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Description
We have used the Spitzer Space Telescope in 2016 February to obtain high cadence, high signal-to-noise, 17hr duration light curves of Neptune at 3.6 and 4.5{mu}m. The light curve duration was chosen to correspond to the rotation period of Neptune. Both light curves are slowly varying with time, with full amplitudes of 1.1mag at 3.6{mu}m and 0.6mag at 4.5{mu}m. We have also extracted sparsely sampled 18hr light curves of Neptune at W1 (3.4{mu}m) and W2 (4.6{mu}m) from the Wide-feld Infrared Survey Explorer (WISE)/NEOWISE archive at six epochs in 2010-2015. These light curves all show similar shapes and amplitudes compared to the Spitzer light curves but with considerable variation from epoch to epoch. These amplitudes are much larger than those observed with Kepler/K2 in the visible (amplitude ~0.02mag) or at 845nm with the Hubble Space Telescope (HST) in 2015 and at 763nm in 2016 (amplitude ~0.2mag). We interpret the Spitzer and WISE light curves as arising entirely from reflected solar photons, from higher levels in Neptune's atmosphere than for K2. Methane gas is the dominant opacity source in Neptune's atmosphere, and methane absorption bands are present in the HST 763 and 845nm, WISE W1, and Spitzer 3.6{mu}m filters.
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