Astronomy190 - Topics in Astronomy
Astronomy and Astrobiology
Lecture 5 : Geology
Questions of the Day
• What is the technique of radiometric dating and why
is it useful?
• What are some major changes that have happened
to Earth since the Hadean Eon?
• How does a planet’s magnetic field affect its
Geology and Astrobiology
• Geology encompasses the study of worlds with
• Geology is important to habitability and life via
three important planetary processes:
– Volcanism, Plate Tectonics, Planet Magnetic Fields
• Additionally, Earth geology has taught us how to
understand and interpret the record of the
Earth’s environment over the past 4.6 billion
• Geologists classify rocks into three different
types, according to how they are made.
– Igneous Rocks: molten rock that cools and solidifies
(e.g. granite, basalt)
– Sedimentary Rock: made from the compression of
sediments, like those found in lakes, rivers and
oceans. (e.g. sandstone, shale)
– Metamorphic Rock: Igneous or Sedimentary rock that
is transformed under high heat and/or pressure. (e.g.
• Rocks are composed of individual crystals or
grains called minerals, which are uniquely
identified by their composition and structure.
• 3000 distinct mineral types are known and are
often grouped by their primary constituents
• Silicates, like quartz and feldspar, contain large amounts of
silicon (Si) and oxygen (O)
• Carbonates, like limestone, contain large amounts of carbon
(C and oxygen (O)
• Rocks can be further classified
– e.g. by grain or crystal size
• Basalt, small crystals (fast cooling)
• Granite, large crystals (slow cooling)
Granite is made of quartz (grey), feldspar (white) and mica (black).
We can find fossils in…
Sedimentary and metamorphic rocks
Forming Sedimentary Rocks
• Sedimentary rocks are particularly important for
understanding life’s history on Earth, because they can:
– contain fossils
– produce a record of time
• Wind, water, ice and plants help break up solid rock into
grains, which is carried to oceans or floodplains and
deposited. Living organisms get buried in the sediment
• Over millions of years, the weight of layers above
compresses the layers below into rock and fossilizes
• More deeply buried layers are usually older
• This allows relative dating of rocks and fossils.
• Mineralogical analysis
– temperature and pressure conditions under which the
• Chemical analysis
– what the rock is made up of
• Isotopic analysis
– shows when a rock is enriched or depleted in a
– radioactive isotopes can tell us when the rock formed
An isotope is an
atom that has the
same number of
protons, but different
numbers of neutrons
• A radioactive isotope or element has a nucleus
that can undergo spontaneous change
– Breaking apart, or capturing or releasing electrons to
turn a neutron into a proton, or vice versa.
• The amount of time it takes 50% of the
radioactive atoms in a sample to change to
something else is called the “half-life”.
– Half lives are different for different types of isotopes.
• The original radioactive material is called the
“parent”. The material it changes into is called
• Radiometric dating compares the amount of
parent and daughter isotopes in a rock to
determine the age of the rock.
– half-lives are determined from lab measurements
– need to rule out daughter products in the rock when it
– rock should be minimally altered for the most
Consider a parent isotope with a half-life of 1,000 years. You
have a rock sample that began its life without any of the daughter
isotope present. The rocks also began its life with 100g of parent
isotope. You measure that, at present, it has 87.5g of daughter
isotope. How old is the sample?
Fossils in the Geological Record
• When an organism dies and gets buried in
sediments, its organic material is gradually
replaced by minerals dissolved in the water.
• Only a small fraction of creatures get fossilized
• Fossils become rarer as we look further back
into the geological record.
– Older fossils more likely to be destroyed by
– Plants and macroscopic animals appear only in the
last 10% of the Earth’s history.
– Prior to this time, all living organisms were
The Geological Time Scale
• Phanerozoic, 540Mya to present day
– “visible life” fossils of plants and animals
• Proterozoic, 2.5Gya to 540Mya
– “earlier life” fossils of single-celled organisms
• Archean, 3.9 to 2.5Gya
• Hadean, 4.6 to 3.9 Gya
The Age of The Earth
• The Earth is ~4.5-4.6 Gy old
• The oldest intact Earth rocks date to 4.0Gya
– Earth rocks older than this have probably been melted and
• Zirconium silicate (zircons), found embedded in
sedimentary rocks, have been dated to 4.4 Gya (using U
and Hf isotopes)
– appear to have been formed when oceans AND continents were
• Lunar rocks are older than 4.4 Gy
– So Earth and Moon must have formed prior to this time
(otherwise they wouldn’t have been around to collide with each
• Meteorites set the maximum age at 4.57+/-0.02 Gy.
• Both the Earth and the Moon formed within 50-70 Myrs
(0.05-0.07 Gy) after the oldest meteorites.
• First 0.5Gy of Earth’s history
• Earth formed with little or no atmosphere, but a
significant fraction of water rich planetesimals
– Trapped gases in the interior
– Released via melted rock/volcanism, moon-forming
– Volcanic gases: H2O, CO2, N2, H2S, SO2, H2
– This “outgassing” provided the water for our oceans
and gases for the atmosphere.
– Atmosphere was likely CO2 and water vapor
dominated. No O2 at that time.
• Zircons show oceans and continents had formed
by 4.4Gya (only 100 My after the planet had
A Planet Under Siege
• In the first few 100 million years of the Solar System’s
history, “leftover” planetesimals would have bombarded
the young planets.
• The Moon’s lunar highlands are heavily cratered and
4.4-4.0 Gy old.
• Sterilizing impacts probably occurred 6-12 times during
– 250-400 km diameter asteroid will vaporize oceans.
– Life may have arisen, and been destroyed, multiple times!
• The lunar maria, formed between 3.9-3.0 Gy ago show
far fewer craters.
• Late Heavy Bombardment 3.9-3.8 Gya
– Possibly due to planetary migration within the Solar System
– Things became much quieter after this…
Crunchy on the outside, chewy on the inside?
• The Earth’s core remains molten due to heating
from radioactive decay.
• the Earth is the only planet in the Solar
System with plate tectonics
• it’s also the only inhabited planet (maybe!)
• actually, probably not…
• but to have it, you need heat-driven mantle
convection and a lithosphere thin enough
to be fractured by the movement of the
• “Plates” are fractured pieces of the lithosphere that are
moved by the underlying convection of the mantle
• Plates spread apart on sea floors, and collide and
“subduct” at ocean trenches.
• Some plates collide and push upwards (Himalayas)
Hot Spot Volcanism
• Present on Earth in the middle of plates, but may be the
ONLY way for some planets to release internal energy.
Global Magnetic Fields
– an interior region of electrically conducting
fluid such as molten metal
– moderately rapid rotation of the planet.
Protecting the Atmosphere
• A magnetic field protects against “solar
wind stripping” of a planetary atmosphere.
Venus and Mars lack strong magnetic fields. However, only Mars
has lost most of its atmosphere to the Solar wind. What trait of
Mars explains this?
its distance from the Sun
its cold temperature
its closeness to Jupiter
its low mass
Questions of the Day
• What is the technique of radiometric dating
and why is it useful?
• What are some major changes that have
happened to Earth since the Hadean Eon?
• How does a planet’s magnetic field affect its