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Of all the myriad environments in our Solar System, the least explored are the distant Ice Giants Uranus and Neptune, and their diverse satellite and ring systems. These ‘intermediate-sized’ worlds are the last remaining class of Solar System planet to be characterised by a dedicated robotic mission, and may shape the paradigm for the most common outcome of planetary formation throughout our galaxy. In response to the 2019 European Space Agency call for scientific themes in the 2030s and 2040s (known as Voyage 2050), we advocated that an international partnership mission to explore an Ice Giant should be a cornerstone of ESA’s science planning in the coming decade, targeting launch opportunities in the early 2030s. This article summarises the inter-disciplinary science opportunities presented in that White Paper [1], and briefly describes developments since 2019.

The ESA-JAXA BepiColombo mission will provide simultaneous measurements from two spacecraft, offering an unprecedented opportunity to investigate magnetospheric and exospheric dynamics at Mercury as well as their interactions with the solar wind, radiation, and interplanetary dust. Many scientific instruments onboard the two spacecraft will be completely, or partially devoted to study the near-space environment of Mercury as well as the complex processes that govern it. Many issues remain unsolved even after the MESSENGER mission that ended in 2015. The specific orbits of the two spacecraft, MPO and Mio, and the comprehensive scientific payload allow a wider range of scientific questions to be addressed than those that could be achieved by the individual instruments acting alone, or by previous missions. These joint observations are of key importance because many phenomena in Mercury's environment are highly temporally and spatially variable. Examples of possible coordinated observations are described in this article, analysing the required geometrical conditions, pointing, resolutions and operation timing of different BepiColombo instruments sensors., Comment: 78 pages, 14 figures, published

The exchange of energy and momentum between the Earth’s upper atmosphere and ionosphere, and its space environment (magnetosphere) is regulated by electric currents (called Birkeland currents) flowing along magnetic field lines that connect these two regions of space1. The associated electric currents flow towards and away from each pole primarily in two concentric conical sheets2. It has been expected that powerful sheets of magnetic-field-aligned electric currents would be found in association with the bright Jovian auroras3. The Juno spacecraft is well positioned to explore Jupiter’s polar magnetosphere and sample Birkeland or field-aligned currents and particle distributions. Since July 2016, Juno has maintained a near-polar orbit, passing over both polar regions every 53 days. From this vantage point, Juno’s complement of science instruments gathers in situ observations of magnetospheric particles and fields while its remote-sensing infrared and ultraviolet spectrographs and imagers map auroral emissions4. Here we present an extensive analysis of magnetic field perturbations measured during Juno’s transits of Jupiter’s polar regions, and thereby demonstrate Birkeland currents associated with Jupiter’s auroral emissions. We characterize the magnitude and spatial extent of the currents and we find that they are weaker than anticipated and filamentary in nature. A significant asymmetry is observed between the field perturbations and the current associated with the northern and the southern auroras. The Juno spacecraft’s observations of magnetic field perturbations in Jupiter’s polar regions show Birkeland currents associated with aurorae that are weaker than anticipated and filamentary in nature. An asymmetry is observed between the northern and southern auroras.

Work at the University of Leicester was supported by STFC grant ST/N000749/1. Work at Imperial College was supported by STFC grant ST/N000692/1. EJB was supported by a Royal Society Wolfson Research Merit Award. MKD was supported by Royal Society Research Professorship RP140004. TJB was supported by STFC Quota Studentship ST/N504117/1. We thank Steve Kellock and the Cassini magnetometer team at Imperial College for access to processed magnetic field data. We also thank the reviewers for useful comments. Calibrated magnetic field data from the Cassini mission are available from the NASA Planetary Data System at the Jet Propulsion Laboratory (https://pds.jpl.nasa.gov/).

During Cassini's Grand Finale proximal orbits, the spacecraft traversed the nightside magnetotail to ∼21 Saturn radii. Clear signatures of Saturn's equatorial current sheet are observed in the magnetic field data. An axisymmetric model of the ring current is fitted to these data, amended to take into account the tilt of the current layer by solar wind forcing, its teardrop‐shaped nature and the magnetotail and magnetopause fringing fields. Variations in ring current parameters are examined in relation to external driving of the magnetosphere by the solar wind and internal driving by the two planetary period oscillations (PPOs), and compared with previous dawn and dayside observations. We find that the relative phasing of the PPOs determines the ring current's response to solar wind conditions. During solar wind compressions when the PPOs are in antiphase, a thick partial ring current is formed on the nightside, dominated by hot plasma injected by tail reconnection. This partial ring current should close partly via magnetopause currents and possibly via field‐aligned currents into the ionosphere. However, during solar wind compressions when the PPOs are in phase, this partial ring current is not detected. During solar wind rarefactions an equatorial “magnetodisc” configuration is observed in the dayside/dawn/nightside regions, with similar total currents flowing at these local times. During very quiet intervals of prolonged solar wind rarefaction, a thin current sheet with an enhanced current density is formed, indicative of a ring current dominated by cool, dense, Enceladus water group ions.

