Retrieval of water in Jupiterʹs deep atmosphere using microwave spectra of its brightness temperature

Despite several spacecraft encounters and numerous groundbased investigations, we still do not know much about Jupiterʹs deep atmosphere; in fact, the Galileo probe results were so different than anyone had anticipated, that we understand even less about this planetʹs atmosphere now than before the Galileo mission. We formulate four basic questions in Section 1.3, which, if solved, would help to better understand the chemistry and dynamics in Jupiterʹs atmosphere. We believe that three out of the four questions (explanation of NH3 altitude profile, characterization of hot spots, altitude below which the atmosphere is uniformly mixed) may be solved from passive sounding of Jupiterʹs deep (∼ tens of bars) atmosphere via a radio telescope orbiting the planet. Question nr. 4 (the water abundance in Jupiterʹs deep atmosphere) has been singled out by the Solar System Exploration Decadal Survey as a key question, since the water abundance in Jupiterʹs deep atmosphere is tied in with planet formation models. In this paper we investigate the sensitivity of microwave retrievals to the composition of Jupiterʹs deep atmosphere, in particular the water abundance. Based upon present uncertainties in the ammonia abundance and other known and unknown absorbers, including uncertainties in clouds (density and index of refraction), and uncertainties in the thermal structure and lineshape profiles, we conclude that the retrieval of water at depth from microwave spectra (disk-averaged and locally) will be highly uncertain. We show that, if the H2O lineshape profile would be accurately known (laboratory data are needed!), an atmosphere with a near-solar H2O abundance can likely be distinguished from one with an abundance of 10–20×solar O based upon the difference in their microwave spectra at wavelengths ≳ 50   cm . This would be sufficient to distinguish between some proposed scenarios by which Jupiter acquired its inventory of volatile elements heavier than helium. If, in addition, limb-darkening measurements are obtained (again, the H2O lineshape profile should be known), tighter constraints on the H2O abundance can be obtained (see also Janssen et al., 2004, this issue).