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Small solar system bodies have widely dispersed orbital poles, posing challenges to dynamical models of solar system origin and evolution. To characterize the orbit pole distribution of dynamical groups of small bodies it helps to have a functional form for a model of the distribution function. Previous studies have used the small-inclination approximation and adopted variations of the normal distribution to model orbital inclination dispersions. Because the orbital pole is a directional variable, its distribution can be more appropriately modeled with directional statistics. We describe the von Mises-Fisher (vMF) distribution on the surface of the unit sphere for application to small bodies' orbital poles. We apply it to the orbit pole distribution of the observed Plutinos. We find a mean pole located at inclination of 3.57 degrees and a longitude of ascending node of 124.38 degrees (in the J2000 reference frame), with a 99.7 per cent confidence cone of half-angle 1.68 degrees. We also estimate a debiased mean pole located 4.6 degrees away, at an inclination of 2.26 degrees and a longitude of ascending node of 292.69 degrees, of similar-size confidence cone. The vMF concentration parameter of Plutino inclinations (relative to either mean pole estimate) is 31.6. This resembles a Rayleigh distribution function with a width parameter of 10.2 degrees. Unlike previous models, the vMF model naturally accommodates all physical inclinations (and no others), whereas Rayleigh or Gaussian models must be truncated to the physical inclination range 0-180 degrees. Further work is needed to produce a theory for the mean pole of the Plutinos against which to compare the observational results., 10 pages, 7 figures. Accepted for Monthly Notices of the Royal Astronomical Society (MNRAS) in April 2023

Introduction: The close approach of asteroid (99942) Apophis on April 13, 2029 presents a unique opportunity to achieve breakthrough science and strengthen planetary defense goals. As discussed in [1], low-frequency (VHF) radar observations can probe the interior structure of small bodies, as demonstrated by CONSERT at comet 67P [2, 3], and the planned JuRa low frequency radar on Hera/Juventas at the Didymos system—target of the DART mission. Radar measurements can determine the distribution of monolithic objects and voids within the body at 10’s of meter scale, which are critical for potential deflection and disruption attempts. This is best accomplished by multi-static, low frequency radar [4]. A mission concept to exploit the Apophis opportunity has been developed in a collaboration between NASA/JPL and CNES. The Distributed Radar Observations of Interior Distributions (DROID) mission would rendezvous with Apophis in late Summer 2028, seven months prior to Earth closest approach (ECA) and escort it through the encounter. A possible asteroid flyby on the way would delay arrival to late 2028 but still provide ample time for pre-ECA characterization. DROID’s measurements would determine the interior structure and properties, the body’s shape, morphology and rotation and observe any resolvable changes. DROID provides unique high fidelity in situ data that complements and enhances Earth-based optical and radar observations of Apophis, as well as data collected by OSIRIS-APEX which is due to rendezvous with Apophis 8 days after ECA. As illustrated in Figure 1, DROID’s architecture calls for three spacecraft: an ESPA Grande-class Mothership and two 6U CubeSats. The Mothership carries the CubeSats to Apophis, achieves the rendezvous cruise trajectory, performs high resolution imaging, and acts as a Direct-to-Earth (DTE) node for the constellation. Once Apophis’s physical characteristics (shape, spin, gravity field) are sufficiently characterized, the Mothership deploys both CubeSats, which then insert themselves into coordinated low orbits to perform monostatic and bistatic radar observations. Mission Goals: The DROID mission has two primary goals. The first goal is to understand the interior structure of a rubble pile asteroid and implications for its formation, evolution and response to a deflection attempt. Objectives include determining shape and density, and determining the internal size, distribution, and arrangement of blocks and voids within Apophis. \ DROID’s second goal is to understand how close planetary encounters affect asteroids. DROID will provide critical pre-ECA imagery of Apophis that are necessary for change detection. Objectives include determining if material moves on the surface of Apophis during the Earth flyby, and determining how the spin state of Apophis changes during ECA. Payload: Given the goals above, DROID employs four types of payloads distributed over three spacecraft (Figure 1). Objectives requiring surface imaging are to be met with a narrow-angle camera (NAC) on-board the Mothership spacecraft, whose focal plane is to be based on the Advanced CASPEX detector [5]. Additional wide-angle cameras (WACs) are carried on the two CubeSats for optical navigation. The objective to map internal structure is achieved using the Low Frequency Radar (LFR) on the CubeSats. The LFR is baselined as a version of JuRa (60 MHz), [6], modified to operate in a bistatic mode [1]. Inter-Spacecraft Link (ISL) S-band transponders on all three spacecraft perform data transfer between CubeSats and Mothership, and synchronize the CubeSat clocks for accurate bistatic radar measurement. ISLs are also used with the Mothership’s DTE link to map the gravity field. Mission Architecture: The DROID mission architecture is compatible with either direct launch or rideshare and will utilize heritage bus designs that can achieve the required propulsion performance. DROID’s 3.54 km/s ΔV requirement is similar to that of ESCAPADE, which uses bipropellant propulsion [7], DROID’s reference mission is constrained by a cruise trajectory insertion (CTI) window of about October-November 2027. Details of launch, CTI and cruise are provided in [8]. Operations: DROID arrives at Apophis around August-September 2028 (~December if it performs a precursor asteroid flyby) and executes a 0.30 km/s burn to reduce its relative velocity. During this phase, the Mothership NAC begins preliminary characterization of Apophis’s shape and spin. Approach imaging is then followed-up by several flyby maneuvers used to characterize the gravity field with DTE communication. The Mothership then deploys the CubeSats, which maneuver into 2-5 body radii altitude, sun-synchronous terminator orbits using their own cold gas propulsion. Following CubeSat deployment, the Mothership positions itself in a 9 body radii altitude orbit where it continues its imaging investigations using the NAC. The CubeSats are positioned antipodally with ±15° margin in their relative position and continuously collect both monostatic and bistatic echoes. A 2-body radii altitude orbit will enable mapping of 20% of the 3D monostatic Doppler sampling at 60 MHz [9], within 40 days. Radar data products include: (1) 3D volumetric backscatter via monostatic/bistatic tomographic SAR, (2) average dielectric constant along interior bistatic ray paths with assessment of internal heterogeneity [10]. The configuration of the DROID constellation during ECA and Post-ECA operations is the subject of on-going studies. Major ECA drivers include positioning of cameras to maximize the likelihood of capturing surface changes and mitigating the risk of collisions with potential ejected debris. The major Post-ECA driver is escaping from Apophis orbit to a safe heliocentric orbit prior to depleting propellent in order to avoid any possibility of impacting the asteroid and perturbing its orbit. Acknowledgments: This work is being carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA (80NM0018D0004), and at CNES. ©2022 California Institute of Technology. Government sponsorship acknowledged. References: [1] Herique, A. et al (this meeting). [2] Barbin, Y. et al (1999) ASR 24. [3] Kofman, W. et al. (2007) SSR 128. [4] Haynes, M. et al (2022) LPSC #1295. [5] Bezine, J. et al. (2021) ICSO 118520V. [6] Herique, A. et al (2020) EPSC. [7] French, R. (2019) AIAA SSC. [8] Amini, R. et al (2022) Apophis T-7 #2012. [9] Haynes, M. et al. (2021) ASR 68. [10] Herique, A. et al (2018) ASR 62.

