Earth and Planetary Astrophysics (astro-ph.EP)

Abstract: Near-Earth asteroids (NEAs) that may evolve into impactors deserve detailed threat assessment studies. Early physical characterization of a would-be impactor may help in optimizing impact mitigation plans. We first detected NEA 2023~DZ$_{2}$ on 27--February--2023. After that, it was found to have a Minimum Orbit Intersection Distance (MOID) with Earth of 0.00005~au as well as an unusually high initial probability of becoming a near-term (in 2026) impactor. We aim to perform a rapid but consistent dynamical and physical characterization of 2023~DZ$_{2}$ as an example of a key response to mitigate the consequences of a potential impact. We use a multi-pronged approach, drawing from various methods (observational/computational) and techniques (spectroscopy/photometry from multiple instruments), and bringing the data together to perform a rapid and robust threat assessment.} The visible reflectance spectrum of 2023~DZ$_{2}$ is consistent with that of an X-type asteroid. Light curves of this object obtained on two different nights give a rotation period $P$=6.2743$\pm$0.0005 min with an amplitude $A$=0.57$\pm$0.14~mag. We confirm that although its MOID is among the smallest known, 2023~DZ$_{2}$ will not impact Earth in the foreseeable future as a result of secular near-resonant behaviour. Our investigation shows that coordinated observation and interpretation of disparate data provides a robust approach from discovery to threat assessment when a virtual impactor is identified. We prove that critical information can be obtained within a few days after the announcement of the potential impactor.

Abstract: The mass and distribution of metals in the interiors of exoplanets are essential for constraining their formation and evolution processes. Nevertheless, with only masses and radii measured, the determination of exoplanet interior structures is degenerate, and so far simplified assumptions have mostly been used to derive planetary metallicities. In this work, we present a method based on a state-of-the-art interior code, recently used for Jupiter, and a Bayesian framework, to explore the possibility of retrieving the interior structure of exoplanets. We use masses, radii, equilibrium temperatures, and measured atmospheric metallicities to retrieve planetary bulk metallicities and core masses. Following results on the giant planets in the solar system and recent development in planet formation, we implement two interior structure models: one with a homogeneous envelope and one with an inhomogeneous one. Our method is first evaluated using a test planet and then applied to a sample of 37 giant exoplanets with observed atmospheric metallicities from the pre-JWST era. Although neither internal structure model is preferred with the current data, it is possible to obtain information on the interior properties of the planets, such as the core mass, through atmospheric measurements in both cases. We present updated metal mass fractions, in agreement with recent results on giant planets in the solar system.

Abstract: We explore experimentally possible explanations of the polarization curves of the sunlight reflected by the Barbarian asteroids. Their peculiar polarization curves are characterized by a large inversion angle, around 30 degrees, which could be related to the presence of FeO-bearing spinel embedded in Calcium-Aluminum Inclusions. In order to test this hypothesis, we have measured the phase function and degree of linear polarization of six samples of Mg-rich olivine and spinel. For each material, we have analyzed the light scattering properties of a millimeter-sized grain and of two powdered samples with size distributions in the micrometer size range. The three spinel samples show a well-defined negative polarization branch with an inversion phase angle located around 24-30 degrees. In contrast, in the case of the olivine samples, the inversion angle is highly dependent on particle size and tends to decrease for larger sizes. We identify the macroscopic geometries as a possible explanation for the evident differences in the polarization curves between olivine and spinel millimeter samples. Although the polarization behaviour in near backscattering of the Barbara asteroid is similar to that of our spinel mm-sized sample in random orientation, this similarity could result in part from crystal retro-reflection rather than composition. This is part of an ongoing experimental project devoted to test separately several components of CV3-like meteorites, representative of the Barbarians composition, to disentangle their contributions to the polarization behavior of these objects.

