arXiv subjects

Michael E. Brown

Publications and source records attributed to Michael E. Brown.

At least 37 records · Page 2Linked to original sources

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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arXiv2024

Generation of Low-Inclination, Neptune-Crossing TNOs by Planet Nine

The solar system's distant reaches exhibit a wealth of anomalous dynamical structure, hinting at the presence of a yet-undetected, massive trans-Neptunian body - Planet 9. Previous analyses have shown how orbital evolution induced by this object can explain the origins of a broad assortment of exotic orbits, ranging from those characterized by high perihelia to those with extreme inclinations. In this work, we shift the focus toward a more conventional class of TNOs, and consider the observed census of long-period, nearly planar, Neptune-crossing objects as a hitherto-unexplored probe of the Planet 9 hypothesis. To this end, we carry out comprehensive $N-$body simulations that self-consistently model gravitational perturbations from all giant planets, the Galactic tide, as well as passing stars, stemming from initial conditions that account for the primordial giant planet migration and sun's early evolution within a star cluster. Accounting for observational biases, our results reveal that the orbital architecture of this group of objects aligns closely with the predictions of the P9-inclusive model. In stark contrast, the P9-free scenario is statistically rejected at a $\sim5\,σ$ confidence-level. Accordingly, this work introduces a new line of evidence supporting the existence of Planet 9 and further delineates a series of observational predictions poised for near-term resolution.

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arXiv2024

JWST Spectrophotometry of the Small Satellites of Uranus and Neptune

We use 1.4-4.6 micron multi-band photometry of the small inner Uranian and Neptunian satellites obtained with the James Webb Space Telescope's near-infrared imager NIRCam to characterize their surface compositions. We find that the satellites of the ice giants have, to first-order, similar compositions to one another, with a 3.0 micron absorption feature possibly associated with an O-H stretch, indicative of water ice or hydrated minerals. Additionally, the spectrophotometry for the small ice giant satellites matches spectra of some Neptune Trojans and excited Kuiper belt objects, suggesting shared properties. Future spectroscopy of these small satellites is necessary to identify and better constrain their specific surface compositions.

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arXiv2024

JWST near-infrared spectroscopy of the Lucy Jupiter Trojan flyby targets: Evidence for OH absorption, aliphatic organics, and CO$_{2}$

We present observations obtained with the Near Infrared Spectrograph on JWST of the five Jupiter Trojans that will be visited by the Lucy spacecraft -- the Patroclus-Menoetius binary, Eurybates, Orus, Leucus, and Polymele. The measured 1.7-5.3 $μ$m reflectance spectra, which provide increased wavelength coverage, spatial resolution, and signal-to-noise ratio over previous ground-based spectroscopy, reveal several distinct absorption features. We detect a broad OH band centered at 3 $μ$m that is most prominent on the less-red objects Eurybates, Patroclus-Menoetius, and Polymele. An additional absorption feature at 3.3-3.6 $μ$m, indicative of aliphatic organics, is systematically deeper on the red objects Orus and Leucus. The collisional fragment Eurybates is unique in displaying an absorption band at 4.25 $μ$m that we attribute to bound or trapped CO$_2$. Comparisons with other solar system small bodies reveal broad similarities in the 2.7-3.6 $μ$m bands with analogous features on Centaurs, Kuiper belt objects (KBOs), and the active asteroid 238P. In the context of recent solar system evolution models, which posit that the Trojans initially formed in the outer solar system, the significant attenuation of the 2.7-3.6 $μ$m absorption features on Trojans relative to KBOs may be the result of secondary thermal processing of the Trojans' surfaces at the higher temperatures of the Jupiter region. The CO$_2$ band manifested on the surface of Eurybates suggests that CO$_2$ may be a major constituent in the bulk composition of Trojans, but resides in the subsurface or deeper interior and is largely obscured by refractory material that formed from the thermophysical processes that were activated during their inward migration.

