Abstract The origins of the giant planet satellites are debated, with scenarios including formation from a protoplanetary disk, sequential assembly from massive rings, and recent accretion after ...major satellite–satellite collisions. Here, we test their predictions by simulating outer solar system bombardment and calculating the oldest surface ages on each moon. Our crater production model assumes the projectiles originated from a massive primordial Kuiper Belt (PKB) that experienced substantial changes from collisional evolution, which transformed its size frequency distribution into a wavy shape, and Neptune’s outward migration, which ejected most PKB objects onto destabilized orbits. The latter event also triggered an instability among the giant planets some tens of Myr after the solar nebula dispersed. We find all giant planet satellites are missing their earliest crater histories, with the likely source being impact resetting events. Iapetus, Hyperion, Phoebe, and Oberon have surface ages that are a few Myr to a few tens of Myr younger than when Neptune entered the PKB (i.e., they are 4.52–4.53 Gyr old). The remaining midsized satellites of Saturn and Uranus, as well as the small satellites located between Saturn’s rings and Dione, have surfaces that are younger still by many tens to many hundreds of Myr (4.1–4.5 Gyr old). A much wider range of surface ages are found for the large moons Callisto, Ganymede, Titan, and Europa (4.1, 3.4, 1.8, and 0.18 Gyr old, respectively). At present, we favor the midsized and larger moons forming within protoplanetary disks, with the other scenarios having several challenges to overcome.
Vesta has a complex cratering history, with ancient terrains as well as recent large impacts that have led to regional resurfacing. Crater counts can help constrain the relative ages of different ...units on Vesta׳s surface, but converting those crater counts to absolute ages requires a chronology function. We present a cratering chronology based on the best current models for the dynamical evolution of asteroid belt, and calibrate it to Vesta using the record of large craters on its surface. While uncertainties remain, our chronology function is broadly consistent with an ancient surface of Vesta as well as other constraints such as the bombardment history of the rest of the inner Solar System and the Ar–Ar age distribution of howardite, eucrite and diogenite (HED) meteorites from Vesta.
•We develop a crater chronology for Vesta based on models of asteroid belt evolution.•Our chronology is consistent with numerous constraints from Vesta.•Application to other large asteroids like Ceres will allow for testing and refinement.
Abstract
NASA's Lucy mission is designed to better understand the unique population of Trojan asteroids. Trojans were probably captured in Jupiter's L4 and L5 points early in the solar system's ...evolution and little altered since then. A critical investigation of Lucy is to use its highest-resolution camera, the Lucy Long Range Reconnaissance Imager (L’LORRI), to image Trojans’ surfaces to understand their geology and impact crater populations. Through crater statistics, the population of smaller bodies that produced those impacts, relative age differences across the bodies, and other comparative investigations between bodies can be studied. Mapping the crater population to the minimum diameters needed to achieve Lucy's objectives might require image subsampling and deconvolution (“processing”) to improve the spatial resolution, a process whereby multiple, slightly offset images are merged to create a single, better-sampled image and deconvolved with L’LORRI's point-spread function. Lucy's first Earth Gravity Assist (EGA1) provided an opportunity to test this process's accuracy using L’LORRI images of the Moon, whose crater population is well characterized and therefore provides ground-truth testing. Specifically, the lunar crater imaging by L’LORRI during EGA1 allowed us to compare crater statistics derived from raw and processed L’LORRI images with ground-truth statistics derived from higher-resolution lunar imaging from other missions. The results indicate the processing can improve impact crater statistics such that features can be identified and measured to ≈70% the diameter that they can otherwise be reliably mapped on native L’LORRI images. This test's results will be used in the observation designs for the Lucy flyby targets.
