arXiv subjects

Michael E. Brown

Publications and source records attributed to Michael E. Brown.

At least 19 recordsLinked to original sources

arXiv2026

JWST/MIRI Finds ISM-like Amorphous Silicates in Interstellar Comet 3I/ATLAS

We present the first spectroscopic mineralogical analysis of the dust coma of an interstellar object (ISO) from JWST mid-infrared spectroscopy of 3I/ATLAS (3I). 3I exhibits a strong 9- to 11-micron emissivity feature due to Si-O vibrational stretching modes that is commonly seen in asteroids, comets, disks, and the interstellar medium. Characterization of this 10-micron emissivity maximum reveals that 3I's dust composition is dominated by amorphous silicates, and that 3I is unlike Solar System comets, which show significant crystalline silicate dust. Instead, 3I's dust composition is more similar to circumstellar transition disks and the interstellar medium. We suggest 3I may have formed in a distant part of its home system out of interstellar medium-like material, without substantial incorporation of silicates condensed near its host star, unlike the mixing scenarios commonly suggested for the Solar System. Alternatively, 3I's original crystalline silicates may have been amorphized during its Gyr-long journey, although we find this alternative less likely due to 3I's mass loss rate and distinct 10-micron feature as compared to observed Solar System comets.

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arXiv2026

Linear Continuum Modelling to Explain The Majority of Bulk Features of Kuiper Belt Object Spectra

The first analyses of the James Webb Space Telescope spectra of trans-Neptunian Objects (TNOs) revealed three discrete types of surfaces. This seems to contradict ground-based spectro-photometric datasets, which suggest a continuum of colors with only two surface types. Here we present linear spectral modelling that reconciles these two results. In our model, the sole parameter is the object's optical slope, and the reflectance spectrum at all wavelengths is linearly proportional to that color, with the slope of that function evaluated from the spectra themselves. When applied to small (H>5 for H2O-types and H>4 for the merged sample of organic-rich and CO2-types) and distant (q>18 au) objects, we find that this model does a reasonable job of reproducing the overall spectral behavior of both samples. Bootstrapping simulations show that if the optical slope were not a good predictor of an object's spectrum, then finding an explained variance of the model that is better than observed occurred in 2.3% of realizations for the H2O-types and 0% of realizations for the organics sample. In a chi^2 sense, the optical color is a better predictor of most spectra as compared to the mean spectrum of a class. The trends of optical color and spectra band-areas exhibited for many key compositional materials are well reproduced, and demonstrate that those materials govern the overall spectral shape within a class. Importantly, these results require that within a given class, the band-areas of those key materials are predictable given only its optical color and its surface type. Unsurprisingly, our simple one-parameter model does not account for the full spectral diversity of TNOs. We speculate that albedo encapsulates much of the remaining diversity.

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arXiv2026

Neptune's Inner Moons and Rings Are Exposed Icy Body Interiors

Neptune's single large moon, Triton, is accompanied by a set of dusty rings and small moons whose composition and origin are uncertain. Using the James Webb Space Telescope, we observed Neptune's rings and three moons: Larissa, Galatea, and Proteus. Remarkably, these moons and rings have spectra that are distinct from other outer solar system objects and show no evidence of water ice, despite hosting deep 3 micron OH absorption bands. Additionally, Larissa, Galatea, and the rings show a compositional signature unique among outer solar system bodies: a 2.72 micron absorption band diagnostic of magnesium-rich phyllosilicates - a signpost of extensive aqueous alteration. These minerals likely formed in the interior of primordial satellites destroyed during Triton's violent capture or a tidally shredded dwarf planet. Our findings suggest that Neptune's present-day inner moons and rings reaccreted from this deep interior material and therefore uniquely access the interior composition of icy outer solar system differentiated bodies.

