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Physics Astrophysics Earth and Planetary Astrophysics

astro-ph.EP Watch Earth and Planetary Astrophysics

Thursday 20 August

Earth and Planetary Astrophysics

Asymmetric Aerosol Distribution on the Terminators of the Warm Saturn WASP-69 b Revealed by JWST NIRISS/SOSS

Le-Chris Wang, Sagnick Mukherjee, Stephen P. Schmidt et al. (+16 more)

How aerosols form, are transported, and cycle between condensation and evaporation across exoplanet temperature regimes remains poorly understood. Recent models and observations suggest that warm giant planets near $800$–$1000$ K may span a transition between homogeneous and longitudinally heterogeneous aerosol distributions. We present a robust detection of aerosol asymmetry in a giant planet with $T_{\rm eq}\lesssim1000$ K, using the $0.86$–$2.82~\mu$m JWST NIRISS/SOSS transmission spectrum of WASP-69 b. The evening limb shows prominent 1.4 $\mu$m H$_2$O absorption ($\Delta\mathrm{BIC}_{\rm H_2O}=+22.7$), whereas H$_2$O is not detected on the morning limb ($\Delta\mathrm{BIC}_{\rm H_2O}=-8.7$). Atmospheric retrievals reveal significant aerosol opacity on both limbs, with high-altitude, optically thick clouds muting molecular features on the morning limb and lower cloud opacity allowing H$_2$O to emerge on the evening limb. The evening terminator is hotter by $304^{+62}_{-91}$ K, consistent with morning-limb condensates partially evaporating during transport toward the evening limb. This mechanism is independently verified with 3D general circulation models. Stellar contamination or aerosols dominated by photochemical haze do not readily explain the asymmetry. From a limb-resolved analysis, we infer a stellar-to-superstellar atmospheric metallicity, with $\rm[M/H]=0.11^{+0.40}_{-0.46}$ from the equilibrium retrieval and [O/H]$=1.38^{+0.44}_{-0.79}$ from the free retrieval. We also detect an escaping metastable-helium tail extending to $3.08^{+0.50}_{-0.45}\,R_p$. WASP-69 b anchors the cooler edge of the emerging population of planets with asymmetric aerosol distributions and suggests that substantial aerosol opacity may persist on both limbs across this transition.
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All authors Le-Chris Wang, Sagnick Mukherjee, Stephen P. Schmidt, Kevin B. Stevenson, Mei Ting Mak, Patrick McCreery, Harry Baskett, Carlos Gascón, David K. Sing, Katharine A. Bennett, Duncan A. Christie, Guangwei Fu, Mercedes López-Morales, Joshua D. Lothringer, Nathan J. Mayne, Lakeisha M. Ramos Rosado, Zafar Rustamkulov, Kevin C. Schlaufman, Kristin S. Sotzen

Earth and Planetary Astrophysics

Diffractive-Sail Single-Impulse Reachable Set for Interplanetary Transfer Design

Shuyue Fu, Jinkai Zhang, Di Wu et al. (+2 more)

Interest in planetary exploration has renewed, and the design of interplanetary transfers has attracted remarkable attention. This paper considers the interplanetary transfer design using a diffractive sail. Considering a nonzero departure hyperbolic excess velocity, the interplanetary transfer problem is transformed into the problem of computing single-impulse reachable sets. Then, based on previous work, a complementary computational method for reachable sets under arbitrary dynamics is proposed using differential algebra combined with adaptive grid refinement. The adaptive grid refinement considers two types of merit scores that reveal dynamical properties and the truncation error of the differential algebra propagation. The proposed method is applied to compute the diffractive-sail reachable sets, and the results verify the effectiveness of the method and merit scores. Finally, a preliminary design of the interplanetary transfers, specified as the Earth-Mars transfers, is performed based on the diffractive-sail reachable sets. The design results are presented. The effects of the corresponding parameters, including transfer time, diffractive angle, and type of diffractive sails, on transfer characteristics are analyzed, providing further insight into parameter selection for interplanetary transfer design.
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All authors Shuyue Fu, Jinkai Zhang, Di Wu, Peng Shi, Shengping Gong

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

The comets discovered from New Zealand, and the astronomers who found them

John Drummond, Wayne Orchiston, Carolyn Brown et al. (+1 more)

Eleven comets were discovered by six New Zealanders from New Zealand shores. New Zealand's important geographical position south of the Equator is highlighted. This location allows observers to discover and observe comets that would be difficult to see from the Northern Hemisphere. The lives of these comet discoverers are explored, as well as the discovery circumstances and an analysis of the morphological changes of their comets. We find that numerous New Zealand observers made many observations of these comets, however, relatively few were included in international publications, this possibly being due to papers about them not being submitted to overseas journals. We also note an interesting trend in that the number of New Zealand newspaper articles relating to these comet discoveries plummeted after the 1946 discovery by Albert Jones. We speculate as to why this may have happened.
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All authors John Drummond, Wayne Orchiston, Carolyn Brown, Jonathan Horner

Wednesday 19 August

Earth and Planetary Astrophysics

Finding Habitable Exoplanets with Binary Relative Astrometry: Planet Detection and Characterization with the Microarcsecond Astrometric Retrieval Algorithm (MARA)

William Roberson, Eric L. Nielsen, Jessie L. Christiansen et al. (+8 more)

Binary relative astrometry is a technique to search for rocky planets in the habitable zone of nearby binary stars using 1D relative astrometry at the microarcsecond level. This unprecedented precision would allow a custom-designed space telescope to directly measure the occurrence rate of these planets. The success of such a mission depends on our ability to recover and characterize planets from the unique format of extreme precision binary relative astrometry data. We present MARA, the Microarcsecond Astrometric Retrieval Algorithm, specifically designed for these data. We describe the design and format of the MARA pipeline, and demonstrate its accuracy and performance with a series of validation tests on simulated data, using the SHERA SMEx mission concept as an example. Our injection/recovery tests show that with these data, MARA is able to detect and characterize rocky planets in the habitable zone of alpha Cen A, down to a coplanar mass of about 1 Earth mass in 1 year orbits. Expanding to a range of input planet masses and periods for the same example mission, we find that the results from these injection/recovery tests generally agree with the analytic predictions of binary relative astrometry sensitivity. We use MARA to map out the expected completeness as a function of planet mass and period, which in this case reaches down to about 0.5 M Earth masses at 3 year orbits around alpha Cen A. These depth-of-search calculations will be a vital ingredient in demographics calculations from the final data from a binary relative astrometry mission.
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All authors William Roberson, Eric L. Nielsen, Jessie L. Christiansen, Gautam Vasisht, Eduardo Bendek, Alex Davis, Eric E. Mamajek, Catherine A. Clark, Kaitlin M. Kratter, Juliette Becker, Michael R. Meyer

