The practice of astronomy is in many ways an intrinsically introspective endeavour. A significant fraction of astronomical motivation is derived from the desire to understand whether a habitable planet such as the Earth is a unique object, and, by extension, whether the inhabitants of the Earth themselves collectively represent a unique phenomenon. By definition, a world is considered potentially habitable if it is theoretically capable of supporting liquid water at its surface. But by focusing solely on strictly Earth-like planets, we risk overlooking other potentially habitable options. In fact, the majority of the worlds known to host liquid water oceans in the solar system are not Earth-like at all. These other worlds do however share a singular defining characteristic: they are the icy moons that orbit the gas giant planets. Their oceans are concealed below kilometers of frozen crust. In the solar system at least three moons are known to host an ocean with a high degree of confidence (Europa, Enceladus, and Titan), and another four are suspected (Ganymede, Callisto, Mimas, and Dione). Hence, any hope of answering the question as to how unique the phenomena of life on Earth really is may hinge predominantly on answering a seemingly unrelated question: how unique are the icy moons? The formation of gas giants appears to be accompanied by the formation of moons, as, at least in the solar system, the two appear inseparable. The gaseous planets Jupiter, Saturn, and Uranus, are each attended by a retinue of moons both regular and irregular. The regular satellites tend to orbit in a single plane, in the same direction, and on nearly circular paths. These peculiar properties are also exhibited by the planets, and hence it is considered likely that some similar process has been at play to form them both. That process is the formation within a swirling disk of gas and dust. Planets form within disks that surrounded a star (a circumstellar disk) while the moons would have formed within a disk surrounding their planet (a circumplanetary disk, or CPD). The last decade of strides made in the observation of circumstellar disks has revolutionized our understanding of the planet formation process. To what extent might the moon and planet formation process be similar? To what extent might we be able to extrapolate our understanding of circumstellar disks down to the scales characteristic of circumplanetary disks? Is there a smooth continuum in physical processes, connecting the formation of large moons, with the formation of the smallest planets? In this work we have extended the theoretical tools used to explore planet formation down into this new regime. On the observational side, as the scale of the astrophysical object shrinks, the capabilities of the instrument must rise commensurately to observe it. We are now at the earliest possible stage of directly observing CPDs to gain insights beyond the theoretical into how giant planet moon formation actually proceeds… @en

Context. The subsurface oceans of icy satellites are among the most compelling among the potentially habitable environments in our Solar System. The question of whether a liquid subsurface layer can be maintained over geological timescales depends on its chemical composition. The composition of icy satellites is linked to that of the circumplanetary disk (CPD) in which they form. The CPD accretes material from the surrounding circumstellar disk in the vicinity of the planet, however, the degree of chemical inheritance is unclear. Aims. We aim to investigate the composition of ices in chemically reset or inherited circumplanetary disks to inform interior modeling and the interpretation of in situ measurements of icy solar system satellites, with an emphasis on the Galilean moon system. Methods. We used the radiation-thermochemical code ProDiMo to produce circumplanetary disk models and then extract the ice composition from time-dependent chemistry, incorporating gas-phase and grain-surface reactions. Results. The initial sublimation of ices during accretion may result in a CO2 -rich ice composition due to efficient OH formation at high gas densities. In the case of a Jovian CPD, the sublimation of accreted ices results in a CO2 iceline between the present-day orbits of Ganymede and Callisto. Sublimated ammonia ice is destroyed by background radiation while drifting towards the CPD midplane. Liberated nitrogen becomes locked in N2 due to efficient self-shielding, leaving ices depleted of ammonia. A significant ammonia ice component remains only when ices are inherited from the circumstellar disk. Conclusions. The observed composition of the Galilean moons is consistent with the sublimation of ices during accretion onto the CPD. In this scenario, the Galilean moon ices are nitrogen-poor and CO2 on Callisto is endogenous and primordial. The ice composition is significantly altered after an initial reset of accreted circumstellar ice. The chemical history of the Galilean moons stands in contrast to the Saturnian system, where the composition of the moons corresponds more closely with the directly inherited circumstellar disk material.

