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We report the identification of compounds on Titan's surface by spatially resolved imaging spectroscopy methods through Titan's atmosphere, and set upper limits to other organic compounds. We present evidence for surface deposits of solid benzene (C6H6), solid and/or liquid ethane (C2H6), or methane (CH4), and clouds of hydrogen cyanide (HCN) aerosols using diagnostic spectral features in data from the Cassini Visual and Infrared Mapping Spectrometer (VIMS). Cyanoacetylene (2‐propynenitrile, IUPAC nomenclature, HC3N) is indicated in spectra of some bright regions, but the spectral resolution of VIMS is insufficient to make a unique identification although it is a closer match to the feature previously attributed to CO2. We identify benzene, an aromatic hydrocarbon, in larger abundances than expected by some models. Acetylene (C2H2), expected to be more abundant on Titan according to some models than benzene, is not detected. Solid acetonitrile (CH3CN) or other nitriles might be candidates for matching other spectral features in some Titan spectra. An as yet unidentified absorption at 5.01‐μm indicates that yet another compound exists on Titan's surface. We place upper limits for liquid methane and ethane in some locations on Titan and find local areas consistent with millimeter path lengths. Except for potential lakes in the southern and northern polar regions, most of Titan appears “dry.” Finally, we find there is little evidence for exposed water ice on the surface. Water ice, if present, must be covered with organic compounds to the depth probed by 1–5‐μm photons: a few millimeters to centimeters.

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The origin of Phoebe, which is the outermost large satellite of Saturn, is of particular interest because its inclined, retrograde orbit suggests that it was gravitationally captured by Saturn, having accreted outside the region of the solar nebula in which Saturn formed. By contrast, Saturn's regular satellites (with prograde, low-inclination, circular orbits) probably accreted within the sub-nebula in which Saturn itself formed. Here we report imaging spectroscopy of Phoebe resulting from the Cassini-Huygens spacecraft encounter on 11 June 2004. We mapped ferrous-iron-bearing minerals, bound water, trapped CO2, probable phyllosilicates, organics, nitriles and cyanide compounds. Detection of these compounds on Phoebe makes it one of the most compositionally diverse objects yet observed in our Solar System. It is likely that Phoebe's surface contains primitive materials from the outer Solar System, indicating a surface of cometary origin.

[1] Reflectance spectra of the organic compounds comprising the alkane series are presented from the ultraviolet to midinfrared, 0.35 to 15.5 mm. Alkanes are hydrocarbon molecules containing only single carbon-carbon bonds, and are found naturally on the Earth and in the atmospheres of the giant planets and Saturn's moon, Titan. This paper presents the spectral properties of the alkanes as the first in a series of papers to build a spectral database of organic compounds for use in remote sensing studies. Applications range from mapping the environment on the Earth, to the search for organic molecules and life in the solar system and throughout the universe. We show that the spectral reflectance properties of organic compounds are rich, with major diagnostic spectral features throughout the spectral range studied. Little to no spectral change was observed as a function of temperature and only small shifts and changes in the width of absorption bands were observed between liquids and solids, making remote detection of spectral properties throughout the solar system simpler. Some high molecular weight organic compounds contain single-bonded carbon chains and have spectra similar to alkanes even when they fall into other families. Small spectral differences are often present allowing discrimination among some compounds, further illustrating the need to catalog spectral properties for accurate remote sensing identification with spectroscopy.

Tidal stress changes within the Earth due to the gravitational attraction of the Sun and Moon can exceed 20 mbar/h, whereas tectonic stress rates are of the order of 0.2 mbar/h. Although the absolute magnitude of tidal stresses is less than 1% of the typical stress drops observed in earthquakes, a correlation between the tidal stress history on a fault plane and the time of failure should exist if (1) tidal and tectonic stresses are combined in a simple manner, (2) the tectonic stress buildup over time is smooth, and (3) failure occurs immediately upon attainment of some critical stress. To search for possible tidal triggering of intermediate and deep focus earthquakes, 7359 events were selected from a global catalog and subdivided into 30 regions. Each event was assigned a tidal phase relative to both the semidiurnal and biweekly tidal stress curves at the time of occurrence. All tidal phase distributions were checked for a preferred phase orientation using the Rayleigh test and for nonrandomness using the chi‐square test. Results of this analysis for the semidiurnal Earth tide show that three regions have a preferred phase direction and four regions display nonrandomness significant at the 90% confidence level, which would be expected for this number of random data sets. The biweekly tide, when normalized for the unequal distribution of biweekly phase over time after correction for clustering of events in some of the data sets, shows only three non‐random data sets at the 95% level as defined by the chi‐square test. Thus intermediate and deep focus earthquakes in most subduction zones occur independently of both the semidiurnal and biweekly tidal stress cycles, and we may conclude that failure below 70 km does not occur at the instant a stress curve, the simple sum of tectonic and tidal stresses, reaches the yield strength of subcrustal rocks. This implies that either tectonic stress rates prior to failure are not smooth and are unaffected by predictable sources of stress or that failure does not necessarily occur when a certain critical stress is reached. These may in turn be due to some time delay characteristic of deep failure, to the stress history of the material, to the diffusion of fluids, or to the mode of failure itself, at present a unique and unexplained phenomenon at such high temperatures and pressures.

H ydrocarbon reservoirs at Titan come in many forms-as gases and condensates in the atmosphere; as surface accumulations of liquid in lakes, slushy soils, and solid sediments; and in the subsurface, perhaps caged within clathrate hydrates and/or as part of a global hydrocarbon aquifer. Because Titan is so far from the Sun and contains multiple atmospheric haze layers, information on its surface features and their composition is extremely difficult to obtain and is acquired via imaging instruments that operate at wavelengths less affected by the haze. Unfortunately, the data are commonly of low resolution, and divergent interpretations abound. However, with the multinational Cassini spacecraft currently orbiting Saturn on its extended Solstice mission, Titan's surface composition is slowly coming into focus. This review will attempt to synthesize the current state of knowledge of hydrocarbon presence and distribution at Titan, emphasizing those observations that have direct compositional relevance to compounds in the atmosphere and on the surface. 6 Curchin, J. M., and R. N. Clark, 2013, Remote sensing of hydrocarbons on Titan, in W. A. Ambrose, J. F. Reilly II, and D. C. Peters, eds., Energy resources for human settlement in the solar system and Earth's future in space: AAPG Memoir 101, p. 115 -140.