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David J. Lawrence

Bio: David J. Lawrence is an academic researcher from Johns Hopkins University Applied Physics Laboratory. The author has contributed to research in topics: Neutron & Physics. The author has an hindex of 56, co-authored 306 publications receiving 12317 citations. Previous affiliations of David J. Lawrence include Johns Hopkins University & Washington University in St. Louis.


Papers
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Journal ArticleDOI
04 Sep 1998-Science
TL;DR: Maps of epithermal- and fast-neutron fluxes measured by Lunar Prospector were used to search for deposits enriched in hydrogen at both lunar poles, and data are consistent with deposits of hydrogen in the form of water ice that are covered by as much as 40 centimeters of desiccated regolith within permanently shaded craters near both poles.
Abstract: Maps of epithermal- and fast-neutron fluxes measured by Lunar Prospector were used to search for deposits enriched in hydrogen at both lunar poles. Depressions in epithermal fluxes were observed close to permanently shaded areas at both poles. The peak depression at the North Pole is 4.6 percent below the average epithermal flux intensity at lower latitudes, and that at the South Pole is 3.0 percent below the low-latitude average. No measurable depression in fast neutrons is seen at either pole. These data are consistent with deposits of hydrogen in the form of water ice that are covered by as much as 40 centimeters of desiccated regolith within permanently shaded craters near both poles.

571 citations

Journal ArticleDOI
05 Jul 2002-Science
TL;DR: Global distributions of thermal, epithermal, and fast neutron fluxes have been mapped during late southern summer/northern winter using the Mars Odyssey Neutron Spectrometer, and portions of the low to middle latitudes indicate subsurface deposits of chemically and/ or physically bound H2O and/or OH.
Abstract: Global distributions of thermal, epithermal, and fast neutron fluxes have been mapped during late southern summer/northern winter using the Mars Odyssey Neutron Spectrometer. These fluxes are selectively sensitive to the vertical and lateral spatial distributions of H and CO2 in the uppermost meter of the martian surface. Poleward of ±60° latitude is terrain rich in hydrogen, probably H2O ice buried beneath tens of centimeter-thick hydrogen-poor soil. The central portion of the north polar cap is covered by a thick CO2 layer, as is the residual south polar cap. Portions of the low to middle latitudes indicate subsurface deposits of chemically and/or physically bound H2O and/or OH.

541 citations

Journal ArticleDOI
11 May 2012-Science
TL;DR: Dawn's results confirm predictions that Vesta differentiated and support its identification as the parent body of the HEDs, and present the mineralogical characterization of Vesta, revealing that this asteroid underwent a complex magmatic evolution that led to a differentiated crust and mantle.
Abstract: The Dawn spacecraft targeted 4 Vesta, believed to be a remnant intact protoplanet from the earliest epoch of solar system formation, based on analyses of howardite-eucrite-diogenite (HED) meteorites that indicate a differentiated parent body. Dawn observations reveal a giant basin at Vesta's south pole, whose excavation was sufficient to produce Vesta-family asteroids (Vestoids) and HED meteorites. The spatially resolved mineralogy of the surface reflects the composition of the HED meteorites, confirming the formation of Vesta's crust by melting of a chondritic parent body. Vesta's mass, volume, and gravitational field are consistent with a core having an average radius of 107 to 113 kilometers, indicating sufficient internal melting to segregate iron. Dawn's results confirm predictions that Vesta differentiated and support its identification as the parent body of the HEDs.

470 citations

Journal ArticleDOI
TL;DR: A lower-limit estimate of the global inventory of near surface hydrogen amounts to a global water layer about 14 cm thick if the reservoir sampled from orbit is assumed to be 1 m thick as mentioned in this paper.
Abstract: [1] Neutron data observed using the Neutron Spectrometer aboard 2001 Mars Odyssey provide a lower limit to the global inventory of Martian water-equivalent hydrogen. Hydrogen-rich deposits ranging between about 20% and 100% water-equivalent by mass are found poleward of ±50° latitude, and less rich, but significant, deposits are found at near-equatorial latitudes. The equatorial deposits between ±45° latitude range between 2% and 10% water-equivalent hydrogen by mass and reach their maximum in two regions that straddle the 0-km elevation contour. Higher water abundances, up to ∼11%, are required in subsurface regolith of some equatorial regions if the upper 10 g/cm2 of regolith is desiccated, as suggested on average by comparison of epithermal and fast neutron data. The hydrogen contents of surface soils in the latitude range between 50° and 80° north and south are equal within data uncertainties. A lower-limit estimate of the global inventory of near surface hydrogen amounts to a global water layer about 14 cm thick if the reservoir sampled from orbit is assumed to be 1 m thick.

