A Giant Planet Spectroscopy Wish List

Last week a colleague of mine (Glenn Orton, senior research scientist at JPL) presented a review talk on outer solar system spectroscopy in Ohio, urging a closer marriage between those taking laboratory spectroscopic data and the end users, I.e., those like me who are desperate for high quality spectroscopic parameters to compare to measured spectra of the giant planets.  Infrared spectroscopy in particular is the only technique for understanding planetary temperatures, composition and aerosols on the giant planets, as we have had only a single direct measurement of Jupiter's soup of atmospheric species from the Galileo probe in 1995.  Here are some notes taken from his talk, leading to a wish list for new spectroscopic measurements at the end.

A Brief History

The history of outer solar system spectroscopy can be traced back to 1932 when Rupert Wildt identified the absorption features of ammonia and methane in Slipher's visible spectra of Jupiter and Saturn from 1905.  Infrared detection of methane by Gerard Kuiper came in 1947, and Spinrad first identified hydrogen absorption in 1963. The 1970s saw the emergence of high resolution infrared spectroscopy with Beer et al. 1974 discovering Jupiter's CH3D; Larson et al. 1974 finding deep water features at 5 microns using the Kuiper Airborne Observatory; and Ridgeway et al. (1974) observing Jupiter's stratospheric hydrocarbons, ethane and acetylene at 10 microns from Kitt Peak.  Gillett et al. (1973) explored the mid infrared in more depth to discover tropospheric absorption features of ammonia and phosphine.  At even longer wavelengths, the first far infrared measurements of the giants came from the radiometer on board Pioneer 10 and 11 in the 1970s, and the IRIS spectroscopic studies of the Voyager spacecraft put infrared giant planet studies on a firm footing in the 1980s.  In more recent times, infrared spectroscopy has been provided by Galileo and Cassini, ground based telescopes along with earth proximal observatories like Spitzer and Herschel.

Temperatures, Composition and Clouds

The primary product of infrared remote sensing is the atmospheric temperature structure from the cloud forming regions up to the stable stratosphere.  These derivations rely on well mixed constituents to serve as thermometers, including methane and hydrogen, although there is mounting evidence that methane is not particularly well mixed on the ice giants.  The different line intensities means that we can probe a broad range of altitudes.  Once we have a good estimate of the temperature, the molecular features can be used to determine composition.  For example, He/H2 can be derived from far-IR spectrum using the differences in absorption between the long-wavelength translational band and broad rotational transitions of H2-H2 versus H2-He collision-induced absorption.  The fact that helium appears deleted on Jupiter and Saturn has been used as evidence for helium segregation within their interiors.  The abundance of deuterium can be used as an indicator of planetary origins, with the D/H ratio in Jupiter and Saturn close to protosolar, whereas that in Uranus and Neptune suggests an enrichment by planetary ices during their formation.

The emergent spectrum of the giant planets is governed by a wide variety of chemical and dynamical processes.  Vertical mixing from the feel troposphere dredges disequilibrium products like phosphine, arisen, germane and CO to the upper troposphere; photochemistry initiated by the UV destruction of methane creates a soup of hydrocarbon species in the stratosphere; and an external influx of oxygenated species to the upper atmosphere provides trace amounts of water, CO and CO2 in the high atmosphere.  

However, determination of chemical abundances and temperatures relies on a knowledge of the broadband contributions to the emergent spectra by aerosols, which are often so broad that the are impossible to identify uniquely.  Ammonia ice, for example, has long been suspected to be the dominant constituent of Jupiter's upper cloud decks, but has only been spectroscopically identified in regions of powerful convective updrafts like the wake of the Great Red Spot.  We sound the vertical properties of clouds by moving in and out of strong methane and hydrogen absorptions in the near infrared - seeing bright clouds in strong absorption bands implies that they are at high altitude, whereas seeing them near the continuum means that they are deep.  An accurate knowledge of the methane line data, as well as the broad spectra of condensates, is therefore essential for accurate interpretations.

A Giant Planet Spectroscopists Wish List

Orton finished his presentation with a spectroscopic wish list for the outer planets community, and calls for a closer connection between the lab spectroscopists and the end users.  His list included:

• Methane: Further studies of methane mostly at the shortest wavelengths where the transitional structures are becoming prohibitively complex, even for modern techniques. 
• Broadening: Collisional-broadening widths and shapes are required for all lines as a function of temperature, particuarly CH4), as we currently use a handful of real measurements and extrapolate them for all lines of a species.  These could be added to a HITRAN/GEISA-like spectroscopic data base for H2/He collisions relevant to the giant planets.
• Hydrocarbons: Characterisation of the full suite of complex hydrocarbons predicted and observed on the giant planets, in addition to nitriles observed on Titan.
• Collision Induced Absorption: Re-examination of collision induced absorptions, including N2-CH4 CIA spectra at appropriate temperatures for Titan, which are known to be off by factors of several; and hydrogen CIA measurements at a broad range of temperatures (I.e., below 40 K and above 400 K).
• Condensates and Hazes: Refractive index spectra of potential condensates in the giant planets.  For example, diphosphene and hydrazine are expected by products of tropospheric photochemistry that could dominate the hazes on Jupiter and Saturn, but their spectra properties are virtually unknown.  We need a library of condensate spectra, e.g. mixtures of NH3 and NH4SH ice with photochemically produced hazes for comparison with broadband planetary observations.
• Hot Giant Planets: Characterisation of “hot spectra” of several constituents for exoplanet spectral analysis.  The brunt of this effort is being led by ab initio quantum modelling (for example the Exomol project led by UCL here in the UK, www.exomol.com), but there is a need for verification by laboratory spectroscopy at temperatures of several thousands of kelvin.

