Compositional Diversity Among Primitive Asteroids

Spectroscopic observations from the ultraviolet to the mid-infrared have revealed new and diagnostic differences among primitive asteroids. We review the spectral characteristics of these asteroids and their inferred compositional and physical properties. Primitive asteroids throughout the belt show carbon-rich compounds, varying degrees of aqueous alteration and even surface ice; recent observations provide significant new constraints on composition, thermal inertia, and other surface properties. New mid-infrared connections between primitive asteroids and interplanetary dust particles indicate that the latter sample a larger fraction of main belt asteroids than meteorites. Links with the composition of comets are consistent with a proposed continuum between primitive asteroids and comets. Two sample-return missions, OSIRIS-REx and Hayabusa 2, will visit primitive near-Earth asteroids (NEAs). Most spacecraft-accessible NEAs originate in the inner asteroid belt, which contains several primitive asteroid families and a background of primitive asteroids outside these families. Initial results from these families offer a tantalizing preview of the properties expected in the NEAs they produce. So far, primitive asteroids in the inner belt fall into two spectral groups. The first group includes the Polana-Eulalia families, which show considerable spectral homogeneity in spite of their dynamical and collisional complexity. In contrast, the Erigone and Sulamitis families are spectrally diverse and most of their members show clear 0.7 microns hydration features. The two sample-return targets (101955) Bennu and (162173) Ryugu, most likely originated in the Polana family.Comment: 31 pages, 11 figures, chapter 5 in Primitive Meteorites and Asteroids, Physical, Chemical, and Spectroscopic Observations Paving the Way to Exploratio

Near-Infrared Mapping and Physical Properties of the Dwarf-Planet Ceres

We study the physical characteristics (shape, dimensions, spin axis direction, albedo maps, mineralogy) of the dwarf-planet Ceres based on high-angular resolution near-infrared observations. We analyze adaptive optics J/H/K imaging observations of Ceres performed at Keck II Observatory in September 2002 with an equivalent spatial resolution of ~50 km. The spectral behavior of the main geological features present on Ceres is compared with laboratory samples. Ceres' shape can be described by an oblate spheroid (a = b = 479.7 +/- 2.3 km, c = 444.4 +/- 2.1 km) with EQJ2000.0 spin vector coordinates RA = 288 +/- 5 deg. and DEC = +66 +/- 5 deg. Ceres sidereal period is measured to be 9.0741 +/- 0.0001 h. We image surface features with diameters in the 50-180 km range and an albedo contrast of ~6% with respect to the average Ceres albedo. The spectral behavior of the brightest regions on Ceres is consistent with phyllosilicates and carbonate compounds. Darker isolated regions could be related to the presence of frost.Comment: 11 pages, 8 Postscript figures, Accepted for publication in A&

The Main Belt Comets and ice in the Solar System

We review the evidence for buried ice in the asteroid belt; specifically the questions around the so-called Main Belt Comets (MBCs). We summarise the evidence for water throughout the Solar System, and describe the various methods for detecting it, including remote sensing from ultraviolet to radio wavelengths. We review progress in the first decade of study of MBCs, including observations, modelling of ice survival, and discussion on their origins. We then look at which methods will likely be most effective for further progress, including the key challenge of direct detection of (escaping) water in these bodies

A small-molecule PI3Kα activator for cardioprotection and neuroregeneration

Harnessing the potential beneficial effects of kinase signalling through the generation of direct kinase activators remains an underexplored area of drug development1,2,3,4,5. This also applies to the PI3K signalling pathway, which has been extensively targeted by inhibitors for conditions with PI3K overactivation, such as cancer and immune dysregulation. Here we report the discovery of UCL-TRO-1938 (referred to as 1938 hereon), a small-molecule activator of the PI3Kα isoform, a crucial effector of growth factor signalling. 1938 allosterically activates PI3Kα through a distinct mechanism by enhancing multiple steps of the PI3Kα catalytic cycle and causes both local and global conformational changes in the PI3Kα structure. This compound is selective for PI3Kα over other PI3K isoforms and multiple protein and lipid kinases. It transiently activates PI3K signalling in all rodent and human cells tested, resulting in cellular responses such as proliferation and neurite outgrowth. In rodent models, acute treatment with 1938 provides cardioprotection from ischaemia–reperfusion injury and, after local administration, enhances nerve regeneration following nerve crush. This study identifies a chemical tool to directly probe the PI3Kα signalling pathway and a new approach to modulate PI3K activity, widening the therapeutic potential of targeting these enzymes through short-term activation for tissue protection and regeneration. Our findings illustrate the potential of activating kinases for therapeutic benefit, a currently largely untapped area of drug development

