Interpretation of Radar Data from the Icy Galilean Satellites and Triton

Details Interpretation of Radar Data from the Icy Galilean Satellites and Triton Interpretation of Radar Data from the Icy Galilean Satellites and Triton

Jan 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........12g&link_type=abstract

Thesis (PH.D.)--STANFORD UNIVERSITY, 1995.Source: Dissertation Abstracts International, Volume: 56-06, Section: B, page: 3353.

Physics

Optics

4

Voyager, Craters, Jupiter, Neptune

Scientific paper

We extend Eshleman's (Science 234, 1986, 587-590) analysis of an icy buried crater model and show that it can explain anomalous 3.5 and 13 cm-lambda radar echoes from the icy Galilean satellites- -radar albedos sigma~ 0.7 -2.6, circular and linear polarization ratios mu C~1.5 and mu L~0.5, and Doppler spectra with cosmTheta scattering law exponents m~1 -2. The model hypothesizes that radio waves are totally internally reflected N times from the walls of buried craters --tens of meters in radii with a water-ice overburden of permittivity varepsilon_1~3.2 varepsilon_0 that is larger than the permittivity varepsilon_2 of the material (probably porous ice) below the crater walls--and are brought to a focus, appearing to come from annular "glory halos" inside the craters, which break up into several coherent glints, each of azimuthal extent H, filling the halo to fraction F. We use geometrical and wave optics to include effects not accounted for by Eshleman, including: the ice overburden, arbitrary crater position, and crater shadowing. The values N = 3 and varepsilon_2/varepsilon _1 = 0.63 give mu_ {C} = 1.6, muL = 0.4, m = 1.9, and spectra that agree well with the general trends in the observations. With FH/ lambda = 10, the areal densities of buried craters on the three satellites required to fit the observed radar albedos are, 0.38, 0.21, and 0.10 for Europa, Ganymede, and Callisto. We determine that Triton's N_2 atmosphere's surface pressure is 1.4 +/- 0.1 Pa and "equivalent isothermal temperature" is 42 +/- 4 K using least squares inversion of the 3.6 and 13 cm-lambda Voyager 2 radio occultation phase data with an exponential model of the atmospheric contribution to the phase (1.7 rad at 3.6 cm-lambda in lower 60 km) and a polynomial model of the nonlinear phase drift (1 rad per 100 km altitude) of the Voyager ultrastable oscillator (USO). Assuming vapor pressure equilibrium between the N_2 gas and ice, the surface temperature is 37.5 +/- 0.5 K, which, together with the Voyager infrared measurements, determines the surface emmisivity is about 0.5. USO noise fluctuations (correlation scale _sp{ ~}> 10 km, alpha ~0.1 rad) do not admit fine resolution temperature and pressure profiles. Atmospheric models with 1.4 Pa surface pressure and various cold thermal profiles, including ones with a surface inversion layer and ones with decreasing temperature with height at either the dry or saturated lapse rates, may be consistent with the data.

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