arXiv subjects

R. P. Binzel

Publications and source records attributed to R. P. Binzel.

At least 19 recordsLinked to original sources

arXiv2026

The Nysa family as the main source of unequilibrated LL ordinary chondrites

Context. The origin of the petrologic diversity observed in ordinary chondrites (OCs), the most common meteorites on Earth, remains debated. Competing models invoke either depth-dependent sampling of a single thermally stratified ("onion-shell") parent body or contributions from multiple distinct parent bodies. Aims. We aim to determine which of the two models is preferred for LL chondrites. These are unique among OCs in exhibiting a bimodal petrologic distribution, with most meteorites being LL3 or LL6. Methods. We compare the spectral and mineralogical properties of LL chondrites and corresponding LL-chondrite-like near-Earth objects (NEOs) with their possible sources in the main asteroid belt. We also model the thermal histories of the proposed parent bodies, based on revised estimates of parent-body sizes. Results. The spectral and mineralogical diversity of LL chondrites is consistent with contributions from the bright, S-type component of the Nysa family (NysaS) and the Flora family, with NysaS supplying mainly low-petrologic-type material and Flora higher-grade material. Unequilibrated, LL3 chondrites appear to originate exclusively from NysaS. Similarly, LL-chondrite-like NEOs form two distinct subpopulations consistent with origins in these same families. Conclusions. Our results favour multiple parent bodies for LL chondrites. The petrologic differences between the NysaS and Flora parent bodies can be explained by differences in their sizes, without requiring different formation times.

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arXiv2026

Apophis source population and Earth encounter frequency of Apophis-like bodies

We provide context for Apophis' 2029 Earth passage by analyzing its possible source populations, in particular, the Flora family, which has a similar composition, corresponding to LL chondrite meteorites. Out of ${\sim}3380$ NEOs larger or equal than Apophis (${\ge}420\,{\rm m}$), $610\pm 140$ are LL-like NEOs from Flora. Their mean encounter probability is $p = 86\times 10^{-18}\,{\rm km}^{-2}\,{\rm y}^{-1}$, corresponding to once per 13000 y frequency of encounters closer than 38000 km. However, this does not apply to Apophis alone, for which the specific encounter probability is higher, $p' = 1603\times 10^{-18}\,{\rm km}^{-2}\,{\rm y}^{-1}$, but the frequency is lower, only once per 430000 y, when we consider it as a single object. Our simulation of the Flora family over $\sim$1 billion years indicates that Apophis-like bodies have orbits that are particularly persistent in near-Earth space. The temporal distribution of encounter probabilities exhibits peaks (up to ${>}10^4$ in the same units) and the specific value for Apophis is not unusual (occurring ${\sim}70\%$ of time). In other words, there is always at least one Apophis-like body among NEOs. We find that such persistence also creates favorable opportunities for temporary capture as Earth coorbitals. Apophis-like bodies are ultimately removed from the inner solar system by approaching the Sun or by impact into one of the terrestrial planets, where the relative split between these outcomes is $(45\pm 2)\,\%$ and $(50\pm 2)\,\%$. While our current knowledge of Apophis' orbit guarantees no threat from Apophis in the next few centuries, we cannot predict any specific outcome for Apophis in the coming thousands or millions of years. Evaluating this statistically over the long term, we find that objects in Apophis-like orbits have a $(19\pm 2)\,\%$ chance of Earth impact over their lifetime of ${\sim}30\,{\rm My}$.

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arXiv2024

Shaking and Tumbling: Short- and Long-Timescale Mechanisms for Resurfacing of Near-Earth Asteroid Surfaces from Planetary Tides and Predictions for the 2029 Earth Encounter by (99942) Apophis

Spectral characterization of near-Earth asteroids (NEAs) has revealed a continuum of space-weathered states for the surfaces of S-complex NEAs, with Q-class NEAs, an S-complex subclass, most closely matching the un-weathered surfaces of ordinary chondrite meteorites. Dynamical calculations of the orbital evolution of S-complex NEAs revealed that Q-class NEAs tend to have close encounters with terrestrial planets, suggesting that planetary tides may play a role in refreshing NEA surfaces. However, the exact physical mechanism(s) that drive resurfacing through tidal encounters and the encounter distance at which these mechanisms are effective, has remained unclear. Through the lens of the upcoming (99942) Apophis encounter with Earth in 2029, we investigate the potential for surface mobilization through tidally-driven seismic shaking over short-timescales during encounter and subsequent surface slope evolution over longer-timescales driven by tumbling. We perform multi-scale numerical modeling and find that the 2029 encounter will induce short-term tidally-driven discrete seismic events that lead to high-frequency (>0.1 Hz) surface accelerations that reach magnitudes similar to Apophis' gravity, and that may be detectable by modern seismometers. It is still unclear if the shaking we model translates to widespread particle mobilization and/or lofting. We also find there will be a significant change in Apophis' tumbling spin state that could lead to longer-term surface refreshing in response to tumbling-induced surface slope changes. We propose that through these mechanisms, space-weathered S-class asteroid surfaces may become refreshed through the exposure of unweathered underlying material. These results will be tested by the future exploration of Apophis by NASA's OSIRIS-APEX.

