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Physics Astrophysics Earth and Planetary Astrophysics

astro-ph.EP Watch Earth and Planetary Astrophysics

Friday 28 August

Earth and Planetary Astrophysics

ExoMOD I. A Forward Model for the Orbital Architecture of Kepler Multi-Planet Systems

David Nesvorny, Daniel A. Yahalomi, David Kipping et al. (+1 more)

Transit observations only detect planets with favorable, near edge-on orientation of orbits as seen by a distant observer, which leaves much freedom for various interpretations in terms of the underlying planetary system architecture. Here we forward model transit observations of the Kepler telescope to characterize the orbital properties of close-in planetary systems. We make sensible choices about the underlying distributions of planet radii, masses and orbital periods, parameterize the orbital excitation with the Angular Momentum Deficit (AMD), and adopt an accurate method to account for transit detection. The fits to Kepler's DR25 data are executed with MultiNest. We find that the orbital period distributions of Kepler singles and multis are statistically different from each other – possibly a consequence of Kepler's observational baseline. The observed gap complexity distribution is reproduced when planets in high multiplicity systems ($m \geq 5$) are assigned ideally correlated period ratios. The high-multiplicity systems probably retained a memory of their formation conditions. The gap complexity metric also helps to constrain the AMD distribution. We find that systems with positive radius monotonicities typically harbor smaller planets, as expected if the radius monotonicity is influenced by non-detections. A FGK dwarf in the Kepler field should host $\simeq 2.4 \pm 0.2$ planets on average with radii $0.5 < R_{\rm pl}/R_\oplus < 7$ and orbital periods $3<P_{\rm orb}<300$ d. For a detected planetary system, there is roughly a 50% chance that Kepler transit observations missed at least one inner or intermediate-period planet.
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All authors David Nesvorny, Daniel A. Yahalomi, David Kipping, Cristian Beauge

Earth and Planetary Astrophysics

A Clearer View of HAT-P-1 b: JWST NIRSpec G395H Reveals Water, Carbon Dioxide, and Possibly Hydrogen Sulfide

Reza Ashtari, Stephen P. Schmidt, Guangwei Fu et al. (+16 more)

As part of JWST's Exoplanet Grand Tour Survey, we use panchromatic transmission spectroscopy to connect HAT-P-1 b's previously studied optical and near-infrared atmosphere to the longer-wavelength molecular bands accessible with JWST. We present JWST NIRSpec G395H transmission spectroscopy of the hot Jupiter HAT-P-1 b over 2.7–5.3~$\mu$m, and combine the new spectrum with archival HST STIS and WFC3 observations for a 0.3–5.3~$\mu$m atmospheric analysis. We independently reduce the JWST data with the Eureka!, FIREFLy, and Tswift pipelines, finding mutually consistent transmission spectra across the G395H bandpass. Atmospheric retrievals yield strong evidence for H$_2$O and CO$_2$ with Bayes factors of $\log_{10}B_{\mathrm{H_2O}}=8.9$ and $\log_{10}B_{\mathrm{CO_2}}=52.3$, while providing tentative evidence for H$_2$S ($\log_{10}B_{\mathrm{H_2S}}=1.4$). The joint H$_2$O and CO$_2$ constraints favor an atmosphere near chemical equilibrium, with $\log_{10} \text{M/H}=0.99^{+0.19}_{-0.14}$, corresponding to $\sim10\times$ Solar or $\sim9\times$ relative to the near-solar metallicity host star, and a 3$\sigma$ upper limit of C/O $<0.52$. Because H$_2$O and CO$_2$ provide a metallicity comparatively insensitive to vertical mixing in this temperature regime, their combined detection suggests the composition is dominated by bulk enrichment rather than strong disequilibrium transport. We find no significant evidence for clouds; instead, the persistence of molecular structure across the spectrum argues against strong cloud muting. The tentative H$_2$S signal, if confirmed, would further suggest limited photochemical processing at the pressures probed. Together, the molecular inventory, enriched metallicity, and low C/O ratio point to an oxygen-rich atmosphere and establish HAT-P-1 b as a benchmark for comparative studies of hot-Jupiter atmospheric composition.
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All authors Reza Ashtari, Stephen P. Schmidt, Guangwei Fu, Avinash Verma, David K. Sing, Kevin B. Stevenson, Jayesh Goyal, Katherine A. Bennett, Joshua D. Lothringer, Jacob Lustig-Yaeger, Sagnick Mukherjee, Carlos Gascón, Natalie H. Allen, Patrick McCreery, Le-Chris Wang, Mei Ting Mak, Kristin S. Sotzen, Lakeisha M. Ramos Rosado, N. J. Mayne

Earth and Planetary Astrophysics · Cross-listed from astro-ph.IM

pyEDITH: the coronagraphic exposure time calculator for the Habitable Worlds Observatory

Eleonora Alei, Miles H. Currie, Corey Spohn et al. (+7 more)

To support the development of next-generation missions for the search and characterization of habitable planets, high-fidelity tools for astrophysical and instrumental noise simulations are needed. In this paper, we introduce pyEDITH, the Python-based coronagraphic exposure time calculator built for the next recommended NASA flagship mission, the Habitable Worlds Observatory (HWO), tasked with searching for signs of habitability and life in dozens of nearby exoplanet systems. pyEDITH is designed to simulate wavelength-dependent exposure times and signal-to-noise ratios (S/N) for synthetic HWO direct imaging observations, considering realistic engineering specifications and user-defined target information. Its modular architecture ensures flexibility as mission requirements evolve. pyEDITH enables a streamlined integration with modern astronomical workflows and was designed to be used by the scientific community at all skill levels for understanding the capabilities and limitations of different HWO architectures for exoplanet analyses. The code has been validated against existing exposure time calculators and released open-source on GitHub and Zenodo, as well as made accessible through a Graphical User Interface. The pyEDITH package includes API documentation, tutorial notebooks, and has been used in forthcoming scientific publications.
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All authors Eleonora Alei, Miles H. Currie, Corey Spohn, Christopher C. Stark, Aki Roberge, Avi M. Mandell, Enrico Biancalani, Samantha Gilbert-Janizek, Jacob Lustig-Yaeger, Sarah Steiger

Earth and Planetary Astrophysics

Inversion Framework of Internal Mass Distribution Parameters of Asteroid Apophis from Dynamical Observations

Yiting Li, Chenyang Huang, Yang Yu et al. (+2 more)

