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The Uranus system hosts a diverse set of large moons, from the enigmatic Miranda with its striking coronae, to the outermost and second-largest moon Oberon with its dark and ancient surface. These moons mostly orbit within Uranus’ magnetosphere, which while relatively depleted in low energy-plasma, was found by Voyager 2 to possess surprisingly intense electron radiation belts. Prominent energetic electron absorptions associated with the Uranian moons were observed by Voyager 2, indicating that these particles interact significantly with the moons. Thus, the surfaces of the moons are continuously bombarded by high energy electrons, which are capable of breaking chemical bonds in surface material, leading to radiolytic chemistry that can alter surface composition. In addition, charged particle bombardment can alter the microstructure of surface ice as well as affect grain sizes by sputtering. Ground-based remote sensing observations have revealed planetocentric and hemispherical trends in several key spectral parameters, including the abundance of CO2 ice, whose concentration on the trailing hemispheres of the moon hint at a possible radiolytic origin. Furthermore, possible signatures of NH3 have been detected on the surfaces of several of the moons, including Ariel, where the sub-observer longitudinal distribution of this species appears to support spatial association with geologic features and terrains. It is known from laboratory experiments that NH3 is readily decomposed by charged particle radiation. If NH3 or NH3-related compounds are present on the moons, it therefore could be an indication of recent emplacement or exposure. Here, we will present possible signatures of charged particle surface modification on the Uranian moons and discuss implications for observations by future missions to the Uranus system.

Titania is the fourth major moon from Uranus and is the largest moon in the Uranian system. It has a diameter of ~1580 km, a density of ~1.7 g/cm3, and was imaged by Voyager 2 at a resolution of 2.9 km/pixel. Even with the low-resolution images, it is apparent that Titania has undergone significant tectonic deformation (Smith et al., 1986) and potentially recent heating events (e.g., Moore et al., 2004). Although there is a significant number craters on Titania, its surfaces exhibit evidence for resurfacing in the chasmata, large (>5 km wide, >1 km deep) canyons that extend for 10s of kilometers, located near the equator. In this work, we will reprocess the Voyager 2 images of Titania and perform new geologic mapping of Titania’s Messina Chasmata region. Using these images and a digital elevation model, we will investigate flexure reflected by Messina to estimate Titania’s heat fluxes in this region.

The results of two radiation test campaigns on a low-cost commercial off-the-shelf magnetometer are presented. The test setup and the total ionization dose (TID) levels studied were designed to meet the requirements of a mission to land on Europa. Based on the Europa Lander Science Definition Team report, instruments inside an aluminum vault at the surface of Europa would need to withstand TID of up to 300 krad(SI). In order to evaluate the performance of the PNI RM3100 magnetometer, nine separate sensors were irradiated at two different facilities during two separate campaigns and under different configurations, including passive and active tests. Of the nine sensors, seven survived the TID of 300 krad(SI) while the other two sensors started presenting failures after reaching 150 krad(SI). Post-irradiation tests showed that eight of the nine sensors continued to work without appreciable degradation after stopping exposure, while one sensor stopped working altogether.

Despite the harsh conditions in the atmosphere of Venus, the possibility for an aerial habitable zone exists. A thermal habitable zone is predicted to exist at an altitude range of 62 to 48 km, above which temperatures drop below the lower thermal limit of cell growth and below which temperatures exceed the evaporation temperature. Many biocidal factors must be considered for the complete definition of an aerial habitable zone; in this study we consider the constraint specifically from the perspective of biocidal solar ultraviolet (UV) intensity in the atmosphere. We simulated the penetration of solar ultraviolet and visible light through the atmosphere using a radiative transfer model, to determine the spectral environment (and thus the UV biocidal effect) as a function of altitude in the atmosphere of Venus. At the top of the thermal aerial habitable zone (62 km) the incoming solar irradiance creates a severely challenging UV environment, with extremophiles such as Deinococcus radiodurans expected to be able to endure these UV conditions for approximately 80 s. At an altitude of around 59 km the biologically-weighted UV irradiance drops below that calculated for the Archean Earth, and continues to fall with decreasing altitude until at 54 km it is less than that found currently at the surface of Earth. Crucially, longer wavelength photosynthetically active light continues to penetrate to these altitudes and below, resulting in a solar radiation environment in the venusian atmosphere below around 54 km that screens biologically-damaging UV radiation yet permits the process of photosynthesis. Whilst not claiming to suggest the existence of an aerial habitable zone in general, by considering thermal conditions, ionising radiation and the UV flux environment of the venusian cloud deck alone, we define a potential habitable zone that extends from 59 km to 48 km. This region should form the focus of future remote and in situ astrobiological investigations of Venus.

The Galileo mission to Jupiter discovered magnetic signatures associated with hidden sub-surface oceans at the moons Europa and Callisto using the phenomenon of magnetic induction. These induced magnetic fields originate from electrically conductive layers within the moons and are driven by Jupiter’s strong time-varying magnetic field. The ice giants and their moons are also ideal laboratories for magnetic induction studies. Both Uranus and Neptune have a strongly tilted magnetic axis with respect to their spin axis, creating a dynamic and strongly variable magnetic field environment at the orbits of their major moons. Although Voyager-2 visited the ice giants in the 1980s, it did not pass close enough to any of the moons to detect magnetic induction signatures. However, Voyager-2 revealed that some of these moons exhibit surface features that hint at recent geologically activity, possibly associated with sub-surface oceans. Future missions to the ice giants may therefore be capable of discovering sub-surface oceans, thereby adding to the family of known “ocean worlds” in our solar system. Here, we assess magnetic induction as a technique for investigating sub-surface oceans within the major moons of Uranus. Furthermore, we establish the ability to distinguish induction responses created by different interior characteristics that tie into the induction response: ocean thickness, conductivity, and depth, and ionospheric conductance. The results reported here demonstrate the possibility of single-pass ocean detection and constrained characterization within the moons of Miranda, Ariel, and Umbriel, and provide guidance for magnetometer selection and trajectory design for future missions to Uranus.

