Structure of 8B from elastic and inelastic 7Be+p scattering

Motivation: Detailed experimental knowledge of the level structure of light weakly bound nuclei is necessary to guide the development of new theoretical approaches that combine nuclear structure with reaction dynamics. Purpose: The resonant structure of 8B is studied in this work. Method: Excitation functions for elastic and inelastic 7Be+p scattering were measured using a 7Be rare isotope beam. Excitation energies ranging between 1.6 and 3.4 MeV were investigated. An R-matrix analysis of the excitation functions was performed. Results: New low-lying resonances at 1.9, 2.5, and 3.3 MeV in 8B are reported with spin-parity assignment 0+, 2+, and 1+, respectively. Comparison to the Time Dependent Continuum Shell (TDCSM) model and ab initio no-core shell model/resonating-group method (NCSM/RGM) calculations is performed. This work is a more detailed analysis of the data first published as a Rapid Communication. [J.P. Mitchell, et al, Phys. Rev. C 82, 011601(R) (2010)] Conclusions: Identification of the 0+, 2+, 1+ states that were predicted by some models at relatively low energy but never observed experimentally is an important step toward understanding the structure of 8B. Their identification was aided by having both elastic and inelastic scattering data. Direct comparison of the cross sections and phase shifts predicted by the TDCSM and ab initio No Core Shell Model coupled with the resonating group method is of particular interest and provides a good test for these theoretical approaches.Comment: 15 pages, 19 figures, 3 tables, submitted to PR

Low-lying states in 8B

Excitation functions of elastic and inelastic 7Be+p scattering were measured in the energy range between 1.6 and 2.8 MeV in the c.m. An R-matrix analysis of the excitation functions provides strong evidence for new positive parity states in 8B. A new 2+ state at an excitation energy of 2.55 MeV was observed and a new 0+ state at 1.9 MeV is tentatively suggested. The R-matrix and Time Dependent Continuum Shell Model were used in the analysis of the excitation functions. The new results are compared to the calculations of contemporary theoretical models.Comment: 6 pages, 5 figures, accepted as Rapid Communication in Phys. Rev.

Mixing model of Phobos' bulk elemental composition for the determination of its origin: Multivariate analysis of MMX/MEGANE data

The formation process of the two Martian moons, Phobos and Deimos, is still debated with two main competing hypotheses: the capture of an asteroid or a giant impact onto Mars. In order to reveal their origin, the Martian Moons eXploration (MMX) mission by Japan Aerospace Exploration Agency (JAXA) plans to measure Phobos' elemental composition by a gamma-ray and neutron spectrometer called MEGANE. This study provides a model of Phobos' bulk elemental composition, assuming the two formation hypotheses. Using the mixing model, we established a MEGANE data analysis flow to discriminate between the formation hypotheses by multivariate analysis. The mixing model expresses the composition of Phobos in 6 key lithophile elements that will be measured by MEGANE (Fe, Si, O, Ca, Mg, and Th) as a linear mixing of two mixing components: material from Mars and material from an asteroid as represented by primitive meteorite compositions. The inversion calculation includes consideration of MEGANE's measurement errors ($E_P$) and derives the mixing ratio for a given Phobos composition, based on which the formation hypotheses are judged. For at least 65\% of the modeled compositions, MEGANE measurements will determine the origin uniquely ($E_P$ = 30\%), and this increases from 74 to 87\% as $E_P$ decreases from 20 to 10\%. Although the discrimination performance depends on $E_P$, the current operation plan for MEGANE predicts an instrument performance for $E_P$ of 20--30\%, resulting in ~70\% discrimination between the original hypotheses. MEGANE observations can also enable the determination of the asteroid type of the captured body or the impactor. The addition of other measurements, such as MEGANE's measurements of the volatile element K, as well as observations by other MMX remote sensing instruments, will also contribute to the MMX mission's goal to constrain the origin of Phobos.Comment: 34 pages, 7 figures, accepted for publication in Icaru