Abstract: Many in situ potassium-argon (K-Ar) dating instruments under development use laser ablation to perform local analyses of several hundred um on rocks. Laser-induced breakdown spectroscopy (LIBS) and noble gas mass spectrometry (MS) are combined to achieve multiple spot analyses of the same rock to obtain K-Ar isochrons. The range and error of the data points on an isochron determine the accuracy and precision of dating. The range of the data on the isochron is governed by the relationship between the laser spot size and mineral size in the target rocks. A smaller laser spot size increases the range of the measured K concentration but decreases the amount of Ar extracted, which deteriorates the measurement accuracy. Because of this trade-off, the optimal laser spot size, which would give the best dating accuracy, would be somewhere in the middle. The mineral composition and spatial distribution in Martian rocks determine this optimum spot size. However, no extensive studies have been conducted to consider optimum laser spot size taking the mineral compositions of Martian rocks into account. Thus, it has been unknown how accurate the LIBS-MS method can be for in situ dating on Mars. In this study, we quantify the precision of dating Martian rocks by the LIBS-MS method and determine the instrumental conditions necessary for achieving the required precision. The dating precision was quantitatively evaluated by simulating isochrons that reflect the mineral composition of Martian rocks, which were obtained with electron probe microanalysis of three Martian meteorites. Our results indicate that a dating precision of 200 Myr could be achieved by reducing the laser spot size to 250 um and improving the measurement accuracy of K and Ar concentrations to 10%. We determined the instrumental conditions necessary to achieve the required dating precision for the LIBS-MS instrument currently developing.

Abstract: Planetesimal formation is still mysterious. One of the ways to form planetesimals is to invoke a gas pressure bump in a protoplanetary disc. In our previous paper, we propose a new scenario in which the piled-up dust at a gas pressure bump created by a migrating planet form planetesimals by streaming instability in a wide region of the disc as the planet migrates inward. In this work, we consider the global time evolution of dust and investigate the detailed conditions and results of the planetesimal formation in our scenario. We use a 1D grid single-sized dust evolution model, which can follow the growth of the particles by their mutual collision and their radial drift and diffusion. We calculate the time-evolution of the radial distribution of the peak mass and surface density of the dust in a gas disc perturbed by an embedded migrating planet and investigate if the dust satisfies the condition for planetesimal formation. We find that planetesimals form in a belt-like region between the snowline and the position where the planet reaches its pebble-isolation mass when the strength of turbulence is $10^{-4}\leq\alpha\leq10^{-3}$, which is broadly consistent with observed value. The mechanism of the formation, streaming instability or mutual collision, depends on the timescale of the streaming instability. The total mass of planetesimals also depends on $\alpha$ and is about $30-100~M_{\rm E}$ if the planetary core has already existed at the beginning and grows by gas accretion, but it decreases as the timing of the formation of the planetary core is later. We also provide simple approximate expressions of the surface density and total mass of the planetesimals and find that the total mass strongly depends on the dust mass. We show that planetesimals form in a belt-like region by the combination of the dust pile-up at the gas pressure bump formed by a planet and its inward migration.

Abstract: Earth is deficient in carbon and nitrogen by up to ${\sim}4$ orders of magnitude compared with the Sun. Destruction of (carbon- and nitrogen-rich) refractory organics in the high-temperature planet forming regions could explain this deficiency. Assuming a refractory cometary composition for these grains, their destruction enhances nitrogen-containing oxygen-poor molecules in the hot gas ($\gtrsim 300$K) after the initial formation and sublimation of these molecules from oxygen-rich ices in the warm gas (${\sim}150$K). Using observations of $37$ high-mass protostars with ALMA, we find that oxygen-containing molecules (CH$_3$OH and HNCO) systematically show no enhancement in their hot component. In contrast, nitrogen-containing, oxygen-poor molecules (CH$_3$CN and C$_2$H$_3$CN) systematically show an enhancement of a factor ${\sim} 5$ in their hot component, pointing to additional production of these molecules in the hot gas. Assuming only thermal excitation conditions, we interpret these results as a signature of destruction of refractory organics, consistent with the cometary composition. This destruction implies a higher C/O and N/O in the hot gas than the warm gas, while, the exact values of these ratios depend on the fraction of grains that are effectively destroyed. This fraction can be found by future chemical models that constrain C/O and N/O from the abundances of minor carbon, nitrogen and oxygen carriers presented here.