The formation of the largest and most ancient lunar impact basin, South Pole–Aitken (SPA), was a defining event in the Moon’s evolution. Using numerical simulations, we show that widespread mantle heating from the SPA impact can catalyze the formation of the long-lived nearside-farside lunar asymmetry in incompatible elements and surface volcanic deposits, which has remained unexplained since its discovery in the Apollo era. The impact-induced heat drives hemisphere-scale mantle convection, which would sequester Th- and Ti-rich lunar magma ocean cumulates in the nearside hemisphere within a few hundred million years if they remain immediately beneath the lunar crust at the time of the SPA impact. A warm initial upper mantle facilitates generation of a pronounced compositional asymmetry consistent with the observed lunar asymmetry.

A near-surface thermal model for Arrokoth is developed based on the recently released 105 facet model of the body. This thermal solution takes into account Arrokoth’s surface reradiation 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  = 2.5 W/m−2 K−1 s1/2. We predict that at New Horizons’ encounter with Arrokoth, its encounter hemisphere surface temperatures were ∼57–59 K in its polar regions, 30–40 K in 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 of 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 Kuiper Belt object origins considerations, we favor the interpretation that New Horizons’ REX instrument’s 29 ± 5 K brightness temperature measurement is consistent with Arrokoth’s near-surface material being made of sub-to-few-millimeter-size tholin-coated amorphous H2O ice grains with 1 W/m−2 K−1 s1/2 <  < 10–20 W/m−2 K−1 s1/2 and which are characterized by an X-band emissivity in the range 0.9 and 1.

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., 43 pages, 8 figure

The dominant topographic features on two-lobed Kuiper Belt object (486958) Arrokoth (provisionally designated 2014 MU69) are scattered, small, circular depressions or pits up to ~1.0 km across and curvilinear troughs observed near the terminator during the New Horizons encounter of 01 January 2019. With important exceptions, evidence for an endogenic origin for pits is lacking and impact remains the most likely origin. Pit depths relative to the local surface are shallower than hypervelocity simple craters on icy moons and consistent with low velocity (~300 m/s) impacts in an icy target (the role of porosity and cohesion in cratering on Arrokoth being uncertain). There is, however, a large scatter in observed d/D, with some pits too shallow to measure reliably. The range of preservation states observed for both pits and troughs is consistent with slow but persistent degradation on the surface of this (and perhaps other) small primitive planetesimal(s) in the Kuiper Belt by agents such as micrometeorite bombardment or volatile loss, indicating that degradation does indeed occur on such bodies. Arrokoth pits could also be related to circular depressions on comet 9P/Tempel (though comparisons of surface features on Arrokoth and comets are intriguing they are limited by resolution differences and the evolved state of cometary surfaces). Two of the curvilinear troughs occur along the terminator of the Large Lobe. One trough is a shallow depression devoid of resolvable structures, the other features several linear scarps a few 10s of meters high, suggesting that coherent failure can occur on the surfaces of small Kuiper Belt objects such as Arrokoth. Several of the deepest small pits are also clustered within this walled trough, similar to tectonically associated pit chains on other planets. These pits and an obliquely viewed linear chain of elongate depressions remain the best candidates for endogenic features on Arrokoth. Subtle undulations and linear features no more than a few 10's of meters in amplitude (and even a few candidate positive relief features) are also evident in the terminator regions, indicating that the long-term evolution of this body may have been complex.