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Context. Gaining a full understanding of the planet and moon formation process calls for observations that probe the circumplanetary environment of accreting giant planets. The mid-infrared ELT imager and spectrograph (METIS) will provide a unique capability to detect warm-gas emission lines from circumplanetary disks. Aims. We aim to demonstrate the capability of the METIS instrument on the Extremely Large Telescope (ELT) to detect circumplanetary disks (CPDs) with fundamental v = 1 0 transitions of 12CO from 4.5 to 5 μm. Methods. We considered the case of the well-studied HD 100546 pre-transitional disk to inform our disk modeling approach. We used the radiation-thermochemical disk modeling code ProDiMo to produce synthetic spectral channel maps. The observational simulator SimMETIS was employed to produce realistic data products with the integral field spectroscopic (IFU) mode. Results. The detectability of the CPD depends strongly on the level of external irradiation and the physical extent of the disk, favoring massive (∼10 MJ) planets and spatially extended disks, with radii approaching the planetary Hill radius. The majority of 12CO line emission originates from the outer disk surface and, thus, the CO line profiles are centrally peaked. The planetary luminosity does not contribute significantly to exciting disk gas line emission. If CPDs are dust-depleted, the 12CO line emission is enhanced as external radiation can penetrate deeper into the line emitting region. Conclusions. UV-bright star systems with pre-transitional disks are ideal candidates to search for CO-emitting CPDs with ELT/METIS. METIS will be able to detect a variety of circumplanetary disks via their fundamental 12CO ro-vibrational line emission in only 60 s of total detector integration time.

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Context. The large icy moons of Jupiter formed in a circumplanetary disk (CPD). CPDs are fed by vertically infalling circumstellar gas and dust which may be shock-heated upon accretion. Accreted material is then either incorporated into moons, falls into the planet, or is lost beyond the disk edge on relatively short timescales. If ices are sublimated during accretion onto the CPD we know there must be sufficient time for them to recondense or moons such as Ganymede or Callisto could not form. The chemical timescale to form sufficiently icy solids places a novel constraint on the dynamical behaviour and properties of CPDs. Aims. We aim to explore the process of ice formation in CPDs to constrain which disk properties (such as the mass, viscosity, and dust-to-gas ratio) are consistent with the formation of an icy moon system. Methods. We use the radiation thermochemical code ProDiMo (Protoplanetary Disk Model) to analyze how the radial ice abundance evolves in CPDs. We consider different initial chemical conditions of the disk to explore the consequences of infalling material being inherited from the circumstellar disk or being reset to atomic conditions by shock-heating. We contrast the timescales of ice formation with disk viscous timescales and radial dust drift. Results. We have derived the radial ice abundance and rate of ice formation in a small grid of model CPDs. Water ice can form very efficiently in the CPD from initially atomic conditions, as a significant fraction is efficiently re-deposited on dust grains within <1 yr. Radial grain drift timescales are in general longer than those of ice formation on grains. Icy grains of size a < 3 mm retain their icy mantles while crossing an optically thin circumstellar disk gap at 5 au for L < 10 L. Conclusions. Three-body reactions play an important role in water formation in the dense midplane condition of CPDs. The CPD midplane must be depleted in dust relative to the circumstellar disk by a factor 10-50 to produce solids with the ice to rock ratio of the icy Galilean satellites. The CPD snowline is not erased by radial grain drift, which is consistent with the compositional gradient of the Galilean satellites being primordial.

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Context. The Galilean satellites are thought to have formed from a circumplanetary disk (CPD) surrounding Jupiter. When it reached a critical mass, Jupiter opened an annular gap in the solar protoplanetary disk that might have exposed the CPD to radiation from the young Sun or from the stellar cluster in which the Solar System formed. Aims. We investigate the radiation field to which the Jovian CPD was exposed during the process of satellite formation. The resulting photoevaporation of the CPD is studied in this context to constrain possible formation scenarios for the Galilean satellites and explain architectural features of the Galilean system. Methods. We constructed a model for the stellar birth cluster to determine the intracluster far-ultraviolet (FUV) radiation field. We employed analytical annular gap profiles informed by hydrodynamical simulations to investigate a range of plausible geometries for the Jovian gap. We used the radiation thermochemical code PRODIMO to evaluate the incident radiation field in the Jovian gap and the photoevaporation of an embedded 2D axisymmetric CPD. Results. We derive the time-dependent intracluster FUV radiation field for the solar birth cluster over 10 Myr. We find that intracluster photoevaporation can cause significant truncation of the Jovian CPD. We determine steady-state truncation radii for possible CPDs, finding that the outer radius is proportional to the accretion rate Ṁ 0.4. For CPD accretion rates Ṁ < 10-12M- yr-1, photoevaporative truncation explains the lack of additional satellites outside the orbit of Callisto. For CPDs of mass MCPD < 10-6.2M-, photoevaporation can disperse the disk before Callisto is able to migrate into the Laplace resonance. This explains why Callisto is the only massive satellite that is excluded from the resonance.

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