arXiv subjects

Bryan Butler

Publications and source records attributed to Bryan Butler.

At least 19 recordsLinked to original sources

arXiv2025

Revealing Callisto's Near Subsurface Thermophysical Properties with ALMA Calibration Data

Thermal images at different wavelengths probe varying subsurface depths of planetary bodies, and therefore can inform us about their compositions, thermophysical properties, and impact histories. We identified six archival observations of Callisto obtained by the Atacama Large Millimeter/submillimeter Array (ALMA) between 2012 July 17 and 2012 November 4 at wavelengths of 0.43-0.47 mm (701.9-641.5 GHz). These wavelengths are shorter than those of nearly all other Callisto ALMA data and are sensitive to subsurface emission at depths (the upper ~cm) between those sounded by millimeter and infrared observations. We estimate the disk-averaged brightness temperature as 133$\pm$15 K, and use a thermophysical mixture model to find that Callisto's thermal emission is best fit by a ~50-50% two-component thermal inertia mixture of $\Gamma_{\text{low}}$~15-40 and $\Gamma_{\text{high}}$~1200-2000 J m$^{-2}$ K$^{-1}$ s$^{-1/2}$, consistent with recent ALMA observations of Callisto at longer wavelengths. Finally, we present several previously unpublished Galileo Photopolarimeter-Radiometer (PPR) observations of Callisto and derive thermal inertia and spectral emissivity maps using the same model. Altogether, these ALMA and PPR maps improve our understanding of the thermal properties and spatial distribution of Callisto's shallow subsurface regolith, and demonstrate the value of ALMA flux density calibrator data for extending frequency coverage of existing science data.

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arXiv2025

A multi-frequency global view of Callisto's thermal properties from ALMA

We present thermal observations of Callisto's leading and trailing hemispheres obtained using the Atacama Large Millimeter/submillimeter Array (ALMA) at 0.87 mm (343 GHz), 1.3 mm (233 GHz), and 3 mm (97 GHz). The angular resolution achieved for these observations ranged from 0.09-0.24 arcseconds, corresponding to ~420-1100 km at Callisto. Global surface properties were derived from the observations using a thermophysical model (de Kleer et al. 2021) constrained by spacecraft data. We find that Callisto's millimeter emissivities are high, with representative values of 0.85-0.97, compared to 0.75-0.85 for Europa and Ganymede at these wavelengths. It is clear that models parameterized by a single thermal inertia are not sufficient to model Callisto's thermal emission, and clearly deviate from the temperature distributions in the data in systematic ways. Rather, more complex models that adopt either two thermal inertia components or that treat electrical skin depth as a free parameter fit the data more accurately than single thermal inertia models. Residuals from the global best-fit models reveal thermal anomalies; in particular, brightness temperatures that are locally 3-5 K colder than surrounding terrain are associated with impact craters. We identify the Valhalla impact basin and a suite of large craters, including Lofn, as key cold anomalies (~3-5 K) and geologic features of interest in these data. These data provide context for Callisto JWST results (Cartwright et al. 2024) as well as the other ALMA Galilean moon observations (de Kleer et al. 2021, Trumbo et al. 2017, 2018, Thelen et al. 2024), and may be useful ground-based context for upcoming Galilean satellite missions (JUICE, Europa Clipper).

