NASA’s Europa Clipper mission will explore Jupiter’s icy moon via multiple flybys in the early 2030s. The ocean world Europa is one of the most promising locations to search for life elsewhere in the solar system, and thus Europa Clipper’s main goal is to characterize Europa’s habitability. In the future, especially if Europa Clipper finds that Europa is habitable, a follow-on landed mission may explore Europa’s surface to search for evidence of life. Here we show that 12 of the 49 prime-mission Europa Clipper flybys are “reconable,” meaning they contain at least one portion of the ground track where the requirements for collecting data necessary for terrain relative navigation (TRN) are fulfilled. TRN would be used by a future mission to navigate to a landing site. Using data from the prior Galileo mission, we study what is currently known about these reconable areas and rank them. Three reconable flybys are ranked highest because of their particular scientific potential, which we discuss based on existing studies. We also identify and rank supporting flybys, which are not themselves reconable but provide data for additional characterization of reconable areas. These current rankings demonstrate the process for assessing reconable areas and reflect present-day knowledge. Future decisions on landing site location(s) would be based on new knowledge from Europa Clipper and would likely yield different priorities than the current rankings. We conclude that there are areas on Europa with particular scientific interest that Europa Clipper will be able to fully characterize for potential future in situ exploration.

The presence of cryovolcanic activity in the form of geyser-like plumes at Jupiter’s moon Europa is a much-debated topic. As an active plume could allow direct sampling by a passing spacecraft of a potentially habitable interior environment, the detection and analysis of ongoing plume activity would be of the highest scientific value. In the past decade, several studies have interpreted different remote and in situ observations as providing evidence for large gaseous plumes at different locations on Europa. However, definitive proof is elusive, and visible imaging data taken during spacecraft flybys do not reveal clear indications of ongoing activity. After arrival at Jupiter in 2030, the NASA Europa Clipper spacecraft will systematically search for and constrain plume activity at Europa utilizing a variety of investigations and methods during, before, and after close flybys. Given the lack of a confirmed plume detection to date, the Europa Clipper science team has adopted a global plume search strategy, not focusing on any specific geographical area or any specific type of observation. This global search strategy assigns enhanced value to data obtained early in the mission, which allows time for further observations and characterization of any observed plume at later times. Here we describe the current state of knowledge on plume activity, the Europa Clipper search strategy, and the role of various instruments on the Europa Clipper payload in this search.

Jupiter’s moon Io is a highly compelling target for future exploration that offers critical insight into tidal dissipation processes and the geology of high heat flux worlds, including primitive planetary bodies, such as the early Earth, that are shaped by enhanced rates of volcanism. Io is important for understanding the development of volcanogenic atmospheres and mass exchange within the Jupiter system. However, fundamental questions remain about the state of Io’s interior, surface, and atmosphere, as well as its role in the evolution of the Galilean satellites. The Io Volcano Observer (IVO) would advance answers to these questions by addressing three key goals: (A) determine how and where tidal heat is generated inside Io, (B) understand how tidal heat is transported to the surface of Io, and (C) understand how Io is evolving. IVO was selected for Phase A study through the NASA Discovery program in 2020, and, in anticipation of the next New Frontiers (NF) opportunity, an enhanced IVO-NF mission concept would increase the Baseline mission from 10 flybys to 20, with an improved radiation design; employ a Ka -band communication system to double IVO’s total data downlink; add a wide-angle camera for color and stereo mapping; add a dust mass spectrometer; and lower the altitude of later flybys to enable new science. This study compares the architecture, instrument suite, and science objectives for Discovery (IVO) and NF (IVO-NF) missions to Io. IVO can achieve outstanding science results at the Discovery level, but we advocate for continued prioritization of Io for NF.

The Mars Reconnaissance Orbiter (MRO) Context Camera (CTX) imaged two newly formed impact craters on the South Polar Layered Deposits (SPLD) of Mars in 2018 and 2020. These two new craters, the first detected on the SPLD, measure similar to 17 m and similar to 48 m in diameter. Follow-up observations were conducted with the High Resolution Imaging Science Experiment (HiRISE), showing seasonal and interannual changes, and providing stereo coverage for the production of digital terrain models (DTMs). Mars Climate Sounder (MCS) data were obtained over the region of these new impacts, giving surface temperature information for the time interval before and after the impacts were detected. Taken together, the optical and infrared observations of these sites reveal craters largely consistent with the morphologies of other small, dated impact craters on Mars, and crater ejecta patterns that suggest a more dust/regolith-dominated upper few meters of the SPLD in contrast to mid-latitude buried ice and lobate debris aprons (LDAs). This supports previous conclusions that the SPLD may have an upper surface depleted in water ice relative to the North PLDs, possibly the result of a widespread deflation event.

