Terrestrial Planets Research Articles

Tidally driven remelting around 4.35 billion years ago indicates the Moon is old

The last giant impact on Earth is thought to have formed the Moon1. The timing of this event can be determined by dating the different rocks assumed to have crystallized from the lunar magma ocean (LMO). This has led to a wide range of estimates for the age of the Moon between 4.35 and 4.51 billion years ago (Ga), depending on whether ages for lunar whole-rock samples2, 3–4 or individual zircon grains5, 6–7 are used. Here we argue that the frequent occurrence of approximately 4.35-Ga ages among lunar rocks and a spike in zircon ages at about the same time8 is indicative of a remelting event driven by the Moon’s orbital evolution rather than the original crystallization of the LMO. We show that during passage through the Laplace plane transition9, the Moon experienced sufficient tidal heating and melting to reset the formation ages of most lunar samples, while retaining an earlier frozen-in shape10 and rare, earlier-formed zircons. This paradigm reconciles existing discrepancies in estimates for the crystallization time of the LMO, and permits formation of the Moon within a few tens of million years of Solar System formation, consistent with dynamical models of terrestrial planet formation11. Remelting of the Moon also explains the lower number of lunar impact basins than expected12,13, and allows metal from planetesimals accreted to the Moon after its formation to be removed to the lunar core, explaining the apparent deficit of such materials in the Moon compared with Earth14.

CALCULATION OF THE SEISMIC RESISTANCE OF THE START-UP AIR SYSTEM OF RDES

The earth beneath our feet is alive, even though it is a rocky planet. Despite the fact that geological activity cannot be felt, sometimes it manifests itself sharply and dangerously. Earthquakes and seismic phenomena are responsible for the transmission of these tectonic shocks and have forced people to be prepared for different situations with the oscillation of the earth's crust. Seismic shocks cause many dangerous phenomena: landslides and avalanches in the mountains, flood waves (tsunamis) on the shores of seas and oceans, cracks and liquefaction on the land surface. Depending on the magnitude of the earthquakes, each year there are from one to two dozen earthquakes with a power of more than 7 points according to the Richter scale. How can the destruction of building equipment, disasters and deaths be prevented and what is "seismic resistance" is the question of our research. In order to confirm that the equipment is guaranteed to perform its functions under seismic conditions, equipment qualification is performed. The assessment of the initial state of seismic qualification consists in comparing the seismic qualification characteristics with the seismic qualification requirements, as well as establishing the availability of information in the factory and project documentation regarding the fastening of the equipment, and allows to establish whether the qualification requirements presented in the process of seismic qualification were taken into account during development and staging for production, manufacturing and installation. Seismic qualification characteristics are contained in the design documentation of equipment developers and manufacturers and can be specified in different forms: values of acceleration amplitudes; the intensity of the earthquake and the level of installation of equipment above the zero mark; test parameters, mechanical performance groups, etc. If the qualification characteristics of the equipment meet the defined or calculated qualification requirements, then the qualification of such equipment for earthquake resistance is established.

Two distinct populations of dark comets delineated by orbits and sizes

Small bodies are capable of delivering essential prerequisites for the development of life, such as volatiles and organics, to the terrestrial planets. For example, empirical evidence suggests that water was delivered to the Earth by hydrated planetesimals from distant regions of the Solar System. Recently, several morphologically inactive near-Earth objects were reported to experience significant nongravitational accelerations inconsistent with radiation-based effects, and possibly explained by volatile-driven outgassing. However, these "dark comets" display no evidence of comae in archival images, which are the defining feature of cometary activity. Here, we report detections of nongravitational accelerations on seven additional objects classified as inactive (doubling the population) that could also be explainable by asymmetric mass loss. A detailed search of archival survey and targeted data rendered no detection of dust activity in any of these objects in individual or stacked images. We calculate dust production limits of [Formula: see text]10, [Formula: see text], and [Formula: see text] kg s[Formula: see text] for 1998 FR[Formula: see text], 2001 ME[Formula: see text], and 2003 RM with these data, indicating little or no dust surrounding the objects during the observations. This set of dark comets reveals the delineation between two distinct populations: larger, "outer" dark comets on eccentric orbits that are end members of a continuum in activity level of comets, and smaller, "inner" dark comets on near-circular orbits that could signify a different different population. These objects may trace various stages in the life cycle of a previously undetected, but potentially numerous, volatile-rich population that may have provided essential material to the Earth.