Carbon on Mercury's Surface - Origin, Distribution, and Concentration

Distinctive low-reflectance material (LRM) was first observed on Mercury in Mariner 10 flyby images. Visible to near-infrared reflectance spectra of LRM are flatter than the average reflectance spectrum of Mercury, which is strongly red sloped (increasing in reflectance with wavelength). From Mariner 10 and early MErcury, Surface, Space, ENvironment, GEochemistry, and Ranging (MESSENGER) flyby observations, it was suggested that a higher content of ilmenite, ulvospinel, carbon, or iron metal could cause both the characteristic dark, flat spectrum of LRM and the globally low reflectance of Mercury. Once MESSENGER entered orbit, low Fe and Ti abundances measured by the X-Ray and Gamma-Ray Spectrometers ruled out ilmenite, and ulvospinel as important surface constituents and implied that LRM was darkened by a different phase, such as carbon or small amounts of micro- or nanophase iron or iron sulfide dispersed in a silicate matrix. Low-altitude thermal neutron measurements of three LRM-rich regions confirmed an enhancement of 1-3 weight-percent carbon over the global abundance, supporting the hypothesis that LRM is darkened by carbon

A Mercury Lander Mission Concept Study for the Next Decadal Survey

Mariner 10 provided our first closeup reconnaissance of Mercury during its three flybys in 1974 and 1975. MESSENGERs 20112015 orbital investigation enabled numerous discoveries, several of which led to substantial or complete changes in our fundamental understanding of the planet. Among these were the unanticipated, widespread presence of volatile elements (e.g., Na, K, S); a surface with extremely low Fe abundance whose darkening agent is likely C; a previously unknown landformhollows that may form by volatile sublimation from within rocks exposed to the harsh conditions on the surface; a history of expansive effusive and explosive volcanism; substantial radial contraction of the planet from interior cooling; offset of the dipole moment of the internal magnetic field northward from the geographic equator by ~20% of the planets radius; crustal magnetization, attributed at least in part to an ancient field; unexpected seasonal variability and relationships among exospheric species and processes; and the presence in permanently shadowed polar terrain of water ice and other volatile materials, likely to include complex organic compounds. Mercurys highly chemically reduced and unexpectedly volatile-rich composition is unique among the terrestrial planets and was not predicted by earlier hypotheses for the planets origin. As an end-member of terrestrial planet formation, Mercury holds unique clues about the original distribution of elements in the earliest stages of the Solar System and how planets (and exoplanets) form and evolve in close proximity to their host stars. The BepiColombo mission promises to expand our knowledge of this planet and to shed light on some of the mysteries revealed by the MESSENGER mission. However, several fundamental science questions raised by MESSENGERs pioneering exploration of Mercury can only be answered with in situ measurements from the planets surface

Olivine or Impact Melt: Nature of the "Orange" Material on Vesta from Dawn

NASA's Dawn mission observed a great variety of colored terrains on asteroid (4) Vesta during its survey with the Framing Camera (FC). Here we present a detailed study of the orange material on Vesta, which was first observed in color ratio images obtained by the FC and presents a red spectral slope. The orange material deposits can be classified into three types, a) diffuse ejecta deposited by recent medium-size impact craters (such as Oppia), b) lobate patches with well-defined edges, and c) ejecta rays from fresh-looking impact craters. The location of the orange diffuse ejecta from Oppia corresponds to the olivine spot nicknamed "Leslie feature" first identified by Gaffey (1997) from ground-based spectral observations. The distribution of the orange material in the FC mosaic is concentrated on the equatorial region and almost exclusively outside the Rheasilvia basin. Our in-depth analysis of the composition of this material uses complementary observations from FC, the visible and infrared spectrometer (VIR), and the Gamma Ray and Neutron Detector (GRaND). Combining the interpretations from the topography, geomorphology, color and spectral parameters, and elemental abundances, the most probable analog for the orange material on Vesta is impact melt

