Showing papers by "Michael E. Brown published in 2017"

The Bimodal Color Distribution of Small Kuiper Belt Objects

Abstract: We conducted a two-night photometric survey of small Kuiper Belt objects (KBOs) near opposition using the wide-field Hyper Suprime-Cam instrument on the 8.2 m Subaru Telescope. The survey covered about 90 deg^2 of sky, with each field imaged in the g and i bands. We detected 356 KBOs, ranging in absolute magnitude from 6.5 to 10.4. Filtering for high-inclination objects within the hot KBO population, we show that the g − i color distribution is strongly bimodal, indicative of two color classes—the red and very red subpopulations. After categorizing objects into the two subpopulations by color, we present the first dedicated analysis of the magnitude distributions of the individual color subpopulations and demonstrate that the two distributions are roughly identical in shape throughout the entire size range covered by our survey. Comparing the color distribution of small hot KBOs with that of Centaurs, we find that they have similar bimodal shapes, thereby providing strong confirmation of previous explanations for the attested bimodality of Centaurs. We also show that the magnitude distributions of the two KBO color subpopulations and the two color subpopulations observed in the Jupiter Trojans are statistically indistinguishable. Finally, we discuss a hypothesis describing the origin of the KBO color bimodality based on our survey results.

Observational Bias and the Clustering of Distant Eccentric Kuiper Belt Objects

Abstract: The hypothesis that a massive Planet Nine exists in the outer solar system on a distant eccentric orbit was inspired by observations showing that the objects with the most distant eccentric orbits in the Kuiper Belt have orbits that are physically aligned, that is, they are clustered in longitude of perihelion and have similar orbital planes. Questions have remained, however, about the effects of observational bias on these observations, particularly on the longitudes of perihelion. Specifically, distant eccentric Kuiper Belt objects (KBOs) tend to be faint and only observable near their perihelia, suggesting that the longitudes of perihelion of the known distant objects could be strongly biased by the limited number of locations in the sky where deep surveys have been carried out. We have developed a method to rigorously estimate the bias in longitude of perihelion for Kuiper Belt observations. We find that the probability that the 10 known KBOs with semimajor axis beyond 230 au are drawn from a population with uniform longitude of perihelion is 1.2%. Combined with the observation that the orbital poles of these objects are also clustered, the overall probability of detecting these two independent clusterings in a randomly distributed sample is 0.025%. While observational bias is clearly present in these observations, it is unlikely to explain the observed alignment of the distant eccentric KBOs.

Effects of vertical loading on lateral screw pile performance

Abstract: The offshore wind energy sector faces new challenges as it moves into deeper water deployment. To meet these challenges, new and efficient foundation solutions are required. One potential solution is to upscale onshore screw piles but they require verification of performance for new geometries and demanding loading regimes. This paper presents a three-dimensional finite-element analysis investigation of screw pile behaviour when subjected to combined vertical and lateral loading in sand. In the investigation, the screw pile length and helical plate diameter were varied on piles with a fixed core diameter while subjecting the piles to combined axial and lateral loading. The results were compared with results from straight shafted piles with the same core diameter. The results of the analysis revealed that vertical compression loads increased the lateral capacity of the screw piles whereas vertical uplift loads marginally reduced the lateral capacity. The downside of this enhanced lateral capacity is that t...

The Density of Mid-sized Kuiper Belt Objects from ALMA Thermal Observations

Abstract: The densities of mid-sized Kuiper Belt objects (KBOs) are a key constraint in understanding the assembly of objects in the outer solar system. These objects are critical for understanding the currently unexplained transition from the smallest KBOs with densities lower than that of water, to the largest objects with significant rock content. Mapping this transition is made difficult by the uncertainties in the diameters of these objects, which maps into an even larger uncertainty in volume and thus density. The substantial collecting area of the Atacama Large Millimeter Array allows significantly more precise measurements of thermal emission from outer solar system objects and could potentially greatly improve the density measurements. Here we use new thermal observations of four objects with satellites to explore the improvements possible with millimeter data. We find that effects due to effective emissivity at millimeter wavelengths make it difficult to use the millimeter data directly to find diameters and thus volumes for these bodies. In addition, we find that when including the effects of model uncertainty, the true uncertainties on the sizes of outer solar system objects measured with radiometry are likely larger than those previously published. Substantial improvement in object sizes will likely require precise occultation measurements.

