NEW CRYOGENIC DIELECTRIC PROPERTY MEASUREMENTS APPLIED TO THE SEAS OF TITA

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.