Elsevier

Icarus

Volume 204, Issue 2, December 2009, Pages 610-618
Icarus

The geology of Hotei Regio, Titan: Correlation of Cassini VIMS and RADAR

https://doi.org/10.1016/j.icarus.2009.07.033Get rights and content

Abstract

Joint Cassini VIMS and RADAR SAR data of ∼700-km-wide Hotei Regio reveal a rich collection of geological features that correlate between the two sets of images. The degree of correlation is greater than anywhere else seen on Titan. Central to Hotei Regio is a basin filled with cryovolcanic flows that are anomalously bright in VIMS data (in particular at 5 μm) and quite variable in roughness in SAR. The edges of the flows are dark in SAR data and appear to overrun a VIMS-bright substrate. SAR-stereo topography shows the flows to be viscous, 100–200 m thick. On its southern edge the basin is ringed by higher (∼1 km) mountainous terrain. The mountains show mixed texture in SAR data: some regions are extremely rough, exhibit low and spectrally neutral albedo in VIMS data and may be partly coated with darker hydrocarbons. Around the southern margin of Hotei Regio, the SAR image shows several large, dendritic, radar-bright channels that flow down from the mountainous terrain and terminate in dark blue patches, seen in VIMS images, whose infrared color is consistent with enrichment in water ice. The patches are in depressions that we interpret to be filled with fluvial deposits eroded and transported by liquid methane in the channels. In the VIMS images the dark blue patches are encased in a latticework of lighter bands that we suggest to demark a set of circumferential and radial fault systems bounding structural depressions. Conceivably the circular features are tectonic structures that are remnant from an ancient impact structure. We suggest that impact-generated structures may have simply served as zones of weakness; no direct causal connection, such as impact-induced volcanism, is implied. We also speculate that two large dark features lying on the northern margin of Hotei Regio could be calderas. In summary the preservation of such a broad suite of VIMS infrared color variations and the detailed correlation with features in the SAR image and SAR topography evidence a complex set of geological processes (pluvial, fluvial, tectonic, cryovolcanic, impact) that have likely remained active up to very recent geological time (<104 year). That the cryovolcanic flows are excessively bright in the infrared, particularly at 5 μm, might signal ongoing geological activity. One study [Nelson, R.M., and 28 colleagues, 2009. Icarus 199, 429–441] reported significant 2-μm albedo changes in VIMS data for Hotei Arcus acquired between 2004 and 2006, that were interpreted as evidence for such activity. However in our review of that work, we do not agree that such evidence has yet been found.

Section snippets

Introduction and background

Hotei Regio (IAU formal name for a ∼700-km-wide region centered at 78°W 26°S) has been flagged as an enigmatic region on Titan in various analyses of Cassini visible-light, short-wavelength-infrared, and radar observations. Hotei Arcus (also an IAU formal name) is a bright arc that forms the southern margin of Hotei Regio; we further clarify this distinction in Section 2. A consensus is emerging that Hotei Regio is a geologically young cryovolcanic region. Cryovolcanism is a term used to

VIMS observations and photometric analysis

The VIMS spectral images described here were acquired on 11/19/2008 during Cassini’s T47 flyby of Titan. Five cubes (CM_1605796665-8513) were acquired of Hotei Regio with resolutions increasing from 18.5 to 12.5 km/pixel. Fig. 1 shows a color mosaic of these data along with individual spectral images for each of eight methane windows taken from the highest-resolution cube (that fortuitously also covers central Hotei Regio). These VIMS cubes were acquired close to the terminator. For the area of

Mapping: correlations between VIMS and SAR

In general, Titan’s surface features as seen in VIMS and SAR images are only weakly correlated (Soderblom et al., 2007b); VIMS senses the composition of the upper few tens of microns of the surface, while SAR is sensitive to surface roughness and the scattering properties of the upper meter or so of the surface. The notable exceptions to this weak correlation are vast belts of longitudinal dunes (Lorenz et al., 2006, Radebaugh et al., 2008) whose patterns are strongly correlated in VIMS and SAR

Discussion regional structures and features – speculations on origin and age

The arcuate and linear structures extend far beyond the region of joint SAR–VIMS coverage shown in Fig. 2, Fig. 3. The larger VIMS mosaic of Fig. 1 was spatially filtered (shown in Fig. 4) to enhance structural details and further suppress regional albedo variations and residual photometric errors. The structural arcs described earlier that lie along the outer southern margin of Hotei Regio are quite evident as are the linear radial segments, in particular one that truncates the western end of

Acknowledgments

We are grateful to Ralph Lorenz and Jani Radebaugh for thoughtful and constructive reviews. This research was carried out under funding from the Cassini Flight Project managed by the Jet Propulsion Laboratory, Caltech for NASA.

References (52)

  • G. Mitri et al.

    Thermal convection in ice-I shells of Titan and Enceladus

    Icarus

    (2008)
  • Giuseppe Mitri et al.

    Resurfacing of Titan by ammonia–water cryomagma

    Icarus

    (2008)
  • G. Tobie et al.

    Titan’s orbit provides evidence for a subsurface NH3–water ocean

    Icarus

    (2005)
  • Barnes, J.W., and 11 colleagues, 2006. Cassini observations of flow-like features in western Tui Regio, Titan. Geophys....
  • Barnes, J.W., and 34 colleagues, 2005. A 5-micron-bright spot on Titan: Evidence for surface diversity. Science 310,...
  • Barnes, Jason W., and 12 colleagues, 2008. Spectroscopy, morphometry, and photoclinometry of Titan’s dunefields from...
  • Berk, A., Anderson, G.P., Acharya, P.K., Bernstein, L.S., Muratov, L., Lee, J., Fox, M., Adler-Golden, S.M., 2006....
  • Brown, R.H., and 21 colleagues, 2004. The Cassini visual and infrared mapping spectrometer (VIMS) investigation. Space...
  • R.H. Brown et al.

    The identification of liquid ethane in Titan’s Ontario Lacus

    Nature

    (2008)
  • William W. Chadwick et al.

    The pattern of circumferential and radial eruptive fissures on the volcanoes of Fernandina and Isabela islands, Galapagos

    Bull. Volcanol.

    (1991)
  • Clark, R.N., and 12 colleagues, submitted for publication. Detection and mapping of hydrocarbon deposits on Titan. J....
  • Coustenis, A., and 25 colleagues, 2007. The composition of Titan’s stratosphere from Cassini/CIRS mid-infrared spectra....
  • Dominic A. Fortes et al.

    Modelling of possible mud volcanism on Titan

    Icarus

    (2006)
  • Griffith, C.A., and 13 colleagues, 2006. Evidence for a polar ethane cloud on Titan. Science 313,...
  • H. Imanaka et al.

    Complex refractive indices of tholins produced from various initial gas mixtures and formation pressures, implications for Titan, the early Earth, and the outer Solar System bodies

    Bull. Am. Astron. Soc.

    (2005)
  • Imanaka, Hiroshi, Khare, Bishun N., Elsila, Jamie E., Bakes, Emma L.O., McKay, Christopher P., Cruikshank, Dale P.,...
  • Cited by (0)

    1

    Deceased.

    View full text