Enceladus, an icy moon of Saturn, possesses an internal water ocean and jets expelling ocean material into space. Cassini investigations indicated that the subsurface ocean could be a habitable environment having a complex interaction with the rocky core. Further investigation of the composition of the plume formed by the jets is necessary to fully understand the ocean, its potential habitability, and what it tells us about Enceladus’s origin. Moonraker has been proposed as an ESA M-class mission designed to orbit Saturn and perform multiple flybys of Enceladus, focusing on traversals of the plume. The proposed Moonraker mission consists of an ESA-provided platform with strong heritage from JUICE and Mars Sample Return and carrying a suite of instruments dedicated to plume and surface analysis. The nominal Moonraker mission has a duration of ∼13.5 yr. It includes a 23-flyby segment with 189 days allocated for the science phase and can be expanded with additional segments if resources allow. The mission concept consists of investigating (i) the habitability conditions of present-day Enceladus and its internal ocean, (ii) the mechanisms at play for the communication between the internal ocean and the surface of the South Polar Terrain, and (iii) the formation conditions of the moon. Moonraker, thanks to state-of-the-art instruments representing a significant improvement over Cassini's payload, would quantify the abundance of key species in the plume, isotopic ratios, and the physical parameters of the plume and the surface. Such a mission would pave the way for a possible future landed mission.

Context. Recent high-contrast imaging surveys, using the Spectro-Polarimetic High contrast imager for Exoplanets REsearch (SPHERE) or the Gemini Planet Imager in search of planets in young, nearby systems, have shown evidence of a small number of giant planets at relatively large separation beyond 10–30 au, where those surveys are the most sensitive. Access to smaller physical separations between 5 and 30 au is the next step for future planet imagers on 10 m telescopes and the next generation of extremely large telescopes in order to bridge the gap with indirect techniques such as radial velocity, transit, and soon astrometry with Gaia. In addition to new technologies and instruments, the development of innovative observing strategies combined with optimized data processing tools is participating in the improvement of detection capabilities at very close angular separation. In that context, we recently proposed a new algorithm, Keplerian-Stacker, which combines multiple observations acquired at different epochs and takes into account the orbital motion of a potential planet present in the images to boost the ultimate detection limit. We showed that this algorithm is able to find planets in time series of simulated images of the SPHERE InfraRed Dual-band Imager and Spectrograph (IRDIS) even when a planet remains undetected at one epoch. Aims. Our goal is to test and validate the K-Stacker algorithm performances on real SPHERE datasets to demonstrate the resilience of this algorithm to instrumental speckles and the gain offered in terms of true detection. This will motivate future dedicated multi-epoch observation campaigns of well-chosen, young, nearby systems and very nearby stars carefully selected to search for planets in emitted and reflected light, respectively, to open a new path concerning the observing strategy used with current and future planet imagers. Methods. To test K-Stacker, we injected fake planets and scanned the low signal-to-noise ratio (S/N) regime in a series of raw observations obtained by the SPHERE/IRDIS instrument in the course of the SPHERE High-contrast ImagiNg survey for Exoplanets. We also considered the cases of two specific targets intensively monitored during this campaign: β Pictoris and HD 95086. For each target and epoch, the data were reduced using standard angular differential imaging processing techniques and then recombined with K-Stacker to recover the fake planetary signals. In addition, the known exoplanets β Pictoris b and HD 95086 b previously identified at lower S/N in single epochs have also been recovered by K-Stacker. Results. We show that K-Stacker achieves a high success rate of ≈100% when the S/N of the planet in the stacked image reaches ≈9. The improvement of the S/N is given as the square root of the total exposure time contained in the data being combined. At S∕N < 6−7, the number of false positives is high near the coronagraphic mask, but a chromatic study or astrophysical criteria can help to disentangle between a bright speckle and a true detection. During the blind test and the redetection of HD 95086 b, and β Pic b, we highlightthe ability of K-Stacker to find orbital solutions consistent with those derived by the current Markov chain Monte Carlo orbital fitting techniques. This confirms that in addition to the detection gain, K-Stacker offers the opportunity to characterize the most probable orbital solutions of the exoplanets recovered at low S/N.

A key feature of the Galilean satellite system is its monotonic decrease in bulk density with growing distance from Jupiter, indicating an ice mass fraction that is zero in the innermost moon Io and about half in the outer moons Ganymede and Callisto. Jupiter-formation models, and perhaps the Juno spacecraft water measurements, are consistent with the possibility that the Jovian system may have formed, at least partly, from ice-poor material. And yet, models of the formation of the Galilean satellites usually assume abundant water ice in the system. Here, we investigate the possibility that the Jovian circumplanetary disk was populated with ice-depleted chondritic minerals, including phyllosilicates. We show that the dehydration of such particles and the outward diffusion of the released water vapor allow condensation of significant amounts of ice in the formation region of Ganymede and Callisto in the Jovian circumplanetary disk. Our model predicts that Europa, Ganymede, and Callisto should have accreted little, if any, volatiles other than water ice, in contrast to the comet-like composition of Saturn’s moon Enceladus. This mechanism allows for the presence of ice-rich moons in water-depleted formation environments around exoplanets as well.