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

Microlensify: a Transformer Based Machine Learning Classifier for Microlensing Events Trained on TESS Light Curves

Atousa Kalantari, Somayeh Khakpash, Sedighe Sajadian et al. (+3 more)

Microlensing can reveal populations of faint compact objects that are otherwise difficult to detect. Depending on their design, all-sky surveys have the potential to search for these objects across the sky. The Transiting Exoplanet Survey Satellite (TESS), primarily designed to detect transiting exoplanets, also provides near all-sky coverage with high cadence. In this work, we use TESS data to search for microlensing candidates using both traditional and machine-learning methods and to identify associated false positives in high-cadence surveys. Microlensify is a physics-informed, transformer-based variational autoencoder trained on simulated single-lens microlensing light curves and real TESS Sector 12 data. The model classifies events, reconstructs light curves, and estimates microlensing event durations. Applied to $\sim 5.6$ million TESS light curves, it identified between $0.036\%$ and $1.89\%$ as microlensing candidates across different TESS pipelines. After applying microlensing detection metrics and cross-matching with SIMBAD, we obtained a final list of candidates and identified false positives including long-period variables, Mira variables, cataclysmic variables, red giants, and transients. We also found Gaussian-like peaks caused by asteroid crossings, a potential source of false positives in high-cadence microlensing surveys. The model also predicts event duration with an accuracy of $R^2 = 0.97$. The model was further tested on published events from different ground-based microlensing surveys, confirming 92.7% as microlensing, demonstrating its applicability across surveys with different cadences.
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All authors Atousa Kalantari, Somayeh Khakpash, Sedighe Sajadian, Hosein Haghi, Willow Fox Fortino, Rosanne Di Stefano

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

Shaping SHAPE - A spectro-polarimeter onboard Chandrayaan-3 to observe Earth as an Exoplanet

Anuj Nandi, Swapnil Singh, Bhavesh Jaiswal et al. (+33 more)

Spectro-polarimetry of HAbitable Planet Earth (SHAPE) is an experimental instrument onboard the Propulsion Module (Orbiter) of the Chandrayaan-3 mission, designed to perform disc-integrated spectro-polarimetric observations of Earth from lunar and highly elliptical Earth orbits. SHAPE is a compact, lightweight spectro-polarimeter comprising three subsystems: the Electro-Optical Detector System (EODS)-Optics, EODS-Electronics, and Radio Frequency Source (RFS). An Acousto-Optic Tunable Filter (AOTF), driven by an in-house-developed 80$-$135 MHz RF source, provides spectral filtering in the near-infrared (NIR) wavelength range of 1.0$-$1.7 $\mu$m and produces two narrow-band beams with mutually perpendicular linear polarization states. The instrument optics, with a field of view of approximately 2.6{\deg}, focus the two beams onto InGaAs detectors. A spectral resolution of 2$-$4 nm is achieved using in-house-designed low-noise front-end electronics. The instrument also incorporates processing and power electronics for signal processing, detector biasing, and subsystem control. We present the overall instrument design, results from pre-launch ground-based testing, and in-orbit operational performance. The current configuration enables SHAPE to measure disc-integrated signatures of Earth over a range of phase angles, providing a test bed for characterizing Earth-like exoplanets and benchmarking future exoplanet observations.
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All authors Anuj Nandi, Swapnil Singh, Bhavesh Jaiswal, Anand Jain, Smrati Verma, Reenu Palawat, Ravishankar B. T., Brajpal Singh, Priyanka Das, Supratik Bose, Supriya Verma, Waghmare Rahul Gautam, Yogesh Prasad K. R., Bijoy Raha, Bhavesh Mendhekar, Sathyanaryana Raju K., Srinivasa Rao Kondapi V., Sumit Kumar, Mukund Kumar Thakur, Vinti Bhatia, Nidhi Sharma, Govinda Rao Yenni, Neeraj Kumar Satya, Venkata Raghavendra, Vivechana M. S., Evangelin Leeja Justin, Praloy Karmakar, Naga Manjusha J., Motamarri Srikanth, Abhishek Singh, Prabakaran B., Honey Gupta, Priyanka Mishra, Chinmay Kumar Rajhans, Kalpana K., Veeramuthuvel P

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

Early Planet Formation in Embedded Disks (eDisk). XXIV: Systematic Investigation of Disk Structures based on Visibility Analysis

Mayank Narang, Jerry Xu, Leslie W. Looney et al. (+11 more)

The dust continuum emission from young protostellar disks encodes key information about their mass distribution and early evolution, yet uniform high-resolution comparative studies remain limited. We present a systematic uv-plane analysis of parametric intensity models applied to ALMA Band-6 (1.3 mm) observations of 23 disks (19 protostellar systems with 4 being in binary) from the eDisk sample, spanning Gaussian profiles to power-law cores with exponential tails (PLCT), including asymmetric extensions. Gaussian models generally fail to reproduce the centrally peaked emission and extended outer structure observed in most disks, whereas the PLCT framework provides a significantly improved description of radial brightness profiles. Incorporating azimuthal asymmetries further reduces residuals in 15 of 17 inclined disks, indicating that departures from axisymmetry are common at early stages. Only two disks, L1489 IRS and Oph IRS63, exhibit clear gap and ring substructures, while most appear smooth at the spatial resolution and sensitivity of our observations. These systems are among the most evolved in the sample, and the absence of flat-spectrum sources limits the evolutionary range probed, {suggesting that the detection of prominent gaps and rings is not common} in the earliest phases of disk evolution. Using a uniform definition of disk radius based on the 95% enclosed flux, we find a positive correlation with stellar mass, $R_{\rm disk} \propto M_{\star}^{1.5 \pm 0.1}$, with disks in binary systems systematically smaller than those around isolated protostars. While the models capture overall morphology and large-scale asymmetries, distinguishing intrinsic structures from radiative transfer effects in optically thick regions remains challenging.
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All authors Mayank Narang, Jerry Xu, Leslie W. Looney, Nagayoshi Ohashi, Anika Khandavalli, Patrick Sheehan, Jonathan P. Williams, Shigehisa Takakuwa, Jes K. Jørgensen, Ilseung Han, Woojin Kwon, Zhi-Yun Li, Nguyen Thi Phuong, John J. Tobin

