Oxygen and Other Volatiles in the Giant Planets and their Satellites

Abstract

Giant planet atmospheric composition and satellite densities provide insights into protoplanetary disk conditions. Abundances of condensable species and noble gases in wellmixed atmospheres can distinguish among several giant planet formation scenarios, and satellite densities are fi rst order measurements of ice:rock ratios. Recent work on protosolar abundances, relying on three-dimensional spectroscopic modeling of the solar photosphere, provides the framework for the interpretation of measurements.Model densities of protoplanetary disk condensates are shown as a function of carbon partitioning between CO, CH 4 and organics. Comparison with observed satellite densities shows that Saturn's icy satellites are inconsistent with solar composition, and must either have formed in a water-rich environment or have suffered a complex collisional history. The larger satellites of the giant planets are consistent with solar composition, with densities that speak of variation in the partitioning of carbon. 68_Oxygen.indb 219 68_Oxygen.indb 219 1/9/2008 11:13:34 PM 1/9/2008 11:13:34 PM 220 Wong et al.Thermochemical equilibrium calculations predict water as the deepest tropospheric cloud on Jupiter, the planet with the best-constrained bulk water abundance. Yet cloud base pressure levels, remote spectroscopic water vapor measurements, and in situ mass-spectral measurements have all been unable to distinguish conclusively between subsolar and supersolar Jovian bulk water abundances, due to modeling assumptions and/or the spatially-variable water vapor distribution in Jupiter's troposphere. Modeling of images of lightning fl ashes is consistent with supersolar water abundances.Galileo probe measurements are consistent with an enrichment factor of 4±2 over the protosolar values for most volatiles other than water (C, N, S, and the noble gases Ar, Kr, and Xe). With that of oxygen unknown, Jupiter's enrichments of other volatiles could be explained in terms of enrichment by heretofore unidentifi ed solar composition icy planetesimals, by planetesimals containing volatiles trapped in water ice clathrates, or by enriched gas in the evolved disk. All models involving delivery of elements by planetesimals require planetesimal formation at temperatures below 40 K, to trap argon and molecular nitrogen. Although atmospheric C/H ratios have been measured for all four giant planets, a conclusive test of the competing formation scenarios cannot be made until O/H is measured on all four planets (extremely diffi cult on Uranus and Neptune), and abundances of the other volatiles and noble gases are measured for the outer three.

Methane, ammonia, and their irradiation products at the surface of an intermediate-size KBO?

“…As described by Desch et al (2009), the presence of only a few percent of ammonia in the interior (by weight, relative to water) can efficiently lower the viscosity of the ammonia-water mixture once is has melted. Please note however that large amounts of ammonia might be incompatible with a high dust/ice ratio such as the one used on our model (Wong et al 2008). Our simulations show that we cannot rule out a cryovolcanic episode in Orcus' past life, which probably supplied most of the observed crystalline water ice of the surface.…”

Jupiter’s Deep Cloud Structure Revealed Using Keck Observations of Spectrally Resolved Line Shapes

“…Since the top is at higher altitude than the base, cloud top constraints provide lower limits to the pressure of the cloud base, or, lower limits to the deep abundance of water. A spectrum requiring a water cloud at P 5  bar would establish a supersolar enrichment of water in Jupiter, better constraining planetary formation models (Wong et al 2008). Figure 4 shows the effect of water cloud pressure level on model fits to a cloudy zone spectrum at 32S.…”

“…The cloud base is therefore found at P 4 5 > -bar. Following Figure 1 in Wong et al (2008), a cloud base at 4-5 bar corresponds to O/H ratios 0.33-1.1×solar (corrected to the new solar O/H ratio of Asplund et al 2009). Our observations thus provide a lower limit to Jupiter's water abundance of 0.33-1.1× solar.…”

“…Ground-based imaging has shown that 5 μm hot spots, such as the one entered by the Galileo probe, cover less than 1% of the surface area of Jupiter (Orton et al 1996). Thus, the H 2 O abundances observed by the probe may be characteristic of all or most hot spots, but they are probably not representative of Jupiter as a whole, especially since a range of other indirect studies (lightning flash depths, the tropospheric CO abundance, and discrete clouds at P 4  bar) point to solar or supersolar water abundances (Wong et al 2008). Since the Galileo probe found that carbon, sulfur, and nitrogen were enriched by ∼4× solar (Wong et al 2004b), oxygen may also be enhanced by the same amount.…”

The comparative exploration of the ice giant planets with twin spacecraft: Unveiling the history of our Solar System

“…The contribution of high-Z elements provided by this phase of late accretion could have contributed to the super-solar abundances of C, N, S, Ar, Kr and Xe in the atmosphere of Jupiter measured by the probe released by the NASA mission Galileo and those measured in the atmospheres of the other giant planets (see Wong et al 2008 for a more in-depth discussion of the measured abundances and the proposed causes and Turrini, Nelson & Barbieri 2014). All these remixing events, moreover, affect the source materials, captured in the form of planetesimals by the circumplanetary disks, from which the regular satellites of the giant planets can form (see for a review).…”