Reassigning CI chondrite parent bodies based on reflectance spectroscopy of samples from carbonaceous asteroid Ryugu and meteorites.

The carbonaceous asteroid Ryugu has been explored by the Hayabusa2 spacecraft to elucidate the actual nature of hydrous asteroids. Laboratory analyses revealed that the samples from Ryugu are comparable to unheated CI carbonaceous chondrites; however, reflectance spectra of Ryugu samples and CIs do not coincide. Here, we demonstrate that Ryugu sample spectra are reproduced by heating Orgueil CI chondrite at 300°C under reducing conditions, which caused dehydration of terrestrial weathering products and reduction of iron in phyllosilicates. Terrestrial weathering of CIs accounts for the spectral differences between Ryugu sample and CIs, which is more severe than space weathering that likely explains those between asteroid Ryugu and the collected samples. Previous assignments of CI chondrite parent bodies, i.e., chemically most primitive objects in the solar system, are based on the spectra of CI chondrites. This study indicates that actual spectra of CI parent bodies are much darker and flatter at ultraviolet to visible wavelengths than the spectra of CI chondrites.

The evolution of Mercury's crust: a global perspective from MESSENGER.

Mapping the distribution and extent of major terrain types on a planet's surface helps to constrain the origin and evolution of its crust. Together, MESSENGER and Mariner 10 observations of Mercury now provide a near-global look at the planet, revealing lateral and vertical heterogeneities in the color and thus composition of Mercury's crust. Smooth plains cover approximately 40% of the surface, and evidence for the volcanic origin of large expanses of plains suggests that a substantial portion of the crust originated volcanically. A low-reflectance, relatively blue component affects at least 15% of the surface and is concentrated in crater and basin ejecta. Its spectral characteristics and likely origin at depth are consistent with its apparent excavation from a lower crust or upper mantle enriched in iron- and titanium-bearing oxides.

Spectroscopic observations of Mercury's surface reflectance during MESSENGER's first Mercury flyby.

During MESSENGER's first flyby of Mercury, the Mercury Atmospheric and Surface Composition Spectrometer made simultaneous mid-ultraviolet to near-infrared (wavelengths of 200 to 1300 nanometers) reflectance observations of the surface. An ultraviolet absorption (<280 nanometers) suggests that the ferrous oxide (Fe2+) content of silicates in average surface material is low (less than 2 to 3 weight percent). This result is supported by the lack of a detectable 1-micrometer Fe2+ absorption band in high-spatial-resolution spectra of mature surface materials as well as immature crater ejecta, which suggests that the ferrous iron content may be low both on the surface and at depth. Differences in absorption features and slope among the spectra are evidence for variations in composition and regolith maturation of Mercury's surface.

Reflectance and color variations on Mercury: regolith processes and compositional heterogeneity.

Multispectral images of Mercury obtained by the MESSENGER spacecraft reveal that its surface has an overall relatively low reflectance with three large-scale units identified on the basis of reflectance and slope (0.4 to 1.0 micrometer). A higher-reflectance, relatively red material occurs as a distinct class of smooth plains that were likely emplaced volcanically; a lower-reflectance material with a lesser spectral slope may represent a distinct crustal component enriched in opaque minerals, possibly more common at depth. A spectrally intermediate terrain probably forms most of the upper crust. Three other spectrally distinct but spatially restricted units include fresh crater ejecta less affected by space weathering than other surface materials; high-reflectance deposits seen in some crater floors; and moderately high-reflectance, relatively reddish material associated with rimless depressions.

Geology of the Caloris basin, Mercury: a view from MESSENGER.

The Caloris basin, the youngest known large impact basin on Mercury, is revealed in MESSENGER images to be modified by volcanism and deformation in a manner distinct from that of lunar impact basins. The morphology and spatial distribution of basin materials themselves closely match lunar counterparts. Evidence for a volcanic origin of the basin's interior plains includes embayed craters on the basin floor and diffuse deposits surrounding rimless depressions interpreted to be of pyroclastic origin. Unlike lunar maria, the volcanic plains in Caloris are higher in albedo than surrounding basin materials and lack spectral evidence for ferrous iron-bearing silicates. Tectonic landforms, contractional wrinkle ridges and extensional troughs, have distributions and age relations different from their counterparts in and around lunar basins, indicating a different stress history.