ASTR Spring 2008 Joel E. Tohline, Alumni Professor 247 Nicholson Hall [Slides from Lecture25]

Chapter 8: Principal Topics How old is the Solar System? Nebular Hypothesis + Planetesimals + Core Accretion: A model that explains how the solar system acquired its key structural properties. –Directions and orientations of planetary orbits –Relative locations of terrestrial and Jovian planets –Size and compositions of planets Observational evidence for extrasolar planets

How old is the Solar System? Radioactive dating First, let’s discuss the idea of radioactive isotopes of atomic elements

How old is the Solar System? Radioactive dating First, let’s discuss the idea of (radioactive & non-radioactive) isotopes of atomic elements

Chemical Elements & Their Isotopes Courtesy of:

Chemical Elements & Their Isotopes Hydrogen

Chemical Elements & Their Isotopes Hydrogen

Chemical Elements & Their Isotopes Helium

Chemical Elements & Their Isotopes Helium

Chemical Elements & Their Isotopes Carbon

Chemical Elements & Their Isotopes Carbon

Chart of Nuclides

C 14

Chart of Nuclides C = 14

How old is the Solar System? Radioactive dating First, let’s discuss the idea of (radioactive & non- radioactive) isotopes of atomic elements Now let’s discuss radioactivity and the concept of “half-life” –“The half-life of an isotope is the time interval in which one-half of the nuclei decay.” [See Box 8-1 in the textbook.] –

Radioactive Decay t 1/2 = half-life Courtesy of:

Color Indicates Approximate “Half-Life”

Some more precise “Half-Life” values

Decay of 14 C to 14 N 14 C 14 N t 1/2 = 5730 years

Some more precise “Half-Life” values

Decay of 87 Rb to 87 Sr t 1/2 = 47 billion years

Some more precise “Half-Life” values

Decay of 238 U to 206 Pb t 1/2 = 4.5 billion years 238 U 206 Pb

Radioactive Decay As the abundance of the radioactive isotope (for example, 14 C) decreases steadily over time, the abundance of the final stable isotope (for example, 14 N) steadily increases.

14 C Dating Suppose an archeologist digs up a primitive weapon made partly of wood and determines that the wood contains an isotopic abundance ratio 14 N/ 14 C = 3. How old is the weapon if we assume that, originally, the wood contained no 14 N ?

14 C Dating 14 N 14 C

14 C Dating 14 N 14 C At what time does the abundance ratio 14 N/ 14 C = 3 ?

14 C Dating 14 N 14 C At what time does the abundance ratio 14 N/ 14 C = 3 ?

14 C Dating 14 N 14 C At what time does the abundance ratio 14 N/ 14 C = 3 ? Age of wood = 2 t 1/2 = 2 x (5730 yrs) = 11,460 yrs

14 C Dating Suppose an archeologist digs up a primitive weapon made partly of wood and determines that the wood contains an isotopic abundance ratio 14 N/ 14 C = 3. How old is the weapon if we assume that, originally, the wood contained no 14 N ? ANSWER: 11,460 years In practice, the ‘dating’ technique is messier than this, but this should give you a general idea of how the radioactive dating technique works.

14 C Dating Suppose an archeologist digs up a primitive weapon made partly of wood and determines that the wood contains an isotopic abundance ratio 14 N/ 14 C = 3. How old is the weapon if we assume that, originally, the wood contained no 14 N ? ANSWER: 11,460 years In practice, the ‘dating’ technique is messier than this, but this should give you a general idea of how the radioactive dating technique works.

238 U Dating Suppose an astronomer discovers a meteorite and determines that the meteorite contains an isotopic abundance ratio 206 Pb/ 238 U = 1. How old is the meteorite if we assume that, when it originally formed, the meteorite contained no 206 Pb ?

Atomic Bombs Rely on spontaneous fission (radioactive decay) of heavy nuclei, such as Uranium and Plutonium A ‘critical mass’ of fissionable material will explode because the decay products from spontaneous fission strike nearby nuclei and induce those nuclei to fission  runaway chain reaction First atomic bombs were constructed during World War II in the so-called ‘Manhattan Project’ centered at Los Alamos, NM