CPT-based design procedure for installation torque prediction for screw piles installed in sand.

Abstract: Screw piles (helical piles) have been used widely as foundations for onshore projects due to their ability to provide high compressive and tensile resistance as well as reduced noise/vibration during installation These types of piles have been proposed as a potential innovative foundation for offshore wind turbines in deeper water In order to adopt the screw pile technique as an offshore foundation, the geometry of the piles would need to be scaled up so they can provide the high capacities required for this application Such a change in size and geometry will lead to uncertainties in predicting the required torque for installation in different soil types and stress histories Without the ability to accurately predict installation torque it is difficult to design screw piles for offshore use or develop appropriate installation plant with the required torque capabilities in different soils This paper presents centrifuge test results of screw piles and CPT tests undertaken in dense sand The installation torque (T) has been correlated to the cone resistance qc to establish a proposed CPT-based design method to predict the required installation torque for modified screw pile geometries

Observational bias and the clustering of distant eccentric Kuiper belt objects

Abstract: The hypothesis that a massive Planet Nine exists in the outer solar system on a distant eccentric orbit was inspired by observations showing that the objects with the most distant eccentric orbits in the Kuiper belt have orbits which are physically aligned, that is, they are clustered in longitude of perihelion and have similar orbital planes. Questions have remained, however, about the effects of observational bias on these observations, particularly on the longitudes of perihelion. Specifically, distant eccentric Kuiper belt objects tend to be faint and only observable near their perihelia, suggesting that the longitudes of perihelion of the known distant objects could be strongly biased by the limited number of locations in the sky where deep surveys have been carried out. We have developed a method to rigorously estimate the longitude of perihelion bias for Kuiper belt observations. We find that the probability that the 10 known Kuiper belt objects with semimajor axis beyond 230 AU are drawn from a population with uniform longitude of perihelion is 1.2%. Combined with the observation that the orbital poles of these object are also clustered, the overall probability of detecting these two independent clusterings in a randomly distributed sample is 0.025%. While observational bias is clearly present in these observations, it is unlikely to explain the observed alignment of the distant eccentric Kuiper belt objects.

The density of mid-sized Kuiper belt objects from ALMA thermal observations

Abstract: The densities of mid-sized Kuiper belt objects are a key constraint into understanding the assembly of objects in the outer solar system. These objects are critical for understanding the currently unexplained transition from the smallest Kuiper belt objects with densities lower than that of water to the largest objects with significant rock content. Mapping this transition is made difficult by the uncertainties in the diameters of these objects, which maps into an even larger uncertainty in volume and thus density. The substantial collecting area of the Atacama Large Millimeter Array allows significantly more precise measurements of thermal emission from outer solar system objects and could potentially greatly improve the density measurements. Here we use new thermal observations of four objects with satellites to explore the improvements possible with millimeter data. We find that effects due to effective emissivity at millimeter wavelengths make it difficult to use the millimeter data directly to find diameters and thus volumes for these bodies. In addition, we find that when including the effects of model uncertainty, the true uncertainties on the sizes of outer solar system objects measured with radiometry are likely larger than those previously published. Substantial improvement in object sizes will likely require precise occultation measurements.

The Color–Magnitude Distribution of Hilda Asteroids: Comparison with Jupiter Trojans

Abstract: Current models of solar system evolution posit that the asteroid populations in resonance with Jupiter are comprised of objects scattered inward from the outer solar system during a period of dynamical instability. In this paper, we present a new analysis of the absolute magnitude and optical color distribution of Hilda asteroids, which lie in 3:2 mean-motion resonance with Jupiter, with the goal of comparing the bulk properties with previously published results from an analogous study of Jupiter Trojans. We report an updated power-law fit of the Hilda magnitude distribution through H = 14. Using photometric data listed in the Sloan Moving Object Catalog, we confirm the previously reported strong bimodality in visible spectral slope distribution, indicative of two subpopulations with differing surface compositions. When considering collisional families separately, we find that collisional fragments follow a unimodal color distribution with spectral slope values consistent with the bluer of the two subpopulations. The color distributions of Hildas and Trojans are comparable and consistent with a scenario in which the color bimodality in both populations developed prior to emplacement into their present-day locations. We propose that the shallower magnitude distribution of the Hildas is a result of an initially much larger Hilda population, which was subsequently depleted as smaller bodies were preferentially ejected from the narrow 3:2 resonance via collisions. Altogether, these observations provide a strong case supporting a common origin for Hildas and Trojans as predicted by current dynamical instability theories of solar system evolution.

