The Density of Mid-sized Kuiper Belt Object 2002 UX25 and the Formation of the Dwarf Planets
The formation of the largest objects in the Kuiper belt, with measured densities of ~1.5 g cm^(–3) and higher, from the coagulation of small bodies, with measured densities below 1 g cm^(–3), is difficult to explain without invoking significant porosity in the smallest objects. If such porosity does occur, measured densities should begin to increase at the size at which significant porosity is no longer supported. Among the asteroids, this transition occurs for diameters larger than ~350 km. In the Kuiper belt, no density measurements have been made between ~350 km and ~850 km, the diameter range where porosities might first begin to drop. Objects in this range could provide key tests of the rock fraction of small Kuiper belt objects (KBOs). Here we report the orbital characterization, mass, and density determination of the 2002 UX25 system in the Kuiper belt. For this object, with a diameter of ~650 km, we find a density of 0.82 ± 0.11 g cm^(–3), making it the largest solid known object in the solar system with a measured density below that of pure water ice. We argue that the porosity of this object is unlikely to be above ~20%, suggesting a low rock fraction. If the currently measured densities of KBOs are a fair representation of the sample as a whole, creating ~1000 km and larger KBOs with rock mass fractions of 70% and higher from coagulation of small objects with rock fractions as low as those inferred from 2002 UX25 is difficult.
Additional Information© 2013 American Astronomical Society. Received 2013 July 10; accepted 2013 October 28; published 2013 November 13. This research has been supported by grant NNX09AB49G from the NASA Planetary Astronomy program. Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the NASA. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. Additional data were obtained from HST. Support for programs 10545 and 10860 were provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of the Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. We thank the anonymous referee for thought-provoking comments which substantially improved the presentation of this manuscript.
Published - 2041-8205_778_2_L34.pdf
Submitted - 1311.0553v1.pdf