Extrasolar planet host stars have been found to be enriched in key planet-building elements. These enrichments have the potential to drastically alter the composition of material available for ...terrestrial planet formation. Here, we report on the combination of dynamical models of late-stage terrestrial planet formation within known extrasolar planetary systems with chemical equilibrium models of the composition of solid material within the disk. This allows us to determine the bulk elemental composition of simulated extrasolar terrestrial planets. A wide variety of resulting planetary compositions are found, ranging from those that are essentially 'Earth like', containing metallic Fe and Mg silicates, to those that are dominated by graphite and SiC. This shows that a diverse range of terrestrial planets may exist within extrasolar planetary systems.
•Bennu most likely came from a low albedo asteroid family in inner main belt.•We modeled the dynamical evolution of candidate source family members.•Semimajor axis distribution of ancient families ...reproduced with stochastic YORP model.•Probable Bennu source families are New Polana (70%, +8/−4%) and Eulalia (30%, +4/−8%).•Most low albedo km-sized NEOs on Earth-like orbits come from these two families.
Asteroid (101955) Bennu, the target of NASA’s OSIRIS-REx sample return mission, is a D≈0.5km diameter low albedo near-Earth object. It has a spectral signature consistent with primitive carbonaceous chondrites, and an orbit similar to that of the Earth. A plausible evolution scenario for Bennu is that it migrated inward across the inner main belt from a low albedo family by Yarkovsky thermal forces over many hundreds of Myr. Eventually, it entered a resonance that took it into the terrestrial planet region, where a combination of planetary encounters and resonances took it to its current orbit over a few Myr to tens of Myr. When it departed the main belt, Bennu probably had an eccentricity 0.1<e<0.2 and an inclination 1°<i<6°. Several low albedo families have the appropriate dynamical, color, albedo, and broad spectral characteristics to produce Bennu: Clarissa, Erigone, Eulalia, New Polana, and Sulamitis.
Here we used a suite of numerical simulations to determine the ages of the families above, how Bennu reached its current orbit, and the most probable source family for Bennu. Specifically, we tracked test Bennu-like asteroids evolving in semimajor axis by the coupled Yarkovsky/YORP effects, incorporating a new formalism for how YORP torques modify the spin vector evolution of small asteroids. Using results and insights provided by Statler (Statler, T.S. 2009. Icarus 202, 502–513), we assumed that modest shape changes to asteroids, produced by a variety of processes (e.g., crater formation, changes to asteroid rotational angular momentum by YORP), caused the test asteroids’ spin rates, but not their obliquities, to undergo a random walk. This “stochastic YORP” mechanism slows down how often asteroids reach YORP endstates (i.e., spinning up so fast that the asteroid sheds mass, spinning down so much the asteroid enters into a tumbling rotation state). This new model allowed us to reproduce the semimajor axis distribution of observed family members from Clarissa, Erigone, Eulalia, New Polana, and Sulamitis. In the process, we derived model family formation ages of ∼60Myr old, 130±30Myr old, 830-100+370Myr old, 1400±150Myr old, and200±40Myr, respectively.
Next, using a Monte-Carlo code to track millions of test asteroids from each of the families above to main belt escape routes capable of producing Bennu-like orbits, we found the most likely parent families for Bennu are Eulalia and New Polana. On average, more than twice as many 0.5km objects from the New Polana family reach Bennu’s orbit as those from the Eulalia family. This corresponds to the New Polana and Eulalia families having a 70-4+8% and 30-8+4% probability of producing Bennu, respectively. Comparable runs to deduce the source of the Hayabusa 2 target, the low albedo 0.87km diameter near-Earth object (162173) 1999 JU3, produced similar probabilities for both families. The former Marco-Polo-R target, the 1.9km asteroid (175706) 1996 FG3, however, has a 85-83+4% probability of coming from the Eulalia family and a 15-4+83% probability of coming from the New Polana family. The reason for this switch is that 1996 FG3 may have been part of Yarkovsky/YORP-produced wave of like-sized bodies that is only now reaching the terrestrial planet region. We suggest that the top-like shape of Bennu is a byproduct of mass wasting and/or mass shedding events produced by YORP spin up during its long journey across the inner main belt.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
(101955) Bennu is a dark, Earth orbit-crossing, asteroid thought to be assembled from the fragments of an ancient collision. Spatially-resolved visible and near-infrared spectra of Bennu provide ...details about its surface properties and composition. In addition to a hydrated phyllosilicate band, we detect a ubiquitous 3.4-micron absorption feature, which we attribute to a mix of organic and carbonate materials. The shape and depth of this absorption feature vary across Bennu’s surface, spanning the range seen among similar main-belt asteroids. Its
distribution does not correlate with the temperature, reflectance, spectral slope, or hydrated
minerals although those characteristics correlate with each other in some cases. The deepest 3.4-micron absorptions occur on individual boulders. The variations may be due to differences in abundance, recent exposure, or space weathering.