416 citations

Journal ArticleDOI
30 Sep 2011-Science
TL;DR: X-ray fluorescence spectra obtained by the MESSENGENGER spacecraft orbiting Mercury indicate that the planet's surface differs in composition from those of other terrestrial planets as discussed by the authors, 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.
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

396 citations


Cited by
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Journal ArticleDOI
TL;DR: The crucial factor that needs to be emphasised with regard to the health effects of selenium is the inextricable U-shaped link with status; whereas additional seenium intake may benefit people with low status, those with adequate-to-high status might be affected adversely and should not take selenum supplements.

2,297 citations

Journal ArticleDOI
28 Feb 2007-JAMA
TL;DR: Treatment with beta carotene, vitamin A, vitamin E, and vitamin E may increase mortality and the potential roles of vitamin C and selenium on mortality need further study.
Abstract: ContextAntioxidant supplements are used for prevention of several diseases.ObjectiveTo assess the effect of antioxidant supplements on mortality in randomized primary and secondary prevention trials.Data Sources and Trial SelectionWe searched electronic databases and bibliographies published by October 2005. All randomized trials involving adults comparing beta carotene, vitamin A, vitamin C (ascorbic acid), vitamin E, and selenium either singly or combined vs placebo or vs no intervention were included in our analysis. Randomization, blinding, and follow-up were considered markers of bias in the included trials. The effect of antioxidant supplements on all-cause mortality was analyzed with random-effects meta-analyses and reported as relative risk (RR) with 95% confidence intervals (CIs). Meta-regression was used to assess the effect of covariates across the trials.Data ExtractionWe included 68 randomized trials with 232 606 participants (385 publications).Data SynthesisWhen all low- and high-bias risk trials of antioxidant supplements were pooled together there was no significant effect on mortality (RR, 1.02; 95% CI, 0.98-1.06). Multivariate meta-regression analyses showed that low-bias risk trials (RR, 1.16; 95% CI, 1.05-1.29) and selenium (RR, 0.998; 95% CI, 0.997-0.9995) were significantly associated with mortality. In 47 low-bias trials with 180 938 participants, the antioxidant supplements significantly increased mortality (RR, 1.05; 95% CI, 1.02-1.08). In low-bias risk trials, after exclusion of selenium trials, beta carotene (RR, 1.07; 95% CI, 1.02-1.11), vitamin A (RR, 1.16; 95% CI, 1.10-1.24), and vitamin E (RR, 1.04; 95% CI, 1.01-1.07), singly or combined, significantly increased mortality. Vitamin C and selenium had no significant effect on mortality.ConclusionsTreatment with beta carotene, vitamin A, and vitamin E may increase mortality. The potential roles of vitamin C and selenium on mortality need further study.

2,195 citations

Journal ArticleDOI
TL;DR: The HiRISE camera as mentioned in this paper provides detailed images (0.25 to 1.3 m/pixel) covering ∼1% of the Martian surface during the 2-year Primary Science Phase (PSP) beginning November 2006.
Abstract: [1] The HiRISE camera features a 0.5 m diameter primary mirror, 12 m effective focal length, and a focal plane system that can acquire images containing up to 28 Gb (gigabits) of data in as little as 6 seconds. HiRISE will provide detailed images (0.25 to 1.3 m/pixel) covering ∼1% of the Martian surface during the 2-year Primary Science Phase (PSP) beginning November 2006. Most images will include color data covering 20% of the potential field of view. A top priority is to acquire ∼1000 stereo pairs and apply precision geometric corrections to enable topographic measurements to better than 25 cm vertical precision. We expect to return more than 12 Tb of HiRISE data during the 2-year PSP, and use pixel binning, conversion from 14 to 8 bit values, and a lossless compression system to increase coverage. HiRISE images are acquired via 14 CCD detectors, each with 2 output channels, and with multiple choices for pixel binning and number of Time Delay and Integration lines. HiRISE will support Mars exploration by locating and characterizing past, present, and future landing sites, unsuccessful landing sites, and past and potentially future rover traverses. We will investigate cratering, volcanism, tectonism, hydrology, sedimentary processes, stratigraphy, aeolian processes, mass wasting, landscape evolution, seasonal processes, climate change, spectrophotometry, glacial and periglacial processes, polar geology, and regolith properties. An Internet Web site (HiWeb) will enable anyone in the world to suggest HiRISE targets on Mars and to easily locate, view, and download HiRISE data products.

