Small Planetary Bodies Research Articles

The Stability of a Dense Crust Situated on Small Bodies

Small planetary bodies in the solar system, including Io, Ganymede, and Callisto, may have a crust denser than their underlying mantle. Despite the inherent gravitational instability of such structures, we show that the growth timescale of the Rayleigh–Taylor (RT) instability can be as long as the age of the solar system, owing to the strong temperature dependence of viscosity. Even in cases where the instability timescale is shorter, the instability is confined to a thin layer at the base of the crust, making the foundering of the entire crust improbable in many scenarios. This study delineates the onset and aftermath of the RT instability, applying a quantitative framework to assess the stability of (i) rock-contaminated crust on icy satellites, and (ii) silicate crust floating on top of a subsurface magma ocean on Io. Notably, for Io the RT instability peels off only 10–100 m from the crust’s base, and thermal diffusion rapidly recovers the crustal thickness through solidification of a magma ocean. Despite recurrent delamination of the crustal base, the initial crustal thickness is maintained by thermal diffusion, virtually stabilizing a floating dense crust. Cracking of the crust also is unlikely to result in the foundering of the crust. A dense crust on a small body is therefore difficult to be overturned, suggesting the potential ubiquity of dense surface layers throughout the solar system.

Strength, plasticity, and spin transition of Fe-N compounds in planetary cores

Elastic and plastic properties of Fe-light element alloys and compounds are needed to determine the compositions and dynamics of planetary cores. Elastic strength and plastic deformation mechanisms and their relationship to electronic properties of ε-Fe7N3 and γ'-Fe4N mixture were investigated by x-ray diffraction and x-ray emission spectroscopy in the diamond anvil cell from 1 bar up to 60 GPa. X-ray diffraction shows that ε-Fe7N3 reaches a pressure of 15–20 GPa before undergoing bulk plasticity at a differential stress of 4.4–10.4 GPa. ε-Fe7N3 is stronger than γ'-Fe4N and hcp-Fe which achieve a flow stress of 1.5–3.6 GPa at 10–15 GPa and 2–3 GPa at ∼20 GPa, respectively. X-ray emission spectroscopy shows that a decrease in electronic spin moment begins before and completes after plastic flow onset for each nitride, suggesting that pressure-driven changes in electronic arrangement do not trigger a plastic response although they may modify the strength and plastic behavior of Fe-N compounds. Plastic deformation in ε-Fe7N3 and hcp-Fe results in a preferred orientation of (0001) normal to maximum compression, while γ'-Fe4N develops a maximum in the (110). These observations may be combined with measurements of elasticity to model seismic properties of cores of small planetary bodies such as Mars, Mercury, and the Moon.

Global geological mapping of Venus and the twenty-first-century legacy of William Smith: identification of challenges and opportunities for future research and exploration

Abstract Geological mapping principles have been applied across planet Earth since William Smith's initial 1815 map. During the Space Age, these principles were applied to smaller moons and planets, appropriately adjusted for remote conditions and data, and revealing ‘one plate planets’ recording Earth's missing chapters. Yet what of Venus, Earth's ‘twin’, shrouded in opaque clouds? How can these geological mapping principles be applied to address fundamental questions of planetary history? We outline how Venus's geological mapping over the last 35 years has proceeded towards a successively integrated more global picture that has revealed a surface and history unlike Earth or the smaller planetary bodies! The current 1 : 10 M global geological map documents geological units that all formed in the last c. 20% of Venus's history. The oldest, Fortunian, involved intense deformation and building of thicker crust (tessera). The Guineverian featured near-global emplacement of vast volcanic plains. The Atlian saw prominent rift zones and large shield volcanoes. The three major phases of activity provide a basis for assessing the geological, atmospheric and geodynamical processes operating earlier in Venus's history that led to the preserved record, raising a series of questions to be investigated in the coming decade by an international armada of Venus missions.