Earth and Planetary Astrophysics

Substructure evolution from protoplanetary to debris disks driven by mutually gravitating planetesimals and implications on Kepler resonances and free-floating planets

Yinuo Han, Konstantin Batygin and Fei Dai

Motivated by recent observations suggesting rings found in debris disks are wider than those in protoplanetary disks, we consider a picture in which planetesimals formed in radially narrow dust traps radially diffuse into debris rings via mutual scattering. Under this picture, evolving the fractional widths ($\Delta r/r$) of resolved debris rings back to a few Myr according to the theoretical $t^{1/5}$ evolutionary trajectory reproduces the protoplanetary ring distribution. We inferred the product of the ring mass and individual planetesimal mass required to reach the observed debris ring widths at their ages, finding that $M_\mathrm{disk}\times m$ ranges from $10^{-3}$ to $10^{3} \, M_\oplus^2$. The distribution of $M_\mathrm{disk} \times m$ appears to correlate with the stellar mass, peaking at 1.5 to 2 $M_\odot$, which resembles the stellar mass dependence of the giant exoplanet occurrence rate. The population of resolved debris rings lie close to the $\Delta r / r = 10 \, h$ equipartition relation expected of a planetesimal ring that formed narrow, with typical resolved debris disks still expected to be broadening radially and vertically at present. If sufficiently massive ($\sim$10 $M_\oplus$), this radial broadening can send a few Mercurys to the terrestrial region within 10 Myr, making debris disks a plausible source of planetesimals disrupting resonant chains among Kepler planets. Within Gyr timescales, outer planetesimal belts can also eject $\sim$1% of their mass into interstellar space if they consist of Moon-sized bodies or above, suggesting that the slow and steady intrinsic evolution of massive debris disks could contribute to the interstellar free-floating population of terrestrial-planet-sized bodies.
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Earth and Planetary Astrophysics

The Effects of M Star Age Dependent Ultraviolet Emission on Detecting and Interpreting Exoplanet Biosignatures

C. Evan Davis, Victoria S. Meadows, Evgenya L. Shkolnik et al. (+5 more)

Given their abundance and observational advantages, M stars will arguably be the best candidates for characterizing and searching for biosignatures on terrestrial exoplanets in the near future. However, photochemistry that can suppress or enhance key biosignature molecules in planetary atmospheres is primarily driven by UV flux from the host M star, which is influenced by stellar activity that decreases with age. Here, we simulate Pre-Industrial Earth-like and Archean Earth-like atmospheres around M4 and M8 stars from 650 Myr to 5 Gyr old. We find that our Pre-Industrial Earth atmospheres around 5 Gyr M stars have up to ten times more CH$_4$ than those around 650 Myr M stars, producing 68% stronger methane bands in NIR transit spectroscopy. Additionally, photochemical shielding from O$_2$ in our Pre-Industrial Earth atmospheres reduces the impact UV-driven photochemistry on composition, while the Archean Earth exhibits larger compositional changes due to weaker shielding from CO$_2$. Lastly, enhanced CO$_2$ photolysis, driven by the strong net UV flux and high Far/Near-UV ratios of 650 Myr and 1 Gyr M stars, cause our Archean Earth-like planets to produce up to 5.4 dex more O$_3$ than when around 5 Gyr M stars. The excess O$_3$ causes the Archean Earth to become half as reflective in the 0.2-0.3 $\mathrm{\mu}$m Hartley band feature in ultraviolet reflectance spectroscopy, which the Habitable Worlds Observatory may be sensitive to. Without the context of the star's real-time, age-dependent UV radiation, this O$_3$ feature could be misinterpreted as a proxy for low, biogenic O$_2$.
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All authors C. Evan Davis, Victoria S. Meadows, Evgenya L. Shkolnik, Andrew P. Lincowski, Sarah Peacock, R. O. Parke Loyd, Adam C. Schneider, Travis Barman

Earth and Planetary Astrophysics

No Persistent Helium Absorption in LHS 1140 b: Four JWST/NIRISS SOSS Transits and Multi-epoch Stellar He I Variability

Amélie Gressier, Charles Cadieux, René Doyon et al. (+4 more)

The search for atmospheres on temperate terrestrial planets is important for understanding how these objects form and evolve, and their potential habitability. Recent transit observations of LHS 1140 b, a temperate ($T_{\rm eq}=226$ K) planet straddling the radius valley ($R_{\rm p}\approx1.7 R_\oplus$), with the WINERED high-resolution spectrograph yielded a detection of planetary atmospheric escape through the measurement of excess absorption ($1.24\pm0.23\%$) in the metastable helium triplet, although a later second visit resulted in a non-detection. We analyze four transits of LHS 1140 b and two of LHS 1140 c observed with JWST/NIRISS SOSS. We find no evidence of helium absorption in either planet, with all amplitudes consistent with zero within $1\sigma$. For LHS 1140 b, we derive $3\sigma$ upper limits of $0.72$–$1.21\%$. Individual visits disfavor the reported WINERED absorption at $2.6$–$3.7\sigma$, while their joint constraint disfavors a persistent signal at $4.5\sigma$. We find that the stellar He I line of LHS 1140 varies substantially between NIRPS and WINERED epochs, with both its strength and fractional variability consistent with the behavior of other M dwarfs. We also identify a moderate correlation between the 2024 He I depth and seeing, suggesting a possible seeing-dependent instrumental contribution. If the high-resolution detection is indeed planetary, the rate of such atmospheric loss events must be relatively low ($f=22_{-12}^{+17}\%$). Alternatively, stellar He I variability may contribute to the reported excess absorption. Additional high-resolution observations, both in and out of transit, are required to distinguish between these scenarios.
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All authors Amélie Gressier, Charles Cadieux, René Doyon, Louis-Philippe Coulombe, Romain Allart, Étienne Artigau, François Bouchy

Earth and Planetary Astrophysics

Wet Removal and Cloud Enhancement: The Microphysics of Cloud-Haze Interactions on Sub-Neptunes

Vighnesh Nagpal, Maria Steinrueck, Diana Powell et al. (+3 more)