Alternative Solvents for Life: Framework for Evaluation, Current Status, and Future Research.

Life is a complex, dynamic chemical system that requires a dense fluid solvent in which to take place. A common assumption is that the most likely solvent for life is liquid water, and some researchers argue that water is the only plausible solvent. However, a persistent theme in astrobiological research postulates that other liquids might be cosmically common and could be solvents for the chemistry of life. In this article, we present a new framework for the analysis of candidate solvents for life, and we deploy this framework to review substances that have been suggested as solvent candidates. We categorize each solvent candidate through the following four criteria: occurrence, solvation, solute stability, and solvent chemical functionality. Our semiquantitative approach addresses all the requirements for a solvent not only from the point of view of its chemical properties but also from the standpoint of its biochemical function. Only the protonating solvents fulfill all the chemical requirements to be a solvent for life, and of those only water and concentrated sulfuric acid are also likely to be abundant in a rocky planetary context. Among the nonprotonating solvents, liquid CO2 stands out as a planetary solvent, and its potential as a solvent for life should be explored. We conclude with a discussion of whether it is possible for a biochemistry to change solvents as an adaptation to radical changes in a planet's environment. Our analysis provides the basis for prioritizing future experimental work to explore potential complex chemistry on other planets. Key Words: Habitability-Alternative solvents for life-Alternative biochemistry. Astrobiology xx, xxx-xxx.

An impact-free mechanism to deliver water to terrestrial planets and exoplanets

Context. The origin of water, particularly on Earth, is still a matter of heated debate. To date, the most widespread scenario is that the Earth originated without water and that it was brought to the planet mainly as a result of impacts by wet asteroids coming from further out in space. However, many uncertainties remain as to the exact processes that supplied an adequate amount of water to inner terrestrial planets. Aims. In this article, we explore a new mechanism that would allow water to be efficiently transported to planets without impacts. We propose that primordial asteroids were icy and that when the ice sublimated, it formed a gaseous disk that could then reach planets and deliver water. Methods. We have developed a new model that follows the sublimation of asteroids on gigayear (Gyr) timescales, taking into account the variable luminosity of the Sun. We then evolved the subsequent gas disk using a viscous diffusion code, which leads to the gas spreading both inwards and outwards in the Solar System. We can then quantify the amount of water that can be accreted onto each planet in a self-consistent manner using our code. Results. We find that this new disk-delivery mechanism is effective and equipped to explain the water content on Earth (with the correct D/H ratio) as well as on other planets and the Moon. Our model shows most of the water being delivered between 20 and 30 Myr after the birth of the Sun, when the Sun’s luminosity increased sharply. Our scenario implies the presence of a gaseous water disk with substantial mass for hundreds of millions of years, which could be one of the key tracers of this mechanism. We show that such a watery disk could be detected in young exo-asteroid belts with ALMA. Conclusions. We propose that viscous water transport is inevitable and more generic than the impact scenario. We also suggest it is a universal process that may also occur in extrasolar systems. The conditions required for this scenario to unfold are indeed expected to be present in most planetary systems: an opaque proto-planetary disk that is initially cold enough for ice to form in the exo-asteroid belt region, followed by a natural outward-moving snow line that allows this initial ice to sublimate after the dissipation of the primordial disk, creating a viscous secondary gas disk and leading to the accretion of water onto the exo-planets.

Phase Equilibria of Sub-Neptunes and Super-Earths

We investigate the consequences of nonideal chemical interaction between silicate and overlying hydrogen-rich envelopes for rocky planets using basic tenets of phase equilibria. Based on our current understanding of the temperature and pressure conditions for complete miscibility of silicate and hydrogen, we find that the silicate-hydrogen binary solvus will dictate the nature of atmospheres and internal layering in rocky planets that garnered H2-rich primary atmospheres. The temperatures at the surfaces of supercritical magma oceans will correspond to the silicate-hydrogen solvus. As a result, the radial positions of supercritical magma ocean–atmosphere interfaces, rather than their temperatures and pressures, should reflect the thermal states of these planets. The conditions prescribed by the solvus influence the structure of the atmosphere, and thus the transit radii of sub-Neptunes. Separation of iron-rich metal to form metal cores in sub-Neptunes and super-Earths is not assured due to prospects for neutral buoyancy of metal in silicate melt induced by dissolution of H, Si, and O in the metal at high temperatures.