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arXiv2024

Young asteroid families as the primary source of meteorites

Understanding the origin of bright shooting stars and their meteorite samples is among the most ancient astronomy-related questions that at larger scales has human consequences [1-3]. As of today, only ${\sim}\,6\%$ of meteorite falls have been firmly linked to their sources (Moon, Mars, and asteroid (4) Vesta [4-6]). Here, we show that ${\sim}\,70\%$ of meteorites originate from three recent breakups of $D > 30\,{\rm km}$ asteroids that occurred 5.8, 7.5 and less than ${\sim}\,40$ million years ago. These breakups, including the well-known Karin family [7], took place in the prominent yet old Koronis and Massalia families and are at the origin of the dominance of H and L ordinary chondrites among meteorite falls. These young families distinguish themselves amidst all main belt asteroids by having a uniquely high abundance of small fragments. Their size-frequency distribution remains steep for a few tens of millions of years, exceeding temporarily the production of metre-sized fragments by the largest old asteroid families (e.g., Flora, Vesta). Supporting evidence includes the existence of associated dust bands [8-10], the cosmic-ray exposure ages of H-chondrite meteorites [11,12], or the distribution of pre-atmospheric orbits of meteorites [13-15].

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arXiv2024

Source regions of carbonaceous meteorites and NEOs

The present work aims to determine the source regions of carbonaceous chondrites (CM, CI, CO, CV, CK, CR, CH, CB, or C-ungrouped). We studied 38 individual asteroid families, including young and old ones, and determined their contributions to the NEO populations at metre and kilometre sizes using collisional and orbital models. Our models are in agreement with spectroscopic observations of NEOs, cosmic-ray exposure ages of meteorites, statistics of bolides, infrared emission from dust bands, composition of interplanetary dust particles (IDPs), or abundance of extraterrestrial helium-3. We identified the Veritas, Polana and Eos families as the primary sources of CM/CR, CI and CO/CV/CK chondrites, respectively. Substantial contributions are also expected from CM-like König and CI-like Clarissa, Misa and Hoffmeister families. The source regions of kilometre-sized bodies are generally different. The Adeona family is by far the main source of CM-like NEOs, whereas the Polana (low-i) and Euphrosyne (high-i) families are at the origin of most CI-like NEOs. The Polana family is the likely source of both Ryugu and Bennu. We were able to link spectroscopically and dynamically several NEOs to the Baptistina family. Finally, it appears that the pre-atmospheric flux of carbonaceous chondrites at metre sizes is about the same as that of ordinary chondrites. Given the difference in fall statistics between the two groups (80\% versus 4.4\%), this implies either substantial atmospheric fragmentation of carbonaceous bodies at the level of ${\sim}0.5\,{\rm MPa}$, or destruction by thermal cracking and water desorption. The source regions of most meteorites and kilometre-sized NEOs have now been determined; including some minor classes like enstatite chondrites and achondrites (Nysa, Hungaria), acapulcoites/lodranites (Iannini).

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arXiv2023

Keck and Gemini spectral characterization of Lucy mission fly-by target (152830) Dinkinesh

Recently, the inner main belt asteroid (152830) Dinkinesh was identified as an additional fly-by target for the Lucy mission. The heliocentric orbit and approximate absolute magnitude of Dinkinesh are known, but little additional information was available prior to its selection as a target. In particular, the lack of color spectrophotometry or spectra made it impossible to assign a spectral type to Dinkinesh from which its albedo could be estimated. We set out to remedy this knowledge gap by obtaining visible wavelength spectra with the Keck telescope on 2022 November 23 and with Gemini-South on 2022 December 27. The spectra measured with the Keck I/Low Resolution Imaging Spectrometer (LRIS) and the Gemini South/Gemini Multi-Object Spectrograph South (GMOS-S) are most similar to the average spectrum of S- and Sq-type asteroids. The most diagnostic feature is the $\approx$15$\pm$1$\%$ silicate absorption feature at $\approx$0.9-1.0~micron. Small S- and Sq-type asteroids have moderately high albedos ranging from 0.17-0.35. Using this albedo range for Dinkinesh in combination with measured absolute magnitude, it is possible to derive an effective diameter and surface brightness for this body. The albedo, size and surface brightness are important inputs required for planning a successful encounter by the Lucy spacecraft.