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arXiv2024

A Pan-STARRS1 Search for Planet Nine

We present a search for Planet Nine using the second data release of the Pan-STARRS1survey. We rule out the existence of a Planet Nine with the characteristics of that predicted in Brown & Batygin (2021) to a 50% completion depth of $V=21.5$. This survey, along with previous analyses of the Zwicky Transient Facility (ZTF) and Dark EnergySurvey (DES) data, rules out 78% of the Brown \& Batygin parameter space. Much of the remaining parameter space is at $V>21$ in regions near and in the area where the northern galactic plane crosses the ecliptic.

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arXiv2023

The distribution of CO$_2$ on Europa indicates an internal source of carbon

Jupiter's moon Europa has a subsurface ocean, the chemistry of which is largely unknown. Carbon dioxide (CO$_2$) has previously been detected on the surface of Europa, but it was not possible to determine whether it originated from subsurface ocean chemistry, was delivered by impacts, or was produced on the surface by radiation processing of impact-delivered material. We map the distribution of CO$_2$ on Europa using observations obtained with the James Webb Space Telescope (JWST) and find a concentration of CO$_2$ within Tara Regio, a recently resurfaced terrain. This indicates the CO$_2$ is derived from an internal carbon source. We propose the CO$_2$ formed in the internal ocean, though we cannot rule out formation on the surface by radiolytic conversion of ocean-derived organics or carbonates.

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arXiv2023

The Spatial Distribution of the Unidentified 2.07 \textmu m Absorption Feature on Europa and Implications for its Origin

A weak absorption feature at 2.07 \textmu m on Europa's trailing hemisphere has been suggested to arise from radiolytic processing of an endogenic salt, possibly sourced from the interior ocean. However, if the genesis of this feature requires endogenic material to be present, one might expect to find a correlation between its spatial distribution and the recently disrupted chaos terrains. Using archived near-infrared observations from Very Large Telescope/SINFONI with a $\sim$1 nm spectral resolution and a linear spatial resolution $\sim$130 km, we examine the spatial distribution of this feature in an effort to explore this endogenic formation hypothesis. We find that while the presence of the 2.07 \textmu m feature is strongly associated with the irradiation pattern on Europa's trailing hemisphere, there is no apparent association between the presence or depth of the absorption feature and Europa's large-scale chaos terrain. This spatial distribution suggests that the formation pathway of the 2.07 \textmu m feature on Europa is independent of any endogenous salts within the recent geology. Instead, we propose that the source of this feature may simply be a product of the radiolytic sulfur cycle or arise from some unidentified parallel irradiation process. Notably, the 2.07 \textmu m absorption band is absent from the Pwyll crater ejecta blanket, suggesting that radiolytic processing has not had enough time to form the species responsible and placing a lower limit on the irradiation timescale. We are unable to find a plausible spectral match to the 2.07 \textmu m feature within the available laboratory data.

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arXiv2023

Science opportunities with solar sailing smallsats

Recently, we witnessed how the synergy of small satellite technology and solar sailing propulsion enables new missions. Together, small satellites with lightweight instruments and solar sails offer affordable access to deep regions of the solar system, also making it possible to realize hard-to-reach trajectories that are not constrained to the ecliptic plane. Combining these two technologies can drastically reduce travel times within the solar system, while delivering robust science. With solar sailing propulsion capable of reaching the velocities of ~5-10 AU/yr, missions using a rideshare launch may reach the Jovian system in two years, Saturn in three. The same technologies could allow reaching solar polar orbits in less than two years. Fast, cost-effective, and maneuverable sailcraft that may travel outside the ecliptic plane open new opportunities for affordable solar system exploration, with great promise for heliophysics, planetary science, and astrophysics. Such missions could be modularized to reach different destinations with different sets of instruments. Benefiting from this progress, we present the "Sundiver" concept, offering novel possibilities for the science community. We discuss some of the key technologies, the current design of the Sundiver sailcraft vehicle and innovative instruments, along with unique science opportunities that these technologies enable, especially as this exploration paradigm evolves. We formulate policy recommendations to allow national space agencies, industry, and other stakeholders to establish a strong scientific, programmatic, and commercial focus, enrich and deepen the space enterprise and broaden its advocacy base by including the Sundiver paradigm as a part of broader space exploration efforts.

