Transcript
7/27/2019 2. Solar Nebula Evolution Condensation
1/14
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
7/27/2019 2. Solar Nebula Evolution Condensation
2/14
Condensation of elements and compounds
7/27/2019 2. Solar Nebula Evolution Condensation
3/14
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
7/27/2019 2. Solar Nebula Evolution Condensation
4/14
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
7/27/2019 2. Solar Nebula Evolution Condensation
5/14
Condensation sequence
7/27/2019 2. Solar Nebula Evolution Condensation
6/14
Condensation sequence
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
7/27/2019 2. Solar Nebula Evolution Condensation
7/14
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
7/27/2019 2. Solar Nebula Evolution Condensation
8/14
Formation of planets
Planets formed from the disc-shaped cloud of gas and dust
left over from the Sun's formation
7/27/2019 2. Solar Nebula Evolution Condensation
9/14
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
7/27/2019 2. Solar Nebula Evolution Condensation
10/14
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
7/27/2019 2. Solar Nebula Evolution Condensation
11/14
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
7/27/2019 2. Solar Nebula Evolution Condensation
12/14
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)
7/27/2019 2. Solar Nebula Evolution Condensation
13/14
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
7/27/2019 2. Solar Nebula Evolution Condensation
14/14
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,
top related