Introduction: Recurring Slope Lineae (RSL) are narrow dark streaks that incrementally lengthen down Martian warm steep slopes [1]. On first approximation, they appear during the Martian summer, disappear in winter and recur annually. This temperature dependence suggested that RSL may be related to flows of liquid water or brines [1-5], possibly fed by groundwater sources [6, 7] or from the deliquescence of hygroscopic salts [8]. Alternatively, RSL have been interpreted as dry granular flows [9, 10, 11, 12], possibly related to aeolian processes [13, 14]. Although a detailed explanation of the RSL nature and formation mechanism is still not complete, the latest evidence points further toward a dry mechanism [15].Here, we provide further evidence for a dry origin of RSL by investigating their colour properties through multiband photometry obtained from 4-filter Colour and Surface Science Imaging System (CaSSIS, [16]) observations at Horowitz crater, Mars. We compare it with multiband photometry of dark tracks left by the passage dust-devils (DDTs), which are dark marks left by the passage of whirlwinds, and regions where surface dust has been partially removed (“dust poor” regions, DPs). Since DDTs and DPs are being formed by ferric dust removal and the exposure of underlying, typically ferrous material, the comparative multiband photometry provided by CaSSIS may help us understand whether RSL are consistent with being formed by the removal of dust. This would contribute to show that RSLs are indeed dry flows of dust and sand. Our comparative photometry is then supported by a photometric modeling approach.Figure 1 CaSSIS NPB (i.e., NIR PAN, BLU) colour composite showing the central peak of Horowitz crater and the location of the RSL, DDTs and “dust-poor” (DP) ROIs. “DR” indicate dust rich regions, “DE” indicate “dust-enriched” regions (i.e., with an intermediate dust content between DP and DR) Methodology: We adapted the technique of [11,12] to compute the relative reflectance of RSL in the NIR (936.7 nm), RED (836.2 nm), PAN (675.0 nm) and BLU (499.9 nm) BLU CaSSIS filters. To do this, we select several regions of interest (ROIs) within RSLs, DDTs and DPs at Horowitz crater. The considered ROIs are shown in Fig. 1. We compute for each filter the RSL relative reflectance as in [11,12] and we repeat the procedure for DDTs and DPs. In our calculations, we apply a first order atmospheric correction by subtracting the I/F of the darkest pixel of each filter before computing the relative reflectance [16,17,18]. The relative albedo profiles are then compared with photometric models of dust deposition derived from the laboratory experiments of [19] and with mixtures of wet and dry martian terrain simulants derived from [20]. Figure 2. Relative albedo of A) RSL B) DDTs and C) DP ROIs with respect to nearby materials and D) corresponding average profileFigure 3. A) RSL and best-fit dust models. The legend reports the estimated dust content for the RSL (numerator) and corresponding nearby material (denominator). B) RSL and best-fit water models. The legend reports the best-fit water contentResults: RSL, DDTs and DPs have similar relative reflectance profiles (Figure 2) within errors, i.e., they are all brighter in the BLU than in the PAN, RED and NIR filters. While there is some variability, there are no significant spectral features that characterize either RSL or DDTs or DPs in the latter three bands, where all show an approximately constant relative reflectance. In addition, photometric models of dust fallout provide significantly better fits than mixtures of dry and wet martian terrain simulants.Discussion and conclusions: The comparison between relative photometry of RSLs, DDTs and DPs in the 4 CaSSIS filters show that these features have a quite similar relative reflectance profile, suggesting that they may have a similar origin. In particular, the higher BLU with respect to PAN, RED and NIR relative reflectance may suggest that the reflectance profiles of both features could be consistent with a ratio between a ferrous material (numerator), exposed by the removal of a ferric surface material such as the Martian dust (denominator). This picture is supported by comparison with photometric model of dust fallout, that consistently provide better fits to the observed relative reflectances than mixtures of dry and wet martian soils.Acknowledgments: The authors wish to thank the spacecraft and instrument engineering teams for the successful completion of the instrument. CaSSIS is a project of the University of Bern and funded through the Swiss Space Office via ESA’s PRODEX programme. The instrument hardware development was also supported by the Italian Space Agency (ASI) (ASI – INAF agreement no. I/018/12/0), INAF/Astronomical Observatory of Padova, and the Space Research Center (CBK) in Warsaw. Support from SGF (Budapest), the University of Arizona (Lunar and Planetary Lab.) and NASA are also gratefully acknowledged. The study has been supported by the Italian Space Agency (ASI-INAF agreement no. 2020-17-HH.0). References: [1] A. S. McEwen et al. (2011), Science (6043), 740–743 [2] Levy, J et al. (2012), Icarus, 219, (1), 1-4 [3] McEwen et al. (2014), Nat. Geosci. 7 (1), 53–58 [4] Stillman et al. (2014), Icarus 233, 328–341 [5] Huber et al., (2020) Icarus 335, 113385. [6] Stilllman et al. (2016), Icarus 265, 125–138. [7] Abotalib and Heggy (2019) ,Nat. Geosci. 12, 235–241 [8] Wang et al. (2020), Icarus 333, 464–480 [9] Dundas et al. (2017), Nat. Geosci. 10 (12), 903–907 [10] Schmidt et al. (2017), Nat. Geosci. 10 (4), 270–273 [11] Schaefer et al. (2019), Icarus 317, 621–648 [12] Munaretto et al. (2020), PSS 187, (2020), 104947 [13] Vincendon et al. (2019), Icarus 325, 115–127 [14] Dundas et al. (2020), Icarus 343, 2020, 113681 [15] McEwen, A. S. et al. (2021), JGR-Planets, in press. [16] Thomas et al. (2017) Space Sci. Rev. 212 (3–4), 1897–1944[17] Chavez Jr, P. S. (1988) .Remote sensing of environment, 24(3), 459-479. [18] Tornabene et al. 2018, SSR,214(1),18 [19] Wells et al., 1984, Icarus 58, 3, 331-338 [20] Gunderson et al 2007, PSS, 55, 10, 1272-1282

