presented whereby a high speed video captures profiles of a crater forming after impact obtained using a vertical laser sheet centered on the impact point. Unlike previous so called 'quarter-space experiments,' where projectiles were launched along a transparent Plexiglas sheet so that growth of half a crater could be viewed, the use of the laser sheet permits viewing changes in crater shape without any physical interference to the cratering process. This technique indicates that for low velocity impacts (45[degrees]) are also seen, and could be due to the low friction angle of the target. Significant crater modification, which is rarely seen in 'quarter-space experiments,' is also observed and appears to be controlled by the dynamic angle of repose of the target. These latter observations indicate that differences in target friction angles may need to be considered when determining near rim ejecta-mass distributions and large-scale crater modification processes on the planets. Author Affiliation: (a) The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723-6099, USA (b) Department of Complexity Sciences and Engineering, University of Tokyo, Kashiwa, Chiba 277-8561, Japan Article History: Received 17 January 2006; Revised 24 October 2006