The Distribution and Origin of Smooth Plains on Mercury

Orbital images from the MESSENGER spacecraft show that ~27% of Mercury's surface is covered by smooth plains, the majority (greater than 65%) of which are interpreted to be volcanic in origin. Most smooth plains share the spectral characteristics of Mercury's northern smooth plains, suggesting they also share their magnesian alkali-basalt-like composition. A smaller fraction of smooth plains interpreted to be volcanic in nature have a lower reflectance and shallower spectral slope, suggesting more ultramafic compositions, an inference that implies high temperatures and high degrees of partial melting in magma source regions persisted through most of the duration of smooth plains formation. The knobby and hummocky plains surrounding the Caloris basin, known as Odin-type plains, occupy an additional 2% of Mercury’s surface. The morphology of these plains and their color and stratigraphic relationships suggest that they formed as Caloris ejecta, although such an origin is in conflict with a straightforward interpretation of crater size-frequency distributions. If some fraction is volcanic, this added area would substantially increase the abundance of relatively young effusive deposits inferred to have more mafic compositions. Smooth plains are widespread on Mercury, but they are more heavily concentrated in the north and in the hemisphere surrounding Caloris. No simple relationship between plains distribution and crustal thickness or radioactive element distribution is observed. A likely volcanic origin for some older terrain on Mercury suggests that the uneven distribution of smooth plains may indicate differences in the emplacement age of large-scale volcanic deposits rather than differences in crustal formational process

Statistical Study of Mercury’s Energetic Electron Events as Observed by the Gamma‐Ray and Neutron Spectrometer Instrument Onboard MESSENGER

We present results from a statistical analysis of Mercury’s energetic electron (EE) events as observed by the gamma‐ray and neutron spectrometer instrument onboard the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. The main objective of this study is to investigate possible anisotropic behavior of EE events using multiple data sets from MESSENGER instruments. We study the data from the neutron spectrometer (NS) and the gamma‐ray spectrometer anticoincidence shield (ACS) because they use the same type of borated plastic scintillator and, hence, they have very similar response functions, and their large surface areas make them more sensitive to low‐intensity EE events than MESSENGER’s particle instrumentation. The combined analysis of NS and ACS data reveals two different classes of energetic electrons: “Standard” events and “ACS‐enhanced” events. Standard events, which comprise over 90% of all events, have signal sizes that are the same in both the ACS and NS. They are likely gyrating particles about Mercury’s magnetic field following a 90° pitch angle distribution and are located in well‐defined latitude and altitude regions within Mercury’s magnetosphere. ACS‐enhanced events, which comprise less than 10% of all events, have signal sizes in the ACS that are 10 to 100 times larger than those observed by the NS. They follow a beam‐like distribution and are observed both inside and outside Mercury’s magnetosphere with a wider range of latitudes and altitudes than Standard events. The difference between the Standard and ACS‐enhanced event characteristics suggests distinct underyling acceleration mechanisms.Key PointsA comprehensive survey of energetic electron (EE) events observed with the neutron spectrometer (NS) and the gamma‐ray spectrometer anticoincidence shield (ACS) is conductedThe majority of EE events detected in the NS are also detected in the ACS and appear to be composed of gyrating, drifting electronsACS‐only and ACS‐enhanced events exhibit a significantly different spatial and temporal characteristics compared with the other EE event classesPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/145319/1/jgra54299_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/145319/2/jgra54299.pd

Comprehensive survey of energetic electron events in Mercury\u27s magnetosphere with data from the MESSENGER Gamma-Ray and Neutron Spectrometer

Data from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) Gamma-Ray and Neutron Spectrometer have been used to detect and characterize energetic electron (EE) events in Mercury\u27s magnetosphere. This instrument detects EE events indirectly via bremsstrahlung photons that are emitted when instrument and spacecraft materials stop electrons having energies of tens to hundreds of keV. From Neutron Spectrometer data taken between 18 March 2011 and 31 December 2013 we have identified 2711 EE events. EE event amplitudes versus energy are distributed as a power law and have a dynamic range of a factor of 400. The duration of the EE events ranges from tens of seconds to nearly 20 min. EE events may be classified as bursty (large variation with time over an event) or smooth (small variation). Almost all EE events are detected inside Mercury\u27s magnetosphere on closed field lines. The precise occurrence times of EE events are stochastic, but the events are located in well-defined regions with clear boundaries that persist in time and form what we call “quasi-permanent structures.” Bursty events occur closer to dawn and at higher latitudes than smooth events, which are seen near noon-to-dusk local times at lower latitudes. A subset of EE events shows strong periodicities that range from hundreds of seconds to tens of milliseconds. The few-minute periodicities are consistent with the Dungey cycle timescale for the magnetosphere and the occurrence of substorm events in Mercury\u27s magnetotail region. Shorter periods may be related to phenomena such as north-south bounce processes for the energetic electrons