Paper No. 4
Presentation Time: 9:45 AM


MITCHELL, Karl L., Jet Propulsion Laboratory, California Institute of Technology, Mail Stop 183-601, 4800 Oak Grove Dr, Pasadena, CA 91109, BARMATZ, Martin B., Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr, Pasadena, CA 91109, JAMIESON, Corey S., SETI Institute, Mountain View, CA 94043 and LORENZ, Ralph, Applied Physics Lab, Johns Hopkins University, Laurel, MD 20723,

In order to model quantitatively the response of Titan’s lakes and seas to Cassini’s RADAR instrument (Ku-band, 13.8 GHz), and thus determine surface reflectivity, attenuation and penetration depth, it is necessary to constrain the dielectric properties of their liquids. Pre-Cassini estimates of methane loss tangents ranged from ~1.6 to 1.7 (real component) and 10-5 to 10-3 (imaginary components). However, a more recent study (Paillou et al., 2008, Geophys. Res. Lett. 35, L05202, doi:10.1029/2007GL032515) used a free-space method to measure the Ku-band dielectric properties of Liquified Natural Gas (a mixture of methane, ethane and other dissolved components) as ε = 1.75 – 0.002i (loss tangent, tan δ = 1.14 x 10-3).

We present a new technique for measuring dielectric properties, used to measure the complex dielectric constant of methane and ethane. A cylindrical cavity containing a cylindrical quartz tube, filled with pure methane or ethane situated along the cavity axis, was excited in TM0n0 modes, where the n = 2 mode had a resonant frequency of ~14 GHz at 90K. Calibration required measurement of cavity resonant frequency and quality factor for the empty cavity, the inserted empty tube, and the tube filled with a hydrocarbon liquid. These quantities were determined by fitting the measured amplitude versus frequency curve with a Lorentzian line shape.

Three sets of measurements have been performed. Averaging these gives complex dielectric constants (ε’ + ε’’ i) of 1.60 + 0.000045i for methane (tan δ = 2.8 x 10-5 ± 26%) and 1.82 + 0.00021i for ethane (tan δ = 1.14 x 10-4 ± 6.0%). Results for ε’ are similar for the fundamental n = 1 mode at 6.4 GHz. Uncertainties in these constants include uncertainties in measured quality factors, Lorentzian line shape fits, cavity radius, and quartz tube dimensions and calculated dielectric constant: for methane, 5% (real) and 26% (imaginary) respectively, and for ethane, 4% and 5%.

A consequence is that Cassini RADAR should be able to probe further through Titan’s lakes and seas by a factor of ~40 (for liquid methane) and a factor of ~10 (for liquid ethane) relative to results of Paillou et al., which predict only a few meters. Future work will include mixtures of ethane and methane (which have differing densities but are miscible) and other dissolved components.