S. R. Floyd

Bio: S. R. Floyd is an academic researcher from Goddard Space Flight Center. The author has contributed to research in topics: Neutron & Near-Earth object. The author has an hindex of 12, co-authored 34 publications receiving 624 citations.

The elemental composition of asteroid 433 Eros: Results of the NEAR-Shoemaker X-ray spectrometer

Abstract: We report major element composition ratios for regions of the asteroid 433 Eros imaged during two solar flares and quiet sun conditions during the period of May to July 2000. Low aluminum abundances for all regions argue against global differentiation of Eros. Magnesium/silicon, aluminum/silicon, calcium/silicon, and iron/silicon ratios are best interpreted as a relatively primitive, chondritic composition. Marked depletions in sulfur and possible aluminum and calcium depletions, relative to ordinary chondrites, may represent signatures of limited partial melting or impact volatilization.

Lunar Exploration Neutron Detector for the NASA Lunar Reconnaissance Orbiter

Abstract: The design of the Lunar Exploration Neutron Detector (LEND) experiment is presented, which was optimized to address several of the primary measurement requirements of NASA’s Lunar Reconnaissance Orbiter (LRO): high spatial resolution hydrogen mapping of the Moon’s upper-most surface, identification of putative deposits of appreciable near-surface water ice in the Moon’s polar cold traps, and characterization of the human-relevant space radiation environment in lunar orbit. A comprehensive program of LEND instrument physical calibrations is discussed and the baseline scenario of LEND observations from the primary LRO lunar orbit is presented. LEND data products will be useful for determining the next stages of the emerging global lunar exploration program, and they will facilitate the study of the physics of hydrogen implantation and diffusion in the regolith, test the presence of water ice deposits in lunar cold polar traps, and investigate the role of neutrons within the radiation environment of the shallow lunar surface.

Compositional mapping with the NEAR X ray/gamma ray spectrometer

Abstract: The X ray/gamma ray spectrometer (XGRS) instrument on board the Near Earth Asteroid Rendezvous (NEAR) spacecraft will map asteroid 433 Eros in the 07 keV to 10 MeV energy region Measurements of the discrete line X ray and gamma ray emissions in this energy domain can be used to obtain both qualitative and quantitative elemental compositions with sufficient accuracy to enable comparison to the major meteorite typies It is believed that Eros is an S-type asteroid, the most common of the near-Earth asteroids The determination of whether Eros consists of either differentiated or undifferentiated materials is an important objective of this mission Observations of Eros during the NEAR mission will contribute significantly to our understanding of the structure and composition of this asteroid The NEAR spacecraft was successfully launched on February 17, 1996 The NEAR XGRS was turned on during the week of April 7, 1996, and all detector systems operated nominally Background spectra have been obtained

Experiment LEND of the NASA Lunar Reconnaissance Orbiter for high-resolution mapping of neutron emission of the Moon.

Abstract: The scientific objectives of neutron mapping of the Moon are presented as 3 investigation tasks of NASA's Lunar Reconnaissance Orbiter mission. Two tasks focus on mapping hydrogen content over the entire Moon and on testing the presence of water-ice deposits at the bottom of permanently shadowed craters at the lunar poles. The third task corresponds to the determination of neutron contribution to the total radiation dose at an altitude of 50 km above the Moon. We show that the Lunar Exploration Neutron Detector (LEND) will be capable of carrying out all 3 investigations. The design concept of LEND is presented together with results of numerical simulations of the instrument's sensitivity for hydrogen detection. The sensitivity of LEND is shown to be characterized by a hydrogen detection limit of about 100 ppm for a polar reference area with a radius of 5 km. If the presence of ice deposits in polar "cold traps" is confirmed, a unique record of many millions of years of lunar history would be obtained, by which the history of lunar impacts could be discerned from the layers of water ice and dust. Future applications of a LEND-type instrument for Mars orbital observations are also discussed.

The Major-Element Composition of Mercury’s Surface from MESSENGER X-ray Spectrometry

Abstract: X-ray fluorescence spectra obtained by the MESSENGER spacecraft orbiting Mercury indicate that the planet's surface differs in composition from those of other terrestrial planets Relatively high Mg/Si and low Al/Si and Ca/Si ratios rule out a lunarlike feldspar-rich crust The sulfur abundance is at least 10 times higher than that of the silicate portion of Earth or the Moon, and this observation, together with a low surface Fe abundance, supports the view that Mercury formed from highly reduced precursor materials, perhaps akin to enstatite chondrite meteorites or anhydrous cometary dust particles Low Fe and Ti abundances do not support the proposal that opaque oxides of these elements contribute substantially to Mercury's low and variable surface reflectance

Production of iron nanoparticles by laser irradiation in a simulation of lunar-like space weathering

Abstract: ‘Space weathering’ is the term applied to the darkening and reddening of planetary surface materials with time, along with the changes to the depths of absorption bands in their optical spectra. It has been invoked to explain the mismatched spectra of lunar rocks and regolith, and between those of asteroids and meteorites1,2,3,4,5,6. The formation of nanophase iron particles on regolith grains as a result of micrometeorite impacts or irradiation by the solar wind has been proposed as the main cause of the change in the optical properties7,8. But laboratory simulations9,10,11,12,13,14 have not revealed the presence of these particles, although nano-second-pulse laser irradiation did reproduce the optical changes12. Here we report observations by transmission electron microscopy of olivine samples subjected to pulse laser irradiation. We find within the amorphous vapour-deposited rims of olivine grains nanophase iron particles similar to those observed in the rims of space-weathered lunar regolith grains15,16. Reduction by hydrogen atoms implanted by the solar wind is therefore not necessary to form the particles. Moreover, the results support the idea that ordinary chondrites came from S-type asteroids5, and thereby provides some constraints on the surface exposure ages of those asteroids.