Full list of authors: Bunce, Emma J.; Martindale, Adrian; Lindsay, Simon; Muinonen, Karri; Rothery, David A.; Pearson, Jim; McDonnell, Ivor; Thomas, Chris; Thornhill, Julian; Tikkanen, Tuomo; Feldman, Charly; Huovelin, Juhani; Korpela, Seppo; Esko, Eero; Lehtolainen, Arto; Treis, Johannes; Majewski, Petra; Hilchenbach, Martin; Väisänen, Timo; Luttinen, Arto; Kohout, Tomas; Penttilä, Antti; Bridges, John; Joy, Katherine H.; Alcacera-Gil, Maria Angeles; Alibert, Guilhem; Anand, Mahesh; Bannister, Nigel; Barcelo-Garcia, Corinne; Bicknell, Chris; Blake, Oliver; Bland, Phil; Butcher, Gillian; Cheney, Andy; Christensen, Ulrich; Crawford, Tony; Crawford, Ian A.; Dennerl, Konrad; Dougherty, Michele; Drumm, Paul; Fairbend, Raymond; Genzer, Maria; Grande, Manuel; Hall, Graeme P.; Hodnett, Rosie; Houghton, Paul; Imber, Suzanne; Kallio, Esa; Lara, Maria Luisa; Balado Margeli, Ana; Mas-Hesse, Miguel J.; Maurice, Sylvestre; Milan, Steve; Millington-Hotze, Peter; Nenonen, Seppo; Nittler, Larry; Okada, Tatsuaki; Ormö, Jens; Perez-Mercader, Juan; Poyner, Richard; Robert, Eddy; Ross, Duncan; Pajas-Sanz, Miriam; Schyns, Emile; Seguy, Julien; Strüder, Lothar; Vaudon, Nathalie; Viceira-Martín, Jose; Williams, Hugo; Willingale, Dick; Yeoman, Tim.-- This is an open access article., The Mercury Imaging X-ray Spectrometer is a highly novel instrument that is designed to map Mercury’s elemental composition from orbit at two angular resolutions. By observing the fluorescence X-rays generated when solar-coronal X-rays and charged particles interact with the surface regolith, MIXS will be able to measure the atomic composition of the upper ∼10-20 μm of Mercury’s surface on the day-side. Through precipitating particles on the night-side, MIXS will also determine the dynamic interaction of the planet’s surface with the surrounding space environment. MIXS is composed of two complementary elements: MIXS-C is a collimated instrument which will achieve global coverage at a similar spatial resolution to that achieved (in the northern hemisphere only – i.e. ∼ 50 – 100 km) by MESSENGER; MIXS-T is the first ever X-ray telescope to be sent to another planet and will, during periods of high solar activity (or intense precipitation of charged particles), reveal the X-ray flux from Mercury at better than 10 km resolution. The design, performance, scientific goals and operations plans of the instrument are discussed, including the initial results from commissioning in space. © 2020, The Author(s)., The MIXS team has gratefully received funding from a consortium of agencies across Europe. The UK Space Agency are the lead funding agency, supported by national contributions from our amazing panEuropean team and the European Space Agency. UK funding has come from, the science and technology facilities council - STFC (grant numbers: PP/E002056/1; PP/E002412/1; ST/L000776/1; ST/M002101/1; ST/M002187/1; ST/N00339X/1; ST/N000471/1; ST/P001963/1; ST/S002596/1; ST/J000213/1), the United Kingdom Space Agency -UKSA (grant numbers: ST/P000908/1; ST/T001542/1; ST/K003194/1; ST/L000318/1; ST/M000702/1) and the European Space Agency -ESA (contract numbers: 4000118221/16/ES/JD; RES-PSS/NR/035-B/200; 20919/07/NF/FG). Images shown in Fig. 4, Fig. 6 and Fig. 9 are adapted from drawings produced by MagnaParva Ltd. under ESA contract number 20919/07/NL/FG. EJB is supported by a Royal Society Wolfson Research Merit Award. The DEPFET X-ray detectors were contributed with the support of dedicated Max Planck Society grants by MPG and MPS as well as ESA contract number 4000108312. The Spanish contribution was funded by MEC/MINECO/MICINN grants; AYA200803467/ESP, AYA2011-24780, AYA2012-39362-C02-01, ESP2014-59789-P and ESP2015-65712-C5-1-R. Finnish hardware contributions were funded by Tekes (now "Business Finland") during the era from 2004 to 2019. Research at the University of Helsinki is supported, in part, by the Academy of Finland (grant number 1325805).

Planetary Space Weather is the discipline that studies the state of the Sun and how it interacts with the interplanetary and planetary environments. It is driven by the Sun’s activity, particularly through large eruptions of plasma (known as coronal mass ejections, CMEs), solar wind stream interaction regions (SIR) formed by the interaction of high-speed solar wind streams with the preceding slower solar wind, and bursts of solar energetic particles (SEPs) that form radiation storms. This is an emerging topic, whose real-time forecast is very challenging because among other factors, it needs a continuous coverage of its variability within the whole heliosphere as well as of the Sun’s activity to improve forecasting. The long cruise of BepiColombo constitutes an exceptional opportunity for studying the Space Weather evolution within half-astronomical unit (AU), as well as in certain parts of its journey, can be used as an upstream solar wind monitor for Venus, Mars and even the outer planets. This work will present preliminary results of the Space Weather conditions encountered by BepiColombo since its launch until mid-2020, which includes data from the solar minimum of activity and few slow solar wind structures. Data come from three of its instruments that are operational for most of the cruise phase, i.e., the BepiColombo Radiation Monitor (BERM), the Mercury Planetary Orbiter Magnetometer (MPO-MAG), and the Solar Intensity X-ray and particle Spectrometer (SIXS). Modelling support for the data observations will be also presented with the so-called solar wind ENLIL simulations.