Pitted cones are positive-relief features that display prominent central craters, and are common features on the northern lowland plains of Mars. Pitted cones have been proposed to form from different mechanisms, including those producing volcanic cinder cones or sedimentary mud volcanoes. Here, we apply a deep learning model to globally map 65,620 pitted cones on Mars using Context Camera images from the Mars Reconnaissance Orbiter. Using model recall and a hand-mapped dataset, we conjecture there could be up to similar to 81,900-162,000 individual pitted cones on Mars. A majority of pitted cones (>97%) occur in three of the northern plains' basins-Utopia, Isidis, and Acidalia Planitiae-and occur within geologic units from the late Hesperian and the early-middle Amazonian. The global dataset shows cone estimated diameter increases moving poleward, but pitted cones largely disappear north of similar to 50 N, perhaps due to polar processes that erase or modify the cones. Approximately 94% of cones overlie a single geologic unit, the Vastitas Borealis Formation (VBF), interpreted as sediment from the highland terrains deposited via outflow channels. This global distribution supports sedimentary volcanism following deposition of sediments by the outflow channels. On Earth, rapidly-deposited sediments make an ideal setting for trapping water in the subsurface that is later erupted as mud flows, and this appears to be feasible on Mars.

The Europa Imaging System (EIS) consists of a Narrow-Angle Camera (NAC) and a Wide-Angle Camera (WAC) that are designed to work together to address high-priority science objectives regarding Europa’s geology, composition, and the nature of its ice shell. EIS accommodates variable geometry and illumination during rapid, low-altitude flybys with both framing and pushbroom imaging capability using rapid-readout, 8-megapixel (4k × 2k) detectors. Color observations are acquired using pushbroom imaging with up to six broadband filters. The data processing units (DPUs) perform digital time delay integration (TDI) to enhance signal-to-noise ratios and use readout strategies to measure and correct spacecraft jitter. The NAC has a 2.3° × 1.2° field of view (FOV) with a 10-μrad instantaneous FOV (IFOV), thus achieving 0.5-m pixel scale over a swath that is 2 km wide and several km long from a range of 50 km. The NAC is mounted on a 2-axis gimbal, ±30° cross- and along-track, that enables independent targeting and near-global (≥90

