Catalogue Search | MBRL
Are you sure you want to remove the book from the shelf?
{{itemTitle}}
131
result(s) for
"Nittler, Larry"
Sort by:
Mercury : the view after Messenger
A study of the planet Mercury, particulary after the Messenger mission. The spacecraft was launched in 2004, orbited Mercury 2008-2015, and crashed onto the surface of Mercury in 2015.
Book
The Presolar Grain Database. I. Silicon Carbide
The Presolar Grain Database (PGD) contains the vast majority of isotope data (published and unpublished) on presolar grains and was first released as a collection of spreadsheets in 2009. It has been a helpful tool used by many researchers in cosmochemistry and astrophysics. However, over the years, accumulated errors compromised major parts of the PGD. Here, we provide a fresh start, with the PGD for silicon carbide (SiC) grains rebuilt from the ground up. We also provide updated rules for SiC grain type classification to unify previous efforts, taking into account newly discovered grain types. We also define a new grain type D, which includes some grains previously classified as ungrouped. Future work will focus on rebuilding the PGD for other kinds of presolar grains: graphite, oxides, silicates, and rarer phases.
Journal Article
The Major-Element Composition of Mercury's Surface from MESSENGER X-ray Spectrometry
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.
Journal Article
Chemical heterogeneity on Mercury's surface revealed by the MESSENGER X-Ray Spectrometer
We present the analysis of 205 spatially resolved measurements of the surface composition of Mercury from MESSENGER's X‐Ray Spectrometer. The surface footprints of these measurements are categorized according to geological terrain. Northern smooth plains deposits and the plains interior to the Caloris basin differ compositionally from older terrain on Mercury. The older terrain generally has higher Mg/Si, S/Si, and Ca/Si ratios, and a lower Al/Si ratio than the smooth plains. Mercury's surface mineralogy is likely dominated by high‐Mg mafic minerals (e.g., enstatite), plagioclase feldspar, and lesser amounts of Ca, Mg, and/or Fe sulfides (e.g., oldhamite). The compositional difference between the volcanic smooth plains and the older terrain reflects different abundances of these minerals and points to the crystallization of the smooth plains from a more chemically evolved magma source. High‐degree partial melts of enstatite chondrite material provide a generally good compositional and mineralogical match for much of the surface of Mercury. An exception is Fe, for which the low surface abundance on Mercury is still higher than that of melts from enstatite chondrites and may indicate an exogenous contribution from meteoroid impacts. Key Points Analysis of spatially resolved X‐ray spectrometry data from MESSENGER Volcanic smooth plains units differ compositionally from older terrains Mercury's surface consists of high‐Mg mafic minerals, plagioclase, and sulfides
Journal Article
Establishing a molecular relationship between chondritic and cometary organic solids
Multidimensional solid-state NMR spectroscopy is used to refine the identification and abundance determination of functional groups in insoluble organic matter (IOM) isolated from a carbonaceous chondrite (Murchison, CM2). It is shown that IOM is composed primarily of highly substituted single ring aromatics, substituted furan/pyran moieties, highly branched oxygenated aliphatics, and carbonyl groups. A pathway for producing an IOM-like molecular structure through formaldehyde polymerization is proposed and tested experimentally. Solid-state 13C NMR analysis of aqueously altered formaldehyde polymer reveals considerable similarity with chondritic IOM. Carbon X-ray absorption near edge structure spectroscopy of formaldehyde polymer reveals the presence of similar functional groups across certain Comet 81P/Wild 2 organic solids, interplanetary dust particles, and primitive IOM. Variation in functional group concentration amongst these extraterrestrial materials is understood to be a result of various degrees of processing in the parent bodies, in space, during atmospheric entry, etc. These results support the hypothesis that chondritic IOM and cometary refractory organic solids are related chemically and likely were derived from formaldehyde polymer. The fine-scale morphology of formaldehyde polymer produced in the experiment reveals abundant nanospherules that are similar in size and shape to organic nanoglobules that are ubiquitous in primitive chondrites.
Journal Article
Evidence for Water Ice Near Mercury's North Pole from MESSENGER Neutron Spectrometer Measurements
Measurements by the Neutron Spectrometer on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft show decreases in the flux of epithermal and fast neutrons from Mercury's north polar region that are consistent with the presence of water ice in permanently shadowed regions. The neutron data indicate that Mercury's radar-bright polar deposits contain, on average, a hydrogen-rich layer more than tens of centimeters thick beneath a surficial layer 10 to 30 cm thick that is less rich in hydrogen. Combined neutron and radar data are best matched if the buried layer consists of nearly pure water ice. The upper layer contains less than 25 weight % water-equivalent hydrogen. The total mass of water at Mercury's poles is inferred to be 2 × 10 16 to 10 18 grams and is consistent with delivery by comets or volatile-rich asteroids.
