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Wind-blown sand or ice dunes are known on Earth, Mars, Venus, Titan, and comet 67P/Churyumov-Gerasimenko. Telfer et al. used images taken by the New Horizons spacecraft to identify dunes in the Sputnik Planitia region on Pluto (see the Perspective by Hayes). Modeling shows that these dunes could be formed by sand-sized grains of solid methane ice transported in typical Pluto winds. The methane grains could have been lofted into the atmosphere by the melting of surrounding nitrogen ice or blown down from nearby mountains. Understanding how dunes form under Pluto conditions will help with interpreting similar features found elsewhere in the solar system. Science , this issue p. 992 ; see also p. 960 Images from New Horizons show dunes on Pluto, probably formed from sand-sized grains of solid methane. The surface of Pluto is more geologically diverse and dynamic than had been expected, but the role of its tenuous atmosphere in shaping the landscape remains unclear. We describe observations from the New Horizons spacecraft of regularly spaced, linear ridges whose morphology, distribution, and orientation are consistent with being transverse dunes. These are located close to mountainous regions and are orthogonal to nearby wind streaks. We demonstrate that the wavelength of the dunes (~0.4 to 1 kilometer) is best explained by the deposition of sand-sized (~200 to ~300 micrometer) particles of methane ice in moderate winds (<10 meters per second). The undisturbed morphology of the dunes, and relationships with the underlying convective glacial ice, imply that the dunes have formed in the very recent geological past.

The objective of the NASA Psyche mission gravity science investigation is to map the mass distribution within asteroid (16) Psyche to elucidate interior structure and to resolve the question of whether this metal-rich asteroid represents a remnant metal core or whether it is a primordial body that never melted. Measurements of gravity will be obtained via the X-band telecommunication system on the Psyche spacecraft, collected from progressively lower mapping altitudes. Orbital gravity will allow an estimate of G M to better than 0.001 km 3  s −2 . A spherical harmonic model of gravity to degree and order 10 will be achievable and, in concert with spherical harmonic data sets from topography and magnetometry, as well as surface composition data, will provide information regarding the spatial and radial distribution of mass that will be used to constrain the origin and evolution of (16) Psyche.

A shake-up for asteroids The 'ordinary chondrite problem' has been a factor in Solar System astronomy for three decades. It refers to the apparent anomaly that whereas about 80% of the meteorites falling to Earth are 'ordinary chondrites', they are rare among asteroids. The usual explanation is that 'space weathering' processes alter ordinary chondrite surfaces, producing reddened 'S-type' asteroids. A mystery remains, though, in the shape of a rare class of asteroids, the Q-types. These are found only near the Earth, and they do display 'fresh' spectral matches to ordinary chondrites. Now the combination of a new data set of 95 asteroid spectra with their detailed orbital histories shows that all Q-type asteroids have recently passed close to Earth at least within the lunar distance. Thus tidal stresses or seismic shaking during these encounters may have exposed new unweathered material on the surface. Intriguingly a test of this hypothesis may be at hand: 99942 Apophis, a potentially Earth-threatening asteroid currently displaying 'weathered' spectral colours, is due to pass within six orbital radii of Earth in 2029. It is predicted that it will experience a seismic 'fresh shake', which should expose new unreddened material on the surface. Telescopic measurements of asteroids' colours rarely match laboratory reflectance spectra of meteorites owing to a 'space weathering' process that rapidly reddens asteroid surfaces. 'Unweathered' asteroids, however, with spectra matching ordinary chondrite meteorites, are seen only among small bodies with orbits that cross inside the orbits of Mars and Earth. Such unweathered asteroids are now shown to have experienced orbital intersections closer than the Earth–Moon distance within the past half-million years. Telescopic measurements of asteroids’ colours rarely match laboratory reflectance spectra of meteorites owing to a ‘space weathering’ 1 , 2 process that rapidly 3 reddens asteroid surfaces in less than 10 6 years. ‘Unweathered’ asteroids (those having spectra matching the most commonly falling ordinary chondrite meteorites), however, are seen among small bodies the orbits of which cross inside Mars and the Earth. Various explanations have been proposed for the origin of these fresh surface colours, ranging from collisions 4 to planetary encounters 5 . Less reddened asteroids seem to cross most deeply into the terrestrial planet region, strengthening 6 the evidence for the planetary-encounter theory 5 , but encounter details within 10 6 years remain to be shown. Here we report that asteroids displaying unweathered spectra (so-called ‘Q-types’ 7 ) have experienced orbital intersections closer than the Earth–Moon distance within the past 5 × 10 5 years. These Q-type asteroids are not currently found among asteroids showing no evidence of recent close planetary encounters. Our results substantiate previous work 5 : tidal stress 8 , strong enough to disturb and expose unweathered surface grains, is the most likely dominant short-term asteroid resurfacing process. Although the seismology details are yet to be worked out, the identification of rapid physical processes that can produce both fresh and weathered asteroid surfaces resolves the decades-long 9 puzzle of the difference in colour of asteroids and meteorites.