The Io Volcano Observer (IVO) [1] is a NASA Discovery mission currently under Phase A study [2]. Its primary goal is a thorough investigation of Io (e.g., [3]), the innermost of Jupiter's Galilean moons and the most volcanically active body in the Solar system. The strategy consists of the observation of Io mainly during ten targeted flybys [4] between August 2033 and April 2037. At this time, IVO will orbit Jupiter on highly eccentric orbits with periods between 78 and 260 days, a minimum Jupiter altitude of ~340000 km, apoapsis distances between 10 and 23 million kilometers, and an orbit inclination of ~45°. Among the remote-sensing and field-and-particle instruments, there are also a narrow-angle camera (NAC; clear aperture of ~15 cm; pixel field-of-view of 10 µrad) and an infrared mapping instrument (TMAP).The irregular moons of Jupiter [5] are a group of Solar system objects which is poorly studied. With 71 currently known objects ranging in size from ~150 x 120 km (Himalia) down to ~2 km, they are the largest sub-group of Jovian moons with respect to quantity. Irregular moons are believed to be remnants from catastrophic collisions of progenitor objects suspected to have been trapped by Jupiter in the early history of the Solar system. Many details and characteristics, including their region of origin and their relationship to other small bodies, are not known [6].The first irregular-moon inventory by a spacecraft orbiting the host planet was successfully performed within the Saturn system by the Cassini spacecraft [7] [8]. Especially in the second half of the mission, approximately one or two days per orbit were used to observe Saturn's irregular moons, resulting in more than 200 observations of these objects. Since Saturn's irregulars were between 4 and >30 million kilometers away from Cassini (except for the targeted Phoebe flyby in June 2004), the objects appear smaller than a pixel in the imaging data. From photometric time series observations, lightcurves were extracted. These lightcurves allowed determination of 24 previously unknown rotation periods and 13 pole solutions, shape models, and phase curves. No such inventory has been done so far at Jupiter, Uranus, or Neptune systems.Irregular moons are difficult to observe from Earth using small and mid-sized telescopes because of their small sizes (most diameters are below 10 km) and thus faint appearance (most are darker than 23rd magnitude). In addition, there is a significant straylight problem from bright Jupiter. Furthermore, since they are quite numerous, thorough inventories are not possible because there are not enough large telescopes available.With a spacecraft like Cassini or IVO orbiting close to these objects, the situation improves fundamentally. No large telescopes are needed anymore because the objects appear much brighter, and because the angular distances to the planet are large. In addition, the long orbits of IVO offer plenty of uninterrupted observation time over many hours and days. In addition, a large phase-angle range as well as out-of-orbit plane observations are possible, adding crucial information that is not possible to achieve from Earth.Similar to the campaign with Cassini at Saturn, the major scientific goal of IVO's observations of irregular moons is to determine fundamental physical properties such as rotation periods, pole directions, sizes, and brightnesses as a function of illumination conditions (phase curves), as well as to provide constraints on object shapes (convex-shape models). This campaign would address many of the fundamental research goals formulated for small Solar system bodies by NASA's Small Bodies Assessment Group [9] [10]. These goals include the role of the irregulars in Solar system formation and dynamical evolution; a more complete understanding of the census, architecture, physical properties (size, shape, mass, density, porosity, spin rate, etc.), surfaces, surface alteration processes, and nature of interiors, as well as relationships to other bodies, events, and processes. With IVO, fundamental remote-sensing data can be obtained to move closer to achieving these goals. Since IVO's orbits are very long, this mission provides an ideal platform for a detailed survey campaign of these intriguing bodies. Performing coordinated observations with other missions (like ESA's JUICE), or combining observations with laboratory measurements (as proposed in the IVO mission as well) will improve the overall value and interpretation of the collected data.Fig. 1: Range (red line) and phase angle (green) of Jupiter as seen from IVO during the orbit tour.Fig. 2: Range, phase angle, and apparent magnitude of Jovian irregular moon Sinope (J9; ø ~ 35 km) as seen from IVO. This plot illustrates the potential of IVO observations of irregular moons. Sinope is the irregular moons where IVO comes closest in the current orbit tour. On 27 Dec 2033, the minimum distance will be 690000 km. In the IVO-NAC, the size of Sinope will be ~6 pxl at this time.References:[1] https://ivo.lpl.arizona.edu/[2] https://www.nasa.gov/press-release/nasa-selects-four-possible-missions-to-study-the-secrets-of-the-solar-system[3] McEwen, A.S., de Kleer, K., Park, R. (2019): Does Io Have a Magma Ocean? eos.org. https://eos.org/features/does-io-have-a-magma-ocean[4] https://youtu.be/7AO4CDwIXv0[5] https://sites.google.com/carnegiescience.edu/sheppard/moons/jupitermoons[6] Nicholson, P.D., Ćuk, M., Sheppard, S.S., Nesvorný, D., Johnson, T.V. (2008): Irregular satellites of the giant planets. In: The Solar System Beyond Neptune (Barucci, M.A. et al., eds.), Space Science Series, The University of Arizona Press, 411-424.[7] Denk, T., Mottola, S., Tosi, F., Bottke, W.F., Hamilton, D.P. (2018): The Irregular Satellites of Saturn. In: Enceladus and the Icy Moons of Saturn (Schenk, P.M., Clark, R.N., Howett, C.J.A., Verbiscer, A.J., Waite, J.H., editors), Space Science Series, The University of Arizona Press, 409-434. doi:10.2458/azu_uapress_9780816537075-ch020.[8] Denk, T., Mottola, S. (2019): Studies of Irregular Satellites: I. Lightcurves and Rotation Periods of 25 Saturnian Moons from Cassini Observations. Icarus 322, 80-103. doi:10.1016/j.icarus.2018.12.040.[9] SBAG (2016): Goals and Objectives for the Exploration and Investigation of the Solar System’s Small Bodies. Version 1, 04 Mar 2016, 41 pp. https://www.lpi.usra.edu/sbag/goals/SBAG_GoalsDoc_ver.1.2.2016.pdf[10] SBAG (2020): Goals and Objectives for the Exploration and Investigation of the Solar System's Small Bodies. Version 2, 19 Feb 2020, 47 pp. https://www.lpi.usra.edu/sbag/goals/SBAG_Goals_Document_2020.pdf