The detection of the internal mass distribution of asteroids is of great significance for understanding their origin, evolution, and mission planning for exploration. Previous approaches rely on indirect density estimates or close-range spacecraft gravity inversion, which have limited applicability. This paper presents a proof-of-concept framework to infer the internal mass properties of asteroid (99942) Apophis during its close Earth flyby in 2029 using dynamical observations collected during the encounter. We establish a dynamical mapping from the evolution of orbital and rotational states to internal structural parameters, formulate it as an inverse problem, and solve it using Particle Swarm Optimization. The algorithm is first validated on a regular ellipsoidal model and then applied to three mass distribution models based on the actual shape of Apophis. Under ideal observation conditions, the relative error of the inverted moment of inertia ratios can be below 0.001%, and the absolute error of the center-of-mass position reaches the order of 10-5 meters. The algorithm successfully distinguishes among different internal structures. When realistic measurement noise is introduced, the inversion accuracy degrades. A sensitivity analysis reveals that the accuracy of the inertia tensor inversion is primarily limited by angular velocity measurement noise, whereas center-of-mass determination is highly sensitive to the precision of position and velocity data. This study provides a proof-of-concept for a technically feasible and cost-effective approach to infer asteroid internal structure during close encounters, and also highlights the critical data accuracy requirements for practical application, offering guidance for future observation campaigns.
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All authors Yiting Li, Chenyang Huang, Yang Yu, Shengping Gong, He Zhang

Earth and Planetary Astrophysics · Cross-listed from physics.space-ph

Solar Magnetic Configuration Control over Radiation Belt Electrons

Ahmad Lalti, Jonathan Rae, Clare Watt et al. (+2 more)

Earth is surrounded by two highly dynamic concentric belts of particle radiation. The outer radiation belt exhibits coherent variability at solar-cycle, seasonal, and 27-day (Carrington) timescales. While the solar-cycle and Carrington variations have been attributed to the recurrence of coronal hole solar wind, the seasonal variation has long been explained by local geometric effects that modulate the solar wind-magnetosphere coupling. Here, we challenge this paradigm by showing that the periodic recurrence of coronal hole solar wind likewise drives the seasonal variation. We further demonstrate that the Alfvénic nature of this solar wind is responsible for the observed electron flux enhancement in the outer radiation belt. These findings provide a unifying framework linking solar magnetic topology, solar wind properties, and magnetospheric dynamics across multiple timescales at Earth and beyond.
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All authors Ahmad Lalti, Jonathan Rae, Clare Watt, Stephanie Yardley, Savvas Raptis

Earth and Planetary Astrophysics · Cross-listed from astro-ph.IM

The PLACID active coronagraphic imager instrument: commissioning status

Ruben Tandon, Lucas Marquis, Liurong Lin et al. (+9 more)

The world's first adaptive stellar coronagraph, the Programmable Liquid-crystal Active Coronagraphic Imager for the 4-m DAG telescope (PLACID) uses a spatial light modulator operating from H- to Ks-band to dynamically adjust the focal-plane phase mask entirely in software. Positioned between the TROIA XAO system and the DIRAC infrared detector, PLACID was installed on the Nasmyth platform of the Turkish 4-m DAG telescope in 2025, followed by optical alignment and successful preliminary acceptance tests with the calibration light source in early 2026. On-sky commissioning is expected, as soon as the TROIA XAO system will be operational. When on-sky, it will enable high-contrast imaging of exoplanets, brown dwarfs, disks, and binary systems, being able to easily deploy any desired phase pattern in the focal plane. Upcoming features include self-calibration of non-common path aberrations, coronagraphic nulling of binary-stars, and coherent differential imaging. We present the preliminary acceptance procedures and internal alignment results, discovery-space estimates, new binary star features and observation tools, to be ready for first science by late 2026/early 2027.
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All authors Ruben Tandon, Lucas Marquis, Liurong Lin, Derya Öztürk Çetni, Axel Potier, Audrey Baur, Iljadin Manurung, Matthias Brändli, Martin Rieder, Daniele Piazza, Laurent Jolissaint, Jonas G. Kühn

Earth and Planetary Astrophysics

Cooling-regulated gas accretion onto gap-opening planets

Ayumu Kuwahara, Michiel Lambrechts, Minghao Zhang et al. (+2 more)

Gas accretion onto forming planets controls the final masses of giant planets and provides observable signatures of ongoing formation. How this process depends on the cooling properties of these newly attracted gas remains poorly constrained. We present long-term, three-dimensional global hydrodynamical simulations to quantify gas accretion onto gap-opening planets in the mass range between 1 and 3 Jupiter masses. We systematically vary the cooling time, $\beta$, from near-isothermal ($\beta=10^{-2}$ in units of orbital time) to near-adiabatic ($\beta=10^{2}$), and follow the evolution until a quasi-steady state is reached. Our simulations show that the gas accretion rate decreases monotonically with increasing $\beta$, as $\dot{M}_{\rm acc}\propto\beta^{-0.18}$, reaching values at $\beta=10^2$ that are approximately an order of magnitude lower than locally isothermal predictions, largely independent of planet mass. The reduction in accretion is traced to thermodynamic restructuring of the circumplanetary region: inefficient cooling weakens shocks, narrows the accretion bands feeding the circumplanetary disk. Our results imply that thermodynamic effects should be taken into account when interpreting observed accretion rates of young planets, and may introduce systematic uncertainties in commonly used locally isothermal assumptions.
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All authors Ayumu Kuwahara, Michiel Lambrechts, Minghao Zhang, Ruobing Dong, Hideko Nomura

Earth and Planetary Astrophysics

Modeling Carbon-Based Planets with MAGRATHEA

Robert Royer III, David R. Rice and Jason H. Steffen

A planet's interior structure not only influences its habitability, but it also provides information about the planet's formation and history. In the quest to characterize the variety of planetary discoveries, the most common practice is to rely on our understanding of the Earth's differentiated interior structure, with an iron core and silicate mantle as a starting point. However, recent work by arXiv:2408.07761 [astro-ph.EP] shows that these assumptions may not be true for all planetary systems as the composition of the condensed material can change drastically towards a carbon mantle instead of a silicate one, once the C/O ratio of the system approaches and exceeds approximately 0.9. In an effort to more fully understand the range of possible planetary bodies, we added the ability to model carbon-based mantles to our open-source planetary interior solver, MAGRATHEA. The newly added carbon mantle includes three relevant allotropes of carbon: graphite, diamond, and BC8 (body centered cubic with eight atoms per cell).
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Thursday 27 August

