Week 2: Our place in the universe Flashcards
13.7 million years ago, the big bang produced a universe full of
Protons (hydrogen) and neutrons
High temperature reactions made other elements in the first 5 minutes
Helium, Lithium, Beryllium
Vast clouds of gas (H > He) collapsed due to gravitational attraction and formed
Spinning disks becoming galaxies (10 trillion)
What causes star formation
- Segregation and gravitational collapse of smaller gas clouds within galaxies creating swirling protostars
- Further collapse and heating initiate nuclear fusion and generate stars (100 billion in one galaxy)
Stars of the main sequence have
A wide range of temperatures and sizes and all undergo slow and steady nuclear fusion of H to He
Star nuclear fusion takes how long for a star
90% of its lifetime (10 billion years for the sun)
Ceclia Payne-Gaposchkin showed that the spectral class of stars depends on
Temperature rather than composition, and that stars are mostly hydrogen
When hydrogen becomes depleted in a stars core
- It contracts and temperature rises (up to 100 million degrees)
- Which enables fusion of helium which generates carbon nitrogen and oxygen
Death of stars occurs when
- Helium is depleted in the core and smaller stars expand into short lived “red giants”
- In larger stars, higher temperatures are achieved so elements of Iron can be made.
Material shed from a star during its red giant stage can form
A planetary nebula
Supergiant stars can produce
Elements heavier than iron by slow addition of neutrons
Eventually supergiants undergo
Cataclysmic explosions known as supernovae (occur 1-3 times per galaxy per century)
Cataclysmic explosions create
Neutron-rich elements such as gold and Uranium from rapid neutron addition
Nucleosynthesis
Process of making new elements by nuclear fusion (and neutron capture) in stars
Elemental abundances in our solar system can be deduced from
- The chemical composition of the sun
- Primitive meteorites called “carbonaceous chondrites”
These also indicate how many star cycles preceded the present solar system
Irrespective of their origin, the starting materials of our solar system collapsed to form
A spinning nebula of hot gas and dust
As the nebula cooled, elements were seperated from another according to
Their condensation temperatures
Refractories > metals > silicates > volatiles
Two steps to explain formation of two very different kinds of planets in our solar system
Condensation and accretion
Planet building: condensation
- Takes place in a radial temperature gradient
- Metals and silicates condense at high temperature and are distributed throughout nebula
- Volatiles condense only at low temperature and so collect mainly in the outer regions (which comprise most of the material) hence explaining the large size of the gas-giant planets
Planet building: accretion
- Gravitational attraction and physical collision leads to mixing and growth of materials, Dust grains -> planetesimals -> planets
- This can explain the varied compositions of particular planets
Remains of plantetoids
Asteroids/meteorites/comets
After accretion, inner rocky planets go through 4 stages of planetary development
- Cratering
- Internal differentiation
- Lava flooding
- Surface modification
Planet development: Cratering
- Planet surface bombarded with debris from the solar system (comets/asteroids/meteorites)
- Continuation of accretion
- Impact rate decreases rapidly through the first billion years of solar system history
Planet development: Internal differentiation
- Early planet interior is molten so minerals and melts separated according to density
- Heaviest iron-nickel metal sinks to make core
- Intermediate iron-magnesium silicates (olivine, pyroxene) crystallize to make mantle
- Low density sodium-calcium-aluminum silicates (feldspar) separate later to make crust
- Volatiles are liberated to make atmosphere
Chondrites in meteorites
- Most abundant
- Metal and volatile rich
- Oldest objects in solar system
Achondrites in meteorites
- Metal and volatile poor
- Younger than chondrites
- Silicates from the mantle of disrupted
- Some from moon and mars
Iron meteorites
Metal-rich samples of the core of a disrupted planetoid
Stony-irons (pallasites)
Core-mantle boundary from a disrupted planetoid
Planet development: lava flooding
- Planets interior partially melted by heat from radioactive decay
- Magmas rise through cracks in the crust and fill impact craters with basaltic lava
- Additional volatiles (h20, co2) escape from the planets interior during these eruptions
- Retention of gases depends on planet size, distance to sun and magnetic field
Planet development: surface modification
- Internal heat (radioactive decay) drives mantle convection and plate tectonics (magmatism and mountain building)
- External heat (sunlight) drives atmospheric and ocean circulation (weathering and erosion)
- Both processes act to shape planets surface
Earths second atmosphere:
- Primeval atmosphere is rich in CO2, N2, H2O, H2 and H2S
- During lava flooding, additional H2O and CO2 are released from the mantle
- Most CO2 dissolves in the ocean where it precipitates as carbonate, thus removing it from the atmosphere
- O2 a recent addition from green plants
Zircon story
- Zircon crystalises from a magma
- Its 18O/16O ratio is high so the magma must have been enriched in 18O
- 18O is enriched during chemical weathering of rocks by liquid water to form clay-rich mud
- So the magma had to be melted mudrock
- Which means there had to be liquid water present before 4400 ma
Distance from earth to the moon
384,400 km
Lunar (moon) features
- Abundant crators indicate no atmosphere or active tectonics
- Light colored highlands cover most of the surface - feldspar rich rock (anorthosite), oldest rocks (up to 4.5 billion years old)
- Dark regions are called maria - basaltic lavas that filled large impact basins, as young as 3 billion years
Origin of the moon
- Earths moon unique in the solar system with its large size relative to the planet
- Leads to three hypotheses for its origin (capture, planetary twin, giant impact)
Giant impact
- Early earth was hit by a large object potentially the size of mars and a large amount of earths mantle was jettisoned into space to make the moon.