The Mars Reconnaissance Orbiter has been orbiting Mars since 2006 and has acquired >80,000 HiRISE images with sub-meter resolution, contributing to over 2000 peer-reviewed publications, and has provided the data needed to enable safe surface landings in key locations by several rovers or landers. This paper describes the changes to science planning, data processing, and analysis tools since the initial Primary Science Phase in 2006-2008. These changes affect the data used or requested by the community and how they should interpret the data. There have been a variety of complications to the dataset over the years, such as gaps in monitoring due to spacecraft and instrument issues and special events like the arrival of new landers or rovers on Mars or global dust storms. The HiRISE optics have performed well except for a period when temperature uniformity was perturbed, reducing the resolution of some images. The focal plane system now has 12 rather than 14 operational detectors. The first failure (2011) was a unit at the edge of the swath width, reducing image width by 10% rather than creating a gap. The recent (2023) failure was in the middle of the swath. An unusual problem with the analog-to-digital conversion of the signal (resulting in erroneous data) has worsened over time; mitigation steps so far have preserved full-resolution imaging over all functional detectors. Soon, full-resolution imaging will be narrowed to a subset of the detectors and there will be more 2 x 2 binned data. We describe lessons received for future very high-resolution orbital imaging. We continue to invite all interested people to suggest HiRISE targets on Mars via HiWish, and to explore the easy-to-use publicly available images.

Carbon dioxide is Mars' most active volatile. The seasonal and diurnal processes of when and where it condenses and sublimates are determined by energy balance between the atmosphere and surface ice in Mars' vapor pressure equilibrium climate. Mars' current obliquity ensures that the polar caps are stable locations for seasonal condensation. The eccentricity of Mars' orbit is the major driver of differences in seasonal behavior of CO2 between the northern vs southern hemisphere. In particular, the current positions of perihelion and aphelion, in addition to the large elevation difference between the poles, dominate the ways seasonal processes transpire in the two hemispheres. We summarize and discuss the unprecedented observations of these processes that have been collected by the Mars Reconnaissance Orbiter over the last 8.5 Mars Years. The longer southern fall and winter allows more time for CO2 ice to accumulate and densify in the southern hemisphere. Northern winter coincides with the perihelion dust storm season, thus the north polar seasonal ice deposits are expected to contain a greater concentration of dust in relation to CO2 and H2O ices. With less time for densification and more contaminants the northern seasonal layer of CO2 ice is likely weaker than the southern layer.

Martian geomorphology and surface features provide links to understanding past geologic processes such as fluid movement, local and regional tectonics, and feature formation mechanisms. Pitted cones are common features in the northern plains basins of Mars. They have been proposed to have formed from upwelling volatile-rich fluids, such as magma or water-sediment slurries. In this study, we map the spatial distributions of pitted cone features across the Utopia Planitia (UP) basin. Using the Average Nearest Neighbor technique, we find that pitted cone features appear to be clustered across the basin, occurring in a narrow band around the circumferential rim of UP. Additionally, we find that pitted cone features also appear to be aligned in chains that are sub-parallel with the basin margin. Parallel bands of cones generally follow elevation contours of the UP basin, which suggests elevation or a correlated factor plays a major role in pitted cone and cone chain formation. We propose that pitted cones and cone chains may be related to vertical fractures formed around the UP basin rim from subsidence of infilling basin material.

The present-day water cycle on Mars has implications for habitability and future human exploration. Water ice clouds and water vapour have been detected above the Tharsis volcanic province, suggesting the active exchange of water between regolith and atmosphere. Here we report observational evidence for extensive transient morning frost deposits on the calderas of the Tharsis volcanoes (Olympus, Arsia and Ascraeus Montes, and Ceraunius Tholus) using high-resolution colour images from the Colour and Stereo Surface Imaging System on board the European Space Agency's Trace Gas Orbiter. The transient bluish deposits appear on the caldera floor and rim in the morning during the colder Martian seasons but are not present by afternoon. The presence of water frost is supported by spectral observations, as well as independent imagery from the European Space Agency's Mars Express orbiter. Climate model simulations further suggest that early-morning surface temperatures at the high altitudes of the volcano calderas are sufficiently low to support the daily condensation of water-but not CO2-frost. Given the unlikely seasonal nature of volcanic outgassing, we suggest the observed frost is atmospheric in origin, implying the role of microclimate in local frost formation and a contribution to the broader Mars water cycle. High-resolution spacecraft imagery has revealed transient deposits that appear in the early mornings of cold seasons at the high altitudes of the Tharsis volcanoes on Mars, consistent with water frost of atmospheric origin.