Earth and Planetary Astrophysics

Stellar Flares Rather than High $T_{int}$ As Possible Cause for the Missing Methane in Young Exoplanets: The Case Study of V1298~Tau\,b

Danica Adams, Pierre-Alexis Roy, Hilke Schlichting et al. (+1 more)

Recent observations of V1298~Tau\,b, a warm sub-Neptune (R$=9.85 \pm 0.3\,R_\oplus$, M$=13.1 \pm 5.3, M_\oplus$, T$_\mathrm{eq} = 670\,$K) orbiting a young (10-30\,Myr) K star, show depleted methane and surprising abundances of CO$_2$ in its atmosphere. This makes the planet an interesting target to study the "missing methane problem", where HST, Spitzer, and JWST have revealed many sub-Jovian planets with effective temperatures cooler than 800\,K to be depleted in methane compared to equilibrium chemistry predictions. On V1298~Tau\,b, an intrinsic temperature (T$_\mathrm{int}$) of 500\,K is required to explain the methane depletion if strong mixing from a warm interior is the reason for the missing methane, but this T$_\mathrm{int}$ is too large for a planet of its age and size according to standard interior structure evolution theories. Here, we show that stellar flares can result in methane depletion in a warm planet's atmosphere with nominal T$_\mathrm{int} \sim 100$\,K. During a stellar flare, fast methane photolysis rates cannot be balanced by diffusion from below into the upper atmospheric layers probed by observations. Young active stars flare frequently, and we show that the timescale of diffusion for the methane to recover from a flare-induced depletion can be slow compared to the flare frequency. Over time, the atmosphere reaches a new steady state set by the average flare frequency and energy. This new steady state shows depleted methane in the 2\,mbar region, and abundant CO, CO${_2}$, and HCN, which results in synthetic transmission spectra that reproduce the main features of the recent JWST observations of V1298~Tau\,b.
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All authors Danica Adams, Pierre-Alexis Roy, Hilke Schlichting, Björn Benneke

Earth and Planetary Astrophysics · Cross-listed from astro-ph.SR

The JWST/NIRCam Scattered Light Disks GTO 2780 program: panchromatic coronagraphic imaging of the HD 10647, HD 32297, HD 61005, HD 107146, and HD 181327 debris disk systems

András Gáspár, Jarron M. Leisenring, Schuyler Grace Wolff et al. (+13 more)

Debris disks, composed of rocks, boulders, planetesimals, and the dust produced in their collisions, present the most readily observable components of mature planetary systems. They also serve as valuable diagnostic tools, enabling studies of planetary dynamical interactions and mineral compositions. Observed from optical to radio wavelengths, each band reveals unique information about the dust populations. Optical and near-infrared observations are specifically sensitive to light scattered off the surfaces of the micron-sized particles. Here, we present results from the JWST/NIRCam GTO program 2780, designed to observe five disk systems previously identified to be exceptionally bright at optical wavelengths (HD 10647, HD 32297, HD 61005, HD 107146, and HD 181327) with six filters using the NIRCam coronagraphs. The NIRCam data complement previous shorter-wavelength images of these same systems. They reveal scattered light from the disks and from the extended halos of tiny grains under the influence of radiative forces, at high resolution and signal to noise. All the systems show evidence for water ice, although it can have differing radial distributions and tends to show stronger signatures in the halos. In the two cases we could analyze, the scattering phase function in the disks resembles the behavior of dust in the Solar System with evidence for enhanced forward scattering in the halos, consistent with the latter being composed of tiny grains. MIRI images for two systems are more centrally concentrated than the shorter wavelength ones, suggesting a role for dragged-in larger grains.
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All authors András Gáspár, Jarron M. Leisenring, Schuyler Grace Wolff, Kellen Lawson, George H. Rieke, Dingshan Deng, Marcia J. Rieke, Antranik A. Sefilian, Charles Beichman, Joshua B. Lovell, John Krist, Marie Ygouf, Jorge Llop-Sayson, Geoffrey Bryden, Christopher C. Stark, Christine H. Chen

Earth and Planetary Astrophysics

An Equation of State for Supercritical Silicate-Hydrogen Mixtures at Sub-Neptune Interior Conditions

Sarah P. Marcum, Lars Stixrude and Edward D. Young

Many sub-Neptunes are expected to contain long-lived molten silicate interiors beneath dense H2-rich envelopes. At pressures and temperatures near the atmosphere-interior boundary, MgSiO3 and H2 may become fully miscible, forming a supercritical silicate-hydrogen fluid. Here we use density functional theory molecular dynamics simulations to construct a self-consistent equation of state for supercritical MgSiO3H4, corresponding to 3.86 wt% hydrogen expressed as equivalent H2. We fit the simulation results with a Helmholtz free-energy formulation that yields density, entropy, heat capacity, thermal expansivity, bulk modulus, and Gruneisen parameter from a single thermodynamic surface. We find that the hydrogen-bearing fluid is lower in density than dry MgSiO3 liquid and deviates significantly from ideal specific-volume mixing between MgSiO3 and H2. The resulting excess volume varies with pressure, indicating that the interaction between hydrogen and the silicate framework evolves with compression. Structural analysis shows increasing Si-H coordination and decreasing persistent H-H bonding at high pressure, consistent with a transition away from molecular H2-like bonding toward a more strongly coupled silicate-hydrogen fluid. We incorporate the MgSiO3H4 equation of state into a composition-dependent MgSiO3-H lookup table and apply it to representative sub-Neptune structure models. The models show that hydrogen partitioning between the atmosphere and condensed interior affects the planetary adiabat relative to the MgSiO3 liquidus. These results demonstrate that supercritical silicate-hydrogen fluids have distinct thermodynamic and structural properties that must be accounted for when modeling sub-Neptune interiors.
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Earth and Planetary Astrophysics