Collisional erosion of mantle silicate during accretion can set the elevated Fe/Mg ratio of Earth

Collisional erosion during terrestrial planets accretion can significantly alter the chemical compositions of planetary bodies relative to their primordial compositions. Notably, the superchondritic Fe/Mg ratio of the bulk Earth could be caused by the preferential stripping of the silicate mantle with respect to the proto-Earth’s core. In this work, we quantify the preferential loss of silicate mantle using SPH numerical simulations of giant collisions between planetary embryos. We span a wide range of impact parameters (impact velocity, impact angle and respective masses of impactor and target). The results are used to set up an erosion model as a function of all impact parameters. This erosion model allows the estimation of (1) the mass of accreted core, (2) the mass of accreted mantle and (3) the mass of escaped mantle for a given collision event. The model is subsequently coupled to N-body numerical simulations to estimate the erosion loss integrated over entire accretion histories of Earth-like embryos. We use collisions files from N-body numerical simulations of accretion within the frame of the classical scenario (Raymond et al., 2009) and of the Grand Tack scenario (Jacobson and Morbidelli, 2014) to track the core mass fractions of growing embryos during the last stages of accretion during 200 Myr. We show that a substantial fraction (10% in average for both dynamically distinct scenarios) of mantle material can be preferentially lost during accretion, increasing accordingly the core to mantle mass ratio. Given the standard deviation, such an amount of preferential mantle loss may be responsible for the superchondritic Fe/Mg bulk Earth ratio, even considering different chondritic contributions.

The Impact of Water Clouds on the Prospective Emission Spectrum of Teegarden’s Star b as Observed by LIFE

Nontransiting terrestrial planets will be accessible by upcoming observatories of which LIFE is an example. Planet b orbiting Teegarden’s Star is one of the optimal targets for such missions. We use a one-dimensional atmospheric model with real gas radiation, a multispecies pseudo-adiabatic convection-condensation scheme, and a water cloud scheme to estimate the impact of the cloud coverage on the emission spectrum of the target, as well as to assess how sensitive LIFE could be to changes in outgoing flux caused by these clouds. Though the emergent flux decreases with a higher cloud coverage, it does not decrease by more than 1 order of magnitude as the coverage increases from 0% to 90%. This allows LIFE to retain a high sensitivity to the cloud cover fraction for wavelengths longer than 7 μm. In this spectral range, with at least 1 bar of N2, LIFE is able to distinguish cloud cover fractions as small as 10% given an integration time of 24 hr, and yields much better precision with a week-long integration. An integration time of 1 week also allows the resolution of local variations in spectral flux, which can lead to easier molecule identification. This ability remains if the planet is a CO2-dominated Venus analog. Partial pressures of N2 lower than 1 bar may create a degeneracy with the cloud cover fraction. LIFE shows promising potential for detecting and characterizing atmospheres even with a high cloud coverage, and retaining a fine sensitivity to relatively small differences in cloud cover fractions.

Observations of the Crustal Mini-magnetopause Reconnection at Mars

Magnetopause reconnection is a fundamental mechanism for mass and energy transfer in terrestrial planetary systems. Unlike Earth, Mars lacks a global magnetic field and instead has localized crustal magnetic fields, which can create structures morphologically similar to a scaled-down intrinsic magnetosphere, potentially forming mini-magnetospheres. In this study, we investigate reconnection in Martian mini-magnetospheres on the dayside, analyzing key characteristics of the reconnecting current sheet, and the associated ion escape rate, and comparing these features to reconnection events at the induced magnetopause documented in previous studies. Our observations reveal that, in the subsolar region, the thickness of the Martian mini-magnetopause is comparable to the upstream proton inertial length and the convective gyroradius of the magnetosheath solar wind. However, at higher solar zenith angles, this thickness increases to several times these characteristic lengths. Reconnection-driven ion escape rates at the induced magnetopause are at least 1 order of magnitude greater than those associated with mini-magnetopause reconnection. These findings provide new insights into reconnection processes at Martian boundary layers.

Did the terrestrial planets of the solar system form by pebble accretion?

