The history of the Hadean Earth (∼4.0-4.5 billion years ago) is poorly understood because few known rocks are older than ∼3.8 billion years old. The main constraints from this era come from ancient ...submillimetre zircon grains. Some of these zircons date back to ∼4.4 billion years ago when the Moon, and presumably the Earth, was being pummelled by an enormous flux of extraterrestrial bodies. The magnitude and exact timing of these early terrestrial impacts, and their effects on crustal growth and evolution, are unknown. Here we provide a new bombardment model of the Hadean Earth that has been calibrated using existing lunar and terrestrial data. We find that the surface of the Hadean Earth was widely reprocessed by impacts through mixing and burial by impact-generated melt. This model may explain the age distribution of Hadean zircons and the absence of early terrestrial rocks. Existing oceans would have repeatedly boiled away into steam atmospheres as a result of large collisions as late as about 4 billion years ago.
Asteroid shapes and hydration levels can serve as tracers of their history and origin. For instance, the asteroids (162173) Ryugu and (101955) Bennu have an oblate spheroidal shape with a pronounced ...equator, but contain different surface hydration levels. Here we show, through numerical simulations of large asteroid disruptions, that oblate spheroids, some of which have a pronounced equator defining a spinning top shape, can form directly through gravitational reaccumulation. We further show that rubble piles formed in a single disruption can have similar porosities but variable degrees of hydration. The direct formation of top shapes from single disruption alone can explain the relatively old crater-retention ages of the equatorial features of Ryugu and Bennu. Two separate parent-body disruptions are not necessarily required to explain their different hydration levels.
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.
We revisit the early evolution of the Moon's bombardment. Our work combines modeling (based on plausible projectile sources and their dynamical decay rates) with constraints from the lunar crater ...record, radiometric ages of the youngest lunar basins, and the abundance of highly siderophile elements in the lunar crust and mantle. We deduce that the evolution of the impact flux did not decline exponentially over the first billion years of lunar history, but also there was no prominent and narrow impact spike ∼3.9Gy ago, unlike that typically envisioned in the lunar cataclysm scenario. Instead, we show the timeline of the lunar bombardment has a sawtooth-like profile, with an uptick in the impact flux near ∼4.1Gy ago. The impact flux at the beginning of this weaker cataclysm was 5–10 times higher than the immediately preceding period. The Nectaris basin should have been one of the first basins formed at the sawtooth. We predict the bombardment rate since ∼4.1Gy ago declined slowly and adhered relatively close to classic crater chronology models (Neukum and Ivanov, 1994). Overall we expect that the sawtooth event accounted for about one-fourth of the total bombardment suffered by the Moon since its formation. Consequently, considering that ∼12–14 basins formed during the sawtooth event, we expect that the net number of basins formed on the Moon was ∼45–50. From our expected bombardment timeline, we derived a new and improved lunar chronology suitable for use on pre-Nectarian surface units. According to this chronology, a significant portion of the oldest lunar cratered terrains has an age of 4.38–4.42Gyr. Moreover, the largest lunar basin, South Pole Aitken, is older than 4.3Gy, and therefore was not produced during the lunar cataclysm.
► We provide a model of the timeline of lunar bombardment in the first billion year. ► Such timeline had a sawtooth-like profile, with an uptick in the impact flux near 4.1Gy ago. ► About one-fourth of the total bombardment suffered by the Moon occurred since this time. ► The bombardment rate since 4.1Gy ago declined slowly as in the classic crater chronology models. ► South Pole Aitken is an old basin, which predates the sawtooth event.
Asteroid crater retention ages have unknown accuracy because projectile-crater scaling laws are difficult to verify. At the same time, our knowledge of asteroid and crater size-frequency ...distributions has increased substantially over the past few decades. These advances make it possible to empirically derive asteroid crater scaling laws by fitting model asteroid size distributions to crater size distributions from asteroids observed by spacecraft. For D > 10 km diameter asteroids like Ceres, Vesta, Lutetia, Mathilde, Ida, Eros, and Gaspra, the best matches occur when the ratio of crater to projectile sizes is f ∼ 10. The same scaling law applied to 0.3 < D < 2.5 km near-Earth asteroids such as Bennu, Ryugu, Itokawa, and Toutatis yield intriguing yet perplexing results. When applied to the largest craters on these asteroids, we obtain crater retention ages of ∼1 billion years for Bennu, Ryugu, and Itokawa and ∼2.5 billion years for Toutatis. These ages agree with the estimated formation ages of their source families and could suggest that the near-Earth asteroid population is dominated by bodies that avoided disruption during their traverse across the main asteroid belt. An alternative interpretation is that f > 10, which would make their crater retention ages much younger. If true, crater scaling laws need to change in a substantial way between D > 10 km asteroids, where f ∼ 10, and 0.3 < D < 2.5 km asteroids, where f > 10.
