A comparison of the laboratory reflectance spectra of meteorites with observations of asteroids revealed that the latter are much 'redder', with the spectral difference explained by 'space ...weathering', though the actual processes and timescales involved have remained controversial. A recent study of young asteroid families concluded that they suffered only minimal space weathering. Here we report additional observations of those families, revealing that space weathering must be a very rapid process-the final colour of a silicate-rich asteroid is acquired shortly after its 'birth' (within 10(6) years of undergoing a catastrophic collision). This rapid timescale favours solar wind implantation as the main mechanism of space weathering, as laboratory experiments have shown that it is the most rapid of several competing processes. We further demonstrate the necessity to take account of composition when evaluating weathering effectiveness, as both laboratory and asteroid data show an apparent dependence of weathering on olivine abundance. The rapid colour change that we find implies that colour trends seen among asteroids are most probably due to compositional or surface-particle-size properties, rather than to different relative ages. Apparently fresh surfaces most frequently seen among small near-Earth asteroids may be the result of tidal shaking that rejuvenates their surfaces during planetary encounters.
The flyby of Pluto and Charon by the New Horizons spacecraft provided high-resolution images of cratered surfaces embedded in the Kuiper belt, an extensive region of bodies orbiting beyond Neptune. ...Impact craters on Pluto and Charon were formed by collisions with other Kuiper belt objects (KBOs) with diameters from ~40 kilometers to ~300 meters, smaller than most KBOs observed directly by telescopes. We find a relative paucity of small craters ≲13 kilometers in diameter, which cannot be explained solely by geological resurfacing. This implies a deficit of small KBOs (≲1 to 2 kilometers in diameter). Some surfaces on Pluto and Charon are likely ≳4 billion years old, thus their crater records provide information on the size-frequency distribution of KBOs in the early Solar System.
The Cold Classical Kuiper Belt, a class of small bodies in undisturbed orbits beyond Neptune, is composed of primitive objects preserving information about Solar System formation. In January 2019, ...the New Horizons spacecraft flew past one of these objects, the 36-kilometer-long contact binary (486958) Arrokoth (provisional designation 2014 MU
). Images from the flyby show that Arrokoth has no detectable rings, and no satellites (larger than 180 meters in diameter) within a radius of 8000 kilometers. Arrokoth has a lightly cratered, smooth surface with complex geological features, unlike those on previously visited Solar System bodies. The density of impact craters indicates the surface dates from the formation of the Solar System. The two lobes of the contact binary have closely aligned poles and equators, constraining their accretion mechanism.
Although petrologic, chemical, and isotopic studies of ordinary chondrites and meteorites in general have largely helped establish a chronology of the earliest events of planetesimal formation and ...their evolution, there are several questions that cannot be resolved via laboratory measurements and/or experiments alone. Here, we propose the rationale for several new constraints on the formation and evolution of ordinary chondrite parent bodies (and, by extension, most planetesimals) from newly available spectral measurements and mineralogical analysis of main-belt S-type asteroids (83 objects) and unequilibrated ordinary chondrite meteorites (53 samples). Based on the latter, we suggest that spectral data may be used to distinguish whether an ordinary chondrite was formed near the surface or in the interior of its parent body. If these constraints are correct, the suggested implications include that: (1) large groups of compositionally similar asteroids are a natural outcome of planetesimal formation and, consequently, meteorites within a given class can originate from multiple parent bodies; (2) the surfaces of large (up to ~200 km) S-type main-belt asteroids mostly expose the interiors of the primordial bodies, a likely consequence of impacts by small asteroids (D < 10 km) in the early solar system; (3) the duration of accretion of the H chondrite parent bodies was likely short (instantaneous or in less than ~10 super(5) yr, but certainly not as long as 1 Myr); (4) LL-like bodies formed closer to the Sun than H-like bodies, a possible consequence of the radial mixing and size sorting of chondrules in the protoplanetary disk prior to accretion.
In May of 2011, NASA selected the
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plorer (OSIRIS-REx) asteroid sample return mission as the third mission ...in the New Frontiers program. The other two New Frontiers missions are
New Horizons
, which explored Pluto during a flyby in July 2015 and is on its way for a flyby of Kuiper Belt object 2014 MU69 on January 1, 2019, and
Juno
, an orbiting mission that is studying the origin, evolution, and internal structure of Jupiter. The spacecraft departed for near-Earth asteroid (101955) Bennu aboard an United Launch Alliance Atlas V 411 evolved expendable launch vehicle at 7:05 p.m. EDT on September 8, 2016, on a seven-year journey to return samples from Bennu. The spacecraft is on an outbound-cruise trajectory that will result in a rendezvous with Bennu in November 2018. The science instruments on the spacecraft will survey Bennu to measure its physical, geological, and chemical properties, and the team will use these data to select a site on the surface to collect at least 60 g of asteroid regolith. The team will also analyze the remote-sensing data to perform a detailed study of the sample site for context, assess Bennu’s resource potential, refine estimates of its impact probability with Earth, and provide ground-truth data for the extensive astronomical data set collected on this asteroid. The spacecraft will leave Bennu in 2021 and return the sample to the Utah Test and Training Range (UTTR) on September 24, 2023.
