Cassini radar observes seasonal change in Titan's south pole

Characterizing seasonal variations in Titan's volatile system is a primary goal of the Cassini spacecraft's Equinox and Solstice missions. Two related studies report new observations by the Cassini radar instrument peering through Titan's thick atmosphere with repeat coverage. Images of the surface at different times show lakes shrinking and disappearing over the course of one to several Earth years. The observations are of interest because they represent Cassini's initial glimpses into Titan's active hydrocarbon-based hydrologic cycle and can be used to test models of Titan's climate. The two related studies are presented by Alex Hayes, California Institute of Technology, and Jonathan Lunine, University of Rome Tor Vergata, at the American Astronomical Society's Division for Planetary Sciences meeting.

Editor's note: Read Astronomy magazine Associate Editor Liz Kruesi's reports from DPS09.

Synthetic Aperture Radar (SAR) data show that a collection of small features, previously identified as lakes, exhibit more than an order of magnitude increase in radar return and have disappearing borders between observations, suggesting surface change. Radiometrically, these changes cannot be explained without invoking temporal variability. The disappearance of these dark features likely represents volatile transport in Titan's methane cycle.

Ontario Lacus is the largest and best characterized lake in Titan's south polar region. Between July 2004 and July 2009, the shorelines of Ontario Lacus have receded, consistent with liquid evaporation and/or infiltration. In June and July 2009, the Cassini radar acquired its first high-resolution SAR images of the lake. Together with closest approach altimetry acquired in December 2008, these observations provide a unique opportunity to study Ontario. Altimetry results are used to estimate shoreline topography and calculate the volume of removed liquid. The imaginary component of the complex index of refraction (k) determines the depth to which radar can penetrate lakes on Titan and is a function of the liquid composition. The radar return from Ontario is observed to follow an exponential decay in a direction perpendicular to the shoreline. This behavior is consistent with attenuation through a deepening liquid medium. If local slopes are known, this variation can be used to estimate k. Altimetry results are coupled with SAR imagery to calculate k for Ontario Lacus. The returned value, k = 0.0004 to 0.0006, represents the first empirical estimate of the complex dielectric properties of liquid hydrocarbon derived from measurements on the surface of Titan, and is consistent with laboratory measurements of liquid natural gas. When accurate laboratory measurements of the relevant material mixtures are available, the observed complex refractive index of Ontario may be used to test compositional models of Titan's lakes. In areas where altimetry does not intersect Ontario's shore, the exponential decay in backscatter is used to create a coarse near-shore bathymetry and slope map.

Evaporation is the most likely scenario for observed changes on Titan's surface. Alternative explanations include freezing, cryovolcanism, and subsurface infiltration. Freezing is thermodynamically discouraged during the summer season in Titan's south pole, and there are no clearly observable cryovolcanic features in the study areas. Infiltration into a static hydrologic system is inconsistent with the observations. However, infiltration into a dynamic hydrologic system with a regionally varying methane/ethane table is possible. If evaporation is responsible, model results suggest rates are about 1m/yr, similar to current GCM estimates of methane evaporation rates for the latitudes and season in question. An analysis of the receding shorelines observed in Ontario Lacus also yield evaporation rates of about 1 m/yr and support the results of the two-layer model for the smaller lakes. These observations constrain volatile fluxes and hence, the evolution of Titan's hydrologic system.