Aerosols are a near-ubiquitous feature of sub-Neptune atmospheres, yet their microphysical nature remains poorly understood. Both condensate clouds and photochemical hazes have been proposed to explain observations, but have largely been studied in isolation. Here we present a new bin-scheme microphysical model, adapted from CARMA, that couples cloud and haze formation through heterogeneous nucleation - the dominant mode of cloud formation in the Solar System - in which haze particles act as cloud condensation nuclei (CCN). Applying this model to KCl clouds on GJ 1214 b-like warm sub-Neptunes, we find that the microphysical contact angle $\theta$ between cloud and haze particles governs distinct regimes of aerosol behavior: at moderate contact angles ($25^\circ \lesssim \theta \lesssim 70^\circ$), hazes are efficiently removed from the upper atmosphere through "wet removal" as they seed gravitationally-settling clouds; at small contact angles ($\theta \lesssim 25^\circ$), heterogeneous nucleation instead produces an enhanced population of mixed cloud-haze particles at high altitudes, dramatically increasing aerosol optical depth ("cloud enhancement"). These structural changes produce differences of up to four scale heights in transmission spectra, with strong effects at optical and near-infrared wavelengths relevant to JWST NIRISS/SOSS, while wavelengths beyond about 3 microns remain comparatively unaffected. We map these effects across orders of magnitude in metallicity, haze production rate, and vertical mixing strength, establishing their generality across sub-Neptune parameter space. Because heterogeneous nucleation is a universal phase-change process, this framework extends naturally to other exoplanet atmospheres and potentially any astrophysical environments where condensation onto foreign substrates may occur, including protoplanetary disks and stellar outflows.
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All authors Vighnesh Nagpal, Maria Steinrueck, Diana Powell, Peter Gao, Dominic Samra, Wolf Cukier

Earth and Planetary Astrophysics

A Search for helium in the atmospheres of three sub-Neptunes and a super-Earth around M-dwarfs

Kiyoe Kawauchi, Norio Narita, Yuichi Ito et al. (+26 more)

Thousands of sub-Neptunes have been discovered mainly through space-based surveys such as Kepler and TESS. Their bulk compositions and internal structures are thought to reflect their formation and evolutionary pathways, and atmospheric observations provide constraints on these processes. The near-infrared helium triplet is a potential tracer of extended, escaping H/He atmospheres. Recent models that include geometric effects suggest that planets orbiting nearby late M dwarfs may offer favorable conditions for detecting this signal. Nevertheless, helium has been reported for only three planets around M dwarfs to date. We conducted high-resolution transmission spectroscopy of three sub-Neptunes (TOI-2136b, TOI-654b, and LP 791-18c) and a super-Earth (TOI-1634b) orbiting M dwarfs with the InfraRed Doppler (IRD) spectrograph on the Subaru Telescope. We find no statistically significant helium absorption in any target; accordingly, we derive 95% confidence upper limits on the helium line depth of 1.36%, 0.60%, 2.07%, and 3.00%, and on the equivalent width of 7.3, 2.1, 7.4, and 9.1 mÅ, for TOI-2136b, TOI-1634b, TOI-654b, and LP 791-18c, respectively. We further explored constraints on the upper-atmospheric temperature and mass-loss rate by comparing these results with isothermal Parker-wind models. While we have compared with self-consistent ATES models of primordial H/He atmospheres spanning a range of assumed X-ray luminosities, changes in the assumed XUV flux do not appear to account for the non-detections. The results suggest that these planets have metal-enriched H/He primary atmospheres or non-primordial atmospheres, such as water-rich envelopes. Future observations of other absorption lines, such as Lyman-$\alpha$, H-$\alpha$, and H$_2$O, may provide further constraints on these atmospheres.
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All authors Kiyoe Kawauchi, Norio Narita, Yuichi Ito, Akifumi Nakayama, Teruyuki Hirano, Masayuki Kuzuhara, Mayuko Mori, Izuru Fukuda, Akihiko Fukui, Yuya Hayashi, Yasunori Hori, Masahiro Ikoma, Kai Ikuta, Taiki Kagetani, Yugo Kawai, Tadahiro Kimura, Vigneshwaran Krishnamurthy, Huan-Yu Teng, Noriharu Watanabe, Yujie Zou, Hiroki Harakawa, Klaus Hodapp, Tomoyuki Kudo, Takashi Kurokawa, Jun Nishikawa, Masashi Omiya, Motohide Tamura, Akitoshi Ueda, Sébastien Vievard

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

Asgard/NOTT: Status of laboratory nulling performance

T. Mattheussen, R. Laugier, D. Defrère et al. (+26 more)

Nulling interferometry enables the direct detection of faint companions and circumstellar structures at angular separations unresolvable by classical, diffraction-limited imagers, whilst dramatically improving the measurable contrast. The Asgard/NOTT nulling instrument aims to achieve a contrast performance of 10^-5 in the L' wavelength band (3.5 - 4.0 {\mu}m), enabling observation and characterization of young giant exoplanets near the snowline and hot exozodiacal dust. Previous studies have verified the nulling capabilities, of the chip in ambient conditions and of the test bench in cryogenic conditions. This work aims to add the first ambient performance assessment of the test bench with spectrally dispersed light. Necessary revisions are made to the data acquisition and calibration pipeline and fringe scans are carried out, modeled and fitted. The splitting ratios of the 4-telescope nulling beam combiner, a photonic Gallium Lanthanum Sulfide (GLS) chip, are moreover characterized on the bench, showing tentative agreement with previous chip characterization. The null performance has worsened, the achieved contrast of ~ 10^-1 being one order of magnitude higher than what earlier characterized performance showed. Multiple future changes to the test bed and to the approach taken promise an improved characterization of performance. In particular, the input beam intensities will be deliberately mismatched to account for the imbalanced splitting ratios of the directional couplers. With the installation of the final cryostat and camera, the developed tools will be leveraged to re-assess the performance in ambient and cryogenic conditions.
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All authors T. Mattheussen, R. Laugier, D. Defrère, G. Garreau, P. Chingaipe, G. Raskin, M. Salman, K. Missiaen, B. Vandenbussche, W. Verstraeten, J. Morren, A. Kuriakose, M. Ireland, M.-A. Martinod, A. Sanny, S. Gross, L. Labadie, A. Bigioli, A. Jóo, S. Madden, A. Mazzoli, G. Medgyesi, S. Ertel, T. A. Stuber, J. P. Scott, S. Kraus, F. Martinache, X. Haubois, N. Schuhler

Earth and Planetary Astrophysics

A metallicity sweet spot for disc fragmentation and planet formation

Ethan J. Carter, Dimitris Stamatellos, George Blaylock-Squibbs et al. (+2 more)

