Abstract
Constraints on the formation of giant planets are provided by comparing elemental and isotopic abundances from Jupiter via the Juno space mission and comets from various observing platforms, extending to other extrasolar Jovian planets thanks to abundance determinations by JWST. Very recent results from Juno suggest some interesting similarities and differences from comets, and from JWST a not-unexpectedly wide range of elemental abundances.
Introduction
Giant planets range in size and type from Jovian gas giants to Uranus/Neptune ice giants (which might, as regards Uranus at least, be a rock giant; Feuchtgruber et al. 2013). The gaseous envelopes of the giant planets contained the residue of solid bodies that contributed to the growth of these planets during their formation, as well as possible post-formation contamination. Identifying the original composition of the planetesimals or smaller pebbles (Lambrechts and Johansen, 2012) and comparison with either nebular models or the measured composition of primitive bodies can constrain where the solid material came from and, to a limited extent, the processes by which it was delivered (e.g., Pacetti et al. 2023; Mousis et al. 2021).
In our own solar system, Uranus and Neptune would seem to provide the clearest signature of solid body accretion given that their hydrogen-helium envelopes are only a minor fraction (of order 10%) of their mass. However, scantily little data are available on their heavy element abundances. In fact, we have a complete inventory of the major elements only for Jupiter, and that was accomplished only over a long period of thirty years from the Galileo Probe to the Juno mission. For comparison with the Jupiter data, many comets and meteorites (as proxies for asteroids) have measured major element abundances and for the latter, noble gases, but noble gas data exist for only one comet.
Beyond the solar system, JWST is providing high resolution, high signal to noise spectra that are allowing major element abundances and metallicities to be determined for giant planets that range over masses from sub-Neptune to super-Jupiter. Of particular interest because of their high abundance as elements and their propensity to condense are carbon and oxygen. Predicting the C/O ratio in models of protoplanetary disks is an important way to connect such models with observations of planets (Öberg et al. 2011). Comparison of the carbon-to-oxygen ratio (hereafter, C/O) in Jupiter to that in comets and extrasolar giant planets is now illuminating how variable formation environments of giant planets might be.
Jovian C/O and metallicity
The ingoing assumption when the Galileo probe was designed and fitted with a mass spectrometer was that the 10-bar level to which the probe could transmit would be (i) sufficiently deep to be below the meteorological layer and (ii) representative of the heavy element abundances throughout a well-mixed hydrogen-helium interior (see Stevenson 2020 for a detailed review). Neither of these assumptions has been borne out by the data. With respect to assumption (i), while the carbon abundance in the envelope through the dominant molecular carrier CH4 was constant with pressure and clearly representative of a non-condensable species with a 3 to 5 times solar value, the molecular carrier of oxygen, water, exhibited a subsolar value that increased weakly down to through the 20-bar depth to which the probe operated (Wong et al. 2004). Since the Galileo probe fell by chance into a so-called 5-micron hot spot which is thought to be a region of overall subsidence, the subsolar value of water measured by the Galileo Probe Mass Spectrometer is generally regarded as reflective of meteorological drying out of water associated with its role as the major condensable in Jupiter’s atmosphere.
The realization that assumption (ii) might be wrong has come much more recently and is a long and delicate argument covered in the following three paragraphs. The Juno mission, proposed and selected in the NASA New Frontiers program, had as a primary goal the measurement of the water abundance deep in Jupiter’s envelope (hundreds of bars pressure) and over a range of latitudes by a microwave radiometer (Janssen et al. 2017). The determination of water is tricky, because the measured brightness temperature is a function of the abundances of microwave absorbers and the physical temperature profile. Ammonia is a very strong absorber of microwaves as well, and the physical temperature profile at any given latitude is not a priori known. After arriving in Jupiter orbit in 2016, Juno found that the ammonia vertical profile is not constant down to significant depths, tens of bars pressure, well below the expected base of the ammonia or mixed ammonia-water clouds. One model involves the formation of large water-ammonia hailstones (“mushballs”) at high altitudes, which survive to well below the cloud base, releasing ammonia and water into the microwave-detected gas phase (Guillot et al. 2020).
As a result, a painstaking effort has been required to remove the vertically and latitudinally variable ammonia profile to reveal the water abundance at deeper levels, in an uncertain physical temperature profile. Being able to measure at different emission angles yields limb darkening which provides additional information to the analysis (Zhang et al. 2020). Only near the equator is the temperature profile such that it can be untangled from the opacity sources in the microwave data, yielding a water abundance between 1.5 and 8.3 times solar (Li et al. 2024).
Juno has other ways to determine the oxygen abundance. In particular, in the deep interior, the shape of the gravitational field measured to exquisite precision by Juno limits the abundance of O (excluding the core) to at most twice solar (Howard et al. 2023a), assuming no other heavy elements. However, the 3-5 times solar carbon measured by Galileo in envelope is, if representative of the deep interior, already the equivalent of twice solar O, or more, and so there is no room even for a solar abundance of water in the deep interior. Put simply, the Z-enrichment in the atmosphere appears to be too large to be consistent with the Z- composition of the deep interior (again, excluding the core).
Various explanations include problems with the equation-of-state, layering and non-adiabaticity, and a higher internal temperature (offsetting the density of the Z-elements). None of these seem to work. Another alternative is that the atmosphere and deep interior have not mixed and have different heavy element abundances. Leaving aside the dynamical challenges of maintaining a denser overlying layer for billions of years (Howard et al. 2023b), one may then ask the question of how the atmosphere became more enriched in Z-elements. The obvious answer given Jupiter’s location is that it accreted solid material after formation, when the gaseous envelope had reached its full mass (Shibata and Helled, 2022).