Introduction: As part of the target prioritisation for the Mercury Imaging X-ray Spectrometer (MIXS), we began by cataloguing all craters that still retain a central peak (or peak-ring) structure. We report on a correlation between Mercury’s peak-ring basins and a region with high Mg/Si values determined by MESSENGER XRS [1]. We explore impact as a mechanism for the elevated Mg/Si values. Complex craters and basins uplift material from deep crustal and upper mantle levels (e.g. [2]). Recent work on Lunar peaked craters confirm that the central peaks act as ‘drill cores’ into the lower strata revealing compositions different from the surrounding terrain [3]. The High-Mg/Si Terrane (HMT) on Mercury exhibits the highest Mg/Si ratios identified on Mercury as well as low Al/Si ratios. It covers an area from approximately 120° W to 45° W and 10° S to 50° N, with an area >5,000,000km2 [1], (Fig. 1). Methods: Three catalogues have been used: A catalogue of all Mercurian craters that retain a central peak structure, created by the authors using [4], for target prioritization for MIXS – the ‘MIXS’ catalogue. Key features were visually categorised as MIXS-T can likely resolve them in the future; Baker et al. [6], which morphometrically catalogues peak-ring basins, protobasins, and ring-cluster basins; Fassett et al. [7], which catalogues all of the craters on Mercury with diameter ≥20km. Excavation depth was calculated from [8] and stratigraphic uplift was calculated for key basins using methods in [9]. Stratigraphic uplift equations are assumed to only hold for material that has not been excavated and are used with caution at Mercury [9]. Crustal thickness data was taken from [10] and combined with estimated basin depths to estimate the depth to the crust-mantle boundary. We use a one-tail test to define precise confidence contours of the HMT (originally identified by [1]) using the mean and error values from [5]. This gives the hypotheses, HC,0: Mg/Si=0.436, HC,1: Mg/Si>0.436. Using the error, σ=0.106, we determine the confidence contours for the HMT (Fig. 1). To assess basin statistics across the planet. 10,000 random points were selected on Mercury. The points acted as the centres for circular buffers for basin-counts, with an area equal to that enclosed by the specific Mg/Si confidence contour. Another one-tailed hypothesis test was then carried out. Let µ be the mean number of basins within the buffer radius and C be the actual number of basins within the specific confidence contour, then we have the general hypotheses, HB,0: C=µ, HB,1: C>µ. The global mean and standard deviation for basin counts were calculated for each crater set, then the hypotheses were tested for each buffer to determine if a greater-than-mean basin-count is present. Results: The greatest overall confidence level (97.7%) is for the logical intersection (∩) of the Baker and MIXS sets, within the 2σ-contour. Figure 2 shows a confidence map for this data set. A high confidence level is also observed for the Baker and MIXS sets independently (>96.7% when using a 2σ-contour), with slightly lower (~1%) confidences when protobasins and ring-cluster basins are also included. The Fassett sets exhibited confidence distributions which do not coincide with the HMT. Uplift and excavation calculations (above) suggest that the 7 basins coincident with the contour (Figure 1) were unlikely to have brought mantle material to the surface, although all but 1 (Praxiteles) uplifted mantle material to within ~10km of the basin floor. Calculations suggest that the impacts excavated material from crustal depths of ~13km to ~20km. Discussion: The probability of a chance correlation between high densities of basins and the HMT is low, therefore, three possible hypotheses are suggested for the observed correlation: high-Mg mantle material has been excavated and/or uplifted to the surface; high-Mg deep crustal material has been excavated and/or uplifted to the surface; impacts caused fracturing in the underlying crust, which facilitated the subsequent extrusion of high-Mg magmas. Excavation and uplift calculations indicate mantle was unlikely to be excavated, or uplifted to the surface. Deep crustal material was almost certainly excavated, although, assuming a ubiquitous deep crustal layer of high-Mg material around Mercury, similar size craters over regions of similar thickness crust would be expected to reveal a high Mg/Si signature. Analysis of the Fassett data did not support this correlation. From the available data, it cannot be ruled out that the giant impactors have fractured the underlying crust and allowed extrusion of basaltic magmas [11]. Given the high-Mg/Si of the HMT, it is expected that these magmas might have compositions similar to magnesian basalts or basaltic komatiites [12]. Conclusion: There is a strong statistical correspondence between peak-ring basins and the HMT. There is insufficient available evidence to assign any definite causal relationship, but it is considered likely that the basin impacts have revealed primitive igneous material with high Mg/Si, which is present at crustal depths not normally revealed by other basin-sized impacts. The material may have been directly revealed or the impact may have facilitated the subsequent extrusion of high-Mg magmas. BepiColombo’s improved geographic coverage, coupled with MIXS’s higher resolution and sensitivity will hopefully answer these questions definitively. References:[1]-Weider, S.Z. et al. (2015) Earth Planet Sc Lett, 416, 109-120. [2]-Melosh, H.J. (1989) Impact cratering: a geologic process, Oxford University Press. [3]-Moriarty, D.P. et al. (2013) J Geophy Res-Planets, 118, 2310-2322. [4]-Denevi, B. W. et al. (2017) Space Sci Rev, 214:2. [5]-Nittler, L.R. et al. in Mercury: The View After MESSENGER, C. Solomon S.R., Nittler L. and Anderson B.J., Editors. Cambridge University Press. [6]-Baker, D.M.H., et al. (2011) Planet Space Sci, 59, 1932-1948. [7]-Fassett, C.I., et al. (2011) Geophys Res Lett, 38, L10202. [8]-Potter, R.W.K. and Head, J.W. (2016) 47th LPSC, Abstract #1117. [9]-Potter, R.W.K. et al. (2013) Geophys Res Lett, 40, 5615-5620. [10]-Smith, D. E., et al. (2012), Science, 316, 214-217. [11]-Frank, E. A., et al. (2017), J Geophy Res-Planets, 122, 614-632. [12]-Phillips, R. J. et al. (2018) in Mercury: The View After MESSENGER, C. Solomon S.R., Nittler L. and Anderson B.J., Editors. Cambridge University Press. [13]-Benkhoff, J., et al. (2010), Planet Space Sci, 58, 2-20.

Uranus and Neptune, and their diverse satellite and ring systems, represent the least explored environments of our Solar System, and yet may provide the archetype for the most common outcome of planetary formation throughout our galaxy. Ice Giants will be the last remaining class of Solar System planet to have a dedicated orbital explorer, and international efforts are under way to realise such an ambitious mission in the coming decades. In 2019, the European Space Agency released a call for scientific themes for its strategic science planning process for the 2030s and 2040s, known as Voyage 2050. We used this opportunity to review our present-day knowledge of the Uranus and Neptune systems, producing a revised and updated set of scientific questions and motivations for their exploration. This review article describes how such a mission could explore their origins, ice-rich interiors, dynamic atmospheres, unique magnetospheres, and myriad icy satellites, to address questions at the heart of modern planetary science. These two worlds are superb examples of how planets with shared origins can exhibit remarkably different evolutionary paths: Neptune as the archetype for Ice Giants, whereas Uranus may be atypical. Exploring Uranus' natural satellites and Neptune's captured moon Triton could reveal how Ocean Worlds form and remain active, redefining the extent of the habitable zone in our Solar System. For these reasons and more, we advocate that an Ice Giant System explorer should become a strategic cornerstone mission within ESA's Voyage 2050 programme, in partnership with international collaborators, and targeting launch opportunities in the early 2030s., Comment: 34 pages, 9 figures, accepted to Planetary and Space Science

