Seasonal change of CH4/H2 ratio in the atmosphere of Uranus

Uranus is mainly composed of elements heavier than hydrogen and helium, such as C, S, even though very little information about these elements has been detected. Methane is one of the most important component of the atmosphere of Uranus. Analysis of the full-disk spectra of Uranus, as observed by the Xinglong 2.16m telescope with BFOSC (Beijing-Faint Object Spectrograph and Camera) on August 26 2016, was performed. A simple empirical model was used to fit the several years of spectrum albedo data to determine the variations of the CH4/H2 ratio in the atmosphere of Uranus. These data include: spectra observed by the 1.52m telescope (1994 and 1995) on La Silla, Chile; spectra observed by the HST-STIS (2002 and 2003); and spectra observed by the Xinglong 2.16m telescope (2016). The interannual change of the CH4/H2 ratio can be derived by comparing the fit results of various spectral albedos. The comparative results indicate that the CH4/H2 ratio in the atmosphere of Uranus declined from 1994 until 2002, and then began to rise thereafter. From the results, we speculate that the variation period of CH4/H2 ratio in the atmosphere of Uranus is longer than twenty years. We were unable to confirm whether the period is 84 or 42 years.