Production of Sulfur Allotropes in Electron Irradiated Jupiter Trojans Ice Analogs

Abstract: In this paper, we investigate sulfur chemistry in laboratory analogs of Jupiter Trojans and Kuiper Belt Objects (KBOs). Electron irradiation experiments of CH_3OH–NH_3–H_2O and H_2S–CH_3OH–NH_3–H_2O ices were conducted to better understand the chemical differences between primordial planetesimals inside and outside the sublimation line of H_2S. The main goal of this work is to test the chemical plausibility of the hypothesis correlating the color bimodality in Jupiter Trojans with sulfur chemistry in the incipient solar system. Temperature programmed desorption (TPD) of the irradiated mixtures allows the detection of small sulfur allotropes (S_3 and S_4) after the irradiation of H2S containing ice mixtures. These small, red polymers are metastable and could polymerize further under thermal processing and irradiation, producing larger sulfur polymers (mainly S_8) that are spectroscopically neutral at wavelengths above 500 nm. This transformation may affect the spectral reflectance of Jupiter Trojans in a different way compared to KBOs, thereby providing a useful framework for possibly differentiating and determining the formation and history of small bodies. Along with allotropes, we report the production of organo-sulfur molecules. Sulfur molecules produced in our experiment have been recently detected by Rosetta in the coma of 67P/Churyumov–Gerasimenko. The very weak absorption of sulfur polymers in the infrared range hampers their identification on Trojans and KBOs, but these allotropes strongly absorb light at UV and Visible wavelengths. This suggests that high signal-to-noise ratio UV–Vis spectra of these objects could provide new constraints on their presence.

ALMA Thermal Observations of a Proposed Plume Source Region on Europa

Abstract: We present a daytime thermal image of Europa taken with the Atacama Large Millimeter Array. The imaged region includes the area northwest of Pwyll Crater, which is associated with a nighttime thermal excess seen by the Galileo Photopolarimeter Radiometer and with two potential plume detections. We develop a global thermal model of Europa and simulate both the daytime and nighttime thermal emission to determine if the nighttime thermal anomaly is caused by excess endogenic heat flow, as might be expected from a plume source region. We find that the nighttime and daytime brightness temperatures near Pwyll Crater cannot be matched by including excess heat flow at that location. Rather, we can successfully model both measurements by increasing the local thermal inertia of the surface.

A New Spectral Feature on the Trailing Hemisphere of Europa at 3.78 microns

Abstract: We present hemispherically resolved spectra of the surface of Europa from ~3.1--4.13 microns, which we obtained using the near infrared spectrometer NIRSPEC on the Keck II telescope. These include the first high-quality L-band spectra of the surface to extend beyond 4 microns. In our data we identify a previously unseen spectral feature at 3.78 microns on the trailing hemisphere. The longitudinal distribution of the feature is consistent with that of a radiolytic product created by electron or Iogenic ion bombardment. This feature is coincident with an absorption feature of SO2 frost seen in both laboratory spectra and spectra of Io. However, the corresponding, typically stronger 4.07 micron feature of SO2 frost is absent from our data. This result is contrary to the suggested detection of SO2 at 4.05 microns in Galileo NIMS data of the trailing hemisphere, which was severely affected by radiation noise. We use simple spectral modeling to argue that the 3.78 micron feature is not easily explained by the presence of SO2 frost on the surface. We explore alternative explanations and discuss other potential candidate species.

