Origin of Earth's Water: Chondritic Inheritance Plus Nebular Ingassing and Storage of Hydrogen in the Core
Jun Wu, Steven J. Desch, Laura Schaefer, Linda T. Elkins‐Tanton, Kaveh Pahlevan
Peter R. Buseck
First published: 09 October 2018
Source/Fonte: NASA
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.
Plain Language Summary
People have long had curiosity in the origin of Earth's
water (equivalently hydrogen). Solar nebula has been given the least
attention among existing theories, although it was the predominating
reservoir of hydrogen in our early solar system. Here we present a first
model for Earth's water origin that quantifies contribution from the
solar nebula in addition to that from chondrites, the primary building
blocks of Earth. The model considers dissolution of nebular hydrogen
into the early Earth's magma oceans and reaction between hydrogen and
iron droplets within the magma ocean. Such processes not only delivered
countless hydrogen atoms from the mantle to the core but also generated
an appreciable difference in hydrogen isotopic composition (2H/1H ratio)
between the mantle and core. Fitting the model to current knowledge
about Earth's hydrogen produces best combinations of nebular and
chondritic contributions to Earth's water. We find that nearly one out
of every 100 water molecules on Earth came from the solar nebula. Our
planet hides majority of its water inside, with roughly two oceans
in the mantle and four to five oceans in the core. These results
suggest inevitable formation of water on sufficiently large rocky
planets in extrasolar systems.
FREE PDF GRATIS: J of Geophysical Research Planets