1,511 citations

Book ChapterDOI
TL;DR: The composition of the primitive mantle derived by as mentioned in this paper shows that Earth was assembled from material that shows many of the same chemical fractionation processes as chondritic meteorites. But the stable isotope record excludes chondrite meteorites as the ‘building blocks’ of Earth.
Abstract: The composition of the primitive mantle derived here shows that Earth was assembled from material that shows many of the same chemical fractionation processes as chondritic meteorites. These processes occurred at the initial stage of the solar system formation, under conditions thought to be present in the solar nebula. But the stable isotope record excludes chondritic meteorites as the ‘building blocks’ of Earth. Meteorites formed in local environments separated from that part of the inner solar system where much of the material forming the terrestrial planets was sourced.

1,196 citations

Journal ArticleDOI
Craig J. Pollock1, T. E. Moore1, A. D. Jacques1, James L. Burch2, U. Gliese1, Yoshifumi Saito, T. Omoto, Levon A. Avanov1, Levon A. Avanov3, A. C. Barrie1, Victoria N. Coffey4, John C. Dorelli1, Daniel J. Gershman5, Daniel J. Gershman1, Daniel J. Gershman3, Barbara L. Giles1, T. Rosnack1, C. Salo1, Shoichiro Yokota, M. L. Adrian1, C. Aoustin, C. Auletti1, S. Aung1, V. Bigio1, N. Cao1, Michael O. Chandler4, Dennis J. Chornay3, Dennis J. Chornay1, K. Christian1, George Clark6, George Clark7, George Clark1, Glyn Collinson1, Glyn Collinson6, T. Corris1, A. De Los Santos2, R. Devlin1, T. Diaz2, T. Dickerson1, C. Dickson1, A. Diekmann4, F. Diggs1, C. Duncan1, A. Figueroa-Vinas1, C. Firman1, M. Freeman2, N. Galassi1, K. Garcia1, G. Goodhart2, D. Guererro2, J. Hageman1, Jennifer Hanley2, E. Hemminger1, Matthew Holland1, M. Hutchins2, T. James1, W. Jones1, S. Kreisler1, Joseph Kujawski8, Joseph Kujawski1, V. Lavu1, J. V. Lobell1, E. LeCompte, A. Lukemire, Elizabeth MacDonald1, Al. Mariano1, Toshifumi Mukai, K. Narayanan1, Q. Nguyan1, M. Onizuka1, William R. Paterson1, S. Persyn2, Benjamin M. Piepgrass2, F. Cheney1, A. C. Rager1, A. C. Rager6, T. Raghuram1, A. Ramil1, L. S. Reichenthal1, H. Rodriguez2, Jean-Noël Rouzaud, A. Rucker1, Marilia Samara1, Jean-André Sauvaud, D. Schuster1, M. Shappirio1, K. Shelton1, D. Sher1, David Smith1, Kerrington D. Smith2, S. E. Smith6, S. E. Smith1, D. Steinfeld1, R. Szymkiewicz1, K. Tanimoto, J. Taylor2, Compton J. Tucker1, K. Tull1, A. Uhl1, J. Vloet2, P. Walpole1, P. Walpole2, S. Weidner2, D. White2, G. E. Winkert1, P.-S. Yeh1, M. Zeuch1 
TL;DR: The Fast Plasma Investigation (FPI) was developed for flight on the Magnetospheric Multiscale (MMS) mission to measure the differential directional flux of magnetospheric electrons and ions with unprecedented time resolution to resolve kinetic-scale plasma dynamics as mentioned in this paper.
Abstract: The Fast Plasma Investigation (FPI) was developed for flight on the Magnetospheric Multiscale (MMS) mission to measure the differential directional flux of magnetospheric electrons and ions with unprecedented time resolution to resolve kinetic-scale plasma dynamics. This increased resolution has been accomplished by placing four dual 180-degree top hat spectrometers for electrons and four dual 180-degree top hat spectrometers for ions around the periphery of each of four MMS spacecraft. Using electrostatic field-of-view deflection, the eight spectrometers for each species together provide 4pi-sr field-of-view with, at worst, 11.25-degree sample spacing. Energy/charge sampling is provided by swept electrostatic energy/charge selection over the range from 10 eV/q to 30000 eV/q. The eight dual spectrometers on each spacecraft are controlled and interrogated by a single block redundant Instrument Data Processing Unit, which in turn interfaces to the observatory’s Instrument Suite Central Instrument Data Processor. This paper describes the design of FPI, its ground and in-flight calibration, its operational concept, and its data products.

1,038 citations