Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009dps....41.2104l&link_type=abstract
American Astronomical Society, DPS meeting #41, #21.04
Astronomy and Astrophysics
Astronomy
Scientific paper
The first SAR observations of Ontario Lacus were made by the Cassini RADAR on passes T57 (June 22, 2009) and T58 (July 8, 2009), providing a nearly complete microwave view of a large lake first seen in ISS images (McEwen et al., BAAS 37, 739, 2005.) Subsequent Cassini VIMS observations of Ontario Lacus indicated the presence of liquid ethane in the lake (Brown et al., Nature, 454, 607, 2008). Comparison of the ISS and RADAR images, taken about 4 Earth years apart, seem to show that the extent of the liquid region--interpreted to be the sharp light-dark boundary at each wavelength--has shrunk. Assuming a topographic slope no larger than 0.1% based on altimetry from the T49 pass of adjacent areas, the shrinkage yields a change in the volume of the liquid of about 15 cu.km.-- an upper limit because the RADAR sees more deeply into the lake than does the ISS. We seek to determine the cause of the shrinkage. The seasonal phase of Titan between 2005 and 2009 permits the hypothesis that the evaporation of methane or ethane from the lake has been responsible. The evaporation of methane will be energy-limited thanks to its large vapor pressure at the southern near-polar temperature of about 92 K (Jennings et al., ApJ, 69, L105, 2009). The maximum evaporative flux at the summer pole is roughly 2 W/sq.meter (Mitchell, JGR, 113, E08015, 2008), leading to a loss over the four years between ISS and RADAR observations of about 20 cu.km of liquid methane. A second approach, assuming advective transport of warm and dry air over the lakes, yields a value several times larger. Ontario Lacus has changed in a way consistent with the hypothesis that it is filled with methane/ethane liquid.
This work is supported by the Cassini Project.
Aharonson Oded
Cassini RADAR Team
Elachi Ch.
Hayes Alex
Lorenz Robert
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