Dust torques for realistic dust size distributions

V. Roatti, G. Picogna and F. Marzari

Previous studies have shown that a population of dust particles with a fixed Stokes number can exert a substantial torque on a low-mass planet embedded in a protoplanetary disk, modifying its migration rate. We aim to characterize the dust torque on a low-mass planet for a realistic distribution of dust grain sizes. We performed 2D hydrodynamical simulations of planet-disk interactions using the PLUTO code, with the addition of Lagrangian superparticles representing dust dynamics. We apply an energy-based criterion to exclude the particles that are gravitationally bound to the planet, to prevent circumplanetary flow to contaminate the torque measurements. We find that the dust torque is dominated by the largest grains in the size distribution and is highly sensitive to the maximum grain size. For typical disk conditions, the torque becomes positive for marginally coupled particles ($\mathrm{St} \gtrsim 10^{-2}$) and can exceed the gas torque in the presence of cm-sized pebbles, leading to outward migration of low-mass planets. Unlike previous studies, the turbulent dust diffusion has a negligible influence on the torque over the explored range of $\alpha = 10^{-4}$ to $3\times 10^{-3}$. The dominant contribution arises from within the planetary Hill sphere, highlighting the need for high spatial resolution and accurate integration of particle trajectories. We derive a scaling law for the dust torque as a function of the maximum grain size and the planetary mass, suitable for implementation in population synthesis models.
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Earth and Planetary Astrophysics · Cross-listed from astro-ph.SR

Panchromatic Spectra of Nearby Low-mass and Sun-like Stars with Directly Imageable Habitable Zones

Sarah Peacock, Breanna A. Binder, Edward W. Schwieterman et al. (+4 more)

Characterizing the high-energy radiation environments of stars plays a critical role in determining which systems' planets are capable of retaining atmospheres and sustaining habitable conditions. X-ray through ultraviolet (UV) radiation drives atmospheric photochemistry, heating, and escape, making accurate characterization of stellar high-energy emission essential for both interpreting future exoplanet observations and identifying the most promising targets for life detection. We construct panchromatic spectral energy distributions (SEDs) spanning the X-ray through radio for 12 nearby low-mass and Sun-like stars with directly imageable habitable zones that are prioritized targets for the Habitable Worlds Observatory (HWO) and Extremely Large Telescopes (ELTs). These SEDs are generated using forward stellar atmosphere models guided and constrained by available archival X-ray and UV observations. We find that many stars in this sample exhibit elevated high-energy radiation environments relative to the modern Sun, with habitable zone X-ray and extreme UV (XUV) fluxes frequently exceeding solar values by 1-2 orders of magnitude. The elevated emission likely reflects a combination of sample selection effects, differences in stellar age and rotation, and intrinsic magnetic variability, with multi-epoch observations demonstrating that variability alone can significantly alter inferred radiation environments. These results highlight high-energy radiation as an important discriminator in identifying the most promising habitable planet hosts and demonstrate the need for expanded X-ray and UV observations to complete the stellar characterization necessary for HWO target prioritization.
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All authors Sarah Peacock, Breanna A. Binder, Edward W. Schwieterman, Margaret C. Turnbull, Stephen R. Kane, Katherine Garcia-Sage, Alison Farrish

Earth and Planetary Astrophysics

Narrow-Shell Stochasticity in Source-Sink Models of the Low Earth Orbit Environment

Jaewon Choi, Souvik Dhara and Harsha Honnappa

Deterministic source-sink models are widely used to assess the long-term evolution, capacity, and sustainability of the low Earth orbit (LEO) environment. These models propagate shell-averaged populations through ordinary differential equations (ODEs), relying on individual collision, disposal, and decay events to average out within sufficiently large altitude shells. As constellation traffic is increasingly organized into kilometer- and sub-kilometer-scale shells, this averaging assumption becomes strained. We formulate the multi-shell, multi-species LEO environment as a Markov jump process and, from this formulation, recover the conventional source-sink ODE as a large-volume limit and derive a stochastic differential equation (SDE) approximation whose fluctuations scale as V^{-1/2} in the per-shell volume V. The diffusion approximation is validated against an exact discrete-event simulation of the underlying jump process. Sweeping V at fixed spatial density over the 450-800 km band, we find that the two descriptions agree at shell volumes comparable to those used in established source-sink models but diverge as shells narrow. At the finest shell volume considered, the mean debris population reaches roughly 4.5 times the ODE prediction, with several realizations undergoing runaway growth absent from the ODE trajectory. This departure is driven by nonlinear collision terms, through which population variance and covariance raise expected collision rates, generating further debris and reinforcing the collision-debris feedback. Narrow-shell stochasticity thus both widens the outcome distribution and shifts the expected trajectory. Because the stochastic model shares its parameterization with the deterministic one, it provides a scale-consistent extension of existing source-sink models for evaluating shell configuration, collision risk, and long-term LEO sustainability.
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Earth and Planetary Astrophysics

Catastrophic tidal disruption of heterogeneous rubble piles: a tale of two regimes

John Wimarsson, Eric Frizzell, Martin Jutzi et al. (+1 more)

The way a rubble-pile body deforms or disrupts under the influence of tidal forces can be directly tied to its internal strength and configuration. Computational modelling of such tidal disruption events provides an indispensable numerical laboratory for constraining the origin and evolution of small bodies in the Solar System. A majority of previous investigations into tidal disruption of rubble piles have mainly considered progenitors consisting of same-sized, spherical elements. Our study attempts to fill the existing gap in studies analysing the effect of aggregate heterogeneity on tidal disruption outcomes by varying element shape and size frequency distribution. Such heterogeneities have been shown to strongly influence rubble pile dynamics for impacts and rotational failure. We performed numerical simulations of parabolic and hyperbolic tidal encounters between six unique rubble-pile progenitors and the Earth using the N-body code GRAINS. The resulting mass distributions of generated fragments and tidal chain morphologies for the different progenitors were further tied to the internal strength of rubble piles. Two regimes of tidal disruption are identified. In the first regime, closest to the planet, the dynamic evolution is dominated by tidal forces. Here, particle shape, size distribution and resolution appear to have little importance for the resulting distribution of fragment masses. In the second, shear-controlled regime, the internal structure of the progenitor begins to strongly influence the resulting tidal chain morphology and properties of the surviving fragments. Heterogeneity originating from the shape and size frequency distribution of elements in rubble pile models has a substantial effect on the outcomes of tidal disruption events. These parameters must be carefully taken into account when future studies attempt to tie results from numerical models to observations.
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All authors John Wimarsson, Eric Frizzell, Martin Jutzi, Fabio Ferrari

Earth and Planetary Astrophysics

Three-dimensional SPH simulations of the Caloris basin-forming impact: basin scaling, the gravity anomaly, and antipodal effects

Thomas Meier, Christian Reinhardt, Martin Jutzi et al. (+1 more)

