4th Onsen Workshop: Formation and Chemical Composition of Cores

Thank you to all participants for coming to Yunishigawa Onsen! We truly enjoyed having all of these wonderful colleagues together for a few days to delve into these grand challenge questions. PS: Don't forget about the jSEDI special issue, submissions are open (see below for details).

Overview

Dates: Tuesday-Friday, 7-10 October 2025  (See below for detailed schedule)

Venue: Honke Bankyu (Yunishigawa hot spring near Nikko). This is a traditional Japanese-style hot-spring hotel (Onsen Ryokan in Japanese). We held the first Onsen Workshop there in 2007, and it was very popular among attendees. There are open-air baths along a river, and we have dinner around the fire.

Scientific Organizating Committee: Kei Hirose (Chair), James Badro (IPGP), John Hernlund (ELSI), Anat Shahar (CIW)

Workshop Theme: Formation and chemical composition of cores (Earth and other planets). We are assembling a diverse international workshop with participants from a variety of disciplines interested in tackling this grand challenge question.

Workshop Fees

Registration Fee: A registration fee of 42,000 Yen will be collected upon arrival to the venue from all participants (not including accompanying persons). Please bring cash to cover this expense. We apologize for the inconvenience. (Note that 7-11 ATM machines in Japan work well with foreign cards, and charge only modest fees.)

Accompanying Persons: Additional fees for guests/non-participants can be paid directly to the venue, either by cash or credit card.

Single Rooms: Those who reserved single rooms can pay the additional fees directly to the venue, either by cash or credit card.

Passport Information

For all non-Japanese participants and accompanying persons: The hotel requires passport information for all non-Japanese participants and accompanying persons. If you have not done so, please send your passport information, passport number and nationality in advance to our secretary Asako (asakosato AT eps.s.u-tokyo.ac.jp) so that your check-in procedure can be as smooth as possible.

Workshop Schedule

Tuesday 7 October (Day 1)

• Transport to Venue. The following options are provided:

14:40 1st Bus Departure from Kinugawa Onsen Station

16:45 2nd Bus Departure from Kinugawa Onsen Station

• Dinner and Welcome Party (18:30-20:30) Please arrive to the dining room by 18:30. Dining closes promptly at 20:30.

• Poster Session and Beer (20:30--)

20:30-- Poster Session (posters will remain up through the end of the workshop):

• James Badro (IPGP) Petrological and geochemical consequences of the solidification of Earth's magma ocean

• Hidenori Genda (ELSI, Science Tokyo) Behavior of Impactor Metallic Cores during Planetary Impacts

• George Helffrich (ELSI, Science Tokyo) Progress towards a phase diagram of the Fe-FeH system

• John Hernlund (ELSI, Science Tokyo) Style of Core Formation and Resulting Planet

• Kei Hirose (Univ Tokyo, ELSI/Science Tokyo) Exploring the Earth’s core composition and temperature

• Shigeru Ida (ELSI, Institute of Science Tokyo) Planet formation and the carbon deficit problem in the terrestrial planets

• Kenji Ohta (Science Tokyo) Electrical and thermal conductivities of Fe alloys under core conditions

• Anat Shahar (Carnegie Science) Water, Water Everywhere

Wednesday 8 October (Day 2)

• Session 1 (9:00-10:20)

9:00-9:20 Martin Bizzarro (Globe Institute, Univ Copenhagen) Interstellar Ices as Carriers of Supernova Material to the Early Solar System

9:20-9:40 Naoya Sakamoto (Hokkaido Univ) Oxygen and Nitrogen isotopes in the Solar System

9:40-10:00 Thorsten Kleine (Max Planck Institute for Solar System Research) Lunar samples may carry a 182W signature of the Moon-forming impactor

10:00-10:20 Discussion

• Morning Break (10:20-10:40)

• Session 2 (10:40-12:00)

10:40-11:00 Alessandro Morbidelli (Collège de France) Chemical evidence for the heterogeneous accretion of Earth: implications on the dynamics of Earth's accretion

11:00-11:40 Anders Johansen (Univ Copenhagen) Hybrid accretion of rocky planets imprinted in volatile depletion

11:20-11:40 Francis Nimmo (UC Santa Cruz) Accretion of carbonaceous materials to the Earth, and implications for core composition

11:40-12:00 Discussion

• Lunch Break (12:00-13:30)

• Session 3 (13:30-15:10)

13:30-13:50 Miki Nakajima (Univ Rochester) Effects of planetary impacts on core and magma ocean dynamos

13:50-14:10 Razvan Caracas (IPGP, CNRS) Core fate during giant impacts

14:10-14:30 Bernard Marty (CRPG/Univ Lorraine, CNRS) Noble gases in the core?

