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The Double Asteroid Redirection Test (DART) spacecraft successfully performed the first test of a kinetic impactor for asteroid deflection by impacting Dimorphos, the secondary of near-Earth binary asteroid (65803) Didymos, and changing the orbital period of Dimorphos. A change in orbital period of approximately 7 min was expected if the incident momentum from the DART spacecraft was directly transferred to the asteroid target in a perfectly inelastic collision 1 , but studies of the probable impact conditions and asteroid properties indicated that a considerable momentum enhancement ( β ) was possible 2 , 3 . In the years before impact, we used lightcurve observations to accurately determine the pre-impact orbit parameters of Dimorphos with respect to Didymos 4 – 6 . Here we report the change in the orbital period of Dimorphos as a result of the DART kinetic impact to be −33.0 ± 1.0 (3 σ ) min. Using new Earth-based lightcurve and radar observations, two independent approaches determined identical values for the change in the orbital period. This large orbit period change suggests that ejecta contributed a substantial amount of momentum to the asteroid beyond what the DART spacecraft carried. The 33 minute change in the orbital period of Dimorphos after the DART kinetic impact suggests that ejecta contributed a substantial amount of momentum to the asteroid compared with the DART spacecraft alone.

Observations of a stellar occultation of Haumea, one of the four known trans-Neptunian dwarf planets, constrain its size, shape and density, and reveal a ring coplanar with Haumea’s largest moon. A ring around Haumea Haumea is a dwarf planet beyond the orbit of Neptune. It is rapidly rotating and very elongated, unlike the other three known trans-Neptunian dwarf planets. Jose Ortiz and collaborators obtained observations from multiple Earth-based telescopes as Haumea passed in front of a background star. This occultation enabled the team to constrain the density of Haumea to an upper limit of about 1,885 kilograms per cubic metre. They also constrained its ellipsoid shape and albedo (0.51). They did not detect an atmosphere around the planet, but found a ring circling it. They determined that the ring is 70 kilometres wide, has a radius of about 2,287 kilometres and lies in the same orbital plane as Haumea's equator and largest moon. It has an orbital period that is three times the spin period of Haumea. The ring absorbed roughly half of the star light coming through, giving it an opacity of 0.5. Haumea—one of the four known trans-Neptunian dwarf planets—is a very elongated and rapidly rotating body 1 , 2 , 3 . In contrast to other dwarf planets 4 , 5 , 6 , its size, shape, albedo and density are not well constrained. The Centaur Chariklo was the first body other than a giant planet known to have a ring system 7 , and the Centaur Chiron was later found to possess something similar to Chariklo’s rings 8 , 9 . Here we report observations from multiple Earth-based observatories of Haumea passing in front of a distant star (a multi-chord stellar occultation). Secondary events observed around the main body of Haumea are consistent with the presence of a ring with an opacity of 0.5, width of 70 kilometres and radius of about 2,287 kilometres. The ring is coplanar with both Haumea’s equator and the orbit of its satellite Hi’iaka. The radius of the ring places it close to the 3:1 mean-motion resonance with Haumea’s spin period—that is, Haumea rotates three times on its axis in the time that a ring particle completes one revolution. The occultation by the main body provides an instantaneous elliptical projected shape with axes of about 1,704 kilometres and 1,138 kilometres. Combined with rotational light curves, the occultation constrains the three-dimensional orientation of Haumea and its triaxial shape, which is inconsistent with a homogeneous body in hydrostatic equilibrium. Haumea’s largest axis is at least 2,322 kilometres, larger than previously thought, implying an upper limit for its density of 1,885 kilograms per cubic metre and a geometric albedo of 0.51, both smaller than previous estimates 1 , 10 , 11 . In addition, this estimate of the density of Haumea is closer to that of Pluto than are previous estimates, in line with expectations. No global nitrogen- or methane-dominated atmosphere was detected.

