TY - JOUR
T1 - Radiative transfer analyses of Titan's tropical atmosphere
AU - Griffith, Caitlin A.
AU - Doose, Lyn
AU - Tomasko, Martin G.
AU - Penteado, Paulo F.
AU - See, Charles
N1 - Funding Information:
Research by C.A. Griffith, L. Doose, C. See, and M. Tomasko are funded by NASA’s Planetary Astronomy and Cassini Data Analysis Programs. We thank Bruno Bézard and Pascal Rannou for their corrections and invaluable help with the manuscript. In addition, we benefitted from the suggestions by Christopher P. McKay and an anonymous reviewer.
PY - 2012/4
Y1 - 2012/4
N2 - Titan's optical and near-IR spectra result primarily from the scattering of sunlight by haze and its absorption by methane. With a column abundance of 92km amagat (11 times that of Earth), Titan's atmosphere is optically thick and only ∼10% of the incident solar radiation reaches the surface, compared to 57% on Earth. Such a formidable atmosphere obstructs investigations of the moon's lower troposphere and surface, which are highly sensitive to the radiative transfer treatment of methane absorption and haze scattering. The absorption and scattering characteristics of Titan's atmosphere have been constrained by the Huygens Probe Descent Imager/Spectral Radiometer (DISR) experiment for conditions at the probe landing site (Tomasko, M.G., Bézard, B., Doose, L., Engel, S., Karkoschka, E. [2008a]. Planet. Space Sci. 56, 624-247; Tomasko, M.G. et al. [2008b]. Planet. Space Sci. 56, 669-707). Cassini's Visual and Infrared Mapping Spectrometer (VIMS) data indicate that the rest of the atmosphere (except for the polar regions) can be understood with small perturbations in the high haze structure determined at the landing site (Penteado, P.F., Griffith, C.A., Tomasko, M.G., Engel, S., See, C., Doose, L., Baines, K.H., Brown, R.H., Buratti, B.J., Clark, R., Nicholson, P., Sotin, C. [2010]. Icarus 206, 352-365). However the in situ measurements were analyzed with a doubling and adding radiative transfer calculation that differs considerably from the discrete ordinates codes used to interpret remote data from Cassini and ground-based measurements. In addition, the calibration of the VIMS data with respect to the DISR data has not yet been tested. Here, VIMS data of the probe landing site are analyzed with the DISR radiative transfer method and the faster discrete ordinates radiative transfer calculation; both models are consistent (to within 0.3%) and reproduce the scattering and absorption characteristics derived from in situ measurements. Constraints on the atmospheric opacity at wavelengths outside those measured by DISR, that is from 1.6 to 5.0μm, are derived using clouds as diffuse reflectors in order to derive Titan's surface albedo to within a few percent error and cloud altitudes to within 5km error. VIMS spectra of Titan at 2.6-3.2μm indicate not only spectral features due to CH 4 and CH 3D (Rannou, P., Cours, T., Le Mouélic, S., Rodriguez, S., Sotin, C., Drossart, P., Brown, R. [2010]. Icarus 208, 850-867), but also a fairly uniform absorption of unknown source, equivalent to the effects of a darkening of the haze to a single scattering albedo of 0.63±0.05. Titan's 4.8μm spectrum point to a haze optical depth of 0.2 at that wavelength. Cloud spectra at 2μm indicate that the far wings of the Voigt profile extend 460cm -1 from methane line centers. This paper releases the doubling and adding radiative transfer code developed by the DISR team, so that future studies of Titan's atmosphere and surface are consistent with the findings by the Huygens Probe. We derive the surface albedo at eight spectral regions of the 8×12km 2 area surrounding the Huygens landing site. Within the 0.4-1.6μm spectral region our surface albedos match DISR measurements, indicating that DISR and VIMS measurements are consistently calibrated. These values together with albedos at longer 1.9-5.0μm wavelengths, not sampled by DISR, resemble a dark version of the spectrum of Ganymede's icy leading hemisphere. The eight surface albedos of the landing site are consistent with, but not deterministic of, exposed water ice with dark impurities.