B‐type asteroids have a negative slope from ∼0.5 to ∼1.1 μm and beyond. What causes this? Visible to near‐infrared reflectance spectra (0.4–2.5 μm) are assembled for 22 B‐type asteroids. The spectra fall naturally into three groups: (1) those with negative (blue) spectral shapes like 2 Pallas (7 objects), (2) those with concave curve shapes like 24 Themis (11 objects), and (3) everything else (4 objects). The asteroid spectra are compared to mineral and meteorite spectra from the Reflectance Experiment Laboratory library of 15,000 samples, in a least squares search for particulate analogs, constrained by spectral brightness. The Pallas group objects show a trend of analogs from the CV, CO, and CK meteorite groups. Only three of the seven Pallas‐like objects are determined to be dynamically related (2, 1508, and 6411). The Themis group objects show a trend of analogs from the CI, CM, CR, CI‐Unusual, and CM‐Unusual meteorites (as expected from the work of Hiroi et al. (1996)). Seven of the 11 Themis‐like objects are dynamically related (24, 62, 222, 316, 379, 383, and 431). Allowing for reasonable uncertainties in the spectral matches, we find no need to invoke mineralogies that do not exist in the meteorite collection to explain B‐type spectra or their negative slopes. Our Themis group results are as expected and are consistent with previous work, but our Pallas group results are new and, in some cases, in conflict with previous work.

Astronomical observations indicate that asteroid (16) Psyche is a large, high‐density (likely >3,400 kg·m−3), metal‐rich (30–55 vol. %) asteroid. Psyche may be remnant core material or it could be a primordial, undifferentiated metal‐rich object. We discuss the science objectives of the upcoming Psyche mission, which will employ three instruments (the Magnetometer, Multispectral Imager, and Gamma‐Ray and Neutron Spectrometer) and will use Doppler tracking of the spacecraft to explore the asteroid. This mission will shed light on the nature and origins of metal‐rich objects in the solar system and beyond, including the cores of the terrestrial planets. Plain Language Summary Asteroid (16) Psyche is the largest known metal‐rich asteroid and is a relic of the building blocks of the planets from the early solar system. We hypothesize that it is either an exposed metallic core of an asteroid or unmelted metal‐rich material. NASA's Psyche mission, launched in October 2023, aims to explore Psyche to understand its formation and evolution. The Psyche spacecraft carries three instruments and will use its radio antenna to study Psyche's magnetic field, surface composition, and interior structure. The Psyche mission offers a historic opportunity to study the processes that led to the formation of the metallic cores of planets. Key Points The Psyche mission will explore the solar system's largest likely metal‐rich asteroid, (16) Psyche Exploration of Psyche offers a historic opportunity to explore small body planetary differentiation and core formation The Psyche spacecraft will study Psyche using imaging, nuclear spectroscopy, magnetometry, and gravity measurements

Once perceived as distant, cold, dark, and seemingly unknowable, Pluto had long been marked as the farthest and most unreachable frontier for solar system exploration. After Voyager accomplished its final planetary reconnaissance at Neptune in 1989, Pluto and its cohort in the Kuiper Belt beckoned as the missing puzzle piece for completing the first reconnaissance of our solar system. In the decades following Voyager, a mission to the Pluto system was not only imagined but also achieved, culminating with the historic 2015 flyby by the New Horizons spacecraft. Pluto and its satellite system (\"the Pluto system\"), including its largest moon, Charon, have been revealed to be worlds of enormous complexity that fantastically exceed preconceptions. The Pluto System After New Horizons seeks to become the benchmark for synthesizing our understanding of the Pluto system. The volume's lead editor is S. Alan Stern, who also serves as NASA's New Horizons Principal Investigator; co-editors Richard P. Binzel, William M. Grundy, Jeffrey M. Moore, and Leslie A. Young are all co-investigators on New Horizons . Leading researchers from around the globe have spent the last five years assimilating Pluto system flyby data returned from New Horizons. The chapters in this volume form an enduring foundation for ongoing study and understanding of the Pluto system. The volume also advances insights into the nature of dwarf planets and Kuiper Belt objects, providing a cornerstone for planning new missions that may return to the Pluto system and explore others of the myriad important worlds beyond Neptune.

