DUNE WIDTHS ON TITAN AS INDICATORS OF SURFACE MOISTURE

Synthetic-aperture radar images of Saturn's moon Titan collected by the Cassini spacecraft show that eolian dunes are abundant on Titan's surface, covering almost 20% (Lorenz, et al, 2006). Nearly all of the dunes are linear and occur near the equator (±30°). These dunes have a general east-west trend controlled by convergent winds. Since Titan's linear dunes appear to be active, they may provide valuable information about present climatic conditions on Titan such as presence and abundance of surface moisture.

Detailed analysis of dune widths spanning latitudes of 10°S to 25°N, imaged mainly in swaths T25, T28, T21 and T23, show a correlation between dune width and latitude. The mean widths of the dunes are statistically different; dunes in the southern hemisphere are wider (1.45 ± 0.40 km) than those in the northern hemisphere (0.95±0.30 km).

There are a number causes for variations in dune widths including sand supply, wind regimes and grain size (Lancaster, 1995). Additionally, dune widths may be affected by liquid in some regions on Titan. River channels give evidence of flowing liquid and possible seasonal precipitation, while lakes, both dry and filled, are evidence that liquid methane is abundant and participates in an active hydrologic cycle. However, few filled methane lakes have been imaged in the southern hemisphere (Lunine, et al. 2008). In contrast, liquid-methane lakes are present in many images of the northern hemisphere. Some climate models of Titan (Mitchell, 2008; Rannou et al., 2006) predict net drying of the surface in the southern hemisphere and wetting of the northern hemisphere during Titan's northern winter, just now ending.

This difference in moisture presence could cause the difference in dune width between the southern and northern hemispheres. Surface moisture—high humidity and/or seasonal precipitation—in the northern hemisphere could prevent sediments from being transported. Terrestrial dune surfaces experimentally soaked with crude oil were immobilized (Kerr and Nigra, 1952). Stabilization of dunes on Titan may not be as complete as seen on oil-soaked terrestrial dunes; however, it may lead to the restricting of dune sizes in certain regions in the northern hemisphere. If this process occurs in the locations under study, we may observe a reversal in the patterns of dune widths with latitude as moisture levels change with Titan's changing seasons.