Not so rare Earth? New developments in understanding the origin of the Earth and Moon

Abstract

A widely accepted model for the origin of the Earth and Moon has been a somewhat specific giant impact scenario involving an impactor to proto-Earth mass ratio of 3:7, occurring 50-60 Ma after T 0 , when the Earth was only half accreted, with the majority of Earth's water then accreted after the main stage of growth, perhaps from comets. There have been many changes to this specific scenario, due to advances in isotopic and trace element geochemistry, more detailed, improved, and realistic giant impact and terrestrial planet accretion modeling, and consideration of terrestrial water sources other than high D/H comets. The current scenario is that the Earth accreted faster and differentiated quickly, the Moon-forming impact could have been mid to late in the accretion process, and water may have been present during accretion. These new developments have broadened the range of conditions required to make an Earth-Moon system, and suggests there may be many new fruitful avenues of research. There are also some classic and unresolved problems such as the significance of the identical O isotopic composition of the Earth and Moon, the depletion of volatiles on the lunar mantle relative to Earth's, the relative contribution of the impactor and proto-Earth to the Moon's mass, and the timing of Earth's possible atmospheric loss relative to the giant impact.3

Experimental studies of metal–silicate partitioning of Sb: Implications for the terrestrial and lunar mantles

“…Second, the Moon's mantle is depleted in volatile elements relative to the Earth's mantle (Wolf and Anders, 1980), and a satisfactory explanation for this difference has yet to be advanced. One possibility is that the Moon's small core formed at lower pressure and reduced conditions relative to the Earth's core, and thus has a different depletion (Righter, 2002), but it is also possible that volatile elements were fundamentally different in the bulk Moon material after the high temperatures of the giant Moon-forming impact (Humayun and Clayton, 1995;Pahlevan and Stevenson, 2007;Righter, 2007).…”

An intrinsic volatility scale relevant to the Earth and Moon and the status of water in the Moon

“…Thus, the lunar interior has long been considered to be extremely depleted in volatile elements (Wolf and Anders ; Papike et al. ; Righter ; Sharp et al. ; Elkins‐Tanton and Grove ; Paniello et al.…”

Why Isn't the Earth Completely Covered in Water?

“…In turbulent eddies the local conditions for gravitational instability can be met (Johansen et al, 2006;2007). The numerical simulations by Johansen et al show the formation of Ceres-mass planetesimals in a few orbital periods.…”

“…But we submit that a significant hydrosphere and a very wet atmosphere already existed on the protoEarth prior to the Moon forming event, due to the accretion of ice laden planetesimals. Since up to 3% water can be dissolved in the modern terrestrial mantle at equilibrium [Righter, 2007] and even more water could have been present "in transit" through the mantle and crust as water worked its way up to the surface from the deeper planetary interior, we see no possible alternative but to accept that the early Earth had quite a large complement of water and may have been a bit more massive than expected when the moon forming collision occurred.…”

“…Typical models for the origin of the Earth begin by assuming the accumulation of some combination of chondritic meteorites (Javoy, 1995;Ringwood, 1979;Wanke, 1981), yet it is highly likely that the asteroids that went into the terrestrial planets are no longer represented to any significant extent in the present-day asteroid population (Nuth, 2008). The argument concerning the composition of the building blocks of the Earth is typically phrased in terms of the chemical composition of the terrestrial mantle as compared to that of primitive meteorites [Righter et al, 2006] or to the isotopic composition of the Earth's oceans as compared to that of cometary water [Righter, 2007]. Both of these considerations yield important constraints on the problem.…”