[Show abstract][Hide abstract] ABSTRACT: The Voyager 1 flyby of Titan in 1980 gave a first glimpse of the chemical
complexity of Titan's atmosphere, detecting many new molecules with the
infrared spectrometer (IRIS). These included propane (C3H8) and propyne
(CH3C2H), while the intermediate-sized C3Hx hydrocarbon (C3H6) was curiously
absent. Using spectra from the Composite Infrared Spectrometer (CIRS) on
Cassini, we show the first positive detection of propene (C3H6) in Titan's
stratosphere (5-sigma significance), finally filling the three-decade gap in
the chemical sequence. We retrieve a vertical abundance profile from 100-250
km, that varies slowly with altitude from ~2 ppbv at 100 km, to ~5 ppbv at 200
km. The abundance of C3H6 is less than both C3H8 and CH3C2H, and we remark on
an emerging paradigm in Titan's hydrocarbon abundances whereby: alkanes >
alkynes > alkenes within the C2Hx and C3Hx chemical families in the lower
stratosphere. More generally, there appears to be much greater ubiquity and
relative abundance of triple-bonded species than double-bonded, likely due to
the greater resistance of triple bonds to photolysis and chemical attack.
Full-text · Article · Sep 2013 · The Astrophysical Journal
[Show abstract][Hide abstract] ABSTRACT: Invariant features are needed for atmospheric normalization of image pairs. Powerful statistical approaches now exist, which are designed to isolate unchanged pixels based on quantitatively evaluating the spectral correlation of pixels in image pairs. This suggests that it should be possible to obtain similar results following an analytical path, and our hypothesis is that the derivation of an analytical procedure will yield some physical insight that is not directly accessible with a stochastic approach. In this paper, we derive an analytical formula that relates pseudoinvariant features (PIFs) to the radiometric properties of the scenes. The formula is then inverted to yield an estimate of the ratio of transmission spectra of the two images given the path radiance for each scene and a set of invariant features, The normalized spectral transmission ratio proved to be quite stable over the 700–1800 nm but was less reproducible outside of that range. The absolute (nonnormalized) values of the transmission ratio implied a disparity in the spectral transmission suggesting that the September sky was clearer in visible light, whereas the August sky was clearer in the infrared. This leads to a discussion of the sensitivity of PIF reflectance to the viewing and illumination angles, suggesting that there may be an inherent uncertainty related to the bidirectional reflectance distribution function of the targets comprising the PIF pixels.
Full-text · Article · Apr 2013 · IEEE Transactions on Geoscience and Remote Sensing
[Show abstract][Hide abstract] ABSTRACT: In this paper we describe the technical implementation of infrared observations of Titan using the Cassini Composite Infrared Spectrometer (CIRS). Due to the distributed operations model of Cassini, instrument teams such as CIRS are responsible for spacecraft pointing during their own observations. We describe the principal factors driving and constraining observation design, and then provide details of the eight standard observation types resulting, followed by a discussion of several later 'evolved' types. We then show some sample science results, and finally conclude with a mention of continuing challenges to CIRS Titan observation design.
[Show abstract][Hide abstract] ABSTRACT: In this paper we describe the first quantitative search for several molecules in Titan's stratosphere in Cassini CIRS infrared spectra. These are: ammonia (NH3), methanol (CH3OH), formaldehyde (H2CO), and acetonitrile (CH3CN), all of which are predicted by photochemical models but only the last of which has been observed, and not in the infrared. We find non-detections in all cases, but derive upper limits on the abundances from low-noise observations at 25 degrees S and 75 degrees N. Comparing these constraints to model predictions, we conclude that CIRS is highly unlikely to see NH3 or CH3OH emissions. However, CH3CN and H2CO are closer to CIRS detectability, and we suggest ways in which the sensitivity threshold may be lowered towards this goal.
