2. solar nebula evolution condensation

7/27/2019 2. Solar Nebula Evolution Condensation

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Inner part of solar nebula began hotfew pre-solar

solids survive; solids condensed from vapor of solar

composition, as temperature decreasedhence the

key to understanding the distribution of elements in the

solar system is the idea of volatilitythe preference ofan element for gaseous species over solids, quantified

by the 50% condensation temperature (e.g., 1650 K for

Al, 970 K for Na, 3 K for He)

Some of the variations in the chemical composition of

primitive meteorites or planets are related to their

temperature of formation

Condensation of elements and compounds


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Condensation of elements and compounds


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Condensation sequence

Some solid phases condense directly from vapor. Others form by

reaction of vapor with previously condensed phases.

Refractory component:First phases to condense: Ca-Al-oxides

(corundum and then perovskite), trace elements: REE, Zr, Hf, Sc

Refractory metals with low vapor pressures, e. g., W, Os, Ir condense atsimilarly high temperatures as metal alloys

Corundum then reacts with vapor to form spinel and melilite which in

turn react to produce diopside at lower T

2. Fe-Mg-silicates:In the reducing environment of the solar nebula Fe

condenses almost entirely as metal, while Mg and Si form forsterite

(Mg2SiO4) most of which is, at lower temperatures converted to

enstatite (MgSiO3) by reaction with gaseous SiO


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Condensation sequence3. Metallic iron condensesat about the same temperature as

forsterite, the sequence depending on pressure

4. Moderately volatile elements:The most abundant elements is

sulfur which condenses by reaction of gaseous S with solid Fe at

710K, independent of pressure. Other moderately volatile elementscondense in solid solution with major phases. Moderately volatile

elements are distributed among sulfides, silicates and metal

5. Highly volatile elements:have condensation temperatures below

FeS. The group of highly volatile elements comprises elements of

very different geochemical affinity, such as the chalcophile Pb and

the atmophile N and rare gases


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Mercury

Venus

Earth

Mars

Jupiter

Saturn

Condensing ices gavethe giant planets the

mass to gravitationallycapture H and He from

nebula

Bulk oxidation state of aplanet is set by how

much O is condensed asFeO and how much H is

retained as H2O


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Among the several classes of chondritic meteorites, relativeabundance of all elements are controlled by volatility; this plotshows the CV/CI chondrites. Presumably similar volatility controlwas active during accretion of the Earth or its source materials.

Volatility controlled element abundances

CV/CI

CV/CI


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Formation of planets

Planets formed from the disc-shaped cloud of gas and dust

left over from the Sun's formation


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PlanetesimalsWithin the solar nebula, dust and ice particles embedded in the gas

moved, occasionally colliding and merging- accretion

Dust accreted into planetesimals with sizes of the order of a kilometer.During this stage the interactions of solid bodies were controlled bythe drag of the nebular gas

In the inner, hotter part of thesolar nebula, planetesimalswere composed mostly of

silicates and metals. In theouter, cooler portion of thenebula, water ice was thedominant component


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Planetary embryosPlanetesimals were massive enough that their gravity influencedmotions of other planetesimals. This increased the frequency ofcollisions, through which the largest bodies grew most rapidly-

runaway growth

At the end of theplanetary formationepoch the inner Solar

System was populated by50100 Moon- to Mars-sized planetary embryos

Further growth occurred when these bodies collided and merged, ontime scales up to 100 million years by mutual gravitationalperturbations

Collision and growth continued until the four terrestrial planets we

know today took shape


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Solar nebula dispersesThe growing proto-Sun accumulated much of the original materialfrom the nebula long before planets formed. A small portion was

incorporated into the planets, but the remainder was swept awaywhen increasing temperatures and pressures initiated nuclearreactions in our Sun's core

The force of the reactioncaused a strong solar wind toexpel the outer layers of the

Sun into space beyond oursolar system. A much weakersolar wind continues to flowfrom our Sun today


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P and T profile in the solar nebula

There was a P and T gradient in

the solar nebula which changed

with time

The inner Solar System (4 AU) was

too warm for volatile moleculeslike water and methane to exist

Planetesimals which formed there was made of compounds with

high melting points, such as metals (like iron, nickel, and aluminum)

and rocky silicates. These rocky bodies would become the

terrestrial planets (Mercury, Venus, Earth, and Mars)


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P and T profile in the solar nebula

There was a P and T gradient

between the inner and outer

parts of the nebula

The gas giant planets (Jupiter, Saturn, Uranus, and Neptune)

formed further out, beyond the frost line, the point between the

orbits of Mars and Jupiter where the material is cool enough for

volatile icy compounds to remain solid


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Density and Size of Planets

We can explain compositionand sizes of planets at various

distances from the sun byconsidering:

Position in the solar nebula(i.e., temperature is >1000 Kat Mercury,

2. solar nebula evolution condensation

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