Clouds on Mars (NASA/JPL/Malin Space Science

Systems)

The Effects of Magma Ocean Depth and Initial Composition on Planetary Differentiation Lindy Elkins-Tanton, MIT

E. M. Parmentier, S. Seager, S. Stanley, B. Weiss, M. Zuber

NSF Astronomy and NASA Mars Fundamental Research

Three stages of early planetary evolution

1. Solidification

Process after Abe and Matsui (1985); Solomatov (2000)

Emissivity parameterizations from Pujol and North (2003), Hodges (2002), Howard and Kasting (unpub)

Different metallic iron core fractions

Three stages of early planetary evolution

1. Solidification

Three stages of early planetary evolution

1. Solidification 2. Cumulate mantle overturns to stability

Elkins-Tanton et al. (2005a, b)

Gravitational overturn: Nonmonotonic density gradients

Overturn creates a laterally heterogeneous mantle

Contours of initial depth (proxy for composition)

Contours of density

Axisymmetic models show: The majority of overturn complete in <2 Myr; small-scale heterogeneities last a long time

Before overturn After overturn

Depths of origin of lunar volcanic rocks

Temperature [C]

Water in cumulates

Elkins-Tanton (2008)

Crustal magnetic field from a Noachian dynamo

Purucker et al. (2001)

Overturn can produce a core dynamo

Stanley et al. (in revision for Science)

Three stages of early planetary evolution

1. Solidification 2. Cumulate mantle overturns to stability

Elkins-Tanton et al. (2005)

Three stages of early planetary evolution

1. Solidification 2. Cumulate mantle overturns to stability

3. Cooling

Planetary surface temperature

Water retained vs. degassed during magma ocean solidification: Super-Earths

Elkins-Tanton and Seager (2008)

Planetary solidification: Time to 98% solid

Elkins-Tanton (2008)

Surface and atmosphere

Surface and atmosphere: 500 km-deep MO, solidification step

1. 10,000 - 1,000,000 years

EARTH

Surface and atmosphere: 500 km-deep MO, overturn and cooling

Minimum water in atmosphere:

3.8×1020 kg water, or 33% of an Earth ocean; 80% of initial water in magma ocean

1. 10,000 - 1,000,000 years

2. overturn: < 1 Myr

3. nearing critical point: (10 Myr)

EARTH

Elkins-Tanton (2008)

Three stages of early planetary evolution

1. Solidification 2. Cumulate mantle overturns to stability

3. Cooling

Planetary surface temperature

Effects of size on cooling time

Planetesimal Moon Earth

• No mafic quench crust is likely to form - without flotation cooling is very fast

• If plagioclase forms and floats, it may significantly slow planetary cooling

r = 10s to 100s km 1736 km 6378 km

Wood et al., 1970; Smith et al., 1970

Effects of size on cooling time

• Water in the magma ocean also suppresses plagioclase stability

• Without a conductive lid, solidification is very fast - faster than it would have been on the Moon

Planetesimal Moon Earth

r = 10s to 100s km 1736 km 6378 km

Elkins-Tanton (2008)

Effects of size on cooling time

Planetesimal Moon Earth

r = 10s to 100s km 1736 km 6378 km

26Al heating melts the body from the inside outHevey and Sanders (2007)Sahipal et al. (2007)

Weiss et al. (submitted)

Conclusions

2. overturn: < 1 Myr

3. nearing critical point: (10 Myr)

1. 60,000 - 1 M years

• Clement surface conditions can be reached within several 10s of Myr of a magma ocean

• Magma oceans may result in magnetic dynamos on bodies of a range of sizes

• Magma ocean solidification creates a stably stratified, laterally-heterogenous, damp mantle

Water degassed from super-Earth magma oceans

Elkins-Tanton and Seager (2008)