•We determined the shape of Bennu from radar and lightcurve observations.•We used the Arecibo and Goldstone radars to select a spacecraft mission target.•Combining radar and lightcurve data makes ...shape modeling simpler and more robust.•Asteroid Bennu has a smoother surface than most spacecraft-visited asteroids.•Bennu has a shape similar to binary near-Earth asteroids.
We determine the three-dimensional shape of near-Earth Asteroid (101955) Bennu based on radar images and optical lightcurves. Bennu was observed both in 1999 at its discovery apparition, and in 2005 using the 12.6-cm radar at the Arecibo Observatory and the 3.5-cm radar at the Goldstone tracking station. Data obtained in both apparitions were used to construct a shape model of this object. Observations were also obtained at many other wavelengths to characterize this object, some of which were used to further constrain the shape modeling. The lightcurve data, along with an initial determination of the rotation period derived from them, simplified and improved the shape modeling.
Below we briefly describe the observations and shape modeling process. We discuss the shape model and the implications for the possible formation and evolution of this object. We also describe the importance and limitations of the shape model in view of the fact that this object is the target of the OSIRIS-REx spacecraft mission.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Carbonaceous chondrites are meteoritic fragments of asteroids that avoided the geological reprocessing of larger planets and allow laboratory probing of early solar-nebula materials. Among these, ...Renazzo-type (CR) chondrites found in Antarctica appear remarkably pristine and are distinguished by abundant organic materials and water-soluble molecules such as amino acids and ammonia. We present a comprehensive analysis of the organic composition of selected CR meteorites of different petrographic classification and compare compounds’ abundance and distribution as they may relate to asteroidal aqueous processing and concomitant evolution of the mineral phases. We found that several CR compounds such as amino acids and sugar alcohols are fully represented in stones with no or minimal water exposure indicating a formation that, if solar, preceded parent body processes. The most pristine CRs also revealed natal enantiomeric excesses (ee) of up to 60%, much larger than ever recorded. However, aqueous alteration appears to affect CR soluble organic composition and abundances, in particular some diastereomeric amino acids may gauge its extent by the consequent racemization of their ee .
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(3200) Phaethon is a compelling object as it has an asteroidal appearance and spectrum, produces a weak dust tail during perihelion at just 0.14 au, and is the parent body of the Geminid Meteor ...Shower. A better understanding of the physical properties of Phaethon is needed to understand the nature of its current and previous activity, relationship to potential source populations, and to plan for the upcoming flyby of the DESTINY+ spacecraft of Phaethon in the 2020s. We performed rotationally resolved spectroscopy of Phaethon at visible and near-infrared wavelengths (0.4-2.5 m) in 2007 and 2017, respectively, to better understand its surface properties. The visible and near-infrared observations both spanned nearly a full rotation or more and were under similar observing geometries, covering the entire surface with the exception of the north pole. The visible wavelengths show blue slopes with only minor slope variations and no absorption features. The NIR data is minimally varying and concave upwards, from very blue to blue-neutral with increasing wavelength. We fit the short-wavelength tail of Phaethon's thermal emission and retrieve an average visible albedo of pv = 0.08 0.01, which is lower than previous measurements but plausible in light of the recent larger radar-measured diameter of Phaethon. We retrieve an average infrared beaming parameter of Phaethon of = 1.70 0.05, which is similar to previous results. We discuss the implications of Phaethon's visible and near-infrared spectrum as well as the lower albedo on its origin, source population, and evolutionary history.
Abstract
Asteroid interiors play a key role in our understanding of asteroid formation and evolution. As no direct interior probing has been done yet, characterisation of asteroids’ interiors relies ...on interpretations of external properties. Here we show, by numerical simulations, that the top-shaped rubble-pile asteroid (101955) Bennu’s geophysical response to spinup is highly sensitive to its material strength. This allows us to infer Bennu’s interior properties and provide general implications for top-shaped rubble piles’ structural evolution. We find that low-cohesion (≲0.78 Pa at surface and ≲1.3 Pa inside) and low-friction (friction angle ≲ 35
∘
) structures with several high-cohesion internal zones can consistently account for all the known geophysical characteristics of Bennu and explain the absence of moons. Furthermore, we reveal the underlying mechanisms that lead to different failure behaviours and identify the reconfiguration pathways of top-shaped asteroids as functions of their structural properties that either facilitate or prevent the formation of moons.