The 27 moons of Uranus (Figure 1) are enigmatic and remain poorly understood. Voyager 2 flew by the Uranus system in 1986, collecting fascinating images of its five largest, tidally-locked ‘classical’ moons (Figure 2), while also discovering a bevy of small moons nestled in its ring system (e.g., [1]) (Figure 3). The surfaces of Uranus’ classical moons Miranda, Ariel, Umbriel, Titania, and Oberon have been modified by endogenic activity, in particular Miranda and Ariel, which exhibit substantial evidence for geologic communication between their interiors and surfaces (e.g., [1-3]) (Figure 2). The available images therefore indicate that these classical moons are candidate ocean worlds, which have, or had, liquid H2O layers beneath their icy exteriors (e.g., [3-5]). Because the Voyager 2 flyby occurred near Uranus’ southern summer solstice (subsolar latitude ~81°S), the collected images are centered near the south poles of these moons, and their northern hemispheres were largely unobservable. Furthermore, only the classical moons and the largest ring moon Puck (Figure 3) were spatially resolved by Voyager 2. The other nine ring moons Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalina, and Belinda were not resolved. Another ring moon, Perdita, was discovered via reanalysis of Voyager 2 data [6], and two more ring moons, Cupid and Mab [7,8], were discovered by space-based telescope observations. All nine known irregular satellites, Francisco, Caliban, Stephano, Trinculo, Sycorax, Margaret, Prospero, Setebos, and Ferdinand, were not detected by Voyager 2 and were discovered later by ground-based observations (e.g., [9-11]). Voyager 2 was not equipped with a near-infrared (NIR) mapping spectrometer, and most of what we know about the compositions of Uranus’ moons has been determined using data collected by ground and space-based telescopes. The surfaces of Uranus’ classical moons are composed of H2O ice mixed with low albedo material that could be rich in organics and silicate minerals (e.g., [12-14]). Carbon dioxide (CO2) has been detected on the classical moons, primarily on their trailing hemispheres, in particular on Ariel [15,16] (Figure 4). Spectrally red material that could be rich in organics has been detected, primarily on the leading hemispheres of these moons (e.g., [17,18]) (Figure 4). Ammonia (NH3) has possibly been detected on the classical moons and may originate from their interiors [18,19]. Although useful, these prior observations are disk-integrated, limiting our ability to constrain the distribution of surface constituents and identify links between volatile species and geologic terrains. Much less is known about the surface compositions of Uranus’ 13 ring moons and nine irregular satellites, which are mostly too faint (Vmag 19.8 - 25.8) for spectroscopic analysis using existing facilities. Spectrophotometric datasets indicate that Uranus’ ring moons have dark surfaces that show hints of H2O ice features [6]. Uranus’ irregular satellites have dark, reddish surfaces (e.g., [20]) but little else is known about their surface compositions, except for Sycorax, which shows hints of H2O ice [21]. An orbiting spacecraft collecting data during close flybys of Uranus’ ring system and classical moons would reveal the surface geologies of these moons, including on their previously unobserved northern hemispheres, determine their surface compositions, and determine whether any of the classical moons are, or were, ocean worlds. Furthermore, an orbiter could spend time looking outward to characterize Uranus’ irregular satellites, providing new insight into these likely captured objects (e.g., Jewitt & Haghighipour 2007). By utilizing a Jupiter gravity assist (2030 - 2034 launch window), a mission could arrive at the Uranian system in the mid 2040’s (∼11 years flight time), using existing chemical propulsion technology [22]. This arrival time frame would allow us to observe these moons’ northern hemispheres. An orbiter making close flybys of the classical moons could search for evidence of ongoing geologic activity and characterize migration of CO2 in response to changes in subsolar heating as the Uranian system transitions into southern spring in 2050. To determine whether liquid H2O layers are present in the interiors of the classical moons, the highest priority instrument onboard an orbiter would be a magnetometer, which could detect and characterize induced magnetic fields emanating from briny subsurface oceans. Visible (VIS, 0.4 - 0.7 µm) and mid-infrared (MIR, 5 - 250 µm) cameras would also be vital to search for plume activity, hot spots, and other signs of geologic communication between the interiors and surfaces of these moons. A spectrometer (0.4 - 5 µm) would be critical for characterizing volatile species that might result from outgassing of material or recently exposed or emplaced surface deposits. The abundant evidence for geologic activity in the recent past on Ariel and Miranda likely makes them the highest priority targets for any mission that aims to characterize Uranus’ satellites. References: [1] Smith, B. A. et al. 1986, Science, 233, 43. [2] Schenk, P. M. 1991, JGR: Solid Earth, 96, 1887. [3] Beddingfield, C. B. & Cartwright, R. J. 2020, Icarus, 113687. [4] Hendrix, A. R. et al. 2019, Astrobiology, 19, 1. [5] Cartwright, R.J. et al. 2021. arXiv preprint arXiv:2105.01164. [6] Karkoschka, E. 2001, Icarus, 151, 51. [7] Showalter, M. R. & Lissauer, J. J. 2006, Science, 311, 973. [8] De Pater, I. et al. 2006, Science, 312, 92. [9] Gladman, B. J. et al. 1998, Nature, 392, 897. [10] Kavelaars, J. et al. 2004, Icarus, 169, 474. [11] Sheppard, S. S. et al. 2005, AJ, 129, 518. [12] Cruikshank, D. et al. 1977, AJ, 217, 1006. [13] Clark, R. N. & Lucey, P. G. 1984, JGR: Solid Earth, 89, 6341. [14] Brown, R. H. & Clark, R. N. 1984, Icarus, 58, 288. [15] Grundy, W. et al. 2006, Icarus, 184, 543. [16] Cartwright, R. J. et al. 2015, Icarus, 257, 428. [17] Buratti, B. J. & Mosher, J. A. 1991, Icarus, 90, 1. [18] Cartwright, R. J. et al. 2018, Icarus, 314, 210. [19] Cartwright, R. J. et al. 2020c, ApJL, 898, L22. [20] Maris, M. et al. 2007, A&A, 472, 311. [21] Romon, J. et al. 2001, A&A, 376, 310. [22] Hofstadter, M. et al. 2019, Planetary and Space Science, 177, 104680.

