Seasonal stratospheric photochemistry on Uranus and Neptune

Abstract

A time-variable 1D photochemical model is used to study the distribution of stratospheric hydrocarbons as a function of altitude, latitude, and season on Uranus and Neptune. The results for Neptune indicate that in the absence of stratospheric circulation or other meridional transport processes, the hydrocarbon abundances exhibit strong seasonal and meridional variations in the upper stratosphere, but that these variations become increasingly damped with depth due to increasing dynamical and chemical time scales.At high altitudes, hydrocarbon mixing ratios are typically largest where the solar insolation is the greatest, leading to strong hemispheric dichotomies between the summer-to-fall hemisphere and winter-to-spring hemisphere. At mbar pressures and deeper, slower chemistry and diffusion lead to latitude variations that become more symmetric about the equator. On Uranus, the stagnant, poorly mixed stratosphere confines methane and its photochemical products to higher pressures, where chemistry and diffusion time scales remain large. Seasonal variations in hydrocarbons are therefore predicted to be more muted on Uranus, despite the planet's very large obliquity. Radiativetransfer simulations demonstrate that latitude variations in hydrocarbons on both planets are potentially observable with future JWST mid-infrared spectral imaging. Our seasonal model predictions for Neptune compare well with retrieved C 2 H 2 and C 2 H 6 abundances from spatially resolved ground-based observations (no such observations currently exist for Uranus), suggesting that stratospheric circulation -which was not included in these modelsmay have little influence on the large-scale meridional hydrocarbon distributions on Neptune, unlike the situation on Jupiter and Saturn.We describe our photochemical models in section 2, present and discuss the results for the seasonal and meridional variations in hydrocarbons on Uranus and Neptune in section 3, compare the theoretical predictions with available observations in section 4, and discuss implications for future observations and modeling in section 5.

Uranus in Northern Midspring: Persistent Atmospheric Temperatures and Circulations Inferred from Thermal Imaging

“…They argued that the discrepant supersaturated mole fraction may have instead indicated spatial heterogeneities in stratospheric methane abundance or temperatures, noting that the Hershchel data probed more global conditions while the Spitzer data were biased towards warmer temperature regions. Interestingly, our data indicate a factor of 5-6 enrichment in acetylene in precisely the coldest regions (i.e., midlatitudes) while mole fraction at warm equatorial regions remain in strong agreement with values derived from Spitzer (Orton et al 2014a;Moses et al 2018), consistent with proposed explanation for the observed discrepancy.…”

“…Higher in the stratosphere, our comparison is limited to a span of nine years. This span is short compared to estimates of long radiative and dynamical timescales on Uranus (Conrath et al 1990;Moses et al 2018), and so it is unsurprising to find little change in the northern hemispheric emission at 13 µm. By the same token, it would be surprising if any physical process could produce the the apparent discrepancies between the 2018 data and 2009 model, further suggesting that this is likely attributable to an error in the assumed profiles of temperatures or acetylene abundances at higher altitudes that go into producing the forward model.…”

“…As long as chemical conversion timescales were short compared to timescales for meridional transport, the concentration of acetylene would peak in the region of upwelling and diminish quickly to the north and south. Moses et al (2018) suggests acetylene loss timescales are roughly 2 years, with net lifetimes of 40 years at 0.2 mbar. Timescales of meridional transport are uncertain but potentially much larger, with Conrath et al (1998) suggesting a dynamical timescale of 700 years in the stratosphere.…”

“…The darkest shading indicates an overlap and a consistency between the two curves within the uncertainties. The dotted horizontal marks the initial condition for the retrieval, while the dashed lines indicate the 0.25-mbar acetylene mole fraction from the model ofMoses et al (2018) for 2009 (Ls∼7) and 2018 (Ls∼46), nearly overlapping at mutual latitudes.…”

“…The right panel shows that contribution to the 13.04 µm (NeII 2) emission peaks in stratosphere at all emission angles, although there is a significant contribution from the upper tropopause near nadir. The dotted line in this panel plots the globally averaged acetylene volume mixing ratio at equinox fromOrton et al (2014b) andMoses et al (2018).…”