2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 132-5
Presentation Time: 9:00 AM-6:30 PM


JOHNSON, Mark, University of California, Riverside, 900 University Avenue, Riverside, CA 92521 and HUGHES III, Richard, San Bernardino Community College District, Crafton Hills College, 11711 Sand Canyon Road, Yucaipa, CA 92399, mjohn024@ucr.edu

The primary delivery mechanisms tasked to explain the origin of Earth's water and deuterium ratios have been the wet and dry models. The wet model simply contends that Earth originally accreted water-rich material directly from the protoplanetary disc thus forming wet. The dry model assumes a hot Earth losing water to space, forming dry, and later receiving additional water via cometary and asteroidal bombardment.

Deuterium, a hydrogen isotope, contains an additional neutron in its nucleus in contrast to hydrogen's one proton and electron. This isotope can chemically bond with hydrogen, deuterium, and oxygen atoms to create increasingly massive water molecules: H2O, HDO, and D2O respectively. Earth's oceans contain approximately one deuterium atom per 6,400 hydrogen atoms, a D/H ratio significantly less than those detected on the majority of comets and asteroids. Recent work shows asteroids to have conflicting elemental and isotopic properties as compared to Earth, suggesting current compositions of comets and asteroids are not uniquely suited as primary delivery mechanisms for terrestrial water.

New research suggests a comet's present composition may not be an accurate analog to its ancient counterpart. A recent study now explains that during comet formation and shortly thereafter, the radioactive decay of 26Al generates enough heat to force the interior temperatures above the melting point of water, permitting stratigraphic cometary differentiation and enrichment of near surface H2O, as well as deuterium-rich HDO and D2O near the core. Recent computational and experimental research conducted at temperatures between 138K and 900K, establishes that the adsorption of water onto olivine grains, which are among the most plentiful minerals in a protoplanetary disc, can cause a planet's mantle to contain multiple oceans worth of water, thus supporting a wet Earth formation.

We present the Gravitational Molecular Hydro-Accretion model (GMHA), positing a succession of mechanisms explaining the origin of incongruent deuterium ratios found within our solar system, while additionally identifying the primary source of Earth's water. Furthermore, we illustrate that GMHA has already made one correct prediction regarding the deuterium ratio of Earth mantle samples, while several other predictions await testing.

  • Mark Johnson 48 x 36 GSA Poster 1.75 MB.pdf (1.8 MB)