Abstract: The recently discovered transiting super-Earth LP 890-9 c is potentially one of the best rocky exoplanets for atmospheric characterization. Orbiting an ultracool M-dwarf at the inner edge of the habitable zone, LP 890-9 c offers a new opportunity to study the climate of rocky planets at the inner edge of the habitable zone. We investigate the molecular detectability with simulated JWST transmission spectra for five potential atmospheres of LP 890-9 c. We find that a small three-transit JWST program can infer evidence of H2O (at 3$\sigma$ confidence) for a full runaway greenhouse scenario. Alternatively, CO2-dominated atmospheres resembling Venus without high-altitude terminator clouds can be identified with eight transits. However, these predictions could be complicated by the impact of clouds and/or unocculted starspots. Nevertheless, JWST observations of LP 890-9 c could provide critical insights and potentially distinguish between models of rocky planets at the inner edge of the habitable zone.

Abstract: Mercury, the innermost planet, formed under highly reduced conditions, based mainly on surface Fe, S, and Si abundances determined from MESSENGER mission data. The minor element Cr may serve as an independent oxybarometer, but only very limited Cr data have been previously reported for Mercury. We report Cr/Si abundances across Mercury's surface based on MESSENGER X-Ray Spectrometer data throughout the spacecraft's orbital mission. The heterogeneous Cr/Si ratio ranges from 0.0015 in the Caloris Basin to 0.0054 within the high-magnesium region, with an average southern hemisphere value of 0.0008 (corresponding to about 200 ppm Cr). Absolute Cr/Si values have systematic uncertainty of at least 30%, but relative variations are more robust. By combining experimental Cr partitioning data along with planetary differentiation modeling, we find that if Mercury formed with bulk chondritic Cr/Al, Cr must be present in the planet's core and differentiation must have occurred at log fO2 in the range of IW-6.5 to IW-2.5 in the absence of sulfides in its interior, and a range of IW-5.5 to IW-2 with an FeS layer at the core-mantle boundary. Models with large fractions of Mg-Ca-rich sulfides in Mercury's interior are more compatible with moderately reducing conditions (IW-5.5 to IW-4) owing to the instability of Mg-Ca-rich sulfides at elevated fO2. These results indicate that if Mercury differentiated at a log fO2 lower than IW-5.5, the presence of sulfides whether in the form of a FeS layer at the top of the core or Mg-Ca-rich sulfides within the mantle would be unlikely.

Abstract: Mercury's orbit can destabilize, resulting in a collision with either Venus or the Sun. Chaotic evolution can cause $g_1$ to decrease to the approximately constant value of $g_5$ and create a resonance. Previous work has approximated the variation in $g_1$ as stochastic diffusion, which leads to a model that can reproduce the Mercury instability statistics of secular and $N$-body models on timescales longer than 10~Gyr. Here we show that the diffusive model underpredicts the Mercury instability probability by a factor of 3-10,000 on timescales less than 5~Gyr, the remaining lifespan of the Solar System. This is because $g_1$ exhibits larger variations on short timescales than the diffusive model would suggest. To better model the variations on short timescales, we build a new subdiffusive model for $g_1$ including a quadratic spring potential above a certain value of $g_1$, which we refer to as a soft upper boundary. Subdiffusion is similar to diffusion, but exhibits larger displacements on short timescales and smaller displacements on long timescales. We choose model parameters based on the short-time behavior of the $g_1$ trajectories in the $N$-body simulations, leading to a tuned model that can reproduce Mercury instability statistics from 1-40~Gyr. This work motivates several questions in planetary dynamics: Why does subdiffusion better approximate the variation in $g_1$ than standard diffusion? Why is a soft upper boundary condition on $g_1$ an appropriate approximation? Why is there an upper bound on $g_1$, but not a lower bound that would prevent it from reaching $g_5$?