The primitive achondrites provide a window into the initial melting of asteroids in the early solar system. The brachinites are olivine-dominated meteorites with a recrystallized texture that we and ...others interpret as evidence of partial melting and melt removal on the brachinite parent body. We present a petrologic, thermodynamic and experimental study of the brachinites to evaluate the conditions under which they formed and test our hypothesis that the precursor material to the brachinites was FeO-rich compared to the precursors of other primitive achondrites. Petrologic analysis of six brachinites (Brachina, Allan Hills (ALH) 84025, Hughes 026, Elephant Moraine (EET) 99402, Northwest Africa (NWA) 3151, and NWA 4969) and one brachinite-like achondrite (NWA 5400) shows that they are meteorites with recrystallized texture that are enriched in olivine (⩾80vol.%) and depleted in other minerals with respect to a chondritic mineralogy. Silicates in the brachinites are FeO-rich (Fa32–36). Brachinite-like achondrite Northwest Africa 5400 is similar in mineralogy and texture to the brachinites but with a slightly lower FeO-content (Fa30). Thermodynamic calculations yield equilibration temperatures above the Fe,Ni–FeS cotectic temperature (∼950°C) for all meteorites studied here and temperatures above the silicate eutectic (∼1050°C) for all but two. Brachina formed at an fO2 of ∼IW, and the other brachinites and NWA 5400 formed at ∼IW−1. All the meteorites show great evidence of formation by partial melting having approximately chondritic to depleted chondritic mineralogies, equilibrated mineral compositions, and recrystallized textures, and having reached temperatures above that required for melt generation. In an attempt to simulate the formation of the brachinite meteorites, we performed one-atmosphere, gas-mixing partial melting experiments of R4 chondrite LaPaz Ice Field 03639. Experiments at 1250°C and an oxygen fugacity of IW−1 produce residual phases that are within the mineralogy and mineral compositions of the brachinites. These experiments provide further evidence for the formation of brachinites as a result of partial melting of a chondritic precursor similar in mineralogy and mineral compositions to the R chondrites.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The Hayabusa2 spacecraft collected samples from the surface of the carbonaceous near-Earth asteroid (162173) Ryugu, and brought them to Earth. The samples were expected to contain organic molecules, ...which record processes that occurred in the early Solar System. We analyzed organic molecules extracted from the Ryugu surface samples. We identify a variety of molecules containing the atoms CHNOS, formed by methylation, hydration, hydroxylation, and sulfurization reactions. Amino acids, aliphatic amines, carboxylic acids, polycyclic aromatic hydrocarbons, and nitrogen-heterocyclic compounds were detected, with properties consistent with an abiotic origin. These compounds likely arose from aqueous reaction on Ryugu’s parent body and are similar to organics in Ivuna-type meteorites. These molecules can survive on the surfaces of asteroids and be transported throughout the Solar System.
The abundances, distributions, enantiomeric ratios, and carbon isotopic compositions of amino acids in two fragments of the Aguas Zarcas CM2 type carbonaceous chondrite fall and a fragment of the CM2 ...Murchison meteorite were determined via liquid chromatography time‐of‐flight mass spectrometry and gas chromatography isotope ratio mass spectrometry. A suite of two‐ to six‐carbon aliphatic primary amino acids was identified in the Aguas Zarcas and Murchison meteorites with abundances ranging from ~0.1 to 158 nmol/g. The high relative abundances of α‐amino acids found in these meteorites are consistent with a Strecker‐cyanohydrin synthesis on these meteorite parent bodies. Amino acid enantiomeric and carbon isotopic measurements in both fragments of the Aguas Zarcas meteorites indicate that both samples experienced some terrestrial protein amino acid contamination after their fall to Earth. In contrast, similar measurements of alanine in Murchison revealed that this common protein amino acid was both racemic (D ≈ L) and heavily enriched in 13C, indicating no measurable terrestrial alanine contamination of this meteorite. Carbon isotope measurements of two rare non‐proteinogenic amino acids in the Aguas Zarcas and Murchison meteorites, α‐aminoisobutyric acid and D‐ and L‐isovaline, also fall well outside the typical terrestrial range, confirming they are extraterrestrial in origin. The detections of non‐terrestrial L‐isovaline excesses of ~10–15% in both the Aguas Zarcas and Murchison meteorites, and non‐terrestrial L‐glutamic acid excesses in Murchison of ~16–40% are consistent with preferential enrichment of circularly polarized light generated L‐amino acid excesses of conglomerate enantiopure crystals during parent body aqueous alteration and provide evidence of an early solar system formation bias toward L‐amino acids prior to the origin of life.
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