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arXiv2025

The Tropical Atmosphere of Jupiter - Shallow Weather, Deep Plumes, and Vortices

Towering storms, swirling clouds, and vortices are the cloud tops manifestation of complex weather systems shaping the atmosphere of Jupiter. We use observations from Juno's MicroWave Radiometer (MWR), the Very Large Array (VLA) and the Hubble Space Telescope (HST) to probe for the first time the depth and impact of weather on Jupiter. We use ammonia, the main source of opacity at radio wavelengths on Jupiter, as the tracer for the weather by fitting ammonia anomalies to the MWR brightness temperature variations. We show that the majority of the weather on Jupiter is confined to regions where the clouds are forming. Both the South Equatorial Belt and the Equatorial Zone have surprisingly shallow weather systems (P < 2 bar), and even in the North Equatorial Belt most of the ammonia variations is above the water condensation level (P ~ 6 bar). This confirms that the water condensation layer plays a crucial role in controlling the dynamics and the weather on Jupiter. However, the shallow nature of the weather cannot explain the deep-seated depletion down to 30 bar that the Juno mission has revealed. We do find three features, however, that extend below the water condensation layer: a vortex in the northern hemisphere reaching down to 30 bar, an ammonia plume down to 20-30 bars, and the signature of precipitation down to 20 bar. This work highlights the interplay of large-scale processes (vortices, plumes) and small-scale processes (storms) are responsible for shaping the atmospheric makeup of Jupiter.

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arXiv2024

Surface properties of the Kalliope-Linus system from ALMA and VLA data

The abundance and distribution of metal in asteroid surfaces can be constrained from thermal emission measurements at radio wavelengths, informing our understanding of planetesimal differentiation processes. We observed the M-type asteroid (22) Kalliope and its moon Linus in thermal emission at 1.3, 9, and 20 mm with the Atacama Large Millimeter/submillimeter Array (ALMA) and the Karl G. Jansky Very Large Array (VLA) over most of Kalliope's rotation period. The 1.3 mm data provide ~30 km resolution on the surface of Kalliope, while both the 1.3 and 9 mm data resolve Linus from Kalliope. We find a thermal inertia for Kalliope of 116$^{+326}_{-91}$ J m$^{-2}$ s$^{-0.5}$ K$^{-1}$ and emissivities of 0.65$\pm$0.02 at 1.3 mm, 0.56$\pm$0.03 at 9 mm, and 0.77$\pm$0.02 at 20 mm. Kalliope's millimeter wavelength emission is suppressed compared to its centimeter wavelength emission, and is also depolarized. We measure emissivities for Linus of 0.73$\pm$0.04 and 0.85$\pm$0.17 at 1.3 and 9 mm respectively, indicating a less metal-rich surface composition for Linus. Spatial variability in Kalliope's emissivity reveals a region in the northern hemisphere with a high dielectric constant, suggestive of enhanced metal content. These results are together consistent with a scenario in which Linus formed from reaggregated ejecta from an impact onto a differentiated Kalliope, leaving Kalliope with a higher surface metal content than Linus, which is distributed heterogeneously across its surface. The low emissivity and lack of polarization suggest a reduced regolith composition where iron is in the form of metallic grains and constitutes ~25% of the surface composition.

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arXiv2024

Observations of Titan's Stratosphere During Northern Summer: Temperatures, CH3CN and CH3D Abundances

Titan's atmospheric composition and dynamical state have previously been studied over numerous epochs by both ground- and space-based facilities. However, stratospheric measurements remain sparse during Titan's northern summer and fall. The lack of seasonal symmetry in observations of Titan's temperature field and chemical abundances raises questions about the nature of the middle atmosphere's meridional circulation and evolution over Titan's 29-yr seasonal cycle that can only be answered through long-term monitoring campaigns. Here, we present maps of Titan's stratospheric temperature, acetonitrile (or methyl cyanide; CH$_3$CN), and monodeuterated methane (CH$_3$D) abundances following Titan's northern summer solstice obtained with Band 9 ($\sim0.43$ mm) ALMA observations. We find that increasing temperatures towards high-southern latitudes, currently in winter, resemble those observed during Titan's northern winter by the Cassini mission. Acetonitrile abundances have changed significantly since previous (sub)millimeter observations, and we find that the species is now highly concentrated at high-southern latitudes. The stratospheric CH$_3$D content is found to range between 4-8 ppm in these observations, and we infer the CH$_4$ abundance to vary between $\sim0.9-1.6\%$ through conversion with previously measured D/H values. A global value of CH$_4=1.15\%$ was retrieved, lending further evidence to the temporal and spatial variability of Titan's stratospheric methane when compared with previous measurements. Additional observations are required to determine the cause and magnitude of stratospheric enhancements in methane during these poorly understood seasons on Titan.