What might this additional solid material have been? An obvious answer would be comets. By way of example, Lunine et al. (AGU meeting presentation, 2023) used the abundances measured in the Jupiter family Comet 67P/Churyumov-Gerasimenko (“67P”) by the Rosetta mission from a variety of published papers. Normalizing relative to solar, they find C/O in the atmosphere to be consistent with that in 67P. Furthermore, 67P is unusual in having noble gas determinations (Ar, Kr, Xe) that are lacking for many outer solar system bodies, but the Galileo Mass Spectrometer measured these in Jupiter. The Kr/Xe ratio in Jupiter and 67P are consistent, but Ar/Xe is not – it is too low in 67P. One explanation is that the relatively volatile argon has been preferentially lost from 67P over time, such that when comets like it added Z elements to Jupiter’s atmosphere, they contained more argon (Figure 1). What is not consistent between Jupiter and 67 P is the isotopic ratios of the stable xenon isotopes. Further, while a nitrogen isotopic ratio 15N/14N has not been published for 67P, the value for other comets is inconsistent with the Jovian envelope.
It is somewhat disappointing that one must contemplate the possibility that the atmospheric C/O ratio measured painstakingly from Galileo and Juno data might not be the bulk value in the deep interior, if the material that seeded the atmosphere with Z-elements were somehow different in that ratio from the bulk of the heavy elements that built up Jupiter. A sampling of many more comets would be helpful, but a perspective is also available now from JWST.
Extrasolar giants with JWST
Hot Jupiters are in principle more straightforward targets for determining C/O ratios because the molecular carriers in these high temperature atmospheres are all in the gas phase. However, results obtained with HST and Spitzer had large error bars in both the C/O ratios (where determined) and the total metallicity (Line et al. 2021). JWST’s performance makes determination of these quantities much more precise (Figure 2). Here are three examples from the GTO program (Lunine #1274), where the data analysis has been performed in collaboration with the group of Professor Jacob Bean at the University of Chicago.
HD149026b, a.k.a Smertrios (Bean et al. 2023). This is a Saturn-mass planet exoplanet with known high bulk metallicity (280 + 26 times solar) based on its density. JWST spectra obtained in secondary eclipse with NIRCAM allow us to obtain an atmospheric metallicity of 59-275 times solar and a C/O ratio of 0.84 + 0.03. Note that the atmospheric metallicity is less than that of the interior, but this does not account for the possibility of a large and pure heavy element core which in the case of Jupiter is diagnosed from gravity data and removed from the comparison with the atmosphere. We cannot perform the same correction for Smertrios in the absence of any gravity data (beyond the radial-velocity-determined mass). We can at least say that the atmospheric and envelope mass could be the same, and that there is no evidence for an atmosphere with elevated metallicity relative to the deep interior. The C/O ratio is larger than the standard quoted value (Lodders 2021), but a recent novel approach to determining elemental ratios in the Sun might lead to an elevated value approaching that of Smertrios (Ngoc Truong et al., in prep. 2024).
Wasp 77Ab (August et al. 2023): This is a hot superJupiter (1.8 Jupiter masses) with a radius larger than Jupiter’s. Our work yields improved metallicity and C/O values from earlier HST and Spitzer observations; our JWST NIRSpec dayside spectra yield a metallicity between 0.08-0.21 solar and a C/O of 0.36 (+0.10, -0.09). The very low metallicity is surprising even for an object with such a massive envelope, suggesting that the gaseous envelope was depleted of Z-elements before collapsing onto the core during formation. However, one might then expect the least volatile materials to be depleted from the gas by condensation, leading to a high C/O as water was preferentially depleted from the gaseous disk.
HD209458b (Xue et al. 2024): This iconic exoplanet was the first to be detected by transit, about 70% the mass of Jupiter and 1.35 times the radius of Jupiter. Conflicting pre-JWST determinations of the abundance of water and detection of other species have been resolved by our primary transit NIRCam observations. Water and carbon dioxide are present, as well as methane, acetylene and hydrogen cyanide. The metallicity is between 2-9 times solar and the C/O an astonishingly low 0.08 (+0.09, -0.05). Thus, the bulk of the metallicity is contributed by O and the resulting O enrichment relative to solar identical to the Juno-derived value for Jupiter.
Taken as a set, including Jupiter, there seems to be a wide variation in metallicity and C/O ratios from system to system. Only one of our objects fall on the atmospheric metallicity-mass trend seen for the solar system’s giants (Figure 3), namely HD209458b. That giant planet, closest in mass to Jupiter, has an identical water enrichment to the equatorial value on Jupiter as determined by Juno. It appears to be one that compositionally would have been much at home in our solar system. Whether this is significant will require determination of O abundances in many more hot Jupiters. But assuming this to be more than coincidence, it argues that the water enrichment in the Jovian atmosphere is indeed the enrichment in the deep interior, and that the discrepancy between the gravity-determined metallicity and that of the atmosphere lies in something other than the metallicity itself – for example, in the equation of state of hydrogen and helium at high pressures. Thanks to the powerful observational capabilities of Juno and JWST, the field of giant planet compositional studies is greatly expanding and in foment.
Acknowledgements
The author’s Juno work is supported by a subcontract from the Southwest Research Institute and the JWST analysis under contract NNX17AL71A through NASA GSFC.
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