Formation of Mercury: A Review 2024

Ryuki HyodoJAXA

Today1. Introduction
2. Mercury’s large core
2.1 impact hypothesis
2.2 condensation hypothesis

3. Planet formation models
4. Late Accretion
5. Super-Mercuries

1. Solar nebula formation(First ~0.1 Myrs)

2. Condensation of solids(First ~1-10 Myrs)

3. Growth of bodies(First ~1-100 Myrs)

4. Giant impact(First ~10-100 Myrs)

Ebel 2006

Benz 2007
Kokubo & Ida 2012

6.1. Long-term internal evolution
6.2. Generation of magnetic elds
(After ~4.5 Ga)

7. Today

5. Late accretion(Until ~4.0 Ga)

Hyodo et al. 2021

fiSlavin et al. 2007

Benz 2007

Mercury’s key features in the context of terrestrial planets
・A large core (~70% by mass) (Hauck et al. 2013)
・Mantle is very reduced, i.e., FeO-free silicate (Warell & Blewett 2004; Ebel & Stewart 2018)
・High abundance of moderately volatile elements in the mantle (Peplowski et al. 2021)

M=0.81M⊕
Core fraction: ~0.23

M=0.055M⊕
Core fraction: ~0.70

Mass is concentrated in Venus and Earth.

EarthVenus

A largeseparation

MarsMercuryAngular Momentum De cit (AMD)The orbits of the terrestrial planets are not highly eccentric.

Eccentricity

Radial Mass Concentration (RMC)

MercuryVenus
EarthSemi-major axis
Real system: RMC=89.9
Numerical simulation: e.g., RMC= 45

AMD =∑i Mi ai 1 −[(1 − ei2) cos ii∑i Mi aifi☑︎☑︎

Mercury’s key featuresin the context of terrestrial planets]

Real system: AMD=0.0018
Numerical simulation: e.g., AMD= 0.0045
RMC = MAX∑i mi

∑i mi [log10 (a /ai)]

2MarsToday
1. Introduction
2. Mercury’s large core
2.1 impact hypothesis
2.2 condensation hypothesis
3. Planet formation models
4. Late Accretion
5. Super-Mercuries

Catastrophic collision (e.g., Benz et al. 2007)

Hit-and-run (e.g., Asphaug & Reufer 2014)

Target becomes Mercury

Impactor becomes Mercury
Grazing
impactfcore~0.3

fcore~0.3

fcore~0.3

fcore~0.7✓ Mtar = 2.25MMercury✓ Mimp ~ 0.1Mtar✓ vimp ~ 20-30km/s (~7vesc)

fcore~0.3

Quick note:— Low probability?
— Re-accretion of the debris (~40% of them?)

fcore~0.7✓ Mtar = 15.4MMercury✓ Mimp = 4.52Mmercury✓ vimp ~ 17km/s (~3vesc)

Quick note:— The fate of largest remnant? Would it be Venus?

Erosion by multiple collisions (Chau et al. 2018)

Cumulative erosive small impacts (Hyodo et al. 2021)

Multiple embryo-embryo (e.g., ~1,000km-sized) collisions

Planetesimals (~100km) are eroded by smaller impactors

fcore~0.3

fcore~0.3

fcore~0.3

fcore~0.7

fcore~0.7

fcore~0.7
fcore~0.3

fcore~0.4

fcore~0.5

fcore~0.7

fcore~0.3

fcore~0.3fcore~0.3✓ Mtar ~ 2.25MMercury✓ Mimp ~ 0.2Mtar✓ vimp ~ 10km/s (~3-4vesc)

Quick note:— This process may be likely during planet accretion.
— Ejecta re-accretion? (ejecta’s self-shielding to non-grav. forces?)— Now, need to be tested in more N-body simulations

fcore~0.7

iron-rich planetesimals

Quick note:— Re-accretion problem solved? (c.f., less ejecta at each impact and thus no self-shielding?)— No one has ever modeled in the N-body simulations (i.e., the likelihood is unknown)

Today1. Introduction
2. Mercury’s large core
2.1 impact hypothesis
2.2 condensation hypothesis
3. Planet formation models
4. Late Accretion
5. Super-Mercuries

“Condensation line is the preferential location
where materials pile up”(Hyodo et al. 2019; 2021, A&A)
Radial diffusion
Vertical diffusion

Recondensation
Dust sticking

Gas accretion
Water vapor

Traffic-jam pile-up of silicate dust
→ gravitational instability?
(formation of rocky planetesimal)

Silicate dust
Pebble-to-gas mass ux

Radial drift due to the gas dragIcy pebbles

Recycling pile-up of icy pebbles→ streaming instability?
(formation of icy planetesimal)

Snow line

Rockvapor

Gas accretion

Rockline

flAguichine et al. 2020
Izidro et al. 2021
Morbidelli+2021
Bogdan et al. 2023
Mah & Bitsch 2023

Snow line
Stevenson & Lunine 1988
Ciesla & Cuzzi 2006
Ros & Johansen 2013
Schoonenberg & Ormel 2017
Drazkowska & Alibert 2017
Hyodo et al. 2019,2021

CO linee.g. Oberg et al. 2011

Condensation of large iron-rich pebbles“Large iron particles (i.e., large stokes number) preferentiallyexperience the streaming instability, forming iron-rich planetesimals.”(Johansen & Dorn 2022)

Surface tension matters!

