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 H2βS into elemental sulfur, here treated as S8β. In the cooler
models, S8β is predicted to condense in optically thick clouds of solid
sulfur particles, whilst in the warmer models S8β 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