Projectile-target interaction and rapid, high-temperature geochemical processes in impact melts

The collision of solid bodies at cosmic encounter velocities produces a variety of distinct physical, morphological, mineralogical, and petrological characteristics that are unique to hypervelocity impacts. Among these is melting of the projectile and substantial parts of the target it strikes. The resulting impact melts are typically intimate mixtures of melts from different precursor lithologies that initially occupied different stratigraphic positions in the crater’s melt zone; at small craters, projectile-derived melts may contribute substantially to the resulting melt mixture. These melts are mixed with each other during crater formation, bringing into contact melts that might have quite diverse physicochemical properties that reflect the different precursor lithologies from which they formed. This thesis investigates these processes by presenting a novel and so-far unique combination of petrologic observations on pristine natural impact glasses and impact melt rocks with results from specifically tailored laser irradiation and hypervelocity impact experiments. This thesis is initially concerned with an in-situ reconstruction of the interface between projectile and target materials in impact experiments that involved impact conditions similar to those during impacts of metallic micrometeoroids into regoliths of asteroids in the main belt. It is shown that under these impact conditions, substantial amounts of projectile melt remain as a continuous melt coating within the crater, and that projectile-coated, heterogeneous melt particles are produced that have a layered structure manifested in distinct layers of decreasing shock metamorphism (ranging from complete melting to below Hugoniot elastic limit; Chapter 5).These melt particles essentially sample the floor of the transient crater at early time steps of crater formation (before the transient crater reaches its final dimensions; Chapter 6), and, thus, preserve the original interface between projectile and target. Processes occurring along this interface are documented and discussed, and it is concluded that impacts of millimeter-size, metallic projectiles into asteroidal regoliths at typical impact velocities in the main belt result in qualitatively similar melt particles, but also that such planetary surfaces might accrete considerable amounts of foreign (i.e. non-endogenic) material. If such melt particles are retained within such regoliths, it is likely that their projectile component might influence surface reflectance spectra obtained from such surfaces. To further explore the chemical interaction between impact melts of diverse composition and structure, the thesis is then concerned with presenting and discussing a novel experimental approach capable of quasi-instantaneously producing, on macroscopic scales, melts and vapors from natural planetary materials by means of direct, continuous-wave laser irradiation (Chapter 7). These experiments simulate post-shock pressure–temperature conditions of hypervelocity impacts in the 4 to 20 km/s range, and experimental products (silicate glasses) are shown to be petrologically and thermodynamically similar to true impact melts (quenched to impact glasses) formed from similar starting materials. This is achieved by “matching” the entropy gains of the laser-generated melts to the entropy gains associated with the thermodynamic states produced in hypervelocity impacts at specific velocities. Thus, macroscopic volumes of melt can be produced that thermodynamically as well as petrologically resemble true impact melts formed at that specific impact velocity. The rest of the thesis is then concerned with investigating petrogenetic processes in impact melts that are so-far only poorly constrained. Chapters 8 and 9 investigate unmixing of silicate impact melts of diverse chemical composition and cooling history due to sensu stricto liquid immiscibility. This represents not only the first comprehensive study of silicate liquid immiscibility in terrestrial impact melts and experimental analogs, but also the first clear evidence of silicate liquid immiscibility in meteorites. Textural evidence of silicate-melt unmixing is presented, and it is shown that the compositions of the conjugate immiscible liquids (Si-rich and Fe-rich) are consistent with phase separation in high- or low-temperature two-liquid fields in the common petrologic (e.g., basaltic) system. Moreover, major-element partition coefficients are correlated with the degree of polymerization of the Si-rich melt. Hence, major-element partitioning between the conjugate liquids as well as the resulting emulsion textures are similar to those known from tholeiitic basalts, lunar basalts, and experimental analogs. However, in impact melts, the high-temperature, “quasi-binary” miscibility gaps (e.g., silica–ferrous oxide; T > 1700 °C) might be encountered that are inaccessible to endogenous magmatic systems. The characteristics of impact melt inhomogeneity produced by melt unmixing in a miscibility gap are then compared to impact melt inhomogeneity caused by incomplete homogenization of different (miscible or immiscible) impact melts, and petrographic tools are presented to distinguish between these two. Chapter 10 presents results from laser irradiation experiments aimed at constraining the fate of carbonates (calcite, dolomite) entrained in superheated silicate melts. Carbonate decomposition by coexisting silicate impact melt is shown to be extremely fast (tens of seconds), and results in contamination of silicate melt with carbonate-derived calcium oxide (and magnesium oxide in the case of dolomite) and release of carbon dioxide at the silicate melt–carbonate interface. Several, partially transient processes are shown to occur at this interface that are largely similar to the formation of calcic skarns during contact metamorphism. It is suggested that upon pressure release, “protracted” decomposition of carbonates by heat influx from coexisting silicate impact melt is an important, if not dominant, process during impact melting of mixed silicate–carbonate targets. However, a combination of these experimental findings, findings from previous studies, and consideration of the phase diagrams of calcite and quartz suggests that, as envisaged by a number of studies, carbonate impact melts are indeed readily produced during adiabatic decompression. Eventually, a working model is presented that suggests that hypervelocity impacts into mixed silicate–carbonate targets may involve both melting and decomposition of carbonates in specific parts of an impact crater, thus bringing together seemingly contrasting previous findings from impactites from terrestrial impact structures. The working model also suggests that impact-induced carbon dioxide degassing of carbonate-bearing targets during hypervelocity impact is more complex than previously thought, which has implications for estimating the net amount of carbon dioxide that is discharged into the atmosphere immediately after such impacts