The Caloris basin is the largest well-preserved impact structure on Mercury, yet its formation conditions, the origin of its positive gravity anomaly, and its relation to the antipodal terrain remain poorly constrained. We present global, three-dimensional smoothed particle hydrodynamics simulations of the Caloris basin-forming impact with the pkdgrav3 code, including material strength. We survey 225 combinations of impactor radius, velocity, angle, and target thermal profile, complemented by simulations of up to two billion particles, which resolve the adopted 40 km crust by ten particle layers. At all resolutions, basin sizes are measured directly from the crust particles. The measured basin diameters follow a single power law in impactor radius, velocity, and angle, systematically steeper than idealized point-source crater scaling. The observed Caloris diameter is reproduced by a broad family of impactors favoring oblique incidence; sensitivity to resolution and crust thickness shifts this family toward smaller or slower impactors. All impactor material remaining in the basin region is vaporized, so our crust-like impactors leave no buried remnant. Instead, the impact thins the mantle and raises a local dome on Mercury's core. The dome's isolated gravity signal is positive and centered on the basin, supporting the mantle-uplift origin proposed for the observed mascon. At the antipode, single seismic pulses cannot loft surface material when strength is included, yet accumulated strain exceeds the elastic limit. A kilometer-scale equivalent thickness of impact-derived material then converges on this weakened surface. Ejecta convergence contributed at least as much as seismic shaking to forming the terrain.
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All authors Thomas Meier, Christian Reinhardt, Martin Jutzi, Joachim Stadel

Earth and Planetary Astrophysics

The true masses of radial-velocity exoplanets constrained by stability

Xander Byrne and Amy Bonsor

Exoplanets discovered with the radial velocity method suffer from a mass-inclination degeneracy: only the minimum mass $M_{\rm min} = M \sin i$ is measured. An exoplanet with an Earth-sized minimum mass could actually therefore be a sub-Neptune, or even a gas giant. However, for compact multi-planet systems, the inclination angle cannot be too low, as this would imply masses so large that the system would be dynamically unstable on time-scales shorter than the age of the system. We present CINEMAS, a Bayesian framework to constrain the inclinations - and hence true masses - of radial-velocity exoplanets in flat multi-planet systems, by accounting for dynamical stability. Applying this framework to five compact exoplanetary systems, we cut upper limits on the true masses of their planets by as much as 45%. As two examples, the masses of the four Barnard's Star planets are constrained to all be below $0.6~M_{\oplus}$ to at least 95% confidence; and the three gas giants orbiting HD 184010 are found to be intermediate in mass between Saturn and Jupiter. With only ~1% of planetary systems being inclined enough to transit, methods of studying non-transiting systems are vital for constraining the properties of nearby exoplanets.
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Earth and Planetary Astrophysics

Alkali lines at extreme densities and their impact on giant planet interior structure

Louis Siebenaler, Nicole F. Allard, Esther van Dijk et al. (+1 more)

Alkali lines, in particular the sodium Na $D$ (5891$\AA$, 5897$\AA$) and potassium K $D$ (7667$\AA$, 7701$\AA$) resonance doublets, are dominant opacity sources in giant planets over a wide range of temperatures ($\gtrsim$1000K). Their strong pressure-broadened wings significantly influence the thermal structure of giant planets, especially at high pressures. Most detailed line-profile calculations have so far been limited to perturber densities up to $10^{21}$cm$^{-3}$. However, conditions in the deep atmospheres and interiors of giant planets can reach significantly higher densities, making the temperature gradients increasingly uncertain. We determined how physically consistent collisional broadening of the Na $D$ and K $D$ lines at extreme densities affects opacity calculations and consequently the inferred interior structure of giant planets. We computed detailed Na $D$ and K $D$ line profiles using unified line theory, extending to molecular hydrogen perturber densities of $n_{\rm H_2} = 5 \times 10^{22}$cm$^{-3}$, which translates to pressures up to $\sim$ 20kbar. The revised cross sections were incorporated into Rosseland mean opacity tables, which were then used to evaluate their effect on planetary thermal structures. At densities $n_{\rm H_2} > 10^{21}$cm$^{-3}$, the line profiles predicted by unified line theory exhibit significantly stronger wings than commonly used Voigt profiles, as well as density-dependent line shifts, which substantially increases Rosseland mean opacities. Consequently, the radiative-convective boundary of warm and hot giant planets can shift to lower pressures, producing warmer interior adiabats and increasing inferred core masses. We further find that Jupiter is unlikely to host a stable radiative layer at the present time or throughout most of its evolution, as the required Na and K abundances for this are well below observational constraints.
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All authors Louis Siebenaler, Nicole F. Allard, Esther van Dijk, Yamila Miguel

Earth and Planetary Astrophysics

Study on the Venusian Atmospheric Thermal Structure: A Comparative Analysis between VEX/Akatsuki and Venus-GRAM/VCD

Soumyaneal Banerjee, R. K. Choudhary, Keshav R. Tripathi et al. (+5 more)

We investigate the thermal structure of the Venusian middle atmosphere between 45 and 80 km using radio occultation (RO) measurements from Venus Express and Akatsuki spanning 2006-2024. Retrieved temperature profiles are compared with climatological predictions from the Venus-GRAM and the Venus Climate Database (VCD). Systematic deviations exceeding 10 K are observed between the RO temperatures and model climatologies, particularly at high latitudes in both hemispheres. The long-term dataset further reveals a possible decadal-scale temporal variability in temperatures across the low-to-mid latitude regions. This variability becomes less coherent at higher altitudes. A combination of post-stratification and bootstrap analysis on the temperature anomalies indicates that the observed temporal variability at low-to-mid and polar latitudes is not readily explained by the latitudinal sampling bias quantified using the adopted analysis, while the trends at mid-to-high latitudes are affected by sparse and uneven RO sounding. Sensitivity tests using modified cloud albedo inputs in VCD simulations show that adjusting cloud radiative forcing partially reconciles the discrepancies, especially at the lower altitudes, below 60 km. However, above the cloud top, at the lower latitudes, the divergence from the observations increases significantly, thereby failing to provide a general reconciliation between VCD and RO. These results highlight the need for updated empirical climatologies incorporating recent RO measurements and for improved physical parameterizations in Venus general circulation models to better capture the global variability in the planet's middle atmosphere.
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All authors Soumyaneal Banerjee, R. K. Choudhary, Keshav R. Tripathi, K. M. Ambili, J. Oschlisniok, Silvia Tellmann, T. Imamura, H. Ando