ALMA Thermal Observations of a Proposed Plume Source Region on Europa

Abstract: We present a daytime thermal image of Europa taken with the Atacama Large Millimeter Array. The imaged region includes the area northwest of Pwyll Crater, which is associated with a nighttime thermal excess seen by the Galileo Photopolarimeter Radiometer and with two potential plume detections. We develop a global thermal model of Europa and simulate both the daytime and nighttime thermal emission to determine if the nighttime thermal anomaly is caused by excess endogenic heat flow, as might be expected from a plume source region. We find that the nighttime and daytime brightness temperatures near Pwyll Crater cannot be matched by including excess heat flow at that location. Rather, we can successfully model both measurements by increasing the local thermal inertia of the surface.

Chalk-steel Interface testing for marine energy foundations

Abstract: To aid deployment and recovery of tidal stream generators, gravity-based foundations rather than fixed-foundation alternatives are being considered in areas where the foundation may be placed directly onto an exposed rock seabed. Horizontal loading is usually critical in such applications, therefore specific knowledge of the interface friction between the foundation (made of steel or concrete) and seabed is important for design. This paper presents results of an interface testing programme of chalk–steel interfaces carried out utilising a computer-controlled interface shear tester under constant normal stress conditions against steel of different roughness. Results indicate that interface strength is significantly affected by the normal stress applied, as interface strength degrades for normal stress levels in excess of 30% of the chalk's tensile strength (∼300 kPa). Large-displacement tests revealed a tendency of the ultimate interface frictional resistance to drop to values very similar to that of the b...

Don't call it a comeback: academic recovery and the timing of educational technology adoption

Abstract: Recent research using learning analytics data to explore student performance over the course of a term suggests that a substantial percentage of students who are classified as academically struggling manage to recover. In this study, we report the result of a hazard analysis based on students' behavioral engagement with different digital instructional technologies over the course of a semester. We observe substantially different adoption and use behavior between students who did and did not experience academic difficulty in the course. Students who experienced moderate academic difficulty benefited the most from using tools that helped them plan their study behaviors. Students who experienced more severe academic difficulty benefited from tools that helped them prepare for exams. We observed that students adopted most tools and system features before they experienced academic difficulty, and students who adopted early were more likely to recover.

A New Spectral Feature on the Trailing Hemisphere of Europa at 3.78 μm

Abstract: We present hemispherically resolved spectra of the surface of Europa from ~3.1–4.13 µm, which we obtained using the near-infrared spectrometer NIRSPEC on the Keck II telescope. These include the first high-quality L-band spectra of the surface to extend beyond 4 µm. In our data, we identify a previously unseen spectral feature at 3.78 µm on the trailing hemisphere. The longitudinal distribution of the feature is consistent with that of a radiolytic product created by electron or Iogenic ion bombardment. This feature is coincident with an absorption feature of SO_2 frost seen in both laboratory spectra and spectra of Io. However, the corresponding, typically stronger 4.07 µm feature of SO_2 frost is absent from our data. This result is contrary to the suggested detection of SO_2 at 4.05 µm in Galileo NIMS data of the trailing hemisphere, which was severely affected by radiation noise. We use simple spectral modeling to argue that the 3.78 µm feature is not easily explained by the presence of SO_2 frost on the surface. We explore alternative explanations and discuss other potential candidate species.

0.7–2.5μm Spectra of Hilda Asteroids

Abstract: The Hilda asteroids are primitive bodies in resonance with Jupiter whose origin and physical properties are not well understood. Current models posit that these asteroids formed in the outer solar system and were scattered along with the Jupiter Trojans into their present-day positions during a chaotic episode of dynamical restructuring. In order to explore the surface composition of these enigmatic objects in comparison with an analogous study of Trojans, we present new near-infrared spectra (0.7–2.5 μm) of 25 Hilda asteroids. No discernible absorption features are apparent in the data. Synthesizing the bimodalities in optical color and infrared reflectivity reported in previous studies, we classify 26 of the 28 Hildas in our spectral sample into the so-called less-red and red sub-populations and find that the two sub-populations have distinct average spectral shapes. Combining our results with visible spectra, we find that Trojans and Hildas possess similar overall spectral shapes, suggesting that the two minor body populations share a common progenitor population. A more detailed examination reveals that while the red Trojans and Hildas have nearly identical spectra, less-red Hildas are systematically bluer in the visible and redder in the near-infrared than less-red Trojans, indicating a putative broad, shallow absorption feature between 0.5 and 1.0 μm. We argue that the less-red and red objects found in both Hildas and Trojans represent two distinct surface chemistries and attribute the small discrepancy between less-red Hildas and Trojans to the difference in surface temperatures between the two regions.