Asteroid 4 Vesta is the only preserved intact example of a large, differentiated protoplanet like those believed to be the building blocks of terrestrial planet accretion. Vesta accreted rapidly from the solar nebula in the inner asteroid belt and likely melted due to heat released due to the decay of 26 Al. Analyses of meteorites from the howardite-eucrite-diogenite (HED) suite, which have been both spectroscopically and dynamically linked to Vesta, lead to a model of the asteroid with a basaltic crust that overlies a depleted peridotitic mantle and an iron core. Vesta’s crust may become more mafic with depth and might have been intruded by plutons arising from mantle melting. Constraints on the asteroid’s moments of inertia from the long-wavelength gravity field, pole position and rotation, informed by bulk composition estimates, allow tradeoffs between mantle density and core size; cores of up to half the planetary radius can be consistent with plausible mantle compositions. The asteroid’s present surface is expected to consist of widespread volcanic terrain, modified extensively by impacts that exposed the underlying crust or possibly the mantle. Hemispheric heterogeneity has been observed by poorly resolved imaging of the surface that suggests the possibility of a physiographic dichotomy as occurs on other terrestrial planets. Vesta might have had an early magma ocean but details of the early thermal structure are far from clear owing to model uncertainties and paradoxical observations from the HEDs. Petrological analysis of the eucrites coupled with thermal evolution modeling recognizes two possible mechanisms of silicate-metal differentiation leading to the formation of the basaltic achondrites: equilibrium partial melting or crystallization of residual liquid from the cooling magma ocean. A firmer understanding the plethora of complex physical and chemical processes that contribute to melting and crystallization will ultimately be required to distinguish among these possibilities. The most prominent physiographic feature on Vesta is the massive south polar basin, whose formation likely re-oriented the body axis of the asteroid’s rotation. The large impact represents the likely major mechanism of ejection of fragments that became the HEDs. Observations from the Dawn mission hold the promise of revolutionizing our understanding of 4 Vesta, and by extension, the nature of collisional, melting and differentiation processes in the nascent solar system.

Asteroids orbit the Sun, most of them in the asteroid belt between Mars and Jupiter. Robotic operated vehicles (ROVs) will need to be developed to explore asteroids, just as astronauts in orbit around or on the surface of Mars will need to command robotic workhorses. Three asteroid-related concepts should be explored: an asteroid survey to find a series of human destinations on the path to Mars while fulfilling the requirements of the 2005 survey act; a competition to test robotic asteroid-deflection methods on which civilization's survival could one day depend; and another to test robotic methods of extracting water or mining other valuable resources from asteroids that might contribute towards sustaining human spacefaring decades from now.

Asteroid discoveries are essential for planetary-defence efforts aiming to prevent impacts with Earth 1 , including the more frequent 2 megaton explosions from decametre impactors 3 , 4 , 5 – 6 . Although large asteroids (≥100 kilometres) have remained in the main belt since their formation 7 , small asteroids are commonly transported to the near-Earth object (NEO) population 8 , 9 . However, owing to the lack of direct observational constraints, their size–frequency distribution (SFD)—which informs our understanding of the NEOs and the delivery of meteorite samples to Earth—varies substantially among models 10 , 11 , 12 , 13 – 14 . Here we report 138 detections of some of the smallest asteroids (≳10 metres) ever observed in the main belt, which were enabled by JWST’s infrared capabilities covering the emission peaks of the asteroids 15 and synthetic tracking techniques 16 , 17 – 18 . Despite small orbital arcs, we constrain the distances and phase angles of the objects using known asteroids as proxies, allowing us to derive sizes through radiometric techniques. Their SFD shows a break at about 100 metres (debiased cumulative slopes of q  = −2.66 ± 0.60 and −0.97 ± 0.14 for diameters smaller and larger than roughly 100 metres, respectively), suggestive of a population driven by collisional cascade. These asteroids were sampled from several asteroid families—most probably Nysa, Polana and Massalia—according to the geometry of pointings considered here. Through further long-stare infrared observations, JWST is poised to serendipitously detect thousands of decametre-scale asteroids across the sky, examining individual asteroid families 19 and the source regions of meteorites 13 , 14 ‘in situ’. Combining the infrared capabilities of JWST and synthetic tracking techniques, the detection of some of the smallest asteroids ever observed in the main belt is reported; their large abundance reveals a population driven by collisional cascade.