Geological investigations planned for the Europa Clipper mission will examine the formation, evolution, and expression of geomorphic structures found on the surface. Understanding geologic features, their formation, and any recent activity are key inputs in constraining Europa’s potential for habitability. In addition to providing information about the moon’s habitability, the geologic study of Europa is compelling in and of itself. Here we provide a high-level, cross-instrument, and cross-discipline overview of the geologic investigations planned within the Europa Clipper mission. Europa’s fascinating collection of ice-focused geology provides an unparalleled opportunity to investigate the dynamics of icy shells, ice-ocean exchange processes, and global-scale tectonic and tidal stresses. We present an overview of what is currently known about the geology of Europa, from global to local scales, highlighting outstanding issues and open questions, and detailing how the Europa Clipper mission will address them. We describe the mission’s strategy for searching for and characterizing current activity in the form of possible active plumes, thermal anomalies, evidence for surface changes, and extremely fresh surface exposures. The complementary and synergistic nature of the data sets from the various instruments and their integration will be key to significantly advancing our understanding of Europa’s geology.

Launched on August 12, 2005, the Mars Reconnaissance Orbiter (MRO) entered Mars orbit on March 10, 2006. Following a period of aerobraking, MRO completed its entry into its primary science orbit in September 2006, initiating a program of systematic observations of Mars that still continues. Five providers, including the Italian Space Agency, provided 6 instruments for flight, observing the surface, atmosphere, and subsurface of Mars with greater spatial resolution and systematic coverage than ever before. Two investigations utilized the spacecraft accelerometers and tracking of the orbiter via the Deep Space Network to study upper atmosphere densities and the gravity field of the planet. The data acquired by MRO have revealed a dynamic planet whose change from an ancient wetter climate to the drier climate of today was a complex transition and not a simple “drying out”. Furthermore, that climate continues to change even today. The diversity of the early habitable environments, the ice ages recorded in the polar cap layering and subsurface ice deposits, the repeating patterns of dust storms, and the revelation of new features at the limit of resolution are all part of the scientific return from MRO during nearly a decade of Mars years. The story of that mission, of the evolution of its capabilities, and its contributions to our current understanding of Mars are the subject of nearly two dozen papers in the Icarus special issue, MRO: Sixteen Years Observing a Changing Mars. Three papers describe in more detail the evolution of instrument operations and data products over the mission; several papers describe new analysis techniques for the radar, including construction of 3-dimensional views, and for the atmospheric sounder, enabling better retrievals in a dusty lower atmosphere. Other papers report on recent research including, but not limited to, dune movement, the roles of water and carbon dioxide ice in surface change, and attempts to understand the formation and fading of the enigmatic recurring slope lineae. This paper describes general aspects of the MRO spacecraft, payload, and mission as context for the special issue papers; it also summarizes scientific results and mission support events on a mission phase by mission phase basis to give a time history of discovery and effort.

Seismic observations of impacts on Mars indicate a higher impact flux than previously measured. Using six confirmed seismic impact detections near the NASA InSight lander and two distant large impacts, we calculate appropriate scalings to compare these rates with lunar-based chronology models. We also update the impact rate from orbital observations using the most recent catalog of new craters on Mars. The snapshot of the current impact rate at Mars recorded seismically is higher than that found using orbital detections alone. The measured rates differ between a factor of 2 and 10, depending on the diameter, although the sample size of seismically detected impacts is small. The close timing of the two largest new impacts found on Mars in the past few decades indicates either a heightened impact rate or a low-probability temporal coincidence, perhaps representing recent fragmentation of a parent body. We conclude that seismic methods of detecting current impacts offer a more complete dataset than orbital imaging.

The goal of NASA’s Europa Clipper mission is to assess the habitability of Jupiter’s moon Europa. After entering Jupiter orbit in 2030, the flight system will collect science data while flying past Europa 49 times at typical closest approach distances of 25–100 km. The mission’s objectives are to investigate Europa’s interior (ice shell and ocean), composition, and geology; the mission will also search for and characterize any current activity including possible plumes. The science objectives will be accomplished with a payload consisting of remote sensing and in-situ instruments. Remote sensing investigations cover the ultraviolet, visible, near infrared, and thermal infrared wavelength ranges of the electromagnetic spectrum, as well as an ice-penetrating radar. In-situ investigations measure the magnetic field, dust grains, neutral gas, and plasma surrounding Europa. Gravity science will be achieved using the telecommunication system, and a radiation monitoring engineering subsystem will provide complementary science data. The flight system is designed to enable all science instruments to operate and gather data simultaneously. Mission planning and operations are guided by scientific requirements and observation strategies, while appropriate updates to the plan will be made tactically as the instruments and Europa are characterized and discoveries emerge. Following collection and validation, all science data will be archived in NASA’s Planetary Data System. Communication, data sharing, and publication policies promote visibility, collaboration, and mutual interdependence across the full Europa Clipper science team, to best achieve the interdisciplinary science necessary to understand Europa.