14:30-14:50 Discussion

14:50-15:00 Group Photo

• Afternoon Break (15:10-16:50)

• Session 4 (16:50-18:10)

16:50-17:10 Julien Siebert (IPGP) Accretion of volatile elements on Earth without the need of a late veneer

17:10-17:30 Ingrid Blanchard (CNRS) Core Formation Resolves Earth's Siderophile Excess Without a Late Veneer

17:30-17:50 Francesca Miozzi (ETH Zürich, Univ Pavia) Magma ocean-atmosphere interaction as an alternative process to deliver water to planetary interiors

17:50-18:10 Discussion

• Dinner (18:30-20:30) Please arrive to the dining room by 18:30. Dining closes promptly at 20:30.

• Evening Debates and Beer (20:30--)

20:30 Open

21:00 Debate Time! Progress is best served when ideas are subject to challenge and debate. We will hold collegial and productive debates on Wednesday and Thursday evenings. The following topics (and more) will be discussed, please send any additional suggestions to the workshop chairs:

• Pebbles vs planetesimals

• Oxidizing vs reducing core formation

• Aftermath of formation: Mixed vs heterogenous?

• Moon formation scenarios

• Core segregation by jets vs diapirs

• Hot vs cool initial core

• Core exsolution/crystallization: Yes or no?

• Favorite core alloy(s)

• Are we making progress?

Thursday 9 October (Day 3)

• Session 5 (9:00-10:20)

9:00-9:20 Arwen Deuss (Utrecht Univ) Seismic structure of the Earth's core

9:20-9:40 Han Hsu (National Central Univ) Superionicity of a novel tetragonal FeH6 phase at high pressure

9:40-10:00 Shuhei Mita (Univ Tokyo) Hydrogen isotope fractionation during core formation

10:00-10:20 Discussion

• Morning Break (10:20-10:40)

• Session 6 (10:40-12:00):

10:40-11:00 John Brodholt (Univ College London) A Case for Si and H as the light elements in the core

11:00-11:40 Shunpei Yokoo (Univ Tokyo) Core-mantle partitioning of light volatile elements

11:20-11:40 Yang Sun (Xiamen Univ) Atomistic Insights into Thermodynamics, Phase Competition, and Growth Processes in Earth's Core

11:40-12:00 Discussion

• Lunch Break (12:00-13:30)

• Session 7 (13:30-15:10)

13:30-13:50 Gerd Steinle-Neumann (Bayerisches Geoinstitut, Univ Bayreuth) Hydrogen in Earth's core – a computational perspective

13:50-14:10 Guillaume Morard (IMPMC/CNRS/Sorbonne Univ) Liquid silicates and iron alloys under extreme conditions

14:10-14:30 Charles-Édouard Boukaré (York Univ) Thermodynamics in the Fe-Si-O system up to 350 GPa

14:30-14:50 Daniele Antonangeli (IMPMC, CNRS, MNHN, Sorbonne Univ)  Melting Curves and Liquid–Liquid Immiscibility in Selected Iron Alloys at High Pressure

14:50-15:10 Discussion

• Afternoon Break (15:10-16:50)

• Session 8 (16:50-18:10)

16:50-17:10 Michael Walter (Earth and Planets Lab, Carnegie Science) Melting and Resistivity of Iron by Pulsed Joule Heating of Iron to >1.5 Mbar

17:10-17:30 Merkel Sébastien (Univ Lille) Revisiting the iron phase diagram

17:30-17:50 Fumiya Sakai (Univ Tokyo) Machine Learning Molecular Dynamics Simulations of Crystallization in Fe-Si-O Liquids

17:50-18:10 Discussion

• Dinner (18:30-20:30) Please arrive to the dining room by 18:30. Dining closes promptly at 20:30.

• Evening Debates and Beer (20:30--)

20:30 Open

21:00 Debate Time! Progress is best served when ideas are subject to challenge and debate. We will hold collegial and productive debates on Wednesday and Thursday evenings. The following topics (and more) will be discussed, please send any additional suggestions to the workshop chairs:

• Pebbles vs planetesimals

• Oxidizing vs reducing core formation

• Aftermath of formation: Mixed vs heterogenous?

• Moon formation scenarios

• Core segregation by jets vs diapirs

• Hot vs cool initial core

• Core exsolution/crystallization: Yes or no?

• Favorite core alloy(s)

• Are we making progress?

