Abstract:
Hydrogen is one of the possible alloying elements in the planetary core, but its siderophile (iron-
loving) nature is debated. The amount of hydrogen in the core is key to the understanding of the
origin of the planets and their surface environment because the reaction of metal-silicate
partitioning of hydrogen could adjust the present ocean mass of water. Furthermore, the kind of
light elements present in the Earth’s core, which is essential to constraint the bulk composition of
the Earth, has been an open question for ~70 years. However, despite its importance, hydrogen
partitioning experiments under core-mantle separation conditions have not been performed due
to experimental difficulties.

In this seminar, I will talk about a recent project to estimate the hydrogen content in the
planetary core (Tagawa et al. Nat. Com. In press.). The collaborators and I examined the
partitioning of hydrogen experimentally between molten iron and silicate melt at 30–60
gigapascals and 3,100–4,600 kelvin, based on diamond-anvil cell experiments and cutting-edge
secondary ion mass spectrometry analyses (SIMS). We find that hydrogen has a metal/silicate
partition coefficient DH  29 and is therefore strongly siderophile at conditions of core formation.
Unless water was delivered only in the final stage of accretion, core formation scenarios suggest
that 0.3–0.6 wt% H was incorporated into the core, leaving a relatively small residual H2O
concentration in silicates. This amount of hydrogen is equivalent to the amount of water in 37–73
ocean mass, which is supported by the recent planet formation scenarios, and explains 30–60% of
the density deficit and sound velocity excess of the outer core relative to pure iron. Hydrogen may
be an important constituent in the Earth’s core and in the metallic cores of any terrestrial planet
or moon having a mass in excess of 10% of the Earth.

Speaker: Dr. Shoh Tagawa, University of Tokyo.

Speaker bio:
Shoh Tagawa investigates hydrogen’s behavior in the planetary interior using a diamond anvil cell
(DAC). He graduated from TokyoTech’s undergraduate and master's courses and then received his
Ph.D. from the University of Tokyo. Recently, he also works as a support in online education.

Host: Kei Hirose, ELSI.

Date: Fri, 26 March, 15:30-16:30 JST (Fri, 26 March, 06:30-07:30 UTC)

Venue: Online