The Realities and Misconceptions of Greek Life

Individual fraternity and sorority chapters vary greatly at every institution. One bond commonly shared by Greek Life Organizations at all institutions are the misconceptions and realities associated with being affiliated with the Greek system. Some realities may persuade a student to consider joining a Greek organization, while many of the misconceptions may deter a potential new member from even considering Greek life as a worthwhile, or rather safe, option. This video pamphlet aims to shed light on some of the common misconceptions and realities of the Greek life system which will in turn allow potential members to make an educated decision as to whether or not Greek life could have a positive impact on their life. The accompanying brochure provides overview of the many benefits to membership in a Greek letter organization as well as prevention and education opportunities implemented to combat many of the Greek Life misconceptions

PACE Solver Description: The KaPoCE Exact Cluster Editing Algorithm

The cluster editing problem is to transform an input graph into a cluster graph by performing a minimum number of edge editing operations. A cluster graph is a graph where each connected component is a clique. An edit operation can be either adding a new edge or removing an existing edge. In this write-up we outline the core techniques used in the exact cluster editing algorithm of the KaPoCE framework (contains also a heuristic solver), submitted to the exact track of the 2021 PACE challenge

PACE Solver Description: KaPoCE: A Heuristic Cluster Editing Algorithm

The cluster editing problem is to transform an input graph into a cluster graph by performing a minimum number of edge editing operations. A cluster graph is a graph where each connected component is a clique. An edit operation can be either adding a new edge or removing an existing edge. In this write-up we outline the core techniques used in the heuristic cluster editing algorithm of the Karlsruhe and Potsdam Cluster Editing (KaPoCE) framework, submitted to the heuristic track of the 2021 PACE challenge

VLBA Observations of Sub-Parsec Structure in Mrk 231: Interaction between a Relativistic Jet and a BAL Wind

We report on the first high frequency VLBI observations of the nearby broad absorption line quasar (BALQSO), Mrk 231. Three epochs of observations were achieved at 15 GHz and 22 GHz, two of these included 43 GHz observations as well. The nuclear radio source is resolved as a compact double. The core component experienced a strong flare in which the flux density at 22 GHz increased by $> 150$ (45 mJy) in three months. Theoretical models of the flare imply that the emission is likely enhanced by very strong Doppler boosting of a highly relativistic ejecta with a kinetic energy flux, $Q \sim 3 \times 10^{43} \mathrm{ergs/sec}$. Combining our data with two previous epochs of 15 GHz data, shows marginal evidence for the slow advance of the secondary component (located $\approx 0.97$ pc from the core) over a 9.4 year span. We estimate that the long term time averaged kinetic energy flux of the secondary at $\bar{Q}\approx 10^{42}\mathrm{ergs/sec}$. Low frequency VLBA observations indicate that the secondary is seen through a shroud of free-free absorbing gas with an emission measure of $\approx 10^{8} \mathrm{cm}^{-6}\mathrm{pc}$. The steep spectrum secondary component appears to be a compact radio lobe that is associated with a working surface between the ram-pressure confined jet, and a dense medium that is likely to be the source of the free-free absorption. The properties of the dense gas are consistent with the temperatures, displacement from the nucleus and the column density of total hydrogen commonly associated with the BAL wind.Comment: To appear in Ap