The orbital and absolute magnitude distribution of the near-Earth objects (NEOs) is difficult to compute, partly because only a modest fraction of the entire NEO population has been discovered so ...far, but also because the known NEOs are biased by complicated observational selection effects. To circumvent these problems, we created a model NEO population which was fit to known NEOs discovered or accidentally rediscovered by Spacewatch. Our method was to numerically integrate thousands of test particles from five source regions that we believe provide most NEOs to the inner Solar System. Four of these source regions are in or adjacent to the main asteroid belt, while the fifth one is associated with the transneptunian disk. The nearly isotropic comets, which include the Halley-type comets and the long-period comets, were not included in our model. Test bodies from our source regions that passed into the NEO region (perihelia
q<1.3 AU and aphelia
Q≥0.983 AU) were tracked until they were eliminated by striking the Sun or a planet or were ejected out of the inner Solar System. These integrations were used to create five residence time probability distributions in semimajor axis, eccentricity, and inclination space (one for each source). These distributions show where NEOs from a given source are statistically most likely to be located. Combining these five residence time probability distributions with an NEO absolute magnitude distribution computed from previous work and a probability function representing the observational biases associated with the Spacewatch NEO survey, we produced an NEO model population that could be fit to 138 NEOs discovered or accidentally rediscovered by Spacewatch. By testing a range of possible source combinations, a best-fit NEO model was computed which (i) provided the debiased orbital and absolute magnitude distributions for the NEO population and (ii) indicated the relative importance of each NEO source region.
Our best-fit model is consistent with 960±120 NEOs having
H<18 and
a<7.4 AU. Approximately 44% (as of December 2000) have been found so far. The limits on this estimate are conditional, since our model does not include nearly isotropic comets. Nearly isotropic comets are generally restricted to a Tisserand parameter (with respect to Jupiter) of
T<2, such that few are believed to have
a<7.4 AU. Our computed NEO orbital distribution, which is valid for bodies as faint as
H<22, indicates that the Amor, Apollo, and Aten populations contain 32±1%, 62±1%, and 6±1% of the NEO population, respectively. We estimate that the population of objects completely inside Earth's orbit (IEOs) arising from our source regions is 2% the size of the NEO population. This value does not include the putative Vulcanoid population located inside Mercury's orbit. Overall, our model predicts that ∼61% of the NEO population comes from the inner main belt (
a<2.5 AU), ∼24% comes from the central main belt (2.5<
a<2.8 AU), ∼8% comes from the outer main belt (
a>2.8 AU), and ∼6% comes from the Jupiter-family comet region (2<
T≲3). The steady-state population in each NEO source region, as well as the influx rates needed to replenish each region, were calculated as a by-product of our method. The population of extinct comets in the Jupiter-family comet region was also computed.
We review the results of an extensive campaign to determine the physical, geological, and dynamical properties of asteroid (101955) Bennu. This investigation provides information on the orbit, shape, ...mass, rotation state, radar response, photometric, spectroscopic, thermal, regolith, and environmental properties of Bennu. We combine these data with cosmochemical and dynamical models to develop a hypothetical timeline for Bennu's formation and evolution. We infer that Bennu is an ancient object that has witnessed over 4.5 Gyr of solar system history. Its chemistry and mineralogy were established within the first 10 Myr of the solar system. It likely originated as a discrete asteroid in the inner Main Belt approximately 0.7–2 Gyr ago as a fragment from the catastrophic disruption of a large (approximately 100‐km), carbonaceous asteroid. It was delivered to near‐Earth space via a combination of Yarkovsky‐induced drift and interaction with giant‐planet resonances. During its journey, YORP processes and planetary close encounters modified Bennu's spin state, potentially reshaping and resurfacing the asteroid. We also review work on Bennu's future dynamical evolution and constrain its ultimate fate. It is one of the most Potentially Hazardous Asteroids with an approximately 1‐in‐2700 chance of impacting the Earth in the late 22nd century. It will most likely end its dynamical life by falling into the Sun. The highest probability for a planetary impact is with Venus, followed by the Earth. There is a chance that Bennu will be ejected from the inner solar system after a close encounter with Jupiter. OSIRIS‐REx will return samples from the surface of this intriguing asteroid in September 2023.
The present population of asteroids in the main belt is largely the result of many past collisions. Ideally, the asteroid fragments resulting from each impact event could help us understand the ...large-scale collisions that shaped the planets during early epochs. Most known asteroid fragment families, however, are very old and have therefore undergone significant collisional and dynamical evolution since their formation. This evolution has masked the properties of the original collisions. Here we report the discovery of a family of asteroids that formed in a disruption event only 5.8 ± 0.2 million years ago, and which has subsequently undergone little dynamical and collisional evolution. We identified 39 fragments, two of which are large and comparable in size (diameters of ∼19 and ∼14 km), with the remainder exhibiting a continuum of sizes in the range 2-7 km. The low measured ejection velocities suggest that gravitational re-accumulation after a collision may be a common feature of asteroid evolution. Moreover, these data can be used to check numerical models of larger-scale collisions.
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