Enceladus and the Icy Moons of Saturn (2016)
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PHOTOMETRY OF ICY MOONS OVER MANY WAVELENGTHS FROM CASSINI VIMS. B J. Buratti1,
R. H. Brown2, R. N. Clark3, K. H. Baines1, P. D. Nicholosn4, J. A. Mosher1. 1Jet Proplusion Laboratory, Callifornia
Institute of Technology, Pasadena, CA 91109, [email protected]; 2University of Arizona, Tucson, AZ;
3
Planetary Science Institute, Tucson, AZ; 4Cornell University, Ithaca, NY.
Introduction: The Visual Infrared Mapping Spectrometer (VIMS) instrument on the Cassini Spacecraft
possesses an unprecented range in wavelength, spanning nearly the entire solar spectrum. Investigation of
photometric properties as a function of wavelength
offers new insights on the physical properties of icy
moons, and it provides tests of various models. Coupled with a planetary tour lasting more than a decade,
this wide range of wavelengths has opened by a number of new areas of study. Among them are investigations of the directional scattering properties of icy
moons spanning a full range of viewing geometries;
accurate measurements of the bolometric Bond albedo,
and maps of that parameter; and more accurate charterizations of physical parameters such as roughness, particle size and regolith porosity based on wavelengths
for which the confounding factor of multiple scattering
is not important.
Phase curves: Ground-based observations of the
icy moons of Saturn are restricted to solar phase angles
less than 6°, and Voyager observations provided a
handful of visible measurements at larger solar phase
angles [1]. Cassini VIMS observations enabled the
measurement of phase curves over a full excursion of
densely spaced phase angles, which have been used to
compute bolometric Bond albedos [2]. Observations at
small solar phase angles over a full range of wavelengths exhibit intriguing behavior that shows a drastic
change at longer wavelengths. This change may be due
to decreased importance of small particles at larger
wavelengths or to the disappearance of multiple scattering. Phase angle measurements at very large angles
have been used to characterize the plumes of Enceladus
and to search for activity on Tethys and Dione by seeking forward scattered radiation [3, 4].
Macroscopic roughness: Another tool that can be
used to probe the physical nature of the surfaces of icy
moons is modeling of rough facets, which range from
clumps of particles to mountains and craters. Larger
wavelengths where the albedo of water ice is low are
particularly effective for understanding roughness because multiple scattering, which partly illuminates primary shadows cast by rough features, is minimal, and
the roughess models are more robust. Models fit to the
low-albedo hemipshere of Iapetus reveal the surface to
be much smoother than is typical for an icy moon, suggesting infilling of features by a dusty deposit [5]. An
analysis of the various terrains of Enceladus shows that
the plains north of the “tiger stripes” are fairly smooth,
and that the stripes themselves are only slightly more
rough, suggesting in-filling of craters with plume particles throughout the entire south polar region of Enceladus.
Maps of bolometric Bond albedo: The Bond albedo is the product of the geometric albedo times the
phase integral. The wide wavelength range of VIMS
offers the opportunity to directly measure the bolometric Bond albedo of icy moons, an important parameter for undertstanding the energy balance on a planetary surface and for doing thermal modeling.. With
both albedo maps and phase integrals over the entire
solar spectrum, a map of the bolometric Bond albedo is
possible. The figure below shows such a map for
Phoebe created from VIMS data.
References: [1] Buratti, B. J. and Veverka, J.
(1984). Icarus 58, 254-264. [2] Pitman, K. M., Buratti, B. J., and Mosher, J. A. (2010). Icarus 206, 537560. [3] Hedman, M. et al. (2013). Nature 500, 182184. [4] Buratti, B. J. et al. (2011). Icarus 214, 534540. [5] Lee, J. S., Buratti, B. J., Hicks, M. D.,
Mosher, J. A. (2010). Icarus 206, 623-630. [6] Buratti, B. J., Dalba, P. A., Hicks, M. D., Chang, J. (2014).
LPS XLV, Abstract # 1777.
Acknowledgments: This research was carried out
at the Jet Propulsion Laboratory, California Institute of
Technology under contract to the National Aeronautics
and Space Administration.