Lecture 2: Ocean Origins
Reading: Garrison Ch. 1
Learning Goals of Lecture 2
•Age of the ocean — how do we know?
•How did Earth get its water (and ocean)?
•Why does Earth have a liquid ocean but not the other
planets in our solar system?
•What did the earliest ocean and atmosphere look like
on Earth?
C
• The Universe is about 13.7 billion years old … that’s 13,700,000,000 years or 13.7
thousand million!
The Universe = all matter and energy that physically exists!
• The Universe is about 13.7 billion years old … that’s 13,700,000,000 years or 13.7
thousand million!
• The beginning of the Universe occurred in an event called the BIG BANG, which
was the start of space and time as a point source of mass and energy followed by
a massive explosion
The Universe = all matter and energy that physically exists!
• The Universe is 13.7 billion years old … that’s 13,700,000,000 years or 13.7
thousand million!
• The beginning of the Universe occurred in an event called the BIG BANG, which
was the start of space and time as a point source of mass and energy followed by
a massive explosion
• This explosion released sub-atomic particles and energy, then (after cooling) the
element hydrogen (H) formed
Age of the Universe: How do we know?
• Radioactive Decay
• Radioactive decay acts like a clock. Why?
6
Radioactive Decay
• Isotopes are forms of a chemical element with the same number of
protons but different numbers of neutrons
esrl.noaa.gov
7
Radioactive Decay
• Isotopes are forms of a chemical element with the same number of
protons but different numbers of neutrons
• Some isotopes are unstable and lose energy by emitting radiation 🡪
this is called radioactive decay
Parent isotope
14
C
🡪
14
N +
Daughter
isotope
-
e +
νe
stable
unstable
Energy
Note: Radiocarbon half life is just 5730 years!
esrl.noaa.gov
8
Radioactive Decay
• Radioactive isotopes decay at a particular rate
Radioactive Decay
• Radioactive isotopes decay at a particular rate
• We need to know the isotope’s half life or the time it takes for one
half of the radioactive parent isotopes to decay to the daughter
isotopes
ck12.org
kgs.ku.edu
Radioactive Decay
• Radioactive isotopes decay at a particular rate
• We need to know the isotope’s half life or the time it takes for one
half of the radioactive parent isotopes to decay to the daughter
isotopes
• We also need to know the amount of parent and daughter isotopes in
the material we want to date
ck12.org
kgs.ku.edu
Radioactive Decay
• Methods similar to measuring radioactive isotopes and their decay
products in rocks can be applied outside the solar system using
spectroscopy
• Absorption spectra of stars tells us about their composition
Missing light at certain
wavelengths that get absorbed
depending on the chemical makeup
of the star
astronomy.nmsu.edu
Radioactive Decay
• Astronomers can measure the concentration of rare radioactive
isotopes in stars by absorption spectroscopy
• Knowing the initial concentration of the radioactive isotope is difficult
and must be indirectly estimated based on our understanding of the
amount of these isotopes produced during the formation of the
Universe
Amount of this element
(thorium) is less than
theory predicts should
have been present at the
time of formation 🡪
radioactive decay
Radioactive Decay
• Astronomers can measure the concentration of rare radioactive
isotopes in stars by absorption spectroscopy
• Knowing the initial concentration of the radioactive isotope is difficult
and must be indirectly estimated based on our understanding of the
amount of these isotopes produced during the formation of the
Universe
Some useful radioactive
isotopes and their half-lives
Uranium-238 4,468,000,000
Rhenium-187 43,500,000,000
Thorium-232 14,100,000,000
Galaxies
• Stars and planets are contained within galaxies
• A galaxy is a huge rotating aggregation of stars, dust, gas, and other debris
• Our galaxy (spiral-shaped) is called the Milky Way
Galaxies
• Stars and planets are contained within galaxies
• A galaxy is a huge rotating aggregation of stars, dust, gas, and other debris
• Our galaxy (spiral-shaped) is called the Milky Way
• Stars are massive spheres of glowing hot gases
• Stars convert hydrogen (H) to helium (He) to heavier elements by nuclear
fusion as they age through a processes called nucleosynthesis
Illustration of the Milky Way
Our solar system is over halfway out from the center in the Orion Arm
Our solar system orbits the
core of the galaxy every
230 million years
100 billion galaxies in the universe!
100 billion stars in each galaxy!
chandra.harvard.edu
Solar System Formation
The Solar System
• Our solar system (Sun and planets) was formed of material made in
older massive stars which accreted (stuck together) and grew larger
• Our solar system is 4.6 billion years old
• Condensation Theory states that our solar system condensed from a cloud of
gas and dust (molecular cloud or solar nebulae) including remnants of older
exploded massive stars (supernovae)
• Planets in our solar system orbit continuously around the Sun (our star)
Gas + Dust =
Nebulae
Makeup of Gas and Dust Cloud
• GASES –
(~98.6%)
• Hydrogen (H), Helium (He)
• ICES –
(~1.1%)
• Water (H2O), Methane (CH4), Ammonia (NH3)
• ROCKS –
(~0.22%)
• Silicates (like SiO42-), Oxides
• METALS –
(~0.08%)
• Iron (Fe), Nickel (Ni)
Protostar Formation
COMPRESSION
HEATING
RAPID SPIN
Figure 1.7
Disks and Rings of Solids Orbiting Around Young Stars
New planets forming?