The lunar crater chronology has been built by connecting radiometric ages of sampled terrains with the areal crater densities on those terrains, particularly the density at diameters (D) ≥ 1 km (aka ...N(>1)). In the past, very few crater chronologies have considered the effect of terrain properties on the crater densities and size-frequency distributions (SFDs) used to build them. This influence is especially important when N(>1) cannot be directly measured. Here we study this influence by using the Model Production Function (MPF) chronology, which incorporates terrain properties into computing N(>1) and absolute model ages (AMAs) through computing crater diameters using standard crater scaling laws. Furthermore, we also gain a better understanding of actual lunar terrain properties by adjusting the MPF to reproduce AMAs close to the radiometric ages of several Apollo sample sites. The properties examined are the consolidation state of the terrain, its effective cratering strength, and density. Overall, we find that the impact melt of Copernicus crater (∼0.8 Ga) is stronger and more consolidated than the mare and highland terrains investigated. The mare become less consolidated and weaker as they age (from 3.1 to 3.8 Ga) – likely due to fracturing and regolith formation by subsequent impacts. The highlands (∼3.8 Ga) are the weakest terrain. The analysis of terrain proprieties allows our MPF computations to reproduce the radiometric ages, and the impact melt and ejecta of Copernicus crater to have the same age, as expected. The new lunar terrain property constraints can be used with the MPF to derive more robust absolute model ages for unsampled terrains. The values presented in this work for impact melt, ejecta, mare, and highlands can serve as references.
•Model Production Function used to infer how terrain properties affect age derivation.•New constraints on strength terrain properties of Apollo sampling sites are given.•Crater model ages should consider terrain properties, particularly for D < 200 m.
•We produced a geologic map Vesta’s Marcia quadrangle to determine regional geologic history.•The ∼58km diameter Marcia crater is youngest large crater on Vesta.•It marks a major vestan ...time-stratigraphic event and is one of the geologically youngest formations.•Marcia reveals pristine bright/dark materials, and terrains suggestive of impact melts and release of volatiles.•Aricia Tholus is a fragment of an ancient impact basin rim topped by a dark-rayed impact crater.
We used Dawn spacecraft data to identify and delineate geological units and landforms in the Marcia quadrangle of Vesta as a means to assess the role of the large, relatively young impact craters Marcia (∼63km diam.) and Calpurnia (∼53km diam.) and their surrounding ejecta field on the local geology. We also investigated a local topographic high with a dark-rayed crater named Aricia Tholus, and the impact crater Octavia that is surrounded by a distinctive diffuse mantle. Crater counts and stratigraphic relations suggest that Marcia is the youngest large crater on Vesta, in which a putative impact melt on the crater floor ranges in age between ∼40 and 60Ma (depending upon choice of chronology system), and Marcia’s ejecta blanket ranges in age between ∼120 and 390Ma (depending upon choice of chronology system). We interpret the geologic units in and around Marcia crater to mark a major vestan time-stratigraphic event, and that the Marcia Formation is one of the geologically youngest formations on Vesta. Marcia crater reveals pristine bright and dark material in its walls and smooth and pitted terrains on its floor. The smooth unit we interpret as evidence of flow of impact melts and (for the pitted terrain) release of volatiles during or after the impact process. The distinctive dark ejecta surrounding craters Marcia and Calpurnia is enriched in OH- or H-bearing phases and has a variable morphology, suggestive of a complex mixture of impact ejecta and impact melts including dark materials possibly derived from carbonaceous chondrite-rich material. Aricia Tholus, which was originally interpreted as a putative vestan volcanic edifice based on lower resolution observations, appears to be a fragment of an ancient impact basin rim topped by a dark-rayed impact crater. Octavia crater has a cratering model formation age of ∼280–990Ma based on counts of its ejecta field (depending upon choice of chronology system), and its ejecta field is the second oldest unit in this quadrangle. The relatively young craters and their related ejecta materials in this quadrangle are in stark contrast to the surrounding heavily cratered units that are related to the billion years old or older Rheasilvia and Veneneia impact basins and Vesta’s ancient crust preserved on Vestalia Terra.
