Astronomy190 - Topics in Astronomy

Astronomy and Astrobiology

Lecture 5 : Geology

Ty Robinson

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 habitability?

Geology and Astrobiology• Geology encompasses the study of worlds with

solid surfaces. • 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 years.

Rocks• 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. slate, marble)

Rock Composition• 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).

Questions?

We can find fossils in…

Sedimentary rocks

Igneous rocks

Metamorphic rocks

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 life’s remains.

Sedimentary Strata

• More deeply buried layers are usually older• This allows relative dating of rocks and fossils.

Rock Analysis• Mineralogical analysis

– temperature and pressure conditions under which the rock formed.

• Chemical analysis– what the rock is made up of

• Isotopic analysis– shows when a rock is enriched or depleted in a

particular isotope– 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

Radiometric Dating • 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 the “daughter”.

Radioactive Decay

Radiometric Dating• 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

formed– rock should be minimally altered for the most

accurate results.

Parent Daughter Half-life

Questions?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?

1,000 years

500 years

3,000 years

2,000 years

• 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 volcanism,

erosion, – Plants and macroscopic animals appear only in the last

10% of the Earth’s history. – Prior to this time, all living organisms were microscopic.

Fossils in the Geological Record

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 recycled

• 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

present.

• 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 other!).

• 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.

The “Hadean”• 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 impact. – 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 formed!)

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 the Hadean– 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!)• coincidence?• 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 mantle.

Plate Tectonics• “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

• Need– 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.

Questions?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 habitability?