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arXiv2022

Isolating the mechanisms for asteroid surface refreshing

Evidence is seen for young, fresh surfaces among Near-Earth and Main-Belt asteroids even though space-weathering timescales are shorter than the age of the surfaces. A number of mechanisms have been proposed to refresh asteroid surfaces on short timescales, such as planetary encounters, YORP spinup, thermal degradation, and collisions. Additionally, other factors such as grain size effects have been proposed to explain the existence of these "fresh-looking" spectra. To investigate the role each of these mechanisms may play, we collected a sample of visible and near-infrared spectra of 477 near-Earth and Mars Crosser asteroids with similar sizes and compositions - all with absolute magnitude H > 16 and within the S-complex and having olivine to pyroxene (ol/(ol+opx)) ratios > 0.65. We taxonomically classify these objects in the Q (fresh) and S (weathered) classes. We find four trends in the Q/S ratio: 1) previous work demonstrated the Q/S ratio increases at smaller sizes down to H<16, but we find a sharp increase near H=19 after which the ratio decreases monotonically 2) in agreement with many previous studies, the Q/S ratio increases with decreasing perihelion distance, and we find it is non-zero for larger perihelia greater than 1.2AU, 3) as a new finding our work reveals the Q/S ratio has a sharp, significant peak near 5 degrees orbital inclination, and 4) we confirm previous findings that the Q/S ratio is higher for objects that have the possibility of encounter with Earth and Venus versus those that don't, however this finding cannot be distinguished from the perihelion trend. No single resurfacing mechanism can explain all of these trends, so multiple mechanisms are required. It is likely that a combination of all four resurfacing mechanisms are needed to account for all observational trends.

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arXiv2022

A Predicted Dearth of Majority Hypervolatile Ices in Oort Cloud Comets

We present new, ice species-specific New Horizons/Alice upper gas coma production limits from the 01 Jan 2019 MU69/Arrokoth flyby of Gladstone et al. (2021) and use them to make predictions about the rarity of majority hypervolatile (CO, N$_2$, CH$_4$) ices in KBOs and Oort Cloud comets. These predictions have a number of important implications for the study of the Oort Cloud, including: determination of hypervolatile rich comets as the first objects emplaced into the Oort Cloud; measurement of CO/N$_2$/CH$_4$ abundance ratios in the proto-planetary disk from hypervolatile rich comets; and population statistical constraints on early (< 20 Myr) planetary aggregation driven versus later (> 50 Myr) planetary migration driven emplacement of objects into the Oort Cloud. They imply that the phenomenon of ultra-distant active comets like C/2017K2 (Jewitt et al. 2017, Hui et al. 2018) should be rare, and thus not a general characteristic of all comets. They also suggest that interstellar object 2I/Borisov did not originate in a planetary system that was inordinately CO rich (Bodewits et al. 2020), but rather could have been ejected onto an interstellar trajectory very early in its natal system's history.

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arXiv2022

A Near Surface Temperature Model of Arrokoth

A near surface thermal model for Arrokoth is developed based on the recently released $10^5$ facet model of the body. This thermal solution takes into account Arrokoth's surface re-radiation back onto itself. The solution method exploits Arrokoth's periodic orbital character to develop a thermal response using a time-asymptotic solution method, which involves a Fourier transform solution of the heat equation, an approach recently used by others. We display detailed thermal solutions assuming that Arrokoth's near surface material's thermal inertia ${\cal I} = $ 2.5 W/m$^{-2}$K$^{-1}$s$^{1/2}$. We predict that at New Horizons' encounter with Arrokoth its encounter hemisphere surface temperatures were $\sim$ 57-59 K in its polar regions, 30-40 K on its equatorial zones, and 11-13 K for its winter hemisphere. Arrokoth's orbitally averaged temperatures are around 30-35 K in its polar regions, and closer to 40 K near its equatorial zones. Thermal reradiation from the surrounding surface amounts to less than 5\% of the total energy budget, while the total energy ensconced into and exhumed out Arrokoth's interior via thermal conduction over one orbit is about 0.5\% of the total energy budget. As a generalized application of this thermal modeling together with other KBO origins considerations, we favor the interpretation that New Horizons' REX instrument's $29 \pm 5$K brightness temperature measurement is consistent with Arrokoth's near surface material's being made of sub-to-few mm sized tholin-coated amorphous \water ice grains with 1 W/m$^{-2}$K$^{-1}$s$^{1/2}$ $< {\cal I} < $10-20 W/m$^{-2}$K$^{-1}$s$^{1/2}$, and which are characterized by an X-band emissivity in the range 0.9 and 1.