Full-text · Article · Nov 2010 · Faraday Discussions
[Show abstract][Hide abstract] ABSTRACT: In this paper we select large spectral averages of data from the Cassini Composite Infrared Spectrometer (CIRS) obtained in limb-viewing mode at low latitudes (30S--30N), greatly increasing the path length and hence signal-to-noise ratio for optically thin trace species such as propane. By modeling and subtracting the emissions of other gas species, we demonstrate that at least six infrared bands of propane are detected by CIRS, including two not previously identified in Titan spectra. Using a new line list for the range 1300-1400cm -1, along with an existing GEISA list, we retrieve propane abundances from two bands at 748 and 1376 cm-1. At 748 cm-1 we retrieve 4.2 +/- 0.5 x 10(-7) (1-sigma error) at 2 mbar, in good agreement with previous studies, although lack of hotbands in the present spectral atlas remains a problem. We also determine 5.7 +/- 0.8 x 10(-7) at 2 mbar from the 1376 cm-1 band - a value that is probably affected by systematic errors including continuum gradients due to haze and also an imperfect model of the n6 band of ethane. This study clearly shows for the first time the ubiquity of propane's emission bands across the thermal infrared spectrum of Titan, and points to an urgent need for further laboratory spectroscopy work, both to provide the line positions and intensities needed to model these bands, and also to further characterize haze spectral opacity. The present lack of accurate modeling capability for propane is an impediment not only for the measurement of propane itself, but also for the search for the emissions of new molecules in many spectral regions. Comment: 7 Figures, 3 Tables. Typeset in Latex with elsart.cls. In press for Planetary and Space Science
Full-text · Article · Sep 2009 · Planetary and Space Science
[Show abstract][Hide abstract] ABSTRACT: Propane gas (C3H8) was first detected in the atmosphere of Titan by the
Voyager 1 IRIS spectrometer, during the 1980 encounter (Maguire et al.,
1981), and remains the heaviest saturated hydrocarbon (alkane) found
there to date. Although the identification was based on the detection of
several bands (including 748, 922, 1054, 1158 cm-1), only the ν26
band at 748 cm-1 has been subsequently modeled to retrieve the
abundance, due to the unique availability of its line parameters in the
GEISA database (Husson et al. 1992). Subsequent measurements from the
ground (Roe et al., 2003) and Earth-orbit (ISO - Coustenis et al. 2003)
have also focused on this one band, deriving an abundance of ~0.5 ppm,
although it remains compromised by coincidence with the R-branch of the
much stronger acetylene (C2H2) gas. The Composite Infrared
Spectrometer (CIRS) instrument carried on-board the Cassini spacecraft
in Saturn orbit has now been observing Titan during more than 50 flybys
over 5 years, and offers a fresh perspective on the prevalence of
propane. With much improved spectral and spatial resolution and
sensitivity over IRIS, CIRS is also able to perform repeated limb
sounding (viewing through the atmosphere above the surface) to increase
signal-to-noise still further. Modeling and removal of the emissions of
other gases now shows clearly for the first time a multitude of propane
bands: including the four seen by IRIS and at least four others (869,
1338, 1376, 1472 cm-1). In addition, a new line atlas for three bands
of propane at shorter wavelengths (1300-1500 cm-1) has now been
compiled, based on the work of Flaud et al. (2001). With this, we now
have the potential to model these weaker bands, and to check the
measurements made by CIRS using the 748 cm-1 band alone. Preliminary
analysis has shown that the retrievals are very sensitive to the
spectral baseline (haze model) assumed, and that existing lab tholin
spectral properties (Khare et al. 1984) do not well match the opacity in
this spectral region. In this paper, we present the CIRS spectra
showing all the visible propane bands, with a view to stimulating
laboratory spectroscopic study of the remaining mid-IR bands (especially
at 869, 922, 1054 and 1158 cm-1). We also report on our progress in the
modeling of the 6-8 and 13 micron bands, and give an update on the
propane abundance at low latitudes. References: Coustenis et al.,
Icarus 161, pp. 383-403, 2003. Flaud et al., J. Chem. Phys. 114, pp.
9361-9366, 2001. Husson et al., J. Quant. Spectro. Rad. Trans. 48, pp.
509-518, 1992. Khare et al., Icarus 60, pp. 127-137, 1984. Maguire et
al, Nature 292, pp. 683-686, 1981. Roe et al., Astrophys. J. 597, pp.
L65-L68, 2003. Yung et al., Astrophys. J. Supp. 55, pp. 465-506, 1984.
[Show abstract][Hide abstract] ABSTRACT: We acquired high spectral and spatial resolution hyperspectral imaging spectrometer observations of Mars from near-UV to near-IR wavelengths (∼300 to 1020 nm) using the STIS instrument on the Hubble Space Telescope during the 1999, 2001, and 2003 oppositions. The data sets have been calibrated to radiance factor (I/F) and map-projected for comparison to each other and to other Mars remote sensing measurements. We searched for and (where detected) mapped a variety of iron-bearing mineral signatures within the data. The strong and smooth increase in I/F from the near-UV to the visible that gives Mars its distinctive reddish color indicates that poorly crystalline ferric oxides dominate the spectral properties of the high albedo regions (as well as many intermediate and low albedo regions), a result consistent with previous remote sensing studies of Mars at these wavelengths. In the near-IR, low albedo regions with a negative spectral slope and/or a distinctive ∼900 nm absorption feature are consistent with, but not unique indicators of, the presence of high-Ca pyroxene or possibly olivine. Mixed ferric–ferrous minerals could also be responsible for the ∼900 nm feature, especially in higher albedo regions with a stronger visible spectral slope. We searched for the presence of several known diagnostic absorption features from the hydrated ferric sulfate mineral jarosite, but did not find any unique evidence for its occurrence at the spatial scale of our observations. We identified a UV contrast reversal in some dark region spectra: at wavelengths shorter than about 340 nm these regions are actually brighter than classical bright regions. This contrast reversal may be indicative of extremely “clean” low albedo surfaces having very little ferric dust contamination. Ratios between the same regions observed during the planet-encircling dust storm of 2001 and during much clearer atmospheric conditions in 2003 provide a good direct estimate of the UV to visible spectral characteristics of airborne dust aerosols. These HST observations can help support the calibration of current and future Mars orbital UV to near-IR spectrometers, and they also provide a dramatic demonstration that even at the highest spatial resolution possible to achieve from the Earth, spectral variations on Mars at these wavelengths are subtle at best.