S. C. Lowry¹, P. R. Weissman², S. R. Duddy¹, B. Rozitis³, A. Fitzsimmons⁴, S. F. Green³, M. D. Hicks², C. Snodgrass⁵, S. D. Wolters³, S. R. Chesley², J. Pittichová² and P. van Oers⁶

¹Centre for Astrophysics and Planetary Science, School of Physical Sciences (SEPnet), The University of Kent, Canterbury, CT2 7NH, UK
²Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
³Planetary and Space Sciences, Department of Physical Sciences, The Open University, Milton Keynes, MK7 6AA, UK
⁴Astrophysics Research Centre, Queens University Belfast, Belfast, BT7 1NN, UK
⁵Max Planck Institute for Solar System Research, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
⁶Isaac Newton Group of Telescopes, 38700 Santa Cruz de la Palma, Canary Islands, Spain

Context. Near-Earth asteroid (25143) Itokawa was visited by the Hayabusa spacecraft in 2005, resulting in a highly detailed shape and surface topography model. This model has led to several predictions for the expected radiative torques on this asteroid, suggesting that its spin rate should be decelerating.
Aims. To detect changes in rotation rate that may be due to YORP-induced radiative torques, which in turn may be used to investigate the interior structure of the asteroid.
Methods. Through an observational survey spanning 2001 to 2013 we obtained rotational lightcurve data at various times over the last five close Earth-approaches of the asteroid. We applied a polyhedron-shape-modelling technique to assess the spin-state of the asteroid and its long term evolution. We also applied a detailed thermophysical analysis to the shape model determined from the Hayabusa spacecraft.
Results. We have successfully measured an acceleration in Itokawa’s spin rate of dω/dt = (3.54 ± 0.38) × 10^-8 rad day^-2, equivalent to a decrease of its rotation period of ~45 ms year^-1. From the thermophysical analysis we find that the centre-of-mass for Itokawa must be shifted by ~21 m along the long-axis of the asteroid to reconcile the observed YORP strength with theory.
Conclusions. This can be explained if Itokawa is composed of two separate bodies with very different bulk densities of 1750 ± 110 kg m^-3 and 2850 ± 500 kg m^-3, and was formed from the merger of two separate bodies, either in the aftermath of a catastrophic disruption of a larger differentiated body, or from the collapse of a binary system. We therefore demonstrate that an observational measurement of radiative torques, when combined with a detailed shape model, can provide insight into the interior structure of an asteroid. Futhermore, this is the first measurement of density inhomogeneity within an asteroidal body, that reveals significant internal structure variation. A specialised spacecraft is normally required for this.

Reference
Lowry SC, Weissman PR, Duddy SR, Rozitis B, Fitzsimmons A, Green SF, Hicks MD, Snodgrass C, Wolters SD, Chesley SR, Pittichová J and van Oers P (2014) The internal structure of asteroid (25143) Itokawa as revealed by detection of YORP spin-up.  Astronomy & Astrophysics 562:A48.
[doi:10.1051/0004-6361/201322175]
Reproduced with permission © ESO

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J.E. Chambers

Department of Terrestrial Magnetism, Carnegie Institution for Science, 5241 Broad Branch Road, NW Washington DC 20015

In the core accretion model for giant planet formation, a solid core forms by coagulation of dust grains in a protoplanetary disk and then accretes gas from the disk when the core reaches a critical mass. Both stages must be completed in a few million years before the disk gas disperses. The slowest stage of this process may be oligarchic growth in which a giant-planet core grows by sweeping up smaller, asteroid-size planetesimals. Here, we describe new numerical simulations of oligarchic growth using a particle-in-a-box model. The simulations include several processes that can effect oligarchic growth: (i) planetesimal fragmentation due to mutual collisions, (ii) the modified capture rate of planetesimals due to a core’s atmosphere, (iii) drag with the disk gas during encounters with the core (so-called “pebble accretion”), (iv) modification of particle velocities by turbulence and drift caused by gas drag, (v) the presence of a population of mm-to-m size “pebbles” that represent the transition point between disruptive collisions between larger particles, and mergers between dust grains, and (vi) radial drift of small objects due to gas drag. Collisions between planetesimals rapidly generate a population of pebbles. The rate at which a core sweeps up pebbles is controlled by pebble accretion dynamics. Metre-size pebbles lose energy during an encounter with a core due to drag, and settle towards the core, greatly increasing the capture probability during a single encounter. Millimetre-size pebbles are tightly coupled to the gas and most are swept past the core during an encounter rather than being captured. Accretion efficiency per encounter increases with pebble size in this size range. However, radial drift rates also increase with size, so metre-size objects encounter a core on many fewer occasions than mm-size pebbles before they drift out of a region. The net result is that core growth rates vary weakly with pebble size, with the optimal diameter being about 10 cm. The main effect of planetesimal size is to determine the rate of mutual collisions, fragment production and the formation of pebbles. 1-km-diameter planetesimals collide frequently and have low impact strengths, leading to a large surface density of pebbles and rapid core growth via pebble accretion. 100-km-diameter planetesimals produce fewer pebbles, and pebble accretion plays a minor role in this case. The strength of turbulence in the gas determines the scale height of pebbles in the disk, which affects the rate at which they are accreted. For an initial solid surface density of 12 g/cm² at 5 AU, with10-cm diameter pebbles and a disk viscosity parameter α=10^-4, a 10-Earth mass core can form in 3 My for 1–10 km diameter planetesimals. The growth of such a core requires longer than 3 My if planetesimals are 100 km in diameter.

Reference
Chambers JE (in press) Giant Planet Formation with Pebble Accretion. Icarus
[doi:10.1016/j.icarus.2014.01.036]
Copyright Elsevier

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