1,721,135 research outputs found
Physicochemical Controls on the Compositions of the Earth and Planets
No full textAbstract Despite the fact that the terrestrial planets all formed from the protoplanetary disk, their bulk compositions show marked departures from that of material condensing from a canonical H 2 -rich solar nebula. Metallic cores fix the oxygen fugacities ( f O 2 s) of the planets to between ∼5 (Mercury) and ∼1 log units below the iron-wüstite (IW) buffer, orders of magnitude higher than that of the nebular gas. Their oxidised character is coupled with a lack of volatile elements with respect to the solar nebula. Here we show that condensates from a canonical solar gas at different temperatures ( T 0 ) produce bulk compositions with Fe/O (by mass) ranging from ∼0.93 ( T 0 = 1250 K) to ∼0.81 ( T 0 = 400 K), far lower than that of Earth at 1.06. Because the reaction Fe(s) + H 2 O(g) = FeO(s) + H 2 (g) proceeds only below ∼600 K, temperatures at which most moderately volatile elements (MVEs) have already condensed, oxidised planets are expected to be rich in volatiles, and vice-versa. That this is not observed suggests that planets i ) did not accrete from equilibrium nebular condensates and/or i i ) underwent additional volatile depletion/ f O 2 changes at conditions distinct from those of the solar nebula. Indeed, MVE abundances in small telluric bodies (Moon, Vesta) are consistent with evaporation/condensation at Δ IW-1 and ∼1400–1800 K, while the extent of mass-dependent isotopic fractionation observed implies this occurred near- or at equilibrium. On the other hand, the volatile-depleted elemental- yet near-chondritic isotopic compositions of larger telluric bodies (Earth, Mars) reflect mixing of bodies that had themselves experienced different extents of volatile depletion, overprinted by accretion of volatile-undepleted material. On the basis of isotopic anomalies in Cr- and Ti in the BSE, such undepleted matter has been proposed to be CI chondrites, which would comprise 40% by mass if the proto-Earth were ureilite-like. However, this would result in an overabundance of volatile elements in the BSE, requiring significant loss thereafter, which has yet to be demonstrated. On the other hand, 6% CI material added late to an enstatite chondrite-like proto-Earth would broadly match the BSE composition. However, because the Earth is an end-member in isotopic anomalies of heavier elements, no combination of existing meteorites alone can account for its chemical- and isotopic composition. Instead, the Earth is most likely made partially or essentially entirely from an NC-like missing component. If so, the oxidised-, yet volatile-poor nature of differentiated bodies in the inner solar system, including Earth and Mars, is a property intrinsic to the NC reservoir.Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung http://dx.doi.org/10.13039/501100001711Staatssekretariat für Bildung, Forschung und Innovation http://dx.doi.org/10.13039/501100007352HORIZON EUROPE European Research Council http://dx.doi.org/10.13039/100019180Ministero dell’Istruzione, dell’Università e della Ricerca http://dx.doi.org/10.13039/501100003407National Science Foundation http://dx.doi.org/10.13039/100000001National Aeronautics and Space Administration http://dx.doi.org/10.13039/100000104Swiss Federal Institute of Technology Zuric
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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Isotopic evolution during Earth-Moon formation and general circulation in Jupiter's middle atmosphere
Full text linkThis thesis is constructed around two distinct topics. The first is the formation history of the Earth and the Moon. The hafnium/tungsten (Hf/W) isotopic system can act as a chronometer for planets forming in the early solar system. To study possible planetary formation scenarios, I model the isotopic evolution of planetary embryos as they form rocky planets during collisions in N-body simulations. In Chapter 2, I show that the fast accretion timescales of the Grand Tack scenario require highly efficient re-equilibration of W to produce an Earth with observed mantle W isotope anomaly (excess of radiogenic tungsten compared to non-radiogenic). Such a high level of re-equilibration is not supported by fluid dynamic experiments, and this result suggests the Grand Tack scenario builds the Earth too quickly. The Earth and Moon share a very similar isotopic fingerprint: many chemical isotopes found in lunar rocks are nearly identical to ones found on Earth. It is particularly interesting that they also share a near-identical W isotope anomaly, because simply starting from similar material is not sufficient. This system evolves depending on the collision history of bodies, so the Earth and Moon sharing W isotopic values require an explanation. The canonical model of the Moon formation holds that it is mostly made up of material from Theia, the impactor into Earth. In Chapter 3, I apply the isotopic evolution model to the canonical Earth-Moon impact formation scenario. Using 242 N-body simulation results, I demonstrate the likelihood of forming an Earth and Moon with near-identical W isotope anomaly is less than 5%. This suggests that an alternate explanation for forming the Moon with Earth material may be necessary to explain the similarity in W isotope anomaly.