An Experimental Study of the Embedment of a Dynamically Installed Anchor in Sand

Abstract: This paper presents a novel dynamically installed anchor concept suitable for sand. The anchor, referred to as the DPAIII, uses a thin ‘blade-like’ design to reduce bearing resistance during penetration, and comprises a lower plate attached to an upper removable follower. The anchor is installed through the kinetic energy it gains during free-fall in water. After embedment, the upper follower is removed leaving the lower plate anchor vertically embedded in the sand. This paper examines the embedment potential of the DPAIII through centrifuge tests conducted at 100 g in both loose and dense sand, using a model DPA III with different follower masses, impacting the model sand bed at two different velocities. The centrifuge tests show promising results, with anchor tip embedment in the range of 0.9 to 2.2 times the lower plate length. The tip embedment is found to be a function of the soil relative density, anchor mass and impact velocity.

$0.7-2.5~\mu$m spectra of Hilda asteroids

Abstract: The Hilda asteroids are primitive bodies in resonance with Jupiter whose origin and physical properties are not well understood. Current models posit that these asteroids formed in the outer Solar System and were scattered along with the Jupiter Trojans into their present-day positions during a chaotic episode of dynamical restructuring. In order to explore the surface composition of these enigmatic objects in comparison with an analogous study of Trojans (Emery et al. 2011), we present new near-infrared spectra (0.7-2.5 $\mu$m) of 25 Hilda asteroids. No discernible absorption features are apparent in the data. Synthesizing the bimodalities in optical color and infrared reflectivity reported in previous studies, we classify 26 of the 28 Hildas in our spectral sample into the so-called less-red and red sub-populations and find that the two sub-populations have distinct average spectral shapes. Combining our results with visible spectra, we find that Trojans and Hildas possess similar overall spectral shapes, suggesting that the two minor body populations share a common progenitor population. A more detailed examination reveals that while the red Trojans and Hildas have nearly identical spectra, less-red Hildas are systematically bluer in the visible and redder in the near-infrared than less-red Trojans, indicating a putative broad, shallow absorption feature between 0.5 and 1.0 $\mu$m. We argue that the less-red and red objects found in both Hildas and Trojans represent two distinct surface chemistries and attribute the small discrepancy between less-red Hildas and Trojans to the difference in surface temperatures between the two regions.

The color-magnitude distribution of Hilda asteroids: Comparison with Jupiter Trojans

Abstract: Current models of Solar System evolution posit that the asteroid populations in resonance with Jupiter are comprised of objects scattered inward from the outer Solar System during a period of dynamical instability. In this paper, we present a new analysis of the absolute magnitude and optical color distribution of Hilda asteroids, which lie in the 3:2 mean motion resonance with Jupiter, with the goal of comparing the bulk properties with previously published results from an analogous study of Jupiter Trojans. We report an updated power law fit of the Hilda magnitude distribution through H=14. Using photometric data listed in the Sloan Moving Object Catalog, we confirm the previously-reported strong bimodality in visible spectral slope distribution, indicative of two sub-populations with differing surface compositions. When considering collisional families separately, we find that collisional fragments follow a unimodal color distribution with spectral slope values consistent with the bluer of the two sub-populations. The color distributions of Hildas and Trojans are comparable and consistent with a scenario in which the color bimodality in both populations developed prior to emplacement into their present-day locations. We propose that the shallower magnitude distribution of the Hildas is a result of an initially much larger Hilda population, which was subsequently depleted as smaller bodies were preferentially ejected from the narrow 3:2 resonance via collisions. Altogether, these observations provide a strong case supporting a common origin for Hildas and Trojans as predicted by current dynamical instability theories of Solar System evolution.