Introduction: Bright slope streaks are enigmatic surface features of increased albedo found on Martian slopes in low thermal inertia regions [1, 2]. These elongated features are thought to form as the more common dark slope streaks [3, 4] gradually fade or brighten with time [5]. In fact, in a few rare cases slope streaks have been observed to have bright and dark sections [1, 5, 6], which could be taken as evidence of this transition. The fading rate of dark slope streaks has been shown to be around 40 years [7] but the contrast reversal rate is unknown. Several hypotheses attempt to explain the origin of dark slope streaks: Dry-based models encompass formation through dust mass wasting, avalanching or granular flows [1, 6, 8]; and aqueous models cite subsurface aquifers as sources, lubricated dust flows and ground staining from saline fluids [9-13]. Various properties of dark slope streak populations were studied in detail to address their origin [14-16]. However, little is known about the physical parameters that would affect their photometry, e.g., surface texture, roughness and grain size. Here, we address this issue by acquiring multi-angular observations of bright slope streaks using the Colour and Stereo Surface Imaging System (CaSSIS) [17] onboard the ExoMars Trace Gas Orbiter (TGO). When necessary and available, we complement these observations with higher-resolution images of our study sites taken by the Mars Reconnaissance Orbiter’s (MRO) High Resolution Imaging Science Experiment (HiRISE). To obtain regional context for our observations, we used a global Context Camera (CTX) mosaic [18] to grid-map the locations of bright slope streaks in the Arabia Terra region [19]. Results: We surveyed the distribution of bright slope streaks in Arabia Terra using a grid map divided into ~16,000 hexagonal facets, each 20 km in diameter, and each qualitatively surveyed. Additionally, we documented ~30 locations of partially dark-bright slope streaks. We targeted one such peculiar case (first observed by the Mars Orbiter Camera (MOC) [1]) with CaSSIS and HiRISE (Fig. 1). This example features a bright apex that begins to darken towards the middle, and at the distal end appears indistinguishable from a dark slope streak. This same albedo reversal is visible under various lighting geometries (23°-86° incidence). We used these multi angular observations as the basis for photometric measurements of this slope streak, which are seen in Fig 2. We measured I/F values (detected irradiance (I) over the solar irradiance at zero incidence (F), such that I/F = 1 for a normally illuminated, perfectly diffuse reflector) of dark and bright slope streak regions-of-interest (ROIs), and ratioed them over the ROIs of the surrounding surface. This method avoids the reflectance dependency on local slope. Fig. 2 indicates that the dark (black squares) and the bright (red circles) part of the slope streak is up to 4.5% darker and 3.5% brighter respectively. It also appears that between years 2009-2014 (three HiRISE observations) the dark slope streak ROI is darker than in the three CaSSIS observations taken in 2020. Discussion: In the past, the contrast reversal of slope streaks was attributed to either physical fading (coverage by atmospheric dust fallout) [5] or viewing geometry effects [6]. Loose snow avalanches on Earth were suggested as a possible analogous process where the latter effect occurs [e.g. 1, 6]. Interestingly, many dark, bright and dark-bright streaks are frequently found on the same slope, which suggests that their albedo is independent of viewing geometry. This is supported by the multiple observations seen in Fig. 1. Reflective behavior appears to be independent of illumination geometry, i.e. the slope streak exhibits the same contrast reversal in all observations. Further, our quantitative photometric measurements (Fig. 2) support this finding. The ratio of the dark ROI and the surroundings within 3 HiRISE observations is almost linear, but the earlier HiRISE image exhibits a higher ratio. The later (2020) 3 CaSSIS images of the dark ROI ratios illustrate a more faded streak. This fading could be a result of atmospheric dust fallout (e.g. dust storm in MY34), but this would not explain the bright ROI behavior. Another hypothesis is that the fading is related to gravity-c­­­ontrolled processes. Particles at the top of the streak might be more easily churned by wind and transported downwards by mass wasting. To investigate these two hypotheses, we will use a CaSSIS-based Digital Terrain Model (DTM) to accurately measure slopes and constrain their photometric effects, and a reflectance model based on Hapke theory [20] to simulate the difference in spectral reflectance between regolith with a variety of grain size distributions. References: [1] Sullivan R. et al. (2001) JGR, 106, 23607-23633. [2] Schorghofer N. et al. (2002) GRL, 29, 2126. [3] Morris, E. C. (1984) JGR, 87, 1164–1178. [4] Ferguson H. M. & Lucchitta B. K. (1984) NASA Tech. Memo. 86246, 188-190. [5] Schorghofer N. et al. (2007) Icarus, 191, 132-140.[6] Baratoux D. et al. (2006) Icarus, 183, 30-45. [7] Bergonio J. R. et al. (2013) Icarus, 225, 194-199. [8] Chuang F. C. et al. (2007) GRL, 34(L20204). [9] Ferris J. C. et al. (2002), GRL, 29. [10] Miyamoto H. et al. (2004) JGR, 109, E06008. [11] Kreslavsky M. A. and Head J. W. (2009) Icarus, 201, 517-527. [12] Head J. W. et al. (2007) AGU Fall Abstracts, (#P22A-08). [13] Bhardwaj A. et al. (2019) Rev. Geophys., 57, 48-77. [14] Schorghofer N. and King C. M. (2011) Icarus, 216, 159-168. [15] Brusnikin E. S. et al. (2016) Icarus, 278, 52-61. [16] Mushkin A. et al (2010), GRL, 37, L22201. [17] Thomas N. et al. (2017), Space Sci. Rev, 212, 1897-1944. [18] Dickson J. L. et al. (2018), LPSC XLIX, Abstract #2083. [19] Ramsdale J. D. et al. (2017) PSS, 140, 49-61. [20] Hapke, B., 2012. Theory of Reflectance and Emittance Spectroscopy, 2nd ed. Cambridge University Press.

