Origin of Earth's Water: Chondritic Inheritance Plus Nebular Ingassing and Storage of Hydrogen in the Core

Abstract

Recent developments in planet formation theory and measurements of low D/H in deep mantle material support a solar nebula source for some of Earth's hydrogen. Here we present a new model for the origin of Earth's water that considers both chondritic water and nebular ingassing of hydrogen. The largest embryo that formed Earth likely had a magma ocean while the solar nebula persisted and could have ingassed nebular gases. The model considers iron hydrogenation reactions during Earth's core formation as a mechanism for both sequestering hydrogen in the core and simultaneously fractionating hydrogen isotopes. By parameterizing the isotopic fractionation factor and initial bulk D/H ratio of Earth's chondritic material, we explore the combined effects of elemental dissolution and isotopic fractionation of hydrogen in iron. By fitting to the two key constraints (three oceans' worth of water in Earth's mantle and on its surface; and D/H in the bulk silicate Earth close to 150 × 10−6), the model searches for best solutions among ~10,000 different combinations of chondritic and nebular contributions. We find that ingassing of a small amount, typically >0–0.5 oceans of nebular hydrogen, is generally demanded, supplementing seven to eight oceans from chondritic contributions. About 60% of the total hydrogen enters the core, and attendant isotopic fractionation plausibly lowers the core's D/H to ~130 × 10−6. Crystallized magma ocean material may have D/H ≈ 110 × 10−6. These modeling results readily explain the low D/H in core‐mantle boundary material and account for Earth's inventory of solar neon and helium.

Controls on the Deep‐Water Cycle Within Three‐Dimensional Mantle Convection Models

“…Beyond 0.9 Gyr the remaining amount of water held within the mantle shows little variation, with the total water content of the mantle ranging from 1.6 to 1.9 OM with a periodicity of around 1 Gyr. This value lies well within the range of classic estimates of mantle water content (Hirschmann, ; Wu et al, ).…”

“…1. For the simplest mantle assumptions which are typically used in mantle modeling studies (incompressible, radially varying viscosity), the anticipated amount of water that the mantle holds is between 1.6 and 1.9 OM, which falls within the range expected from petrological observations, some of the other simpler modeling studies, and recent estimates from recent planetary formation study (Wu et al, 2018). 2.…”

“…The results of the work undertaken in this paper may be summarized as follows. For the simplest mantle assumptions which are typically used in mantle modeling studies (incompressible, radially varying viscosity), the anticipated amount of water that the mantle holds is between 1.6 and 1.9 OM, which falls within the range expected from petrological observations, some of the other simpler modeling studies, and recent estimates from recent planetary formation study (Wu et al, ). The water storage capacity of the mantle only varies by a few tenths of an OM for many of the dynamic variations. Exceptions are for a particularly stiff mantle (thicker thermal boundary layers reduce ocean influx opportunity) and a very weak lithosphere (low upper boundary layer viscosity results in major heat loss from mantle) giving rise to drier and wetter mantles, respectively. Our models suggest the mantle can efficiently go from a dry mantle to a wet mantle over the course of 3.6 Gyr, suggesting that the present mantle water content could be insensitive to its starting water content. Adjusting the values used for water solubility in the midmantle ambient material to account for a DHMS phase H does not cause a significant change in the total mantle water budget in our 3‐D model. We observe a close link between the lower mantle's maximum water saturation value and the upper mantle's most common water content. …”

Hydrogen isotopic evidence for early oxidation of silicate Earth

“…The impact probability of comets with Earth is small (~ppm) such that the inferred mass of cometary material can only contribute <10% of a terrestrial ocean, even with complete volatile retention during impact (Dauphas and Morbidelli, 2014). A nebular origin for terrestrial water has been discussed (Ikoma and Genda, 2006) but the solar neon inventory of the Earth only implies ingassing of a small fraction (<10%) of Earth's water inventory into an early magma ocean in the presence of the solar nebula (Wu et al, 2018). Moreover, a nebular source for terrestrial water would require a ~6x deuterium enrichment in the oceans (Fig.…”

Viroids-First—A Model for Life on Earth, Mars and Exoplanets

“…Evaporated water condensed and rained back down-one of the sources of water on Earth. Other possible sources of the Earth's water are asteroids or the solar nebula, the gaseous cloud after formation of the Sun and planets [6].…”