- This would explain earths size and why its made of differentiated material (now the most widely accepted hypothesis)
Planets within a stars habitable zone recieve
An amount of solar energy appropriate for liquid water to be present (not too much and not too little). Whether or not a planet it does depends on additional factors
Mars is much different from earth in terms of
Size (smaller), mass (lighter) density (less dense) and distance from the sun (further from sun)
Mars surface features
- Largest volcano (Olympus Mons, 3x bigger than Mauna Kea)
- Largest canyon (Valles Marineris. 6km deep, stretches from Dunedin to Perth)
Mars atmosphere
- Mostly CO2 (similar in composition to venus, very different to earth)
- Very thin (weak gravitational field due to low mass means mars cannot hold onto light gases)
- Weak greenhouse effect as a result (average temperature -63 degrees)
- Polar ice caps contain abundant CO2
Massive land slips and dry river channels give evidence of
Past liquid H2O on mars
Where did mars water go
- UV radiation splits atmospheric H2O into H2 and O2
- Both of these easily escape due to weak gravitational field
- O2 also consumed by oxidization of iron on surface, hence Mars red color
- H2O frozen in the polar ice caps and beneath the surface
Mars probe landings
Viking 1 and 2 (1976), Pathfinder (1997), Mars exploration rovers (2004), Curiosity (2012), Perseverance (2021)
Venus is the most similar planet to Earth in terms of
Size, mass and density
Are there plate tectonics on Venus
- Mantle convection may deform crust and produce large surface bulges (coronae, some 2x higher than Everest)
- Low density of impact craters indicates planet-wide lava resurfacing within last billion years
- No trenches or arcs = no subduction zones
- Thus no plate tectonics despite many volcanos and mountains
Venus atmosphere traits
- Very thick - pressure at surface is 100x that of earth
- Almost entirely CO2 with smaller amounts of N2 and SO2
- A secondary atmosphere that has originated from volcanic eruptions
Venus greenhouse
- Co2 rich atmosphere creates greenhouse effect 300,000 time that of earth.
- Combined with its position closer to the sun, this results in Venus average surface temperature of 460 degrees.
- Both Earth and Venus had early atmospheres that were CO2 rich
Earths CO2 history: earth, liquid water was stable very early resulting in
- An ocean into which atmospheric CO2 dissolves
- Also promotes chemical weathering of crust which supplies dissolved Calcium to ocean
Earths CO2 history: in the ocean, dissolved Calcium and CO2 combine to precipitate
Solid carbonate (accumulates as limestone and is stored in the crust for long time scales)
Earths CO2 history: Plate tectonics return CO2 to the atmosphere through
- Metamorphism and volcanism associated with subduction of oceanic crust transfers Co2 from limestone back into the atmosphere
- Also prevents “runaway icehouse” as on mars e.g snowball earth 700 mya
Venus CO2 history: On venus, liquid may never have been stable due to
- High initial surface temperature causing all surface water to evaporate and gradually be lost to space
- The absence of an ocean into which CO2 could dissolve means no storage of carbonate in the crust
- So all CO2 is in the atmosphere where it has generated a “runaway greenhouse”
Source of excess CO2
Changes in atmospheric carbon is consistent with burning of fossil fuel carbon
C + O2 = CO2
