47th Lunar and Planetary Science Conference (2016)
1910.pdf
The Edge of Xanadu : Investigation with Altimetry and Nadir Emissivity. R. D. Lorenz1, A. Le Gall2, E. P.
Turtle1, M. Mastrogiuseppe3, V. Poggiali4, A. G. Hayes3, M. A. Janssen5 M. Malaska5, R. Lopes5 P. Callahan5, J.
Radebaugh6, J. Barnes7, R. Kirk8 , E. Stofan9. Johns Hopkins Univ. Applied Physics Lab. 2LATMOS-UVSQ, IPSL,
Paris, France. 3Cornell Univ., Ithaca, NY. 4Sapienza Università di Roma, Italy. 5Jet Propulsion Laboratory, Pasadena, CA. 6Brigham Young Uni., Provo, UT. 7U. Idaho, Moscow, ID. 8USGS, Flagstaff, AZ. 9NASA, Washington, DC.
[email protected]
Introduction: The leading hemisphere of Titan
was recognized to be brighter than the rest at both radar and near-infrared wavelengths from lightcurve data
even before Titan's surface was imaged (e.g. [1]). The
terrain responsible was the first to be named on Titan –
Xanadu.
This region is apparently old, retaining a relatively
large number of impact craters, and heavily tectonized,
featuring numerous mountains [2]. It also has some of
the best-developed fluvial networks on Titan. A key
current question is how Xanadu is differentiated from
the Shangri-La dunefields to the west : why does sand
not continue migrating into Xanadu. Topographic
blocking is one possibility, but seemingly unlikely
since Xanadu is actually low in elevation overall [3,4].
Another possibility [5] is that fluvial transport sweeps
sand away to the north or south.
Xanadu has not only high backscatter, but unusually low emissivity [3] indicating a substantial volume
scattering component [4].
Observations during the T77 Cassini flyby (figure
1) give a new perspective (figure 2) on this distinctive
terrain through altimetry measurements. These not only
provide topographic information, but also yield radiometric data (emissivity and backscatter). Backscatter
at nadir provides independent information from that
measured at higher incidence in SAR, since different
reflection mechanisms can dominate.
References: [1] R. D. Lorenz and J. I. Lunine, Planetary and Space Science, 45 981-992, 1997
[2] J. Radebaugh et al, Icarus, 211, 672-685, 2011
[3] B. Stiles et al., Icarus, 202, 584-598, 2009
[4] R. Lorenz et al., Icarus, 225, 367-377, 2013
[5] J. Barnes et al., Planetary Science, 4, 1-19, 2015
[6] F. Paganelli, et al., Icarus, 191, 211-222, 2007
[7] M. A. Janssen et al., Icarus, 200, 222-239, 2009.
[8] H. Zebker et al..,
Acknowledgments: This work was supported by
NASA Grant NNX13AH14G "Cassini RADAR Science Support"
Figure 1. Location map showing the T77 altimetry swath on a basemap of SAR and VIMS data. In addition to the
optical and SAR boundary of Xanadu at 140oW, note also a small bright feature seen in low-resolution HiSAR at
155oW (arguably part of Oahu Facula – the bright/dark boundaries in ISS data, on which that feature was named, are
not so obvious in the SAR data). The width of the yellow line reflects the spacecraft altitude as it receded westwards
and the altimeter footprint grew in diameter, a factor that needs to be taken into account in interpreting altimeter data
[8].
47th Lunar and Planetary Science Conference (2016)
1910.pdf
Figure 1. Profiles along the equator at the Shangri-La / Xanadu boundary. Top-to-bottom plots show relative brightness in the ISS mosaic map; microwave emissivity and backscatter (dB) sensed at nadir, the threshold height (m),
and the range span (m) in the altimetry echo. The microwave properties do not have as well-defined an 'edge' as the
near-infrared reflectivity. It is also noteworthy that while there is a large-scale anticorrelation of emissivity and
backscatter (as one might expect from Kirchoff's Law), on the local scale of mountains often the two properties are
positively correlated. As noted further south from SARtopo, the mountains are superposed on a lower base level
than the dunefields to the West – here there is a ~150m drop. Strikingly, this drop is further East than the nearinfrared boundary. The feature at 155 oW has striking contrast in nadir backscatter but little topographic expression.