Fragmentation of gravitationally unstable discs offers an alternate formation mechanism for gas giant planets and brown dwarfs on wide orbits. Metallicity plays a key role in disc evolution from the onset of gravitational instability to the formation of planets. We aim to determine the effect of metallicity on disc fragmentation and on the properties of disc-instability planets. We model gravitationally unstable discs with varying metallicity ($0.01-10 \,\rm Z_{\odot}$) using the Smoothed Particle Hydrodynamics code PHANTOM. Our simulations reveal a "sweet spot" for fragmentation at $0.3 \,\rm Z_{\odot}$, where cooling is most efficient, with fragmentation also happening less vigorously at higher and lower metallicities. However, further away from the sweet spot, fragmentation becomes more difficult and is eventually suppressed at extreme low and high metallicities ($0.01 \,\rm Z_{\odot}$ and $10 \,\rm Z_{\odot}$, respectively), where the disc cools inefficiently. Discs with metallicities close to the sweet spot form more planets per disc, faster, and with lower initial masses than fragmenting discs with higher or lower metallicities. Our results may explain the slight overabundance of wide-orbit giant planets observed around metal-poor stars; these planets may have formed via disc fragmentation.
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All authors Ethan J. Carter, Dimitris Stamatellos, George Blaylock-Squibbs, Alison Young, Ken Rice

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

The Birth of Radar Meteor Astronomy at Jodrell Bank: The Collaboration between Bernard Lovell and Manning Prentice

Jeremy Shears

The discovery of radar echoes from meteor trains at Jodrell Bank in December 1945 heralded the beginning of a new era in meteor astronomy. This paper examines the early collaboration between Bernard Lovell, whose expertise in radar and physics transformed wartime technology into a new astronomical technique, and J. P. Manning Prentice, Director of the British Astronomical Association Meteor Section, whose knowledge of visual meteor observation proved essential to interpreting the discoveries. Drawing on previously unpublished correspondence from the Lovell Archive at the University of Manchester, the paper traces the development of their partnership, including the pioneering 1946 Perseid and Giacobinid observations, the contribution of BAA observers, and the establishment of radar meteor research at Jodrell Bank. It reveals how complementary professional and amateur expertise combined to create a new field of astronomy and a lasting scientific friendship.
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Earth and Planetary Astrophysics

Looking for TRAPPIST-1 external planets' emission in JWST archival data

Louis-Julien Cartigny, Alice Maurel, Elsa Ducrot et al. (+1 more)

The TRAPPIST-1 system has been thoroughly observed with JWST. Unfortunately, stellar contamination issues strongly limit the interpretation of transit observations. As for emission observations, only the two closest planets have been observed through five dedicated JWST programs. We gathered all these emission observations and tried to detect the combined emission of the external planets in previous JWST MIRI observations of TRAPPIST-1; this paper presents our approach and our results. We could not achieve sufficient precision to detect the thermal emission of the outer planets and to discriminate between an all-bare rocks and an all-atmospheres scenario. However, we show that a~60 hours continuous observations at a specific phase range, when the gradient of thermal flux from the outer planets is maximum, could allow us to achieve this goal.
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All authors Louis-Julien Cartigny, Alice Maurel, Elsa Ducrot, Taylor J. Bell

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

Investigating the properties of nearby young moving groups using GaiaDR3

Mamta Lad, Ujjwal Krishnan, Blesson Mathew et al. (+4 more)

Moving groups, which are gravitationally unbound collections of stars spread over large portions of the sky, pose challenges to their identification. Analyzing the three-dimensional spatial motion of stars is one method to identify their members. Despite several studies in the past, a reliable and conclusive catalog of moving group stars is currently lacking. Our objective is to present a most recent and updated catalog of nearby young moving group (NYMG) candidates in the solar neighborhood and investigate their properties using updated science data from the Gaia Data Release 3 (DR3). We searched for candidates of twelve NYMGs within a distance of 150 pc using Gaia DR3. To determine the membership, we employed the Bayesian algorithm BANYAN-Sigma. We compiled a total list of 3,153 NYMG candidates, of which 1,651 were new candidates. We also assessed the credibility of the literature defined 'Good-box' criterion with the latest comprehensive catalog of NYMGs from our study. We homogeneously estimated the ages of the NYMG candidates using Gaia DR3 photometry data. The analysis revealed that our NYMG candidates display a large scatter on the CMDs resulting in a large range of estimated ages from isochrones. Additionally, we conducted an infrared (IR) excess analysis to identify disk candidates among our final sample. Our spectral energy distribution (SED) analysis found 51 stars with IR excess. We present a largely inclusive list of all the NYMG candidates within 150 pc of Solar neighbourhood using Gaia DR3. The wide range of ages obtained from isochrone fitting underscores the need for more robust age analysis techniques to accurately determine the ages of NYMG members. The presence of IR excess in the 51 stars confirms the existence of disks indicating that they could be potential candidates for exoplanet detections.
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All authors Mamta Lad, Ujjwal Krishnan, Blesson Mathew, Raksha Olakal, Arun Roy, Sreeja Kartha, Vineeth Valsan

Earth and Planetary Astrophysics

Stellar Abundances as Probes of Rocky Exoplanet Interiors: The Mantle Composition and Mineralogy of GJ 486b

Harshit Krishna, Asmit Gupta, Sneha Bhowmik et al. (+3 more)

Over the past three decades, hundreds of rocky exoplanets have been discovered. Some of these show atmospheric signatures indicative of diverse chemical compositions. Interpreting these atmospheres requires a physically grounded understanding of planetary interiors, as interior composition and mineralogy govern the formation and evolution of secondary atmospheres. Rocky terrestrial exoplanets are expected to inherit the refractory composition of their host stars, providing a direct pathway to constrain their bulk composition and mineralogy. However, a large fraction of these planets orbit M-type stars, whose compositions remain poorly constrained because of limited and uncertain stellar abundance measurements. Here, we present a physically consistent framework that connects stellar abundances to the interior structure and mineralogy of rocky exoplanets by combining stellar abundance inference, devolatilization modeling to estimate bulk and mantle elemental abundances, interior structure calculations to derive pressure-temperature profiles, and thermodynamic equilibrium modeling of mantle mineralogy. We first benchmark the framework against Earth and then apply it to the super-Earth GJ 486b using chemically consistent abundances derived from ensembles of similar M-dwarf hosts. We find that GJ 486b likely hosts an iron-rich and silica-poor mantle relative to Earth while preserving the major mantle phase transitions. Sensitivity analyses show that the overall mineralogical structure is robust to variations in bulk composition and pressure-temperature profiles, with temperature primarily modulating phase proportions near key transitions. Finally, our results show that stellar-abundance inference combined with devolatilization models constrains the interior composition of rocky exoplanets and provides a foundation for linking planetary interiors to atmospheric characterization.
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All authors Harshit Krishna, Asmit Gupta, Sneha Bhowmik, Chandan K. Sahu, Sudipta Mridha, Liton Majumdar

