Shelley's Biblical Extracts

Probing the origin of the dark material on Iapetus

“…We sample this compound at 4.26 μm, the absorption being broad enough to prevent a significant error to be committed in evaluating the band depth. The aromatic C–H stretch on Phoebe mostly occurs at 3.25 μm (Clark et al 2005; Coradini et al 2008) while on Iapetus is reported at 3.29 μm (Cruikshank et al 2008), i.e. shifted by ∼40 nm or two spectral channels of VIMS‐IR.…”

“…Interestingly, the ν 3 asymmetric stretch of CO 2 is slightly shifted in position, moving – on an average – from 4.247 μm on Hyperion to 4.254 μm on Iapetus and 4.260 μm on Phoebe (Clark et al 2005; Buratti et al 2005a; Cruikshank et al 2007; Filacchione et al 2009), i.e. a maximum difference of 13 nm, which is anyway comprised into one spectral channel of VIMS‐IR (whose spectral resolution at those wavelengths is ∼22 nm).…”

“…We also include the CH aliphatic stretch at 3.41 μm and its overtone at 1.75 μm (Cruikshank et al 2008); and the 2.44 broad feature possibly being the overtone of a cyanide compound (Clark et al 2005) or the fundamental of molecular hydrogen trapped in dark material (Clark et al 2009). According to the first interpretation, the fundamental of CN varies in position as a result of variations in its chemical environment; assuming the RC15 calibration, in the Phoebe spectrum a feature is seen in the 4.5–4.6 μm range (Clark et al 2005) and we evaluate this bond at 4.53 μm. Finally, assuming the RC15 calibration we have also considered the 4.42 μm signature, reported by Coradini et al (2008) in homogeneous types identified on the surface of Phoebe and suggested to be related to a nitrile compound like HC 3 N or HNCO.…”

“…The sensitivity function allows to convert the raw signal of each pixel inside the IR image into radiance (divided by the IR integration time and the flat‐field) and then into reflectance I / F , where I is the intensity of reflected light (uncorrected for the specific observational geometry and the thermal emission) and π F is the plane‐parallel flux of sunlight incident on the satellite (Thekaekara 1973), scaled for its heliocentric distance (for details about the VIMS calibration; see McCord et al 2004). It should be noted that the latest unofficial RC17 sensitivity function of VIMS‐IR, derived in 2008 and currently under test, is different from the RC15 calibration used in Clark et al (2005) by the use of additional standard stars and data from the VIMS solar port. It was found that the RC15 calibration, based on comparison to Galileo Near‐Infrared Mapping Spectrometer (NIMS) data with VIMS data from the Cassini Jupiter encounter, the Moon flyby plus ground calibration data contained a residual 3‐μm absorption and spectral structure in the 2–2.5 μm region due to ringing in the VIMS order sorting filters (Clark, personal communication).…”

“…Cruikshank et al (2008) also found, in association with the aromatic band, two weaker bands at 3.42 and 3.52 μm, attributed to the asymmetric stretching modes of the ‐CH2‐ group in aliphatic hydrocarbons. Compounds containing the nitrile (‐C≡N) functional group were initially suspected as the cause of the 2.42–2.44 μm band, clearly correlated to the low‐albedo material on Phoebe, Iapetus and Dione (Cruikshank et al 1991; Clark et al 2005, 2008; Brown et al 2006), and possibly some of the indistinct spectral structure in the region 4.5–4.8 μm (Buratti et al 2005a); although recently Clark et al (2009) revised this interpretation and suggested that molecular hydrogen trapped in dark material would be a better candidate for this feature. From the analysis of far‐ultraviolet (far‐UV) spectra returned by the Cassini Ultraviolet Imaging Spectrograph (UVIS), Hendrix & Hansen (2008) found that water ice amounts increase within the dark material away from the apex (at 90 W° longitude, the centre of the dark leading hemisphere), consistently with thermal segregation of water ice; yet the fact that water ice is present also at the lowest, darkest and warmest latitudes, where it is not expected to be stable, may be a sign of ongoing or recent emplacement of the dark material from an exogenous source (ibid).…”