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Detection and characterization of 0.5â8 MeV neutrons near Mercury: Evidence for a solar origin
Data from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) Neutron Spectrometer (NS) have been used to identify energetic neutrons (0.5â8âMeV energy) associated with solar events that occurred on 4 June 2011. Multiple lines of evidence, including measurements from the NS and the MESSENGER Gamma-Ray Spectrometer, indicate that the detected neutrons have a solar origin. This evidence includes a lack of time-coincident, energetic (>45âMeV) charged particles that could otherwise create local neutrons from nearby spacecraft material and a lack of proton-induced gamma rays that should be seen if energetic protons were present. NS data cannot rule out the presence of lower-energy ions (<30âMeV) that can produce local neutrons. However, the ion spectral shape required to produce the measured neutron count rate locally is softer than any known ion spectral shape. The neutron energy spectrum shows a relative enhancement in the energy range 0.8â3âMeV compared with cosmic-ray-generated neutrons from the spacecraft or Mercury. The spectral shape of the measured neutron fluence spectrum is consistent with a previously modeled fluence spectrum of neutrons that originate at the Sun and are propagated through the MESSENGER spacecraft to the NS. These measurements provide strong evidence for a solar origin of the detected neutrons and suggest that a large number of low-energy threshold ion evaporation reactions were taking place on the Sun during the neutron event
Aluminum Abundance on the Surface of Mercury: Application of a New Background-Reduction Technique for the Analysis of Gamma-Ray Spectroscopy Data
A new technique has been developed for characterizing gamma-ray emission from a planetary surface in the presence of large background signals generated in a spacecraft. This technique is applied to the analysis of Al gamma rays measured by the MESSENGER Gamma-Ray Spectrometer to determine the abundance of Al on the surface of Mercury. The result (Al/Si = 0.29-0.13+0.05) is consistent with Al/Si ratios derived from the MESSENGER X-Ray Spectrometer and confirms the finding of low Al abundances. The measured abundance rules out a global, lunar-like feldspar-rich crust and is consistent with previously suggested analogs for surface material on Mercury, including terrestrial komatiites, low-iron basalts, partial melts of CB chondrites, and partial melts of enstatite chondrites. Additional applications of this technique include the measurement of other elements on Mercury's surface as well as the analysis of data from other planetary gamma-ray spectrometer experiments
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 () 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 ( = 30\%), and this increases from 74 to 87\% as decreases
from 20 to 10\%. Although the discrimination performance depends on , the
current operation plan for MEGANE predicts an instrument performance for
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
Mineralogy of the Mercurian Surface
The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft orbited Mercury for four years until April 2015, revealing its structure, chemical makeup, and compositional diversity. Data from the mission have confirmed that Mercury is a compositional end-member among the terrestrial planets. The X-Ray Spectrometer (XRS) and Gamma-Ray Spectrometer (GRS) on board MESSENGER provided the first detailed geochemical analyses of Mercury's surface. These instruments have been used in conjunction with the Neutron Spectrometer and the Mercury Dual Imaging System to classify numerous geological and geochemical features on the surface of Mercury that were previously unknown. Furthermore, the data have revealed several surprising characteristics about Mercury's surface, including elevated S abundances (up to 4 wt%) and low Fe abundances (less than 2.5 wt%). The S and Fe abundances were used to quantify Mercury's highly reduced state, i.e., between 2.6 and 7.3 log10 units below the Iron-Wustite (IW) buffer. This fO2 is lower than any of the other terrestrial planets in the inner Solar System and has important consequences for the thermal and magmatic evolution of Mercury, its surface mineralogy and geochemistry, and the petrogenesis of the planet's magmas. Although MESSENGER has revealed substantial geochemical diversity across the surface of Mercury, until now, there have been only limited efforts to understand the mineralogical and petrological diversity of the planet. Here we present a systematic and comprehensive study of the potential mineralogical and petrological diversity of Mercury
Statistical Study of Mercuryâs Energetic Electron Events as Observed by the GammaâRay and Neutron Spectrometer Instrument Onboard MESSENGER
We present results from a statistical analysis of Mercuryâs energetic electron (EE) events as observed by the gammaâray 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 gammaâray 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 lowâintensity EE events than MESSENGERâs particle instrumentation. The combined analysis of NS and ACS data reveals two different classes of energetic electrons: âStandardâ events and âACSâenhancedâ 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 Mercuryâs magnetic field following a 90° pitch angle distribution and are located in wellâdefined latitude and altitude regions within Mercuryâs magnetosphere. ACSâenhanced 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 beamâlike distribution and are observed both inside and outside Mercuryâs magnetosphere with a wider range of latitudes and altitudes than Standard events. The difference between the Standard and ACSâenhanced event characteristics suggests distinct underyling acceleration mechanisms.Key PointsA comprehensive survey of energetic electron (EE) events observed with the neutron spectrometer (NS) and the gammaâray 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 electronsACSâonly and ACSâenhanced 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 20âmin. 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 âquasi-permanent structures.â 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