Mass-dependent nickel isotopic variations in achondrites and lunar rocks

We present high-precision mass-dependent nickel isotopic data for a comprehensive suite of achondrites and lunar rocks, providing key insights into the early planetary differentiation and Earth-Moon system formation. The primitive achondrites display high Ni contents and invariant Ni isotopic compositions. Incomplete core-mantle differentiation in primitive achondrite parent bodies resulted in the retention of metal in the mantle, which dominated the Ni budget and accounted for the bulk chondritic Ni isotopic values. The highly reduced differentiated achondrites, aubrites and an ungrouped achondrite (NWA 8409), have variable, and extremely light Ni isotopic compositions. Acid leaching experiments demonstrate that the sulfides are a significant host of light Ni isotopes in aubrites. The most extreme Ni isotope values of aubrites may be due to large Ni isotope fractionation accompanied by silicate-sulfide-metal separation during differentiation of the parent bodies, and subsequent global disruptive collision and reassembly with variably high proportions of sulfides enriched in the mantle. The howardite-eucrite-diogenite (HED) meteorites show Ni isotopic variations that are positively correlated with Ni/Co ratios, a feature that cannot be produced by igneous differentiation. Late accretion of high-Ni and high-Ni/Co chondritic materials after core formation of their likely parent body, Vesta, could have accounted for this correlation. Thus, the primitive silicate mantle of Vesta may have sub-chondritic Ni isotopic compositions, implying possible Ni isotope fractionation during core-mantle differentiation of small planetary bodies. The lunar breccia meteorites have homogenously chondritic Ni isotope values, together with their high Ni/Co of bulk rock and metals therein, suggesting impact contamination. Lunar basalt meteorites have low Ni/Co ratios and are systematically isotopically lighter than the breccias, displaying a positive correlation between Ni isotope value and Ni/Co ratio, as that seen in the HEDs. Therefore, the Ni isotopic systematics in lunar rocks also indicates the effect of late accretion, with the primitive lunar mantle having sub-chondritic Ni isotope values. This implies that the Moon-forming impactor, Theia, was likely an aubrite-like differentiated planetary body whose mantle was enriched in light Ni isotopes. We suggest that there was significant Ni isotope fractionation between core and mantle during differentiation of early formed small planetary bodies, but this signature can be obscured by late accretion in the bulk achondrite records.

High Pressure Melting Curve of Fe Determined by Inter‐Metallic Fast Diffusion Technique

AbstractThe heat extracted from the core by the overlying mantle across the core‐mantle boundary controls the thermal evolution of the core. This in turn leads to the solidification of the inner core in association with the exsolution of light alloying elements into the liquid outer core. Although the temperature (T) at the inner core boundary (ICB) would be adjusted to account for the effects of the light elements, the melting T of Fe places an upper bound at the ICB and it is a vital point in the thermal profile of the core. Here, we determine the melting T of Fe in the multi‐anvil press by characterizing the interface of Fe‐W interaction. Our data place a tighter constraint on the melting curve of Fe between 8 and 21 GPa, that is directly applicable to small planetary bodies and serves as an anchor for melting curve of Fe at higher pressure.

Birth and Decline of Magma Oceans in Planetesimals: 2. Structure and Thermal History of Early Accreted Small Planetary Bodies

AbstractThis is the second of two companion papers that present a theoretical and experimental study of the thermal history of planetesimals in which heating by short‐lived radioactive isotopes generates an internal magma ocean and the subsequent cooling and crystallization thereof. We study the conditions required to form and preserve basal cumulates and flotation crusts, and the implications for the thermal evolution of planetary bodies. Our model predicts that planetesimals larger than 30 km can reach 1300°C and a melt fraction of 40 vol%, producing a solid‐like to liquid‐like rheological transition that triggers an internal magma ocean. In the magma ocean regime core‐mantle differentiation occurs very quickly and the mantle convects under a relic of chondritic material whose thickness is controlled by the temperature of rheological transition. We show that the magma ocean episode is associated with time‐dependent crystal segregation and no re‐entrainment. Segregation of crystals is essentially constrained by their size and by their density difference with respect to the melt, the latter being fully determined by the planetesimal's initial composition. Olivine cumulates are likely to form at the core‐mantle boundary. Under certain particular conditions, a flotation crust can also form, which reduces the efficiency of heat evacuation by convection, thereby enhancing the magma ocean's lifetime and the efficiency of crystal segregation. Two types of large‐scale mantle structure are possible outcomes: a well‐mixed upper mantle above an olivine cumulate, or a more finely layered “onion‐shell” structure.