The dominant accretion process leading to the formation of the terrestrial planets of the Solar System is a subject of intense scientific debate. Two radically different scenarios have been proposed. The classic scenario starts from a disk of planetesimals which, by mutual collisions, produce a set of Moon to Mars-mass planetary embryos. After the removal of gas from the disk, the embryos experience mutual giant impacts which, together with the accretion of additional planetesimals, lead to the formation of the terrestrial planets on a timescale of tens of millions of years. In the alternative, pebble accretion scenario, the terrestrial planets grow by accreting sunward-drifting mm-cm sized particles from the outer disk. The planets all form within the lifetime of the disk, with the sole exception of Earth, which undergoes a single post-disk giant impact with Theia (a fifth protoplanet formed by pebble accretion itself) to form the Moon. To distinguish between these two scenarios, we revisit all available constraints: compositional (in terms of nucleosynthetic isotope anomalies and chemical composition), dynamical and chronological. We find that the pebble accretion scenario is unable to match these constraints in a self-consistent manner, unlike the classic scenario.

Zircon trace element evidence for early hydrothermal activity on Mars.

Finding direct evidence for hydrous fluids on early Mars is of interest for understanding the origin of water on rocky planets, surface processes, and conditions essential for habitability, but it is challenging to obtain from martian meteorites. Micro- to nanoscale microscopy of a unique impact-shocked zircon from the regolith breccia meteorite NWA7034 reveals textural and chemical indicators of hydrothermal conditions on Mars during crystallization 4.45 billion years ago. Element distribution maps show sharp alternating zoning defined by marked enrichments of non-formula elements, such as Fe, Al, and Na, and ubiquitous nanoscale magnetite inclusions. The zoning and inclusions are similar to those reported in terrestrial zircon crystallizing in the presence of aqueous fluid and are here interpreted as primary features recording zircon growth from exsolved hydrous fluids at ~4.45 billion years. The unique record of crustal processes preserved in this grain survived early impact bombardment and provides previously unidentified petrological evidence for a wet pre-Noachian martian crust.

Puffy Venuses: The Mass–Radius Impact of Carbon-rich Atmospheres on Lava Worlds

The recent advancements in exoplanet observations enable the potential detection of exo-Venuses, rocky planets with carbon-rich atmospheres. How extended these atmospheres can be, given high carbon abundances, has not been studied. To answer this, we present a model for a theoretical class of exoplanets—puffy Venuses—characterized by thick, carbon-dominated atmospheres in equilibrium with global magma oceans (MOs). Our model accounts for carbon and hydrogen partition between the atmosphere and the MO, as well as the C–H–O equilibrium chemistry throughout a semi-gray, radiative-convective atmosphere. We find that radius inflation by puffy Venus atmospheres is significant on small and irradiated planets: carbon content of 1200 ppm (or that of ordinary chondrites) can generate an atmosphere of ∼0.16–0.3 R ⊕ for an Earth-mass planet with equilibrium temperatures of 1500–2000 K. We identify TOI-561 b as an especially promising puffy Venus candidate, whose underdensity could be attributed to a thick C-rich atmosphere. We also advocate for a puffy Venus interpretation of 55 Cancri e, where a recent JWST observation indicates the presence of a CO/CO2 atmosphere. Puffy Venuses may thus constitute a testable alternative interpretation for the interior structure of underdense low-mass exoplanets.

Hic Sunt Dracones: Uncovering Dynamical Perturbers within the Habitable Zone

The continuing exploration of neighboring planetary systems is providing deeper insights into the relative prevalence of various system architectures, particularly with respect to the solar system. However, a full assessment of the dynamical feasibility of possible terrestrial planets within the habitable zones (HZs) of nearby stars requires detailed knowledge of the masses and orbital solutions of any known planets within these systems. Moreover, the presence of as-yet undetected planets in or near the HZ will be crucial for providing a robust target list for future direct imaging surveys. In this work, we quantify the distribution of uncertainties on planetary masses and semimajor axes for 1062 confirmed planets, finding median uncertainties of 11.1% and 2.2%, respectively. We show the dependence of these uncertainties on stellar mass and orbital period and discuss the effects of these uncertainties on dynamical analyses and the locations of mean motion resonance. We also calculate the expected radial velocity (RV) semiamplitude for a Neptune-mass planet in the middle of the HZ for each of the proposed Habitable Worlds Observatory target stars. We find that for more than half of these stars, the RV semiamplitude is less than 1.5 m s−1 rendering them unlikely to be detected in archival RV data sets and highlighting the need for further observations to understand the dynamical viability of the HZ for these systems. We provide specific recommendations regarding stellar characterization and RV survey strategies that work toward the detection of presently unseen perturbers within the HZ.