Ganymede's surface exhibits great geological diversity, with old dark terrains, expressed through the surface composition, which is known to be dominated by two constituents: H2O-ice and an unidentified darkening agent. In this paper, new investigations of the composition of Ganymede's surface at global scale are presented. The analyses are derived from the linear spectral modeling of a high spectral resolution dataset, acquired with the near-infrared (1.40–2.50 μm) ground-based integral field spectrometer SINFONI (SINgle Faint Object Near-IR Investigation) of the Very Large Telescope (VLT hereafter) located in Chile. We show that, unlike the neighboring moon Europa, photometric corrections cannot be performed using a simple Lambertian model. However, we find that the Oren-Nayar (1994) model, generalizing the Lambert's law for rough surfaces, produces excellent results. Spectral modeling confirms that Ganymede's surface composition is dominated by H2O-ice, which is predominantly crystalline, as well as a darkening agent, but it also clearly highlights the necessity of secondary species to better fit the measurements: sulfuric acid hydrate and salts, likely sulfates and chlorinated. A latitudinal gradient and a hemispherical dichotomy are the strongest spatial patterns observed for the darkening agent, the H2O-ice, and the sulfuric acid: the darkening agent is by far the major compound at the equator and mid-latitudes (≤ ± 35°N), especially on the trailing hemisphere, while the H2O-ice and the sulfuric acid are mostly located at high latitudes and on the leading hemisphere. This anti-correlation is likely a consequence of the bombardment of the constituents in the Jovian magnetosphere which are much more intense at latitudes higher than ±35°N. Furthermore, the modeling confirms that polar caps are enriched in small, fresh, H2O-ice grains (i.e. ≤50 μm) while equatorial regions are mostly composed of larger grains (i.e. ≥200 μm, up to 1 mm). Finally, the spatial distribution of the salts is neither related to the Jovian magnetospheric bombardment nor the craters. These species are mostly detected on bright grooved terrains surrounding darker areas. Endogenous processes, such as freezing of upwelling fluids going through the ice shell, may explain this distribution. In addition, a small spectral residue that might be related to brines and/or hydrated silica-bearing minerals are located in the same areas.

The 27 satellites of Uranus are enigmatic, with dark surfaces coated by material that could be rich in organics. Voyager 2 imaged the southern hemispheres of Uranus' five largest 'classical' moons Miranda, Ariel, Umbriel, Titania, and Oberon, as well as the largest ring moon Puck, but their northern hemispheres were largely unobservable at the time of the flyby and were not imaged. Additionally, no spatially resolved datasets exist for the other 21 known moons, and their surface properties are essentially unknown. Because Voyager 2 was not equipped with a near-infrared mapping spectrometer, our knowledge of the Uranian moons' surface compositions, and the processes that modify them, is limited to disk-integrated datasets collected by ground- and space-based telescopes. Nevertheless, images collected by the Imaging Science System on Voyager 2 and reflectance spectra collected by telescope facilities indicate that the five classical moons are candidate ocean worlds that might currently have, or had, liquid subsurface layers beneath their icy surfaces. To determine whether these moons are ocean worlds, and investigate Uranus' ring moons and irregular satellites, close-up observations and measurements made by instruments onboard a Uranus orbiter are needed., Accepted in AAS Planetary Science Journal. arXiv admin note: text overlap with arXiv:2007.07284

Icy bodies are the most numerous and diverse bodies in the Solar System, but only a few have been visited or will be visited by a space probe. The Galilean satellites are one of those, especially Ganymede which is the primary target of the future L-class mission JUICE (launch scheduled in 2022) in ESA’s Cosmic Vision programme. Ganymede's surface visually exhibits an important geological diversity, with young bright areas to older dark terrains. This diversity also expresses itself through the moon’s surface composition, which was studied extensively in situ by the NIMS instrument of the Galileo mission (NASA) in the late 90s; like the majority of giant planet satellites, Ganymede's surface is dominated by H2O-ice and some non-icy components, very likely hydrated salts based on the distorted shape of the spectral signatures (McCord et al., 2001). However, this binary composition has been obtained with a spectral sampling (~25 µm) not allowing to detect specific absorptions for the non-icy materials, thus not allowing their identification. Hence, many questions about Ganymede’s surface composition remain unanswered while important technical advances have been made since. In preparation of the JUICE mission, and specifically of the near-infrared imaging spectrometer MAJIS of the JUICE mission, a ground-based campaign was performed using an instrument with a much finer, i.e. better, spectral sampling: SINFONI (SINgle Faint Object Near-IR Investigation). SINFONI is installed on the UT4 of the Very Large Telescope (VLT hereafter) at the European Southern Observatory (ESO hereafter) in Chile. It combines one adaptive optics module and an integral field spectrometer operating in the near-infrared covering from the beginning of the J-band (~1.1 µm) to the end of the K-band (~2.45 µm). Here we present the results derived from the analysis and the modeling of four observations acquired at different dates, from October 2012 to March 2015, all covering the 1.45 – 2.45 µm wavelength range with a spectral resolution about 0.5 µm and a spatial sampling of 12.5 x 12.5 mas2. These results were recently published in Icarus (Ligier et al., 2019). The first result we obtained concerns the physical properties of Ganymede’s surface. Indeed, the data reduction process highlights that the Lambertian model is not sufficient to remove the photometric effects due to observations geometry. Instead, the Oren-Nayar model (Oren & Nayar, 1994), which generalizes the Lambertian law for rough surfaces, produces excellent results where no inclination residuals are observed up to inclination angles around 65°. The quality of the photometric correction is thus used as proxy to infer Ganymede’s surface roughness: from 16° ± 6° to 21° ± 6° depending on the observations. Then, concerning the surface composition of the moon, our modeling confirms that it is dominated by H2O-ice, predominantly the crystalline form. The abundance maps of the ice show two main patterns: (1) a latitudinal gradient in terms of abundance, with large polar caps, and (2) a latitudinal gradient in terms of grain size, where the smaller grains (>50 µm) are located at the highest latitudes, showing a sharp transition around ±35°N coinciding with the transition between open and closed field lines of Ganymede’s own magnetic field (figure 1a). Ice sublimation explains this distribution, redistributing H2O-ice from the “hot” equator to the colder poles, very likely redeposited as finer grains. Apart from the ice, another major compound is required to fit Ganymede’s spectra: a darkening agent. Similarly to previous studies, this darkening agent could not be identified, but we were able to provide new constraints on it. First of all, this unknown material cannot be organic matter since its reflectance level, about 0.25, is more or less five times higher than that of organic matter. Instead, the reflectance level suggests a silicate-type material, as already mentioned in a previous study about Callisto (Calvin & Clark, 1991). Its abundance map shows the highest abundances (up to 0.85) at equatorial latitudes in the trailing hemisphere (figure 1b). The well-known surface sputtering engendered by Jupiter’s magnetosphere is the simplest process to explain such distribution. Even with the magnetic field of the moon, it is possible for corotating singly-charged ions to become neutralized near the moon and continue to the surface as neutrals, sputtering mostly around the trailing apex. Last but not least, our study highlights the necessity of secondary species, i.e. >10% overall, to better fit the measurements: sulfuric acid hydrate and salts, likely sulfates and chlorinated. While the sulfuric acid hydrate is, like H2O-ice, mostly located at high latitudes (figure 1c), the abundance map of the salts shows a heterogeneous distribution which seems neither related to the Jovian magnetospheric bombardment nor craters (figure 1d). These species are mostly detected on bright grooved terrains surrounding darker areas. Endogenous processes, such as freezing of upwelling fluids going through the moon’s ice shell, may explain this heterogeneous distribution.