Pyroxene reflectance spectra: Minor absorption bands and effects of elemental substitutions

Reflectance spectra of a suite of compositionally diverse pyroxenes exhibit variable spectral properties which are associated with various elemental substitutions. Those associated with transition series elements, such as Cr, Ti, V, and Mn, give rise to a number of minor absorption bands in the visible spectral region and, in some cases (e.g., Mn), at longer wavelengths. Substitutions by other cations, such as Li and Zr, do not result in distinct absorption bands. The spectra of these pyroxenes are dominated by the transition series elements that may be present in the samples. The visible wavelength region of low-calcium pyroxene reflectance spectra exhibit a number of absorption bands which are attributable to Fe2+ spin-forbidden crystal field transitions. Fe2+-Fe3+ intervalence charge transfer absorption bands near 0.77 μm are common in terrestrial pyroxenes which contain both ferrous and ferric iron. Collectively, these results indicate that the presence of various transition series elements can be detected in reflectance spectra and that their oxidation states and site occupancies can also be determined on the basis of unique spectral characteristics present in their reflectance spectra, thereby greatly increasing the range of compositional information which can be derived from analysis of their reflectance spectra.This study was supported by grants-in-aid of research from the Natural Sciences and Engineering Research Council of Canada (NSERC), Sigma Xi, The Scientific Research Society, the Geological Society of America, a research contract from the Canadian Space Agency, and a start-up grant from the University of Winnipeg.This study was supported by grants-in-aid of research from the Natural Sciences and Engineering Research Council of Canada (NSERC), Sigma Xi, The Scientific Research Society, the Geological Society of America, a research contract from the Canadian Space Agency, and a start-up grant from the University of Winnipeg.https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2001JE00159

EVIDENCE FOR A COMPOSITIONAL RELATIONSHIP BETWEEN ASTEROIDS AND METEORITES FROM INFRARED SPECTRAL REFLECTANCES

High-resolution Fourier spectra in the 0.9-2.5 μm region were measured for sixteen asteroids. These data were combined with 0.3-1.1 μm spectrophotometry and 3.0-3.5 μm narrowband photometry for compositional analysis. Comparison spectra of meteorites and terrestrial minerals were measured in the laboratory, some under simulated asteroidal conditions of pressure and temperature. Spectra of eleven representative S-type asteroids show a range of olivine/pyroxene ratios overlapping those of ordinary and carbonaceous chondrites, but not approaching those of common differentiated meteorite types. The reddening in the asteroid spectra implies the presence of metallic iron, but if the metal is finely divided its abundance could be low. S-type asteroids have spectra most consistent with undifferentiated compositions, and some of them, especially 8 Flora, could be ordinary chondrite parent bodies. 4 Vesta and 349 Dembowska are unusual asteroids which have spectra resembling those of achondritic meteorites. Vesta has a pyroxene-feldspar mineralogy intermediate in composition between eucrites and howardites. If shergottite-like basalts are present, they must be in low abundance. Dembowska has an olivine-pyroxene mineralogy similar in some ways to ordinary chondrites, but there is considerable evidence that it is actually a fragment of the mantle of a differentiated Vesta-like parent body. The most diagnostic spectral feature seen on three low-albedo asteroids is the 3 μm band due to water of hydration. 1 Ceres must consist mostly of a low-iron clay mineral with some hydrated salts. 2 Pallas has a low abundance of hydrated minerals relative to Ceres, with the bulk of its composition being anhydrous iron-free silicates. 324 Bamberga probably contains clay minerals, but its spectrum is dominated by abundant magnetite. These and other C-type asteroids have surface compositions consistent with massive aqueous alteration of primary carbonaceous chondrite minerals. These results all indicate that the compositions of main belt asteroids are more closely related to the compositions of meteorites than was previously believed. S-type and C-type asteroids are undifferentiated assemblages of which ordinary and carbonaceous chondrites are an incomplete sample. Differentiated meteorites could be derived from the other relatively rare asteroid types