S. Mazrouei^a, M.G. Daly^a, O.S. Barnouin^b, C.M. Ernst^b, I. DeSouza^a

^aThe Centre for Research in Earth and Space Science, York University, Toronto, Ont., Canada, M3J 1P3
^bThe Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723-6099, USA

Asteroid 25143 Itokawa is a small elongated asteroid with two distinct parts. The evolution of this two-part body has been the source of speculation. The scenarios for the formation of this asteroid include: two-body capture, catastrophic disruption and rapid reaccretion, YORP spin-up and mass shedding, and disruption (or partial disruption) with two-body reaccretion. In this paper we use the global and regional analyses of block populations and size-frequency distributions as evidence of the probable evolutionary history of Itokawa. The block sample used in this study is believed to be complete for blocks of size >6m and consists of a sample more than twice as large as previous known studies.
Although block size frequency distributions hint at different evolutionary paths for the head and the body, their differences are not statistically significant. The distribution of blocks across each body provides clues as to the histories of each body. The head is populated in a spherically symmetric fashion while the body has a distinct equatorial peak. When considering that the head and the body may have been separate entities for a period of time and estimating a rotational axis using minimum rotational energy considerations, the preferential equatorial distribution becomes even more pronounced. We interpret this as excellent evidence for the partial disruption of a proto-Itokawa, subsequent planarization of a debris field and reaccretion of the head and the body into its present configuration.

Reference
Mazrouei S, Daly MG, Barnouin OS, Ernst CM and DeSouza I (in press) Block Distributions on Itokawa. Icarus
[doi:10.1016/j.icarus.2013.11.010]
Copyright Elsevier

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Kenji Furuya¹, Yuri Aikawa¹, Hideko Nomura^2,3,4, Franck Hersant^5,6 and Valentine Wakelam^5,6

¹Department of Earth and Planetary Sciences, Kobe University, Kobe 657-8501, Japan
²Department of Astronomy, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
³National Astronomical Observatory of Japan, Osawa, Mitaka, Tokyo 181-8588, Japan
⁴Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo 152-8551, Japan
⁵University of Bordeaux, LAB, UMR 5804, F-33270 Floirac, France
⁶Centre National de la Recherche Scientifique, LAB, UMR 5804, F-33270 Floirac, France

We investigate water and deuterated water chemistry in turbulent protoplanetary disks. Chemical rate equations are solved with the diffusion term, mimicking turbulent mixing in a vertical direction. Water near the midplane is transported to the disk atmosphere by turbulence and is destroyed by photoreactions to produce atomic oxygen, while the atomic oxygen is transported to the midplane and reforms water and/or other molecules. We find that this cycle significantly decreases column densities of water ice at r  30 AU, where dust temperatures are too high to reform water ice effectively. The radial extent of such region depends on the desorption energy of atomic hydrogen. Our model indicates that water ice could be deficient even outside the sublimation radius. Outside this radius, the cycle decreases the deuterium-to-hydrogen (D/H) ratio of water ice from ~2 × 10^-2, which is set by the collapsing core model, to 10^-4–10^-2 in 10⁶ yr, without significantly decreasing the water ice column density. The resultant D/H ratios depend on the strength of mixing and the radial distance from the central star. Our finding suggests that the D/H ratio of cometary water (~10^-4) could be established (i.e., cometary water could be formed) in the solar nebula, even if the D/H ratio of water ice delivered to the disk was very high (~10^-2).

Reference
Furuya K, Aikawa Y, Nomura H, Hersant F and Wakelam V (2013) Water in Protoplanetary Disks: Deuteration and Turbulent Mixing. The Astrophysical Journal 779:11.
[doi:10.1088/0004-637X/779/1/11]

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