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arXiv2023

Masses and densities of dwarf planet satellites measured with ALMA

We have used the Atacama Large Millimeter Array (ALMA) to measure precise absolute astrometric positions and detect the astrometric wobble of dwarf planet Orcus and its satellite Vanth over a complete orbit. We also place upper limits to the astrometric wobble induced by Dysnomia on dwarf planet Eris around its orbit. From the Vanth-Orcus barycentric motion, we find a Vanth-Orcus mass ratio of 0.16$\pm$0.02 -- the highest of any known planet or dwarf planet. This large ratio is consistent with the hypothesis that Vanth is a largely-intact impactor from a giant collision in the system, and that the system has likely evolved to a double synchronous state. We find only an upper limit of the barycenter motion of Eris, which implies a one sigma upper limit to the Dysnomia-Eris mass ratio of 0.0085, close to the modeled transition region between giant impact generated satellites which are largely intact remnants of the original impactor and those which form out of reaccreted disk material left over post-impact. The low albedo of Dysnomia leads us to marginally favor the intact impactor scenario. We find that Dysnomia has density of <1.2 g cm$^{-3}$, significantly lower than the 2.4 g cm$^{-3}$ of Eris.

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arXiv2023

The state of CO and CO2 ices in the Kuiper belt as seen by JWST

JWST has shown that CO2 and CO are common on the surfaces of objects in the Kuiper belt and have apparent surface coverages even higher than that of water ice, though water ice is expected to be significantly more abundant in the bulk composition. Using full Mie scattering theory, we show that the high abundance and the unusual spectral behaviour around the 4.26 micron v1 band of CO2 can be explained by a surface covered in a few micron thick layer of ~ 1-2 micron CO2 particles. CO is unstable at the temperatures in the Kuiper belt, so the CO must be trapped in some more stable species. While hydrate clathrates or amorphous water ice are often invoked as a trapping mechanism for outer solar system ices, the expected spectral shift of the absorption line for a CO hydrate clathrates or trapping in amorphous ice is not seen, nor does the H2O abundance appear to be high enough to explain the depth of the CO absorption line. Instead, we suggest that the CO is created via irradiation of CO2 and trapped in the CO2 grains during this process. The presence of a thin surface layer of CO2 with embedded CO suggests volatile differentiation driving CO2 from the interior as a major process driving the surface appearance of these mid-sized Kuiper belt objects, but the mechanisms that control the small grain size and depth of the surface layer remain unclear.

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arXiv2023

The Optical Aurorae of Europa, Ganymede and Callisto

The tenuous atmospheres of the Galilean satellites are sourced from their surfaces and produced by a combination of plasma-surface interactions and thermal processes. Though thin, these atmospheres can be studied via their auroral emissions, and most work to date has focused on their aurora at UV wavelengths. Here we present the first detections of Ganymede's and Callisto's optical aurorae, as well detections of new optical auroral lines at Europa, based on observations of the targets over ten Jupiter eclipses from 1998 to 2021 with Keck/HIRES. We present measurements of OI emission at 6300/6364, 5577, 7774, and 8446 A and place upper limits on hydrogen at 6563 A. These constitute the first detections of emissions at 7774 and 8446 A at a planetary body other than Earth. The simultaneous measurement of multiple emission lines provides robust constraints on atmospheric composition. We find that the eclipse atmospheres of Europa and Ganymede are composed predominantly of O2 with average column densities of (4.1 \pm 0.1) x 10^{14} cm^{-2} and (4.7 \pm 0.1) x 10^{14} cm^{-2}, respectively. We find weak evidence for H2O in Europa's bulk atmosphere at an H2O/O2 ratio of $\sim$0.25, and place only an upper limit on H2O in Ganymede's bulk atmosphere, corresponding to H2O/O2 < 0.6. The column density of O2 derived for Callisto is (4.0 \pm 0.9 x 10^{15} cm^{-2} for an assumed electron density of 0.15 cm^{-3}, but electron properties at Callisto's orbit are very poorly constrained.

