BepiColombo is a joint mission between the European Space Agency, ESA, and the Japanese Aerospace Exploration Agency, JAXA, to perform a comprehensive exploration of Mercury. Launched on 20^th October 2018 from the European spaceport in Kourou, French Guiana, the spacecraft is now en route to Mercury. Two orbiters have been sent to Mercury and will be put into dedicated, polar orbits around the planet to study the planet and its environment. One orbiter, Mio, is provided by JAXA, and one orbiter, MPO, is provided by ESA. The scientific payload of both spacecraft will provide detailed information necessary to understand the origin and evolution of the planet itself and its surrounding environment. Mercury is the planet closest to the Sun, the only terrestrial planet besides Earth with a self-sustained magnetic field, and the smallest planet in our Solar System. It is a key planet for understanding the evolutionary history of our Solar System and therefore also for the question of how the Earth and our Planetary System were formed. The scientific objectives focus on a global characterization of Mercury through the investigation of its interior, surface, exosphere, and magnetosphere. In addition, instrumentation onboard BepiColombo will be used to test Einstein’s theory of general relativity. Major effort was put into optimizing the scientific return of the mission by defining a payload such that individual measurements can be interrelated and complement each other.

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).

BepiColombo has a larger and in many ways more capable suite of instruments relevant for determination of the topographic, physical, chemical and mineralogical properties of Mercury’s surface than the suite carried by NASA’s MESSENGER spacecraft. Moreover, BepiColombo’s data rate is substantially higher. This equips it to confirm, elaborate upon, and go beyond many of MESSENGER’s remarkable achievements. Furthermore, the geometry of BepiColombo’s orbital science campaign, beginning in 2026, will enable it to make uniformly resolved observations of both northern and southern hemispheres. This will offer more detailed and complete imaging and topographic mapping, element mapping with better sensitivity and improved spatial resolution, and totally new mineralogical mapping. We discuss MESSENGER data in the context of preparing for BepiColombo, and describe the contributions that we expect BepiColombo to make towards increased knowledge and understanding of Mercury’s surface and its composition. Much current work, including analysis of analogue materials, is directed towards better preparing ourselves to understand what BepiColombo might reveal. Some of MESSENGER’s more remarkable observations were obtained under unique or extreme conditions. BepiColombo should be able to confirm the validity of these observations and reveal the extent to which they are representative of the planet as a whole. It will also make new observations to clarify geological processes governing and reflecting crustal origin and evolution. We anticipate that the insights gained into Mercury’s geological history and its current space weathering environment will enable us to better understand the relationships of surface chemistry, morphologies and structures with the composition of crustal types, including the nature and mobility of volatile species. This will enable estimation of the composition of 12 DiSTAR, Università degli Studi di Napoli, Naples, Italy 13 Aberystwyth University, Aberystwyth, Wales, UK 14 Institut für Geophysik und extraterrestrische Physik, Technische Universität Braunschweig, Braunschweig, Germany 15 Institut für Planetologie, Westfälische Wilhelms Universität Münster, 48149 Münster, Germany 16 Earth-Life Science Institute, Tokyo Institute of Technology, Tokyo, Japan 17 Department of Physics, University of Helsinki, Finland & Institute of Geology, The Czech Academy of Sciences, Prague, Czech Republic 18 Institute for Space Research, Profsojuznaja 84/32, 117997 Moscow, Russian Federation 19 Dipartimento di Ingegneria e Geologia, Università G. D’Annunzio, Chieti, Italy 20 Department of Physics, University of Helsinki, Helsinki, Finland 21 Nordic Optical Telescope, Santa Cruz de La Palma, Canary Islands, Spain 22 Institute of Space Astrophysics and Planetology, INAF, Rome, Italy 23 Finnish Geospatial Research Institute FGI, National Land Survey of Finland, Helsinki, Finland 24 Department of Earth and Environmental Sciences, KU Leuven, 3001 Leuven, Belgium 25 ESTEC, European Space Agency, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands 26 Earth and Planets Laboratory, Carnegie Institution of Washington, Washington DC, USA 27 ESA/ESTEC, SCI-S, Noordwijk, The Netherlands 28 CITEUC, Geophysical & Astronomical Observatory, University of Coimbra, Coimbra, Portugal Rationale for BepiColombo Studies of Mercury’s Surface and Composition Page 3 of 46 66 the mantle from which the crust was derived, and lead to tighter constraints on models for Mercury’s origin including the nature and original heliocentric distance of the material from which it formed.

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.

A full Geant4 model for the BepiColombo MIXS X-ray telescope has been implemented in order to simulate the propagation of fluorescence X-rays through a state-of-the-art micro channel optics (MCP). Our model includes a realistic 3D geometrical description of the instrument, down to micrometer scale for the optics, as well as the physics processes for the reflection of X-ray photons at grazing angles combined with the standard Geant4 electromagnetic interactions. The model has been preliminary validated against design reference simulation software and with experimental test data.

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 Alfvén 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.

While the terrestrial aurorae are known to be driven primarily by the interaction of the Earth's magnetosphere with the solar wind, there is considerable evidence that auroral emissions on Jupiter and Saturn are driven primarily by internal processes, with the main energy source being the planets' rapid rotation. Prior observations have suggested there might be some influence of the solar wind on Jupiter's aurorae and indicated that auroral storms on Saturn can occur at times of solar wind pressure increases. To investigate in detail the dependence of auroral processes on solar wind conditions, a large campaign of observations of these planets has been undertaken using the Hubble Space Telescope, in association with measurements from planetary spacecraft and solar wind conditions both propagated from 1 AU and measured near each planet. The data indicate a brightening of both the auroral emissions and Saturn kilometric radiation at Saturn close in time to the arrival of solar wind shocks and pressure increases, consistent with a direct physical relationship between Saturnian auroral processes and solar wind conditions. At Jupiter the correlation is less strong, with increases in total auroral power seen near the arrival of solar wind forward shocks but little increase observed near reverse shocks. In addition, auroral dawn storms have been observed when there was little change in solar wind conditions. The data are consistent with some solar wind influence on some Jovian auroral processes, while the auroral activity also varies independently of the solar wind. This extensive data set will serve to constrain theoretical models for the interaction of the solar wind with the magnetospheres of Jupiter and Saturn.

We report progress in the design of the BepiColombo Mercury Imaging X-ray Spectrometer (MIXS). This instrument consists of two modules; a Wolter I soft X-ray telescope based on radially packed microchannel plate optics (MIXS-T) and a profiled collimator which uses a square pore square packed microchannel plate array to restrict its field of view (MIXS-C). Both instrument modules have identical focal planes (DEPFET macropixel array) providing an energy resolution of better than 200 eV FWHM throughout the mission. The primary science goal of MIXS is to perform X-ray fluorescence spectroscopy of the Hermean surface with unprecedented spatial and energy resolution. This allows discrimination between different regolith types, and by combining with data from other instruments, between competing models of crustal evolution and planetary formation. MIXS will also probe the complex coupling between the planet's surface, exosphere and magnetosphere by observing Particle Induced X-ray Emission (PIXE).

The coupled upper atmospheres, ionospheres and magnetospheres of the planets were the subject of a joint RAS–G/MIST discussion meeting hosted by the RAS on 10 January 2003, organized by Ingo Mueller-Wodarg and Emma Bunce. The scientific programme consisted of 10 presentations by speakers from the UK, France, USA and Australia.