Lunar X-ray fluorescence observations by the Chandrayaan-1 X-ray Spectrometer (C1XS): Results from the nearside southern highlands

The Chandrayaan-1 X-ray Spectrometer (C1XS) flown on-board the first Indian lunar mission Chandrayaan-1, measured X-ray fluorescence spectra during several episodes of solar flares during its operational period of ∼9 months. The accompanying X-ray Solar Monitor (XSM) provided simultaneous spectra of solar X-rays incident on the Moon which are essential to derive elemental chemistry. In this paper, we present the surface abundances of Mg, Al, Si, Ca and Fe, derived from C1XS data for a highland region on the southern nearside of the Moon. Analysis techniques are described in detail including absolute X-ray line flux derivation and conversion into elemental abundance. The results are consistent with a composition rich in plagioclase with a slight mafic mineral enhancement and a Ca/Al ratio that is significantly lower than measured in lunar returned samples. We suggest various possible scenarios to explain the deviations.     

Analysis of MESSENGER Gamma-Ray Spectrometer data from the Mercury flybys

During its three flybys of Mercury, the MESSENGER spacecraft made the first detection of gamma-ray emission from the planet's surface. With a closest approach distance of ∼200 km, the flybys provided an opportunity to measure elemental abundances of Mercury's near-equatorial regions, which will not be visited at low altitude during MESSENGER's orbital mission phase. Despite being limited by low planetary photon flux, sufficient counts were accumulated during the first two flybys to estimate bounds on abundances for some elements having relatively strong gamma-ray spectral peaks, including Si, Fe, Ti, K, and Th. Only for Si is the standard deviation σ sufficiently small to conclude that this element was detected with 99% confidence. Iron and potassium are detected at the 2−σ (95% confidence) level, whereas only upper bounds on Ti and Th can be determined. Relative to a Si abundance assumed to be 18 weight percent (wt%), 2−σ upper bounds have been estimated as 9.7 wt% for Fe, 7.0 wt% for Ti, 0.087 wt% for K, and 2.2 ppm for Th. The relatively low upper bound on K rules out some previously suggested models for surface composition for the regions sampled. Upper bounds on Fe/Si and Ti/Si ratios are generally consistent with Ti and Fe abundances estimated from the analysis of measurements by the MESSENGER Neutron Spectrometer during the flybys but are also permissive of much lower concentrations.     

Elemental,isotopic, and structural changes in Tagish Lake insoluble organic matter produced by parent body processes

Here, we present the results of a multitechnique study of the bulk properties of insoluble organic material (IOM) from the Tagish Lake meteorite, including four lithologies that have undergone different degrees of aqueous alteration. The IOM C contents of all four lithologies are very uniform and comprise about half the bulk C and N contents of the lithologies. However, the bulk IOM elemental and isotopic compositions vary significantly. In particular, there is a correlated decrease in bulk IOM H/C ratios and δD values with increasing degree of alteration—the IOM in the least altered lithology is intermediate between CM and CR IOM, while that in the more altered lithologies resembles the very aromatic IOM in mildly metamorphosed CV and CO chondrites, and heated CMs. Nuclear magnetic resonance (NMR) spectroscopy, C X‐ray absorption near‐edge (XANES), and Fourier transform infrared (FTIR) spectroscopy confirm and quantitate this transformation from CR‐like, relatively aliphatic IOM functional group chemistry to a highly aromatic one. The transformation is almost certainly thermally driven, and probably occurred under hydrothermal conditions. The lack of a paramagnetic shift in ¹³C NMR spectra and 1s‐σ* exciton in the C‐XANES spectra, both typically seen in metamorphosed chondrites, shows that the temperatures were lower and/or the timescales were shorter than experienced by even the least metamorphosed type 3 chondrites. Two endmember models were considered to quantitatively account for the changes in IOM functional group chemistry, but the one in which the transformations involved quantitative conversion of aliphatic material to aromatic material was the more successful. It seems likely that similar processes were involved in producing the diversity of IOM compositions and functional group chemistries among CR, CM, and CI chondrites. If correct, CRs experienced the lowest temperatures, while CM and CI chondrites experienced similar more elevated temperatures. This ordering is inconsistent with alteration temperatures based on mineralogy and O isotopes.     