The Solar Intensity X-ray and particle Spectrometer (SIXS) on the BepiColombo Mercury Planetary Orbiter (“Bepi”) measures the direct solar X-rays, energetic protons, and electrons that bombard, and interact with, the Hermean surface. The interactions result in X-ray fluorescence and scattering, and particle induced X-ray emission (PIXE), i.e. “glow” of the surface in X-rays. Simultaneous monitoring of the incident and emitted radiation enables derivation of the abundances of some chemical elements and scattering properties of the outermost surface layer of the planet, and it may reveal other sources of X-ray emission, due to, for example, weak aurora-like phenomena in Mercury’s exosphere. Mapping of the Hermean X-ray emission is the main task of the MIXS instrument onboard BepiColombo. SIXS data will also be used for investigations of the solar X-ray corona and solar energetic particles (SEP), both in the cruise phase and the passes of the Earth, Venus and Mercury before the arrival at Mercury’s orbit, and the final science phase at Mercury’s orbit. These observations provide the first-ever opportunity for in-situ measurements of the propagation of SEPs, their interactions with the interplanetary magnetic field, and space weather phenomena in multiple locations throughout the inner solar system far away from the Earth, and more extensively at Mercury’s orbit.In this paper we describe the scientific objectives, design and calibrations, operational principles, and scientific performance of the final SIXS instrument launched to the mission to planet Mercury onboard BepiColombo. We also provide the first analysis results of science observations with SIXS, that were made during the Near-Earth Commissioning Phase and early cruise phase operations in 2018–19, including the background X-ray sky observations and “first light” observations of the Sun with the SIXS X-ray detection system (SIXS-X), and in-situ energetic electron and proton observations with the SIXS Particle detection system (SIXS-P).

We compare Chandra and XMM-Newton X-ray observations of Jupiter during 2007 with a rich multi-instrument data set including upstream in situ solar wind measurements from the New Horizons spacecraft, radio emissions from the Nancay Decametric Array and Wind/Waves, and ultraviolet (UV) observations from the Hubble Space Telescope. New Horizons data revealed two corotating interaction regions (CIRs) impacted Jupiter during these observations. Non-Io decametric bursts and UV emissions brightened together and varied in phase with the CIRs. We characterize three types of X-ray aurorae: hard X-ray bremsstrahlung main emission, pulsed/flared soft X-ray emissions, and a newly identified dim flickering (varying on short time scales, but quasi-continuously present) aurora. For most observations, the X-ray aurorae were dominated by pulsed/flaring emissions, with ion spectral lines that were best fit by iogenic plasma. However, the brightest X-ray aurora was coincident with a magnetosphere expansion. For this observation, the aurorae were produced by both flickering emission and erratic pulses/flares. Auroral spectral models for this observation required the addition of solar wind ions to attain good fits, suggesting solar wind entry into the outer magnetosphere or directly into the pole for this particularly bright observation. X-ray bremsstrahlung from high energy electrons was only bright for one observation, which was during a forward shock. This bremsstrahlung was spatially coincident with bright UV main emission (power > 1 TW) and X-ray ion spectral line dusk emission, suggesting closening of upward and downward current systems during the shock. Otherwise, the bremsstrahlung was dim, and UV main emission power was also lower (

The 2007-2009 solar minimum was the longest of the space age. We present the first of two companion papers on Chandra and XMM-Newton X-ray campaigns of Jupiter through February-March 2007. We find that low solar X-ray flux during solar minimum causes Jupiter's equatorial regions to be exceptionally X-ray dim (0.21 GW at minimum; 0.76 GW at maximum). While the Jovian equatorial emission varies with solar cycle, the aurorae have comparably bright intervals at solar minimum and maximum. We apply atomic charge exchange models to auroral spectra and find that iogenic plasma of sulphur and oxygen ions provides excellent fits for XMM-Newton observations. The fitted spectral S:O ratios of 0.4-1.3 are in good agreement with in situ magnetospheric S:O measurements of 0.3-1.5, suggesting that the ions that produce Jupiter's X-ray aurora predominantly originate inside the magnetosphere. The aurorae were particularly bright on 24-25 February and 8-9 March, but these two observations exhibit very different spatial, spectral, and temporal behavior; 24-25 February was the only observation in this campaign with significant hard X-ray bremsstrahlung from precipitating electrons, suggesting this may be rare. For 8-9 March, a bremsstrahlung component was absent, but bright oxygen O(6+)lines and best-fit models containing carbon, point to contributions from solar wind ions. This contribution is absent in the other observations. Comparing simultaneous Chandra ACIS and XMM-Newton EPIC spectra showed that ACIS systematically underreported 0.45- to 0.6-keV Jovian emission, suggesting quenching may be less important for Jupiter's atmosphere than previously thought. We therefore recommend XMM-Newton for spectral analyses and quantifying opacity/quenching effects.

We examine the azimuthal magnetic field signatures associated with Saturn's northern hemisphere auroral field‐aligned currents observed in the dawn sector during Cassini's Proximal orbits (April 2017 and September 2017). We compare these currents with observations of the auroral currents from near noon taken during the F‐ring orbits prior to the Proximal orbits. First, we show that the position of the main auroral upward current is displaced poleward between the two local times (LTs). This is consistent with the statistical position of the ultraviolet auroral oval for the same time interval. Second, we show the overall average ionospheric meridional current profile differs significantly on the equatorward boundary of the upward current with a swept‐forward configuration with respect to planetary rotation present at dawn. We separate the planetary period oscillation (PPO) currents from the PPO‐independent currents and show their positional relationship is maintained as the latitude of the current shifts in LT implying an intrinsic link between the two systems. Focusing on the individual upward current sheets pass‐by‐pass, we find that the main upward current at dawn is stronger compared to near noon. This results in the current density being ~1.4 times higher in the dawn sector. We determine a proxy for the precipitating electron power and show that the dawn PPO‐independent upward current electron power is ~1.9 times higher than at noon. These new observations of the dawn auroral region from the Proximal orbits may show evidence of an additional upward current at dawn likely associated with strong flows in the outer magnetosphere.