Journal Article
Explosive Nucleosynthesis in Core-collapse Type II Supernovae: Insights from New C, N, Si, and Al–Mg Isotopic Compositions of Presolar Grains
We report C, N, Si, and Al–Mg isotope data for 39 presolar X silicon carbide (SiC) and four silicon nitride grains—a group of presolar grains that condensed in the remnants of core-collapse Type II supernovae (CCSNe)—isolated from the Murchison meteorite. Energy dispersive X-ray data were used to determine the Mg and Al contents of the X SiC grains for comparison with the Mg/Al ratios determined by secondary ion mass spectroscopy (SIMS). Previous SIMS studies have used O-rich standards in the absence of alternatives. In this study, the correlated isotopic and elemental data of the X SiC grains enabled accurate determination of the initial 26Al/27Al ratios for the grains. Our new grain data suggest that (i) the literature data for X grains are affected to varying degrees by asteroidal/terrestrial contamination, and (ii) the Al/Mg ratios in SiC are a factor of 2 (with ±6% 1σ uncertainties) lower than estimated based on the SIMS analyses that used O-rich standards. The lowered Al/Mg ratios result in proportionally higher inferred initial 26Al/27Al ratios for presolar SiC grains. In addition, the suppression of asteroidal/terrestrial contamination in this study leads to the observation of negative trends for 12C/13C–30Si/28Si and 26Al/27Al–30Si/28Si among our CCSN grains. We discuss these isotope trends in the light of explosive CCSN nucleosynthesis models, based on which we provide new insights into several nontraditional CCSN nucleosynthesis processes, including explosive H burning, the existence of a C/Si zone in the outer regions of CCSNe, and neutrino–nucleus reactions in deep CCSN regions.
Journal Article
Magnesium-rich crustal compositions on Mercury: Implications for magmatism from petrologic modeling
We have conducted petrologic modeling of MESSENGER‐derived compositions and analog compositions to gain a better understanding of the petrogenesis of the crust of Mercury. Analog compositions included a 1425°C partial melt of the Indarch (EH4) meteorite and a range of Mg‐rich terrestrial rocks (magnesian basalt, basaltic komatiite, and peridotitic komatiite). All models were held at the iron‐wüstite buffer to simulate the reducing conditions that likely existed during Mercury's formation. We then compared modeled mineral compositions and abundances, liquidus temperatures, and viscosities to better constrain the characteristics of the lavas that erupted on Mercury's surface. Our results show that the surface composition of Mercury is most similar to that of a terrestrial magnesian basalt (with lowered FeO), composed mainly of Mg‐rich orthopyroxene and plagioclase. Because the model magmas are Mg‐rich, their counterparts on Mercury would have erupted at high temperatures and displayed low viscosities. Producing melts of these compositions would have required high temperatures at the mantle source regions on Mercury. The inferred low‐viscosity lavas would have erupted as thin, laterally extensive flows (depending upon their effusion rate) and would be expected to display surficial flow features that might be preserved to the present. Key Points We conducted petrologic modeling of MESSENGER‐derived and analog compositions Mercury's surface is composed mainly of Mg‐rich orthopyroxene and plagioclase
Journal Article
Rationale for BepiColombo Studies of Mercury’s Surface and Composition
BepiColombo has a larger and in many ways more capable suite of instruments relevant for determination of the topographic, physical, chemical and mineralogical properties of Mercury’s surface than the suite carried by NASA’s MESSENGER spacecraft. Moreover, BepiColombo’s data rate is substantially higher. This equips it to confirm, elaborate upon, and go beyond many of MESSENGER’s remarkable achievements. Furthermore, the geometry of BepiColombo’s orbital science campaign, beginning in 2026, will enable it to make uniformly resolved observations of both northern and southern hemispheres. This will offer more detailed and complete imaging and topographic mapping, element mapping with better sensitivity and improved spatial resolution, and totally new mineralogical mapping.
We discuss MESSENGER data in the context of preparing for BepiColombo, and describe the contributions that we expect BepiColombo to make towards increased knowledge and understanding of Mercury’s surface and its composition. Much current work, including analysis of analogue materials, is directed towards better preparing ourselves to understand what BepiColombo might reveal. Some of MESSENGER’s more remarkable observations were obtained under unique or extreme conditions. BepiColombo should be able to confirm the validity of these observations and reveal the extent to which they are representative of the planet as a whole. It will also make new observations to clarify geological processes governing and reflecting crustal origin and evolution.
We anticipate that the insights gained into Mercury’s geological history and its current space weathering environment will enable us to better understand the relationships of surface chemistry, morphologies and structures with the composition of crustal types, including the nature and mobility of volatile species. This will enable estimation of the composition of the mantle from which the crust was derived, and lead to tighter constraints on models for Mercury’s origin including the nature and original heliocentric distance of the material from which it formed.
Journal Article
Radioactive Elements on Mercury's Surface from MESSENGER: Implications for the Planet's Formation and Evolution
The MESSENGER Gamma-Ray Spectrometer measured the average surface abundances of the radioactive elements potassium (K, 1150 ± 220 parts per million), thorium (Th, 220 ± 60 parts per billion), and uranium (U, 90 ± 20 parts per billion) in Mercury's northern hemisphere. The abundance of the moderately volatile element K, relative to Th and U, is inconsistent with physical models for the formation of Mercury requiring extreme heating of the planet or its precursor materials, and supports formation from volatile-containing material comparable to chondritic meteorites. Abundances of K, Th, and U indicate that internal heat production has declined substantially since Mercury's formation, consistent with widespread volcanism shortly after the end of late heavy bombardment 3.8 billion years ago and limited, isolated volcanic activity since.
Journal Article