Context.
Understanding the conditions that lead to the formation of planetesimals – the building blocks of planets - and their initial size distribution is a central problem of modern planetology. ...While most of these original planetesimals were accreted onto the terrestrial planets and the cores of the giant planets, some were also stranded in the main belt, where 4.5 Gyr of collisional evolution broke most of them into families of collisional asteroid fragments. However, some planetesimals survived, and are still hidden amongst asteroid fragments in the main belt.
Aims.
We make use of astronomical data to identify these leftover planetesimals amongst all other asteroids. Our search is based on separating planetesimal survivors from families of asteroids generated by collisions. Namely, we aim to identify and “clean” the main belt of collisional family members: by doing so, we would be left with the surviving members of the original planetesimals.
Methods.
We focus here on the inner portion of the main belt for asteroids with intermediate to high albedo. It is known that current asteroid family catalogs are not suitable for the aforementioned cleaning; they are conservative and only one-quarter of the known asteroids are associated with the approximately 120 distinct asteroid families. We therefore developed methods to inclusively link asteroids to known collisional families in order to better capture their extent. Namely, we apply a hierarchical clustering method (HCM) on asteroids filtered according to the V-shape of the Yarkovsky drift of each family in order to reassess family membership (V-shape-constrained HCM). The identified families were removed and the remaining background population was searched for previously undetected collisional families.
Results.
We succeed in using our V-shape-constrained HCM to link family “halos” to their cores. After removing these reassessed families from the asteroid population, our V-shape search reveals a previously unknown collisional family of S-type asteroids in the inner main belt with an age of 4.3 ± 1.7 Gyr and a significance level of 3.4
σ
. When this ancient collisional family is removed, 34 planetesimals are identified and their size-frequency distribution is presented.
Conclusions.
The asteroid belt has two components: planetesimals and collisional fragments. The cumulative size-frequency distribution of planetesimals has a steep power-law index for bodies larger than 100 km in diameter and a much smaller power-law index for planetesimals smaller than 100 km.
Near‐Earth asteroid (101955) Bennu is an active asteroid experiencing mass loss in the form of ejection events emitting up to hundreds of millimeter‐ to centimeter‐scale particles. The close ...proximity of the Origins, Spectral Interpretations, Resource Identification, and Security–Regolith Explorer spacecraft enabled monitoring of particles for a 10‐month period encompassing Bennu's perihelion and aphelion. We found 18 multiparticle ejection events, with masses ranging from near zero to hundreds of grams (or thousands with uncertainties) and translational kinetic energies ranging from near zero to tens of millijoules (or hundreds with uncertainties). We estimate that Bennu ejects ~104 g per orbit. The largest event took place on 6 January 2019 and consisted of ~200 particles. The observed mass and translational kinetic energy of the event were between 459 and 528 g and 62 and 77 mJ, respectively. Hundreds of particles not associated with the multiparticle ejections were also observed. Photometry of the best‐observed particles, measured at phase angles between ~70° and 120°, was used to derive a linear phase coefficient of 0.013 ± 0.005 magnitudes per degree of phase angle. Ground‐based data back to 1999 show no evidence of past activity for Bennu; however, the currently observed activity is orders of magnitude lower than observed at other active asteroids and too low be observed remotely. There appears to be a gentle decrease in activity with distance from the Sun, suggestive of ejection processes such as meteoroid impacts and thermal fracturing, although observational bias may be a factor.
Plain Language Summary
We measured the brightness of pebble‐sized particles in the vicinity of near‐Earth asteroid Bennu to better understand their physical characteristics and the events that launched them from Bennu's surface. Our measurements spanned 10 months, encompassing Bennu's closest and farthest distances from the Sun, so that we could assess how the level of ejection activity changes with solar distance. We observed 18 multiparticle ejection events containing anywhere from a few to 200+ particles. Individual particles ranged from millimeters to centimeters in diameter. The energy of the events and a possible decrease in activity with larger distances from the Sun suggest that meteoroid impacts, fracturing of surface boulders due to solar heating, or both may be responsible for ejecting the particles. We estimate that Bennu releases ~10,000 g of material over one orbit or 1.2 years. Although mass loss has been remotely observed for other asteroids, the comparatively low level of particle ejection activity at Bennu was only observable thanks to the close proximity of the Origins, Spectral Interpretations, Resource Identification, and Security–Regolith Explorer spacecraft.