Mercury has a global dayside exosphere, with measured densities of 10−2 cm−3 at ~1500 km. Here we report on the inferred enhancement of neutral densities (, Mercury has a global dayside exosphere that is very tenuous and does not extend far from the planet. Here, the authors show enhancement of neutral densities at high altitudes inferred from pickup ions that is most likely caused by the impact of a meteroid.

We investigated whether ammonia-rich constituents are present on the surface of the Uranian moon Ariel by analyzing 32 near-infrared reflectance spectra collected over a wide range of sub-observer longitudes and latitudes. We measured the band areas and depths of a 2.2-{\micron} feature in these spectra, which has been attributed to ammonia-bearing species on other icy bodies. Ten spectra display prominent 2.2-{\micron} features with band areas and depths > 2{\sigma}. We determined the longitudinal distribution of the 2.2-{\micron} band, finding no statistically meaningful differences between Ariel's leading and trailing hemispheres, indicating that this band is distributed across Ariel's surface. We compared the band centers and shapes of the five Ariel spectra displaying the strongest 2.2-{\micron} bands to laboratory spectra of various ammonia-bearing and ammonium-bearing species, finding that the spectral signatures of the Ariel spectra are best matched by ammonia-hydrates and flash frozen ammonia-water solutions. Our analysis also revealed that four Ariel spectra display 2.24-{\micron} bands (> 2{\sigma} band areas and depths), with band centers and shapes that are best matched by ammonia ice. Because ammonia should be efficiently removed over short timescales by ultraviolet photons, cosmic rays, and charged particles trapped in Uranus' magnetosphere, the possible presence of this constituent supports geologic activity in the recent past, such as emplacement of ammonia-rich cryolavas and exposure of ammonia-rich deposits by tectonism, impact events, and mass wasting.

Lightning was detected by Voyager 2 at Uranus and Neptune, and weaker electrical processes also occur throughout planetary atmospheres from galactic cosmic ray (GCR) ionisation. Lightning is an indicator of convection, whereas electrical processes away from storms modulate cloud formation and chemistry, particularly if there is little insolation to drive other mechanisms. The ice giants appear to be unique in the Solar System in that they are distant enough from the Sun for GCR-related mechanisms to be significant for clouds and climate, yet also convective enough for lightning to occur. This paper reviews observations (both from Voyager 2 and Earth), data analysis and modelling, and considers options for future missions. Radio, energetic particle and magnetic instruments are recommended for future orbiters, and Huygens-like atmospheric electricity sensors for in situ observations. Uranian lightning is also expected to be detectable from terrestrial radio telescopes., Revised version for Space Science Reviews

Jupiter has the most complex and energetic radiation belts in our Solar System and one of the most challenging space environments to measure and characterize in-depth. Their hazardous environment is also a reason why so many spacecraft avoid flying directly through their most intense regions, thus explaining how Jupiter’s radiation belts have kept many of their secrets so well hidden, despite having been studied for decades. In this paper we argue why these secrets are worth unveiling. Jupiter’s radiation belts and the vast magnetosphere that encloses them constitute an unprecedented physical laboratory, suitable for interdisciplinary and novel scientific investigations: from studying fundamental high energy plasma physics processes which operate throughout the Universe, such as adiabatic charged particle acceleration and nonlinear wave-particle interactions, to exploiting the astrobiological consequences of energetic particle radiation. The in-situ exploration of the uninviting environment of Jupiter’s radiation belts presents us with many challenges in mission design, science planning, instrumentation, and technology. We address these challenges by reviewing the different options that exist for direct and indirect observations of this unique system. We stress the need for new instruments, the value of synergistic Earth and Jupiter-based remote sensing and in-situ investigations, and the vital importance of multi-spacecraft in-situ measurements. While simultaneous, multi-point in-situ observations have long become the standard for exploring electromagnetic interactions in the inner Solar System, they have never taken place at Jupiter or any strongly magnetized planet besides Earth. We conclude that a dedicated multi-spacecraft mission to Jupiter is an essential and obvious way forward for exploring the planet’s radiation belts. Besides guaranteeing numerous discoveries and huge leaps in our understanding of radiation belt systems, such a mission would also enable us to view Jupiter, its extended magnetosphere, moons, and rings under new light, with great benefits for space, planetary, and astrophysical sciences. For all these reasons, in-situ investigations of Jupiter’s radiation belts deserve to be given a high priority in the future exploration of our Solar System. This article is based on a White Paper submitted in response to the European Space Agency’s call for science themes for its Voyage 2050 programme.

We investigate the average configuration and structure of Saturn's magnetosphere in the nightside equatorial and high‐latitude regions. Electron data from the Cassini Plasma Spectrometer's Electron Spectrometer (CAPS‐ELS) is processed to produce a signal‐to‐noise ratio for the entire CAPS‐ELS time of operation at Saturn's magnetosphere. We investigate where the signal‐to‐noise ratio falls below 1, to identify regions in the magnetosphere where there is a significant depletion in the electron content. In the nightside equatorial region we use this to find that the most planetward reconnection x‐line location is at 20 – 25 RS downtail from the planet in the midnight to dawn sector. We also find an equatorial dawn‐dusk asymmetry at a radial distance of >20 RS which may indicate the presence of plasma depleted flux tubes returning to the dayside after reconnection in the tail. Furthermore, we find that the high‐latitude magnetosphere is predominantly in a state of constant plasma depletion and located on open field lines. We map the region of high‐latitude magnetosphere that is depleted of electrons to the polar cap to estimate the size and open flux content within the polar caps. The mean open flux content for the northern and southern polar caps are found to be 25±5 and 32±5 GWb, respectively. The average location of the open‐closed field boundary is found at invariant colatitudes of 12.7±0.6° and 14.5±0.6°. The northern boundary is modulated by planetary period oscillations more than the southern boundary.

