Atmospheres of Solar System: a paradigm for physics and methodologies

in the study of exoplanets

The Giant Planets

D. Grassi, G. Sindoni, E. D'Aversa, F. Oliva,A. Adriani, M.L. Moriconi, G. Piccioni, G. Bellucci, A. Mura, N.I. Ignatiev

Outline

● Giant Planets as 'analogues' of exoplanets● Main physical properties of the atmospheres of

exoplanets● Hints form solar system studies on the nature of

exosystems● Italian expertise and space mission involvement

Giant Planets as 'analogues' of exoplanets

● For their radius and mass, giant planets of the solar system represents an obvious analogue of exoplanets

● Nonetheless, the very different orbital conditions (i.e. energy input from host star) suggest very different evolutionary pattern and physical state

Data from exoplanets.eu

Main physical properties of giant planets

● Mostly composed of light elements (but rocky/metal cores may reach several Earth masses) See Formisano et al. talk

● No proper 'solid' surface: the region observable from MW to UV is a gaseous atmosphere

● Ubiquitous occurrence of aerosols of different composition and sizes (hazes/clouds)

● Strong magnetic fields (of different origin)

Credit: D. E. Gary

The two classes

The 'Gas' Giants

● Smaller

● Higher content of C, N and O (x20)

● In the observable atmospheres: – Depleted in He

– Enriched in heavier elements x3-5

w.r.t solar abundance

The 'Icy' Giants

Different formation history => different composition => different structure

Uranus seen by Keck

Credit: Sromovsky et al., 2012

Neptune (Voyager 2)

Main drivers for atmospheric dynamics

● Rotation– Giant planets have fast 'rotation periods': strong zoned structure of the

atmosphere

– Vertical extent of zonal circulation is still unknown

● Internal heat– Jupiter has a net emission of energy: the heat drives vigorous convective

activity at different time scales

● Solar illumination– Despite remoteness of the Sun, in Uranus becomes a major driver in dynamic

due to lack of internal heat

● Input from magnetosphere– Is a main driver for Uranus upper atmosphere

Complex dynamics

● Absence of a surface (damping) allows the development of very complex atmospheric phenomena, including free-body resonances

● Nonetheless, Global Circulation models are available to properly reproduce a large array of observed phenomena

The Saturn hexagon (Cassini)NASA/JPL-Caltech/Space Science Institute

The Jupiter Red Spot (Voyager 1)Reprocessing by Bjorn Jonsson

Air composition

● H2 and He represent the bulk of observable atmospheres, but...

● Several minor species become important:– Different species are present in their

reduced forms (H2O, NH3, CH4, H2S), including very exotic ones (PH3, AsH3, GeH4)

– Hydrocarbon species (e.g.: C2H2, C2H6) are observed in the stratospheres, due to a rich CH4-based photochemistry

– Stratospheric gases are also subject to intense irradiation by magnetospheric ion and electrons Data from Taylor et al., 2004

Observable atmosphere

● Spectra of giant planets is dominated by the CH4

absorption bands● Ultimate sources of opacity

are represented by Rayleigh scattering toward visible and H2 CIA

● Visible and IR can not probe below 10 bar even in an aerosol-free atmosphere Taylor et al., 2004

Aerosols● H2O, H2S, NH3 condense in the cold upper troposphere to

create multiple and optically thick aerosol decks

● Organic photochemistry make ices far from being puure

● Icy giants are so cold that also CH4 condense to leave behind an almost pure H2 and He atmosphere

Jupiter (data from Seiff et al., 1998) Uranus (note different scales)

Analogues?

● Distance from host star plays a major role– Energy input by stellar radiation (much warmer

atmospheres, different aerosol composition)

– Tidal forces (likely tidal locking, solar-antisolar circulation, minor role of Coriolis)

– Sputtering from stellar wind (rises the issue of atmosphere lifetime)

● Even if basic physics remains the same, exoplanets allow us to explore a variety of regimes not met in Solar System

Conclusions

● Giant planets in the solar system are the closest available analogues to study exoplanets

● Close distances of exoplanets from host stars do not allow to extend much further the comparison

● A number of methodologies developed for the study of giant planets are being successfully extended to exoplanets

Export skills from Giant Planet community

● Global circulation models● Radiative transfer in optically-thick scattering

atmospheres● Scattering properties of aerosols● Microphysics of cloud formation ● Gaseous opacities at high T and P ● Optimal inverse retrieval schemes (Bayesian)● Atmospheric sputtering by energetic particles

Expertise available in the Italian planetary community

Mission involvement

● Italian community has grown in recent years, from experience maturated during Venus Express and Cassini missions, and in view of forthcoming Juno and JUICE mission to Jupiter

● Italian community actively involved in further mission proposals to giant planets (Uranus Pathfinder for M4)

● Contact: davide.grassi@iaps.inaf.it