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arXiv2026

Deuterated water and the formation of the satellites of Uranus

The satellites of Uranus orbit in a low-eccentricity, equatorial plane that is tilted by 98 degrees relative to the solar system -- a geometry that mirrors Uranus's extreme axial tilt. Although a giant impact could have tipped Uranus, how the satellites came to share this orientation remains uncertain. Proposed formation pathways include primordial accretion followed by reorientation, formation from debris generated by the tilting impact, and reaccretion from a massive ring produced by the tidal disruption of passing bodies from the outer solar system. Current observations do not discriminate among these scenarios. Using the James Webb Space Telescope, we measured the deuterium-to-hydrogen (D/H) ration in the water ice of the five regular satellites of Uranus. We find an average D/H ratio of $2.1\pm 0.2 \times 10^{-4}$, nearly five times higher than that of Uranus and comparable to the values measured in comets. This enrichment is inconsistent with with any formation scenario in which substantial Uranian material was incorporated into the satellites, thereby excluding models that require significant mixing in an impact-derived vapor disk. The observed D/H ratios are instead compatible with models in which the satellites accreted from material that remained largely separate from Uranus, such as debris from a disrupted pre-existing satellite system or from a tidally captured outer solar system body. The innermost regular satellite, Miranda, exhibits a marginally elevated D/H ratio (2.8 $σ$ above the average of the other satellites), potentially indicating a distinct formation history or source of water and offering an important clue for distinguishing amount competing models.

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arXiv2026

Nereid as a Regular Satellite of Neptune

Nereid, Neptune's third largest moon, is considered to be a captured irregular satellite due to its highly eccentric orbit. However, among irregular satellites, Nereid is an outlier: it is the largest, the closest to its host planet, and the most eccentric. We present James Webb Space Telescope near-infrared spectroscopy of Nereid that demonstrates that its composition is inconsistent with its suggested captured origin. We then simulate Nereid's early orbital history subsequent to Triton's capture to demonstrate a plausible dynamical pathway for a regular satellite formed in-situ around Neptune to evolve to Nereid's present-day orbit. Based upon the available spectroscopic and dynamical evidence, we propose that Nereid is not a body captured from the Kuiper belt, but rather the sole surviving intact regular satellite of Neptune.

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arXiv2026

Formation and Trapping of CO2 from Cryogenic Irradiation of Carbonate

The detection of CO2 on the Jovian satellite Europa by Galileo NIMS and recent mapping of the leading side by JWST has revealed that it is most concentrated in geologically young terrains, and its v3 asymmetric stretch appears as a spectral doublet centered at 4.25 and 4.27 um. Since crystalline CO2 is unstable at Europan surface conditions, this observation implies an active source and a trapping medium, which may be separate. To this end, several hypotheses have been proposed, but no laboratory work has successfully reproduced the spectral features of CO2 on Europa so far. Radiolyzed carbonates have also been discussed as plausible precursors and host materials for CO2, though their role has not been experimentally validated in a Europa-like environment. Here, we report the first laboratory experiments investigating CO2 production from carbonate salts exposed to 10 keV electron irradiation at 50, 100, and 120 K in ultrahigh vacuum. Using diffuse reflectance FTIR spectroscopy, we observe the emergence, growth, and saturation of an absorption doublet centered near 4.25 and 4.27 um, consistent with the CO2 v3 band. Postirradiation thermal desorption studies using residual gas analysis reveal that the radiolytically formed CO2 is stable at temperatures beyond Europa's surface. This work provides the first experimental evidence that low-energy electron irradiation of carbonates in cryogenic, vacuum conditions can produce and retain CO2, and suggests that carbonates can serve as endogenous reservoirs of CO2 on irradiated icy bodies in the outer solar system.

