P11C-2108
Explanation of Europa’s Unusual Polarization Properties: The Regolith is Sub-micron, Fine-Grained, High Porosity Material

Monday, 14 December 2015
Poster Hall (Moscone South)
Robert M. Nelson1, Mark D Boryta2, Bruce W Hapke3, Kenneth S. Manatt4, Adaeze Nebedum2, Desiree Olivia Kroner5, Yuriy Shkuratov6, Vladimir Psarev6, Kurt Vanderoort7 and William D Smythe8, (1)Planetary Science Institute Tucson, Tucson, AZ, United States, (2)Mt San Antonio College, Walnut, CA, United States, (3)University of Pittsburgh, Pittsburgh, PA, United States, (4)Jet Propulsion Laboratory, Earth and Space Science Division, Pasadena, CA, United States, (5)University of California at Los Angeles, Department of Chemical Engineering, Los Angeles, CA, United States, (6)Kararzin University, Institute for Astronomy, Kharkiv, Ukraine, (7)California Polytechnic State University at Pomona, Department of Physics, Pomona, CA, United States, (8)NASA Jet Propulsion Laboratory, Pasadena, CA, United States
Abstract:
For several decades, unusual reflectance and polarization phase curves have been reported on Europa by experienced ground based astronomers (Rosenbush et al., 1997, 2015). The observed reflectance phase curve is consistent with the phase curves reported in the laboratory in fine grained particulate media (Nelson et al., 2000, 2002, Shkuratov et al., 2002). Shkuratov et al. (2002) also measured polarization properties of fine grained media showing that they relate to the coherent backscatter enhancement phenomenon and are consistent with the astronomical data.

We have reconfigured a goniometric photopolarimeter (GPP) (Nelson et al., 2000, 2002) to measure in the laboratory the polarization phase curves of highly reflective particulate materials that simulate the Europa’s predominately water ice regolith. We apply the Helmholtz Reciprocity Principle - we present our samples with linearly polarized light and measure the change in the intensity of the reflected component with phase angle from 0.05 to 15 degrees. This is physically equivalent to the astronomical polarization measurements. We report here the polarization phase curves for a suite of high albedo particulates of size 0.1<D< 30 µm, five of which are 1.5 µm or smaller in diameter. The depth and position of the polarization minimum, the crossover point, and the slope at the crossover point all correlate with particle size. We find no polarization effects for the particle sizes greater than 2 µm.

Our laboratory polarization measurements are remarkably consistent with the astronomical data. These observations can be explained by a European regolith that is extremely fine grained and with very high porosity, perhaps with void space exceeding 90%.

If a reflectance phase curve and a polarization phase curve of solar system object can be obtained (even at a very small range of phase angles), it will soon be possible to determine (or at least constrain) important regolith properties. Future missions to the Jovian system (particularly Europa) would derive great benefit from including polarization measurement capability.

 This work is supported by NASA’s Cassini Saturn Orbiter mission.

<< Previous Abstract | Next Abstract >>