Full list of authors: Blanc, Michel; Prieto-Ballesteros, Olga; André, Nicolas; Gomez-Elvira, Javier; Jones, Geraint; Sterken, Veerle; Desprats, William; Gurvits, Leonid I.; Khurana, Krishan; Balmino, Georges; Blöcker, Aljona; Broquet, Renaud; Bunce, Emma; Cavel, Cyril; Choblet, Gaël; Colins, Geoffrey; Coradini, Marcello; Cooper, John; Dirkx, Dominic; Fontaine, Dominique; Garnier, Philippe; Gaudin, David; Hartogh, Paul; Hussmann, Hauke; Genova, Antonio; Iess, Luciano; Jäggi, Adrian; Kempf, Sascha; Krupp, Norbert; Lara, Luisa; Lasue, Jérémie; Lainey, Valéry; Leblanc, François; Lebreton, Jean-Pierre; Longobardo, Andrea; Lorenz, Ralph; Martins, Philippe; Martins, Zita; Marty, Jean-Charles; Masters, Adam; Mimoun, David; Palumba, Ernesto; Parro, Victor; Regnier, Pascal; Saur, Joachim; Schutte, Adriaan; Sittler, Edward C.; Spohn, Tilman; Srama, Ralf; Stephan, Katrin; Szegő, Károly; Tosi, Federico; Vance, Steve; Wagner, Roland; Van Hoolst, Tim; Volwerk, Martin; Wahlund, Jan-Erik; Westall, Frances; Wurz, Peter, Europa is the closest and probably the most promising target to search for extant life in the Solar System, based on complementary evidence that it may fulfil the key criteria for habitability: the Galileo discovery of a sub-surface ocean; the many indications that the ice shell is active and may be partly permeable to transfer of chemical species, biomolecules and elementary forms of life; the identification of candidate thermal and chemical energy sources necessary to drive a metabolic activity near the ocean floor. In this article we are proposing that ESA collaborates with NASA to design and fly jointly an ambitious and exciting planetary mission, which we call the Joint Europa Mission (JEM), to reach two objectives: perform a full characterization of Europa's habitability with the capabilities of a Europa orbiter, and search for bio-signatures in the environment of Europa (surface, subsurface and exosphere) by the combination of an orbiter and a lander. JEM can build on the advanced understanding of this system which the missions preceding JEM will provide: Juno, JUICE and Europa Clipper, and on the Europa lander concept currently designed by NASA (Maize, report to OPAG, 2019). We propose the following overarching goals for our Joint Europa Mission (JEM): Understand Europa as a complex system responding to Jupiter system forcing, characterize the habitability of its potential biosphere, and search for life at its surface and in its sub-surface and exosphere. We address these goals by a combination of five Priority Scientific Objectives, each with focused measurement objectives providing detailed constraints on the science payloads and on the platforms used by the mission. The JEM observation strategy will combine three types of scientific measurement sequences: measurements on a high-latitude, low-altitude Europan orbit; in-situ measurements to be performed at the surface, using a soft lander; and measurements during the final descent to Europa's surface. The implementation of these three observation sequences will rest on the combination of two science platforms: a soft lander to perform all scientific measurements at the surface and sub-surface at a selected landing site, and an orbiter to perform the orbital survey and descent sequences. We describe a science payload for the lander and orbiter that will meet our science objectives. We propose an innovative distribution of roles for NASA and ESA; while NASA would provide an SLS launcher, the lander stack and most of the mission operations, ESA would provide the carrier-orbiter-relay platform and a stand-alone astrobiology module for the characterization of life at Europa's surface: the Astrobiology Wet Laboratory (AWL). Following this approach, JEM will be a major exciting joint venture to the outer Solar System of NASA and ESA, working together toward one of the most exciting scientific endeavours of the 21st century: to search for life beyond our own planet. © 2020, The authors received support from the sponsors of their home institutions during the development of their projects, particularly at the two institutes leading this effort: at IRAP, Toulouse, MB and NA acknowledge the support of CNRS, University Toulouse III - Paul Sabatier and CNES. At CAB, Madrid, OPB and JGE acknowledge the support of INTA and Spanish MINECO project ESP2014-55811-C2-1-P and ESP2017-89053-C2-1-P and the AEI project MDM-2017-0737 Unidad de Excelencia "Maria de Maeztu". We would also like to extend special thanks to the PASO of CNES for its precious assistance and expertise in the design of the mission scenario.

The thickness and variability of the Saturnian magnetodisc current sheet is investigated using the Cassini magnetometer data set. Cassini performed 66 fast, steep crossings of the equatorial current sheet where a clear signature in the magnetic field data allowed for a direct determination of its thickness and the offset of its center. The average, or nominal, current sheet half‐thickness is 1.3 R S , where R S is the equatorial radius of Saturn, equal to 60,268 km. This is thinner than previously calculated, but both spatial and temporal dependencies are identified. The current sheet is thicker and more variable by a factor ∼2 on the nightside compared to the dayside, ranging from 0.5–3 R S . The current sheet is on average 50% thicker in the nightside quasi‐dipolar region (≤15 R S ) compared to the dayside. These results are consistent with the presence of a noon‐midnight electric field at Saturn that produces a hotter plasma population on the nightside compared to the dayside. It is also shown that the current sheet becomes significantly thinner in the outer region of the nightside, while staying approximately constant with radial distance on the dayside, reflecting the dayside compression of the magnetosphere by the solar wind. Some of the variability is well characterized by the planetary period oscillations (PPOs). However, we also find evidence for non‐PPO drivers of variability.

We examine the azimuthal magnetic field signatures associated with Saturn’s northern hemisphere auroral field-aligned currents observed in the dawn sector during Cassini’s Proximal orbits (April 2017 and September 2017). We compare these currents with observations of the auroral currents from near noon taken during the F-ring orbits prior to the Proximal orbits. First, we show that the position of the main auroral upward current is displaced poleward between the two local times (LT). This is consistent with the statistical position of the ultraviolet auroral oval for the same time interval. Second, we show the overall average ionospheric meridional current profile differs significantly on the equatorward boundary of the upward current with a swept-forward configuration with respect to planetary rotation present at dawn. We separate the planetary period oscillation (PPO) currents from the PPO-independent currents and show their positional relationship is maintained as the latitude of the current shifts in LT implying an intrinsic link between the two systems. Focusing on the individual upward current sheets pass-by-pass we find that the main upward current at dawn is stronger compared to near-noon. This results in the current density been ~1.4 times higher in the dawn sector. We determine a proxy for the precipitating electron power and show that the dawn PPO-independent upward current electron power is ~1.9 times higher than at noon. These new observations of the dawn auroral region from the Proximal suggest the possibility of an additional upward current at dawn likely associated with strong flows in the outer magnetosphere. These findings provide new insights into the dawn sector of giant planet magnetospheres.

The presence of a substantial azimuthal current sheet in Saturn’s magnetosphere was identified in Voyager and Pioneer magnetometer data. Data from these spacecraft showed depressions in the strength of the field below that expected for the internal field of the planet alone. This ring current was modelled as a simple axisymmetric current system by Connerney et al. [1980, 1983]. In this study we utilise the Connerney ring current model to look at the size, shape, current density and total current of Saturn’s ring current as observed during the Cassini proximal orbits. We compare the variations in these parameters with the phases of the planetary period oscillations and with the occurrence of magnetospheric storms as determined from propagated solar wind data and LEMMS electron and proton data. Overall, we find that Saturn’s ring current is a dynamical environment which varies in size and magnitude due to both planetary period oscillations and solar-driven storms.