In summary, infrared spectroscopy of the giant planets can reveal a great deal about their temperatures, composition and aerosols, but the results will only ever be as good as the underlying radiative transfer modelling.  Those models are totally dependent on accurate and precise databases of spectroscopic parameters, and laboratory studies should continue to play a major role in supporting missions and ground based observations.

Depth of Zonal Winds on the Giants

Planetary scientists studying giant planet atmospheres continue to struggle with the most basic of questions:  are the jets confined to a shallow layer within and immediately below the clouds (e.g., Hess and Panofsky, 1951), or do the cloud-level jets extend through the molecular envelopes?  Or is it some hybrid of the two?

The "deep structure" scenario stems from the observation that internal heat fluxes are transported by convection, and convection homogenises entropy and density to create near-barotropic conditions in the molecular envelope.  Taylor-Proudman theorem then implies that the jets will be constant on cylinders parallel to the rotation axis (Taylor columns), extending far below the cloud-decks.  However, at cloud-level, the conditions are non-barotropic (temperature gradients and wind shears exist) and winds in this outer baroclinic zone would not necessarily represent the speeds of Taylor columns in the interior barotropic zone.

But even with this deep structure, there must be some depth to which strong zonal jets can no longer penetrate.  Lorentz forces in the metallic hydrogen regions (i.e., the dynamo-generating region responsible for the magnetic fields) would brake strong zonal flows (Kirk and Stevenson, 1987) at pressures exceeding 1-2 Mbar.  Liu et al. (2008) also suggest that the jets cannot reach the base of the molecular envelope, where a smooth transition to metallic, conducting properties would be occurring.  Indeed, Liu calculated that Ohmic dissipation would be much larger than the observed luminosities of Jupiter and Saturn if the Taylor columns penetrated deeper than 96% (Jupiter) or 86% (Saturn) of the radius, and hence favoured a scenario with winds confined to the weather layer.  But there are methods to reduce the Ohmic dissipation with full 3D magnetohydrodynamic calculations (e.g., Glatzmaier 2008), so the jury is still out on how deep Taylor columns could extend without Ohmic dissipation becoming a substantial problem.

Showman et al. (2006) showed that deep jets can result from both shallow forcing (e.g., jet pumping by eddy momentum flux convergence from thunderstorms or baroclinic instabilities) and deep forcing (e.g., convection in the molecular envelope).  In their models, they get a baroclinic thermal wind region overlying a barotropic jet region to the base of the model, suggesting that the deep winds measured by the Galileo probe at 20 bar on Jupiter (Atkinson et al., 1998) doesn't imply a deep source of the winds.  Conversely, deep forcing (e.g., waves generated by convection in the molecular envelope) could induce zonal winds at shallow depths.  Del Genio et al. (2009) emphasise that deep vs. shallow structure is a different (but related) problem to the question of deep vs. shallow forcing of the zonal jets, and there are a wide variety of models out there that choose one approach or the other, each trying (often successfully) to reproduce the cloud-level wind field.  But the basic question remains - how do the zonal winds vary with depth, and do they remain constant down to the expected water cloud and below?

The problem is that almost all our information (temperatures, winds, dynamical tracers) comes from the cloud-forming region, where the complex transition from barotropic interior to baroclinic outer zone is taking place.  Here we believe that moist adiabatic ascent (i.e., powered by latent heat from water condensation) powers the overturning circulation and eddies power the zonal jets.  But the picture remains complicated, as the typical idea of air rising in anticyclonic zones and descending in warm, dry cyclonic belts could in fact be reversed at the depth of the water cloud (zones are warm, belts are cool so that moist adiabatic ascent takes place in belts).  This overturning circulation in the upper tropospheric cloud forming region, well above the deep molecular envelope, will be the topic of a future post.

Just recently, Kaspi et al. (2013) have used gravity field data from the Voyager 2 flybys of Uranus and Neptune to suggest that their atmospheric wind structure must be shallow, and they suggest that shallow forcings (e.g., moisture driven motions) are responsible for their windfields.  This sort of gravitational field mapping will be performed by Juno and Cassini for Jupiter and Saturn, respectively, towards the end of this decade.  Combined with microwave radiometry, this is one promising approach for peering beneath the clouds to understand how deep these zonal jets penetrate.