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On 12 November 2014, the Philae lander descended towards comet 67P/Churyumov–Gerasimenko, bounced twice off the surface, then arrived under an overhanging cliff in the Abydos region. The landing process provided insights into the properties of a cometary nucleus 1 – 3 . Here we report an investigation of the previously undiscovered site of the second touchdown, where Philae spent almost two minutes of its cross-comet journey, producing four distinct surface contacts on two adjoining cometary boulders. It exposed primitive water ice—that is, water ice from the time of the comet’s formation 4.5 billion years ago—in their interiors while travelling through a crevice between the boulders. Our multi-instrument observations made 19 months later found that this water ice, mixed with ubiquitous dark organic-rich material, has a local dust/ice mass ratio of 2.3 − 0.16 + 0.2 : 1 , matching values previously observed in freshly exposed water ice from outbursts 4 and water ice in shadow 5 , 6 . At the end of the crevice, Philae made a 0.25-metre-deep impression in the boulder ice, providing in situ measurements confirming that primitive ice has a very low compressive strength (less than 12 pascals, softer than freshly fallen light snow) and allowing a key estimation to be made of the porosity (75 ± 7 per cent) of the boulders’ icy interiors. Our results provide constraints for cometary landers seeking access to a volatile-rich ice sample. When the Philae lander bounced on the surface of comet 67P/Churyumov–Gerasimenko, it exposed primitive icy-dust material within cometary boulders; the intrinsic strength and porosity of this material is reported.

A drop in the ocean Earth's bulk composition is similar to that of a group of oxygen-poor meteorites called enstatite chondrites, thought to have formed in the early solar nebula. This leads to the suggestion that proto-Earth was dry, and that volatiles including water were delivered by asteroid and comet impacts. The deuterium-to-hydrogen (D/H) ratios measured in six Oort cloud comets are much higher than on Earth, however, apparently ruling out a dominant role for such bodies. Now the Herschel Space Telescope has been used to determine the D/H ratio in the Kuiper belt comet 103P/Hartley 2. The ratio is Earth-like, suggesting that this population of comets may have contributed to Earth's ocean waters. For decades, the source of Earth's volatiles, especially water with a deuterium-to-hydrogen ratio (D/H) of (1.558 ± 0.001) × 10 −4 , has been a subject of debate. The similarity of Earth’s bulk composition to that of meteorites known as enstatite chondrites 1 suggests a dry proto-Earth 2 with subsequent delivery of volatiles 3 by local accretion 4 or impacts of asteroids or comets 5 , 6 . Previous measurements in six comets from the Oort cloud yielded a mean D/H ratio of (2.96 ± 0.25) × 10 −4 . The D/H value in carbonaceous chondrites, (1.4 ± 0.1) × 10 −4 , together with dynamical simulations, led to models in which asteroids were the main source of Earth's water 7 , with ≤10 per cent being delivered by comets. Here we report that the D/H ratio in the Jupiter-family comet 103P/Hartley 2, which originated in the Kuiper belt, is (1.61 ± 0.24) × 10 −4 . This result substantially expands the reservoir of Earth ocean-like water to include some comets, and is consistent with the emerging picture of a complex dynamical evolution of the early Solar System 8 , 9 .

The largest asteroid of the Solar System, (1) Ceres, has been thought to have an icy surface; here it is observed to be emitting water vapour. Water vapour on the asteroid Ceres The presence of hydrated minerals on the surface of Ceres, the largest body in the Solar System's main asteroid belt, suggested that there may be water there too. Now infrared spectra obtained by ESA's Herschel Space Observatory provide unambiguous evidence that there is water ice at or near the surface of Ceres. Water vapour is issuing at a rate of at least 10 26 molecules per second from sources on Ceres localized to mid-latitude regions. The water evaporation could be due to comet-like sublimation or to cryo-volcanism, in which volcanoes erupt volatiles such as water instead of molten rocks. This finding supports models that propose that the icy bodies such as comets may have migrated into the asteroid belt from beyond the notional 'snowline' dividing the early Solar System into a 'dry' inner and 'icy' outer regions. The ‘snowline’ conventionally divides Solar System objects into dry bodies, ranging out to the main asteroid belt, and icy bodies beyond the belt. Models suggest that some of the icy bodies may have migrated into the asteroid belt 1 . Recent observations indicate the presence of water ice on the surface of some asteroids 2 , 3 , 4 , with sublimation 5 a potential reason for the dust activity observed on others. Hydrated minerals have been found 6 , 7 , 8 on the surface of the largest object in the asteroid belt, the dwarf planet (1) Ceres, which is thought to be differentiated into a silicate core with an icy mantle 9 , 10 , 11 . The presence of water vapour around Ceres was suggested by a marginal detection of the photodissociation product of water, hydroxyl (ref. 12 ), but could not be confirmed by later, more sensitive observations 13 . Here we report the detection of water vapour around Ceres, with at least 10 26 molecules being produced per second, originating from localized sources that seem to be linked to mid-latitude regions on the surface 14 , 15 . The water evaporation could be due to comet-like sublimation or to cryo-volcanism, in which volcanoes erupt volatiles such as water instead of molten rocks.