Jupiter’s moon Europa, which is thought to possess a large liquid water ocean beneath its icy crust, is one of the most compelling targets in the search for life beyond Earth. Its geologically young surface, along with a number of surface features, indicate that material from Europa’s interior may be emplaced on the surface. However, the surface is affected by the harsh radiation environment of Jupiter’s magnetosphere, which over time may lead to chemical alteration and destruction of potential biosignatures. We show that radiation dose rates are highly dependent on surface location. Radiation processing and destruction of potential biosignatures is found to be significant down to depths of ~1 cm in mid- to high-latitude regions, and to depths of 10–20 cm within ‘radiation lenses’ centred on the leading and trailing hemispheres. These results indicate that future missions to Europa’s surface do not need to excavate material to great depths to investigate the composition of endogenic material and search for potential biosignatures. A model reconstructs the radiation dose from both protons and electrons on Europa’s surface. Using laboratory data on irradiated amino acids, it shows that organics can be preserved at detectable levels at depths of just a few centimetres at mid-to-high latitudes and in young (

The current lattice configuration of the water ice on the surfaces of the inner satellites of Jupiter and Saturn is likely shaped by many factors. But laboratory experiments have found that energetic proton irradiation can cause a transition in the structure of pure water ice from crystalline to amorphous. It is not known to what extent this process is competitive with other processes in solar system contexts. For example, surface regions that are rich in water ice may be too warm for this effect to be important, even if the energetic proton bombardment rate is very high. In this paper, we make predictions, based on particle flux levels and other considerations, about where in the magnetospheres of Jupiter and Saturn the ∼MeV proton irradiation mechanism should be most relevant. Our results support the conclusions of Hansen and McCord (2004), who related relative level of radiation on the three outer Galilean satellites to the amorphous ice content within the top 1 mm of surface. We argue here that if magnetospheric effects are considered more carefully, the correlation is even more compelling. Crystalline ice is by far the dominant ice state detected on the inner Saturnian satellites and, as we show here, the flux of bombarding energetic protons onto these bodies is much smaller than at the inner Jovian satellites. Therefore, the ice on the Saturnian satellites also corroborates the correlation.

With the public and scientific community’s growing interest in ocean worlds, the icy moons of Uranus offer an ideal opportunity to explore a native ice giant satellite system. Although it is uncertain whether any of the Uranian moons currently host subsurface oceans, there is tantalizing evidence—including geologically young surface features and volatiles that are not stable—that this could be the case, making these objects possible ocean worlds in their own right. Determining whether subsurface oceans are present in the interiors of these moons would increase our understanding of the conditions under which subsurface oceans are formed and maintained over the history of the solar system. The presence and stability of a subsurface ocean in the interiors of any icy body is key to identifying its potential as a habitable environment. In this work, we describe a midsize (New Frontiers class) mission concept: the Uranian Magnetosphere and Moons Investigator. The magnetosphere and moons are tightly coupled parts of the Uranian system, complementary to study, and best analyzed together in order to investigate the Uranian moons as potential ocean worlds. Additionally, this mission concept includes study of Uranus’s unique rings and magnetosphere–solar wind interaction. With a future, more detailed trade study, there also could be opportunities for studies of Uranus itself.

Saturn's inner mid-size moons are exposed to a number of external weathering processes, including charged particle bombardment and UV photolysis, as well as deposition of E-ring grains and interplanetary dust. While optical remote sensing observations by several instruments onboard the Cassini spacecraft have revealed a number of weathering patterns across the surfaces of these moons, it is not entirely clear which external process is responsible for which observed weathering pattern. Here we focus on Saturn's moon Mimas and model the effect of energetic electron bombardment across its surface. By using a combination of a guiding center, bounce-averaged charged particle tracing approach and a particle physics code, we investigate how the radiation dose due to energetic electrons is deposited with depth at different locations. We predict a lens-shaped electron energy deposition pattern that extends down to ∼cm depths at low latitudes centered around the apex of the leading hemisphere (90° W). These results are consistent with previous remote sensing observations of a lens-shaped color anomaly observed by the Imaging Science Subsystem (ISS) instrument as well as a thermal inertia anomaly observed by the Visual and Infrared Mapping Spectrometer (VIMS) and the Composite Infrared Spectrometer (CIRS). Our results confirm that these features are produced by MeV electrons that have a penetration depth into the surface comparable to the effective sampling depths of these instruments. On the trailing hemisphere we predict a similar lens-shaped electron energy deposition pattern, whose effects have to date not been observed by the Cassini remote sensing instruments. We suggest that no corresponding lens-shaped weathering pattern has been observed on the trailing hemisphere because of the comparatively short range of lower energy (

The early prediction and subsequent detection of an $\mbox{O}_{2}$ atmosphere on Europa, coupled with the discovery that Europa has an ocean under its ice mantle, has made this moon a prime astrobiologic target, soon to be visited by the JUICE and Europa Clipper spacecraft. In spite of the considerable number of observational, modeling, and laboratory efforts, understanding the physics leading to the observed morphology of Europa’s near-surface $\mbox{O}_{2}$ atmosphere has been problematic. This is the case as the observed emissions depend on the local incident plasma ion flux, the local temperature and composition of the regolith, as well as on the near-surface electron temperature and density. Here we rely heavily on earlier reviews briefly summarizing the observational, laboratory and simulation efforts. Although it is agreed that radiolysis of the surface ice by the incident Jovian plasma is the ultimate source of observed O2, a recent, simple model of thermal desorption from a regolith permeated with $\mbox{O}_{2}$ has changed the usual paradigm. In that model, the observed orbital dependence of the local source of the near-surface O2 atmosphere is suggested to be due to the release of $\mbox{O}_{2}$ likely trapped on the ice grains at dangling bonds by the solar flux with a smaller contribution due to direct sputtering. This assumes that Europa’s icy regolith is permeated with trapped $\mbox{O}_{2}$ , which could also affect our understanding of the suggestion that the radiolytic products in Europa’s regolith might be a source of oxidants for its underground ocean.

In this article, we summarize the work of the NASA Outer Planets Assessment Group (OPAG) Roadmaps to Ocean Worlds (ROW) group. The aim of this group is to assemble the scientific framework that will guide the exploration of ocean worlds, and to identify and prioritize science objectives for ocean worlds over the next several decades. The overarching goal of an Ocean Worlds exploration program as defined by ROW is to “identify ocean worlds, characterize their oceans, evaluate their habitability, search for life, and ultimately understand any life we find.” The ROW team supports the creation of an exploration program that studies the full spectrum of ocean worlds, that is, not just the exploration of known ocean worlds such as Europa but candidate ocean worlds such as Triton as well. The ROW team finds that the confirmed ocean worlds Enceladus, Titan, and Europa are the highest priority bodies to target in the near term to address ROW goals. Triton is the highest priority candidate ocean world to target in the near term. A major finding of this study is that, to map out a coherent Ocean Worlds Program, significant input is required from studies here on Earth; rigorous Research and Analysis studies are called for to enable some future ocean worlds missions to be thoughtfully planned and undertaken. A second finding is that progress needs to be made in the area of collaborations between Earth ocean scientists and extraterrestrial ocean scientists.

