Enceladus: Implications of its unusual photometric properties

Abstract We have investigated the photometric properties of Enceladus up to phase angles of 43°, using Voyager 1 and 2 images. Among the satellite's unusual photometric characteristics are its very high geometric albedo (1.0 ± 0.1 near 0.47 μ), which extends down to at least 0.34 μ, and the uniformity of albedos, colors, and scattering properties over the geologically varied surface. Although some albedo variations of up to 15% occur in low resolution Voyager 1 images, the albedos of the four major geological units imaged in the Voyager 2 near-encounter sequence differ by 1–2% or less, even though the ages of these units probably differ by at least a factor of 10 (3.8 billion years to a few hundred million years). The lack of correlation of spectrophotometric properties with terrain type suggests that the optical characteristics of Enceladus are determined by a recently deposited ubiquitous surface layer, possibly originating in Saturn's E-ring. The high geometric albedo implies that the surface layer is remarkably free of opaque material and is much more backscattering than is common for natural or laboratory frost layers on Earth. The unique phase and photometric functions of Enceladus can be explained by the high degree of multiple scattering in its surface. The existing observations suggest that the textural characteristics of Enceladus' regolith are similar to those of other icy Saturnian satellites.

[1]  Roger N. Clark,et al.  Water frost and ice - The near-infrared spectral reflectance 0.65-2.5 microns. [observed on natural satellites and other solar system objects , 1981 .

[2]  J. Veverka,et al.  Photometry of rough planetary surfaces: The role of multiple scattering , 1985 .

[3]  W. Hartmann.  Surface evolution of two-component stone/ice bodies in the Jupiter region , 1980 .

[4]  L. Soderblom,et al.  Global multispectral mosaics of the icy Galilean satellites , 1983 .

[5]  B. Buratti.  Application of a radiative transfer model to bright icy satellites , 1984 .

[6]  S. Squyres,et al.  Color photometry of surface features on Ganymede and Callisto , 1981 .

[7]  D. Cruikshank.  Physical properties of the satellites of Saturn , 1978 .

[8]  C. Porco,et al.  Saturn's rings - Structure, dynamics, and particle properties , 1984 .

[9]  J. Spencer.  Icy Galilean satellite reflectance spectra: Less ice on Ganymede and Callisto? , 1987 .

[10]  J. Veverka,et al.  Voyager photometry of Rhea, Dione, Tethys, Enceladus and Mimas , 1984 .

[11]  J. Burns,et al.  Six-color photometry of Iapetus, Titan, Rhea, Dione and Tethys , 1974 .

[12]  M. Minnaert.  Photometry of the Moon , 1961 .

[13]  William M. Irvine,et al.  The shadowing effect in diffuse reflection , 1966 .

[14]  B. Hapke.  Bidirectional reflectance spectroscopy: 1. Theory , 1981 .

[15]  B. Hapke.  Bidirectional reflectance spectroscopy , 1984 .

[16]  J. Lissauer,et al.  Ring torque on Janus and the melting of Enceladus , 1984 .

[17]  O. Franz,et al.  Photometry of Dione, Tethys, and Enceladus on the UBV system , 1975 .

[18]  M. Minnaert.  The reciprocity principle in lunar photometry , 1941 .

[19]  D. Cruikshank.  The surfaces and interiors of Saturn's satellites , 1979 .

[20]  S. Squyres,et al.  The evolution of Enceladus , 1983 .

[21]  Bruce Hapke,et al.  Interpretations of optical observations of Mercury and the moon , 1977 .

[22]  G. Danielson,et al.  Saturn's E ring: I. CCD observations of March 1980 , 1981 .

[23]  E. Grün,et al.  Saturn's E, G, and F rings - Modulated by the plasma sheet? , 1983 .

[24]  R. L. Millis,et al.  UBV photometry of the Galilean satellites , 1975 .

[25]  L. Soderblom,et al.  Radiometric performance of the Voyager cameras , 1981 .

[26]  B. Buratti.  Voyager disk resolved photometry of the Saturnian satellites , 1984 .

[27]  Kari Lumme,et al.  Radiative transfer in the surfaces of atmosphereless bodies. I. Theory. , 1981 .

[28]  J. Boyce,et al.  Crater numbers and geological histories of Iapetus, Enceladus, Tethys and Hyperion , 1983, Nature.

[29]  P. Oetking.  Photometric studies of diffusely reflecting surfaces with applications to the brightness of the Moon , 1966 .

[30]  J. Veverka.  The photometric properties of natural snow and of snow-covered planets , 1973 .

[31]  J. Goguen.  A Theoretical and Experimental Investigation of the Photometric Functions of Particulate Surfaces. , 1981 .

[32]  K. Pang,et al.  The E ring of Saturn and satellite Enceladus , 1984 .

[33]  J. L. Mitchell,et al.  A New Look at the Saturn System: The Voyager 2 Images , 1982, Science.