Abstract
We use our new light curves, along with historical data, to determine the rotation state, photometric properties, and convex shape models of the targets of the Lucy mission (3548) Eurybates ...and (21900) Orus. We determine a retrograde spin for both targets, with sidereal rotation periods of 8.7027283 ± 0.0000029 h and 13.486190 ± 0.000017 h, respectively. The phase curves of both objects are nearly linear in the phase-angle range observable from Earth and lack a pronounced opposition effect. Unsupervised classification of these phase curves by the Penttilä et al. tool suggests that Eurybates and Orus belong to the C and D taxonomic types, respectively, thereby independently confirming past classifications based on their spectral slope. Time-resolved color-index measurements show no systematic color variations correlated with rotation for either target at the 1% level, suggesting that no variegation is present on a hemispherical scale for any of the objects. Comparison of the shape models with stellar occultation data available for the two objects from the program by Buie et al. allows us to resolve the longitude ambiguity of the orientations of the spin axes and derive unique pole solutions for both targets. Furthermore, scaling the shape models to match the occultation chords produces accurate sizes and geometric albedos for both objects. The derived surface-equivalent spherical diameters are
D
s
= 69.3 ± 1.4 km and
D
s
= 60.5 ± 0.9 km for Eurybates and Orus, respectively, while the geometric albedo in the
H
,
G
1
,
G
2
system is
p
V
(
H
,
G
1
,
G
2
) = 0.044 ± 0.003 and
p
V
(
H
,
G
1
,
G
2
) = 0.040 ± 0.002 for Eurybates and Orus, respectively.
Due to the rapidly expanding body of biomedical literature, biologists require increasingly sophisticated and efficient systems to help them to search for relevant information. Such systems should ...account for the multiple written variants used to represent biomedical concepts, and allow the user to search for specific pieces of knowledge (or events) involving these concepts, e.g., protein-protein interactions. Such functionality requires access to detailed information about words used in the biomedical literature. Existing databases and ontologies often have a specific focus and are oriented towards human use. Consequently, biological knowledge is dispersed amongst many resources, which often do not attempt to account for the large and frequently changing set of variants that appear in the literature. Additionally, such resources typically do not provide information about how terms relate to each other in texts to describe events.
This article provides an overview of the design, construction and evaluation of a large-scale lexical and conceptual resource for the biomedical domain, the BioLexicon. The resource can be exploited by text mining tools at several levels, e.g., part-of-speech tagging, recognition of biomedical entities, and the extraction of events in which they are involved. As such, the BioLexicon must account for real usage of words in biomedical texts. In particular, the BioLexicon gathers together different types of terms from several existing data resources into a single, unified repository, and augments them with new term variants automatically extracted from biomedical literature. Extraction of events is facilitated through the inclusion of biologically pertinent verbs (around which events are typically organized) together with information about typical patterns of grammatical and semantic behaviour, which are acquired from domain-specific texts. In order to foster interoperability, the BioLexicon is modelled using the Lexical Markup Framework, an ISO standard.
The BioLexicon contains over 2.2 M lexical entries and over 1.8 M terminological variants, as well as over 3.3 M semantic relations, including over 2 M synonymy relations. Its exploitation can benefit both application developers and users. We demonstrate some such benefits by describing integration of the resource into a number of different tools, and evaluating improvements in performance that this can bring.
