Explore the vast range of titles available.

Impact craters, which can be considered the lunar equivalent of fossils, are the most dominant lunar surface features and record the history of the Solar System. We address the problem of automatic crater detection and age estimation. From initially small numbers of recognized craters and dated craters, i.e., 7895 and 1411, respectively, we progressively identify new craters and estimate their ages with Chang’E data and stratigraphic information by transfer learning using deep neural networks. This results in the identification of 109,956 new craters, which is more than a dozen times greater than the initial number of recognized craters. The formation systems of 18,996 newly detected craters larger than 8 km are estimated. Here, a new lunar crater database for the mid- and low-latitude regions of the Moon is derived and distributed to the planetary community together with the related data analysis. Using Chang’E data, the authors here identify more than 109,000 previously unrecognized lunar craters and date almost 19,000 craters based on transfer learning with deep neural networks. A new lunar crater database is derived and distributed to the planetary community.

The lack of pyrimidine diversity in meteorites remains a mystery since prebiotic chemical models and laboratory experiments have predicted that these compounds can also be produced from chemical precursors found in meteorites. Here we report the detection of nucleobases in three carbonaceous meteorites using state-of-the-art analytical techniques optimized for small-scale quantification of nucleobases down to the range of parts per trillion (ppt). In addition to previously detected purine nucleobases in meteorites such as guanine and adenine, we identify various pyrimidine nucleobases such as cytosine, uracil, and thymine, and their structural isomers such as isocytosine, imidazole-4-carboxylic acid, and 6-methyluracil, respectively. Given the similarity in the molecular distribution of pyrimidines in meteorites and those in photon-processed interstellar ice analogues, some of these derivatives could have been generated by photochemical reactions prevailing in the interstellar medium and later incorporated into asteroids during solar system formation. This study demonstrates that a diversity of meteoritic nucleobases could serve as building blocks of DNA and RNA on the early Earth.

C-type asteroids 1 are considered to be primitive small Solar System bodies enriched in water and organics, providing clues to the origin and evolution of the Solar System and the building blocks of life. C-type asteroid 162173 Ryugu has been characterized by remote sensing 2 – 7 and on-asteroid measurements 8 , 9 with Hayabusa2 (ref.  10 ). However, the ground truth provided by laboratory analysis of returned samples is invaluable to determine the fine properties of asteroids and other planetary bodies. We report preliminary results of analyses on returned samples from Ryugu of the particle size distribution, density and porosity, spectral properties and textural properties, and the results of a search for Ca–Al-rich inclusions (CAIs) and chondrules. The bulk sample mainly consists of rugged and smooth particles of millimetre to submillimetre size, confirming that the physical and chemical properties were not altered during the return from the asteroid. The power index of its size distribution is shallower than that of the surface boulder observed on Ryugu 11 , indicating differences in the returned Ryugu samples. The average of the estimated bulk densities of Ryugu sample particles is 1,282 ± 231 kg m −3 , which is lower than that of meteorites 12 , suggesting a high microporosity down to the millimetre scale, extending centimetre-scale estimates from thermal measurements 5 , 9 . The extremely dark optical to near-infrared reflectance and spectral profile with weak absorptions at 2.7 and 3.4 μm imply a carbonaceous composition with indigenous aqueous alteration, matching the global average of Ryugu 3 , 4 and confirming that the sample is representative of the asteroid. Together with the absence of submillimetre CAIs and chondrules, these features indicate that Ryugu is most similar to CI chondrites but has lower albedo, higher porosity and more fragile characteristics. The Hayabusa2 spacecraft returned 5.4 g of material from the asteroid Ryugu. A first analysis of the samples found an estimated density of 1,282 ± 231 kg m −3 , considerably lower than even the most porous meteorites. Together with preliminary spectral analysis, these results indicate that Ryugu is similar to CI chondrites, but darker, more porous and more brittle.

Phosphorus (P) is critical to modern biochemical functions and can control ecosystem growth. It was presumably important as a reagent in prebiotic chemistry. However, on the early Earth, P sources may have consisted primarily of poorly soluble calcium phosphates, which may have rendered phosphate as a minimally available nutrient or reagent if these minerals were the sole source. Here, we review aqueous P availability on the early Earth (>2.5 Gyr ago), considering both mineral sources and geochemical sinks relevant to its solvation, and activation by abiotic and biological pathways. Phosphorus on Earth’s early surface would have been present as a mixture of phosphate minerals, as a minor element in silicate minerals, and in reactive, reduced phases from accreted dust, meteorites and asteroids. These P sources would have weathered and plausibly furnished the prebiotic Earth with abundant and potentially reactive P. After the origin of a biosphere, life evolved to draw on not just reactive available P sources, but also insoluble and unreactive sources. The rise of an ecosystem dependent on this element at some point forged a P-limited biosphere, with evolutionary stress forcing the efficient extraction and recycling of P from both abiotic and biotic sources and sinks.A review of aqueous phosphorus availability on the Earth’s early surface suggests a range of phosphorus sources supplied the prebiotic Earth, but that phosphorus availability declined as life evolved and altered geochemical cycling.