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arXiv2020

On the Origin and Thermal Stability of Arrokoths and Plutos Ices

We discuss in a thermodynamic, geologically empirical way the long-term nature of the stable majority ices that could be present in Kuiper Belt Object 2014 MU69 after its 4.6 Gyr residence in the EKB as a cold classical object. Considering the stability versus sublimation into vacuum for the suite of ices commonly found on comets, Centaurs, and KBOs at the average ~40K sunlit surface temperature of MU69 over Myr to Gyr, we find only 3 common ices that are truly refractory: HCN, CH3OH, and H2O (in order of increasing stability). NH3 and H2CO ices are marginally stable and may be removed by any positive temperature excursions in the EKB, as produced every 1e8 - 1e9 yrs by nearby supernovae and passing O/B stars. To date the NH team has reported the presence of abundant CH3OH and evidence for H2O on MU69s surface (Lisse et al. 2017, Grundy et al. 2020). NH3 has been searched for, but not found. We predict that future absorption feature detections will be due to an HCN or poly-H2CO based species. Consideration of the conditions present in the EKB region during the formation era of MU69 lead us to infer that it formed "in the dark", in an optically thick mid-plane, unable to see the nascent, variable, highly luminous Young Stellar Object-TTauri Sun, and that KBOs contain HCN and CH3OH ice phases in addition to the H2O ice phases found in their Short Period comet descendants. Finally, when we apply our ice thermal stability analysis to bodies/populations related to MU69, we find that methanol ice may be ubiquitous in the outer solar system; that if Pluto is not a fully differentiated body, then it must have gained its hypervolatile ices from proto-planetary disk sources in the first few Myr of the solar systems existence; and that hypervolatile rich, highly primordial comet C/2016 R2 was placed onto an Oort Cloud orbit on a similar timescale.

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arXiv2020

Initial results from the New Horizons exploration of 2014 MU69, a small Kuiper Belt Object

The Kuiper Belt is a distant region of the Solar System. On 1 January 2019, the New Horizons spacecraft flew close to (486958) 2014 MU69, a Cold Classical Kuiper Belt Object, a class of objects that have never been heated by the Sun and are therefore well preserved since their formation. Here we describe initial results from these encounter observations. MU69 is a bi-lobed contact binary with a flattened shape, discrete geological units, and noticeable albedo heterogeneity. However, there is little surface color and compositional heterogeneity. No evidence for satellites, ring or dust structures, gas coma, or solar wind interactions was detected. By origin MU69 appears consistent with pebble cloud collapse followed by a low velocity merger of its two lobes.

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arXiv2020

The Geology and Geophysics of Kuiper Belt Object (486958) Arrokoth

The Cold Classical Kuiper Belt, a class of small bodies in undisturbed orbits beyond Neptune, are primitive objects preserving information about Solar System formation. The New Horizons spacecraft flew past one of these objects, the 36 km long contact binary (486958) Arrokoth (2014 MU69), in January 2019. Images from the flyby show that Arrokoth has no detectable rings, and no satellites (larger than 180 meters diameter) within a radius of 8000 km, and 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.

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arXiv2020

Color, Composition, and Thermal Environment of Kuiper Belt Object (486958) Arrokoth

The outer Solar System object (486958) Arrokoth (provisional designation 2014 MU$_{69}$) has been largely undisturbed since its formation. We study its surface composition using data collected by the New Horizons spacecraft. Methanol ice is present along with organic material, which may have formed through radiation of simple molecules. Water ice was not detected. This composition indicates hydrogenation of carbon monoxide-rich ice and/ or energetic processing of methane condensed on water ice grains in the cold, outer edge of the early Solar System. There are only small regional variations in color and spectra across the surface, suggesting Arrokoth formed from a homogeneous or well-mixed reservoir of solids. Microwave thermal emission from the winter night side is consistent with a mean brightness temperature of 29$\pm$5 K.

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arXiv2019

Impact Craters on Pluto and Charon Indicate a Deficit of Small Kuiper Belt Objects

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 less than approximately 13 kilometers in diameter, which cannot be explained solely by geological resurfacing. This implies a deficit of small KBOs (less than 1 to 2 kilometers in diameter). Some surfaces on Pluto and Charon are likely greater than 4 billion years old, thus their crater records provide information on the size-frequency distribution of KBOs in the early Solar System.