Following an approach similar to that used for the Moon, Mercury's surface units were subdivided into five time-stratigraphic systems based on geologic mapping using Mariner 10 images. The absolute ...time scale originally suggested for the time periods associated with these systems was based on the assumption that the lunar impact flux history applied to Mercury. However, we find that the duration and onset of corresponding periods in the stratigraphic sequences on Mercury and the Moon are not the same. Using high-resolution and multiband image data obtained by the MErcury Surface, Space ENviroment, GEochemistry, and Ranging (MESSENGER) spacecraft, we identify and catalog fresh impact craters interpreted to have formed during Mercury's two most recent periods, the Mansurian and Kuiperian. We use the densities of the inferred Kuiperian- and Mansurian-aged crater populations to estimate new limits for the age boundaries of these time intervals. Results suggest that both the Mansurian and Kuiperian periods began more recently and extended for significantly shorter durations of time than previously suggested. The Kuiperian is estimated to have initiated as recently as approximately 280 plus or minus 60 Ma (million years) and the Mansurian as recently as approximately 1.7 plus or minus 0.2 Ga (billion years).
While we now know much about the volatile‐rich world of Ceres from the Dawn mission, the deep interior remains something of an enigma, shrouded by a crust composed of water ice, carbonates, ...phyllosilicates, salts and clathrate hydrates. While smaller than most active moons or planets, Ceres has many features commonly associated with active, icy bodies including: hydrothermal, cryovolcanic, and tectonic features. Yet on active icy moons tidal heating is a significant component of the thermal budget; it is unclear whether radiogenic heating alone would be sufficient to supply the heat necessary for Ceres' interior to undergo solid‐state convection. Here we show that transient asymmetric convection develops as the temperature within the body rises from heat generated by the decay of long‐lived radionuclides (e.g., U, Th, K). The onset of transient asymmetric convection may reconcile a number of puzzling features on Ceres including: the missing large craters, Hanami Planum—the region of thickened crust, the gravity and crustal thickness, and the lithospheric stress state represented by the Samhain Catenae. Hemispheric‐scale instabilities may also be important in the evolution of small bodies with small cores throughout the solar system, including the small icy moons of Saturn and Uranus as well as Kuiper belt objects.
Plain Language Summary
Ceres is the largest body in the asteroid belt. Because Ceres is small, there was not enough gravitational energy when it formed to heat the interior. We investigate whether heat generated by the decay of radiogenic elements can power the tectonism, ice‐volcanism, and evidence for past hydrothermal activity that have been documented by the Dawn mission. Using computer modeling, we find a planet‐scale asymmetric instability (one hemisphere up, one hemisphere down) forms as a small spherical body heats due to the decay of radiogenic elements within the interior. We show that this planet‐scale instability can explain many puzzling features on Ceres including: the high topographic plateau, fracture zones, and the absence of large craters. We suggest that planet‐scale instabilities may play a role in the dynamics of other small icy bodies in the solar system.
Key Points
As small bodies warm due to radiogenic heating, they undergo a hemisphere‐scale instability with one hemisphere rising and the other sinking
On Ceres, a hemispheric‐scale instability can explain the absence of large craters, large topographic plateaus, and tectonic features
Hemisphere‐scale instabilities may play an important role in the dynamics of small icy bodies throughout the solar system
Within the “Museo Virtuale della Musica BellinInRete” project, a corpus of letters, written by the renowned composer Vincenzo Bellini (1801 - 1835) from Catania, will be encoded and made publicly ...available. This contribution aims at illustrating the part of the project regarding the implementation of the prototype for the metadata and text encoding, indexing and visualisation of Bellini’s correspondence. The encoding scheme has been defined according to the latest guidelines of the Text Encoding Initiative and it has been instantiated on a sample of letters. Contextually, a first environment has been implemented by customizing two open source tools: Edition Visualization Technology and Omega Scholarly platform. The main objective of the digital edition is to engage general public with the cultural heritage held by the Belliniano Civic Museum of Catania. This wide access to Bellini’s correspondence has been conceived preserving the scholarly transcriptions of the letters edited by Seminara within her most recent critical edition (Olschki, 2017). The digital edition of the corpus takes care of handling the correspondence metadata by means of the correspDesc TEI tagset. Finally, Bellini's letters will be accessible via the Web platform as well as integrated into a forthcoming interactive and multimedia tour hosted at the museum.
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