Mathematics – Logic
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p54c..02h&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P54C-02
Mathematics
Logic
[5419] Planetary Sciences: Solid Surface Planets / Hydrology And Fluvial Processes, [5462] Planetary Sciences: Solid Surface Planets / Polar Regions, [5464] Planetary Sciences: Solid Surface Planets / Remote Sensing, [5470] Planetary Sciences: Solid Surface Planets / Surface Materials And Properties
Scientific paper
Ephemeral hydrocarbon liquids have been observed in Titan's south polar region by the Cassini RADAR during southern summer. A collection of features, whose morphologies match that of previously identified partially-filled lakes, show more than an order of magnitude increase in backscatter. The morphologic boundaries of these features are transient between observations, suggesting surface change. Radiometrically, these changes are inconsistent with common scattering models. If not due to observational effects, the disappearance of these features represents volatile transport in Titan's hydrologic cycle. Lacustrine features identified on Titan have been grouped into three classes; empty lake basins, partially-filled lakes, and dark or liquid-filled lakes. Partially-filled lakes have radar returns consistent with incident radiation penetrating a liquid layer and interacting with the lakebed, while dark lakes completely reflect and absorb incident microwave energy. Empty lakes are brighter than their exteriors in both nadir and off-nadir observations, suggesting a strong volume scattering component. Backscatter models that include diffuse and quasi-specular scattering components are required to explain the incidence angle dependence of empty lakes. These scattering models are consistent with both individual empty lakes observed at multiple incidence angles and the collective set of empty lakes observed to date. Partially-filled lakes have lower backscatter, forbidding a significant diffusive component, and steeper slopes at lower angles. The increase in radar brightness between these feature classes suggests the exposure of diffusively scattering lakebeds that were previously covered by an attenuating liquid medium. A simple two-layer model is used to explain backscatter variations and estimate liquid depth changes in the ephemeral features. Changes in surface wave height may also be considered a possible explanation, but does not naturally explain the transient lake boundaries. In addition, the wave heights required to match radar returns are large compared with recent limits placed on the smoothness of Ontario Lacus and backscatter constraints from other lakes. Potential explanations for the observed surface changes include freezing, cryovolcanism, infiltration, and liquid evaporation. 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 phreatic surface is possible. Model results suggest evaporation rates are ~1 m/yr, similar to current GCM estimates of methane evaporation rates for the latitudes (60°S-65°S) and times (Ls between 309° and 360°) in question. An analysis of receding shorelines observed in Ontario Lacus also yield evaporation rates of ~1 m/yr and support the results of the two-layer model. These observations constrain volatile fluxes and hence, the seasonal evolution of Titan's hydrologic system.
Aharonson Oded
Elachi Ch.
Hayes Alexander G.
Lunine Jonathan I.
Mitri Giuseppe
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