Wednesday 26 August

Earth and Planetary Astrophysics

Scaling law for the diffusion coefficient in vibration-driven crater relaxation

Hayato Narita and Hiroaki Katsuragi

Impact craters relax over long timescales, and this process is commonly described by a diffusion model. The diffusion coefficient determines the relaxation rate, and it has been estimated from observed crater shapes. This coefficient, however, represents the combined effect of several physical mechanisms. Its physical basis has not been revealed yet. In this study, we isolate the contribution of seismic vibration in a controlled experiment. Using a quasi-two-dimensional setup, we first confirm that a linear diffusion equation reproduces the crater relaxation. We then measure the diffusion coefficient and obtain the scaling form through systematic experiments. We find that the diffusion coefficient is proportional to the crater diameter. This dependence is not expected for simple diffusion. We interpret this dependence as arising because the thickness of the vibro-fluidized granular layer scales with the crater depth. This scaling relation is the main result of this study. We then apply it to the Moon with a parameterized model of impact-driven seismic spreading in the regolith layer. Integrating over the impact flux yields a macroscopic coefficient nearly proportional to crater diameter for shallow-layer-like spreading. Its magnitude depends on uncertain model parameters but is consistent with observations for plausible values. These results indicate that vibration is a physically plausible contributor to crater relaxation on the Moon.
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Earth and Planetary Astrophysics · Cross-listed from physics.flu-dyn

Fluxes through a uniform double-diffusive staircase at low Prandtl number

Arstanbek Tulekeyev and Pascale Garaud

Double-diffusive convection in the diffusive regime is often observed to be in a long-lived layered state, well-known in the Arctic ocean and in volcanic lakes, and thought to exist in the interiors of giant planets. The density profile resembles a `staircase', with stacks of convective layers of uniform density separated by strongly stratified interfaces. The governing parameters are the Prandtl number $\mathrm{Pr}$, diffusivity ratio $\tau$, density ratio $R_\rho$, and Rayleigh number $\mathrm{Ra}$. In this work, we perform direct numerical simulations in triply periodic domains to systematically investigate the properties of statistically stationary double-diffusive staircases with $\tau \le \mathrm{Pr} \ll 1$. We demonstrate that they can exist for significantly larger density ratios than previously reported, and propose a new criterion for the existence of long-lived layered states. Notably, this criterion depends weakly on $\mathrm{Ra}$. In all simulations, we measure the total fluxes of density $F^{\mathrm{tot}}_\rho$, temperature $F^{\mathrm{tot}}_T$ and composition $F^{\mathrm{tot}}_C$, and the flux ratio $\gamma =F^{\mathrm{tot}}_C/F^{\mathrm{tot}}_T$. We confirm that $\gamma$ is approximately constant for moderate to large $R_\rho$ but also depends weakly on $\mathrm{Ra}$. We show that the Nusselt numbers for composition and temperature scale as $\mathrm{Ra}^{1/3}$ in this limit, which notably differs from the $(\mathrm{RaPr})^{1/3}$ scaling previously found at lower $R_\rho$. We propose a new model for these findings that is based on the rise and turbulent mixing of density anomalies that form at the edges of the interfaces. The model explains the existing low $\mathrm{Pr}$ data with reasonable accuracy.
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Earth and Planetary Astrophysics · Cross-listed from astro-ph.IM

The MegaWave Radio Surveyor

T. Joseph W. Lazio, Evgenya Shkolnik, James Aguirre et al. (+40 more)

Several Decadal-level questions in astrophysics, exoplanets, astrobiology, and cosmology can be addressed only at low radio frequencies inaccessible from Earth. The MegaWave Radio Surveyor would open this largely-unexplored region of the electromagnetic spectrum with a space-based interferometer to (1)~Track the space weather of other stars; (2)~Detect magnetically-generated emission from exoplanets to probe their interiors and assess magnetic shielding of their atmospheres; (3)~Probe the Universe's evolution during the Dark Ages via the highly-redshifted HI hyperfine line; and (4)~Assess the role of cosmic rays and magnetic fields in the cosmic web. An Astrophysics Strategic Technology & Research Accelerator (ASTRA) Initiative concept, the MegaWave Radio Surveyor's science objectives respond to the Pathways to Discovery Decadal Survey and three other National Academies studies, and it would serve as a Formative Era mission in the Enduring Quests, Daring Visions roadmap. Developments in U.S. space industries enable this observatory to be realized. The MegaWave Radio Surveyor would offer a versatile, scalable, and resilient architecture capable of sensitive and simultaneous observations below 45~MHz and unprecedented angular resolution at these frequencies. The concept builds upon NASA's Sun Radio Interferometer Space Experiment (SunRISE), Star-Planet Activity Research CubeSat (SPARCS), and Lunar Surface Electromagnetics Experiment (LuSEE-Night). The MegaWave Radio Surveyor could leverage multiple elements of the Artemis program, such as access to and beyond cislunar space and communications, and there are opportunities to infuse new autonomy/AI modes for mission operations. By opening one of the last windows in the electromagnetic spectrum and pioneering space interferometry at unprecedented scales, the MegaWave Radio Surveyor would establish a transformational capability.
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All authors T. Joseph W. Lazio, Evgenya Shkolnik, James Aguirre, Stuart D. Bale, Judd Bowman, Ruby Byrne, Joseph Callingham, Tracy E. Clarke, Ivey Davis, Tim Dolch, Peter Driscoll, Anastasia Fialkov, Steven Furlanetto, Simona Giacintucci, Ravit Helled, Jacqueline Hewitt, Phil Hopkins, Andrea Isella, Daniel Jacobs, Eloy de Lera Acedo, Melodie M. Kao, Mary Knapp, L.V.E. Koopmans, Nicholas Kern, Jasmina Lazendic-Galloway, Susan T. Lepri, R. O. Parke Loyd, James P. Lux, James P. Mason, Raul Monsalve, Miguel Morales, Julian B. Muñoz, Rachel Osten, J. Sebastian Pineda, Jonathan Pober, Sam B. Ponnada, Leslie A. Rogers, Saurabh Singh, Jake D. Turner, Jackie Villadsen, Philippe Zarka, John ZuHorne, Ellen Zweibel

Earth and Planetary Astrophysics

Non-equilibrium condensation of the first Solar System solids

Sebastien Charnoz, Jérome Aleon, Marc Chaussidon et al. (+3 more)

