Statistics – Computation
Scientific paper
May 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998icar..133....1r&link_type=abstract
Icarus, Volume 133, Issue 1, pp. 1-24.
Statistics
Computation
38
Scientific paper
In this paper, we study the effect of target size on the fragmentation outcome of rock targets using a 2D numerical hydrocode. After comparing our hydrocode calculations to laboratory data (including explosive disruption experiments) to validate the results, we use the code to calculate how the critical specific energy (Q*) needed to catastrophically fracture a body varies with target size in the regimes not accessible to experiment. Impact velocity is generally kept constant at about 2.0 km s^-1, although some higher velocity (~5 km s^-1) simulations were run to determine a velocity dependence for the fragmentation outcome. To reflect the asteroid population, target diameters range from 10 cm to 1000 km, spanning the regimes where strength and self-gravity (radially varying lithostatic stress) each dominate resistance to fragmentation. We find that there is a significant difference in fragmentation outcome when the lithostatic stress is included in the computations. As expected, surface layers fragment more easily, while the strength of the central regions is greatly enhanced. We derive the Q* versus size relationship for three materials, (basalt, strong-, and weak-cement mortar) each having different static compressive strengths and representing a range of asteroid materials. The hydrocode results showed that Q* decreased with increasing target size in the strength regime, with slopes of 0.43, 0.59, and 0.6 for basalt, strong and weak mortar, respectively. This decrease is directly related to the decrease in strain rate as target size grows. In the gravity regime, Q* increases with increasing target size, with a slope equal to 2.6 for all three of the materials modeled. These values are much steeper than those previously derived from scaling theories. Ejecta velocity distributions as a function of target size are examined as well. For large bodies, resultant ejecta speeds tend to be well below escape velocity, implying that these asteroids are likely to be reaccumulated rubble piles. In simulating the creation of the asteroid family Eos, we find that the code-calculated fragment size distribution is similar in character to the observed data, but secondary fragment sizes are significantly underestimated. More importantly, the determined ejecta speeds were too low for these fragments to have achieved escape velocity, and thus we fail to actually form the separate bodies comprising the Eos family, and are left instead with a single rubble pile conglomerate.
Melosh Henry Jay
Ryan Eileen V.
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