Planet Size Controls Fe Isotope Fractionation Between Mantle and Core

AbstractAs an element ubiquitous in the Solar system, the isotopic composition of iron exhibits rich variations in different planetary reservoirs. Such variations reflect the diverse range of differentiation and evolution processes experienced by their parent bodies. A key in deciphering iron isotope variations among planetary samples is to understand how iron isotopes fractionate during core formation. Here we report new Nuclear Resonant Inelastic X‐ray Scattering experiments on silicate glasses of bulk silicate Earth compositions to measure their force constants at high pressures of up to 30 GPa. The force constant results are subsequently used to constrain iron isotope fractionation during core formation on terrestrial planets. Using a model that integrates temperature, pressure, core composition, and redox state of the silicate mantle, we show that core formation might lead to an isotopically light mantle for small planetary bodies but a heavy one for Earth‐sized terrestrial planets.

Time-energy optimal landing on planetary bodies via theory of functional connections

In this paper, we propose a unified approach to solve the time-energy optimal landing problem on planetary bodies (e.g. planets, moons, and asteroids). In particular, the indirect optimization method, based on the derivation of the first order necessary conditions from the Hamiltonian, is exploited and the Two-Point Boundary Value Problem arising from the application of the Pontryagin Minimum Principle is solved using the Theory of Functional Connections. The optimal landing trajectories are accurately computed with a computational time on the order of 10–100 ms, using a MATLAB implementation. The speed and accuracy of the proposed method makes it suitable for real time applications. The algorithm is applied and validated for the landing on large (Mars and Moon) and small (asteroids Gaspra and Bennu) planetary bodies.

Possible thermal evolutionary pathways of irregular shaped small asteroids and planetesimals

Two distinct thermal evolutionary pathways of irregular shaped small planetesimals in the early solar system have been studied. We have taken a case study of two S-type asteroids; (243) Ida and (951) Gaspra, on the basis of their precise physical dimensions accessed by the Philip Stooke small body 3-D shape models of the NASA Planetary Data System. The 3- D shape models for the two asteroids are based on the Galileo spacecraft fly-by mission. Based on our novel thermal evolutionary code for the precise shape of the asteroids we found that the small planetary bodies that accreted within the initial 2-3 million years (Myr) experienced sintering, whereas, the bodies formed afterwards were left unconsolidated, e.g., as a rubble pile. The former set of bodies could have formed by direct aggregation of nebula dust. Whereas, the majority of the rubble pile type small planetary bodies accreted latter by the assemblage of the fragmented debris of the initially existing planetesimals. These bodies cooled over tens of million years. Generations of small planetesimals formed over time in the early solar system evolved from an ensemble of compact consolidated bodies to rubble pile bodies due to collision induced fragmentation and re-accretion.

Small All-Range Lidar for Asteroid and Comet Core Missions

We report the development of a new type of space lidar specifically designed for missions to small planetary bodies for both topographic mapping and support of sample collection or landing. The instrument is designed to have a wide dynamic range with several operation modes for different mission phases. The laser transmitter consists of a fiber laser that is intensity modulated with a return-to-zero pseudo-noise (RZPN) code. The receiver detects the coded pulse-train by correlating the detected signal with the RZPN kernel. Unlike regular pseudo noise (PN) lidars, the RZPN kernel is set to zero outside laser firing windows, which removes most of the background noise over the receiver integration time. This technique enables the use of low peak-power but high pulse-rate lasers, such as fiber lasers, for long-distance ranging without aliasing. The laser power and the internal gain of the detector can both be adjusted to give a wide measurement dynamic range. The laser modulation code pattern can also be reconfigured in orbit to optimize measurements to different measurement environments. The receiver uses a multi-pixel linear mode photon-counting HgCdTe avalanche photodiode (APD) array with near quantum limited sensitivity at near to mid infrared wavelengths where many fiber lasers and diode lasers operate. The instrument is modular and versatile and can be built mostly with components developed by the optical communication industry.