We conduct a statistical analysis of 2,094 reconnection events in Saturn's near-equatorial magnetotail previously identified in Cassini magnetometer data from intervals during 2006 and 2009/2010. These consist of tailward propagating plasmoids and planetward propagating dipolarizations, with approximately twice as many plasmoids as dipolarizations. We organize these by three related planetary period oscillation (PPO) phase systems, the northern and southern PPO phases relative to noon, the same phases retarded by a radial propagation delay, and the local retarded phases that take account of the azimuth (local time) of the observation. Clear PPO modulation is found for both plasmoid and dipolarization events, with local retarded phases best organizing the event data with the modulation in event frequency propagating across the tail as the PPO systems rotate. This indicates that the events are localized in azimuth, rather than simultaneously affecting much of the tail width. Overall, events occur preferentially by factors of ~3 at northern and southern phases where the tail current sheet is expected locally to be thinnest in the PPO cycle, with field lines contracting back from their maximum radial displacement, compared with the antiphase conditions. Separating the events into those representing the start of independent reconnection episodes, occurring at least 3 hr after the last, and events in subsequent clusters, shows that the above phases are predominantly characteristic of the majority cluster events. The phases at the start of independent reconnection episodes are typically ~60° earlier.

International audience; Throughout 2017, the Hubble Space Telescope (HST) observed the northern far-ultraviolet aurorae of Saturn at northern solstice, during the Cassini Grand Finale. These conditions provided a complete viewing of the northern auroral region from Earth and a maximal solar illumination, expected to maximize the ionosphere-magnetosphere coupling. In this study, we analyze 24 HST images concurrently with Cassini measurements of Saturn's Kilometric Radiation and solar wind parameters predicted by two MHD models. The aurorae reveal highly variable components, down to timescales of minutes, radiating 7 to 124 +/-11 GW. They include a nightside-shifted main oval, unexpectedly frequent and bright cusp emissions and a dayside low latitude oval. On average, these emissions display a strong Local Time dependence with two maxima at dawn and pre-midnight, the latter being newly observed and attributed to nightside injections possibly associated with solstice conditions. These results provide a reference frame to analyze Cassini in situ measurements, whether simultaneous or not.

We determine properties of Saturn's planetary period oscillations from Cassini magnetic measurements over the ~2‐year interval from September 2015 to end of mission in September 2017, spanning Saturn northern summer solstice in May 2017. Phases of the northern system oscillations are derived over the whole interval, while those of the southern system are not discerned in initial equatorial data due to too low amplitude relative to the northern, but are determined once southern polar data become available from inclined orbits beginning May 2016. Planetary period oscillation periods are shown to be almost constant over these intervals at ~10.79 hr for the northern system and ~10.68 hr for the southern, essentially unchanged from values previously determined after the periods reversed in 2014. High cadence phase and amplitude data obtained from the short‐period Cassini orbits during the mission's last 10 months newly reveal the presence of dual modulated oscillations varying at the beat period of the two systems (~42 days) on nightside polar field lines in the vicinity (likely either side) of the open‐closed field boundary. The modulations differ from those observed previously in the equatorial region, indicative of a reversal in sign of the radial component oscillations, but not of the colatitudinal component oscillations. Brief discussion is given of a possible theoretical scenario. While weak equatorial beat modulations indicate a north/south amplitude ratio >5 early in the study interval, polar and equatorial region modulations suggest a ratio ~1.4 during the later interval, indicating a significant recovery of the southern system.

We have discovered pulsating emission within Jupiter's main auroral oval, providing evidence of the auroral signature of Jovian ULF wave processes. The form comprises a 1° × 2° spot located directly on the main emission, whose intensity oscillates with a period of ∼10 min throughout the 45 min observation. The feature appears on the duskward edge of the discontinuity, maps to ∼13–14 h LT and ∼20–50 RJ, and rotates at around a half of rigid corotation. We show that the period of the oscillation is similar to the expected Alfven travel time between the ionosphere and the upper edge of the equatorial plasma sheet in the middle magnetosphere, and we thus suggest that the pulsating aurora is driven by a mode confined to the low-density region outside the plasma sheet. This significant new observation shows that Jupiter's auroras present an important remote sensing window on Jovian magnetospheric wave processes.

Recent results of the first ever orbit through Jupiter's auroral region by NASA's Juno spacecraft did not show evidence of coherent acceleration in the auroral or polar region. However, in this letter, we show energetic particle data from Juno's Jupiter Energetic-particle Detector Instrument instrument during the third auroral pass that exhibits conclusive evidence of downward parallel electric fields in portions of Jupiter's polar region. The energetic particle distributions show inverted-V ion and electron structures in a downward electric current region with accelerated peaked distributions in hundreds of keV to ~1 MeV range. The origin of these large electric potential structures is investigated and discussed within the current theoretical framework of current-voltage relationships at both Earth and Jupiter. Parallel electric fields responsible for accelerating particles to maintain the aurora/magnetospheric circuit appear to be a common phenomenon among strongly magnetized planets with conducting ionospheres; however, their origin and generation mechanisms are subjects of ongoing research.

We present the first comparison of Jupiter's auroral morphology with an extended, continuous and complete set of near-Jupiter interplanetary data, revealing the response of Jupiter's auroras to the interplanetary conditions. We show that for ∼1-3 days following compression region onset the planet's main emission brightened. A duskside poleward region also brightened during compressions, as well as during shallow rarefaction conditions at the start of the program. The power emitted from the noon active region did not exhibit dependence on any interplanetary parameter, though the morphology typically differed between rarefactions and compressions. The auroras equatorward of the main emission brightened over ∼10 days following an interval of increased volcanic activity on Io. These results show that the dependence of Jupiter's magnetosphere and auroras on the interplanetary conditions are more diverse than previously thought.

