Global mapping and characterization of Titan’s dune fields with Cassini: Correlation between RADAR and VIMS observations

Abstract

Vast fields of linear dunes have been observed in the equatorial regions of Titan, Saturn's largest moon. As the Cassini mission, in orbit around Saturn since July 2004 and extended until May 2017, carries on, the high-resolution coverage of Titan's surface increases, revealing new dune fields and allowing refinements in the examination of their properties. In this paper, we present the joint analysis of Cassini's microwave and infrared global scale observations of Titan. Integrating within an up-to-date global map of Titan all the Cassini RADAR and VIMS (Visual and Infrared Mapping Spectrometer) images-the latter being empirically corrected for atmospheric scattering and surface photometry, from July 2004 through July 2013 and June 2010 respectively, we found very good qualitative and quantitative spatial matching between the geographic distribution of the dune fields and a specific infrared spectral unit (namely the ''dark brown'' unit). The high degree of spatial correlation between dunes and the ''dark brown'' unit has important implications for Titan's geology and climate. We found that RADAR-mapped dunes and the ''dark brown'' unit are similarly confined within the equatorial belt (±30°in latitudes) with an equivalent distribution with latitude, suggesting an increasing sediment availability and mobility at Titan's tropics relative to higher latitudes, compatible with the lower ground humidity predicted in equatorial regions by General Circulation Models. Furthermore, the strong correlation between RADAR-mapped dunes and the VIMS ''dark brown'' unit (72%) allows us to better constrain the total surface area covered by dune material, previously estimated from the extrapolation of the RADAR observations alone. According to our calculations, dune material cover 17.5 ± 1.5% of Titan's surface area, equivalent to a total surface area of 14.6 ± 1.2 million km 2 ($1.5 times the surface area of Earth's Sahara desert). The VIMS ''dark brown'' coloration of the dune material is here confirmed at large spatial scale. If the sand particle composition is dominated by solid organics produced in and settling from the atmosphere, as supported by our spectral modeling and by previous spectral analysis, microwave radiometric data and atmospheric modeling, dune fields are one of the major surface hydrocarbon reservoirs on Titan. Assuming two possible scenarios for the sand distribution (either the sand is (1) entirely trapped in dune landforms, or (2) trapped in dunes at places where dune landforms are firmly observed and in sand sheets elsewhere), we estimate the volume of hydrocarbons trapped in the dune sediment to be comprised between 1.

Introduction to the Planetary Dunes special issue, and the aeolian career of Ronald Greeley

“…Lorenz (2014) summarizes the key parameters involved with aeolian processes on Titan, including the sensitivity of saltation to these parameters, and also evaluates the dunebuilding timescales for Titan. Rodriguez et al (2014) …”

Acetylene on Titan’s Surface

“…Radar measurements also confirm that the dunes' composition cannot be composed of mainly water ice. Barnes et al (2008) and Rodriguez et al (2014) show that the spectrum of this area broadly matches various organics. Our results indicate the presence of the C 2 H 2 absorption band in the dark terrain observed in the Fensal-Aztlan/Quivira region where the dunes are observed.…”

“…Shangri La is a large dark dune field located close to the Adiri bright region. The Huygens probe landed in 2005 January at the border between Adiri and Shangri La on a dry lake-bed (Tomasko et al 2005;Soderblom et al 2007), in what appears as the dark-blue terrain in VIMS data (Rodriguez et al 2006(Rodriguez et al , 2014 Keller 2008), close to the one we report in the present paper, the GCMS detected traces of C 2 H 2 close to the surface (Niemann et al 2010). The region investigated here is the eastern part of Shangri La that shares a boundary with the bright Xanadu region.…”

“…It comprises the Fensal and Aztlan dune fields (Rodriguez et al 2014) as well as high-albedo areas such as the Quivira or Sinlap crater (Le Mouélic et al 2008), where darkblue infrared units are suspected to be enriched in fine grain water ice compared to to the rest of Titan's surface (Rodriguez et al 2006). McCord et al (2008), Soderblom et al (2007), Barnes et al (2008), and Rodriguez et al (2014) showed that the dunes' infrared spectrum is inconsistent with that of water ice. Radar measurements also confirm that the dunes' composition cannot be composed of mainly water ice.…”

Temporal variations of Titan’s surface with Cassini/VIMS

“…Porco et al, 2005;Lorenz et al, 2006;Soderblom et al, 2007a,b;Jaumann et al, 2008Jaumann et al, , 2009Lopes et al, 2010;Langhans et al, 2012;Rodriguez et al, 2014). Most of the surface features observed by the Cassini-Huygens mission today have been discovered and monitored by the Visual and Infrared Mapping http://dx.doi.org/10.1016/j.icarus.2015.05.003 0019-1035/Ó 2015 Elsevier Inc. All rights reserved.…”