Granular flow on a rotating and gravitating elliptical body

Abstract

Hayabusa2 extended mission: New voyage to rendezvous with a small asteroid rotating with a short period

Hayabusa2 is the Japanese Asteroid Return Mission and targeted the carbonaceous asteroid Ryugu, conducted by the Japan Aerospace Exploration Agency (JAXA). The goal of this mission was to conduct proximity operations including remote sensing observations, material sampling, and a Small Carry-On Impact experiment, as well as sample analyses. As of September 2020, the spacecraft is on the way back to Earth with samples from Ryugu with no critical issues after the successful departure in November 2019. Here, we propose an extended mission in which the spacecraft will rendezvous with a small asteroid with ~30 m - ~40 m in diameter that is rotating at a spin period of ~10 min after an additional ~10-year cruise phase. We introduce that two scenarios are suitable for the extended mission. In the first scenario, the spacecraft will perform swing-by maneuvers at Venus once and Earth twice to arrive at asteroid 2001 AV43. In the second scenario, it will perform swing-by maneuvers at Earth twice to reach asteroid 1998 KY26. In both scenarios, the mission will continue until the early 2030s. JAXA recently released the decision that the spacecraft will rendezvous with 1998 KY26. This paper focuses on our scientific assessments of the two scenarios but leaves the decision process to go to 1998 KY26 for future reports. Rendezvous operations will be planned to detail the physical properties and surrounding environments of the target, one of the smallest elements of small planetary bodies. By achieving the planned operations, the mission will provide critical hints on the violent histories of collisions and accumulations of small bodies in the solar system. Furthermore, the established scientific knowledge and techniques will advance key technologies for planetary defense.

The radial structure of planetary bodies formed by the streaming instability

Comets and small planetesimals are believed to contain primordial building blocks in the form of millimeter to centimeter sized pebbles. One of the viable growing mechanisms to form these small bodies is through the streaming instability (SI) in which pebbles cluster and gravitationally collapse toward a planetesimal or comet in the presence of gas drag. However, most SI simulations are global and lack the resolution to follow the final collapse stage of a pebble cloud within its Hill radius. We aim to track the collapse of a gravitationally bound pebble cloud subject to mutual collisions and gas drag with the representative particle approach. We determine the radial pebble size distribution of the collapsed core and the impact of mutual pebble collisions on the pebble size distribution. We find that virial equilibrium is never reached during the cloud evolution and that, in general, pebbles with a given Stokes number (St) collapse toward an optically thick core in a sequence from aerodynamically largest (St ~ 0.1) to aerodynamically smallest (St ~ 2 × 10−3). We show that at the location where the core becomes optically thick, the terminal velocity vt,* ~ 60 m s−1St2 is well below the fragmentation threshold velocity. While collisional processing is negligible during cloud evolution, the collisions that do occur are sticking. These results support the observations that comets and small planetary bodies are composed of primordial pebbles in the millimeter to centimeter size range.

Seismic waves in the asteroid environment

Through numerical simulations, we investigate impact generated seismic wave transmission in granular media under extremely low pressure. This mimics the conditions in the interior of asteroids and other small planetary bodies. We find a dependency not only on the overburden pressure on the medium, but also on the velocity of the impact that generates the wave. This is, at extremely low values of overburden pressure, the wave speed depends no only on the imposed pressure, but also on the increment in pressure created by the passing of the wave. We study crystalline and random packings and find very similar behaviour though with different wave speeds as expected. We then relate our results to different mission-related events on asteroids.