Chronology of aqueous activity on chondrite parent bodies constrains their accretion times and thermal histories. Radiometric 53 Mn– 53 Cr dating has been successfully applied to aqueously formed carbonates in CM carbonaceous chondrites. Owing to the absence of carbonates in ordinary (H, L and LL), and CV and CO carbonaceous chondrites, and the lack of proper standards, there are no reliable ages of aqueous activity on their parent bodies. Here we report the first 53 Mn– 53 Cr ages of aqueously formed fayalite in the L3 chondrite Elephant Moraine 90161 as Myr after calcium–aluminium-rich inclusions (CAIs), the oldest Solar System solids. In addition, measurements using our synthesized fayalite standard show that fayalite in the CV3 chondrite Asuka 881317 and CO3-like chondrite MacAlpine Hills 88107 formed and Myr after CAIs, respectively. Thermal modelling, combined with the inferred conditions (temperature and water/rock ratio) and 53 Mn– 53 Cr ages of aqueous alteration, suggests accretion of the L, CV and CO parent bodies ∼1.8−2.5 Myr after CAIs. The parent bodies of many chondritic meteorites experienced aqueous alteration, the chronology of which helps constrain their histories. Here, the authors synthesize a fayalite standard and report reliable ages of secondary fayalite, from which model accretion ages are determined and the place of accretion is inferred.

The origin of most chondrules (small, previously molten spherules inside meteorites) is shown to be impact jetting; chondrules form from the shock-melted material ejected from a protoplanet on impact, making meteorites a byproduct of planet formation. Chondrule formation The origin of chondrules, the millimetre-scale, once-molten droplets found in most meteorites, has been an enduring mystery in the science of meteoritics. Using a hydrocode model and a dynamical accretion model to simulate protoplanetary impacts, Brandon Johnson et al . show that large-scale accretionary impacts can produce massive sprays of millimetre-scale molten droplets in sufficient quantities during the first 5 Myr of planetary accretion to explain the observed abundance of chondrules. This finding supports an impact origin for chondrules and implies that meteorites are a byproduct of planet formation rather than leftover building material. Chondrules are the millimetre-scale, previously molten, spherules found in most meteorites 1 . Before chondrules formed, large differentiating planetesimals had already accreted 2 . Volatile-rich olivine reveals that chondrules formed in extremely solid-rich environments, more like impact plumes than the solar nebula 3 , 4 , 5 . The unique chondrules in CB chondrites probably formed in a vapour-melt plume produced by a hypervelocity impact 6 with an impact velocity greater than 10 kilometres per second. An acceptable formation model for the overwhelming majority of chondrules, however, has not been established. Here we report that impacts can produce enough chondrules during the first five million years of planetary accretion to explain their observed abundance. Building on a previous study of impact jetting 7 , we simulate protoplanetary impacts, finding that material is melted and ejected at high speed when the impact velocity exceeds 2.5 kilometres per second. Using a Monte Carlo accretion code, we estimate the location, timing, sizes, and velocities of chondrule-forming impacts. Ejecta size estimates 8 indicate that jetted melt will form millimetre-scale droplets. Our radiative transfer models show that these droplets experience the expected cooling rates of ten to a thousand kelvin per hour 9 , 10 . An impact origin for chondrules implies that meteorites are a byproduct of planet formation rather than leftover building material.