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arXiv2024

Atacama Large Aperture Submillimeter Telescope (AtLAST) Science: Planetary and Cometary Atmospheres

The study of planets and small bodies within our Solar System is fundamental for understanding the formation and evolution the Earth and other planets. Compositional and meteorological studies of the giant planets provide a foundation for understanding the nature of the most commonly observed exoplanets, while spectroscopic observations of the atmospheres of terrestrial planets, moons, and comets provide insights into the past and present-day habitability of planetary environments, and the availability of the chemical ingredients for life. While prior and existing (sub)millimeter observations have led to major advances in these areas, progress is hindered by limitations in the dynamic range, spatial and temporal coverage, as well as sensitivity of existing telescopes and interferometers. Here, we summarize some of the key planetary science use cases that factor into the design of the Atacama Large Aperture Submillimeter Telescope (AtLAST), a proposed 50-m class single dish facility: (1) to more fully characterize planetary wind fields and atmospheric thermal structures, (2) to measure the compositions of icy moon atmospheres and plumes, (3) to obtain detections of new, astrobiologically relevant gases and perform isotopic surveys of comets, and (4) to perform synergistic, temporally-resolved measurements in support of dedicated interplanetary space missions. The improved spatial coverage (several arcminutes), resolution ($\sim1.2''-12''$), bandwidth (several tens of GHz), dynamic range ($\sim10^5$) and sensitivity ($\sim1$ mK km s$^{-1}$) required by these science cases would enable new insights into the chemistry and physics of planetary environments, the origins of prebiotic molecules and the habitability of planetary systems in general.

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arXiv2023

Thermal properties of the leading hemisphere of Callisto inferred from ALMA observations

We present a thermal observation of Callisto's leading hemisphere obtained using the Atacama Large Millimeter/submillimeter Array (ALMA) at 0.87 mm (343 GHz). The angular resolution achieved for this observation was $\sim$$0.16^{\prime\prime}$, which for Callisto at the time of this observation ($D\sim 1.05^{\prime\prime}$) was equivalent to $\sim$6 elements across the surface. Our disk-integrated brightness temperature of 116 $\pm$ 5 K (8.03 $\pm$ 0.40 Jy) is consistent with prior disk-integrated observations. Global surface properties were derived from the observation using a thermophysical model (de Kleer et al. 2021) constrained by spacecraft data. We find that models parameterized by two thermal inertia components more accurately fit the data than single thermal inertia models. Our best-fit global parameters adopt a lower thermal inertia of 15-50 $\text{J}\:\text{m}^{-2}\:\text{K}^{-1}\:\text{s}^{-1/2}$ and a higher thermal inertia component of 1200-2000 $\text{J}\:\text{m}^{-2}\:\text{K}^{-1}\:\text{s}^{-1/2}$, with retrieved millimeter emissivities of 0.89-0.91. We identify several thermally anomalous regions, including spots $\sim$3 K colder than model predictions co-located with the Valhalla impact basin and a complex of craters in the southern hemisphere; this indicates the presence of materials possessing either a higher thermal inertia or a lower emissivity. A warm region confined to the mid-latitudes in these leading hemisphere data may be indicative of regolith property changes due to exogenic sculpting.

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arXiv2023

Evidence of a Polar Cyclone on Uranus from VLA Observations

We present observations of Uranus in northern spring with the VLA from 0.7 cm to 5 cm. These observations reveal details in thermal emission from Uranus' north pole at 10s of bars, including a dark collar near 80N and a bright spot at the polar center. The bright central spot resembles observations of polar emission on Saturn and Neptune at shallower pressures. We constrain the variations in temperature and NH3/H2S abundances which could explain these features. We find that the brightness temperature of the polar spot can be recreated through 5 K temperature gradients and/or 10x depletion of NH3 or H2S vapor between 10-20 bars, both consistent with the presence of a cyclonic polar vortex. The contrast of the polar spot may have increased since 2015, which would suggest seasonal evolution of Uranus' polar circulation at depth.