Understanding the nature and origin of the asteroid population in Earth's vicinity (near-Earth asteroids, and its subset of potentially hazardous asteroids) is a matter of both scientific interest ...and practical importance. It is generally expected that the compositions of the asteroids that are most likely to hit Earth should reflect those of the most common meteorites. Here we report that most near-Earth asteroids (including the potentially hazardous subset) have spectral properties quantitatively similar to the class of meteorites known as LL chondrites. The prominent Flora family in the inner part of the asteroid belt shares the same spectral properties, suggesting that it is a dominant source of near-Earth asteroids. The observed similarity of near-Earth asteroids to LL chondrites is, however, surprising, as this meteorite class is relatively rare (∼8 per cent of all meteorite falls). One possible explanation is the role of a size-dependent process, such as the Yarkovsky effect, in transporting material from the main belt.
The New Horizons spacecraft flew past the Kuiper Belt object (486958) Arrokoth (also known as 2014 MU69) in January 2019. Because of the great distance to the outer Solar System and limited ...bandwidth, it will take until late 2020 to downlink all the spacecraft's observations back to Earth. Three papers in this issue analyze recently downlinked data, including the highest-resolution images taken during the encounter (see the Perspective by Jewitt). Spencer et al. examined Arrokoth's geology and geophysics using stereo imaging, dated the surface using impact craters, and produced a geomorphological map. Grundy et al. investigated the composition of the surface using color imaging and spectroscopic data and assessed Arrokoth's thermal emission using microwave radiometry. McKinnon et al. used simulations to determine how Arrokoth formed: Two gravitationally bound objects gently spiraled together during the formation of the Solar System. Together, these papers determine the age, composition, and formation process of the most pristine object yet visited by a spacecraft.
ABSTRACT Meteorites have long been considered as reflections of the compositional diversity of main belt asteroids and consequently they have been used to decipher their origin, formation, and ...evolution. However, while some meteorites are known to sample the surfaces of metallic, rocky and hydrated asteroids (about one-third of the mass of the belt), the low-density icy asteroids (C-, P-, and D-types), representing the rest of the main belt, appear to be unsampled in our meteorite collections. Here we provide conclusive evidence that the surface compositions of these icy bodies are compatible with those of the most common extraterrestrial materials (by mass), namely anhydrous interplanetary dust particles (IDPs). Given that these particles are quite different from known meteorites, it follows that the composition of the asteroid belt consists largely of more friable material not well represented by the cohesive meteorites in our collections. In the light of our current understanding of the early dynamical evolution of the solar system, meteorites likely sample bodies formed in the inner region of the solar system (0.5-4 AU) whereas chondritic porous IDPs sample bodies that formed in the outer region (>5 AU).
► We show how mid-IR emission spectra of asteroids can constrain their surface composition and structure. ► We demonstrate that a large fraction of the large main belt asteroids possess a highly ...porous surface. ► A new laboratory sample preparation method allows us to constrain their surface composition.
Emission features in the mid-IR domain (7–25μm) are quite ubiquitous among large asteroids and therefore offer the potential to uncover their surface composition. However, when comparing these spectra with the actual laboratory spectra of both minerals and meteorites, they do not necessarily match. Here, and in a companion paper by King et al. (in preparation, 2012), we show that by modifying the sample preparation – typically by suspending meteorite and/or mineral powder (<30μm) in IR-transparent KBr (potassium bromide) powder – we are able to reproduce the spectral behavior of those main-belt asteroids with emissivity features. This resulting good match between KBr-diluted meteorite spectra and asteroid spectra suggests an important surface porosity (>90%) for the first millimeter for our asteroid sample. It therefore appears that mid-IR emission spectra of asteroids do not only carry information about their surface composition but they can also help us constraining their surface structure (under-dense versus compact surface structure), as suggested by Emery et al. (Emery, J.P., Cruikshank, D.P., van Cleve, J. 2006. Icarus 182, 496–512) in the case of the Jupiter Trojans. The large surface porosity inferred from the mid-IR spectra of certain asteroids is also implied by two other independent measurements, namely their thermal inertia and their radar albedo. We further clarified how much compositional information can be retrieved from the mid-IR range by focusing our analysis on a single object, 624 Hektor. We showed that the mid-IR range provides critical constraints (i) on its origin and of that of the red Trojans that we locate in the formation regions of the comets, and (ii) on the primordial composition of the dust present in the outer region (>10AU) of the Solar System’s protoplanetary disk. Future investigations should focus on finding the mechanism responsible for creating such high surface porosity.
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.
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