Context. Cosmic rays (CRs), which constantly bombard planetary magnetic fields and atmospheres, are the primary driver of atmospheric spallation processes. The higher the energy of these particles, the deeper they penetrate the planetary atmosphere, and the more likely interactions become with the ambient atmospheric material and the evolution of secondary particle showers. Aims. As recently discussed in the literature, CRs are the dominant driver of the Venusian atmospheric ionization and the induced radiation dose below ~100 km. In this study, we model the atmospherically absorbed dose and the dose equivalent to the effect of cosmic rays in the context of Venusian habitability. Methods. The Atmospheric Radiation Interaction Simulator was used to model the altitude-dependent Venusian absorbed dose and the Venusian dose equivalent. For the first time, we modeled the dose rates for different shape-, size-, and composition-mimicking detectors (phantoms): a CO2-based phantom, a water-based microbial cell, and a phantom mimicking human tissue. Results. Based on our new model approach, we give a reliable estimate of the altitude-dependent Venusian radiation dose in water-based microorganisms here for the first time. These microorganisms are representative of known terrestrial life. We also present a detailed analysis of the influence of the strongest ground-level enhancements measured at the Earth’s surface, and of the impact of two historic extreme solar events on the Venusian radiation dose. Our study shows that because a phantom based on Venusian air was used, and because furthermore, the quality factors of different radiation types were not taken into account, previous model efforts have underestimated the radiation hazard for any putative Venusian cloud-based life by up to a factor of five. However, because we furthermore show that even the strongest events would not have had a hazardous effect on putative microorganisms within the potentially habitable zone (51–62 km), these differences may play only a minor role.

Enceladus is unique as an astrobiology target in that it hosts an active plume sourced directly from its habitable subsurface ocean. Ice particles from the plume contain geochemical constituents that are diagnostic of the ocean conditions, and may hold traces of life and/or biosignatures, if they exist. Up to 93% of the plume particles fall back onto the surface of Enceladus. The low radiation environment and present-day activity are favorable to the preservation of any complex organics and putative biosignatures contained within these particles. Laboratory experiments and modeling suggest that plume deposits would likely be weakly consolidated and relatively easy to sample. Sampling systems like a dual rasp, under development to achieve technology readiness level (TRL) 5 in 2021, would enable a landed mission on Enceladus’ surface to acquire large amounts of surface materials, a requirement for analysis of trace constituents. A landed mission on Enceladus could greatly enhance our understanding of the chemical makeup of plume particles and the subsurface ocean, and seek traces of life and/or biosignatures.

Understanding what processes govern atmospheric escape and the loss of planetary water is of paramount importance for understanding how life in the universe can exist. One mechanism thought to be important at all planets is an “ambipolar” electric field that helps ions overcome gravity. We report the discovery and first quantitative extraterrestrial measurements of such a field at the planet Venus. Unexpectedly, despite comparable gravity, we show the field to be five times stronger than in Earth's similar ionosphere. Contrary to our understanding, Venus would still lose heavy ions (including oxygen and all water-group species) to space, even if there were no stripping by the solar wind. We therefore find that it is possible for planets to lose heavy ions to space entirely through electric forces in their ionospheres and such an “electric wind” must be considered when studying the evolution and potential habitability of any planet in any star system.

We investigated whether sulfur-bearing species are present on the icy Galilean moon Callisto by analyzing eight near-infrared reflectance spectra collected over a wide range of sub-observer longitudes. We measured the band areas and depths of a 4-micron feature in these spectra, which has been attributed to sulfur dioxide (SO2), as well as carbonates, in previously collected datasets of this moon. All eight spectra we collected display the 4-micron band. The four spectra collected over Callisto's leading hemisphere display significantly stronger 4-micron bands compared to the four trailing hemisphere spectra (> 3-sigma difference). We compared the central wavelength position and shape of Callisto's 4-micron band to laboratory spectra of various sulfur-bearing species and carbonates. Our comparison demonstrates that Callisto's 4-micron band has a spectral signature similar to thermally-altered sulfur, as well as a 4.025 micron feature attributed to disulfanide (HS2). Our analysis therefore supports the presence of S-bearing species on Callisto but is not consistent with the presence of SO2. The significantly stronger 4-micron band detected on Callisto's leading hemisphere could result from collisions with H2S-rich dust grains that originate on Jupiter's retrograde irregular satellites or implantation of magnetospheric S ions that originate from volcanic activity on Io. Alternatively, S-bearing species could be native to Callisto and are exposed by dust collisions and larger impacts that drive regolith overturn, primarily on its leading side.

In 2011 a thermally anomalous region was discovered on Mimas, Saturn's innermost major icy satellite (Howett et al., 2011). The anomalous region is a lens-like shape located at low latitudes on Mimas' leading hemisphere. It manifests as a region with warmer nighttime temperatures, and cooler daytime ones than its surroundings. The thermally anomalous region is spatially correlated with a darkening in Mimas' IR/UV surface color (Schenk et al., 2011) and the region preferentially bombarded by high-energy electrons (Paranicas et al., 2012, Paranicas et al., 2014; Nordheim et al., 2017). We use data from Cassini's Composite Infrared Spectrometer (CIRS) to map Mimas' surface temperatures and its thermophysical properties. This provides a dramatic improvement on the work in Howett et al. (2011), where the values were determined at only two regions on Mimas (one inside, and another outside of the anomalous region). We use all spatially-resolved scans made by CIRS' focal plane 3 (FP3, 600 to 1100 cm−1) of Mimas' surface, which are largely daytime observations but do include one nighttime one. The resulting temperature maps confirm the presence and location of Mimas' previously discovered thermally anomalous region. No other thermally anomalous regions were discovered, although we note that the surface coverage is incomplete on Mimas' leading and anti-Saturn hemisphere. The thermal inertia map confirms that the anomalous region has a notably higher thermal inertia than its surroundings: 98 ± 42 J m−2 K−1 s-1/2 inside of the anomaly, compared to 34 ± 32 J m−2 K−1 s-1/2 outside. The albedo inside and outside of the anomalous region agrees within their uncertainty: 0.45 ± 0.08 inside compared to 0.41 ± 0.07 outside the anomaly. Interestingly the albedo appears brighter inside the anomaly region, which may not be surprising given this region does appear brighter at some UV wavelengths (0.338 μm, see Schenk et al., 2011). However, this result should be treated with caution because, as previously stated, statistically the albedo of these two regions is the same when their uncertainties are considered. These thermal inertia and albedo values determined here are consistent with those found by Howett et al. (2011), who determined the thermal inertia inside the anomaly to be 66 ± 23 J m−2 K−1 s-1/2 and