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arXiv2021

Ganymede's Surface Properties from Millimeter and Infrared Thermal Emission

We present thermal observations of Ganymede from the Atacama Large Millimeter Array (ALMA) in 2016-2019 at a spatial resolution of 300-900 km (0.1-0.2'' angular resolution) and frequencies of 97.5, 233, and 343.5 GHz (wavelengths of 3, 1.3, and 0.87 mm); the observations collectively covered all Ganymede longitudes. We determine the global thermophysical properties using a thermal model that considers subsurface emission and depth- and temperature-dependent thermophysical and dielectric properties, in combination with a retrieval algorithm. The data are sensitive to emission from the upper $\sim$0.5 meter of the surface, and we find a millimeter emissivity of 0.75-0.78 and (sub)surface porosities of 10-40%, corresponding to effective thermal inertias of 400-800 J m^{-2} K^{-1} s^{-1/2}. Combined with past infrared results, as well as modeling presented here of a previously-unpublished Galileo PPR nighttime infrared observation, the multi-wavelength constraints are consistent with a compaction profile whereby the porosity drops from ~85% at the surface to 10{+30/-10}% at depth over a compaction length scale of tens of cm. We present maps of temperature residuals from the best-fit global models which indicate localized variations in thermal surface properties at some (but not all) dark terrains and at impact craters, which appear 5-8 K colder than the model. Equatorial regions are warmer than predicted by the model, in particular near the centers of the leading and trailing hemispheres, while the mid-latitudes (~30-60 degrees) are generally colder than predicted; these trends are suggestive of an exogenic origin.

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arXiv2020

No evidence of phosphine in the atmosphere of Venus by independent analyses

The detection of phosphine (PH3) in the atmosphere of Venus has been recently reported based on millimeter-wave radio observations (Greaves et al. 2020), and its re-analyses (Greaves et al. 2021a/b). In this Matters Arising we perform an independent reanalysis, identifying several issues in the interpretation of the spectroscopic data. As a result, we determine sensitive upper-limits for PH3 in Venus' atmosphere (>75 km, above the cloud decks) that are discrepant with the findings in G2020 and G2021a/b. The measurements target the fundamental first rotational transition of PH3 (J=1-0) at 266.944513 GHz, which was observed with the James Clerk Maxwell Telescope (JCMT) in June 2017 and with the Atacama Large Millimeter/submillimeter Array (ALMA) in March 2019. This line's center is near the SO2 (J=309,21-318,24) transition at 266.943329 GHz (only 1.3 km/s away from the PH3 line) which represents a potential source of contamination. The JCMT and ALMA data, as presented in G2020, are at spectral resolutions comparable to the frequency separation of the two lines. Moreover, the spectral features identified are several km/s in width, and therefore do not permit distinct spectroscopic separation of the candidate spectral lines of PH3 and SO2. We present the radiative transfer modelling we have performed and then discuss the ALMA and JCMT analyses in turn.