Saturn’s moon Enceladus harbours a global 1 ice-covered water ocean 2 , 3 . The Cassini spacecraft investigated the composition of the ocean by analysis of material ejected into space by the moon’s cryovolcanic plume 4 – 9 . The analysis of salt-rich ice grains by Cassini’s Cosmic Dust Analyzer 10 enabled inference of major solutes in the ocean water (Na + , K + , Cl – , HCO 3 – , CO 3 2– ) and its alkaline pH 3 , 11 . Phosphorus, the least abundant of the bio-essential elements 12 – 14 , has not yet been detected in an ocean beyond Earth. Earlier geochemical modelling studies suggest that phosphate might be scarce in the ocean of Enceladus and other icy ocean worlds 15 , 16 . However, more recent modelling of mineral solubilities in Enceladus’s ocean indicates that phosphate could be relatively abundant 17 . Here we present Cassini’s Cosmic Dust Analyzer mass spectra of ice grains emitted by Enceladus that show the presence of sodium phosphates. Our observational results, together with laboratory analogue experiments, suggest that phosphorus is readily available in Enceladus’s ocean in the form of orthophosphates, with phosphorus concentrations at least 100-fold higher in the moon’s plume-forming ocean waters than in Earth’s oceans. Furthermore, geochemical experiments and modelling demonstrate that such high phosphate abundances could be achieved in Enceladus and possibly in other icy ocean worlds beyond the primordial CO 2 snowline, either at the cold seafloor or in hydrothermal environments with moderate temperatures. In both cases the main driver is probably the higher solubility of calcium phosphate minerals compared with calcium carbonate in moderately alkaline solutions rich in carbonate or bicarbonate ions. Cassini’s Cosmic Dust Analyzer mass spectra of ice grains emitted by Enceladus show the presence of sodium phosphates, suggesting that phosphorus is readily available in Enceladus’s ocean in the form of orthophosphates.

Saturn’s moon Enceladus harbours a global water ocean 1 , which lies under an ice crust and above a rocky core 2 . Through warm cracks in the crust 3 a cryo-volcanic plume ejects ice grains and vapour into space 4 – 7 that contain materials originating from the ocean 8 , 9 . Hydrothermal activity is suspected to occur deep inside the porous core 10 – 12 , powered by tidal dissipation 13 . So far, only simple organic compounds with molecular masses mostly below 50 atomic mass units have been observed in plume material 6 , 14 , 15 . Here we report observations of emitted ice grains containing concentrated and complex macromolecular organic material with molecular masses above 200 atomic mass units. The data constrain the macromolecular structure of organics detected in the ice grains and suggest the presence of a thin organic-rich film on top of the oceanic water table, where organic nucleation cores generated by the bursting of bubbles allow the probing of Enceladus’ organic inventory in enhanced concentrations. The detection of complex organic molecules with masses higher than 200 atomic mass units in ice grains emitted from Enceladus indicates the presence of a thin organic-rich layer on top of the moon’s subsurface ocean.

Analysis of silicon-rich, nanometre-sized dust particles near Saturn shows them to consist of silica, which was initially embedded in icy grains emitted from Enceladus’ subsurface waters and released by sputter erosion in Saturn’s E ring; their properties indicate their ongoing formation and transport by high-temperature hydrothermal reactions from the ocean floor and up into the plume of Enceladus. Evidence of hydrothermal activity on Enceladus Hsiang-Wen Hsu et al . have analysed the silicon-rich, nanometre-sized dust stream particles in the Saturnian system using the Cosmic Dust Analyser (CDA) onboard the Cassini spacecraft. With the help of experiments and modelling, the particles are interpreted as silica grains that were initially embedded in the icy plume emitted from subsurface waters on Enceladus and released by sputter erosion in Saturn's E ring. Their properties indicate their formation and transport by high-temperature hydrothermal reactions from the ocean floor and up into the plume of Enceladus. Detection of sodium-salt-rich ice grains emitted from the plume of the Saturnian moon Enceladus suggests that the grains formed as frozen droplets from a liquid water reservoir that is, or has been, in contact with rock 1 , 2 . Gravitational field measurements suggest a regional south polar subsurface ocean of about 10 kilometres thickness located beneath an ice crust 30 to 40 kilometres thick 3 . These findings imply rock–water interactions in regions surrounding the core of Enceladus. The resulting chemical ‘footprints’ are expected to be preserved in the liquid and subsequently transported upwards to the near-surface plume sources, where they eventually would be ejected and could be measured by a spacecraft 4 . Here we report an analysis of silicon-rich, nanometre-sized dust particles 5 , 6 , 7 , 8 (so-called stream particles) that stand out from the water-ice-dominated objects characteristic of Saturn. We interpret these grains as nanometre-sized SiO 2 (silica) particles, initially embedded in icy grains emitted from Enceladus’ subsurface waters and released by sputter erosion in Saturn’s E ring. The composition and the limited size range (2 to 8 nanometres in radius) of stream particles indicate ongoing high-temperature (>90 °C) hydrothermal reactions associated with global-scale geothermal activity that quickly transports hydrothermal products from the ocean floor at a depth of at least 40 kilometres up to the plume of Enceladus.