Galactic cosmic rays (GCRs) are highly energetic particles that can have significant effects on the atmospheres and potentially also surfaces of (exo)planets and moons. Their propagation through the Sun’s heliosphere and their interaction with planetary bodies have been widely studied in the solar system (e.g., Earth, Mars, Venus, and Europa). There is currently much interest in exoplanetary science, particularly in terms of characterizing the potential habitability of exoplanetary environments. As a consequence of this, models have been developed to quantify the effect of GCRs on exoplanet systems. However, many such studies assume Earth-like (1 au) GCR fluxes. Here we will demonstrate why this is not a reasonable assumption. We briefly discuss the journey that GCRs make from their birth to the arrival at an exoplanet, and discuss the various implications this will have on GCR fluxes. We demonstrate the importance of understanding the specific local interstellar medium (ISM) that an exoplanetary system resides within, as this determines the size of the astrosphere of the host star. This has strong implications for the modulation of GCR fluxes throughout an astrosphere. We estimate how GCR proton fluxes at 1 au (from the Sun) would be different from current values if the solar system was embedded in a different ISM environment. Furthermore, we provide estimates of the wide range of possible GCR proton fluxes at the exoplanets Kepler-20f and Kepler-88c using previously published estimates for the local ISM parameters at these bodies.

While the present-day surface of Venus is certainly incompatible with terrestrial biology, the planet may have possessed oceans in the past and provided conditions suitable for the origin of life. Venusian life may persist today high in the atmosphere where the temperature and pH regime is tolerable to terrestrial extremophile microbes: an aerial habitable zone. Here we argue that on the basis of the combined biological hazard of high temperature and high acidity this habitable zone lies between 51 km (65 °C) and 62 km (−20 °C) altitude. Compared to Earth, this potential venusian biosphere may be exposed to substantially more comic ionising radiation: Venus has no protective magnetic field, orbits closer to the Sun, and the entire habitable region lies high in the atmosphere – if this narrow band is sterilised there is no reservoir of deeper life that can recolonise afterwards. Here we model the propagation of particle radiation through the venusian atmosphere, considering both the background flux of high-energy galactic cosmic rays and the transient but exceptionally high-fluence bursts of extreme solar particle events (SPE), such as the Carrington Event of 1859 and that inferred for AD 775. We calculate the altitude profiles of both energy deposition into the atmosphere and the absorbed radiation dose to assess this astrophysical threat to the potential high-altitude venusian biosphere. We find that at the top of the habitable zone (62 km altitude; 190 g/cm2 shielding depth) the radiation dose from the modelled Carrington Event with a hard spectrum (matched to the February 1956 SPE) is over 18,000 times higher than the background from GCR, and 50,000 times higher for the modelled 775 AD event. However, even though the flux of ionising radiation can be sterilizing high in the atmosphere, the total dose delivered at the top of the habitable zone by a worst-case SPE like the 775 AD event is 0.09 Gy, which is not likely to present a significant survival challenge. Nonetheless, the extreme ionisation could force atmospheric chemistry that may perturb a venusian biosphere in other ways. The energy deposition profiles presented here are also applicable to modelling efforts to understand how fundamental planetary atmospheric processes such as atmospheric chemistry, cloud microphysics and atmospheric electrical systems are affected by extreme solar particle events. The companion paper to this study, Constraints on a potential aerial biosphere on Venus: II. Solar ultraviolet radiation (Patel et al., in preparation), considers the threat posed by penetration of solar UV radiation. The results of these twin studies are based on Venus but are also applicable to extrasolar terrestrial planets near the inner edge of the circumstellar habitable zone.

Lunar meteorites provide important new samples of the Moon remote from regions visited by the Apollo and Luna sample return missions. Petrologic and geochemical analysis of these meteorites, combined with orbital remote sensing measurements, have enabled additional discoveries about the composition and age of the lunar surface on a global scale. However, the interpretation of these samples is limited by the fact that we do not know the source region of any individual lunar meteorite. Here, we investigate the link between meteorite and source region on the Moon using the Lunar Prospector gamma ray spectrometer remote sensing data set for the elements Fe, Ti, and Th. The approach has been validated using Apollo and Luna bulk regolith samples, and we have applied it to 48 meteorites excluding paired stones. Our approach is able broadly to differentiate the best compositional matches as potential regions of origin for the various classes of lunar meteorites. Basaltic and intermediate Fe regolith breccia meteorites are found to have the best constrained potential launch sites, with some impact breccias and pristine mare basalts also having reasonably well-defined potential source regions. Launch areas for highland feldspathic meteorites are much less well constrained and the addition of another element, such as Mg, will probably be required to identify potential source regions for these.

On 26 September 2005, Cassini conducted its only close targeted flyby of Saturn's small, irregularly shaped moon Hyperion. Approximately 6 min before the closest approach, the electron spectrometer (ELS), part of the Cassini Plasma Spectrometer (CAPS) detected a field-aligned electron population originating from the direction of the moon's surface. Plasma wave activity detected by the Radio and Plasma Wave instrument suggests electron beam activity. A dropout in energetic electrons was observed by both CAPS-ELS and the Magnetospheric Imaging Instrument Low-Energy Magnetospheric Measurement System, indicating that the moon and the spacecraft were magnetically connected when the field-aligned electron population was observed. We show that this constitutes a remote detection of a strongly negative (~ −200 V) surface potential on Hyperion, consistent with the predicted surface potential in regions near the solar terminator.

We have modeled an electron precipitation pattern expected on Mimas, Tethys, and Dione, using two different approaches. In the first approach, we adapt a previously developed model to compute an integrated energy flux into the surfaces of Mimas, Tethys, and Dione. This is a guiding-center, bounce-averaged model. In the second approach, we track individual particles in an electromagnetic field for an inert or slightly magnetized satellite. This second approach allows us to include the effects of electron pitch angle and gyrophase on the weathering pattern. Both methods converge on an enhanced dose pattern on each satellite’s leading hemisphere that is lens-shaped. We also present mission-averaged electron energy spectra obtained near these satellites by Cassini’s Magnetosphere Imaging Instrument (MIMI). These data are interpreted using our current understanding of both the environment and the instrument’s response. Fits to the data are integrated to find an energy flux into each satellite’s surface, as a function of longitude and latitude. Using positions on the moon accessible to energetic electrons from the modeling and the integrated energy flux based on data, we find lens patterns that fall off with increasing moon latitude. The predicted patterns are qualitatively consistent with some but not all of the optical observations made of these hemispheres.