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arXiv2020

Tropospheric Composition and Circulation of Uranus with ALMA and the VLA

We present ALMA and VLA spatial maps of the Uranian atmosphere taken between 2015 and 2018 at wavelengths from 1.3 mm to 10 cm, probing pressures from $\sim$1 to $\sim$50 bar at spatial resolutions from 0.1'' to 0.8''. Radiative transfer modeling was performed to determine the physical origin of the brightness variations across Uranus's disk. The radio-dark equator and midlatitudes of the planet (south of $\sim$50$^\circ$ N) are well fit by a deep H$_2$S mixing ratio of $8.7_{-1.5}^{+3.1}\times10^{-4}$ ($37_{-6}^{+13}\times$ Solar) and a deep NH$_3$ mixing ratio of $1.7_{-0.4}^{+0.7}\times10^{-4}$ ($1.4_{-0.3}^{+0.5}\times$ Solar), in good agreement with literature models of Uranus's disk-averaged spectrum. The north polar region is very bright at all frequencies northward of $\sim$50$^\circ$N, which we attribute to strong depletions extending down to the NH$_4$SH layer in both NH$_3$ and H$_2$S relative to the equatorial region; the model is consistent with an NH$_3$ abundance of $4.7_{-1.8}^{+2.1} \times 10^{-7}$ and an H$_2$S abundance of $<$$1.9\times10^{-7}$ between $\sim$20 and $\sim$50 bar. Combining this observed depletion in condensible molecules with methane-sensitive near-infrared observations from the literature suggests large-scale downwelling in the north polar vortex region from $\sim$0.1 to $\sim$50 bar. The highest-resolution maps reveal zonal radio-dark and radio-bright bands at 20$^\circ$S, 0$^\circ$, and 20$^\circ$N, as well as zonal banding within the north polar region. The difference in brightness is a factor of $\sim$10 less pronounced in these bands than the difference between the north pole and equator, and additional observations are required to determine the temperature, composition and vertical extent of these features.

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arXiv2019

First ALMA Millimeter Wavelength Maps of Jupiter, with a Multi-Wavelength Study of Convection

We obtained the first maps of Jupiter at 1-3 mm wavelength with the Atacama Large Millimeter/Submillimeter Array (ALMA) on 3-5 January 2017, just days after an energetic eruption at 16.5S jovigraphic latitude had been reported by the amateur community, and about 2-3 months after the detection of similarly energetic eruptions in the northern hemisphere, at 22.2-23.0N. Our observations, probing below the ammonia cloud deck, show that the erupting plumes in the SEB bring up ammonia gas from the deep atmosphere. While models of plume eruptions that are triggered at the water condensation level explain data taken at uv-visible and mid-infrared wavelengths, our ALMA observations provide a crucial, hitherto missing, link in the moist convection theory by showing that ammonia gas from the deep atmosphere is indeed brought up in these plumes. Contemporaneous HST data show that the plumes reach altitudes as high as the tropopause. We suggest that the plumes at 22.2-23.0N also rise up well above the ammonia cloud deck, and that descending air may dry the neighboring belts even more than in quiescent times, which would explain our observations in the north.

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arXiv2018

Potential for Solar System Science with the ngVLA

Radio wavelength observations of solar system bodies are a powerful method of probing many characteristics of those bodies. From surface and subsurface, to atmospheres (including deep atmospheres of the giant planets), to rings, to the magnetosphere of Jupiter, these observations provide unique information on current state, and sometimes history, of the bodies. The ngVLA will enable the highest sensitivity and resolution observations of this kind, with the potential to revolutionize our understanding of some of these bodies. In this article, we present a review of state-of-the-art radio wavelength observations of a variety of bodies in our solar system, varying in size from ring particles and small near-Earth asteroids to the giant planets. Throughout the review we mention improvements for each body (or class of bodies) to be expected with the ngVLA. A simulation of a Neptune-sized object is presented in Section 6. Section 7 provides a brief summary for each type of object, together with the type of measurements needed for all objects throughout the Solar System.

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arXiv2018

Science with an ngVLA: The ngVLA Reference Design

The next-generation Very Large Array (ngVLA) is an astronomical observatory planned to operate at centimeter wavelengths (25 to 0.26 centimeters, corresponding to a frequency range extending from 1.2 to 116 GHz). The observatory will be a synthesis radio telescope constituted of approximately 244 reflector antennas each of 18 meters diameter, and 19 reflector antennas each of 6 meters diameter, operating in a phased or interferometric mode. We provide a technical overview of the Reference Design of the ngVLA. This Reference Design forms a baseline for a technical readiness assessment and the construction and operations cost estimate of the ngVLA. The concepts for major system elements such as the antenna, receiving electronics, and central signal processing are presented.