Tuesday 18 August

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

Evolution of lunar wake potentials: structure, energy conversion, and their imprints on velocity distributions

Xin An, Vassilis Angelopoulos, Jasper S. Halekas et al. (+4 more)

We study the evolution of electric potentials in the lunar wake. The wake potential exhibits two distinct spatial scales. The macroscopic scale arises from solar wind expansion into the vacuum, with a potential length-scale growing with distance from the Moon; the microscopic scales arises from ion acoustic shocks near the wake center, with transition layers spanning tens of local Debye lengths. This two-scale potential mediates energy conversion between ions and electrons during wake refilling. The macroscale potential retards electrons and accelerates ions to supersonic velocities, converting electron thermal energy to ion kinetic energy. The microscale potential then decelerates ions to subsonic velocities and heats both species, converting ion kinetic energy back to thermal energy. Together, the two-scale potential imprints distinct signatures on velocity distributions, including ion beams and electron flat-top distributions, consistent with ARTEMIS observations.
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All authors Xin An, Vassilis Angelopoulos, Jasper S. Halekas, Terry Z. Liu, Shaosui Xu, Andrew R. Poppe, Ferdinand Plaschke

Earth and Planetary Astrophysics

The Inbound Gas and Dust Evolution of Interstellar Comet 3I/ATLAS from Optical Spectroscopy and Multi-band Photometry

Toni Santana-Ros, Alexey Sergeyev, Oleksandra Ivanova et al. (+9 more)

Interstellar objects (ISOs) provide direct constraints on the composition and evolution of extrasolar planetary systems. As the third confirmed ISO, comet 3I/ATLAS offers a rare opportunity to study pristine material from another star system. We present a systematic multi-band monitoring campaign conducted from 2025 July to September. To minimize systematic uncertainties, we utilized a homogeneous dataset of SDSS $g', r', i', z_s$ and Johnson-Cousins $BVR$ imaging obtained with MuSCAT3/4 on the 2.0-m Faulkes Telescopes and the 1.5-m Maidanak telescope. We employed a novel multi-aperture technique providing a robust, distance-independent characterization of the coma, alongside optical spectroscopy from SALT and the Nordic Optical Telescope. 3I/ATLAS exhibited a stable reddish color with no significant secular variation during its inbound journey. Spectroscopic analysis reveals carbon-chain depletion, with CN as the only prominent molecular emission. Coma morphology, analyzed via azimuthal averaging and rotational gradient filters, shows a persistent sunward jet at a position angle of $\sim$280$^{\circ}$, indicating sustained and localized outgassing from a high-latitude active region. Light-scattering modeling demonstrates that the observed colors and polarization are consistent with dense aggregates of organic matter and silicate minerals, while water ice sublimation remained insufficient to alter the optical properties. The stability of the coma structures and the homogeneous nature of our dataset indicate that 3I/ATLAS underwent a steady activation phase, preserving a volatile-rich but carbon-chain-poor composition. This characterization points to an object formed in a distinct protoplanetary environment and provides a definitive record of its pre-perihelion evolution.
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All authors Toni Santana-Ros, Alexey Sergeyev, Oleksandra Ivanova, Sofiia Mykhailova, Johannes Markkanen, Igor Luk'yanyk, Tomasz Kwiatkowski, Dagmara Oszkiewicz, Antti Penttilä, Nicolas Erasmus, Kamoliddin Ergashev, Yurij Krugly

Earth and Planetary Astrophysics

The Tidal Venus Phenomenon: Demographics and Case Studies

Stephen R. Kane and Emma L. Miles

The demographics of terrestrial planets and their orbits reveal a vast diversity in overall planetary energy budgets. Terrestrial exoplanets in short-period or eccentric orbits can experience intense tidal heating that, combined with stellar irradiation, may trigger runaway greenhouse conditions analogous to Venus. We calculate tidal heating rates for 143 terrestrial-sized exoplanets with measured eccentricities and find that, under adopted archive default eccentricities and constant-$Q$ assumptions, 70% exceed the extreme volcanism threshold in tidal flux and 96% exceed the runaway greenhouse limit in total zero-albedo flux. We develop a three-category taxonomy of tidal influence on climate: flux-driven Venus analogs, tidally dominated planets, and historically compromised Habitable Zone (HZ) worlds, and apply this framework to five case studies. TOI-6716 b and TOI-912 b may have exceptionally high tidal fluxes, potentially serving as examples where tidal dissipation alone causes them to exceed the runaway greenhouse threshold. TOI-700 d and LHS 1140 b, though currently below the threshold, were exposed to above-threshold stellar irradiation during their host stars' $\sim$0.5–3~Gyr pre-main-sequence phases; whether their atmospheres survived is testable with JWST. GJ 12 b, already above the threshold from stellar flux alone, experiences a tidal heat flux of $\sim$5~W/m$^2$ (comparable to Io) that drives an independent volcanic pathway to a runaway greenhouse. Three-dimensional climate simulations show that a temperate atmosphere for GJ 12 b fails to achieve radiative balance, while a Venus-like CO$_2$-dominated atmosphere converges to a stable state. We consider observational prospects for these systems and connections to forthcoming Venus in-situ missions.
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Earth and Planetary Astrophysics · Cross-listed from astro-ph.GA

Accretion of AGN Stars under Influence of Disk Geometry II: The Adiabatic Regime and Runaway Collapse Induced by Self-gravity

Yi-Xian Chen, Yan-Fei Jiang and Jeremy Goodman

Accretion onto massive stars embedded in Active Galactic Nuclei (AGN) disks around supermassive black holes (SMBHs) is regulated to the stellar Eddington rate in the fast-diffusion or radiatively-efficient limit, $c/\tau > c_s$, where $\tau$ is the optical depth of the accretion flow and $c_s$ the sound speed. However, when the ambient density is sufficiently high, the opposite slow-diffusion limit applies. In this regime, accretion proceeds quasi-adiabatically and forms a hydrostatic circumstellar envelope (CSE) that stalls further mass inflow in the absence of self-gravity. We perform 3D hydrodynamic simulations in the adiabatic limit to investigate the structure and evolution of such envelopes. For low thermal mass ratios, $q_{\rm th} \equiv M_\star/M_{\rm th}$ where $M_{\rm th}=c_s^3/(G\Omega)$ is the thermal mass, the CSE boundary smoothly matches the ambient disk entropy and density without forming a shock. In contrast, when $q_{\rm th} \gg 1$, a strong shock develops at the envelope boundary, substantially increasing the entropy of the envelope and thereby regulating its structure and mass, $M_{\rm env}$. In marginally self-gravitating disks with Toomre parameter $Q \sim 1$, we find that at sufficiently large $q_{\rm th}$ the envelope mass satisfies $M_{\rm env}/M_\star \gtrsim 1$. This condition is equivalent to stating that the post-shock material entering the envelope possesses lower radiation entropy than the characteristic stellar value, which triggers dynamical runaway growth on a dynamical timescale once envelope self-gravity is included in our simulations. In realistic AGN disk environments with SMBH mass $\sim 10^8M_\odot$, runaway may occur close to the minimum self-gravitating radii and produce supermassive stars of $\sim 10^5M_\odot$.
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Earth and Planetary Astrophysics · Cross-listed from astro-ph.IM

