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| Titel |
Bathymetry and Absorptivity of Titan's Ontario Lacus |
| VerfasserIn |
Alexander Hayes, Oded Aharonson, Howard Zebker, Ralph Lorenz, Randolph Kirk, Philippe Paillou, Jonathan Lunine, Philip Callahan, Stephen Wall, Charles Elachi |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250034482
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| Zusammenfassung |
| Ontario Lacus is the largest and best characterized lake in Titan’s south polar region. In June
and July 2009, the Cassini RADAR acquired its first high resolution Synthetic Aperture
Radar (SAR) images of the area. Together with closest approach altimetry acquired in
December 2008, these observations provide a unique opportunity to study the lake’s
near-shore bathymetry and complex refractive properties. This work represents the first
empirical estimate of the imaginary dielectric properties of liquid hydrocarbons on Titan
using Cassini observations.
The normalized radar backscatter cross-section (Ïo) is observed to decrease exponentially
with distance from the local shoreline. This behavior is consistent with attenuation by a
deepening layer of liquid and, if local topography is known, can be used to derive absorptive
optical properties. Accordingly, we estimate near-shore topography from radar altimetric
profiles and then analyze SAR backscatter to determine the imaginary component of the
liquid’s complex index of refraction (κ). The complex refractive index at microwave
wavelength determines the depth to which radar waves can penetrate lakes on Titan, and is a
function of liquid composition. The derived value, κ = (6.6-1.2+1.6) à 10-4, corresponds
to a loss tangent of tanÎ = (1.0-0.18+0.25) Ã 10-3 and is compatible with a
composition dominated by liquid hydrocarbons. This value is consistent with previous
laboratory estimates of simple hydrocarbons and will help constrain Ontario’s liquid
composition when additional laboratory data become available. Significant amounts of
highly absorptive material, such as tholins, are inconsistent with the measured loss
tangent.
In areas that do not intersect altimetry profiles, relative slopes can be calculated assuming
the index of refraction is constant throughout the liquid. Slope estimations are provided for
eight areas, in addition to the two which contain altimetry intersections, forming a coarse
bathymetry map. The derived bathymetric slopes vary from (0.5 - 2.5) Ã 10-3 and correlate
well with observed shoreline morphologies. Slope values on the eastern shore, which exhibits
a beachhead morphology, are characteristically steeper than the western shore, where a more
complex shoreline exists. The shallowest slope occurs in the southwestern tip of Ontario,
where the greatest magnitude of shoreline recession has been observed between 2005 and
2009 ISS and Radar observations. A linear relationship is observed between shoreline
recession magnitude and the inverse of near-shore slope, consistent with a depth reduction of
4.0 ± 1.3Â m over the four years between observations. Together, these observations provide
a new understanding of Ontario Lacus and its role in Titan’s hydrocarbon cycle. |
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