Identification and measurement of neutron-absorbing elements on Mercury’s surface

MESSENGER Neutron Spectrometer (NS) observations of cosmic-ray-generated thermal neutrons provide the first direct measurements of Mercury’s surface elemental composition. Specifically, we show that Mercury’s surface is enriched in neutron-absorbing elements and has a measured macroscopic neutron-absorption cross section of 45-81 × 10⁻⁴ cm²/g, a range similar to the neutron absorption of lunar basalts from Mare Crisium. The expected neutron-absorbing elements are Fe and Ti, with possible trace amounts of Gd and Sm. Fe and Ti, in particular, are important for understanding Mercury’s formation and how its surface may have changed over time through magmatic processes. With neutron Doppler filtering - a neutron energy separation technique based on spacecraft velocity - we demonstrate that Mercury’s surface composition cannot be matched by prior models, which have characteristically low abundances of Fe, Ti, Gd, and Sm. While neutron spectroscopy alone cannot separate the relative contributions of individual neutron-absorbing elements, these results provide strong new constraints on the nature of Mercury’s surface materials. For example, if all the measured neutron absorption were due to the presence of an Fe-Ti oxide and that oxide were ilmenite, then Mercury’s surface would have an ilmenite content of 7-18 wt.%. This result is in general agreement with the inference from color imaging and visible-near-infrared spectroscopy that Mercury’s overall low reflectance is consistent with a surface composition that is enriched in Fe-Ti oxides. The incorporation of substantial Fe and Ti in oxides would imply that the oxygen fugacity of basalts on Mercury is at the upper range of oxygen fugacities inferred for basalts on the Moon.     

Mapping iron abundances on the surface of Mercury: Predicted spatial resolution of the MESSENGER Gamma-Ray Spectrometer

To illustrate the spatial resolution of measurements of Mercury's surface elemental composition by the Gamma-Ray Spectrometer on the MESSENGER spacecraft after one year of orbital observations, we have simulated a global coverage map of the 846-keV iron gamma-ray count rate. The simulated map suggests that distinct geologic units larger than 800 km in horizontal dimension will be discernable when the difference in Fe abundance between adjacent geologic units exceeds 4 wt%. These results imply that the MESSENGER Gamma-Ray Spectrometer dataset will provide useful information for regional geological studies of the surface of Mercury.     

Observations of suprathermal electrons in Mercury's magnetosphere during the three MESSENGER flybys

In 2008 the MESSENGER spacecraft made the first direct observation of Mercury's magnetosphere in the more than 30 years since the Mariner 10 encounters. During MESSENGER's first flyby on 14 January 2008, the interplanetary magnetic field (IMF) was northward immediately prior to and following MESSENGER's equatorial passage through this small magnetosphere. The Energetic Particle Spectrometer (EPS), one of two sensors on the Energetic Particle and Plasma Spectrometer instrument that responds to electrons from ∼35 keV to 1 MeV and ions from ∼35 keV to 2.75 MeV, saw no increases in particle intensity above instrumental background (∼5 particles/cm²/sr/s/keV at 45 keV) at any time during the probe's magnetospheric passage. During MESSENGER's second flyby on 6 October 2008, there was a steady southward IMF, and intense reconnection was observed between the planet's magnetic field and the IMF. However, once again EPS did not observe bursts of energetic particles similar to those reported by Mariner 10 from its March 1974 encounter. On 29 September 2009, MESSENGER flew by Mercury for the third and final time before orbit insertion in March 2011. Although a spacecraft safe-hold event stopped science measurements prior to the outbound portion of the flyby, all instruments recorded full observations until a few minutes before the closest approach. In particular, the MESSENGER Magnetometer documented several substorm-like signatures of extreme loading of Mercury's magnetotail, but again EPS measured no energetic ions or electrons above instrument background during the inbound portion of the flyby. MESSENGER's X-Ray Spectrometer (XRS) nonetheless observed photons resulting from low-energy (∼10 keV) electrons impinging on its detectors during each of the three flybys. We infer that suprathermal plasma electrons below the EPS energy threshold caused the bremsstrahlung seen by XRS. In this paper, we summarize the energetic particle observations made by EPS and XRS during MESSENGER's three Mercury flybys, and we revisit the observations reported by Mariner 10 in the context of these new results.     