(as the nucleus formation rate is “exponentially” depends on σ)σFe = 1.8 J m−2, σFe+S = 1.2 J m−2−2−2σMgSiO3 = 0.4 J m , σFeO = 0.6 J mNuclei
Free energy

rcri

Fe(g)

Al(g) H2O(g) H2(g)Mg(g)
at~0.4au

SiO(g)
Temp.
decreases

Fe(s) MgSiO (s)3FeO(s)Al2O3(s)Streaming
Instability

rcri ∝ σParticle size

*Adding sulfur leads to condensation of Fe+S at a higher temperature than FeO.

And more from other ideas...
• Mantle evaporation (Cameron 1985; Fegley &Cameron 1987)• Silicate/metal separation by photophoresis (Wurm et al. 2013)• A compressed planetary core (Mocquet et al. 2014).
Cameron 1985; Fegley &Cameron 1987

Wurm et al. 2013
Photophoresis can separate high thermal conductivity materials (iron)from lower thermal conductivity solids (silicate)Today1. Introduction
2. Mercury’s large core
2.1 impact hypothesis
2.2 condensation hypothesis
3. Planet formation models
4. Late Accretion
5. Super-Mercuries

Classical model— a smooth continuous disk —(Wetherill 1978; Hayashi 1981)

0.4au

0.7au

1.0au

1.5au

• Mass concentration (to Earth/Venus) is hardly explained.
• Fragmentation can explain real AMD, but not RMC.

Chambers 2013; see also Chambers 2001

Narrow ring model

Real system
Simulations(e.g., Hansen 2009; Walsh & Levison 2016, Woo 2024)

Earth
0.4au

0.7au

1.0au

1.5au

Venus

MarsMercury• Compact system• Good AMD (the system is not too excited)• Good RMC (Mass concentration at Venus and Earth)

Hansen 2009

Now scientists are studying…
✓ Distribution w/w.o various gas disks
Example1
Example2
Example3
Semi-major axis

e.g.,Chambers 20
Hansen 2009
Walsh & Levison 2016
Izidoro et al. 2021
Clement et al. 2001
Woo 2024✓ Instability models

5au

10au

5au

10au

5au

10au

e.g.,
Chambers 2013
Raymond et al. 2016
Clement et al. 2019,2023
Franco et al. 2022
Woo et al. 2024

✓ Fragmentation models (not a single giant impact to form Mercury)
Destructive collisions among e.g. embryos
e.g.,Marcus 2009
Stewart & Leignhardt 2009
Leinhardt & Stewart 2012
Carter et al. 2018
Gabriel et al. 2020

Cumulative erosion via numerous cratering impacts

e.g.,Melosh 1989
Hyodo & Genda 2021,2022

See also
Reinhardt et al. 2022
Dou et al. 2024

Today1. Introduction
2. Mercury’s large core
2.1 impact hypothesis
2.2 condensation hypothesis
3. Planet formation models
4. Late Accretion
5. Super-Mercuries

What is the late accretion?
Late accretion is inevitable; i.e., a numerous crater-forming impacts.
*a giant impact is a stochastic process

Leftover small bodies

e.g., Brasser+2016

Late accretion depends on models
Late accretion is inevitable, a numerous crater-forming impacts.
*a giant impact is a stochastic process

Leftover small bodies

e.g., Brasser+2016

Late Accretion to Mercury(Hyodo et al. 2021)

Late accretion is inevitable (~4 Ga):• Surface composition would be heterogeneously modi ed.
→ i.e., volatiles can be provided even in a giant impact hypothesis.

fi• Global resurfacing with local crust melting would take place.

Today1. Introduction
2. Mercury’s large core
2.1 impact hypothesis
2.2 condensation hypothesis
3. Planet formation models
4. Late Accretion
5. Super-Mercuries

Super-Mercuries✓ Mercury is unique in the solar system.
✓ Super-Mercuries seem also unique in the exoplanet population.
→ An unique formation idea may be required for planet formation.

The inner most planet is not always the densest one.
(Bonomo et al. 2019)

Mass=1.24Ms

Kepler-107b

Mass = 3.8MEarthdensity=5.3 g/cm3

Kepler-107c

Mass ~ 10.0MEarth, which is big!!
Density=12.6 g/cm3• Not always the inner planet has a large core
→ Evaporation, separation, simple condensation models
may not be preferred.
• A big planet can have a large core
→ A simple giant impact model may not be preferred.
From Johansen & Dorn 2022

Summary• Condensation, accretion, fragmentation, and giant impact can all change the core fraction.
→ the dominant process is not yet understood.
• Late accretion can change the surface composition.
• Beyond Mercury’s formation, the modeling needs to cover also super-Mercuries

8. Super-Mercuries➕