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C. A. Griffith (Northern Arizona University), T. C. Owen (Inst. for Astronomy), T. R. Geballe (Genimi Project), G. Miller (Northern Arizona University)
We report the first evidence of methane condensation clouds in Titan's troposphere. This discovery results from observations of unprecedented enhancements in Titan's flux within 4 narrow spectral regions where the satellite's atmosphere is otherwise transparent. On September 4, 1995, Titan's albedos exceeded those previously measured (at the same Titan longitude) by 14%, 17%, 30% and 200% at 1.3, 1.6, 2.0 and 2.9 um respectively. Observations taken a day earlier show a 14% increase in reflectivity at 2 um, the most transparent of the two windows (1.6 and 2 um) recorded on that date. Because the enhancements extend into spectral regions insensitive to Titan's surface, e.g. at 2.14-2.15 and 1.62-1.625 um, Titan's surface is not the culprit. The spectral characteristics of the flux enhancements in each window independently point to the same cause: a thick cloud deck located at 15 km altitude and covering 7-9% of Titan's disk.
Several lines of evidence support this interpretation. Clouds at 15 km altitude enhance Titan's flux at precisely the spectral ranges observed. Deeper clouds affect a narrower spectral range for each window; higher clouds would be observed over greater spectral ranges. Secondly, the same ~7-9% coverage of clouds located at 15 km reproduces the observed flux enhancements at all windows. In contrast, low clouds (e.g. at 5 km altitude) require 9% covering at 1.28, 1.59, and 2.0 um, and 15% at 2.9 um. This is a consequence of the increased methane absorption at 2.9 um. Finally, a cloud deck at 15 km altitude with 9% coverage interprets the 2.9 um spectrum well; deeper (higher) clouds indicate features stronger (weaker) than observed.
Presently, it is too early to understand the cause for a sudden appearance of clouds on Titan. Whether clouds are occasional, though quick, occurrences that punctuate otherwise dry conditions can be addressed by future observations. Additional near-IR spectra are needed to study the evolution of these clouds, how they grow in size and altitude, and how long they are sustained.