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arXiv2019

Convection in a volatile nitrogen-ice-rich layer drives Pluto's geological vigor

The vast, deep, volatile-ice-filled basin informally named Sputnik Planum is central to Pluto's geological activity[1,2]. Composed of molecular nitrogen, methane, and carbon monoxide ices[3], but dominated by N2-ice, this ice layer is organized into cells or polygons, typically ~10-40 km across, that resemble the surface manifestation of solid state convection[1,2]. Here we report, based on available rheological measurements[4], that solid layers of N2 ice approximately greater than 1 km thick should convect for estimated present-day heat flow conditions on Pluto. More importantly, we show numerically that convective overturn in a several-km-thick layer of solid nitrogen can explain the great lateral width of the cells. The temperature dependence of N2-ice viscosity implies that the SP ice layer convects in the so-called sluggish lid regime[5], a unique convective mode heretofore not definitively observed in the Solar System. Average surface horizontal velocities of a few cm/yr imply surface transport or renewal times of ~500,000 years, well under the 10 Myr upper limit crater retention age for Sputnik Planum[2]. Similar convective surface renewal may also occur on other dwarf planets in the Kuiper belt, which may help explain the high albedos of some of them.

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arXiv2019

Reorientation of Sputnik Planitia implies a Subsurface Ocean on Pluto

The deep nitrogen-covered Sputnik Planitia (SP; informal name) basin on Pluto is located very close to the longitude of Pluto's tidal axis[1] and may be an impact feature [2], by analogy with other large basins in the solar system[3,4]. Reorientation[5-7] due to tidal and rotational torques can explain SP's location, but requires it to be a positive gravity anomaly[7], despite its negative topography. Here we argue that if SP formed via impact and if Pluto possesses a subsurface ocean, a positive gravity anomaly would naturally result because of shell thinning and ocean uplift, followed by later modest N2 deposition. Without a subsurface ocean a positive gravity anomaly requires an implausibly thick N2 layer (greater than 40 km). A rigid, conductive ice shell is required to prolong such an ocean's lifetime to the present day[8] and maintain ocean uplift. Because N2 deposition is latitude-dependent[9], nitrogen loading and reorientation may have exhibited complex feedbacks[7].

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arXiv2019

Pluto's Haze as a Surface Material

Pluto's atmospheric haze settles out rapidly compared with geological timescales. It needs to be accounted for as a surface material, distinct from Pluto's icy bedrock and from the volatile ices that migrate via sublimation and condensation on seasonal timescales. This paper explores how a steady supply of atmospheric haze might affect three distinct provinces on Pluto. We pose the question of why they each look so different from one another if the same haze material is settling out onto all of them. Cthulhu is a more ancient region with comparatively little present-day geological activity, where the haze appears to simply accumulate over time. Sputnik Planitia is a very active region where glacial convection, as well as sublimation and condensation rapidly refresh the surface, hiding recently deposited haze from view. Lowell Regio is a region of intermediate age featuring very distinct coloration from the rest of Pluto. Using a simple model haze particle as a colorant, we are not able to match the colors in both Lowell Regio and Cthulhu. To account for their distinct colors, we propose that after arrival at Pluto's surface, haze particles may be less inert than might be supposed from the low surface temperatures. They must either interact with local materials and environments to produce distinct products in different regions, or else the supply of haze must be non-uniform in time and/or location, such that different products are delivered to different places.

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arXiv2019

Formation of Charon's Red Poles From Seasonally Cold-Trapped Volatiles

A unique feature of Pluto's large satellite Charon is its dark red northern polar cap. Similar colours on Pluto's surface have been attributed to organic macromolecules produced by energetic radiation processing of hydrocarbons. The polar location of this material on Charon implicates the temperature extremes that result from Charon's high obliquity and long seasons. The escape of Pluto's atmosphere provides a potential feed stock for production of complex chemistry. Gas from Pluto that is transiently cold-trapped and processed at Charon's winter pole was proposed as an explanation on the basis of an image of Charon's northern hemisphere, but not modelled quantitatively. Here we report images of the southern hemisphere illuminated by Pluto-shine and also images taken during the approach phase showing the northern polar cap over a range of longitudes. We model the surface thermal environment on Charon, the supply and temporary cold-trapping of material escaping from Pluto, and, while cold-trapped, its photolytic processing into more complex and less volatile molecules. The model results are consistent with the proposed mechanism producing the observed colour pattern on Charon.

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