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arXiv2026

The Volatile Inventory of 3I/ATLAS as seen with JWST/MIRI

We present the first spectroscopic characterization of an interstellar object at mid-infrared wavelengths. Post-perihelion observations of 3I/ATLAS using the JWST/MIRI medium-resolution spectrometer were obtained on 2025 December 15--16 and 27 when the object was at heliocentric distances of 2.20 and 2.54 au, respectively. Our 5--28 micron spectra exhibit fluorescence features from several gaseous species, including the $ν_2$ band of water at 5.8--7.0 microns. the primary $ν_2$ and associated hot bands of carbon dioxide around 15 microns, and a forbidden transition of atomic nickel at 7.507 microns. We also report the first direct detection of methane in an interstellar object. The delayed onset of methane production relative to water suggests past depletion from the outermost layers, with the observed methane emerging from unprocessed subsurface material. Comparison of the volatile production rates measured during the two epochs indicate a significant reduction in overall outgassing over 12 days, with the measured water activity level dropping more steeply than other species. As shown through near-nucleus coma mapping, 3I continues to display an extended source of water production from icy grains entrained within the coma. Our production rate measurements confirm that 3I exhibits a strongly enhanced CO$_2$:H$_2$O mixing ratio relative to typical solar system comets, as well as a somewhat enriched CH$_4$:H$_2$O value.

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arXiv2026

The Fate of Frozen Carbonated Water at Europa-like Conditions

We present the results of experiments probing the retention of CO2 in crystalline water ice, frozen sodium chloride (NaCl) brines, and flash-frozen carbonated water using diffuse reflectance infrared spectroscopy. Characteristic absorptions alluding to the formation of clathrate hydrates in crystalline ices and frozen brines are observed. NaCl in frozen brines does not affect qualitatively affect the formation of clathrate hydrates. Generation and stability of clathrates in crystalline ice transiently subjected to pressure-temperature (P-T) conditions in the stability region is observed, despite conditions being unviable at the onset of freezing. Retention of CO2 in flash-frozen carbonated water is observed to be dependent on the temperature of the substrate during freezing. The state of CO2 retained in the resulting ices differs from clathrate hydrates, as inferred from the respective infrared spectra. Both mechanisms of CO2 retention are stable up to 140 K and under evacuated conditions. In the context of Europa, the P-T states traversed by the samples plausibly represent the typical conditions around endogenous CO2 if it is indeed transported from the subsurface ocean to the surface while being retained in ice/frozen brines and/or liquid emerging on the surface. However, the absorptions of CO2 in the laboratory infrared spectra do not match those detected on the leading side of Europa by the NIRSpec instrument on board JWST. Therefore, it is unlikely that the endogenous CO2 observed at the surface of Europa is sourced directly from the ocean, unless additional processes affect the observed bands of CO2 on Europa.

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arXiv2026

Satellites and small bodies with ALMA: Insights into Solar System formation & evolution

Our understanding of the formation and evolution of planetary systems has made major advances in the past decade. This progress has been driven in large part by the Atacama Large Millimeter/submillimeter Array (ALMA), which has given us an unprecedented view of Solar System bodies themselves, and of the structure and chemistry of forming exoplanetary systems. Within our own Solar System, ALMA has enabled the detection of new molecules and isotopologues across moons and comets, as well as placing new constraints on the compositions and histories of small bodies through thermal emission observations. In this article, we highlight some key areas where ALMA has contributed to a deeper understanding of our Solar System's formation and evolution, and place these discoveries in the context of our evolving understanding of protoplanetary disks.

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arXiv2025

JWST Spectroscopy of a Blue Binary Cold Classical Kuiper Belt Object

We present observations of two binary systems within the cold classical region of the Kuiper belt$-$2001 XR254 and 2016 BP81$-$obtained with the JWST Near-Infrared Spectrograph. The measured reflectance spectrum of 2001 XR254 is characteristic of the red cold classicals, with strong features due to carbon dioxide, carbon monoxide, and methanol ices. In contrast, 2016 BP81 is a blue binary, with a water-ice-rich surface composition. The two components of the 2016 BP81 binary display identical spectral profiles, consistent with coeval formation from gravitational collapse. Through qualitative and quantitative comparisons of water-ice-rich Kuiper belt objects observed with JWST, we identify a small subclass, including 2016 BP81, that appears to differ in systematic ways from the rest of the population. The relatively deep carbon dioxide ice absorption bands and enhanced signatures of aliphatic organics suggest that objects within this subclass may have originated in a distinct formation environment from the other water-ice-rich Kuiper belt objects. The implications of our findings are discussed within the context of recent models of Kuiper belt formation and evolution.