Primitive meteorites (chondrites) consist of an out-of-equilibrium assemblage of minerals formed during the assembling of our Solar Nebula. The conditions under which their precursors condensed remain unclear as a result of subsequent re-processing in the protoplanetary disk or in asteroidal parent bodies. Chondrites are classified into three main classes enstatite (EC), ordinary (OC), and carbonaceous (CC) distinguished by different bulk composition and oxidation state. While equilibrium condensation models explain the composition of some of their refractory components they do not explain the emergence of three mineralogical classes. Moreover, the low pressures, steep temperature gradients, and short dynamical transport timescales in forming protoplanetary disks likely hindered equilibrium. Here we test the hypothesis that chondrite precursors formed via kinetic non-equilibrium condensation. Using a new time-dependent condensation model, we show that varying the cooling rate and pressure produce only three types of mineralogies. Departure from equilibrium yields increasingly oxidized and hydrous mineralogies. When projected into a Urey-Craig diagram, the predicted mineralogical types fall close to the redox states of EC, OC, and CC chondrites. These results suggest that the mineralogical diversity of chondrites may reflect, in part, local condensation kinetics, offering an alternative to large-scale variations of oxidation conditions.
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All authors Sebastien Charnoz, Jérome Aleon, Marc Chaussidon, Paolo Sossi, Yves Marrocchi, Patrick Franco

Earth and Planetary Astrophysics

SOAR TESS Survey III: Mapping the Stellar Companion Deficit around TESS Planet Candidates

Carl Ziegler, Andrei Tokovinin, César Briceño et al. (+3 more)

We present the cumulative SOAR TESS Survey, based on HRCam speckle observations of 2,982 targets, and report 433 companion measurements obtained after Survey II. In a demographic sample of 1,199 dwarf primaries, we compare detected companions with secondary-to-primary mass ratios $q=M_2/M_1\geq0.4$ to a field-star population after accounting for each observation's detection limits. We find 24 companions within 50 au versus 84.8 expected and 48 within 100 au versus 124.4 expected. The companion frequencies relative to the field expectation are $S_{50\,{\rm au}}=0.291^{+0.062}_{-0.054}$ and $S_{100\,{\rm au}}=0.391^{+0.058}_{-0.053}$. A continuous model in which the deficit weakens with separation reaches the midpoint between close-binary suppression and the field rate at $a_{50}=74$ au (57–101 au). The Gaia-matched subset contains 88 companions versus 69.6 expected, a difference consistent with the model once uncertainties in the field comparison and Gaia selection are included. A model that preserves the total number of binaries but shifts the field distribution toward 100 au fits poorly. Under the limiting assumption that all retained planets orbit the catalog primary, the fitted law implies reductions of 17.0% in volume-limited planet yield over $q\geq0.4$ and 23.3% when extended to all mass ratios. M-dwarf hosts show a similar close-companion deficit, but the smaller sample prevents a precise comparison with solar-type hosts.
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All authors Carl Ziegler, Andrei Tokovinin, César Briceño, Nicholas M. Law, Andrew W. Mann, Seth Batson

Earth and Planetary Astrophysics · Cross-listed from astro-ph.IM

Alfred: A Flexible, User-Friendly Python Package for Exoplanet Confirmation

Maxwell A. Kroft

In the era of TESS, exoplanet follow-up and confirmation have become a massive community effort. With only about an eighth of the over 8,000 TESS exoplanet candidates confirmed, astronomers need flexible software to allow them to characterize many different kinds of systems with data from multiple telescopes and instruments. In this work, I present Alfred (the Awesome Library For Robust Exoplanet Detection), an open-source Python package for exoplanet detection and confirmation. Alfred is designed to be flexible and easy to use, allowing users to fit transit and radial velocity data for an arbitrary system of exoplanets. The code is released under an MIT license on GitHub (https://github.com/maxkroft/Alfred ) and is installable via pip, with documentation under active development (https://alfred-exoplanets.readthedocs.io/en/latest/index.html# ).
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Earth and Planetary Astrophysics · Cross-listed from astro-ph.IM

Best practices for obtaining astrometric observations from JWST (274) NIRCam data

Marco Micheli and Bryan J. Holler

The James Webb Space Telescope (JWST), with its near-infrared imager NIRCam, is capable of obtaining detections of small bodies down to what would correspond to a visual magnitude of about 31. However, the complexity of NIRCam introduces multiple subtle effects in the way the images are obtained, and they in turn affect the astrometric information that can be extracted from the images. In the following, we very briefly summarize these peculiarities, and provide some basic information on how to extract meaningful astrometric positions from NIRCam images.
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Earth and Planetary Astrophysics · Cross-listed from astro-ph.SR

The Isotopic and Elemental Abundances of Planet-Host Star TRAPPIST-1

Darío González Picos, Ian J.M. Crossfield, David Coria et al. (+6 more)

Elemental and isotopic abundances are key tracers of planet formation, stellar evolution, and Galactic chemical evolution. Very low-mass stars are particularly interesting in this regard, because unlike more massive or evolved stars their photospheric abundances retain the star's natal composition. Cool dwarf spectra have historically been challenging to use for measurements of chemical abundances because of the blending of molecular and atomic features. However, recent advances in molecular line lists, atmospheric models, fitting techniques and IR spectrographs have enabled the successful measurement of elemental and isotopic ratios in a few dozen very low-mass stars. Here, we present near-infrared high-resolution spectroscopy of TRAPPIST-1 covering the fundamental and overtone bands of carbon monoxide and its prominent isotopologues. From the joint analysis of the $K$-band (CFHT/SPIRou) and $M$-band (Keck/NIRSPEC) spectra, we derive the first stellar C/O ratio and the first carbon and oxygen isotope ratios for this star. We obtain a metallicity of $[\mathrm{M/H}] = 0.00 \pm 0.06$, a C/O ratio of $0.60 \pm 0.02$, $^{12}\mathrm{C}/^{13}\mathrm{C} = 154{}^{+17}_{-16}$, and $^{16}\mathrm{O}/^{18}\mathrm{O} = 490{}^{+78}_{-64}$. TRAPPIST-1 has generally solar-like elemental abundances, with a $^{12}\mathrm{C}/^{13}\mathrm{C}$ ratio that is higher than the solar value and may be modestly elevated relative to some nearby cool dwarfs at similar metallicity. While C/O is most tightly constrained by the $K$-band, the isotopic detections are driven by the $M$-band data.
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All authors Darío González Picos, Ian J.M. Crossfield, David Coria, Joshua Lothringer, Eric Gaidos, Elisabeth A.C. Mills, Sam de Regt, Donatella Romano, Ignas Snellen

Earth and Planetary Astrophysics

Benchmark Brown Dwarfs as Chemical Laboratories: Linking System Bulk Properties to Atmospheric Retrievals

Viktória Kecskeméthy, Ben Burningham, Fei Wang et al. (+8 more)

