Photolytic Hazes in the Atmosphere of 51 Eri b

We use a 1D model to address photochemistry and possible haze formation in the irradiated warm Jupiter, 51 Eridani b. The intended focus was to be carbon, but sulfur photochemistry turns out to be important. The case for organic photochemical hazes is intriguing but falls short of being compelling. If organic hazes form, they are likeliest to do so if vertical mixing in 51 Eri b is weaker than in Jupiter, and they would be found below the altitudes where methane and water are photolyzed. The more novel result is that photochemistry turns H$_2$S into elemental sulfur, here treated as S$_8$. In the cooler models, S$_8$ is predicted to condense in optically thick clouds of solid sulfur particles, whilst in the warmer models S$_8$ remains a vapor along with several other sulfur allotropes that are both visually striking and potentially observable. For 51 Eri b, the division between models with and without condensed sulfur is at an effective temperature of 700 K, which is within error its actual effective temperature; the local temperature where sulfur condenses is between 280 and 320 K. The sulfur photochemistry we have discussed is quite general and ought to be found in a wide variety of worlds over a broad temperature range, both colder and hotter than the 650-750 K range studied here, and we show that products of sulfur photochemistry will be nearly as abundant on planets where the UV irradiation is orders of magnitude weaker than it is on 51 Eri b.Comment: 24 pages including 11 figures and a tabl

Nitrogen and Oxygen Photochemistry following SL9

The collision of Shoemaker Levy 9 (SL9) with Jupiter caused many new molecular species to be deposited in the Jovian stratosphere. We use a photochemical model to follow the evolution of the impact derived species. Our results regarding the nitrogen and oxygen compounds are presented here. NH3 photolysis initiates the nitrogen photochemistry. Much of the nitrogen ends up in N2, nitrogen-sulfur compounds, and HCN, but NH3 and nitriles such as C2H3CN may also exist in observable quantities for a year or so after the impacts. Oxygen species survive for a long time in the Jovian stratosphere. The only major oxygen containing compounds that exhibit dramatic changes in the lower stratosphere in the first year following the impacts are SO, SO2, and OCS - H2O, CO2, and CO are comparatively stable. We discuss the important photochemical processes operating on the nitrogen and oxygen species in the Jovian stratosphere, make prediction concerning the temporal variation of the major species, and identify molecules that might act as good tracers for atmospheric dynamics

An Analysis of Neptune's Stratospheric Haze Using High-Phase-Angle Voyager Images

We have inverted high-phase-angle Voyager images of Neptune to determine the atmospheric extinction coefficient as a function of altitude and the scattering phase function at a reference altitude. Comparisons between theoretical model and observations help separate the contributions from molecular Rayleigh and aerosol scattering and help determine the variation of the aerosol size, concentration, and scattering properties with altitude. Further comparisons between models and data allow us to place constraints on the location and composition of the hazes, the concentration and downward flux of certain condensible hydrocarbon gases, the eddy diffusion coefficient in the lower stratosphere, and the thermal profile in parts of Neptune's stratosphere. We find that a distinct stratospheric haze layer exists near 12(sub -1, sup +1) mbar in Neptune's lower stratosphere, most probably due to condensed ethane. The derived stratospheric haze production rate of 1.0(sub -0.3, sup +0.2) x 10(exp -15) g cm(exp -2) sec(exp -1) is substantially lower than photochemical model predictions. Evidence for hazes at higher altitudes also exists. Unlike the situation on Uranus, large particles (0.08-0.11 microns) may be present at high altitudes on Neptune (e.g., near 0.5 mbar), well above the region in which we expect the major hydrocarbon species to condense. Near 28 mbar, the mean particle size is about 0.13(sub -0.02, sup +0.02) microns with a concentration of 5(sub -3, sup +3) particles cm(exp -3). The cumulative haze extinction optical depth above 15 mbar in the clear filter is approx. 3 x 10(exp -3), and much of this extinction is due to scattering rather than absorption; thus, if our limb-scan sites are typical, the hazes cannot account for the stratospheric temperature inversion on Neptune and may not contribute significantly to atmospheric heating. We compare the imaging results with the results from other observations, including those of the Voyager Photopolarimeter Subsystem, and discuss differences between Neptune and Uranus

Neptune's visual albedo variations over a solar cycle: A pre-Voyager look at ion-induced nucleatlon and cloud formation in Neptune's troposphere

High-energy galactic cosmic rays can penetrate to deep levels within Neptune's atmosphere to form a substantial ionospheric layer in the lower stratosphere and upper troposphere of the planet. Because cosmic-ray modulation in the interplanetary medium creates an inverse relationship between cosmic-ray intensity and solar activity, the ionization rate in the lower atmosphere will vary with the 11-year solar cycle in such a way that maximum ionization will occur at sunspot minimum and minimum ionization at sunspot maximum. This variable ionization may, by the process of ion-induced nucleation, regulate the formation and optical properties of an upper tropospheric haze in the atmosphere of Neptune and could thus provide a mechanism for modulating the planet's visual brightness over a solar cycle. We estimate the rates of cosmic-ray ionization at solar maximum and minimum for a range of proposed Neptune dipole magnetic field strengths; discuss the physics of formation of ion clusters and eventual aerosol particles; and, using a simple radiative transfer model, discuss the ways in which this variable aerosol formation can modulate the geometric albedo of Neptune over a solar cycle. If Voyager spacecraft observations reveal that Neptune's dipole magnetic field is about 1 Gauss or less, then our modeling indicates that variations in an aerosol layer due to differences in cosmic ray ionization may help explain some of the observed brightness variation of Neptune over a solar cycle

Hydrogen and Deuterium Loss from the Terrestrial Atmosphere: A Quantitative Assessment of Nonthermal Escape Fluxes

A comprehensive one-dimensional photochemical model extending from the middle atmosphere (50 km) to the exobase (432 km) has been used to study the escape of hydrogen and deuterium from the Earth's atmosphere. The model incorporates recent advances in chemical kinetics as well as atmospheric observations by satellites, especially the Atmosphere Explorer C satellite. The results suggest: (1) the escape fluxes of both H and D are limited by the upward transport of total hydrogen and total deuterium at the homopause (this result is known as Hunten's limiting flux theorem); (2) about one fourth of total hydrogen escape is thermal, the rest being nonthermal; (3) escape of D is nonthermal; and (4) charge exchange and polar wind are important mechanisms for the nonthermal escape of H and D, but other nonthermal mechanisms may be required. The efficiency to escape from the terrestrial atmosphere for D is 0.74 of the efficiency for H. If the difference between the D/H ratio measured in deep-sea tholeiite glass and that of standard sea water, δD = −77‰, were caused by the escape of H and D, we estimate that as much water as the equivalent of 36% of the present ocean might have been lost in the past