The second topic is understanding the temperature, zonal wind, and general circulation that occurs in the middle atmosphere of Jupiter. The Voyager and Cassini spacecraft, along with many ground-based telescopic observations, have provided zonally averaged distributions of temperature, gaseous species, and haze particles in Jupiter’s upper troposphere and stratosphere. Measurements of wind speed are derived from cloud movement near the 0.5 – 1 bar pressure level, but we have no measurements of circulation in the stratosphere. Historical models of this region have used 2D linearized equations of motion and simple radiative calculations. In Chapter 4, I present a state-of-the-art 2D dynamical model of Jupiter’s middle atmosphere with realistic radiative transfer. A dynamical model with a simple frictional drag force, representing eddy forces that damp the mean zonal wind, is able to reproduce some of the small latitudinal temperature variations seen on Jupiter. However, none of the models tested were able to produce strong >5 K variations observed in the low-to-mid latitudes between 1 – 500 mbar. This suggests that localized wave forcing plays a dominant role in shaping the temperature distribution in the middle atmosphere of Jupiter. I also show that polar temperatures are strongly dependant on the chosen optical properties of stratospheric haze, and further work constraining haze opacity is needed to accurately model heating and cooling at the poles
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The Internal History of the Moon and Kuiper Belt Objects from Gravity and Topography
Full text linkThis thesis comprises two separate but interesting projects that attempt to constrain the internal history of planetary bodies. The first set attempts to interpret the Moon's internal thermal history from the relaxation state of lunar impact basins. As the Moon cools, impact structures degrade at a slower and slower rate. This can be observed in maps of lunar topography and crustal thickness. This analysis, however, was greatly enhanced by the GRAIL spacecraft mission to the Moon. In Chapter 2,I present the first relaxation analysis of the most up-to-date complete lunar impact basin catalog. With the addition of ~6 new impact basins and the re-qualification of other basins, a basin relaxation transition is clearly observed in the lunar impact record. This relaxation transition signal can be used to constrain and link lunar solidification and cooling models with impact chronology models. In that study, I find that if the lunar surface experienced a lull in basin-class impacts it must have solidified and cooled rapidly following its formation.The second project involves two studies that try to understand the thermal history of Pluto and its moon Charon. In July 2015, the field of Kuiper Belt Objects was greatly widened with the arrival of the New Horizons spacecraft at the Pluto-Charon system. One of the major discoveries of that mission was the prevalence of extensional tectonic features on both worlds, a likely signal of a frozen-out (or possibly still freezing) subsurface ocean. In Chapter 3, I characterize the large extensional tectonics features in the encounter hemisphere on Pluto. Then by comparing the features to topographic flexure models, I was able to constrain the maximum surface heat flux experienced by Pluto. This showed that Pluto's internal evolution matches thermal models that primarily use a radiogenic heat source.Although Chapter 3 put maximum heat flux constrains on the thermal history of Pluto, the constraints can be improved upon and expanded to include analysis of Charon's surface. In Chapter 4, I create and use limb profile topography of Pluto andCharon to understand the differences in the morphological and interior history of the two worlds. This is achieved by calculating the topographic variance spectra from limb profiles, which typically results in a single-power law spectrum. While this typical case holds for Pluto, it does not for Charon which displays a characteristic wavelength. My analysis further constrains an upper limit for Pluto's maximum surface heat flux, but it also sets a range for the absolute maximum heat flux for Charon that cannot be solely explained by radiogenic heating. This implies that an extra heat source, probably tidal heating, was necessary
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The Physics and Chemistry of Terrestrial Planetary and Satellite Accretion
Full text linkThis dissertation examines the influence which a geophysical process (giant impacts) has on a geochemical marker (composition) during terrestrial planet formation. Simultaneously studying all planets maximizes the available constraints and permits examination of controls on the overall composition of the Earth. I also examine the Galilean satellite system to determine the universality of the terrestrial conclusions.The late stages of planetary accretion involve stochastic, large collisions. Impact-related erosion and fragmentation can have profound consequences for the rate and style of accretion and the bulk chemistries of terrestrial planets. However, the previous predominate assumption in computer models of accretion was that all collisions resulted in perfect merging despite the likelihood of these collisions producing a range of outcomes (e.g., hit-and-run, removal of material from target, or production of several post-collision bodies). In this work, I investigate the effects of late-stage accretion with multiple collision types and the consequences on the bulk (mantle/core) and isotopic (Hf--W) composition. My model is composed of two parts: (1) N-body accretion code tracks orbital and collisional evolution of the bodies and (2) geochemical post-processing evolves composition in light of impact-related mixing, partial equilibration and radioactive decay. For terrestrial planets, Part (1) is Chambers (2013, Icarus) and incorporates multiple collisional outcomes. For Galilean satellites, Part (1) is Ogihara & Ida (2012, Icarus) and assumes perfect merging for all collisions thus the model is not self-consistent (it likely overestimates compositional changes).For the terrestrial planets, the results are consistent with observed mantle/core ratios and tungsten isotopic anomalies. A moderate (approx. 