PACE solver description: The KaPoCE exact cluster editing algorithm

The cluster editing problem is to transform an input graph into a cluster graph by performing a minimum number of edge editing operations. A cluster graph is a graph where each connected component is a clique. An edit operation can be either adding a new edge or removing an existing edge. In this write-up we outline the core techniques used in the exact cluster editing algorithm of the KaPoCE framework (contains also a heuristic solver), submitted to the exact track of the 2021 PACE challenge

PACE solver description: KaPoCE: A heuristic cluster editing algorithm

The cluster editing problem is to transform an input graph into a cluster graph by performing a minimum number of edge editing operations. A cluster graph is a graph where each connected component is a clique. An edit operation can be either adding a new edge or removing an existing edge. In this write-up we outline the core techniques used in the heuristic cluster editing algorithm of the Karlsruhe and Potsdam Cluster Editing (KaPoCE) framework, submitted to the heuristic track of the 2021 PACE challenge

Increasing Neff with particles in thermal equilibrium with neutrinos

Recent work on increasing the effective number of neutrino species (Neff) in the early universe has focussed on introducing extra relativistic species (`dark radiation'). We draw attention to another possibility: a new particle of mass less than 10 MeV that remains in thermal equilibrium with neutrinos until it becomes non-relativistic increases the neutrino temperature relative to the photons. We demonstrate that this leads to a value of Neff that is greater than three and that Neff at CMB formation is larger than at BBN. We investigate the constraints on such particles from the primordial abundance of helium and deuterium created during BBN and from the CMB power spectrum measured by ACT and SPT and find that they are presently relatively unconstrained. We forecast the sensitivity of the Planck satellite to this scenario: in addition to dramatically improving constraints on the particle mass, in some regions of parameter space it can discriminate between the new particle being a real or complex scalar.Comment: 10 pages, 5 figures v2 matches version to appear in JCA

Reconstructing the primordial power spectrum from the CMB

We propose a straightforward and model independent methodology for characterizing the sensitivity of CMB and other experiments to wiggles, irregularities, and features in the primordial power spectrum. Assuming that the primordial cosmological perturbations are adiabatic, we present a function space generalization of the usual Fisher matrix formalism, applied to a CMB experiment resembling Planck with and without ancillary data. This work is closely related to other work on recovering the inflationary potential and exploring specific models of non-minimal, or perhaps baroque, primordial power spectra. The approach adopted here, however, most directly expresses what the data is really telling us. We explore in detail the structure of the available information and quantify exactly what features can be reconstructed and at what statistical significance.Comment: 43 pages Revtex, 23 figure

Stagnation and Infall of Dense Clumps in the Stellar Wind of tau Scorpii

Observations of the B0.2V star tau Scorpii have revealed unusual stellar wind characteristics: red-shifted absorption in the far-ultraviolet O VI resonance doublet up to +250 km/s, and extremely hard X-ray emission implying gas at temperatures in excess of 10^7 K. We describe a phenomenological model to explain these properties. We assume the wind of tau Sco consists of two components: ambient gas in which denser clumps are embedded. The clumps are optically thick in the UV resonance lines primarily responsible for accelerating the ambient wind. The reduced acceleration causes the clumps to slow and even infall, all the while being confined by the ram pressure of the outflowing ambient wind. We calculate detailed trajectories of the clumps in the ambient stellar wind, accounting for a line radiation driving force and the momentum deposited by the ambient wind in the form of drag. We show these clumps will fall back towards the star with velocities of several hundred km/sec for a broad range of initial conditions. The infalling clumps produce X-ray emitting plasmas with temperatures in excess of (1-6)x10^7 K in bow shocks at their leading edge. The infalling material explains the peculiar red-shifted absorption wings seen in the O VI doublet. The required mass loss in clumps is 3% - 30% ofthe total mass loss rate. The model developed here can be generally applied to line-driven outflows with clumps or density irregularities. (Abstract Abridged)Comment: To appear in the ApJ (1 May 2000). 24 pages, including 6 embedded figure