Composition of Titan's Surface

Abstract

The Huygens Probe returned the first in situ data on Titan's surface composition in January 2005. Although Huygens landed on a dry plain, the Gas Chromatograph Mass Spectrometer (GCMS) showed evidence of methane moisture in the near subsurface suggesting methane precipitation at some time in the past. Heavier organic molecules were not found to be abundant in the atmosphere or at the surface, but the GCMS surface results did show ethane to be present and tentatively identified cyanogen, benzene, and carbon dioxide. During descent, aerosol particles were processed with the Aerosol Collector and Pyroliser; results suggested that the aerosols contain both nitriles and hydrocarbons. The Descent Imager/Spectral Radiometer (DISR also carried by the probe) measured the visible and near-infrared spectral reflectance of the dark plain surface at the landing site. Those data suggest a mixture of water ice, tholin-like materials, and dark neutral material with a blue slope in the near infrared; identification of water ice is suggested but inconclusive. Most remarkably DISR did not detect spectral features, beyond those for methane, for a wide range of spectrally active hydrocarbon and nitrile compounds that had been expected to be present on the surface.

The Cassini Visual and Infrared Mapping Spectrometer (VIMS) observes the spectral properties of Titan's surface through atmospheric windows between intense methane absorption bands. VIMS data show Titan's dark blue units (in RGB composites of 2.0, 1.6, and 1.3 μm) to exhibit lower relative albedos in the 1.6, 2.0, and 5 μm windows interpreted (though not unambiguously) to result from enhancement in water ice. Spectra for bright units do not exhibit depressed albedo in these windows. This gives strong evidence that the bright units are bright organic solids and not exposed water ice. The other dark equatorial unit, the dark brown unit, correlates with the vast seas of dunes discovered in the Cassini RADAR SAR (Synthetic Aperture Radar) images, suggesting that the dunes are composed of dark organic grains. If the bright materials and dark dunes are both largely organics, then they appear to consist of physically and/or chemically different hydrocarbons and/or nitriles. The VIMS and RADAR data together lead to a model where a dark blue substrate is mantled by the seas of dark organic dunes seen in SAR images and by thinner units of bright organic solids that are invisible to SAR.

Carbon dioxide has been suggested as a reasonable compositional component of Titan's surface. The GCMS did tentatively identify CO₂ at the surface. VIMS observations of south mid-latitude 5 m m bright spots Hotei Regio and Tui Regio have been suggested as attributable to carbon dioxide. CO₂ might explain both an unusual spectral slope in the 2.7–2.8 m m spectral region and an absorption band near 4.92 m m. However the VIMS 4.92 m m band is shifted significantly in wavelength from the position observed in the laboratory rendering the CO₂ identification in VIMS Tui Regio spectra inconclusive. An alternate suggestion for the source of the 4.92 μm feature in the VIMS Tui Regio spectrum is the nitrile cyanoacetylene (HC₃ N); it offers a better spectral match than does CO₂. Cyanoacetylene is a known thermospheric product detected by both the Composite Infrared Spectrometer (CIRS) and the Ion and Neutral Mass Spectrometer (INMS) Cassini instruments. If Tui Regio in fact shows a high abundance of cyanoacetylene it raises questions as to by what processes such materials are concentrated.

Other surface absorption features in the 4.8–5.2 μm spectral region have been attributed to various aromatic and aliphatic hydrocarbons including benzene. Because of the low signal precision of VIMS data at these wavelengths, these features are difficult to detect, particularly in Titan's dark regions. As a result there is a debate over the certainty of their existence. One such argued absorption feature near 5.05 μm most closely matches laboratory spectra of benzene, a compound detected both at the surface by the GCMS and at high altitude by INMS in greater abundance than expected. Another absorption feature at 4.97 μ m, also in debate, is best matched by spectra of the low-molecular-weight alkanes, methane and ethane, suggestive of moist surfaces wetted with such liquids consistent with GCMS observations of subsurface methane moisture.

The Cassini RADAR measurements constrain electrical properties related to Titan's surface composition in its scat-terometry and radiometry modes. Analysis of the scatterom-etry observations yields an average dielectric constant of ε ~ 2.2. The global passive microwave radiometry map shows the effective ε to be quite uniform over the globe; >95% of the surface shows a narrow range of ε ~1.5 ± 0.3. Both data sets suggest a high degree of volume scattering indicating substantial porosity making higher- ε materials including fractured, porous water ice, possible. At the same time, these data preclude substantial exposures of solid sheets of water ice (ε ~ 3.1) in the near surface except perhaps as local outcrops as at Sinlap crater (ε ~ 2.5). The radiometry analysis also yields global maps of thermal emissivity and of volume scattering. These properties show Titan's surface on the global scale to be consistent with fluffy blankets and veneers of organics, perhaps with graded density increasing with depth. The higher emissivity of the radar-dark dunes is consistent with grains having hydrocarbon and/or nitrile rich materials.

Cassini SAR images showed the north-polar region (>70°N) to exhibit a plethora of features resembling terrestrial lakes and seas. Further support for their being liquid is provided from analysis of high-resolution microwave radi-ometry that shows the north polar lakes to have high emis-sivity (~0.985) and low equivalent dielectric constant (~1.6) consistent with methane-ethane liquid. Most significant VIMS found absorption bands in south polar lake Ontario Lacus that evidence the presence of ethane, probably in liquid solution with methane, nitrogen, and other low-molecular-weight hydrocarbons.