ALMA Thermal Observations of Europa

Abstract: We present four daytime thermal images of Europa taken with the Atacama Large Millimeter Array. Together, these images comprise the first spatially resolved thermal data set with complete coverage of Europa's surface. The resulting brightness temperatures correspond to a frequency of 233 GHz (1.3 mm) and a typical linear resolution of roughly 200 km. At this resolution, the images capture spatially localized thermal variations on the scale of geologic and compositional units. We use a global thermal model of Europa to simulate the ALMA observations in order to investigate the thermal structure visible in the data. Comparisons between the data and model images suggest that the large-scale daytime thermal structure on Europa largely results from bolometric albedo variations across the surface. Using bolometric albedos extrapolated from Voyager measurements, a homogenous model reproduces these patterns well, but localized discrepancies exist. These discrepancies can be largely explained by spatial inhomogeneity of the surface thermal properties. Thus, we use the four ALMA images to create maps of the surface thermal inertia and emissivity at our ALMA wavelength. From these maps, we identify a region of either particularly high thermal inertia or low emissivity near 90° west and 23° north, which appears anomalously cold in two of our images.

Toward Less Conservative Flotation Criteria for Lightweight Cables and Pipelines.

Abstract: This paper aims to give a new perspective on possible criteria for the pipeline and cable flotation phenomenon. Laboratory flotation and pullout tests have been conducted in high moisture content kaolin clay using a model pipeline or cable analogue of 63mm diameter installed at different embedment depths. Uncertainty over the definition of “flotation” has led to comparison of data from previous studies and those obtained from the pull-out tests described herein. Different definitions of flotation may lead to a significant increase in the magnitude of available resistance used in design which can directly affect the commercial choices made for commercial pipelines and cables (e.g. coating type and thickness and minimum burial depth).

Exploring the Relationship between the Use of Learning Technologies and Student Success in the Engineering Classroom

Abstract: Exploring the Relationship between the Use of Learning Technologies and Student Success in the Engineering Classroom Mr. Robert Matthew DeMonbrun, University of Michigan Matt DeMonbrun is a Ph.D. Candidate at the Center for the Study of Higher and Postsecondary Education (CSHPE) in the School of Education at the University of Michigan. His research interests include college student development theory, intergroup interactions, and teaching and learning practices and how they relate to student learning outcomes in engineering education. Mr. Michael Geoffrey Brown, University of Michigan Michael Brown is a doctoral candidate in the Center for the Study of Higher and Post-Secondary Education at the University of Michigan.

Improvement of Seabed Cable Plough Tow Force Prediction Models

Abstract: As the use of offshore renewable energy sources such as wind and wave energy systems expands, they represent an increasing portion of the energy mix. Subsea cables connecting these resources are subject to many hazards, making cable burial using cable ploughing an important tool in preventing unplanned outages and downtime. This represents a significant installation cost for offshore renewables and there is a need to improve the accuracy of tow force, plough speed predictions and final burial depth of the cable. These are currently made using semiempirical models as the large soil displacements involved cause instabilities in the finite element analysis. This paper provides an overview of work to develop new material point method numerical software to allow cable ploughing to be analysed and optimised. The focus of this paper is the physical modelling carried out to provide validation of the software, which includes both 1g and centrifuge testing, as well as considering interim ways to improve the existing semi-empirical models.

The bimodal color distribution of small Kuiper Belt objects

Abstract: We conducted a two-night photometric survey of small Kuiper Belt objects (KBOs) near opposition using the wide-field Hyper Suprime-Cam instrument on the 8.2 m Subaru Telescope. The survey covered about 90 deg^2 of sky, with each field imaged in the g and i bands. We detected 356 KBOs, ranging in absolute magnitude from 6.5 to 10.4. Filtering for high-inclination objects within the hot KBO population, we show that the g-i color distribution is strongly bimodal, indicative of two color classes - the red and very red subpopulations. After categorizing objects into the two subpopulations by color, we present the first dedicated analysis of the magnitude distributions of the individual color subpopulations and demonstrate that the two distributions are roughly identical in shape throughout the entire size range covered by our survey. Comparing the color distribution of small hot KBOs with that of Centaurs, we find that they have similar bimodal shapes, thereby providing strong confirmation of previous explanations for the attested bimodality of Centaurs. We also show that the magnitude distributions of the two KBO color subpopulations and the two color subpopulations observed in the Jupiter Trojans are statistically indistinguishable. Finally, we discuss a hypothesis describing the origin of the KBO color bimodality based on our survey results.