The depths of clouds on Jupiter: Observational constraints on the O/H ratio

The oxygen abundance in Jupiter is an important constraint on planet formation and conditions in protoplanetary disks. Oxygen, in the form of water, is also dynamically significant in Jupiter's atmosphere: as a tracer of circulation and as a carrier of latent heat. We have developed a technique to measure the depth of opaque cloud tops in Jupiter's atmosphere (Bjoraker et al. 2015; this meeting; also Astrophysical Journal in press). We measure resolved CH3D line shapes in the 5-μm window of Jupiter's spectrum to distinguish between cloud-top pressure levels of about 3 to 10 bars. We will use the retrieved cloud top pressure levels to place lower limits on the O/H ratio in Jupiter, based on Keck/NIRSPEC spectra acquired in January 2013. Since our spectra do not directly give the temperature/pressure profile in the cloud layer, constraining the O/H ratio requires independent atmospheric structure data. We will review observational and theoretical constraints on Jupiter's thermal structure, which lead to uncertainty bounds on the O/H ratio we derive. Preliminary work to date suggests that our technique may be able to determine whether or not the Galileo Probe Mass Spectrometer O/H measurement can be representative of the planet's bulk abundance, and whether O is supersolar in Jupiter like the other volatile elements C, N, and S. If we can distinguish between O/H lower limits of 10x and 3x solar, we will be able to test the hypothesis that Jupiter's volatiles must have been delivered via water ice clathrates. [This conference abstract is supported by NASA grant NNX11AM55G issued through the Outer Planets Research Program, and by grants NNX11AJ47G, NNX14AJ43G, and NNX1AJ41G through the Planetary Astronomy and Solar System Observations Programs.]