High-eccentricity migration of planetesimals around polluted white dwarfs

ABSTRACT Several white dwarfs (WDs) with atmospheric metal pollution have been found to host small planetary bodies (planetesimals) orbiting near the tidal disruption radius. We study the physical properties and dynamical origin of these bodies under the hypothesis that they underwent high-eccentricity migration from initial distances of several astronomical units. We examine two plausible mechanisms for orbital migration and circularization: tidal friction and ram-pressure drag in a compact disc. For each mechanism, we derive general analytical expressions for the evolution of the orbit that can be rescaled for various situations. We identify the physical parameters that determine whether a planetesimal’s orbit can circularize within the appropriate time-scale and constrain these parameters based on the properties of the observed systems. For tidal migration to work, an internal viscosity similar to that of molten rock is required, and this may be naturally produced by tidal heating. For disc migration to operate, a minimal column density of the disc is implied; the inferred total disc mass is consistent with estimates of the total mass of metals accreted by polluted WDs.

Speckle noise attenuation in orbital laser vibrometer seismology

The interior structures of small planetary bodies such as asteroids and comets are an enigma. Determining the internal structure is important for understanding the formation and evolution of the solar system and also has implications for in-situ resource exploitation and planetary defense. Despite the immense value, no detailed investigation of the interior properties of an asteroid has yet been made. Although orbital radar and lander based seismological approaches have been proposed to make direct interior observations, it has not yet been demonstrated that radar could transmit through a rocky asteroid, nor that landing multiple seismometers is feasible or affordable. An elegant alternative to landed seismometers is orbital laser Doppler vibrometry, which could record seismic shaking of a small body without contact with the surface. Laser Doppler vibrometers (LDVs) are mature instruments for terrestrial applications and could be adapted for a space environment. However, when incident on a rough surface like an asteroid regolith, an LDV is subject to laser speckle noise which may be misinterpreted as seismic shaking. We address the challenge of making LDV measurements on naturally rough surfaces by quantifying the laser speckle noise that an orbital LDV would encounter during a hypothetical orbital measurement. Specifically, we simulate an LDV measurement of a seismic signal generated by an impact source on the asteroid 101955 Bennu. We demonstrate that speckle noise can be attenuated by combining multiple signals recorded by an orbital seismometer equipped with multiple LDV sensors. By mitigating laser speckle, we demonstrate that an orbital LDV can record seismic signals on a natural asteroid surface, which would enable an orbital seismometer to achieve the dense global coverage necessary for high resolution interior imaging.

Formation of primitive achondrites by partial melting of alkali-undepleted planetesimals in the inner solar system

Acapulcoites-lodranites, ureilites, brachinites, brachinite-like achondrites and winonaites are the main groups of primitive achondrites. They are variably depleted in incompatible lithophile elements (Al, Na, K and rare earth elements) and siderophile/chalcophile elements relative to chondrites and are interpreted as the residual mantle of planetesimals from which silicate melts and sulfide/metal melts were extracted. We use a series of melting experiments conducted with various chondritic compositions (CV, CM, CI, H and LL) to constrain the oxygen fugacity (fO2), the temperature, extent of melting and the initial bulk composition of the parent bodies of primitive achondrites. They melted at different and variable fO2: ΔIW −0.5/−1.0 for brachinites, ΔIW −1.3/−2.5 for ureilites, ΔIW −1.6/−2.7 for acapulcoites/lodranites and ΔIW −2.5/−3.0 for winonaites (with ΔIW = log fO2 – (log fO2)IW; IW being the iron-wustite buffer). Those main groups of primitive achondrites, which have nucleosynthetic anomalies characteristic of the “non-carbonaceous” reservoir and the inner solar system, were not initially depleted in Na2O and K2O relative to the sun’s photosphere. This suggests, in accordance with the enrichment in the heavy isotopes of Zn, Rb and K in eucrites, that the depletion of moderately volatile elements in planetesimals that melted to a larger extent (e.g. Vesta, the angrite parent body) resulted from evaporative losses during partial melting. The depletion of moderately volatile elements in terrestrial planets is likely inherited from partial melting and differentiation of small planetary bodies rather than from the incomplete condensation of the solar nebula.