Observations are reported of a permanent, asymmetric dust cloud around the Moon, caused by impacts of high-speed cometary dust particles on eccentric orbits, as opposed to particles of asteroidal origin following near-circular paths striking the Moon at lower speeds. A permanent dust ring around the Moon Before its planned demise on lunar impact in April 2014, NASA's Lunar Atmosphere and Dust Environment Explorer (LADEE) spent some seven months orbiting the Moon's equator, collecting dust particles for spectroscopic analysis. Sketches made by the Apollo 17 astronauts famously showed a lunar horizon glow, triggering suggestions that electrostatic lofting might be generating dense clouds of small dust particles high above the lunar surface. In this first report on the observations made by the Lunar Dust Experiment (LDEX) onboard LADEE, Mihaly Horànyi et al . find no evidence for such clouds. However, they have detected a permanent asymmetric dust cloud around the Moon, supplied by secondary ejecta dust particles produced by the continual surface impacts of high-speed cometary dust particles in eccentric orbits, as opposed to particles of asteroidal origin following near-circular paths and striking the Moon at lower speeds. The lunar surface is exposed to the same stream of interplanetary dust particles as the Earth, and the LDEX data show that the density of the lunar ejecta cloud increases during meteor showers such as the Geminids. Interplanetary dust particles hit the surfaces of airless bodies in the Solar System, generating charged 1 and neutral 2 gas clouds, as well as secondary ejecta dust particles 3 . Gravitationally bound ejecta clouds that form dust exospheres were recognized by in situ dust instruments around the icy moons of Jupiter 4 and Saturn 5 , but have hitherto not been observed near bodies with refractory regolith surfaces. High-altitude Apollo 15 and 17 observations of a ‘horizon glow’ indicated a putative population of high-density small dust particles near the lunar terminators 6 , 7 , although later orbital observations 8 , 9 yielded upper limits on the abundance of such particles that were a factor of about 10 4 lower than that necessary to produce the Apollo results. Here we report observations of a permanent, asymmetric dust cloud around the Moon, caused by impacts of high-speed cometary dust particles on eccentric orbits, as opposed to particles of asteroidal origin following near-circular paths striking the Moon at lower speeds. The density of the lunar ejecta cloud increases during the annual meteor showers, especially the Geminids, because the lunar surface is exposed to the same stream of interplanetary dust particles. We expect all airless planetary objects to be immersed in similar tenuous clouds of dust.

Io experiences tidal deformation as a result of its eccentric orbit around Jupiter, which provides a primary energy source for Io’s continuing volcanic activity and infrared emission 1 . The amount of tidal energy dissipated within Io is enormous and has been suggested to support the large-scale melting of its interior and the formation of a global subsurface magma ocean. If Io has a shallow global magma ocean, its tidal deformation would be much larger than in the case of a more rigid, mostly solid interior 2 . Here we report the measurement of Io’s tidal deformation, quantified by the gravitational tidal Love number k 2 , enabled by two recent flybys of the Juno spacecraft. By combining Juno 3 , 4 and Galileo 5 , 6 – 7 Doppler data from the NASA Deep Space Network and astrometric observations, we recover Re( k 2 ) of 0.125 ± 0.047 (1 σ ) and the tidal dissipation parameter Q of 11.4 ± 3.6 (1 σ ). These measurements confirm that a shallow global magma ocean in Io does not exist and are consistent with Io having a mostly solid mantle 2 . Our results indicate that tidal forces do not universally create global magma oceans, which may be prevented from forming owing to rapid melt ascent, intrusion and eruption 8 , 9 , so even strong tidal heating—such as that expected on several known exoplanets and super-Earths 10 —may not guarantee the formation of magma oceans on moons or planetary bodies. By measuring the tidal deformation of Io as it orbits Jupiter using Juno Doppler and historically available data, the hypothesis of a shallow global magma ocean in Io is shown to be false.

