New constraints on the CH₄vertical profile in Uranus and Neptune fromHerschelobservations

Abstract

Dedicated line observations of CH 4 rotational lines performed with Herschel/PACS and HIFI in 2009-2011 provide new inferences of the mean methane profile in the upper tropospheres and stratospheres of Uranus and Neptune. At Uranus, CH 4 is found to be near saturation, with a ∼9 × 10 −4 tropopause/lower stratosphere mole fraction. This is nominally six times larger than inferred from Spitzer in 2007, although reconciliation may be possible if the CH 4 abundance decreases sharply from ∼100 to 2 mbar. This unexpected situation might reflect heterogeneous conditions in Uranus' stratosphere, with local CH 4 depletions and heating associated with downwelling motions. Higher CH 4 abundances compared to values inferred under solstitial conditions by Voyager in 1989 suggest that atmospheric mixing is effectively subdued at high latitudes and/or is time-variable. At Neptune, the mid-stratosphere CH 4 abundance is (1.15 ± 0.10) × 10 −3 , in agreement with earlier determinations and indicative of either leakage through a warmer polar region or upwelling at low or middle latitudes. On both planets, spatially resolved observations of temperature and methane in the stratosphere are needed to further identify the physical processes at work.

Seasonal stratospheric photochemistry on Uranus and Neptune

“…The results for both planets as a function of latitude are then presented, followed by the column densities as a function of both latitude and season. (Caldwell et al, 1988;Bézard et al, 1991;Bishop et al, 1992;Orton et al, 1992;Kostiuk et al, 1992;Yelle et al, 1993;Bézard et al, 1998;Schulz et al, 1999;Meadows et al, 2008;Fletcher et al, 2010;Greathouse et al, 2011;Lellouch et al, 2015),…”

“…A total of ∼70 hydrocarbon and oxy- Moses et al, 2000b). Multiple Rayleigh scattering of the dominant gas-phase constituents is included, but aerosol scattering and absorption are not -the stratospheric aerosols are optically thin (Pollack et al, 1987;Moses et al, 1995;Tomasko, 2009, 2011 Fletcher et al, 2010;Lellouch et al, 2010Lellouch et al, , 2015. This assumption leads to an inaccurate methane profile in the troposphere and therefore inaccurate tropospheric hydrocarbon chemistry (which is relatively unimportant at these deeper altitudes, in any case), so the tropospheric results are ignored throughout the paper; only the stratospheric results are discussed.…”

Detection of CS in Neptune’s atmosphere from ALMA observations

“…These models are compared with the averaged spectrum shown in Fig. 1. and also used for Neptune (Lellouch et al 2005(Lellouch et al , 2015. Opacity sources of CS, CO, and HCN were included (using line parameters from Pickett et al 1998), also included was the collision-induced absorption opacity due to the main compounds of Neptune's atmosphere, H 2 -H 2 , H 2 -He, and H 2 -CH 4 , using codes developed by Borysow et al (1985Borysow et al ( , 1988, Borysow & Frommhold (1986).…”

“…Opacity sources of CS, CO, and HCN were included (using line parameters from Pickett et al 1998), also included was the collision-induced absorption opacity due to the main compounds of Neptune's atmosphere, H 2 -H 2 , H 2 -He, and H 2 -CH 4 , using codes developed by Borysow et al (1985Borysow et al ( , 1988, Borysow & Frommhold (1986). We adopted a He mole fraction of 0.149 (Burgdorf et al 2003) and the CH 4 vertical distribution of Lellouch et al (2015). The Neptune thermal profile, taken from Lellouch et al (2010), is shown in Fig.…”

Upper Atmospheres and Ionospheres of Planets and Satellites

“…In contrast to Jupiter and Saturn, temperatures on Uranus and Neptune are cold enough for methane to condense in the troposphere. On Uranus, the stratospheric mixing ratio of CH 4 is only 10 #5 -10 #4 , and the abundance decreases further above the 0.1 mbar level (e.g., Lellouch et al 2015), possibly explaining the relatively deep base of the thermosphere on Uranus. On Neptune, the stratospheric CH 4 mixing ratio of about 10 #3 is several times higher than allowed by the mean tropospheric cold trap.…”