Space missions and thermal infrared observations have shown that small asteroids (kilometre-sized or smaller) are covered by a layer of centimetre-sized or smaller particles, which constitute the ...regolith. Regolith generation has traditionally been attributed to the fall back of impact ejecta and by the break-up of boulders by micrometeoroid impact. Laboratory experiments and impact models, however, show that crater ejecta velocities are typically greater than several tens of centimetres per second, which corresponds to the gravitational escape velocity of kilometre-sized asteroids. Therefore, impact debris cannot be the main source of regolith on small asteroids. Here we report that thermal fatigue, a mechanism of rock weathering and fragmentation with no subsequent ejection, is the dominant process governing regolith generation on small asteroids. We find that thermal fragmentation induced by the diurnal temperature variations breaks up rocks larger than a few centimetres more quickly than do micrometeoroid impacts. Because thermal fragmentation is independent of asteroid size, this process can also contribute to regolith production on larger asteroids. Production of fresh regolith originating in thermal fatigue fragmentation may be an important process for the rejuvenation of the surfaces of near-Earth asteroids, and may explain the observed lack of low-perihelion, carbonaceous, near-Earth asteroids.
•Occator crater on dwarf planet 1 Ceres hosts high albedo faculae that may have been deposited by a hydrothermal system.•Impact induced heating during the formation Occator produced a hydrothermal ...system with volumes sufficient to form the faculae.•This hydrothermal system lasted between 0.4 and 4 Myr, implying that the faculae are largely contemporaneous with the formation of the crater.
Occator crater is perhaps the most distinct surface feature observed by NASA's Dawn spacecraft on the Cerean surface. Contained within the crater are the highest albedo features on the planet, Cerealia Facula and Vinalia Faculae, and relatively smooth lobate flow deposits. We present hydrocode simulations of the formation of Occator crater, varying the water to rock ratio of our pre-impact Cerean surface. We find that at water to rock mass ratios up to 0.3, sufficient volumes of Occator's post-impact subsurface would be above the melting point of water to allow for the deposition of faculae-like deposits via impact-heat driven hydrothermal effusion of brines. This reservoir of hydrothermally viable material beneath the crater is composed of a mixture of impactor material and material uplifted from 10′s of kilometers beneath the pre-impact surface, which could sample a deep subsurface volatile reservoir, if present. Using a conductive cooling model, we estimate that the lifetime of hydrothermal activity within such a system, depending on choice of material constants, is between 0.4 and 4 Myr. Our results suggest that impact heating from the Occator forming impact provides a viable mechanism for the creation of the observed faculae, with the proviso that the faculae formed within a relatively short time window after the crater itself formed.
The early bombardment history of Mars may have drastically shaped its crustal evolution. Impact-induced melting of crustal and mantle materials leads to the formation of local and regional melt ...ponds, and the cumulative effects of the impact flux could result in widespread melting of the crust. To quantify impact-melt production, its provenance and final distribution as a function of impact conditions, we carried out a systematic parameter study using the iSALE shock physics code. In contrast to simplified scaling laws for estimating the amount of melt generated by shock compression, we take the planet's thermal state at the time of impact into account. In addition, we consider decompression melting as a consequence of lithostatic uplift of initially deep-seated material. We find that the geothermal profile has a strong effect on melt production, and that melt volumes are significantly increased by up to a factor of seven in comparison to existing analytical estimates. Enhanced melting occurs at impactor sizes (and velocities) that deposit most of their energy at a depth close to the base of the lithosphere. Impactors larger than 10 km penetrate through the lithosphere and can generate a significant amount of melt by decompression due to lithostatic uplift, which can make up to 40% of the total melt volume. In some cases, the total melt volume exceeds the volume of the transient (and final) crater and the surface expression of these collisions may resemble large igneous provinces rather than typical craters.
•Melt production on early hot Mars is increased by up to 7× compared to scaling laws.•Decompression can notably contribute to melting in large impacts on early Mars.•Paucity of large carters on Mars may explained by craters drowning in their melt.•Classical scaling laws fail to estimate melt production by large impactors (> 10 km).
Abstract
Crater chronologies are a fundamental tool to assess the relative and absolute ages of planetary surfaces when direct radiometric dating is not available. Martian crater chronologies are ...derived from lunar crater spatial densities on terrains with known radiometric ages, and thus they critically depend on the Moon-to-Mars extrapolation. This extrapolation requires knowledge of the time evolution of the impact flux, including contributions from various impactor populations, factors that are not trivially connected to the dynamical evolution of the early Solar System. In this paper, we will present a new Martian crater chronology based on current dynamical models, and consider the main sources of uncertainties (e.g., impactor size–frequency distribution; dynamical models with late and early instabilities, etc.). The resulting “envelope” of Martian crater chronologies significantly differs from previous chronologies. The new Martian crater chronology is discussed using two interesting applications: Jezero crater’s dark terrain (relevant to the NASA Mars 2020 mission) and the southern heavily cratered highlands. Our results indicate that Jezero’s dark terrain may have formed ∼3.1 Ga, i.e., up to 0.5 Gyr older than previously thought. In addition, available crater chronologies (including our own) overestimate the number of craters larger than 150 km on the southern highlands, suggesting either that large craters have been efficiently erased over Martian history or that dynamical models need further refinement. Further, our chronology constrains the age of Isidis basin to be 4.05–4.2 Ga and that of the Borealis basin to be 4.35–4.40 Ga; these are predictions that can be tested with future sample and return missions.
