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 photons D +H = 3 He + … 3 He + 3 He = 4 He + 2H + … 3 He + 4 He = 7 Be + … (and similar reactions to make Li and B) Products quickly decay: 7 Be + e - = 7 Li 7 Li + P = 8 Be 8 Be = 2.4 He Timescale ~ 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: 4 He + 4 He + 4 He = 12 C Opens possibility of many similar reactions: 12 C + 4 He = 16 O 16 O + 4 He = 20 Ne 20 Ne + 4 He = 24 Mg 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 12 C + 12 C = 23 Na + H 16 O + 16 O = 28 Si + 4 He CNO cycle 12 C + P = 13 N = 13 C 13 C + P = 14 N 14 N + P = 15 O = 15 N 15 N + P = 12 C + 4 He

The E process (for ‘Equilibrium’): why the cores of planets are Fe-rich A 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 abundance Consumed in He burning Overall ~ exponential drop in abundance with increasing Z Drop in bonding energy per nucleon w/ increasing Z Even Z > odd Z Memory of He burning Fe and neighbors are anomalously abundant Maximum 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 which is most like the earth?

Much of the diversity in meteorite composition reflects variations in oxidation state of solar nebula (H 2 O/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 earth CI 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 earth CI chondrites 0.1 Are they simply missing, or hiding somewhere in the earth? We’ll revisit this question next lecture

Entry speed estimated by thermally stepped He release method >18 km/s probable comet origin <14 km/s probable asteroid origin Stratospheric IDPs 2µm- 25µm et particles Collected by NASA U2/ER2/WB-57F aircraft

1 ng IDP >10 5 components Contents: anhydrous silicate minerals amorphous silicate (glass) Fe, Ni sulfides oxides Fe, Ni metal grains organic materials 1 µm bulk composition ~ solar

“Kuiper Belt” around HR 4796A Samples from the Kuiper Belt (Neptune and beyond) The ‘Stardust’ mission

Wild 2 encounter 1/2/2004 direction of interstellar flux Launch 2/7/99 Earth gravity Assist 1/15/2001 Earth return 1/15/2006 Wild 2 orbit Loop #1 Loops #2 & 3 Interstellar dust Collection periods Stardust’s Wild Ride - 3 loops around the Sun

January 15, 2006

A piece of ‘star dust’