The first extensive study of interplanetary magnetic field (IMF) characteristics and stability at Mercury is undertaken using MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) magnetometer data. Variations in IMF and solar wind conditions have a direct and rapid effect upon Mercury's highly dynamic magnetosphere; hence, understanding of the time scales over which these variations occur is crucial because they determine the duration of magnetospheric states. We characterize typical distributions of IMF field strength, clock angle, and cone angle throughout the duration of MESSENGER's mission. Clock and cone angle distributions collected during the first Earth year of the mission indicate that there was a significant north-south asymmetry in the location of the heliospheric current sheet during this period. The stability of IMF magnitude, clock angle, cone angle, and IMF Bz polarity is quantified for the entire mission. Changes in IMF Bz polarity and magnitude are found to be less likely for higher initial field magnitudes. Stability in IMF conditions is also found to be higher at aphelion (heliocentric distance r ∼ 0.31 AU) than at perihelion (r ∼ 0.47 AU).

We evaluate the expected effects of magnetosphere-ionosphere coupling at Jupiter along the Juno Perijove 1 (PJ1) trajectory using an axisymmetric physical model. As found at Saturn, the model predicts distributed downward field-aligned currents over polar regions mapping to the tail and outer magnetosphere, closed principally through a ring of upward current mapping to the middle magnetosphere, which requires downward acceleration of magnetospheric electrons generating Jupiter's main auroral emission. Auroral location, width, intensity, electron energy, and current density are in accord with values derived from previous ultraviolet imaging, such that the model forms an appropriate baseline for comparison with Juno data. We evaluate the azimuthal field perturbations during six anticipated near-planet encounters with middle magnetosphere field lines at radial distances between ~1.6 and ~16 Jovian radii, discuss the expected form of the accelerated electron distributions, and comment briefly on model expectations in relation to first results derived from Juno PJ1 data.

We analyze periapsis pass magnetic field data from the final 23 orbits of the Cassini spacecraft at Saturn, uniquely encompassing auroral, subauroral, ring region, and intra-ring field lines, to determine the planetary period oscillations (PPOs) and mean residual fields in these regions. Dual modulation by northern and southern PPO systems is found almost continuously, demonstrating for the first time the presence of PPOs on and inside ring region field lines. The azimuthal component displays the largest ~10–15nT PPO amplitudes on auroral field lines, falling across the subauroral region to ~3–5 nT on main ring field lines in the northern hemisphere, less in the southern hemisphere, while increasing to ~5–8 nT on D ring and intra-D ring field lines. Auroral and subauroral amplitudes mapped along field lines are in good agreement with previous analyses in regions of overlap. Colatitudinal and radial field oscillations generally have a half and a quarter the amplitude of the azimuthal component, respectively. Inner region oscillation phases are typically several tens of degrees “earlier” than those of outer subauroral and auroral regions. Mean residual poloidal fields (internal and ring current fields subtracted) show quasi-sinusoidal latitude variations of ~2.5nT amplitude, with radial and colatitudinal fields approximately in quadrature. Mean azimuthal fields peaking at ~15 nT are approximately symmetrical about the equator on and inside D ring field lines as previously reported, but are unexpectedly superposed on ~3–5nT “lagging” fields which extend continuously through the ring region onto subauroral field lines north and south.

Saturn's main aurorae are thought to be generated by plasma flow shears associated with a gradient in angular plasma velocity in the outer magnetosphere. Dungey cycle convection across the polar cap, in combination with rotational flow, may maximize (minimize) this flow shear at dawn (dusk) under strong solar wind driving. Using imagery from Cassini's Ultraviolet Imaging Spectrograph, we surprisingly find no related asymmetry in auroral power but demonstrate that the previously observed “dawn arc” is a signature of quasiperiodic auroral plasma injections commencing near dawn, which seem to be transient signatures of magnetotail reconnection and not part of the static main aurorae. We conclude that direct Dungey cycle driving in Saturn's magnetosphere is small compared to internal driving under usual conditions. Saturn's large-scale auroral dynamics hence seem predominantly controlled by internal plasma loading, with plasma release in the magnetotail being triggered both internally through planetary period oscillation effects and externally through solar wind compressions.

During the final 22 full revolutions of the Cassini mission in 2017, the spacecraft passed at periapsis near the noon meridian through the gap between the inner edge of Saturn's D ring and the denser layers of the planet's atmosphere, revealing the presence of an unanticipated low‐latitude current system via the associated azimuthal perturbation field peaking typically at ~10‐30 nT. Assuming approximate axisymmetry, here we use the field data to calculate the associated horizontal meridional currents flowing in the ionosphere at the feet of the field lines traversed, together with the exterior field‐aligned currents required by current continuity. We show that the ionospheric currents are typically~0.5–1.5 MA per radian of azimuth, similar to auroral region currents, while the field‐aligned current densities above the ionosphere are typically ~5‐10 nA m^(‐2), more than an order less than auroral values. The principal factor involved in this difference is the ionospheric areas into which the currents map. While around a third of passes exhibit unidirectional currents flowing northward in the ionosphere closing southward along exterior field lines, many passes also display layers of reversed northward field‐aligned current of comparable or larger magnitude in the region interior to the D ring, which may reverse sign again on the innermost field lines traversed. Overall, however, the currents generally show a high degree of north‐south conjugacy indicative of an interhemispheric system, certainly on the larger overall spatial scales involved, if less so for the smaller‐scale structures, possibly due to rapid temporal or local time variations.

International audience; We compare periods and phases of Saturn planetary period oscillations determined from Cassini magnetic field and Saturn kilometric radiation (SKR) data from the beginning of 2016 to the end of mission in mid‐September 2017, encompassing northern summer solstice in May 2017. Both data sets show that the periods are almost unchanging, varying by only ~ ±0.01 hr about 10.79 hr for the northern system and 10.68 hr for the southern system, close to values attained by mid‐2015 after period coalescence between mid‐2013 and mid‐2014. The mean absolute differences between the magnetic and SKR periods are ~0.0036 hr (~13 s), consistent with estimated magnetic measurement uncertainties, while the overall mean difference is less than 0.001 hr (~2–3 s), at the limit of resolution. The relative phasing between magnetic and SKR modulations is correspondingly near constant and such that the equatorial planetary period oscillation fields of the northern/southern systems point radially outward near‐oppositely at ~14.3/2.5 hr local time at corresponding SKR maxima, with upward planetary period oscillation currents located ~2 hr postdawn for both systems, consistent with previous intervals having dawnside spacecraft apoapsides. Southern SKR emissions are found to be significantly dual modulated at both southern and northern periods in data limited to lie well within the southern shadow zone of the northern sources. These northern period modulations are shown to be approximately in phase with those in the northern emissions, consistent with a recent suggestion that bidirectional auroral electron acceleration may generate in phase SKR emissions in both hemispheres.