Images from Kitt Peak
Observatory and
Spitzer Space Telescope
document the outburst
of HOPS 383, a young
protostar in the Orion
Star Formation
Complex, March 2015
Nasa.gov
Birth of a Star
• Nebulae: Clouds of H, He, and dust
• Gravitational collapse leads to compression, heating, and rotation
• Leads to a protostar
Birth of a Star
• Nebulae: Clouds of H, He, and dust
• Gravitational collapse leads to compression, heating, and rotation
• Leads to a protostar
• Small particles in the nebulae clump into larger masses (accretion)
and planets begin to form
Birth of a Star
• Nebulae: Clouds of H, He, and dust
• Gravitational collapse leads to compression, heating, and rotation
• Leads to a protostar
• Small particles in the nebulae clump into larger masses (accretion)
and planets begin to form
• Ignition of nuclear fusion: H 🡪 He generates a stellar wind that clears
away the remaining gases but not solids
BIG BANG
(Star Phases)
MAIN
SEQUENCE
SUPERGIANT STARS
SUPERNOVA
NUCLEOSYNTHESIS
NUCLEOSYNTHESIS
Atoms in our bodies were formed in supernova (massive
star explosions) billions of years ago!
“The nitrogen in our DNA, the calcium in our teeth, the
iron in our blood, the carbon in our apple pies were made
in the interiors of collapsing stars. We are made of
starstuff.”
- Carl Sagan, astronomer
Temperature Gradient in the Spinning Disk
Gas giants
Rocky inner planets
Uranus
Mercury
Jupiter
Neptune
Saturn
Sun
Pluto
(demotednot a planet)
The Young Earth
Mutual gravitational
attraction of particles
causes planet to grow
Gravitational attraction
causes pressure inside,
melting rock
Light matter rises,
Dense matter sinks
Core, mantle and crust
form 🡪Density Stratification
Figure 1.10
Dramatic Early History
IMPACTS add energy (Heat)
A massive early impact with Theia
formed the Moon (~4.4 Ga)
Ga = billion years
Giant Impact Hypothesis
Figure 1.11
Where did the (surface) water come from?
• Option 1: Outgassing of volatiles from Earth’s interior
• Volatiles are compounds with low boiling points (H2O, O2, other gases)
Where did the water come from?
• Option 2: Icy comets hitting Earth bring water
The answer? probably a combination of both sources
Our Earliest Atmosphere
• Primary Atmosphere
• H and He from pre-solar nebula
• Stripped away by radiation (solar wind) and lost to space
H2 He
2 4
CH4 NH3 H2O
16 17 18
Molecular mass
N2 CO2
28 44
Our Earliest Atmosphere
• Primary Atmosphere
• H and He from pre-solar nebula
• Stripped away by radiation (solar wind) and lost to space
• Photodissociation by UV light
H2 He
2 4
2H2O
CH4
2NH3
2H2
2H2
3H2
CH4 NH3 H2O
16 17 18
+ O2
+ C
+ N2
N2 CO2
28 44
Our Early Atmosphere
• Secondary Atmosphere
Know these
compounds!
• Accumulated after the nebula dispersed
• Gas from comet ice — H2O, CO2, CO, CH4, NH3
• Erupting volcanoes — H2O, CO2, N2, H2S, SO2
• Heavy molecules held by gravitational attraction to the planet
H2O = water,
CO2 = carbon dioxide,
CH4 = methane,
NH3 = ammonia,
SO2 = sulfur dioxide
What controls the Earth’s temperature?
• Inverse square law: heat depends on distance (1/Distance2) from the
Sun
• Heat flux to Earth is 1368 Watts per m2 (solar constant)
Mercury: 0.4 AU
Earth :1 AU
Venus: 0.72 AU
1 / (0.4)2 = 6.25
1 / (0.72)2 = 1.93
Modern atmospheric composition
High P (90 atm)
P = 1 atm
low P (0.007 atm)
What happened to Venus?
• Rocky planet 95% Earth’s size
• Thick, cloudy atmosphere (90 atm)
• Extremely high, invariant temperature of 462°C
Russian Venera Lander Photos 1970s
Probes survived just 2 hours
May once have had an
ocean…
over 700 million years ago
What happened to Mars?
• Rocky planet 53% Earth’s size (and 1/10th the mass)
• 0.006 atm pressure
• Surface temperature on Mars is -55 °C (much colder than Earth)
• Similar atmospheric components as Venus (CO2 and N2)
What happened to Mars?
• Atmosphere of secondary origin (outgassing and comets)
• Mars gravity too weak to hold onto these gases
Polar ice caps of CO2 (dry
ice) and water
nasa.gov
Water on Mars
Features caused by running water
Back to Earth
Surface temperature on Earth
is average of +14°C
Earth’s surface eventually
cooled enough that rains
fell due to gravitational
attraction, which led to
the formation of the
oceans
Water on Earth
• Ocean formed early in Earth’s history (about 4 billion years ago)
• The same types of sediments as found at the bottom of oceans today
have been dated to 3.8 Ga (so the oceans must have been there to
deposit them)
Isua Formation, Greenland
• Uniformitarianism: the idea that the
physical and chemical processes that
operate on Earth today behaved the
same way in the past
Water on Earth
• CO2 from the atmosphere dissolved into the oceans and was
ultimately removed as carbonate sediments (limestone) in Earth’s
crust
• The oceans are therefore responsible for converting a potentially
massive CO2 atmosphere like on Venus into rock
• Our present atmosphere is primarily N2 and O2 (accumulated later
due to biology)
• CO2 is a trace gas (less than 0.1%)
Evolution of Earth’s Atmosphere
This is because
of LIFE