Evolution of stratospheric chemistry in the Saturn storm beacon region

Seasonal stratospheric photochemistry on Uranus and Neptune

“…Less stable species such as C 2 H 4 , CH 3 C 2 H, C 4 H 2 , and CH 3 that have shorter chemical lifetimes at depth continue to experience seasonal varia-tions at all altitudes above their condensation regions (i.e., for the first three aforementioned species, which condense). The seasonal behavior of these species can be complicated, as both diffusion and in situ photochemistry affect the behavior, photodestruction of other seasonally variable species such as C 2 H 2 and C 2 H 6 contribute to their production, and photochemically produced radicals such as atomic H affect both production and destruction (see Moses et al, 2000aMoses et al, , 2015, for details). This more complicated chemistry (as well as uncertainties in the thermal structure for the observational abundance determinations) contributes to the apparent disagreements between the CH 3 C 2 H and C 4 H 2 abundances predicted by the models and inferred from observations.…”

“…Because many of the hydrocarbon photochemical products condense in the cold lower stratospheres of these planets, any (likely) changes in temperature with season could strongly affect the total column density of the condensable products, due to the high sensitivity of the species' vapor pressures to temperatures, although the gas-phase chemistry variations themselves are not very sensitive to temperature (e.g., Moses and Greathouse, 2005;Moses et al, 2015). Differences in the methane abundance at high versus low latitudes have already been identified in the tropospheres of Uranus and Neptune Tomasko, 2009, 2011;Sromovsky et al, 2011Sromovsky et al, , 2014Irwin et al, 2012;Tice et al, 2013;de Kleer et al, 2015;Luszcz-Cook et al, 2016), and if such differences extend into the stratosphere, would have a major effect on photochemical production rates.…”

On the Composition of Young, Directly Imaged Giant Planets

“…As an example, the column abundance of ethane (C 2 H 6 ) above 100 mbar on Saturn (Moses et al 2015), which is ∼10 au from the Sun, is five orders of magnitude larger than that of the generic 10 au young Jupiter shown in Figure 7, despite the greater H Lyα and overall UV flux received by the 10 au generic young Jupiter around its brighter star. The main source of the ethane is still the same on both planets-the three-body reaction CH 3 + CH 3 + M  C 2 H 6 + M-but the CH 3 on the 10 au young Jupiter goes back to recycle the CH 4 more than 99.9% of the time, because the higher atmospheric temperatures lead to a more efficient reaction of CH 3 with H 2 to form CH 4 + H. Still, the total stratospheric column production rate of C 2 H 6 is larger on the 10 au young Jupiter than on Saturn due to the brightness of the star and the larger UV flux; however, C 2 H 6 is also more readily destroyed on the warmer young Jupiter through H + C 2 H 6  C 2 H 5 + H 2 , with a much larger percentage of the carbon ending up back in CH 4 rather than in C 2 H x and other higher-order hydrocarbons.…”

Dust ablation on the giant planets: Consequences for stratospheric photochemistry

“…The first is the meteoroid ablation code described in Moses (1992), with updates from Moses (1997). The second is the Caltech/JPL one-dimensional (1D) KINET-ICS photochemical model developed by Yuk Yung and Mark Allen (e.g., Allen et al, 1981;Yung et al, 1984), most recently updated for the giant planets by Moses et al (2015).…”

“…The resulting gas production rate profiles from the ablation process are then included as a source of oxygen species to stratospheric photochemical models (e.g., Moses et al, 2000bMoses et al, , 2005Moses et al, , 2015 that consider coupled hydrocarbonoxygen chemistry (see sections 3.2-3.5). In sections 3.2-3.5, we compare the photochemical model results with observations and discuss the implications with respect to the origin of the observed oxygen species on each planet, and in section 4 we discuss the likely importance of thermochemistry and high-energy collisions during the meteor phase in securing the high inferred CO/H 2 O ratio in the stratospheres of these planets.…”