Determining the composition of the Earth

Abstract

A long-standing question in the planetary sciences asks what the Earth is made of. For historical reasons, volatile-depleted primitive materials similar to current chondritic meteorites were long considered to provide the 'building blocks' of the terrestrial planets. But material from the Earth, Mars, comets and various meteorites have Mg/Si and Al/Si ratios, oxygen-isotope ratios, osmium-isotope ratios and D/H, Ar/H2O and Kr/Xe ratios such that no primitive material similar to the Earth's mantle is currently represented in our meteorite collections. The 'building blocks' of the Earth must instead be composed of unsampled 'Earth chondrite' or 'Earth achondrite'.

Water on Planets

“…Based on modeling and observations of the heliocentric distance for the onset of cometary activity, the snow line for H 2 O in the solar system today is around 2.0-2.5 AU (e.g., at 160K T 200K approximately;see review in Encrenaz, 2008). The water snow line was likely to have been farther out (perhaps near 4-5 AU, near where proto-Jupiter formed) in the early solar system (e.g., Stevenson and Lunine, 1988;Encrenaz, 2008), and of course there are different, similarly more distant "snow" lines for molecular disk volatiles like CH 4 , NH 3 , CO 2 , and others that have lower condensation temperatures based on their saturation vapor pressure behavior.…”

“…Combined with the ubiquity of water in general, this implies that water ice is likely to be the dominant volatile in a typical solar nebula, and that the snow line marks an important boundary in a typical solar system, beyond which significantly greater abundances of solids (ice) are available for planetary accretion and growth (e.g., Stevenson and Lunine, 1988;Encrenaz, 2008). The concept of a hard "line" beyond which water condenses is likely to be unrealistic, however, as local, synopticscale variations in nebular conditions (e.g., pressure, temperature, density, composition, grain size) almost certainly would have resulted in the water ice condensation occurring within a more fuzzy zone rather than at a specific, narrow boundary.…”

“…Cometary comae reveal prodigious evidence for water vapor and water ice, and modeling of the available data on water and dust abundances suggests water mass fractions around 80% for typical comet nuclei (e.g., Festou et al, 2004;Encrenaz, 2008). Until very recently, however, water ice had not been directly measured on comet nuclei; as mentioned above, their surfaces were instead observed to be apparently mantled by a dark, reddish layer of refractories and/or macromolecular hydrocarbons.…”

“…Aspects of physics, chemistry, astronomy, geology, and even biology all come into play when considering the origin and influence of water on the history and evolution of planetary (and satellite, asteroidal, cometary, and even dust grain) surfaces, interiors, and atmospheres. Outstanding reviews of this topic have been presented recently by Chambers, 2004, Drake, 2005, Encrenaz, 2008, Owen, 2008, and Albarede, 2009. Any review of this broad topic is almost certain to be biased by the reviewer's own background, experience, and perception of the role of water in their own specific scientific studies.…”

Mineral Radioactivity in Sands as a Mechanism for Fixation of Organic Carbon on the Early Earth

“…Evidence that the inner planets of the Solar System are formed from relatively narrow accretion zones (Drake and Righter, 2002;Taylor et al, 2003) suggests that we may not expect uranium/thorium concentrations to be at Earth levels in all rocky planets, although their budget on Venus may be similar to that on Earth (Taylor, 1999). The lack of plate tectonics on Mars except perhaps in infancy (Kerr, 1998;Nimmo and Stevenson, 2000), and on other terrestrial planets, limits the potential for crustal differentiation.…”