Maps of Mercury’s surface composition, derived from MESSENGER X-Ray Spectrometer (XRS) data, reveal large-scale variations in major element abundances. The Mg/Si map has near-global coverage and a large dynamic range. The lowest Mg/Si values (~0.2) are found for the volcanic smooth plains within the Caloris impact basin. With this new Mg/Si map we confirm previous results, that showed large expanses of smooth plains on Mercury have low and relatively uniform Mg/Si compared with older, more heavily cratered terrains. The highest Mg/Si values (~0.8) are found for the low-reflectance ejecta deposits that surround the Rachmaninoff impact basin, and in a large (>5×106 km2) region centered at ~30°N, 290°E. This large high-Mg region (HMR) is also distinctive in other elemental abundances, with the lowest Al/Si and highest S/Si and Ca/Si ratios observed on the planet’s surface. In this study, we discuss possible causes of Mercury’s compositional variations and the origin of the HMR in particular.
The Global Contraction of Mercury
Mercury is replete with tectonic structures interpreted to be the result of planetary cooling and contraction, but the number and distribution of these structures, and their relation to topography, have not been well understood. Moreover, previous estimates of the amount of global contraction inferred from spacecraft images (0.8–3 km) were far less than predicted by models of interior thermal evolution (~5–10 km). Here we use orbital observations acquired by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft to develop a synthesis of the global contraction of the innermost planet, and we show that Mercury has experienced much greater contraction than has previously been recognized.
Primitive COs are of considerable interest because they preserve the highest abundances of presolar silicates in their matrices. This is presumably because they experienced low degrees of both met- amorphism and aqueous alteration. The insoluble organic matter (IOM) is a sensitive indicator of metamorphism, with IOM becoming less abundant and more ‘graphitic’ with increasing metamorphism. Since IOM is the major C-bearing component of chondrites and the C isotopic composition of IOM tends to also change during metamorphism, bulk C abundances and isotopic compositions may be useful preliminary indicators of petrologic type. Similarly, bulk H abundances and isotopes can be useful indicators of the degree of aqueous alteration. Bulk C and H abundances and isotopes in a small suite of CO3s were used to identify DOM 08006 as a potentially very primitive CO3. A subsequent presolar grain search showed that it has the highest matrix-normalized presolar silicate abundance of any chondrite. Hence, we have begun a more extensive survey to look for more very primitive COs, as well as to possibly identify relatively unaltered CMs, like Paris, that have been misclassified.
The detection of magmatic water in lunar volcanic glasses, and the high pre-eruptive abundance of water in melt inclusions from one of these samples, has provided the first definitive evidence for the accretion and retention in the Moon’s interior of one of the most volatile elements in the solar system. This surprising result, the culmination of over four decades of intensive geochemical investigation, provides a severe constraint on high-temperature models that seek to explain the formation and evolution of the Moon. With increasing consideration of the orbital dynamics of the Earth-Moon-Sun system, there now exists a very wide parameter space for physical models to explain the origin of the Moon by a giant impact, with the Moon formed from a circum-terrestrial disc of molten debris ejected largely from the Earth. This class of models can explain the Earth-Moon angular momentum and early thermal history of the Moon. However, as currently formulated, all of these models predict wholesale melting and partial vaporization of the silicate material that enters proto-lunar orbit, and total evaporation of the most volatile elements. Thus all of these models fail to account for the presence of water in the Moon’s interior.
Water in the lunar interior is at odds not only with existing formation models, it is also counter to one of the longest-standing observations in lunar geochemistry, namely the volatile-depleted nature of the Moon compared with the Earth. There exists a large body of evidence that the abundances of volatile elements in lunar basalts are present at levels that are 10-100x lower than their abundances in terrestrial mid-ocean ridge and ocean-island basalts. This fundamental observation was not extended to the highly-volatile atmophile elements due to a combination of factors; the barely-detectable concentrations in lunar samples, the implantation of hydrogen and other volatiles by solar wind, and suspected contamination by micrometeorite material and terrestrial atmosphere. Nevertheless, the detection and abundance of magmatic water in lunar glasses and minerals, as well as other volatiles like fluorine and chlorine, is an apparently contradictory result in the context of prior laboratory studies of lunar samples. This contradiction has led to the suggestion that perhaps the water-bearing samples are a lunar anomaly, and do not say anything particularly fundamental about the formation and evolution of the Moon.
Here we will demonstrate that most of these apparent contradictions – the geochemical ones at least – have arisen due to the previously unappreciated importance of a single widespread process, magmatic degassing. Degassing occurs in all eruptions of magma, with consequent release of volatile elements into an exsolved vapor phase, and has thus affected all lunar volcanic samples.