Friday 10 October (Day 4)

• Session 9 (9:20-10:40)

9:20-9:40 Stéphane Labrosse (ENS Lyon) Long term core evolution with extraction of light elements at the core mantle boundary

9:40-10:00 Henri Samuel (CNRS, IPGP, Univ Paris Cité) Mars' present-day structure and long-term evolution from InSight data

10:00-10:20 Takashi Yoshizaki (Ignition Point Inc., Tohoku Univ) Cosmochemical Core Models—Now Subject to InSight's Peer Review

10:20-10:40 Discussion

• Morning Break (10:40-10:50)

• Session 10 (10:50-12:10):

10:50-11:10 Yingwei Fei (Earth & Planets Lab, Carnegie Science) An integrated approach to narrow down the compositions of planetary cores

11:10-11:30 Kevin Righter (Univ Rochester) Siderophile element depletions in planetary mantles and comparative planetology of core formation

11:30-11:50 Caroline Dorn (ETH Zürich) Coupled Interiors and Atmospheres as Keys to Super-Earth and Sub-Neptune Diversity

11:50-12:10 Discussion

• Lunch Break (12:10-13:00)

• Bus Departure (13:00-14:00)

13:35-14:45 Bus Stop at Kinugawa Onsen Station

14:15-15:15 Bus Arrives at Nikko Toshogu Shrine (World Heritage Site)

Location/Access

The workshop will be held at Honke Bankyu 749 Yunishigawa, Nikko, Tochigi 321-2601 (click here for Google Map location). Bus service to the venue departing from Kinugawa Onsen Station is provided. The first bus leaves at 14:40 and the second bus leaves at 16:45. If you are unable to take either of these buses, you will need to find other arrangements to reach the venue (contact one of the organizers if you need assistance). Kinugawa Onsen Station is operated by Tobu Railways, several express trains are available from different parts of Tokyo, for more information click here for the Tobu access page for Kinugawa Onsen.

Registration

All attendees are requested to fill the following registration form, this will enable us to assemble a coherent program, share information, collect your suggestions, and stay in touch in the future.

Note: if your browser is having difficulty rendering this embedded registration form below, please click here to go directly to the registration form.

Journal Special Issue

Announcement: We are organizing a special issue titled Formation and Composition of Cores to be published for this workshop in our new community journal, jSEDI. There is no fee to publish, authors retain copyright, and published papers are released under a creative commons (CC-BY) license (complete open access).

Instructions: Please submit through the jSEDI website and note in the cover letter that the manuscript is intended to be included as part of the special issue on the "Formation and Composition of Cores."

Practicalities: Submission is already open, and there is no deadline. Submitted manuscripts will immediately be available on one of the host public archives utilized by the journal (Arxiv, Zenodo, HAL) during peer review and prior to publication. Papers will be published by jSEDI immediately after acceptance, there will be no delays or waiting for other papers submitted to the issue to be published. A special section/listing will be arranged for this special issue upon publication of the first paper, and all others will be added as they are accepted. The Co-Chairs of this workshop will help with the editorial workload of handling manuscripts for peer review.

Venue Etiquette

 Honke Bankyu is a traditional Japanese-style hot-spring hotel ("Onsen Ryokan" in Japanese). There are a number of customs that guests typically follow, however, some are more strict than others. We describe some of these below:

• Footwear: Outdoor shoes should be removed and stored at the front entrance to the hotel. Indoor slippers are provided for guests to wear inside the hotel. It is customary to remove general house slippers before entering any room having a tatami (straw) floor. Special slippers are often provided for use in toilets, please change into these when entering the toilet room, and please remember to take them off when leaving the toilet. Outdoor slippers may also be available for use in outdoor areas in the hotel premises.

• Robes ("Yukata"): Ryokans typically provide traditional robes/yukata to wear inside the hotel. You are not required to wear them, however, it may be fun to relax in the traditional way. If you need a different size than the one provided, please ask the hotel staff. Please wear the robes over your underclothing. There are a number of customs (see this video for a detailed discussion), however, the most important is that the left side should go over the right side. This manner of wearing the yukata shows respect to others, the opposite sense (right over left) holds the reverse meaning (you may notice that shoji blinds and sliding doors have a similar connotation).

• Hot Spring: Please remove house slippers before entering the bath area, and store your belongings and clothes in the provided storage bins. Please use the wash stations to clean up before entering the hot spring water. No cloth or clothing, no soap or article or anything of any kind (except your body) is allowed in the common bath water. Long hair is also tied up to prevent it from soaking in the common bath water.

• Meals: Traditional Japanese ryokan meals are world famous, and Honke Bankyu is exceptional among them. Please enjoy your meals with an open mind, which will incorporate regional dishes and ingredients which you may not be familiar with.

• Drinking: Although it is not strictly observed, nor necessary, it is considered kind and is customary to pour drinks for others, and for others to pour for you.

• Discussion: Please avoid shouting/yelling or holding a discussion at volumes louder than necessary. Telephone calls should be held in private, rather than public, places.

Code of Conduct

Harassment of any kind will not be tolerated. All event attendees and staff have the right to a space that is free of all forms of discrimination, harassment, bullying, and/or retaliation. Participants who are requested to stop any harassing behaviour are expected to comply immediately. If you wish to report harassment, suspect that someone else is being harassed, or have any other concerns, please contact a member of the organizing committee as soon as possible.