A model of the Moon’s tidal evolution, starting from the fast-spinning, high-obliquity Earth that would be expected after a giant impact, reveals that solar perturbations on the Moon’s orbit naturally produce the current lunar inclination and Earth’s low obliquity. An explanation of Earth's low obliquity Matija Ćuk et al . show that tidal dissipation due to lunar obliquity may have been an important effect during the Moon's tidal evolution, in which case the past lunar inclination would have been larger than can be explained by present theoretical models. They instead propose a tidal evolution model that starts with the Moon in an equatorial orbit around an initially fast-spinning, high-obliquity Earth—a plausible outcome of giant impacts. In this model the solar perturbations on the Moon's orbit naturally induce a large lunar inclination and remove angular momentum from the Earth–Moon system. In the giant-impact hypothesis for lunar origin, the Moon accreted from an equatorial circum-terrestrial disk; however, the current lunar orbital inclination of five degrees requires a subsequent dynamical process that is still unclear 1 , 2 , 3 . In addition, the giant-impact theory has been challenged by the Moon’s unexpectedly Earth-like isotopic composition 4 , 5 . Here we show that tidal dissipation due to lunar obliquity was an important effect during the Moon’s tidal evolution, and the lunar inclination in the past must have been very large, defying theoretical explanations. We present a tidal evolution model starting with the Moon in an equatorial orbit around an initially fast-spinning, high-obliquity Earth, which is a probable outcome of giant impacts. Using numerical modelling, we show that the solar perturbations on the Moon’s orbit naturally induce a large lunar inclination and remove angular momentum from the Earth–Moon system. Our tidal evolution model supports recent high-angular-momentum, giant-impact scenarios to explain the Moon’s isotopic composition 6 , 7 , 8 and provides a new pathway to reach Earth’s climatically favourable low obliquity.

Saturn is orbited by dozens of moons, and the intricate dynamics of this complex system provide clues about its formation and evolution. Tidal friction within Saturn causes its moons to migrate outwards, driving them into orbital resonances that pump their eccentricities or inclinations, which in turn leads to tidal heating of the moons. However, in giant planets, the dissipative processes that determine the tidal migration timescale remain poorly understood. Standard theories suggest an orbital expansion rate inversely proportional to the power 11/2 in distance 1 , implying negligible migration for outer moons such as Saturn’s largest moon, Titan. Here, we use two independent measurements obtained with the Cassini spacecraft to measure Titan’s orbital expansion rate. We find that Titan rapidly migrates away from Saturn on a timescale of roughly ten billion years, corresponding to a tidal quality factor of Saturn of Q ≃ 100, which is more than a hundred times smaller than most expectations. Our results for Titan and five other moons agree with the predictions of a resonance-locking tidal theory 2 , sustained by excitation of inertial waves inside the planet. The associated tidal expansion is only weakly sensitive to orbital distance, motivating a revision of the evolutionary history of Saturn’s moon system. In particular, it suggests that Titan formed much closer to Saturn and has migrated outward to its current position. Titan is migrating away from Saturn on a much shorter timescale than expected, lending support to the resonance-locking tidal theory. This result motivates a revision of the evolutionary history of Saturn’s moon system and may be relevant to other giant planets.

Moons potentially harbouring a global ocean are tending to become relatively common objects in the Solar System 1 . The presence of these long-lived global oceans is generally betrayed by surface modification owing to internal dynamics 2 . Hence, Mimas would be the most unlikely place to look for the presence of a global ocean 3 . Here, from detailed analysis of Mimas’s orbital motion based on Cassini data, with a particular focus on Mimas’s periapsis drift, we show that its heavily cratered icy shell hides a global ocean, at a depth of 20–30 kilometres. Eccentricity damping implies that the ocean is likely to be less than 25 million years old and still evolving. Our simulations show that the ocean–ice interface reached a depth of less than 30 kilometres only recently (less than 2–3 million years ago), a time span too short for signs of activity at Mimas’s surface to have appeared. An analysis of the orbital motion of Saturn’s moon Mimas shows that a recently formed global subsurface ocean lies beneath its cratered icy shell and that this ocean is probably still evolving.