Sound velocities and thermal equation of state of fcc-iron-nickel alloys at high pressure and high temperature: Implications for the cores of Moon and several planets

Fcc-Fe-Ni alloy is believed to be the most dominant solid constitute of moderate-sized terrestrial planetary cores. Investigating the physical properties, especially the density and sound velocity of Fe-Ni alloys and comparing them with seismic observations is an indispensable approach to constructing compositional models for planetary interiors. In this study, we conducted sound velocity measurements on Fe-Ni alloys with 10 wt.% and 20 wt.% Ni up to ∼13.5 GPa and 1073 K, using the ultrasonic interferometry technique in a multi-anvil apparatus in conjunction with synchrotron radiation. By fitting the experimental data to finite strain equations, the bulk and shear moduli and their pressure and temperature derivatives are derived, yielding KS0 =145.8(14) GPa, G0 = 73.2(7) GPa, KS0’ = 5.89(24), G0’ = 2.89(8), (∂KS/∂T)P = -0.0181(12) GPa/K and (∂G/∂T)P = -0.0393(10) GPa/K for fcc-Fe80Ni20. An examination of the density-velocity relationship shows that compressional wave velocity is insensitive to temperature within the current pressure and temperature range, while shear wave velocity exhibits a large reduction with increasing temperature. Extrapolation of the sound velocities following the finite strain theories suggests that much slower Vs should be expected at pressure and temperature conditions corresponding to those of the lunar core. Possible core density and velocity profiles for other moderate planets and satellites, such as Mars, Mercury, and Ganymede are also calculated.

Development of Gravity Theories in the View of TRAPPIST-1e

Abstract Contrary to the solar system, most exoplanet systems detected hitherto are close-in and compact. One typical system is TRAPPIST-1, which has seven nearly co-planar terrestrial planets all within the orbit of Mercury, including three in the habitable zone.
To evaluate the differences in development of sophisticated gravity theories from the solar system, we use N-body integrations to simulate ephemeris and reproduce some important astronomy phenomena observed on the potentially habitable planet TRAPPIST-1e.
Retrograde motions of other planets last 1–2 orders of magnitude shorter than in the solar system, but occur much more frequently. Transit events of all inner planets can be observed steadily. 
Except Kepler's first law is hard to notice for low eccentricities of planets, the other two laws can then be precisely verified in 102 days, because the areas swept by planets vary by <~ 0.01% and the observed semimajor axes and periods result in constants with theoretical and observation accuracies both <~ 2%. However, the mean motion correlation implies the Great Inequality does not keep apparent between one pair of planets like Jupiter and Saturn.
Furthermore, general relativity can hardly be discovered because it gives rise to perihelion precession of inner planets only ∼0.1% of gravity precession, dozens of times smaller than Mercury. 
Our results supports the possibility of developing part of gravity theories by potential exo-civilizations in compact systems like TRAPPIST-1.

A magnetohydrodynamic mechanism for the formation of solar polar vortices

Polar vortices are ubiquitous features of planetary atmospheric flows, from the Earth-like rocky planets to Jupiter- and Saturn-like gas giant planets. Very little is known about their existence or dynamics on the Sun. What should be expected near the Sun's pole for the upcoming solar multi-viewpoint and polar missions? Here, we report the magnetohydrodynamic (MHD) nonlinear simulations for the formation and evolution of solar polar vortices using a near-surface MHD shallow-water model. Our findings indicate that the rush to the poles, the migration of magnetic fields toward the pole following the Sun's magnetic cycle, can positively contribute to the formation of polar vortices. The mechanism proposed here for the formation of polar vortices involves the role of magnetic fields and may be relevant to any star with a magnetic cycle. The Sun's polar vortices resulting from this mechanism are predominantly MHD, consisting of a tight pair of cyclonic and anticyclonic swirls. This mechanism is likely to operate during all solar cycle phases except the peak, when the polar field reverses. Polar vortices can impact dynamical evolution of global flows and polar fields, which seed the next activity cycle, hence better knowledge of physics of polar regions may lead to improved solar cycle and space weather forecasts.

