Element Classification & Solar System Origin

Question 1

what does the cosmochemical element classification classify based on

Answer 1

the condensation temperature (Tc) at which they are expected to condense in the solar nebula at 10^-4 bar.

Question 2

what are the four cosmochemical classification groups? give their condensation temperatures

Answer 2

refractory >1400K
main component 1400-1250K
moderately volatile 1250-650K
highly volatile <650K

Question 3

how is the boundary between moderately volatile and highly volatile cosmochemical groups divided?

Answer 3

the boundary at which Iron Sulphide condenses (FeS)

Question 4

give an example of a refractory element

Answer 4

refractory metals (Os, Ir, W) and metal-oxides (CaO, Al2O3, TiO3, REE oxides)

Question 5

give an example of a main component element

Answer 5

forsterite (olv) and enstatite (pyx) - magnesium silicates

Question 6

give an example of moderately and highly volatile elements

Answer 6

alkali elements Na, Rb, Cs, metals such as Nz, Cd and non-metals including S, Se, Sb and the noble gases. formation of FeS at 650K encourages volatile elements to react with FeS and condense

Question 7

what are the two geochemical element classification schemes?

Answer 7

1. Goldschmidt’s classification
2. compatible vs incompatible

Question 8

what are the four Goldschmidt’s classification groups? give their affinities

Answer 8

lithophile (silicate rocks)
siderophile (metallic liquids)
chalcophile (sulphide liquids)
atmophile (atmosphere)

Question 9

what is a compatible element?

Answer 9

D > 1
a compatible element has the correct ionic charge and/or ionic radius to preferentially partition into a mineral rather than remaining in the partial melt

Question 10

what is an incompatible element?

Answer 10

D < 1
an incompatible element has the incorrect ionic charge and/or ionic radius and preferentially partitions into the partial melt rather than going into the mineral

Question 11

how wide was the protoplanetary disc that formed the Sun?

Answer 11

100-200AU

Question 12

what is a T-Tauri star?

Answer 12

a star which emits enormous amounts of radiation and produce strong stellar winds

Question 13

why did the nebula temperatures drop when most of the material had been accreted to the Sun?

Answer 13

there was less gas and dust to absorb radiation

Question 14

how did the Sun’s state as a T-Tauri star help to clear the nebula and cool it?

Answer 14

the Sun as a T-Tauri nearly halted the Sun’s accretion by clearing the protosuns original cocoon of gas and dust helping the cool the originally hot inner solar system

Question 15

in what phase of the protosun did planetary accretion begin?

Answer 15

the T-Tauri phase, which helped to cool the hot solar system. therefore more and more gas condensed to dust which sticked together and accumulated into planetesimals and eventually planets.

Question 16

when is the T-Tauri phase complete?

Answer 16

when gravitational heating within the sun is sufficient to ignite nuclear fusion reactions in the core.

Question 17

for planets to form, why is it important for gas to condense and sufficient dust to accumulate into larger bodies

Answer 17

so that the gas/dust isnt accreted into the Sun, or completely dissipated by the T-Tauri winds of the young Sun

Question 18

what sort of timescale were dust particles produced?

Answer 18

quickly - over a few 1000s of years

Question 19

why were volatile constituents lost from the inner disk before condensation?

Answer 19

it was too hot for them to condense (~1000-2000K) and so were accreted or swept away in the inner disk.
the outer disk was colder ~10-100K and were therefore able to condense.

Question 20

what is the snow line/frost line

Answer 20

the minimum radial distance from the Sun, at which the temperature was low enough (~170K) such that water ice could condense from the solar nebula gas.

Question 21

between which 2 planets is (and was) the snow line positioned?

Answer 21

Mars and Jupiter

Question 22

what is the phase of water inwards of the snow line?

Answer 22

water is a vapour

Question 23

why were the Jovian planets able to grow into huge gas giants composed primarily of H and He.

Answer 23

the outer solar system was colder and allowed for water ice and clathrate condensation, therefore they had more material to accrete. their silicate rocky cores were able to quickly (>10 Earth masses after <1Ma) and grew sufficiently large to accrete uncondensed gases (H and He) by gravity before these gases were lost completely.

they also had extra material to accrete because a lot of material was swept away by the strong T-Tauri wind of the young Sun.

Question 24

why are the inner planets much smaller than outer?

Answer 24

inner terrestrial planets positioned inwards of snow line where water ice/clathrates not condensed. therefore they had to accrete from small amounts of silicates and metals. additionally, prominity to Sun meant a lot of material was swept out or accreted by Sun.

Question 25

what are the 5 steps of planetary accretion?

Answer 25

1. collisional growth (agglomeration)
2. gravitational instability
3. runaway accretion
4. oligarchic accretion
5. chaotic growth

Question 26

what happens during the collisional growth phase of planetary accretion?

Answer 26

agglomeration of micrometer dust to mm-sized pebbles due to low-velocity collisions and sticking of fluffy surfaces/electrostatic attraction

Question 27

what happens during the gravitational instability phase of planetary accretion?

Answer 27

pebbles (mm) -> planetesimals (>1km) is poorly understood but thought…
pebbles from stage 1 settled into a thin layer that was gravitationally unstable and concentrated into filaments and knots which collapsed further into planetesimal

Question 28

what happens during the runaway accretion phase of planetary accretion?

Answer 28

gravitational focusing - planetesimals are large enough to attract other bodies by mutual gravity (both attracting each other)
they accrete by collisional growth = runaway accretion forming moon sized bodies (>1000km diameter)

Question 29

what happens during the oligarchic accretion phase of planetary accretion?

Answer 29

there are now several hundred large bodies - oligarchs - which clear their local neighbourhood up of material producing ~100 moon to mars sized bodies

Question 30

what happens during the chaotic growth phase of planetary accretion?

Answer 30

oligarchs perturb orbits of other bodies ejecting some and others colliding with each other in highly energetic collisions. differentiated planetary embryos produce particularly energetic collisions. after 10-100Ma of chaotic growth the terrestrial planets settled into orbit.

Question 31

why is the gravitational instability phase (pebbles to planetesimals) poorly understood?

Answer 31

the bodies are:
1. too dense to stick following impact
2. too large for electrostatic attraction
3. too small for gravity to play a role