Key Points
Asteroid (101955) Bennu is active from perihelion through aphelion with a possible decrease in activity further from the Sun
Bennu's activity is less than that detected by telescope for other active asteroids and is only observable up close
The particles' shallow phase functions resemble those of similarly sized individual rocks rather than those of ensemble asteroid surfaces
NASA's OSIRIS‐REx mission observed millimeter‐ to centimeter‐scale pebbles being ejected from the surface of asteroid (101955) Bennu, indicating that Bennu is an active asteroid. About 30% of these ...particles escape from Bennu, and the minimum orbital intersection distance (MOID) between Bennu and Earth suggest the possibility of a “Bennuid” particle flux at Earth. We characterize the evolution of Bennu's particle stream and potential for meteor flux by simulating weekly particle ejections between the years 1780 and 2135 continuing their dynamical evolution until 2200. Ejections are modeled as a discrete release of 95 particles every week. The meteoroid stream is found to be fully distributed around Bennu's orbital path in 80±40 years. Individual particles and streams remain associable to Bennu for the entire 420 years simulated. Particle flux at Earth is predicted to begin in 2101, as the Bennu‐Earth MOID reaches minimum values. The year of highest particle flux, 2182, experiences 161 Earth intersections and accounts for ∼1/4 of our predicted meteors. Our methods can be expanded to study the history and structure of the general meteoroid population and to estimate flux from specific near‐Earth asteroids.
Plain Language Summary
NASA's OSIRIS‐REx asteroid sample return mission observed coin‐sized rock fragments launching from the surface of the near‐Earth asteroid Bennu. Although many of these particles fall back down to the surface of Bennu, about 30% escape the gravitational influence of this small celestial body and enter orbits around the Sun. By simulating the motion of small particles ejected from Bennu over the years 1780–2200, we test whether they eventually encounter Earth's atmosphere. The predicted particle flux ranges from undetectable to ∼1 meteor per 10 h, a rate which is comparable with the weakest known meteor showers. We find that ejected particles spread out along Bennu's orbit and occupy positions around the entire circle within 80 years. For the 420 years simulated, the particles can be easily associated with Bennu by the similarities in their orbits. Particles we simulated being ejected from Bennu are not found to impact Earth until 2101. We predict a maximum flux in the year 2182, with around 161 intersecting meteors potentially visible as shooting stars. Our methods can be used to investigate the possibility of meteoroid streams from other near‐Earth asteroids to identify sources of known meteoroid streams and meteor showers.
Key Points
We simulate 355 years (1780–2135) of particle ejection from asteroid (101955) Bennu
Meteor flux (at Earth) of particles from Bennu is <1/yr until 2100 AD, peaking in 2182 AD at 161 Bennuids
The Bennu particle stream encircles in 80 years, and 99% of stream members remain associable for all 420 years of the simulation
A new femtosecond correlation nonlinear optical profilometer with micron spatial resolution is proposed, in order to increase the measurement accuracy of the heterogeneity profile of a surface at a ...distance up to 1 km. The principle of operation of this profilometer is based on determining the distance to the scanned object by measuring the time interval between sequences of the base (probing) and reflected femtosecond laser pulses. Thereafter, the profile of a rough surface is reestablished by using the one-dimensional spatial distribution of second-harmonic radiation as the space-time coupling of the sequences of the probing and reflected pulses in a nonlinear crystal is restored. It was shown that the spatial resolution of the profilometer is no greater than 5 μm.
A new nonlinear optical laser surface profilometer with micron surface resolution is considered. It is shown that with a resolution of the spectrometer of 0.23 nm the spatial resolution of the ...proposed method may reach 22.5 μm.
We study the compact stars internal structure and observable characteristics alterations due to the quark deconfinement phase transition. To proceed with, we investigate the properties of ...isospin-asymmetric nuclear matter in the improved relativistic mean-field (RMF) theory, including a scalar-isovector \delta-meson effective field. In order to describe the quark phase, we use the improved version of the MIT bag model, in which the interactions between u, d and s quarks inside the bag are taken into account in the one-gluon exchange approximation. We compute the amount of energy released by the corequake for both cases of deconfinement phase transition scenarios, corresponding to the Maxwellian type ordinary first-order phase transition and the phase transition with formation of a mixed quark-hadron phase (Glendenning scenario).
The kinetics of asphaltene aggregation in toluene–heptane mixtures has been studied by photon correlation spectroscopy (PCS). The PCS technique was adapted for investigation of opaque liquids. We ...examined the natural asphaltenes of various origin as well as the same ones with chemically modified metal-porphyrin kernels. The concentration of asphaltenes in toluene varied from 1 to 10 g/l. The theoretical model developed employs Smoluchowski fundamental approach. It allowed us to describe quantitatively the experimental curves for which the universal behavior was found. At asphaltene concentrations below the critical micelle concentration the diffusion limited aggregation (DLA) solely is observed. Above the CMC the reaction limited aggregation (RLA) occurs in the initial stage of particle growth. Then, the aggregation mechanism goes to the DLA. The results obtained prove asphaltene aggregation in hydrocarbon solutions is similar to that of typical colloids.
PDF
