JUNO

 

A comprehensive calibration and mapping of thethermal microwave emission from Titan’s surface is

reported based on radiometric data obtained at 2.18-cm wavelength by the passive radiometer included

in the Cassini RADAR instrument. Compared to previouswork, the present results incorporate the much

larger data set obtained in the approximately ten years following Saturn Orbit Insertion. Brightness

temperature data including polarization were accumulated by segments in Titan passes from Ta

(October 2004) through T98 (February 2014). The observational segments were analyzed to produce a

mosaic of effective dielectric constant based on the measurement of thermal polarization covering 76%

of the surface, and brightness temperature at normal incidence covering Titan’s entire surface. As part

of the mosaicking process we also solved for the seasonal variation of physical temperature with latitude,

which we found to be smaller by a factor of 0.87 ± 0.05 in relative amplitude compared to that reported in

the thermal infrared by Cassini’s Composite Infrared Spectrometer (CIRS). We used the equatorial temperature

obtained by the Huygens probe and the seasonal dependence with latitude from CIRS to convert

the brightness mosaic to absolute emissivity, from which we could infer global thermophysical properties

of the surface in combination with the dielectric mosaic. We see strong evidence for subsurface (volume)

scattering as a dominant cause of the radar reflectivity in bright regions, and elsewhere a surface

composition consistent with the slow deposition and processing of organic compounds from the

atmosphere. The presence of water ice in the near subsurface is strongly indicated by the high degree

of volume scattering observed in radar-bright regions (e.g., Hummocky/mountainous terrains)

constituting 10% of Titan’s surface. A thermal analysis allowed us to infer a mean 2.18-cm emission

depth in the range 40–100 cm for the dominant radar-dark terrains (the remainder of Titan’s surface)

at all latitudes of Titan, consistent with the deposition and possible processing and redistribution of

tholin-like atmospheric photochemical products.