Comets
- small bodies in elliptical orbits
- ice and rock and dust particles
- May represent samples of early solar system
- Kuiper belt and Oort Cloud
Asteroids and meteors
- small bodies of variable composition
- rock, metallic iron
- sometimes strike earth (meteorites)
- asteroid belt, Trojans, Greeks
Origin of asteroids and meteors
- primordial planet remnants
- samples of early solar system material
- some meteorites are ejected from mars
Origins: Nebular Hypothesis
- Gravitational collapse of rotating nebula of gas and dust
- With collapse, gas becomes denser and hotter at centre, nuclear fusion begins and sun starts
- Condensation of material in rotating disk formed building blocks that through accretion formed planets, moons and other objects
- Distance from sun (temperature) dictated what condensed, and thus the different compositions of the inner (terrestrial) and outer (Jovian) planets
Five key factors critical in evolution of terrestrial planets:
- Melting, impacts, and differentiation
- as planet accretion reached maximum 4.56 billion years ago, terrestrial planets partly melted as collisions took place. Led to partial melting so that denser materials sank to center (differentiation)
- during and after partial melting, moon and terrestrial planets struck by numerous meteorites until about 4 billion years ago. Since then, terrestrial planets and the moon have behaved as independent closed systems
Five key factors critical in evolution of terrestrial planets:
- Volcanism
- interior of planets remained hot as a result of radioactive elements that produce heat as they decay: this is the driving force behind volcanic activity
- rate of cooling of planets depends on size. The largest planets (Earth and Venus) are still cooling whereas others have cooled down. There are indications of past volcanic activity on the moon and mars.
Five key factors critical in the evolution of terrestrial planets:
- Planetary mass
- mass determines orbit and number of moons the planet can capture
- also determines if planet has gravitational pull to keep atmosphere
Five key factors critical in the evolution of terrestrial planets:
- Distance from sun
-determines if water can exist as liquid water (critical factor in volcanism and surface composition)
Five key factors critical in the evolution of terrestrial planets:
- Biosphere
- presence or absence of a biosphere plays an essential role in the development of the biogeochemical cycles that control that composition of Earth’s atmosphere
- photosynthetic organisms convert carbon dioxide and liquid water into oxygen gas and organic matter
Age of solar system
4.56 billion years
How do we know the age of the solar system
Via radiometric dating of primitive meteorites
Radioactive decay occurs at fixed rates, providing an internal clock. We measure amounts of parent and daughter atoms
REVIEW GRAPHS
Radiometric dating gives
Absolute or numerical ages
Relative ages
Used to use before the discovery of radiometric dating and development of instruments to measure numbers (or ratios) of atoms very precisely
Shows up in rock records
Geologic time in perspective
Review this slide
Precambrian/Cambrian boundary
- 542 MA
- originally denoted where fossil-bearing sedimentary deposits started
- there are Precambrian sediments
- we now know of numerous Precambrian fossils (e.g. stromatolites 3.5 GA)
Is there a location where the entire rock record is in place
- no
- for e.g. there are Precambrian rocks covered by Pleistocene (0.13-1.80 MA) in northern Alberta
- these gaps are caused by events such as mountain building, sea level changes, weathering
Structure of earth is determined by 3 major compositional layers:
Core: metallic, iron rich, solid inner core. Liquid outer core (solid and liquid are same composition)
Mantle: dense rocky matter
Crust: thin, less dense rocky matter (because of differentiation)
Two contrasting types of Earth’s crust:
- Continental: thickness averages 45 km but varies 30-70 km
- Oceanic: 8 km thick
Two types have distinctly different compositions (reflecting different origins)
Layers of different strength: lithosphere
Crust and upper most mantle
Cool, rigid, brittle, breaks
Known by samples and geophysical measurement
Asthenosphere
Upper mantle
Hot, weak, plastic, flows
Known by geophysical measurements and experiments (limited samples)
Mesosphere
Deeper mantle
Hot, but stronger than atmosphere. Flows
How do we know composition of core
Geophysical measurements, experiments, meteorites
All three layers (lithosphere, asthenosphere and mesosphere) are
Solid
Internal structure and composition of Earth
Draw out and review diagram in notes