AB - Titan's optical and near-IR spectra result primarily from the scattering of sunlight by haze and its absorption by methane. With a column abundance of 92km amagat (11 times that of Earth), Titan's atmosphere is optically thick and only ∼10% of the incident solar radiation reaches the surface, compared to 57% on Earth. Such a formidable atmosphere obstructs investigations of the moon's lower troposphere and surface, which are highly sensitive to the radiative transfer treatment of methane absorption and haze scattering. The absorption and scattering characteristics of Titan's atmosphere have been constrained by the Huygens Probe Descent Imager/Spectral Radiometer (DISR) experiment for conditions at the probe landing site (Tomasko, M.G., Bézard, B., Doose, L., Engel, S., Karkoschka, E. [2008a]. Planet. Space Sci. 56, 624-247; Tomasko, M.G. et al. [2008b]. Planet. Space Sci. 56, 669-707). Cassini's Visual and Infrared Mapping Spectrometer (VIMS) data indicate that the rest of the atmosphere (except for the polar regions) can be understood with small perturbations in the high haze structure determined at the landing site (Penteado, P.F., Griffith, C.A., Tomasko, M.G., Engel, S., See, C., Doose, L., Baines, K.H., Brown, R.H., Buratti, B.J., Clark, R., Nicholson, P., Sotin, C. [2010]. Icarus 206, 352-365). However the in situ measurements were analyzed with a doubling and adding radiative transfer calculation that differs considerably from the discrete ordinates codes used to interpret remote data from Cassini and ground-based measurements. In addition, the calibration of the VIMS data with respect to the DISR data has not yet been tested. Here, VIMS data of the probe landing site are analyzed with the DISR radiative transfer method and the faster discrete ordinates radiative transfer calculation; both models are consistent (to within 0.3%) and reproduce the scattering and absorption characteristics derived from in situ measurements. Constraints on the atmospheric opacity at wavelengths outside those measured by DISR, that is from 1.6 to 5.0μm, are derived using clouds as diffuse reflectors in order to derive Titan's surface albedo to within a few percent error and cloud altitudes to within 5km error. VIMS spectra of Titan at 2.6-3.2μm indicate not only spectral features due to CH 4 and CH 3D (Rannou, P., Cours, T., Le Mouélic, S., Rodriguez, S., Sotin, C., Drossart, P., Brown, R. [2010]. Icarus 208, 850-867), but also a fairly uniform absorption of unknown source, equivalent to the effects of a darkening of the haze to a single scattering albedo of 0.63±0.05. Titan's 4.8μm spectrum point to a haze optical depth of 0.2 at that wavelength. Cloud spectra at 2μm indicate that the far wings of the Voigt profile extend 460cm -1 from methane line centers. This paper releases the doubling and adding radiative transfer code developed by the DISR team, so that future studies of Titan's atmosphere and surface are consistent with the findings by the Huygens Probe. We derive the surface albedo at eight spectral regions of the 8×12km 2 area surrounding the Huygens landing site. Within the 0.4-1.6μm spectral region our surface albedos match DISR measurements, indicating that DISR and VIMS measurements are consistently calibrated. These values together with albedos at longer 1.9-5.0μm wavelengths, not sampled by DISR, resemble a dark version of the spectrum of Ganymede's icy leading hemisphere. The eight surface albedos of the landing site are consistent with, but not deterministic of, exposed water ice with dark impurities.
KW - Infrared observations
KW - Radiative transfer
KW - Satellites, atmospheres
KW - Spectroscopy
KW - Titan
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U2 - 10.1016/j.icarus.2011.11.034
DO - 10.1016/j.icarus.2011.11.034
M3 - Article
AN - SCOPUS:84857931782
SN - 0019-1035
VL - 218
SP - 975
EP - 988
JO - Icarus
JF - Icarus
IS - 2
ER -