Interior Structures of Planets

Transcription

1 Interior Structures of Planets Thematic Questions about Planetary Interiors Planetary interiors tend to be layered structures How does differentiation of planetary materials to form a layered structure occur? Is the process continuous? What triggers this development? Evidence for core, mantle, and crust How can the interior of a planet be assessed? Characteristics of Earth s Interior How does it compare with other planets? What was the sequence of its formation? Interior of the Earth Layered Core, mantle, crust Lithosphere, asthenosphere 1

2 Probing Earth s Interior Evidence of Interior Structure Seismic Waves: P and S waves Primary pass through liquids Secondary cannot travel in liquids P: compressional, fast S: shear, slow Seismic Refraction differences in density creates shadows in the occurrence of seismic waves crust mantle core refraction of seismic waves Composition and Density of Earth Layers Crust silicates Mantle silicates phase Core Fe, Ni outer: liquid inner: solid 2

3 Differentiation of Earth s Interior Processes and Planetary Comparisons Differentiated into core, mantle and crust from either unsorted or layered protoplanet Resultant structure: Mercury dependent on composition Venus Earth Moon core mantle Mars unsorted mantle carbonaceous chondrite core dust cloud layered primitive crust residual mantle Fe,Ni Earth: History of Melting Requirements for Core Formation Initial interior temperature insufficient Heated by radioactive decay until melting begins at shallow depth Collapse or big burp with volatile release collapse Fe volatiles Fe sinking 3

4 Radioactive Clocks and Time Methods of Age Determination Absolute Ages measured by radioactive decay assesses time elapsed since formation direct evidence of date of birth Relative Ages assessment of age by reference to other dated materials, or within framework evidence of comparative age (younger, older) evidence of probable age range e.g. within geological eon, era, period, epoch Geological Time Scale Names, Divisions Eons, Eras Periods, Epochs Basis: observed differences or major changes in biota, rock types boundaries calibrated by absolute ages 4

5 Radioactive Decay Disintegration Process and Rates Parent isotopes decay into daughters Rate of decay: proportional to total number of radioactive atoms Decay constant: defines rate of decay differs for different isotopes (slope of curve) Half-life: time for 1 / 2 of original material to decay Length of time unit varies Radioactive Decay Comparable initial amounts Illustration Different rates of decay with increasing half-lives Amount Increasing 1/2 life Time 5

6 Rates of Radioactive Decay Disintegration Rates and Half-Lives Function of nuclear stability decay rates and constants vary greatly microseconds to billions of years many isotopes have geologically useful half-lives Decay Half- Decay Half- Parent Daughter constant life Parent Daughter constant life 14 C 14 N 1.21x a 1 Re 1 Os 1.52x Ga Rb Sr 1.42x Ga 230 Th 226 Ra 9.22x ka 40 K 40 Ca 4.96x Ga 232 Th 208 Pb 4.95x Ga 40 K 40 Ar 5.81x Ma 234 U 230 Th 2.79x ka 138 La 138 Ce 6.54x Ga 235 U 207 Pb 9.85x Ma 147 Sm 143 Nd 6.42x Ga 238 U 206 Pb 1.55x Ga 176 Lu 176 Hf 1.96x Ga Approaches to Age Dating Half-Lives and Isochrons Decay is defined 0 Variations in original abundances define isochrons Daughter isotope (% of atoms) 50 4 half-lives 3 decrease in parent 2 isochrons 1 increase in daughter 100 time = Parent isotope (% of atoms) 6

7 Radioactive Dating using Rb/Sr Rubidium/Strontium Isochrons Abundance of parent ( Rb) and daughter ( Sr) considered relative to stable isotope 86 Sr as ratios. Different ratios of Sr/ 86 Sr permit determination of age from slope of isochron. Sr/ 86 Sr Initial Sr/ 86 Sr ratio M 1 M 2 M 1 M 2 Isochron time today decrease M 4 in Rb M 3 M 3 Isochron at time of formation Increase in Sr M 4 t=0 Rb/ 86 Sr Examples of Rb/Sr Dating Granite and Lunar Basalt Isochrons give age Isochron = 3.30 ± 0.08 Ga ilmenite Sr/ 86 Sr Isochron = 1725Ma granite from Sudbury, Ont Rb/ 86 Sr Sr/ 86 Sr olivine pyroxene total rock (2) plagioclase t=0 Lunar rock Rb/ 86 Sr 7

8 Examples of Rb/Sr Dating Tieschitz 4.52 ± 0.03 Ga Sr/ 86 Sr Meteorite Age of Solar System Rb/ 86 Sr 8