We present the first atmospheric retrieval analysis of two compositional benchmark mid-L dwarfs – SDSSJ141659.78+500626.4 and GJ 499 C – using the Brewster retrieval framework, where the wide benchmark nature of these systems provides independent constraints on age and bulk composition from their stellar primaries. These targets were observed with JWST using NIRSpec Prism and MIRI LRS, providing low-resolution ($R$ $\sim$ 100) spectra between 0.6–14.0 $\mathrm{\mu}$m with high signal-to-noise ratios (SNR $\sim$100 – 600). For SDSSJ141659.78+500626.4, the retrieved cloud combination of a high-altitude enstatite slab and low-altitude iron deck clouds matches phase-equilibrium predictions based on the primary star's Mg/Si ratio. We retrieve a super-solar C/O = 0.71$^{+0.01}_{-0.01}$ and slightly metal-rich [M/H] = 0.22$^{+0.03}_{-0.03}$. This C/O ratio can only be reconciled with the value inferred for the primary if additional oxygen sequestration beyond the retrieved cloud mass is present, or if there are uncertainties in the adopted opacities or other model deficiencies. The inferred [C/H] and [O/H] are 0.31$\pm$0.03 and 0.19$\pm$0.03, respectively, which are consistent within the relatively large uncertainties of the host star abundances. For GJ 499 C, the retrieved silicon-monoxide and forsterite slab clouds are difficult to explain with simple phase-equilibrium assumptions, yielding Mg/Si $\sim$ 1.9. We estimate C/O = 0.69$^{+0.02}_{-0.02}$ and [M/H] = 0.13$^{+0.04}_{-0.06}$. For both objects, the inferred radii of 0.85$^{+0.01}_{-0.01}$ $R_{\mathrm{Jup}}$ and 1.00$^{+0.02}_{-0.02}$ $R_{\mathrm{Jup}}$ and masses of 73.7$^{+4.9}_{-9.2}$ $M_{\mathrm{Jup}}$ and 68.0$^{+8.5}_{-12.7}$ $M_{\mathrm{Jup}}$ are consistent with evolutionary models and system ages, highlighting the plausibility of our results.
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All authors Viktória Kecskeméthy, Ben Burningham, Fei Wang, Austin J. Rothermich, Jacqueline K. Faherty, Genaro Suárez, Caprice L. Phillips, Melanie Rowland, Channon Visscher, Johanna M. Vos, Daniella C. Bardalez Gagliuffi

Earth and Planetary Astrophysics

JWST color sequence of transiting exoplanets and directly imaged substellar objects

Guangwei Fu, Joshua D. Lothringer, Sagnick Mukherjee et al. (+4 more)

Color-magnitude diagrams (CMDs) have been foundational across astrophysics, from galaxies and stars to brown dwarfs, and JWST now extends them to transiting exoplanets and self-luminous substellar objects at overlapping temperatures. We compile JWST dayside emission spectra for 13 transiting giant planets, 57 self-luminous objects, and the irradiated brown dwarf ZTF J0038+2030 B, spanning ~350-2600 K and log g = 2.5-5.5, plus ~2150 SPHEREx ultracool dwarfs, converted to synthetic photometry in 2MASS J/Ks and five NIRCam medium bands isolating H2O, CH4, CO2, and CO. Low gravity and inflated radii make transiting planets resemble young substellar objects, with shallower features and redder colors from high-altitude clouds that raise the photosphere. Transiting planets follow the L-dwarf sequence on the J versus J-K CMD, with WASP-80 b (T_eq~800 K) showing no T-dwarf-like blueward turn, suggesting a delayed or suppressed L/T transition in irradiated, low-gravity atmospheres. Methane onset is delayed from substellar to transiting-planet atmospheres, and cloudy radiative-convective models indicate that lower gravity and cloud back-warming shift atmospheres toward CO-dominated chemistry, suppressing photospheric CH4. The irradiated but old, high-gravity brown dwarf ZTF J0038+2030 B (T_day = 1049 K, log g = 5.4) shows a deep, field-T-dwarf-like dayside methane band, implicating gravity rather than irradiation as the dominant control. CO2-to-CO diagnostics place transiting and directly imaged planetary-mass companions at stronger relative CO2 absorption than field brown dwarfs, consistent with metallicity enhancement from planetesimal accretion. These CMD sequences provide a unified, model-testable map for self-luminous and irradiated atmospheres.
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All authors Guangwei Fu, Joshua D. Lothringer, Sagnick Mukherjee, David K. Sing, Elena Manjavacas, Kevin B. Stevenson, William Balmer

Earth and Planetary Astrophysics

An Approximately 70-Year Core-Related Modulation of Earth Rotation and Its Implications for the Leap Second

Zewen Zhang, Yuanwei Wu, Xishun Li et al. (+4 more)

Recent observations of Universal Time (UT1) indicate an acceleration in Earth's rotation. If sustained under the current leap-second framework, this behavior could eventually prompt consideration of a negative leap second. We examine whether the recent acceleration is consistent with an approximately 70-year, core-related modulation of length of day (LOD). After removal of modeled tidal, surface-fluid, and secular contributions, residual LOD contains a near-70-year component, and a similar component is present in core angular momentum (CAM)-derived equivalent LOD inferred from geomagnetic observations. All harmonic, spectral, and LOD-CAM analyses reported here use the common 1883–2022 interval. Harmonic regression over trial periods of 50–100 yr gives periods of 69.7 yr for residual LOD and 71.8 yr for CAM-derived equivalent LOD, with amplitudes of 2.87 and 1.94 ms, respectively. Lomb–Scargle spectra show peaks near 67.8 and 70.5 yr. The annual series have a zero-lag correlation of 0.918. Their lagged correlation has a broad maximum for a CAM lead of approximately 1-3 yr, with a numerical maximum of 0.932 at 2 yr. Because both records are strongly autocorrelated, these coefficients are used to characterize their correspondence rather than to assess predictive significance. The results are consistent with a core-related contribution to low-frequency rotational variability, but they do not uniquely separate the contributions of electromagnetic, topographic, gravitational, and viscous core–mantle coupling mechanisms. Within the fitted model, the multidecadal component alone does not indicate sustained near-term shortening of the day that would, by itself, require a negative leap second. This is a model-dependent geophysical assessment, not an operational prediction of future UTC adjustments.
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All authors Zewen Zhang, Yuanwei Wu, Xishun Li, Dang Yao, Xuan Cheng, Xuhai Yang, Shougang Zhang