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arXiv2025

Deuterated water ice on the satellites of Saturn

The deuterium to hydrogen ratio in water ice in a planetary body carries important information on the history of water processing and delivery in the protostellar nebula. For a giant planet satellite, the D/H ratio is also affected by the processes and temperatures of the circumplanetary or circumstellar environment in which the satellites formed. Here we present robust JWST spectroscopic detections of the 4.14 $μ$m O-D stretch absorption line (analogous to the 3 $μ$m water O-H stretch) on the mid-sized Saturnian satellites and use these detections to infer a D/H ratio on each satellite. Within the limitations of the technique, we find that all of the satellites are consistent with having a D/H ratio of about $1.5 \times$ Vienna Standard Mean Ocean Water (VSMOW), which is about an order of magnitude higher than the value of the atmosphere of Saturn. A much higher previously reported D/H ratio for Phoebe is ruled out at the 10$σ$ level, and a 3$σ$ upper limit of 2.3 $\times$ VSMOW is obtained. The elevated D/H ratios demonstrate that the solid planetesimals and pebbles that built the satellites never sublimed and re-equilibrated with the gaseous circumplanetary disk. The similarity of the D/H measurements across all satellites suggest that the D/H ratio of water ice in the vicinity of Saturn at the time of satellite formation was also approximately 1.5 $\times$ VSMOW.

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arXiv2025

A JWST study of CO$_2$ on the satellites of Saturn

Solid state CO$_2$ has been detected throughout the outer solar system, even at temperatures where crystalline CO$_2$ is unstable, requiring that the CO$_2$ be trapped in a separate host material. The Saturnian satellites provide an ideal laboratory for the study of this trapped CO$_2$, allowing us to examine objects with identical insolation, but with a range of environments, ice exposure, organic abundance, and formation locations. Here, we present JWST spectra of 8 mid-sized satellites of Saturn, including Mimas, Enceladus, Tethys, Dione, and Rhea interior to Titan, and Hyperion, Iapetus, and Phoebe exterior. The $\sim$4.26 $μ$m CO$_2$ $ν_3$ band is detected on each satellite, and the $\sim$2.7 $μ$m $ν_1+ν_3$ band is detected on all but Phoebe and the leading hemisphere of Iapetus. Based on the wavelength shifts of these bands, we find four separate types of trapped CO$_2$ on the satellites. On the inner satellites, CO$_2$ appears trapped in amorphous ice sourced from Saturn's E-ring, and a second component of CO$_2$ is associated with the dark material most prominent on the trailing hemispheres of Dione and Rhea. On the outer satellites, CO$_2$ appears to be produced by irradiation of organics on Phoebe, which are then transported to the dark leading hemisphere of Iapetus and onto the dark regions of Hyperion. CO$_2$ is also trapped by water ice on the trailing hemisphere of Iapetus and on Hyperion. These observations point to the continued need for laboratory studies to better understand the sources and trapping mechanisms of CO$_2$ throughout the outer solar system.