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arXiv2018

The Next Generation Very Large Array: A Technical Overview

The next-generation Very Large Array (ngVLA) is an astronomical observatory planned to operate at centimeter wavelengths (25 to 0.26 centimeters, corresponding to a frequency range extending from 1.2 GHz to 116 GHz). The observatory will be a synthesis radio telescope constituted of approximately 214 reflector antennas each of 18 meters diameter, operating in a phased or interferometric mode. We provide an overview of the current system design of the ngVLA. The concepts for major system elements such as the antenna, receiving electronics, and central signal processing are presented. We also describe the major development activities that are presently underway to advance the design.

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arXiv2017

Atmospheric waves and dynamics beneath Jupiter's clouds from radiowavelength observations

We observed Jupiter at wavelengths near 2 cm with the Karl G. Jansky Very Large Array in February 2015. These frequencies are mostly sensitive to variations in ammonia abundance and probe between ~0.5-2.0 bars of pressure in Jupiter's atmosphere; within and below the visible cloud deck which has its base near 0.7 bars. The resultant observed data were projected into a cylindrical map of the planet with spatial resolution of ~1500 km at the equator. We have examined the data for atmospheric waves and observed a prominent bright belt of radio hotspot features near 10N, likely connected to the same equatorial wave associated with the 5-micron hotspots. We conducted a passive tracer power spectral wave analysis for the entire map. The power spectra analysis revealed that the atmosphere sampled in our observation (excluding the NEB region) is in a 2-D turbulent regime and its dynamics are predominately governed by the shallow water equations. The GRS is also very prominent and has a noticeable meridional asymmetry and we compare it, and nearby storms, with optical images. We find that the meridional radio profile has a global north-south hemisphere distinction and find correlations of it to optical intensity banding and to shear zones of the zonal wind profile over select regions of latitude. Amateur optical images taken before and after our observation complemented the radio wavelength map to investigate dynamics of the equatorial region in Jupiter's atmosphere. We find that two radio hotspots at 2 cm are well correlated with optical plumes in the NEB, additionally revealing they are not the same 5 micron hotspot features correlated with optical dark patches between adjacent plumes. This analysis exploits the VLA's upgraded sensitivity and explores the opportunities now possible when studying gas giants, especially atmospheric dynamics of layers beneath upper level clouds.

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arXiv2015

Next Generation Very Large Array Memo No. 9 Science Working Group 4: Time Domain, Fundamental Physics, and Cosmology

We report here on key science topics for the Next Generation Very Large Array in the areas of time domain, fundamental physics, and cosmology. Key science cases considered are pulsars in orbit around the Galactic Center massive black hole, Sagittarius A*, electromagnetic counterparts to gravitational waves, and astrometric cosmology. These areas all have the potential for ground-breaking and transformative discovery. Numerous other topics were discussed during the preparation of this report and some of those discussions are summarized here, as well. There is no doubt that further investigation of the science case will reveal rich and compelling opportunities.

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arXiv2015

Next Generation Very Large Array Memo No. 6, Science Working Group 1: The Cradle of Life

This paper discusses compelling science cases for a future long-baseline interferometer operating at millimeter and centimeter wavelengths, like the proposed Next Generation Vary Large Array (ngVLA). We report on the activities of the Cradle of Life science working group, which focused on the formation of low- and high-mass stars, the formation of planets and evolution of protoplanetary disks, the physical and compositional study of Solar System bodies, and the possible detection of radio signals from extraterrestrial civilizations. We propose 19 scientific projects based on the current specification of the ngVLA. Five of them are highlighted as possible Key Science Projects: (1) Resolving the density structure and dynamics of the youngest HII regions and high-mass protostellar jets, (2) Unveiling binary/multiple protostars at higher resolution, (3) Mapping planet formation regions in nearby disks on scales down to 1 AU, (4) Studying the formation of complex molecules, and (5) Deep atmospheric mapping of giant planets in the Solar System. For each of these projects, we discuss the scientific importance and feasibility. The results presented here should be considered as the beginning of a more in-depth analysis of the science enabled by such a facility, and are by no means complete or exhaustive.

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