Radiation damage to the Hubble Space Telescope has been several years out of phase with the Solar cycle

Gavin Leroy, Juan Paolo Lorenzo Gerardo Barrios, Maximilian von Wietersheim-Kramsta et al. (+8 more)

As well as obtaining beautiful images of the Universe, the Hubble Space Telescope's CCD detectors are sensitive radiation dosimeters that have been monitored in Low Earth Orbit for more than 24 years. The rate of radiation damage they received has varied over each Solar cycle, but several years out of phase with the appearance of sunspots or coronal mass ejections. We investigate functional forms that successfully fit the time series of damage to telescopes elsewhere in the Solar system. We obtain remarkably accurate fits to Hubble data but with physically absurd parameter values. During image post-processing, such fits can be used empirically, to correct more than 99.5% of the radiation damage's effect on image quality. However, fits to the time series with physically reasonable parameters produce worse performance. Our results highlight the diversity of radiation environments in different parts of our Solar system, and the complexity of Low Earth Orbit in particular. Our results also motivate continued monitoring of radiation damage to currently operational spacecraft, to more reliably predict the rate of degradation in (and useful lifespan of) future missions.
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All authors Gavin Leroy, Juan Paolo Lorenzo Gerardo Barrios, Maximilian von Wietersheim-Kramsta, Richard Massey, Richard G. Hayes, Jacob A. Kegerreis, David Lagattuta, Zane D. Lentz, James W. Nightingale, Jesper Skottfelt, Felix Vecchi

2608.18214 · 18 Aug · Paper presented at SPIE Astronomical Telescopes + Instrumentation 2026. Accompanying poster can be found at https://spie.org/astronomical-telescopes-instrumentation/presentation/Radiation-damage-to-the-Hubble-Space-Telescope-has-been-several/14145-131

Abstract PDF $TSCP ×3

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

The Roman Coronagraph Community Participation Program: data reduction pipeline astrometric calibration

Amanda Chavez, Jason Wang, Vanessa P. Bailey et al. (+5 more)

The Nancy Grace Roman Space Telescope will be equipped with a Technology Demonstration Coronagraph Instrument that will push the current limits of high contrast imaging for exoplanets ($10^{-9}$ contrast). The Roman Coronagraph Community Participation Program has developed corgidrp, a python-based data reduction pipeline for the Roman Coronagraph Instrument that will perform essential data processing and calibration steps for coronagraphic observations. The astrometric calibration function within corgidrp allows us to understand the on-sky angular size and distance scale of science observations by characterizing essential detector parameters: boresight, plate scale, north angle, and optical distortion. Measuring these astrometric calibration products not only helps us understand the science output of our data, but it is what allows us to point the Roman coronagraph accurately at our science target in the first place. Here, we describe the techniques used within the astrometric calibration and demonstrate that our algorithm meets Technology Demonstration Threshold Requirements: (1) compute the on-sky location of the center of CGI EXCAM detector to better than 30 [mas] and (2) compute the on-sky position angles of the camera axes to within 0.3 [deg].
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All authors Amanda Chavez, Jason Wang, Vanessa P. Bailey, Maxwell Millar-Blanchaer, Julien Girard, John Krist, Julia Milton, Marie Ygouf

Earth and Planetary Astrophysics

Hydrodynamics modeling of the water snow line in young protoplanetary disks with dust-size-dependent opacities

Eduard I. Vorobyov, Anastasiia Topchieva, Aleksandr Skliarevskii et al. (+2 more)

Aims. We investigated the properties of the water snow line during the early stages of disk evolution, paying particular attention to the effects of gravitational instability and dust growth on the snow line's shape and position. Methods. We used the FEOSAD numerical hydrodynamics code to simulate the disk formation and evolution in the thin-disk limit. The simulations incorporate the coevolution of gas, dust, and volatiles, including dust growth, volatile phase transitions, and dust-size-dependent opacities. Results. The position of the water snow line is highly nonsteady during the considered disk evolution period, first moving outward during the disk build-up and then retreating back as the disk cools. Its form in the disk midplane deviates strongly from a circular shape in the early gravitationally unstable phase of disk evolution. An increase in the amounts of grown dust and water ice as well as in the maximum dust size just beyond the snow line, as is readily observed in one-dimensional viscous disk evolution models, in our hydrodynamic models occurs only after gravitational instability diminishes. Dust-growth-induced opacity changes have a profound effect on the position of the water snow line, shifting it closer to the star by almost a factor of two compared to models that do not take this effect into account. Conclusions. The shape, position, and properties of the water snow line in young, gravitationally unstable disks differ from those of older, axisymmetric disks. Our results highlight the importance of taking into account the dependence of opacity on dust size when studying disk evolution.
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All authors Eduard I. Vorobyov, Anastasiia Topchieva, Aleksandr Skliarevskii, Yaroslav Pavlyuchenkov, Konstanze Zwintz

Earth and Planetary Astrophysics

Triggered Fragmentation in Self-Gravitating Protoplanetary discs: Cooling, Mass Movement and Instability

Pratishtha Rawat, Farzana Meru and Rebecca Nealon

Previous three-dimensional hydrodynamical simulations of gravitationally unstable discs have shown that the formation of a single fragment can trigger the formation of subsequent fragments. This behaviour was attributed to changes in the surface mass density caused by the interaction between the first fragment and the disc material. This study reanalyses those simulations to see if the surface mass density was the sole driver. Our results reveal that both surface mass density and sound speed can contribute to the formation of additional fragments. Beyond the general cooling typical of such discs, the inwards movement of cool material from the outer disc, driven by the formation of the first fragment, can enhance fragmentation in the inner regions. We also identify that interactions between the midplane gas and the cooler upper layers of the disc can facilitate additional cooling. Triggered fragmentation can create a unique planet-formation environment by redistributing material across the disc, potentially leading to chemically distinct fragments from those formed in-situ.
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Earth and Planetary Astrophysics · Cross-listed from astro-ph.IM