The large domes found on the dwarf planet Ceres may not result from cryovolcanism, but from solid-state flow analogous to salt doming on Earth, according to numerical simulations of gravitational loading.

The Dawn spacecraft finds evidence for organic material and water ice on Ceres A fundamental question in the evolution of the early Earth is the origin of the oceans and of some of the organic molecules that were required for the formation of life. Earth formed in the protoplanetary disk, a mixture of gas and dust. At the location of Earth, temperatures were too high for water vapor and some more volatile organic components to condense. This led to the idea that those materials may have been delivered to Earth by asteroids and/or comets from the outer solar system. Recent spacecraft studies of Comet 67P/Churyumov-Gerasimenko with Rosetta ( 1 , 2 ), and of Ceres on page 719 of this issue by De Sanctis et al. ( 3 ) and by Prettyman et al. ( 4 ) with the Dawn space probe, provide evidence that complex organic molecules and even amino acids are ubiquitous on small bodies in the solar system and that water ice is abundant in the asteroid belt.

Here we describe the novel, multi-point Comet Interceptor mission. It is dedicated to the exploration of a little-processed long-period comet, possibly entering the inner Solar System for the first time, or to encounter an interstellar object originating at another star. The objectives of the mission are to address the following questions: What are the surface composition, shape, morphology, and structure of the target object? What is the composition of the gas and dust in the coma, its connection to the nucleus, and the nature of its interaction with the solar wind? The mission was proposed to the European Space Agency in 2018, and formally adopted by the agency in June 2022, for launch in 2029 together with the Ariel mission. Comet Interceptor will take advantage of the opportunity presented by ESA’s F-Class call for fast, flexible, low-cost missions to which it was proposed. The call required a launch to a halo orbit around the Sun-Earth L2 point. The mission can take advantage of this placement to wait for the discovery of a suitable comet reachable with its minimum Δ V capability of 600  ms − 1 . Comet Interceptor will be unique in encountering and studying, at a nominal closest approach distance of 1000 km, a comet that represents a near-pristine sample of material from the formation of the Solar System. It will also add a capability that no previous cometary mission has had, which is to deploy two sub-probes – B1, provided by the Japanese space agency, JAXA, and B2 – that will follow different trajectories through the coma. While the main probe passes at a nominal 1000 km distance, probes B1 and B2 will follow different chords through the coma at distances of 850 km and 400 km, respectively. The result will be unique, simultaneous, spatially resolved information of the 3-dimensional properties of the target comet and its interaction with the space environment. We present the mission’s science background leading to these objectives, as well as an overview of the scientific instruments, mission design, and schedule.

Critical measurements for understanding accretion and the dust/gas ratio in the solar nebula, where planets were forming 4.5 billion years ago, are being obtained by the GIADA (Grain Impact Analyser and Dust Accumulator) experiment on the European Space Agency’s Rosetta spacecraft orbiting comet 67P/Churyumov-Gerasimenko. Between 3.6 and 3.4 astronomical units inbound, GIADA and OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) detected 35 outflowing grains of mass 10 −10 to 10 −7 kilograms, and 48 grains of mass 10 −5 to 10 −2 kilograms, respectively. Combined with gas data from the MIRO (Microwave Instrument for the Rosetta Orbiter) and ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) instruments, we find a dust/gas mass ratio of 4 ± 2 averaged over the sunlit nucleus surface. A cloud of larger grains also encircles the nucleus in bound orbits from the previous perihelion. The largest orbiting clumps are meter-sized, confirming the dust/gas ratio of 3 inferred at perihelion from models of dust comae and trails.

Images of comet 67P/Churyumov-Gerasimenko acquired by the OSIRIS (Optical, Spectroscopic and Infrared Remote Imaging System) imaging system onboard the European Space Agency’s Rosetta spacecraft at scales of better than 0.8 meter per pixel show a wide variety of different structures and textures. The data show the importance of airfall, surface dust transport, mass wasting, and insolation weathering for cometary surface evolution, and they offer some support for subsurface fluidization models and mass loss through the ejection of large chunks of material.

The OSIRIS cameras (optical, spectroscopic, and infrared remote imaging system) onboard the European Space Agency's Rosetta spacecraft observed comet 9P/Tempel 1 for 17 days continuously around the time of NASA's Deep Impact mission. The cyanide-to-water production ratio was slightly enhanced in the impact cloud, compared with that of normal comet activity. Dust particles were flowing outward in the coma at >160 meters per second, accelerated by the gas. The slope of the brightness increase showed a dip about 200 seconds after the impact. Dust Af[rho] values before and long after the impact confirm the slight decrease of cometary activity. The dust-to-water mass ratio was much larger than 1.

Images from the OSIRIS scientific imaging system onboard Rosetta show that the nucleus of 67P/Churyumov-Gerasimenko consists of two lobes connected by a short neck. The nucleus has a bulk density less than half that of water. Activity at a distance from the Sun of >3 astronomical units is predominantly from the neck, where jets have been seen consistently. The nucleus rotates about the principal axis of momentum. The surface morphology suggests that the removal of larger volumes of material, possibly via explosive release of subsurface pressure or via creation of overhangs by sublimation, may be a major mass loss process. The shape raises the question of whether the two lobes represent a contact binary formed 4.5 billion years ago, or a single body where a gap has evolved via mass loss.