The mass-independent calcium isotope composition of inner-Solar-System bodies is correlated with their masses and accretion ages, indicating a rapid growth for the precursors of Earth and the Moon during the protoplanetary disk’s lifetime. Like Earth, like moon Variation in the isotopic composition of material within the early inner Solar System is usually thought to reflect spatial heterogeneity in the protoplanetary disk. Martin Schiller and co-authors find that the calcium isotope composition of samples from the parent bodies of ureilite and angrite meteorites, as well as from Vesta, Mars and Earth, are correlated to the masses of their inferred parent asteroids and planets. This provides a proxy for their accretion timescales and implies a rapid 'secular' evolution of the bulk calcium isotope composition of the disk in the rocky-planet-forming region. The authors infer that this secular evolution reflects the introduction of pristine outer-Solar-System material to the thermally processed inner protoplanetary disk associated with the accretion of mass to the proto-Sun. They also conclude that the indistinguishable calcium isotope composition of the Earth and the Moon implies that the Moon-forming impact involved protoplanets that completed their accretion near the end of the disk's lifetime. Nucleosynthetic isotope variability among Solar System objects is often used to probe the genetic relationship between meteorite groups and the rocky planets (Mercury, Venus, Earth and Mars), which, in turn, may provide insights into the building blocks of the Earth–Moon system 1 , 2 , 3 , 4 , 5 . Using this approach, it has been inferred that no primitive meteorite matches the terrestrial composition and the protoplanetary disk material from which Earth and the Moon accreted is therefore largely unconstrained 6 . This conclusion, however, is based on the assumption that the observed nucleosynthetic variability of inner-Solar-System objects predominantly reflects spatial heterogeneity. Here we use the isotopic composition of the refractory element calcium to show that the nucleosynthetic variability in the inner Solar System primarily reflects a rapid change in the mass-independent calcium isotope composition of protoplanetary disk solids associated with early mass accretion to the proto-Sun. We measure the mass-independent 48 Ca/ 44 Ca ratios of samples originating from the parent bodies of ureilite and angrite meteorites, as well as from Vesta, Mars and Earth, and find that they are positively correlated with the masses of their parent asteroids and planets, which are a proxy of their accretion timescales. This correlation implies a secular evolution of the bulk calcium isotope composition of the protoplanetary disk in the terrestrial planet-forming region. Individual chondrules from ordinary chondrites formed within one million years of the collapse of the proto-Sun 7 reveal the full range of inner-Solar-System mass-independent 48 Ca/ 44 Ca ratios, indicating a rapid change in the composition of the material of the protoplanetary disk. We infer that this secular evolution reflects admixing of pristine outer-Solar-System material into the thermally processed inner protoplanetary disk associated with the accretion of mass to the proto-Sun. The identical calcium isotope composition of Earth and the Moon reported here is a prediction of our model if the Moon-forming impact involved protoplanets or precursors that completed their accretion near the end of the protoplanetary disk’s lifetime.

Why Venus lacks plate tectonics remains an unanswered question in terrestrial planet evolution. There is observational evidence for subduction—a requirement for plate tectonics—on Venus, but it is unclear why the features have characteristics of both mantle plumes and subduction zones. One explanation is that mantle plumes trigger subduction. Here we compare laboratory experiments of plume-induced subduction in a colloidal solution of nanoparticles to observations of proposed subduction sites on Venus. The experimental fluids are heated from below to produce upwelling plumes, which in turn produce tensile fractures in the lithosphere-like skin that forms on the upper surface. Plume material upwells through the fractures and spreads above the skin, analogous to volcanic flooding, and leads to bending and eventual subduction of the skin along arcuate segments. The segments are analogous to the semi-circular trenches seen at two proposed sites of plume-triggered subduction at Quetzalpetlatl and Artemis coronae. Other experimental deformation structures and subsurface density variations are also consistent with topography, radar and gravity data for Venus. Scaling analysis suggests that this regime with limited, plume-induced subduction is favoured by a hot lithosphere, such as that found on early Earth or present-day Venus. Venus lacks plate tectonics, but some trenches on Venus resemble subduction zones. Laboratory experiments suggest that upwelling plumes can initiate localized subduction of a thin lithosphere such as the one on Venus.

The formation of a primordial crust is a critical step in the evolution of terrestrial planets but the timing of this process is poorly understood. The mineral zircon is a powerful tool for constraining crust formation because it can be accurately dated with the uranium-to-lead (U–Pb) isotopic decay system and is resistant to subsequent alteration. Moreover, given the high concentration of hafnium in zircon, the lutetium-to-hafnium ( 176 Lu– 176 Hf) isotopic decay system can be used to determine the nature and formation timescale of its source reservoir 1 – 3 . Ancient igneous zircons with crystallization ages of around 4,430 million years (Myr) have been reported in Martian meteorites that are believed to represent regolith breccias from the southern highlands of Mars 4 , 5 . These zircons are present in evolved lithologies interpreted to reflect re-melted primary Martian crust 4 , thereby potentially providing insight into early crustal evolution on Mars. Here, we report concomitant high-precision U–Pb ages and Hf-isotope compositions of ancient zircons from the NWA 7034 Martian regolith breccia. Seven zircons with mostly concordant U–Pb ages define 207 Pb/ 206 Pb dates ranging from 4,476.3 ± 0.9 Myr ago to 4,429.7 ± 1.0 Myr ago, including the oldest directly dated material from Mars. All zircons record unradiogenic initial Hf-isotope compositions inherited from an enriched, andesitic-like crust extracted from a primitive mantle no later than 4,547 Myr ago. Thus, a primordial crust existed on Mars by this time and survived for around 100 Myr before it was reworked, possibly by impacts 4 , 5 , to produce magmas from which the zircons crystallized. Given that formation of a stable primordial crust is the end product of planetary differentiation, our data require that the accretion, core formation and magma ocean crystallization on Mars were completed less than 20 Myr after the formation of the Solar System. These timescales support models that suggest extremely rapid magma ocean crystallization leading to a gravitationally unstable stratified mantle, which subsequently overturns, resulting in decompression melting of rising cumulates and production of a primordial basaltic to andesitic crust 6 , 7 . Isotopic compositions of ancient zircons from the NWA 7034 Martian meteorite suggest that Mars must have formed its primordial crust extremely swiftly, less than 20 million years after the formation of the Solar System.