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First sample return mission in 34 years Traveled ~ 3 billion miles Retrieved samples of comet Wild-2 Flew through the comets tail Collected dust particles in aerogel Stardust

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Geologic Time Image source:

Fundamental geologic observations Relative vs. absolute dating Economic implications Philosophical dimensions Earth’s place in the Solar System Meteorites Geologic Time

Nicolaus Steno (1669) –Observations regarding sedimentary rocks Principle of Superposition Principle of Original Horizontality Principle of cross-cutting relationships –Developed for dating igneous rocks Law of Faunal succession (~1800) –Allows correlation of rocks of same age Relative dating techniques

Principle of Original Horizontality (Grand Canyon, USA) Source: © The McGraw-Hill Companies, Inc./Doug Sherman, photographer. Principle of Superposition (Grand Canyon, USA)

Schist and Granite Source:Courtesy of Carla W. Montgomery.

Still many unanswered questions… –Absolute age, length of geologic processes “The Present is the Key to the Past” James Hutton, late 18 th century Uniformitarianism

Source: © The McGraw-Hill Companies, Inc./Doug Sherman, photographer.

Age of the Earth

How old is the Earth? Bishop Usher (16 th century) Bible…..~6,000 years old (4004 BC) Lord Kelvin (19 th century) Cooling…20-40 million years old John Joly (1899) Salinity of oceans…100 million Age of the Earth

Radioactivity

Radiometric Dating Radioactive isotopes in rocks decay at a known rate U, Rb, K, Th, C, others…. Counting atoms in the rocks tells time Parent-daughter ratio Concept of half-life How long it takes for half the parent isotope to decay Parent-daughter ratios become smaller in time Isotope clocks are useful for ~ 7 ½ lives Age of the Earth

Carbon 14 N  14 C in the atmosphere, then we eat the carbon 14 C isotopic half-life = ~6,000 years U-Pb 238 U  206 Pb, and 235 U  207 Pb 238 U isotopic half-life = ~4,500,000,000 years 4.5 billion years old (4.5 byo) Other ‘old’ isotopic clocks Rb  Sr, K  Ar, Th  Pb Absolute dating: isotopic clocks

Meteorites ~ 4.57 byo Earth ~ 4.56 byo Oldest rocks on Earth ~ 4.0 byo Oldest minerals ~ 4.4 byo Oldest sediments ~ 3.8 byo Oldest fossils ~ 3.5 byo Billions, not millions! A very old Earth…

50 um The Oldest Minerals on Earth: Zircon (ZrSiO 4 )

Subdivisions of geologic time –Eon, Era, Period, Epoch –Eons Precambrian: 4.5 b.y. to ~0.5 b.y. Phanerozoic: ~0.5 b.y. to today Geologic time scale

Subdivisions of geologic time –Eon, Era, Period, Epoch –Eras Paleozoic: ~560 m.y. to ~250 m.y. Mesozoic: ~250 m.y. to ~65 m.y. Cenozoic: ~65 m.y. to today Geologic time scale

Subdivisions of geologic time –Eon, Era, Period, Epoch –Today (your lifetime) Eon: Phanerozoic (~550 m.y.  now Era: Cenozoic (65 m.y.  now Period: Quaternary (2 m.y.  now Epoch: Holocene (10 k.y  now Geologic time scale

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