• Experimental study on the collisional disruption of porous gypsum spheres

      Okamoto, C.; Arakawa, M. (The Meteoritical Society, 2009-01-01)
      In order to study the catastrophic disruption of porous bodies such as asteroids and planetesimals, we conducted several impact experiments using porous gypsum spheres (porosity: 50%). We investigated the fragment mass and velocity of disrupted gypsum spheres over a wide range of specific energies from 3 x 10^3 J/kg to 5 x 10^4 J/kg. We compared the largest fragment mass (ml/Mt) and the antipodal velocity (Va) of gypsum with those of non-porous materials such as basalt and ice. The results showed that the impact strength of gypsum was notably higher than that of the non-porous bodies; however, the fragment velocity of gypsum was slower than that of the non-porous bodies. This was because the micro-pores dispersed in the gypsum spheres caused a rapid attenuation of shock pressure in them. From these results, we expect that the collisional disruption of porous bodies could be significantly different from that of non-porous bodies.
    • Simultaneous spectroscopic and photometric observations of binary asteroids

      Polishook, D.; Brosch, N.; Prialnik, D.; Kaspi, S. (The Meteoritical Society, 2009-01-01)
      We present results of visible wavelengths spectroscopic measurements (0.45 to 0.72 microns) of two binary asteroids, obtained with the 1-m telescope at the Wise Observatory on January 2008. The asteroids 90 Antiope and 1509 Esclangona were observed to search for spectroscopic variations correlated with their rotation while presenting different regions of their surface to the viewer. Simultaneous photometric observations were performed with the Wise Observatorys 0.46 m telescope, to investigate the rotational phase behavior and possible eclipse events. 90 Antiope displayed an eclipse event during our observations. We could not measure any slope change of the spectroscopic albedo within the error range of 3%, except for a steady decrease in the total light flux while the eclipse took place. We conclude that the surface compositions of the two components do not differ dramatically, implying a common origin and history. 1509 Esclangona did not show an eclipse, but rather a unique lightcurve with three peaks and a wide and flat minimum, repeating with a period of 3.2524 hours. Careful measurements of the spectral albedo slopes reveal a color variation of 7 to 10 percent on the surface of 1509 Esclangona, which correlates with a specific region in the photometric lightcurve. This result suggests that the different features on the lightcurve are at least partially produced by color variations and could perhaps be explained by the existence of an exposed fresh surface on 1509 Esclangona.
    • The formation of the Baptistina family by catastrophic disruption: Porous versus non-porous parent body

      Jutzi, M.; Michel, P.; Benz, W.; Richardson, D. C. (The Meteoritical Society, 2009-01-01)
      In this paper, we present numerical simulations aimed at reproducing the Baptistina family based on its properties estimated by observations. A previous study by Bottke et al. (2007) indicated that this family is probably at the origin of the K/T impactor, is linked to the CM meteorites and was produced by the disruption of a parent body 170 km in size due to the head-on impact of a projectile 60 km in size at 3 km s^(-1). This estimate was based on simulations of fragmentation of non-porous materials, while the family was assumed to be of degrees C taxonomic type, which is generally interpreted as being formed from a porous body. Using both a model of fragmentation of non-porous materials, and a model that we developed recently for porous ones, we performed numerical simulations of disruptions aimed at reproducing this family and at analyzing the differences in the outcome between those two models. Our results show that a reasonable match to the estimated size distribution of the real family is produced from the disruption of a porous parent body by the head-on impact of a projectile 54 km in size at 3 km s^(-1). Thus, our simulations with a model consistent with the assumed dark type of the family requires a smaller projectile than previously estimated, but the difference remains small enough to not affect the proposed scenario of this family history. We then find that the break-up of a porous body leads to different outcomes than the disruption of a non-porous one. The real properties of the Baptistina family still contain large uncertainties, and it remains possible that its formation did not involve the proposed impact conditions. However, the simulations presented here already show some range of outcomes and once the real properties are better constrained, it will be easy to check whether one of them provides a good match.
    • The impact origin of Eunomia and Themis families

      Leliwa-Kopystyński, J.; Burchell, M. J.; Włodarczyk, I. (The Meteoritical Society, 2009-01-01)
      Criteria for finding asteroid families (Zappala et al. 1995) are applied to a large (205,770 member) data set of asteroid orbital elements. The cases of the Eunomia and Themis families are considered as examples. This is combined with the cratering criteria for catastrophic disruption of small bodies in the solar system (Leliwa-Kopystyński et al. 2008). We find that the Eunomia parent body itself was not catastrophically disrupted in the family-generating impact event; after impact, the current body contains as much as 70% of its primordial mass. However, by contrast with Eunomia, the present mass of 24 Themis is only about 21% of that of its primordial body. Limits are placed on the sizes of the impactors in both examples, and for the case of Eunomia, the radius of the just sub-critical crater (which may be present on 15 Eunomia) is predicted as 58 km.