0.4) core equilibration factor is preferred due to protracted accretion time. It is important to include multi-modal collisions when modeling planet formation if composition, timescales, or spatial distribution of mass are being investigated. I could not reproduce the observed ice fraction gradient of the Galilean satellites, even with an initial compositional gradient and vaporization of water ice. Some other physical process(es) are needed, perhaps tidally-driven volatile loss at Io and Europa. Extensive inward radial migration smooths out initial compositional gradients
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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The Crusts of Mars, Tethys, and Mimas: Geophysical Exploration of Historic Heat Flow
Full text linkThe evolution of a planetary body often determines and is determined by its thermal properties. In my first project, I explore the consequences of heating upon pore closure, allowing me to estimate the heat flow through the Martian crust during the latest significant pore generation event—likely large basin-forming impacts. We apply a pore closure model developed for the Moon to Mars and take into account the geological processes that may alter the depth of a transition between porous and competent crust. If the 8–11 km deep discontinuity in seismic wave speed detected by the InSight lander marks the base of the uppermost porous layer of the Martian crust, then the heat flux at the time the porosity was created must exceed 60 mW m^−2, indicating a time prior to 4 Ga. Then, I explore how the global shape of an icy satellite allows us to infer its heat budget and interior—including the presence or absence of a subsurface global ocean. I apply this method in my second and third projects to Tethys and Mimas, respectively. We assume spatial variations in tidal heating are responsible for thickness or temperature variations in an isostatic ice shell, which manifests as surface topography. For Saturn’s moon Tethys, our best-fit models require Pratt isostasy and obliquity tides, with a normalized moment of inertia 0.340-0.345 and an average surface heat flux 1-2 mW m^−2. Then, we find that to account for its hydrostatic shape, Mimas’ normalized moment of inertia is 0.375, indicating a relatively undifferentiated world. Its remaining topography is consistent with a ∼30 km thick conductive ice shell in Airy isostasy atop a weakly convecting ∼30 km thick layer that itself mantles a ∼140 km radius ice-rock interior. For neither satellite do we find an ocean. However, the total power and pattern inferred to produce both satellites’ shapes from tidal heating indicate an ancient era of high obliquity. The common thread of all three projects is the flow of heat, and how our understanding of it can be revealed by or can reveal properties of the planetary bodies we study
Planetary Heat: Exploring how Planetary Surfaces are Shaped
Full text linkThis thesis consists of three loosely related projects exploring the physics of planetary bodies. The throughline in this research is that I explore how a planetary body's interior influences its exterior -- in particular how heat migrating outward drives evolution and leaves detectable traces of that evolution.
Chapter One describes a novel form of volcanism -- volcanism on iron bodies, which we call ferrovolcanism. We predict that metallic bodies were able to host volcanism, making metal the third major type of crustal material capable of being volcanic, in addition to ice and silicate planets. We discuss the potential for its observation by the Psyche mission, its role in the evolution of metallic bodies, and its potential influence on the metallic meteorite record.
Chapter Two lays out a way to significantly improve Europa Clipper's ability to measure Europa's global shape, without requiring any extra measurements. By using stellar occultations, measurements that Europa Clipper was already planning to collect, we can supplement radar altimetry to obtain more complete global coverage of Europa. We demonstrated the potential for this combined dataset to significantly improve global fits, which would allow Europa Clipper to better constrain the thickness, rheology, and history of Europa's ice shell.
Chapter Three explores the relationship between rotation rate and tidal dissipation in the interior of Jupiter's moon Io. This is motivated by two separate lines of thinking: 1) Io's volcanoes appear to be offset in longitude from where tidal dissipation models predict they should form, and 2) if a satellite is sufficiently fluid - plausible for Io because it is so strongly heated - it is expected to rotate slightly faster than the synchronous rotation rate we see across solar system satellites. We find that because of the rigidity of its lithosphere, we do not expect Io to rotate nonsynchronously on geophysically relevant timescales
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