Three-Dimensional Rapid Orbit Transfer of Diffractive Sail with a Littrow Transmission Grating-Propelled Spacecraft

A diffractive solar sail is an elegant concept for a propellantless spacecraft propulsion system that uses a large, thin, lightweight surface covered with a metamaterial film to convert solar radiation pressure into a net propulsive acceleration. The latter can be used to perform a typical orbit transfer both in a heliocentric and in a planetocentric mission scenario. In this sense, the diffractive sail, proposed by Swartzlander a few years ago, can be considered a sort of evolution of the more conventional reflective solar sail, which generally uses a metallized film to reflect the incident photons, studied in the scientific literature starting from the pioneering works of Tsander and Tsiolkovsky in the first decades of the last century. In the context of a diffractive sail, the use of a metamaterial film with a Littrow transmission grating allows for the propulsive acceleration magnitude to be reduced to zero (and then, the spacecraft to be inserted in a coasting arc during the transfer) without resorting to a sail attitude that is almost edgewise to the Sun, as in the case of a classical reflective solar sail. The aim of this work is to study the optimal (i.e., the rapid) transfer performance of a spacecraft propelled by a diffractive sail with a Littrow transmission grating (DSLT) in a three-dimensional heliocentric mission scenario, in which the space vehicle transfers between two assigned Keplerian orbits. Accordingly, this paper extends and generalizes the results recently obtained by the author in the context of a simplified, two-dimensional, heliocentric mission scenario. In particular, this work illustrates an analytical model of the thrust vector that can be used to study the performance of a DSLT-based spacecraft in a three-dimensional optimization context. The simplified thrust model is employed to simulate the rapid transfer in a set of heliocentric mission scenarios as a typical interplanetary transfer toward a terrestrial planet and a rendezvous with a periodic comet.

Linking the primordial composition of planet building disks to the present-day composition of rocky exoplanets

The composition of rocky planets is strongly driven by the primordial materials in the protoplanetary disk, which can be inferred from the abundances of the host star. Understanding this compositional link is crucial for characterizing exoplanets. We aim to investigate the relationship between the compositions of low-mass planets and their host stars. We determined the primordial compositions of host stars using high-precision present-day stellar abundances and stellar evolutionary models. These primordial abundances were then input into a stoichiometric model to estimate the composition of planet-building blocks. Additionally, we employed a three-component planetary interior model (core, mantle, and water in different phases) to estimate planetary compositions based only on their radius and mass. We find that although stellar abundances vary over time, relevant abundance ratios such as Fe/Mg remain relatively constant during the main sequence evolution for low temperature stars. A strong correlation is found between the iron-to-silicate mass fraction of protoplanetary disks and planets, while no significant correlation was observed for water mass fractions. The Fe/Mg ratio varies significantly between planets and their stars, indicating substantial disk-driven compositional diversity, and this ratio also correlates with planetary radius. While stellar abundances, as a proxy of the composition of protoplanetary disk, provide a baseline for planetary composition, significant deviations arise due to complex disk processes, challenging the assumption of a direct, one-to-one elemental relationship between stars and their planets.

The Evolution of Angular Momentum from Terrestrial Planets during their Planetary Formation

Abstract. It has been a long run on searching for the angular momentum development and its origin for the terrestrial planets, in contrast to a general theory or equation that can write the whole picture of the angular momentum development. Generally, planetary angular momentum is built up by numerous rocky or dust contributors fused together and affected by later interactions of different sources. In other words, it requires tremendous calculation on the demotion of each debris on a complex system, meaning various theories to study the physics and fit into present observational data. The case of terrestrial planets is studied by computer simulation with models that contain different strategies of interacting bodies inside the space, and these simulations are conducted under various initial conditions, for instance, the number of involving bodies and the sizes of the rocky debris. This article briefly gives a picture of the theories simulating and predicting the development and origin of terrestrial planets by literature view of listed references, aiming to summarize and discuss the differences of several simulations and their conditions.

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 unweathered 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 have 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 the encounter and subsequent surface slope evolution over longer timescales driven by tumbling. We perform multiscale 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’s 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 that there will be a significant change in Apophis’s 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.