International audience; The interior structure of the Moon is constrained by its mass, moment of inertia, and k 2 and h 2 tidal Love numbers. We infer the likely radius, density, and (elastic limit) rigidity of all interior layers by solving the inverse problem using these observational constraints assuming spherical symmetry. Our results do not favor the presence of a low rigidity transition layer between a liquid outer core and mantle. If a transition layer exists, its rigidity is constrained to 43 +26 −9 GPa, with a preference for the high rigidity values. Therefore, if a transition layer exists, it is more likely to have a rigidity similar to that of the mantle (∼70 GPa). The total (solid and liquid) core mass fraction relative to the lunar mass is constrained to 0.0098 +0.0066 −0.0094 and 0.0198 +0.0026 −0.0049 for interior structures with and without a transition layer, respectively, narrowing the range of possible giant impact formation scenarios.

A high-resolution gravity map over the entire lunar surface has been derived from data acquired by the Gravity Recovery and Interior Laboratory (GRAIL) mission. Soderblom et al. (2015) showed that crater Bouguer gravity anomalies scale with crater diameter and porosity for craters in the lunar highlands. Here we extend this study globally, examining complex craters in each of the three lunar terranes: highlands, maria, and the South Pole-Aitken basin. We find that craters within South Pole-Aitken basin and in the lunar maria have statistically different Bouguer anomalies from those in the lunar highlands. These differences are best explained by differences in crustal porosity among the three terranes. Though there is still much unresolved scatter in the data, we find that no other lunar material properties (crustal thickness, density gradient, etc.) are able to improve our model fit to the data.

We report measurements of the oxidation state of Fe nanoparticles within lunar soils that experienced varied degrees of space weathering. We measured >100 particles from immature, submature, and mature lunar samples using electron energy-loss spectroscopy (EELS) coupled to an aberration-corrected transmission electron microscope. The EELS measurements show that the nanoparticles are composed of a mixture of Fe0, Fe2+, and Fe3+ oxidation states, and exhibit a trend of increasing oxidation state with higher maturity. We hypothesize that the oxidation is driven by the diffusion of O atoms to the surface of the Fe nanoparticles from the oxygen-rich matrix that surrounds them. The oxidation state of Fe in the nanoparticles has an effect on modeled reflectance properties of lunar soil. These results are relevant to remote sensing data for the Moon and to the remote determination of relative soil maturities for various regions of the lunar surface.

As first noted 200 years ago by Laplace, the Moon's rotational and tidal bulges are significantly larger than expected, given the Moon's present orbital and rotational state. This excess deformation has been ascribed to a fossil figure, frozen in when the Moon was closer to the Earth. However, the observed figure is only consistent with an eccentric and non-synchronous orbit, contrary to our understanding of the Moon's formation and evolution. Here, we show that lunar mascons and impact basins have a significant contribution to the observed lunar figure. Removing their contribution reveals a misaligned fossil figure consistent with an early epoch of true polar wander (driven by the formation of the South Pole-Aitken Basin) and an early low-eccentricity, synchronous lunar orbit. This new self-consistent model solves a long-standing problem in planetary science and will inform future studies of the Moon's dynamical evolution and early dynamo.

The Orientale basin is the youngest and best-preserved major impact structure on the Moon. We used the Gravity Recovery and Interior Laboratory (GRAIL) spacecraft to investigate the gravitational field of Orientale at 3- to 5-kilometer (km) horizontal resolution. A volume of at least (3.4 ± 0.2) × 106 km3 of crustal material was removed and redistributed during basin formation. There is no preserved evidence of the transient crater that would reveal the basin’s maximum volume, but its diameter may now be inferred to be between 320 and 460 km. The gravity field resolves distinctive structures of Orientale’s three rings and suggests the presence of faults associated with the outer two that penetrate to the mantle. The crustal structure of Orientale provides constraints on the formation of multiring basins.

Multiring basins, large impact craters characterized by multiple concentric topographic rings, dominate the stratigraphy, tectonics, and crustal structure of the Moon. Using a hydrocode, we simulated the formation of the Orientale multiring basin, producing a subsurface structure consistent with high-resolution gravity data from the Gravity Recovery and Interior Laboratory (GRAIL) spacecraft. The simulated impact produced a transient crater, ~390 kilometers in diameter, that was not maintained because of subsequent gravitational collapse. Our simulations indicate that the flow of warm weak material at depth was crucial to the formation of the basin’s outer rings, which are large normal faults that formed at different times during the collapse stage. The key parameters controlling ring location and spacing are impactor diameter and lunar thermal gradients.