Ceres, the largest body in the asteroid belt (940 km diameter), provides a unique opportunity to study the interior structure of a volatile-rich dwarf planet. Variations in a planetary body's ...subsurface rheology and density affect the rate of topographic relaxation. Preferential attenuation of long wavelength topography (≥150 km) on Ceres suggests that the viscosity of its crust decreases with increasing depth. We present finite element (FE) geodynamical simulations of Ceres to identify the internal structures and compositions that best reproduce its topography as observed by the NASA Dawn mission. We infer that Ceres has a mechanically strong crust with maximum effective viscosity ∼1025 Pa s. Combined with density constraints, this rheology suggests a crustal composition of carbonates or phyllosilicates, water ice, and at least 30 volume percent (vol.%) low-density, high-strength phases most consistent with salt and/or clathrate hydrates. The inference of these crustal materials supports the past existence of a global ocean, consistent with the observed surface composition. Meanwhile, we infer that the uppermost ≥60 km of the silicate-rich mantle is mechanically weak with viscosity <1021 Pa s, suggesting the presence of liquid pore fluids in this region and a low temperature history that avoided igneous differentiation due to late accretion or efficient heat loss through hydrothermal processes.
•We use Dawn mission data and FE models to understand the interior of Ceres.•The crust has low density and high strength, suggesting substantial hydrated salts.•Any dehydrated mantle is at least 100 km deep, implying low peak temperatures <600 °C.•The salt and volatile-rich crust is most likely due to freezing of global surface fluids.•The low deep interior temperatures suggest late accretion or convective heat loss.
The earliest bombardment history of the Moon potentially provides powerful constraints for solar system evolution models. A major uncertainty, however, is how much of this history is actually ...recorded in lunar craters. For example, some argue that most ancient lunar craters and basins were produced by a declining bombardment of leftover planetesimals produced by terrestrial planet formation processes. Others believe that most lunar craters and large basins were formed in a narrow time interval between 3.8 and 4.0Ga, the so-called lunar cataclysm. In the light of recent improvements in our understanding of early solar system evolution, it is possible that the contributions from both scenarios could be represented in the lunar crater record. If so, when did the declining bombardment end and the lunar cataclysm begin?
Here we show, using new counts of 15–150km diameter craters on the most ancient lunar terrains, that the craters found on or near Nectaris basin appear to have been created by projectiles hitting twice as fast as those that made the oldest craters on various Pre-Nectarian-era terrains. This dramatic velocity increase is consistent with the existence of a lunar cataclysm and potentially with a late reconfiguration of giant planet orbits, which may have strongly modified the source of lunar impactors. This work also suggests that the lunar cataclysm may have started near the formation time of Nectaris basin. This possibility implies that South Pole-Aitken basin (SPA), the largest lunar basin and one of the oldest by superposition, was not created during the cataclysm. This view is strengthened by our interpretation that a substantial fraction of ancient craters on SPA were made by low velocity impactors. Finally, we believe these results shed new light on the impact history of the primordial Earth.
► Craters on ancient lunar terrains have been analyzed. ► We find new evidences supporting the lunar cataclysm. ► The cataclysm was less intense than previously thought.
The most heavily cratered terrains on Mercury have been estimated to be about 4 billion years (Gyr) old, but this was based on images of only about 45 per cent of the surface; even older regions ...could have existed in the unobserved portion. These terrains have a lower density of craters less than 100 km in diameter than does the Moon, an observation attributed to preferential resurfacing on Mercury. Here we report global crater statistics of Mercury's most heavily cratered terrains on the entire surface. Applying a recent model for early lunar crater chronology and an updated dynamical extrapolation to Mercury, we find that the oldest surfaces were emplaced just after the start of the Late Heavy Bombardment (LHB) about 4.0-4.1 Gyr ago. Mercury's global record of large impact basins, which has hitherto not been dated, yields a similar surface age. This agreement implies that resurfacing was global and was due to volcanism, as previously suggested. This activity ended during the tail of the LHB, within about 300-400 million years after the emplacement of the oldest terrains on Mercury. These findings suggest that persistent volcanism could have been aided by the surge of basin-scale impacts during this bombardment.
Dawn's global mapping of Vesta reveals that its observed south polar depression is composed of two overlapping giant impact features. These large basins provide exceptional windows into impact ...processes at planetary scales. The youngest Rheasilvia, is 500 kilometers wide and 19 kilometers deep and finds its nearest morphologic analog among large basins on low-gravity icy satellites. Extensive ejecta deposits occur, but impact melt volume is low, exposing an unusual spiral fracture pattern that is likely related to faulting during uplift and convergence of the basin floor. Rheasilvia obliterated half of another 400-kilometer-wide impact basin, Veneneia. Both basins are unexpectedly young, roughly 1 to 2 billion years, and their formation substantially reset Vestan geology and excavated sufficient volumes of older compositionally heterogeneous crustal material to have created the Vestoids and howardfte-eucrite-diogerrite meteorites.
Abstract
NASA’s Gravity Recovery and Interior Laboratory (GRAIL) spacecraft revealed the crust of the Moon is highly porous, with ~4% porosity at 20 km deep. The deep lying porosity discovered by ...GRAIL has been difficult to explain, with most current models only able to explain high porosity near the lunar surface (first few kilometers) or inside complex craters. Using hydrocode routines we simulated fracturing and generation of porosity by large impacts in lunar, martian, and Earth crust. Our simulations indicate impacts that produce 100–1000 km scale basins alone are capable of producing all observed porosity within the lunar crust. Simulations under the higher surface gravity of Mars and Earth suggest basin forming impacts can be a primary source of porosity and fracturing of ancient planetary crusts. Thus, we show that impacts could have supported widespread crustal fluid circulation, with important implications for subsurface habitable environments on early Earth and Mars.
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