Isotopic and chemical variation of organic nanoglobules in primitive meteorites

Organic nanoglobules are microscopic spherical carbon‐rich objects present in chondritic meteorites and other astromaterials. We performed a survey of the morphology, organic functional chemistry, and isotopic composition of 184 nanoglobules in insoluble organic matter (IOM) residues from seven primitive carbonaceous chondrites. Hollow and solid nanoglobules occur in each IOM residue, as well as globules with unusual shapes and structures. Most nanoglobules have an organic functional chemistry similar to, but slightly more carboxyl‐rich than, the surrounding IOM, while a subset of nanoglobules have a distinct, highly aromatic functionality. The range of nanoglobule N isotopic compositions was similar to that of nonglobular ¹⁵N‐rich hotspots in each IOM residue, but nanoglobules account for only about one third of the total ¹⁵N‐rich hotspots in each sample. Furthermore, many nanoglobules in each residue contained no ¹⁵N enrichment above that of bulk IOM. No morphological indicators were found to robustly distinguish the highly aromatic nanoglobules from those that have a more IOM‐like functional chemistry, or to distinguish ¹⁵N‐rich nanoglobules from those that are isotopically normal. The relative abundance of aromatic nanoglobules was lower, and nanoglobule diameters were greater, in more altered meteorites, suggesting the creation/modification of IOM‐like nanoglobules during parent‐body processing. However, ¹⁵N‐rich nanoglobules, including many with highly aromatic functional chemistry, likely reflect preaccretionary isotopic fractionation in cold molecular cloud or protostellar environments. These data indicate that no single formation mechanism can explain all of the observed characteristics of nanoglobules, and their properties are likely a result of multiple processes occurring in a variety of environments.     

The composition of 433 Eros: A mineralogical—chemical synthesis

Abstract— The near‐Earth asteroid rendezvous (NEAR) mission carried x‐ray/gamma‐ray spectrometers and multi‐spectral imager/near‐infrared spectrometer instrument packages which gave complementary information on the chemistry and mineralogy, respectively, of the target asteroid 433 Eros. Synthesis of these two data sets provides information not available from either alone, including the abundance of non‐mafic silicates, metal and sulfide minerals. We have utilized four techniques to synthesize these data sets. Venn diagrams, which examine overlapping features in two data sets, suggest that the best match for 433 Eros is an ordinary chondrite, altered at the surface of the asteroid, or perhaps a primitive achondrite derived from material mineralogically similar to these chondrites. Normalized element distributions preclude FeO‐rich pyroxenes and suggest that the x‐ray and gamma‐ray data can be reconciled with a common silicate mineralogy by inclusion of varying amounts of metal. Normative mineralogy cannot be applied to these data sets owing to uncertainties in oxygen abundance and lack of any constraints on the abundance of sodium. Matrix inversion for simultaneous solution of mineral abundances yields reasonable results for the x‐ray‐derived bulk composition, but seems to confirm the inconsistency between mineral compositions and orthopyroxene/clinopyroxene ratios. A unique solution does not seem possible in synthesizing these multiple data sets. Future missions including a lander to fully characterize regolith distribution and sample return would resolve the types of problems faced in synthesizing the NEAR data.     

Coordinated isotopic and mineralogic analyses of planetary materials enabled by in situ lift‐out with a focused ion beam scanning electron microscope

Abstract— We describe a focused ion beam scanning electron microscope (FIB‐SEM) technique that enables coordinated isotopic and mineralogic analysis of planetary materials. We show that site‐specific electron‐transparent sections can be created and extracted in situ using a microtweezer and demonstrate that they are amenable to analysis by secondary ion mass spectrometry (SIMS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). These methods greatly advance the ability to address several fundamental questions in meteoritics, such as accretion and alteration histories of chondrules and the origin and history of preserved nebular and presolar materials.