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arXiv2022

Spectroscopic mapping of Io's surface with HST/STIS: SO$_2$ frost, sulfur allotropes, and large-scale compositional patterns

Io's intense volcanic activity results in one of the most colorful surfaces in the solar system. Ultraviolet and visible-wavelength observations of Io are critical to uncovering the chemistry behind its volcanic hues. Here, we present global, spatially resolved UV-visible spectra of Io from the Space Telescope Imaging Spectrograph on the Hubble Space Telescope (HST), which bridge the gap between previous highly resolved imagery and disk-integrated spectroscopy, to provide an unprecedented combination of spatial and spectral detail. We use this comprehensive dataset to investigate spectral endmembers, map observed spectral features associated with SO$_2$ frost and other sulfur species, and explore possible compositions in the context of Io surface processes. In agreement with past observations, our results are consistent with extensive equatorial SO$_2$ frost deposits that are stable over multi-decade timescales, widespread sulfur-rich plains surrounding the SO$_2$ deposits, and the enrichment of Pele's pyroclastic ring and the high-latitude regions in metastable short-chain sulfur allotropes.

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arXiv2022

Photometric Confirmation and Characterization of the Ennomos Collisional Family in the Jupiter Trojans

Collisional families offer a unique window into the interior composition of asteroid populations. Previous dynamical studies of the Jupiter Trojans have uncovered a handful of potential collisional families, two of which have been subsequently confirmed through spectral characterization. In this paper, we present new multiband photometric observations of the proposed Ennomos family and derive precise $g-i$ colors of 75 candidate family members. While the majority of the targets have visible colors that are indistinguishable from background objects, we identify 13 objects with closely grouped dynamical properties that have significantly bluer colors. We determine that the true Ennomos collisional family is tightly confined to $a'_{p} > 5.29$ au and $0.45 < \sin{i_{p}} < 0.47$, and the majority of its confirmed members have near-solar spectral slopes, including some of the bluest objects hitherto discovered in the Trojan population. The property of distinctly neutral colors that is shared by both the Ennomos family and the previously characterized Eurybates family indicates that the spectral properties of freshly exposed surfaces in the Jupiter region are markedly different than the surfaces of uncollided Trojans. This implies that the processes of ice sublimation and space weathering at 5.2 au yield a distinct regolith chemistry from the primordial environment within which the Trojans were initially accreted. It also suggests that the Trojans were emplaced in their present-day location from elsewhere sometime after the initial population formed, which is a key prediction of recent dynamical instability models of solar system evolution.

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arXiv2022

Limits on the Detection of Planet Nine in the Dark Energy Survey

Studies of the clustering of the most distant Kuiper belt objects in the outer solar system have hinted at the possible existence of a planet beyond Neptune referred to as Planet Nine (P9). Recent efforts have constrained the parameter space of the orbital elements of P9, allowing for the creation of a synthetic catalog of hypothetical P9s. By examining the potential recovery of such a catalog within numerous sky surveys, it is possible to further constrain the parameter space for P9, providing direction for a more targeted search. We examine the ability of the full six years of the Dark Energy Survey (DES) to recover a synthetic Planet Nine population presented in Brown and Batygin (2021a) [arXiv:2108.09868]. We find that out of 100,000 simulated objects, 11,709 cross the wide DES survey footprint of which 10,187 (87.0%) are recovered. This rules out an additional 5% of the parameter space after accounting for Planets Nine that would have been detected by both the Zwicky Transient Facility and DES.