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arXiv2025

JWST Near Infrared Spectroscopy of High Albedo Jupiter Trojans: A New Surface Type in the Trojan Belt

We present 0.8 to 5 $μ$m JWST spectra of four $\sim$20 km diameter Jupiter Trojans known to have albedos elevated above the values typical in the remaining Trojan population. The spectra of these four high albedo Jupiter Trojans are all similar, with red slopes in the optical-IR transition region, a break to lower slopes at 1.3 $μ$m, and a broad absorptions from 2.8 to 4 $μ$m. The 0.8 to 2.5 $μ$m spectra of these objects match the spectra of neither the well-known "red" and "less-red" Jupiter Trojans nor of any known asteroid taxonomic class. The reflecticity of these objects does not rise redward of 4 $μ$m, a property that is seen in the previous JWST observations of Jupiter Trojans only in Polymele. Indeed, the high albedo Jupiter Trojan spectra are a good match to that of Polymele, and Polymele is both the smallest Jupiter Trojan in the previous JWST sample and has the highest albedo of the objects in that sample. We conclude that Polymele and the other high albedo Jupiter Trojans represent a third class of Jupiter Trojans not represented in the more heavily-studied larger objects and are perhaps the products of recent disruptions. The Lucy flyby of Polymele in September 2027 will give a direct view of one of this new class of Jupiter Trojans.

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arXiv2025

Spectroscopic Mapping of Callisto with HST/STIS and Implications for its Surface Composition

We present global, spatially resolved ultraviolet-visible spectra of Callisto obtained with HST/STIS and explore possible compositions of Callisto's surface material. We map the strength of a widespread downturn toward the near-UV and the NIR spectral slope from 700 to 1000 nm, which varies from slightly blue (reflectance decreasing from 700 to 1000 nm) to red (reflectance increasing) across Callisto's surface. Globally, bright water-ice-rich regions tend to have neutral or blue NIR slopes and a shallower near-UV downturn, while darker material is associated with red NIR slopes and stronger near-UV absorption. Broad absorptions near 820 and 930 nm are spatially correlated with the Asgard and Valhalla impact basins and may be associated with iron-bearing silicates. An absorption edge near 275 nm maps primarily to Callisto's trailing hemisphere, and a 320 nm absorption most prevalent within and surrounding Asgard and Valhalla may be related to organics. We report two new absorption features near 230 and 450 nm which might be attributed to irradiated NaCl. We find little evidence for sulfur-bearing species at UV-visible wavelengths and suggest that a 280 nm band seen only in leading/trailing hemisphere ratio spectra and previously attributed to SO2 is better explained as a consequence of dividing the unrelated 320 nm leading hemisphere band by the trailing hemisphere 275 nm absorption edge. Spatial variations in spectral features suggest that Callisto's dark material composition varies regionally, reflecting a mix of endogenic and exogenic sources and radiolytic alteration.

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arXiv2025

Saturnian Irregular Satellites as a Probe of Kuiper Belt Surface Evolution

We present the JWST NIRSpec PRISM 0.7-5.3 micron spectra of Albiorix and Siarnaq and the NIRSpec G235H/G395M 1.7-5.3 micron spectra of Phoebe, the three largest Saturnian irregular satellites. The irregular satellites of the giant planets are thought to be captured planetesimals from the same population as Kuiper belt objects. They are emplaced inside Saturn's Hill sphere during the giant-planet instability described by the Nice Model, and are thus valuable tracers of Kuiper belt surface evolution. Phoebe's JWST spectrum matches the global average from Cassini VIMS, and by comparing the spectrum to the library of Kuiper belt object spectra from JWST, we demonstrate Phoebe's compositional similarity to water-rich KBOs. On the smaller Albiorix and Siarnaq, we observe a broad 3 micron O-H band but do not see a Fresnel peak or the 1.5/2.0 micron features characteristic of H$_2$O ice. We posit that after capture, the frequent high-velocity collisions between smaller irregular satellites sublimate the water ice, while the much larger Phoebe is resistant to disruption and retains its water ice. We suggest that the presence of CO$_2$ on the smaller satellites, despite the lack of water ice, indicates later formation of CO$_2$ on these surfaces through irradiation of organic compounds.

