Future Exploration of Neptune's Atmosphere: Three Big Questions

Neptune stands apart from the other giant planets, possessing the most meteorologically active atmosphere in our solar system despite its great distance from the Sun.   Unlike sluggish Uranus, which was radically altered by collisional processes, Neptune may be typical of a whole class ice giant planets being discovered beyond our solar system.   And yet some of the basic dynamical, chemical and cloud-forming processes at work within this churning atmosphere, along with the competing influences of seasonally changing insolation and internal heat flux on atmospheric structure, remain an unresolved mystery. The European astronomy community is currently assembling a series of white papers to inform ESA's large cornerstone missions in the coming decades, and I've been asked to contribute ideas for Neptune orbital exploration.  In my opinion, future exploration of Neptune’s atmosphere must focus on what makes this world unique, so here's my list of top questions.

Neptune through the wavelengths, from visible-light imaging from Hubble, to near-infrared imaging from Keck (Credit I. de Pater) and thermal-infrared imaging from VLT (Credit:  G. Orton)

What powers the circulation and meteorology of an ice giant? Neptune provides an excellent test for models balancing seasonally dependent insolation (due to the 26^o axial tilt and the 165-year orbit) and excess internal heat flux (emission exceeding solar inputs by a factor of 2.6, the largest in the solar system).  The source of this intrinsic luminosity is uncertain, but it likely drives the complex meteorology of the troposphere and is the key factor distinguishing Neptune from Uranus, which has a negligible internal heat.  Neptune has a different relation between the banded cloud structures, atmospheric temperatures and the zonal wind structure when compared to Jupiter or Saturn.  Rapidly evolving convective cloud activity prevails at cooler mid-latitudes, with retrograde flow at the warmer equator and a high-latitude prograde jet confining a seasonally variable polar vortex of unusually high temperatures and unique chemical composition.  Dark ovals (such as the Great Dark Spot observed by Voyager 2) are sometimes associated with bright white orographic clouds at higher altitudes.  Neptune’s zonal winds are among the strongest in the solar system, possibly as a result of less atmospheric turbulence dissipating the energy when compared to Jupiter.  A future mission must correlate visible changes to cloud albedo, winds, eddies and vortices with environmental changes (e.g., latent heat release from cloud condensation, conversion between different spin states of molecular hydrogen, long term seasonal variability in temperature and composition) to understand the processes controlling the changing face of Neptune.

What is the origin and distribution of the zoo of chemical species in Neptune’s atmosphere?  Neptune’s atmospheric composition is determined by condensation chemistry (removing volatiles such as CH₄, NH₃, H₂S, and H₂O to the condensate phase), vertical mixing (dredging CO and possibly other species from the warmer interior), external influx of oxygenated species from infalling comets and dust, and a rich hydrocarbon photochemistry due to the UV destruction of methane.  Measurements of elemental enrichments (C/H, N/H, O/H), isotopic ratios (D/H, ¹³C/¹²C, ¹⁵N/¹⁴N) and noble gas abundances (via an entry probe) would provide constraints on the delivery of these materials to the forming proto-Neptune and conditions in the early solar system.  Furthermore, mapping the spatial distributions of cloud-forming volatiles, disequilibrium species and photochemical products teaches us about the chemical processes and cloud formation at work within the ice giant, and their variability from equator to pole.  The latitudinal distribution of methane will reveal whether it is enhanced by tropical uplift or by warming of the cloud trap at the seasonally heated poles. Indeed, the polar vortices are the sites of unique conditions due to a close connection with the planet’s magnetosphere, and require exploration via a high-inclination orbital phase. 

 

The highly variable atmpsphere imaged by Hubble in 2011 (left); and
high-altitude clouds of methane ice observed by Voyager in 1989 (right).

What are the atmospheric structure and cloud properties from the troposphere to the thermosphere?  Determinations of the three dimensional profiles of temperature, density, gaseous composition and aerosols provides the key to understanding the balance between internal heating, convective mixing, latent heat release and radiative heating and cooling throughout an ice giant atmosphere.  Vertical sounding should reveal the circulation regimes, zonal winds and turbulence characteristics at a variety of depths both within and above the tropospheric clouds; the nature and spatiotemporal variability of ice giant clouds and hazes; meridional motions in the stratosphere; and the importance of wave activity in redistributing energy and material with altitude.  The importance of wave breaking and ionosphere-magnetosphere drag processes should be determined in studying the abnormally high temperatures of the ionosphere and thermosphere.  This three-dimensional planetary-scale characterization of an ice giant atmosphere will provide a bridge between the deep circulation and the external magnetospheric environment, to be directly compared with Galileo, Cassini and JUICE reconnaissance of the gas giants.