In the beginning…

The composition of the solar system and earth

Interior of the Genesis sample collection module

The Genesis sample collection module after ‘landing’

Picking through the pieces

Features that demand an explanation:• H and He are by far most abundant elements• Li, Be and B are anomalously low in abundance• Overall ~ exponential drop in abundance with increasing Z• Even Z > odd Z• Fe and neighbors are anomalously abundant

“Hydrogen as food’ hypothesis: Burbidge et al., 1957(built on ideas of Gamow re. nucleosynthesis in big bang)

I. H burning

H + H = D + + + +

positron neutrino

photonsD +H = 3He + …

3He + 3He = 4He + 2H + …

3He + 4He = 7Be + … (and similar reactions to make Li and B)

Products quickly decay:7Be + e- = 7Li

7Li + P = 8Be8Be = 2.4He

Timescale ~ 10-16 s { Stuck; no way to elements heavier than B

(rxn. discovered by H. Bethe, 1939)

Willie Fowler, Salpeter and Hoyle

“Would you not say to yourself, 'Some super- calculating intellect must have designed the properties of the carbon atom, otherwise the chance of my finding such an atom through the blind forces of nature would be utterly minuscule.' Of course you would . . .. A common sense interpretation of the facts suggests that a superintellect has monkeyed with physics, as well as with chemistry and biology, and that there are no blind forces worth speaking about in nature. The numbers one calculates from the facts seem to me so overwhelming as to put this conclusion almost beyond question.”

F. Hoyle

Show the solution is the following reaction in red giant stars:

4He + 4He + 4He = 12C

Opens possibility of many similar reactions:

12C + 4He = 16O16O + 4He = 20Ne

20Ne + 4He = 24Mg

Collectively referred to as ‘He burning’

“We do not argue with the critic who urges that stars are not hot enough for this process; we tell him to go and find a hotter place.”A. Eddington

Advanced burning:

origin of the 2nd quartile of the mass range

12C + 12C = 23Na + H

16O + 16O = 28Si + 4He

CNO cycle

12C + P = 13N = 13C13C + P = 14N14N + P = 15O = 15N15N + P = 12C + 4He

The E process (for ‘Equilibrium’): why the cores of planets are Fe-richA quasi-equilibrium between proton+neutron addition + photo-degradation

Promotes nuclei with high binding energy per nucleon

Neutron capture as a mode of synthesizing heavy elements

Occurs in environments rich in high-energy neutrons, such as super-novae

Features that demand an explanation:• H and He are by far most abundant elements

H primordial; He consequence of 1˚ generation H burning

• Li, Be and B are anomalously low in abundanceConsumed in He burning

• Overall ~ exponential drop in abundance with increasing ZDrop in bonding energy per nucleon w/ increasing Z

• Even Z > odd ZMemory of He burning

• Fe and neighbors are anomalously abundantMaximum in bonding energy per nucleon at Fe

These factors are directly responsible for the fact that terrestrial planets are made of silicates and oxides (‘rocks’) with magnetic Fe cores.

N

Primitive meteorites look a lot like the sun (minus the gas and all the hotness)

II. Accretion of the Earth

(and inheritance of interstellar dust)

letters indicate compositional fields of various

types of primitive meteorites

Earth is somewhere near here

But primitive meteorites are diverse; how are we to know whichis most like the earth?

Much of the diversity in meteorite composition reflects variations in oxidation state of solar nebula (H2O/CO ratio)

Infer the earth based on ‘geochemical’ vs. ‘cosmochemical’diversity in composition

Broad groupings of elements in geochemical processes

The earth’s mantle is mostly chondritic, but depleted in moderately volatile elements (K, Na)

Silicate earthCI chondrites

Are they simply missing, or hiding somewhere in the earth? We’ll revisit this question in two lectures

1

The earth’s mantle is also depleted in siderophile elements (Ni, Cu, Au)

Silicate earthCI chondrites

0.1

Are they simply missing, or hiding somewhere in the earth? We’ll revisit this question next lecture

Entry speed estimated bythermally stepped He release method

>18 km/s probable comet origin

<14 km/s probable asteroid origin

Stratospheric IDPs

2µm- 25µm et particlesCollected by NASA

U2/ER2/WB-57F aircraft

1 ng IDP >105 components Contents: anhydrous silicate minerals

amorphous silicate (glass)Fe, Ni sulfidesoxidesFe, Ni metal grainsorganic materials

1 µm

bulk composition ~ solarbulk composition ~ solar

“Kuiper Belt” around HR 4796A

Samples from the Kuiper Belt(Neptune and beyond)

The ‘Stardust’ mission

Wild 2 encounter1/2/2004

direction of interstellar flux

Launch2/7/99

Earth gravity

Assist1/15/2001

Earth return

1/15/2006

Wild 2 orbit

Loop #1

Loops #2 & 3

Interstellar dustCollection periods

Stardust’s Wild Ride - 3 loops around the Sun

January 15, 2006

A piece of ‘star dust’