Jupiter's moon Europa is exposed to Galactic cosmic rays (GCRs), highly energetic particles from deep space that constantly bombard its surface. Here we have investigated the effect of GCRs on Europa's surface by carrying out particle physics simulations of secondary particle cascades that occur within surface ice due to incident GCR ions. We find that shielding by the Jovian magnetosphere prevents a significant fraction of the GCR spectrum from reaching Europa. Furthermore, we find that while GCRs are capable of affecting the surface down to depths of at least 10 m, the radiation dose due to these particles is small. Within the uppermost meter of Europa's surface, radiation due to magnetospheric particle bombardment dominates over the GCR dose except for surface locations at high latitudes, where the magnetospheric radiation flux is small.

Context.Cosmic ray bombardment represents a major source of ionization in planetary atmospheres. The higher the energy of the primary cosmic ray particles, the deeper they can penetrate into the atmosphere. In addition, incident high energy cosmic ray particles induce extensive secondary particle cascades (“air showers”) that can contain up to several billion secondary particles per incoming primary particle. To quantify cosmic ray-induced effects on planetary atmospheres it is therefore important to accurately model the entire secondary particle cascade. This is particularly important in thick planetary atmospheres where the secondary particle cascades can develop extensively before being absorbed by the surface.Aims.Inside the Venusian atmosphere, cosmic rays are the dominant driver for the ionization below an altitude of ~100 km. In this work we revisit the numerical modeling of the galactic and solar cosmic-ray induced atmospheric ionization for cosmic ray ions from Hydrogen (Z= 1) to Nickel (Z= 28) and investigate the influence of strong solar energetic particle events inside the Venusian atmosphere.Methods.The Atmospheric Radiation Interaction Simulator (AtRIS), a newly developed simulation code to model the interaction of the near-(exo)planet particle and radiation field with the (exo)planetary atmosphere, was used to revisit the modeling of the altitude-dependent Venusian atmospheric ionization. Thereby, spherical geometry, the newest version of Geant4 (10.5) as well as the newest Geant4-based hadronic and electromagnetic interaction models were utilized.Results.Based on our new model approach we show that previous studies may have underestimated the galactic cosmic ray-induced atmospheric ion pair production by, amongst others, underestimating the influence of galactic cosmic ray protons above 1 TeV/nuc. Furthermore, we study the influence of 71 exceptionally strong solar particle events that were measured as Ground Level Enhancements at the Earth’s surface, and show a detailed analysis of the impact of such strong events on the Venusian ionization.

The International Space University—Balloon Air traffic control Technology Experiment (I-BATE 1 ) has flown on board two stratospheric balloons and has tracked nearby aircraft by receiving their Automatic Dependent Surveillance-Broadcast (ADS-B) transmissions. Air traffic worldwide is facing increasing congestion. It is predicted that daily European flight volumes will more than double by 2030 compared to 2009 volumes. ADS-B is an air traffic management system being used to mitigate air traffic congestion. Each aircraft is equipped with both a GPS receiver and an ADS-B transponder. The transponder transmits an equipped aircraft's unique identifier, position, heading, and velocity once per second. The ADS-B transmissions can then be received by ground stations for use in traditional air traffic management. Airspace not monitored by these ground stations or other traditional means remains uncontrolled and poorly monitored. A constellation of space-based ADS-B receivers could close these gaps and provide global air traffic monitoring. By flying an ADS-B receiver on a stratospheric balloon, I-BATE has served as a precursor to a constellation of ADS-B-equipped Earth-orbiting satellites. From the ∼30 km balloon altitude, I-BATE tracked aircraft ranging up to 850 km. The experiment has served as a proof of concept for space-based air traffic management and supports a technology readiness level 6 of space-based ADS-B reception.

The planet Uranus, one of the two ice giants in the Solar System, has only been visited once by the Voyager 2 spacecraft in 1986. Ice giants represent a fundamental class of planets, and many known exoplanets fall within this category. Therefore, a dedicated mission to an ice giant is crucial to improve the understanding of the formation, evolution and current characteristics of such planets in order to extend the knowledge of both the Solar System and exoplanetary systems. In the study at hand, the rationale, selection, and conceptual design for a mission to investigate the Uranian system, as an archetype for ice giants, is presented. A structured analysis of science questions relating to the Uranian system is performed, categorized by the themes atmosphere, interior, moons and rings, and magnetosphere. In each theme, science questions are defined, with their relative importance in the theme quantified. Additionally, top-level weights for each theme are defined, with atmosphere and interior weighted the strongest, as they are more related to both exoplanetary systems and the Uranian system, than the other two themes (which are more specific for the planet itself). Several top level mission architecture aspects have been defined, from which the most promising concepts were generated using heuristic methods. A trade-off analysis of these concepts is presented, separately, for engineering aspects, such as cost, complexity, and risk, and for science aspects. The science score for each mission is generated from the capability of each mission concept to answer the science questions. The trade-off results in terms of relative science and engineering weight are presented, and competitive mission concepts are analyzed based on the preferred mission type. A mission design point for a typical flagship science mission is selected from the trade space. It consists of a Uranus orbiter with a dry mass of 2073 kg including 402 kg of payload and a Uranus entry probe, which is to perform measurements down 100 bar atmospheric pressure. The orbiter science phase will consist of a Uranus orbit phase of approximately 2 years in a highly elliptical orbit, during which 36 Uranus orbits are performed. Subsequently, a moon phase is performed, during which the periapsis will be raised in five steps, facilitating 9 flybys of each of Uranus’ major moons. A preliminary vehicle design is presented, seeking the best compromise between the design drivers, which basically derive from the large distance between Uranus and the Earth (e.g., high thermal load during Venus flyby, low thermal load during Uranus science phase, low data-rate during Uranus science phase, the need of radioisotope power source, etc). This paper is the result of a study carried out during the Alpbach Summer School 2012 “Exploration of the icy planets and their systems” and a one-week follow-up meeting in Graz, Austria. The results of this study show that a flagship ESA L-class mission – consisting of an orbiter with a single atmospheric entry probe and flybys of the main satellites – would be able to address the set of science questions which are identified in the study at hand as the most essential for the understanding of Uranus and its system. The spacecraft, as currently designed, could be launched with an Ariane 5, in 2026, arriving at Uranus in 2044, and operating until 2050. The development of a radioactive power source is the main requirement for feasibility for this mission.

著者人数: 114名, Accepted: 2014-08-07, 資料番号: SA1005021000