The nebular hypothesis

The solar nebula (gas) contracted, cooled and condensed into dust sized particles that accreted (stuck together as the result of collisions) into protoplanets (asteroid sized bodies) and then larger planets

http://meteorites.asu.edu/

Evidence that chondrites are representative of early solar system materials:

• 1:1 ratio of non-volatile elements to those in the sun• “sedimentary”, non-equilibrated texture - formed by

accretion of condensed particles with little subsequent heating or alteration

• presence of minerals predicted to be formed by condensation of solar nebula, such as corundum, Fe metal, troilite, etc.

Also inferred: achondrites, iron and stony-meteorites represent fragments of accreted planetesimals that had heated internally, melted and differentiated

The nebular hypothesis

The solar nebula (gas) contracted, cooled and condensed into dust sized particles that accreted (stuck together as the result of collisions) into protoplanets (asteroid sized bodies) and then larger planets

http://meteorites.asu.edu/

Earth (also, Mercury, Venus, Mars) is layered (core, mantle, crust). How did it form, and what was its original bulk composition?

Hypotheses for planet formation:

1) Heterogeneous accretion (layers form in condensation sequence from refractory to volatile. Since iron condenses early – did the core form this way?)

2) Homogeneous accretion - whole planet accreted from the same mixture of condensing materials, and then differentiated by internal melting

3) “Chondrite mixing” - accretion of the constituents of chondrites in varying proportions depending on time and distance from the sun, followed by interior melting

Sample chondrite mixing model for Earth and Moon:

Chondrite mixing model: Use each element that can be estimated to determine the proportion of a particular chondrite consitituent:

1) Refractory elements in Calcium Aluminum Inclusions - U2) Silicates - Si3) Metal - Fe4) Troilite (sulfide) - S5) Volatile elements – K, Tl

Then add together all the other elements in those constituents:

1) Refractory elements: U, also Ca, Al, Ba, Sr, Sc, Ti, Pt, Re, Os……..2) a) Elements in refractory silicates: Si, Mg…..

b) Elements in late condensing silicates: Na, K, Rb, Cs……..3) Elements in metal phase: Fe, also Ni, Cr, Co, Au, P, As……..4) Elements in troilite: Fe and S, also Zn, Cu, Ga, Se, Te……….5) Highly volatile elements: Tl, Bi, Pb, Hg, Cl, I…………

What do we know about the bulk composition of the Earth (and other planets and moons). What can we use to constrain the proportions of chondrite constituents?

1) Amount of Fe can be estimated from size of core (if detectible), mass and density of planet/moon

2) Amount of U can be estimated from surface heat flow measurements and alpha particle emission (for planets and moons), and, on Earth, by assuming most U is in the crust (which can be analyzed directly)

3) On Earth, amount of K can be estimated from amount of 40Ar in the atmosphere, for Earth and differentiated planets and moons, also that most K is in the crust (which can be analyzed directly).

http://earthref.org/GERM/

Estimate the composition of the Earth’s Core:

Fe-Ni based on Earth’s density and observations of iron meteorites

One or more light elements needed to account for lower than expected density inferred from seismic wave velocities – S, O most likely

Trace elements with “siderophile” behavior (partition into molten metal) added to core composition in accord with their “preference” for metallic versus silicate melts (and subtracted from silicate earth)

(worked problem 12.1 in McSween et al.)

Result is: “silicate earth” and “primitive mantle” – the composition of the silicate Earth following internal melting and core formation

For Tuesday:

The silicate earth (crust and mantle together) contains about 45% SiO2 by weight (gram SiO2/gram rock X 100). Calculate the concentration of K2O in weight percent and U, Sr and Tl in parts per million (micrograms element/gram rock) in the silicate earth assuming abundances relative to Si are the same as the solar (or “cosmic”) abundances (use Table 2.1 in Faure).