Adaptive generalized ZEM-ZEV feedback guidance for planetary landing via a deep reinforcement learning approach

Precision landing on large and small planetary bodies is a technology of utmost importance for future human and robotic exploration of the solar system. In this context, the Zero-Effort-Miss/Zero-Effort-Velocity (ZEM/ZEV) feedback guidance algorithm has been studied extensively and is still a field of active research. The algorithm, although powerful in terms of accuracy and ease of implementation, has some limitations. Therefore with this paper we present an adaptive guidance algorithm based on classical ZEM/ZEV in which machine learning is used to overcome its limitations and create a closed loop guidance algorithm that is sufficiently lightweight to be implemented on board spacecraft and flexible enough to be able to adapt to the given constraint scenario. The adopted methodology is an actor-critic reinforcement learning algorithm that learns the parameters of the above-mentioned guidance architecture according to the given problem constraints.

Seismology on small planetary bodies by orbital laser Doppler vibrometry

The interior structure of small planetary bodies holds clues about their origin and evolution, from which we can derive an understanding of the solar system’s formation. High resolution geophysical imaging of small bodies can use either radar waves for dielectric properties, or seismic waves for elastic properties. Radar investigation is efficiently done from orbiters, but conventional seismic investigation requires landed instruments (seismometers, geophones) mechanically coupled to the body.We propose an alternative form of seismic investigation for small bodies using Laser Doppler Vibrometers (LDV). LDVs can sense motion at a distance, without contact with the ground, using coherent laser beams reflected off the body. LDVs can be mounted on orbiters, transforming seismology into a remote sensing investigation, comparable to making visual, thermal or electromagnetic observations from space. Orbital seismometers are advantageous over landed seismometers because they do not require expensive and complex landing operations, do not require mechanical coupling with the ground, are mobile and can provide global coverage, operate from stable and robust orbital platforms that can be made absolutely quiet from vibrations, and do not have sensitive mechanical components.Dense global coverage enables wavefield imaging of small body interiors using high resolution terrestrial exploration seismology techniques. Migration identifies and positions the interior reflectors by time reversal. Tomography constrains the elastic properties in-between the interfaces. These techniques benefit from dense data acquired by LDV systems at the surface, and from knowledge of small body shapes. In both cases, a complex body shape, such as a comet or asteroid, contributes to increased wave-path diversity in its interior, and leads to high (sub-wavelength) imaging resolution.

Testing accretion mechanisms of the H chondrite parent body utilizing nucleosynthetic anomalies

AbstractPlanetary bodies a few hundred kilometers in radii are the precursors to larger planets but it is unclear whether these bodies themselves formed very rapidly or accreted slowly over several millions of years. Ordinary H chondrite meteorites provide an opportunity to investigate the accretion time scale of a small planetary body given that variable degrees of thermal metamorphism present in H chondrites provide a proxy for their stratigraphic depth and, therefore, relative accretion times. We exploit this feature to search for nucleosynthetic isotope variability of 54Cr, which is a sensitive tracer of spatial and temporal variations in the protoplanetary disk's solids, between 17 H chondrites covering all petrologic types to obtain clues about the parent body accretionary rate. We find no systematic variability in the mass‐biased corrected abundances of 53Cr or 54Cr outside of the analytical uncertainties, suggesting very rapid accretion of the H chondrite parent body consistent with turbulent accretion. By utilizing the μ54Cr–planetary mass relationship observed between inner solar system planetary bodies, we calculate that the H chondrite accretion occurred at 1.1 ± 0.4 or 1.8 ± 0.2 Myr after the formation of calcium‐aluminum‐rich inclusions (CAIs), assuming either the initial 26Al/27Al abundance of inner solar system solids determined from angrite meteorites or CAIs from CV chondrites, respectively. Notably, these ages are in agreement with age estimates based on the parent bodies’ thermal evolution when correcting these calculations to the same initial 26Al/27Al abundance, reinforcing the idea of a secular evolution in the isotopic composition of inner disk solids.