New limits on H+3 abundance on Neptune using Keck NIRSPEC

Neptune and Uranus are observed with Keck II NIRSPEC in an attempt to detect H+3 emission from Neptune. In this set of observations, H+3 emission remains undetected at Neptune, whereas line-resolved emission from Uranus was observed with a signal-to-noise ratio of ∼100. Using this, we have derived an upper limit of the column-integrated H+3 density on Neptune of 1.5 (+4.8−0.9)× 1013 m−2, assuming a temperature of 550±100 K. This value improves the previous established limit by a factor of 20 and shows that the H+3 density predicted by the best available model overestimate the density by at least a factor of 3. In addition, the solar reflection continuum of Neptune in the K and L′ bands is seen to be brighter on the Northern hemisphere by a factor of ∼2, whereas previous observations had noted the solar reflection as being brighter on the Southern hemisphere.