The supersolar abundances of volatiles observed in giant planets suggest that a compositional gradient was present at the time of their formation in the protosolar nebula. To explain this gradient, several studies have investigated the radial transport of trace species and the effect of icelines on the abundance profiles of solids and vapors formed in the disk. However, these models only consider the presence of solids in the forms of pure condensates or amorphous ice during the evolution of the protosolar nebula. They usually neglect the possible crystallization and destabilization of clathrates, along with the resulting interplay between the abundance of water and those of these crystalline forms. This study is aimed at pushing this kind of investigation further by considering all possible solid phases together in the protosolar nebula: pure condensates, amorphous ice, and clathrates. To this end, we used a one-dimensional (1D) protoplanetary disk model coupled with modules describing the evolution of trace species in the vapor phase, as well as the dynamics of dust and pebbles. Eleven key species are considered here, including H2O, CO, CO2, CH4, H2S, N2, NH3, Ar, Kr, Xe, and PH3. Two sets of initial conditions are explored for the protosolar nebula. In a first scenario, the disk is initially filled with icy grains in the forms of pure condensates. In this case, we show that clathrates can crystallize and form enrichment peaks up to about ten times the initial abundances at their crystallization lines. In a second scenario, the volatiles were delivered to the protosolar nebula in the forms of amorphous grains. In this case, the presence of clathrates is not possible because there is no available crystalline water ice in their formation region. Enrichment peaks of pure condensates also form beyond the snowline up to about seven times the initial abundances. Our model can then be used to compare the compositions of its different volatile reservoirs with those of comet C/2016 R2 PanSTARRS, Jupiter, Uranus, and Neptune. We find that the two investigated scenarios provide compositions of solids and vapors consistent with those observed in the bodies considered.

&lt;p&gt;How volatiles were incorporated in the building blocks of planets and small bodies in the protosolar nebula (PSN) remains an outstanding question. Some scenarios consider that planetesimals formed from a mixture of refractory material and volatiles trapped in amorphous ice in the outer nebula, while others hypothesize that volatiles have been incorporated in clathrates or formed pure condensates [1,2]. Here, we study the evolution of volatiles species in the PSN (H2O, CO, CO2, CH4, H2S, N2, NH3, Ar, Kr, Xe and PH3) considering two possible volatiles reservoir in the initial state: amorphous ice (see Figure 1) or pure condensates (see Figure 2).&amp;#160; To do so, we use a 1D disk accretion model [3]&amp;#160; with radial transport of trace species to compute the radial distribution of volatiles in the PSN. This model includes condensation/sublimation rates of pure condensates, as well as clathration/release rates when enough crystalline water is available. Figure 1 represents the case where volatiles are initially delivered to the PSN in the form of pure condensates. Figure 2 represents the case where volatiles are delivered to the PSN by amorphous ice. Species are released when amorphous grains cross the ACTZ region. Once delivered to the disk, the phase (solid or gaseous) of each species is ruled by the positions of its corresponding condensation and clathration lines. Clathration lines of the considered volatiles are closer to the Sun than their respective condensation lines, except for CO wich have its clathration line further from the sun than its condensation line. Gaseous volatiles condense or become entrapped (depending on the availability of water ice) when diffusing outward the locations of their lines. Conversely, volatiles condensed/entrapped in grains or pebbles are released in gaseous forms when drifting inward their lines. Peaks of abundances form close to each line.&amp;#160; Our simulations show that a significant fraction of volatiles can be trapped in clathrates, only if they have initially been delivered in pure condensate forms to the disk. We also show that several regions in the protosolar nebula share a metallicity that is consistent with those measured in the atmospheres of the ice giants [3,4]. These findings have important implications for the formation history of the outer planets&lt;/p&gt; &lt;p&gt;&lt;img src=&quot;&quot; alt=&quot;&quot; /&gt;&lt;/p&gt; &lt;p&gt;Figure 1 : Scheme showing the disk at initialization and at a given time. The volatiles are initially delivered under pure condensates and vapor. The vapor will condensate into clathrate hydrate, if there is enough crystalline water available. Grains drift inward while vapor undergo diffusion inward and outward. Leading to an accumulation of species at the place of condensation lines.&lt;/p&gt; &lt;p&gt;&amp;#160;&lt;/p&gt; &lt;p&gt;&lt;img src=&quot;&quot; alt=&quot;&quot; /&gt;&lt;/p&gt; &lt;p&gt;Figure 2 Scheme showing the disk at initialization and at a given time. The volatiles are initially delivered trapped into amorphous ice and vapor released from amorphous ice at the Amorphous to Crystalline Transition Zone (ACTZ). The clathration line is further than the ACTZ, since there no crystalline water after the ACTZ, clathration cannot happen, or is marginal if the clathration line is close to the ACTZ.&lt;/p&gt; &lt;p&gt;[1] : Gautier, D., Hersant, F., Mousis, O., et al. 2001, ApJL, 550, L227&amp;#160;&amp;#160;&lt;/p&gt; &lt;p&gt;[2] : Mousis, O., Ronnet, T., &amp; Lunine, J. I. 2019, ApJ, 875, 9.&lt;/p&gt; &lt;p&gt;[3] : Aguichine, A., Mousis, O., Devouard, B., et al. 2020, ApJ, 901, 97.&lt;/p&gt; &lt;p&gt;[4] : Asplund, M., Grevesse, N., Sauval, A. J., et al. 2009, ARA&amp;A, 47, 481.&lt;/p&gt; &lt;p&gt;[5] : Irwin, P. G. J., Toledo, D., Garland, R., et al. 2018, Nature Astronomy, 2, 420.&lt;/p&gt;