CRATERING EFFICIENCY ON AN ASTEROID
SURFACE COVERED WITH A BLOCK LAYER.
E. Tatsumi1 and S. Sugita1. 1Graduate School of Frontier Science,
Univ. of Tokyo. E-mail: tatsumi@astrobio.k.u-tokyo.ac.jp.
Introduction: Recent detailed geochemical analyses of returned Itokawa sample have yielded important age esimations.
For example, the cosmic ray exposure (CRE) age of the particles
from Itokawa are less than 10 Myr [3,4], and Ar40/Ar39 age is reported as 1.26±0.24 Ga [5]. The geologic meaning of these radioisotope ages, however, is not fully understood; their links tospecific geologic events on Itokawa and its parent body have notbeen established yet. One of the obstacles against such geologic
interpretation of these radiometric ages is the great ambiguity inthe resurfacing processes. More specifically, crater retention age
has been estimated to be 75 Myr - 1 Gyr based on strength scaling [2]. This large uncertainty in surface age estimation comesfrom that in crater size scaling. Furthermore, this uncertainty
could be underestimated because cratering on rubble piles, such
as Itokawa may not be controlled by target strength.
Accurate understanding of radiometric age estimates would
require more accurate crater size scaling on rubble-pile structureswith possible mechanical stratification. Although crater experiments on uniformly coarse-grained targets have been conducted[6, 7], Itokawa may have a boulder-rich layer superimposed on afiner regolith-rich substrate [1]. Such structure may also beformed via so-called Brazilian-nuts effect. In this study, we investigate the crater scaling on such layered structures.
Experiments: Impacts of cm-size bodies at the mean impact
velocity (~5 km/s) in the asteroid main belt, meter-sized boulders
on asteroid surfaces can be easily disrupted. To reproduce thisdisruption at low velocities (<200 m/s) in a laboratory, we used
weakly sintered glass beads blocks (8 - 15 mm) as boulder simulants. We also used 200 µm glass beads as regolith simulant.
Three types of targets were used: (1) a 20-mm-thick block layer
on a regolith substrate, (2) a 40-mm-thick block layer on a regolith substrate, and (3) a uniform regolith layer. Polycarbonate
projectiles 10 mm in diameter were launched at ~70 - 190 m/s.
Result & Discussion: For low impact energies, crater sizesof targets 1 and 2 are smaller by ~20 - 40% than target 3 (gravityscaling), but still much larger than the strength scaling for unfractured targets of sintered glass beads. When impact energy becomes large enough for projectiles to penetrate through the top
block layer, crater sizes approach the gravity scaling quickly.
Strength crater scaling rules were used for the estimation ofItokawa surface age by [2].Our experimental results, however,indicate that the crater scaling on coarse-grain targets is rather
close to the gravity scaling than the strength scaling. This result
is rather consistent with a prediction based on impact experiments with much large impact energy on uniformly coarsegrained targets [7]. Then, the crater retention age of Itokawa
would be estimated to be <10 Myr. This age is comparable toCRE age and much shorter than the Ar40/Ar39 age (~1 Gy) [5].
This suggests the CRE age may correspond to the Itokawa resurfacing age and that Ar40/Ar39 age may correspond to a much older
event: a catastrophic impact event on Itokawa’s parent body [5].
References: [1] Hirata N. et al. 2009. Icarus 200: 486-502.
[2] Michel P. 2009. Icarus 200: 503-513. [3] Nagao K. et al.
2011. Science 333: 1128-1131 [4]Meier M. M. M. et al. 2014.
#1247, 45th LPSC. [5] Park J. et al. 2014. #5190 77th Ann. Met.
Soc. Mtg. [6]Güttler et al. 2012. Icarus 220: 1040-1049. [7]Holsapple K. A. and Housen K. R. 2014. #2538 45th LPSC.