The Cassini mission entered the Grand Finale phase in April 2017 and executed 22.5 highly inclined, close-in orbits around Saturn before diving into the planet on September 15th 2017. Here we present our analysis of the Cassini Grand Finale magnetometer (MAG) dataset, focusing on Saturn's internal magnetic field. These measurements demonstrate that Saturn's internal magnetic field is exceptionally axisymmetric, with a dipole tilt less than 0.007 degrees (25.2 arcsecs). Saturn's magnetic equator was directly measured to be shifted northward by ~ 0.0468 +/- 0.00043 (1-sigma) $R_S$, 2820 +/- 26 km, at cylindrical radial distances between 1.034 and 1.069 $R_S$ from the spin-axis. Although almost perfectly axisymmetric, Saturn's internal magnetic field exhibits features on many characteristic length scales in the latitudinal direction. Examining Br at the a=0.75 $R_S$, c=0.6993 $R_S$ isobaric surface, the degrees 4 to 11 contributions correspond to latitudinally banded magnetic perturbations with characteristic width similar to that of the off-equatorial zonal jets observed in the atmosphere of Saturn. Saturn's internal magnetic field beyond 60 degrees latitude, in particular the small-scale features, are less well constrained by the available measurements, mainly due to incomplete spatial coverage in the polar region. A stably stratified layer thicker than 2500 km likely exists above Saturn's deep dynamo to filter out the non-axisymmetric internal magnetic field. A heat transport mechanism other than pure conduction, e.g. double diffusive convection, must be operating within this layer to be compatible with Saturn's observed luminosity. The latitudinally banded magnetic perturbations likely arise from a shallow secondary dynamo action with latitudinally banded differential rotation in the semi-conducting layer., Comment: Accepted for publication in Icarus

International audience; We investigate planetary period oscillations (PPOs) in Saturn's magnetosphere using Cassini magnetic field and Saturn kilometric radiation (SKR) data over the interval from late 2012 to the end of 2015, beginning 3 years after vernal equinox and ending 1.5 years before northern solstice. Previous studies have shown that the northern and southern PPO periods converged across equinox from southern summer values 10.8 h for the southern system and 10.6 h for the northern system and near coalesced 1 year after equinox, before separating again with the southern period 10.69 h remaining longer than the northern 10.64 h. We show that these conditions ended in mid-2013 when the two periods coalesced at 10.66 h and remained so until mid-2014, increasing together to longer periods 10.70 h. During coalescence the two systems were locked near magnetic antiphase with SKR modulations in phase, a condition in which the effects of the generating rotating twin vortex flows in the two ionospheres reinforce each other via hemisphere-to-hemisphere coupling. The magnetic-SKR relative phasing indicates the dominance of postdawn SKR sources in both hemispheres, as was generally the case during the study interval. In mid-2014 the two periods separated again, the northern increasing to 10.78 h by the end of 2015, similar to the southern period during southern summer, while the southern period remained fixed near 10.70 h, well above the northern period during southern summer. Despite this difference, this behavior resulted in the first enduring reversal of the two periods, northern longer than southern, during the Cassini era.

We present a statistical survey of ultralow‐frequency wave activity within the Hermean magnetosphere using the entire MErcury Surface, Space ENvironment, GEochemistry, and Ranging magnetometer data set. This study is focused upon wave activity with frequencies, Key Points Evidence that the Kelvin‐Helmholtz instability is driving Hermean ULF wave activityObservations suggest that Earth‐like field line resonance is possible at MercuryWave power mapped to Mercury's surface reveals the likely average polar cap boundary location

We examine and compare the magnetic field perturbations associated with field-aligned ionosphere-magnetosphere coupling currents at Saturn, observed by the Cassini spacecraft during two sequences of highly inclined orbits in 2006/2007 and 2008 under late southern summer conditions. These sequences explore the southern currents in the dawn-noon and midnight sectors, respectively, thus allowing investigation of possible origins of the local time (LT) asymmetry in auroral Saturn kilometric radiation (SKR) emissions, which peak in power at ~8 h LT in the dawn-noon sector. We first show that the dawn-noon field data generally have the same four-sheet current structure as found previously in the midnight data and that both are similarly modulated by “planetary period oscillation” (PPO) currents. We then separate the averaged PPO-independent (e.g., subcorotation) and PPO-related currents for both LT sectors by using the current system symmetry properties. Surprisingly, we find that the PPO-independent currents are essentially identical within uncertainties in the dawn-dusk and midnight sectors, thus providing no explanation for the LT dependence of the SKR emissions. The main PPO-related currents are, however, found to be slightly stronger and narrower in latitudinal width at dawn-noon than at midnight, leading to estimated precipitating electron powers, and hence emissions, that are on average a factor of ~1.3 larger at dawn-noon than at midnight, inadequate to account for the observed LT asymmetry in SKR power by a factor of ~2.7. Some other factors must also be involved, such as a LT asymmetry in the hot magnetospheric auroral source electron population.

The recently completed MESSENGER mission to Mercury has detected X-ray fluorescence events on the nightside surface of the planet, induced by the precipitation of electrons. We expand upon previously reported catalogues of such events, using a filter based on elemental fluorescence lines to construct a catalogue covering the full five years of the MESSENGER mission. We find that the locations of the majority of these events are ordered in two clear latitudinal bands on the dawn side of the planet centred at ~50°N and ~20°S. Electron precipitation is implied to be either stable or occurring repeatedly on timescales of up to several minutes, long in relation to characteristic times of the Mercury magnetospheric environment. Conversely, X-ray fluorescence events are observed on only ~40% of MESSENGER orbits, although we note that some events are inevitably lost during the filtering process. We suggest that the regions of most intense precipitation are determined by the location of the relevant magnetic field line footprints on the surface. We are able to place speculative limits on the energies of electrons precipitating in this manner based on fluorescence lines in the observed X-ray spectra. The poleward boundaries of the regions of most intense precipitation are found to be collocated with the open-closed field line boundary. We use a magnetic field model to trace field lines from these fluorescence sites to implied locations of origin in the magnetotail.