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arXiv2024

Europa's H$_2$O$_2$: Temperature Insensitivity and a Correlation with CO$_2$

H$_2$O$_2$ is part of Europa's water-ice radiolytic cycle and a potential source of oxidants to Europa's subsurface ocean. However, factors controlling the concentration of this critical surface species remain unclear. Though laboratory experiments suggest that Europa's H$_2$O$_2$ should be concentrated in the coldest, most ice-rich regions toward the poles, Keck adaptive optics observations have shown the strongest H$_2$O$_2$ signatures in comparatively warm, salt-bearing terrain at low latitudes. As a result, it was suggested that the local non-ice composition of these terrains -- particularly hypothesized enrichments of CO$_2$ -- may be a more dominant control on H$_2$O$_2$ than temperature or water-ice abundance. Here, we use observations of Europa from the NASA Infrared Telescope Facility, Keck Observatory, and JWST to disentangle the potential effects of temperature and composition. In order to isolate the effect of temperature on Europa's H$_2$O$_2$, we use the ground-based observations to assess its response to temperature changes over timescales associated with Europa's daily eclipse and diurnal cycle. We use JWST Cycle 1 data to look for any geographic correlation between Europa's H$_2$O$_2$ and CO$_2$. Both changes in Europa's 3.5-$μ$m H$_2$O$_2$ absorption band from pre to post eclipse and across a local day suggest minimal effects of the local temperature on these timescales. In contrast, the JWST observations show a strong positive correlation between Europa's H$_2$O$_2$ and CO$_2$ bands, supporting the previously suggested possibility that the presence of CO$_2$ in the ice may enhance H$_2$O$_2$ concentrations via electron-scavenging.

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arXiv2024

The Trojan-like Colors of Low-Perihelion Kuiper Belt Objects

An important testable prediction of dynamical instability models for the early evolution of the Solar System is that Jupiter Trojans share a source population with the Kuiper belt. Concrete evidence of this prediction remains elusive, as Kuiper belt objects (KBOs) and Jupiter Trojans appear to have different surface compositions. We address the long-standing question of Trojan origin by finding a dynamical sub-population in the Kuiper belt with Trojan-like colors. Combining existing photometric data with our own surveys on Keck I and Palomar P200, we find that the low-perihelion ($q<30 $AU, $a>30 $AU) component of the Kuiper belt has colors that bifurcate similarly to the Jupiter Trojans, unlike Centaurs ($a<30 $AU) which have redder, Kuiper belt-like colors. To connect the Jupiter Trojans to the Kuiper belt, we test whether the distinct Trojan-like colors of low-perihelion KBOs result from surface processing, or are sourced from a specific population in the Kuiper belt. By simulating the evolution of the Canada-France Ecliptic Plane Survey synthetic population of KBOs for four billion years, we find that differences in heating timescales cannot result in a significant depletion of Very Red low-perihelion KBOs as compared to the Centaurs. We find that the neutrally-colored scattered disk objects ($e>0.6$ KBOs) contribute more to the low-perihelion KBO population rather than Centaurs, resulting in their different colors.

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arXiv2024

Pwyll and Manannán Craters as a Laboratory for Constraining Irradiation Timescales on Europa

We examine high spatial resolution Galileo/NIMS observations of the young (~1 My - 20 My) impact features, Pwyll and Manannán craters, on Europa's trailing hemisphere in an effort to constrain irradiation timescales. We characterize their composition using a linear spectral modeling analysis and find that both craters and their ejecta are depleted in hydrated sulfuric acid relative to nearby older terrain. This suggests that the radiolytic sulfur cycle has not yet had enough time to build up an equilibrium concentration of H2SO4, and places a strong lower limit of the age of the craters on the equilibrium timescale of the radiolytic sulfur cycle on Europa's trailing hemisphere. Additionally, we find that the dark and red material seen in the craters and proximal ejecta of Pwyll and Manannán show the spectroscopic signature of hydrated, presumably endogenic salts. This suggests that the irradiation-induced darkening and redenning of endogenic salts thought to occur on Europa's trailing hemisphere has already happened at Pwyll and Manannán, thereby placing an upper limit on the timescale by which salts are irradiation reddened.

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