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arXiv2022

An ALMA search for high albedo objects among the mid-sized Jupiter Trojan population

We use ALMA measurements of 870 $μ$m thermal emission from a sample of mid-sized (15-40 km diameter) Jupiter Trojan asteroids to search for high albedo objects in this population. We calculate the diameters and albedos of each object using a thermal model which also incorporates {contemporaneous} Zwicky Transient Facility photometry to accurately measure the absolute magnitude at the time of the ALMA observation. We find that while many albedos are lower than reported from WISE, several small Trojans have high albedos independently measured both from ALMA and from WISE. The number of these high albedo objects is approximately consistent with expectations of the number of objects that recently have undergone large-scale impacts, suggesting that the interiors of freshly-crated Jupiter Trojans could contain high albedo materials such as ices.

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arXiv2022

The Influence of Temperature and Photobleaching on Irradiated Sodium Chloride at Europa-like Conditions

Europa's leading hemisphere chaos regions have a spectral feature at 450 nm that has been attributed to absorption by crystal defects in irradiated sodium chloride, known as F-centers. Some discrepancies exist between the laboratory data of irradiated sodium chloride and the observations, including a $\sim$10 nm shift in central wavelength of the F-center band and the lack of the prominent 720 nm absorption on Europa from M-centers, which result from the coalescence of pairs of F-centers. Here, we perform irradiation experiments on sodium chloride in an attempt to understand these discrepancies. We show that careful control of the temperature of the sample at a temperature of 120 K yields F-centers with an absorption wavelength comparable to that of Europa. In addition, we measure the effect of photobleaching -- the destruction of F-centers by photons -- and show that at the energetic particle and photon flux on Europa, an equilibrium will be reached where only a modest F-center absorption develops. The density of F-centers never reaches high enough values for the creation of secondary M-centers. Our experiments predict that F-centers grow during the night on Europa in the absence of photobleaching and then partially decay during the daytime. We show observations from the Hubble Space Telescope consistent with this prediction. All observations of the 450 nm F-center on Europa are now consistent with laboratory measurements of sodium chloride, confirming the presence of this salt on Europa.

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arXiv2022

A New UV Spectral Feature on Europa: Confirmation of NaCl in Leading-hemisphere Chaos Terrain

Recent visible-wavelength observations of Europa's surface obtained with the Hubble Space Telescope revealed the presence of an absorption feature near 450 nm that appears spatially correlated with leading-hemisphere chaos terrain. This feature was interpreted to reflect the presence of irradiated sodium chloride ultimately sourced from the interior. Here, we use ultraviolet spectra also collected with the Hubble Space Telescope to detect an additional previously unseen absorption near 230 nm, which spatially correlates with the 450 nm feature and with the same leading-hemisphere chaos terrain. We find that the new ultraviolet feature is also well-matched by irradiated sodium chloride at Europa-like conditions. Such confirmation of sodium chloride within geologically young regions has important implications for Europa's subsurface composition.

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arXiv2022

The mid-UV spectrum of irradiated NaCl at Europa-like conditions

Recent observations from the Hubble Space Telescope show a mid-UV absorption feature localized to leading hemisphere chaos regions on Europa. The same regions were previously found to have a visible absorption at 450 nm that was attributed to the presence of irradiated NaCl. The lack of any additional diagnostic absorptions for NaCl in the visible spectrum of these terrains made confirmation of this identification difficult. Here we use laboratory experiments to show that NaCl irradiated at Europa's surface temperatures develops an absorption at $\sim$220 nm consistent with the new detection in Europa's mid-UV spectrum, strongly supporting the NaCl identification. Irradiated NaCl in leading-hemisphere chaos terrain would suggest that sodium and chlorine are important components of Europa's subsurface ocean.

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