Accurately simulating gain and clock-induced charge production in the EMCCD gain register

Kevin J. Ludwick

An electron-multiplying charge-coupled device (EMCCD) is capable of precise detections in low-signal environments, able to detect a single photon through electron multiplication. It has many applications, such as faint-target astronomy, quantum optics, molecule tracing, and others, and it will be used for faint companion detection in the Roman Telescope's coronagraph instrument. In an EMCCD, photons hit the pixels, and photo-electrons are created; these are multiplied via impact ionization as they travel through the gain register from one gain stage to the next. A high gain means a high multiplication factor, and this is achieved through a high voltage difference across a gain stage. If the gain is high enough, the chance of clock-induced charge (CIC) production in the gain register increases. The probability distribution function governing the gain process typically used only accounts for charge multiplication if one or more electrons enter the gain register. I discuss my implementation of the simulation of this effect and its customization in emccd_detect, the EMCCD detector simulator used for the Roman Telescope. In addition, the simulator has been updated to use the exact binomial distribution for EM gain instead of the approximate Gamma distribution usually used in the literature, which is only valid for large counts. I also examine some EMCCD data and show through maximum likelihood estimation with CIC_gain_register that the data conform better to the binomial distribution versus the approximate Gamma/Erlang distribution. The use of the modified distribution would in principle improve the fidelity of Roman's testing and lead to better EMCCD calibration and more accurate signal extraction from a frame.
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Earth and Planetary Astrophysics · Cross-listed from astro-ph.SR

A Catalog of Homogeneously Derived Stellar Parameters for Spectroscopic Survey Stars

Stefan Y. Stefanov, Evelina Zaharieva, Djemma Ruseva et al. (+9 more)

Uniformly derived stellar parameters are vital for exoplanet demographic studies because they directly influence the inferred planetary masses, radii, and bulk densities. This study presents a new homogeneous catalog of physical stellar parameters for 5533 single stars observed by the HARPS, HIRES, and CARMENES radial-velocity (RV) surveys. Stellar parameters are determined using a Bayesian framework with two independent sets of stellar evolutionary models: MIST and PARSEC, using published spectroscopic parameter estimates and catalog values as priors. The resulting stellar effective temperatures, masses, radii, and surface gravities are compared and evaluated for consistency with stellar parameters listed in major exoplanet catalogs. While our estimates show consistency with those listed in external catalogs, we identify method-dependent differences in stellar masses for low-mass and pre-main-sequence stars, as well as G and K giants, where isochrone-based solutions may be affected by age-mass degeneracies. For low-mass stars such as M-dwarfs, the catalog provides masses derived from established mass-luminosity empirical relations, which tend to be more reliable. This catalog provides the largest uniformly derived stellar reference sample for Doppler survey targets and also illustrates the applicability and limitations of stellar-parameter homogenisation methods for future RV exoplanet demographics studies.
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All authors Stefan Y. Stefanov, Evelina Zaharieva, Djemma Ruseva, Milen Minev, Denitza Stoeva, Volker Perdelwitz, Lev Tal-Or, Jerusalem T. Teklu, Elena Vchkova Bebekovska, Desislava Antonova, Vladimir Bozhilov, Trifon Trifonov

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

Astrobiology and the Transformation of Scientific Epistemology

Kristina Šekrst

Astrobiology occupies an unusual position within the philosophy of science. Confronted with the n = 1 problem - having only a single example of life to study - it attempts to investigate life beyond Earth while relying entirely on Earth's biosphere as its reference point, a constraint that creates unique epistemic challenges. Unlike traditional sciences with clear predictive frameworks, astrobiology operates as what we might call a transient science: a discipline functioning without foundational certainties, relying predominantly on abductive reasoning, and confronting hypotheses that may remain untestable for decades. It is, in essence, a science of absence - of evidence, certainty, and analogy - where progress lies in refining conceptual and experimental tools to recognize unfamiliar forms of life. This positions astrobiology alongside emerging fields like artificial intelligence and cognitive science within a broader transformation of how scientific knowledge is constructed when dealing with phenomena that transcend direct empirical access.
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Earth and Planetary Astrophysics

Radar observations of Europa in 2011-2024: New insights into radar scattering properties

Tunhui Xie, Jean-Luc Margot, Sebastiano Padovan et al. (+5 more)

Three of Jupiter's Galilean satellites - Europa, Ganymede, and Callisto - are of particular scientific interest due to their icy shells and suspected subsurface oceans. However, the radar properties of the icy satellites have not been measured since observations in 1987-1991. Because radio waves can penetrate pure ice to considerable depths, radar observations provide a powerful means of characterizing the subsurface properties of the icy shells of these satellites, offering key insights into planetary evolution. We have observed Europa using the Goldstone 3.5-cm Solar System Radar and the Green Bank Telescope (GBT) in 2011-2024 in order to address a longstanding gap in the radar studies of these moons. In this paper, we present the most longitudinally comprehensive set of radar measurements of Europa to date and describe its disk-integrated radar properties. On the basis of monostatic Goldstone data, we find radar albedo values in two circular polarizations of $\hat\sigma_{\rm OC}$ = 0.92 $\pm$ 0.11 and $\hat\sigma_{\rm SC}$ = 1.35 $\pm$ 0.13 (unweighted mean and root-mean-square dispersion), with a circular polarization ratio of $\mu_c$ = 1.44 $\pm$ 0.12 (weighted mean and root-mean-square dispersion). Values obtained bistatically at the GBT are similar. The $\mu_c$ values suggest a leading-vs-trailing side dichotomy in radar scattering properties on Europa. Our results support the existence of the coherent backscatter opposition effect (CBOE), currently the most widely accepted physical mechanism that explains the unusual radar scattering properties of the icy Galilean satellites. Because we observed Europa with a bistatic configuration, we can place a lower bound on the width of Europa's CBOE peak equal to 36 arcsec, which provides an upper bound of 32 m ($\sim$1000 wavelengths) on the penetrating depth of X-band radar waves at Europa.
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All authors Tunhui Xie, Jean-Luc Margot, Sebastiano Padovan, Frank Ghigo, Will Armentrout, Joseph S. Jao, Jon D. Giorgini, Joseph Lazio