Probing the atmospheres of Uranus comma Neptune and Titan with CH4 lines

Methane is a key species in the Outer Planets. It is the third most abundant molecule in all four Giant Planets, with an abundance ofabout 2 % in Uranus and Neptune, and reaching 5% (of N2) at the surface of Titan. Because of its large abundance, methane plays a dominant role in governing the atmospheric chemistry of all these planets. Indeed, the photolysis of methane by solar photons initiates a complex chemistry, giving rise to a wealth of hydrocarbons. In the case of Titan, the photochemistry of methane is even more complex, because of the coupled CH4N2 chemistry taking place in Titan.s upper atmosphere. Initial observations of Neptune and Titan, performed in the framework of the GTKP ..Water and related chemistry in the Solar System. (PI: P. Hartogh) have allowed the detection of CH4 emission at 119.6 micron and in several other lines, but with a low spectral resolution. These measurements have constrained the stratospheric abundance of CH4. In the case of Uranus, the PACS measurements of the CH4 line at 159 microns shows only tropospheric absorption, and with a low signaltonoise ratio.The goal of this proposal is to use the high spectral resolution of HIFI in order to resolve the 1882 GHz methane lines on Neptune and Titan. These optically thick lines will allow to constrain the vertical temperature profiles in their stratospheres, and in Neptune.s case, the verticaldistribution of methane. For Uranus, we propose to observe again the methane line at 159 micron with PACS, but with a gain of a factor 3 in sensitivity, in order to confirm the detection and better constrain its abundance. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]