CaSSIS is a high-resolution visual telescope onboard the ExoMars Trace Gas Orbiter. The mission started the primary science phase in April 2018. The relatively small single image footprint (typically 40 km x 9.5 km) when compared to the total surface area of Mars demands that images should be targeted and target selection is key for the science return. This paper describes the science planning concept set around the target selection, and the process followed in order to generate the CaSSIS commands. The tools used are described as well as all the iterations and teams involved. Finally, special cases and the handling of contingencies are discussed. The procedures may serve as a guideline for future high-resolution instruments on missions to planetary objects.

We present an overview of the radiation environment monitoring program planned for the Europa Clipper mission. The harsh radiation environment of Jupiter will be measured by a dedicated Radiation Monitor (RadMon) subsystem, yielding mission accumulative Total Ionizing Dose (TID) and instantaneous electron flux measurements with a 1-Hz cadence. The radiation monitoring subsystem is comprised of a stand alone sensor assembly along with distributed TID assemblies at various locations on the spacecraft. The sensor assembly itself is made of a TID sensor stack using the Metal-Oxide Semiconducting Field-Effect Transistor (MOSFET) and a Charge Rate Monitor (CRM) that uses a stack of bulk charge collection plates. The TID measurements will provide the critical information about the overall radiation levels relevant to the degradation of electronics over time, and the electron flux data can serve as a proxy for the Internal ElectroStatic Discharge (IESD) environment by measuring the >∼1 MeV electron environment. In addition, the radiation monitoring subsystem data will be augmented by serendipitous radiation data from science instruments onboard. This will be enabled by careful modeling and analysis of opportunistic background data from potentially the following instruments: Europa Imaging System (EIS), Europa-Ultraviolet Spectrograph (Europa-UVS), Mapping Imaging Spectrometer for Europa (MISE), MAss Spectrometer for Planetary EXploration (MASPEX), Plasma Instrument for Magnetic Sounding (PIMS), and SUrface Dust Analyzer (SUDA). Based on the current analysis, these instruments will be most sensitive to >1 MeV electrons. As such, the high-energy electron data obtained by the radiation monitoring subsystem will be qualitatively and quantitatively enhanced by the high-energy electron data acquired by the instruments. The holistic radiation monitoring program for the mission will be an extensive collaboration among many teams across the flight and payload systems. Although the radiation monitoring subsystem itself is an engineering resource for the mission, the collective data from the mission can also be used to improve the scientific understanding of the Jovian magnetosphere and the high-energy electron environment near Europa, where the motion of charged particles is perturbed by the local electromagnetic environment. The data could also help in the understanding of the radiation modification of Europa surface compounds, which could subsequently help guide lab experiments to aid in understanding the origin and evolution of surface materials and in constraining the interpretation of observational data. To this end, the radiation monitoring subsystem is a useful resource for helping address the Europa Clipper mission’s primary goal of assessing the habitability of Europa.