Elsevier

Icarus

Volume 176, Issue 1, July 2005, Pages 184-191
Icarus

Diverse albedos of small trans-neptunian objects

https://doi.org/10.1016/j.icarus.2005.01.007Get rights and content

Abstract

Discovery of trans-neptunian object (TNO) satellites and determination of their orbits has recently enabled estimation of the size and albedo of several small TNOs, extending the size range of objects having known size and albedo down into the sub-100 km range. In this paper we compute albedo and size estimates or limits for 20 TNOs, using a consistent method for all binary objects and a consistent method for all objects having reported thermal fluxes. As is true for larger TNOs, the small objects show a remarkable diversity of albedos. Although the sample is limited, there do not yet appear to be any trends relating albedo to other observable properties or to dynamical class, with the possible exception of inclination. The observed albedo diversity of TNOs has important implications for computing the size-frequency distribution, the mass, and other global properties of the Kuiper belt derived from observations of objects' apparent magnitudes and may also point the way toward an improved compositional taxonomy based on albedo in addition to color.

Introduction

Considerable progress has been made in understanding the dynamical structure of the trans-neptunian Solar System, but the physical characteristics of the small bodies that inhabit that region remain poorly determined. Measurements of colors, absolute magnitudes, and lightcurves have accumulated for a number of objects, and near infrared spectra have been reported for some of the brightest ones, but knowledge of fundamental properties such as size, mass, albedo, and density remain woefully sparse. Determining these properties for a representative sample of TNOs is crucial for estimating the total mass of material in the trans-neptunian region, for relating magnitude-frequency distributions to size- and mass-frequency distributions, for elucidating patterns of compositional taxonomy, for quantitative interpretation of infrared spectra, and for constraining the internal compositions of these objects. All of these objectives have important implications for physical and chemical conditions in the outer proto-planetary nebula and for the accretion of solid objects in the outer Solar System.

Data constraining TNO sizes, albedos, and colors are compiled in this paper in an effort to search for possible patterns related to TNO origins or subsequent surface processing. We use a consistent set of models to derive size and albedo constraints from thermal emission observations and from binary orbits reported by various authors, thereby expanding the range of diameters included in the sample to more than an order of magnitude.

Although they likely do derive from the TNO population, Centaurs (by which we mean non-resonant objects which cross the orbits of major planets) and comet nuclei were excluded from this study because we lack knowledge about what part of the trans-neptunian system they originated from. Comet nuclei also have systematically different colors from TNOs (e.g., Jewitt, 2002), inferred to result from volatile loss processes which may also affect Centaurs.

Section snippets

Size and albedo

Size and albedo are fundamental properties of bodies which constrain composition and internal structure. If systematic patterns were found to exist among these properties, they could reveal much about the accretion of TNOs in the proto-planetary nebula and about the subsequent processing of their surfaces. Different types of TNOs could have formed at different rates or in different source regions resulting in different bulk compositions. Objects in different types of orbits could experience

Data processing

By using a consistent set of thermal models to re-compute sizes and albedos from reported thermal and visual fluxes and a consistent set of densities to estimate sizes and albedos from system masses and visual photometry we attempted to provide as consistent as possible a basis for comparison. We also used photometry from the literature to compute the spectral slope parameter s, defined as the percent increase in reflectance per 100 nm of wavelength relative to the V central wavelength 550 nm,

Comparisons

Comparing R-band albedo with size d, spectral slope s, and inclination i (with respect to the invariable plane), no clear albedo trends are evident with size or with color, especially if the planet Pluto is excluded (Pluto's high albedo being attributed to surface–atmosphere interactions unlikely to affect the smaller TNOs). Higher albedos among smaller TNOs and among TNOs having higher inclinations and eccentricities could result from higher collisional erosion rates on such objects, if more

Conclusion

Statistical study of TNO albedos is limited by the small number of measurements, but useful hints are beginning to emerge. Contrary to expectation, convincing evidence is not seen for dependence of albedo on object size for diameters spanning an order of magnitude. Similarly, no clear correlation of color with albedo is observed. A wide range of albedos is evident among Scattered, Resonant, and Classical objects, as well as among both small and large, and gray and red objects. Models of

Acknowledgments

This work was made possible by NASA/ESA Hubble Space Telescope programs #9386, #9585, and #9991, support for which was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Two anonymous referees contributed useful input. We are also grateful to M.W. Buie and E.I. Chiang for help with dynamical classification, and to the free and open source software

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    1

    Now at Johns Hopkins University, Baltimore, MD 21218, USA.

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