Phys 214. Planets and Life

Dr. Cristina Buzea

Department of Physics

Room 259

E-mail: cristi@physics.queensu.ca

(Please use PHYS214 in e-mail subject)

Lecture 10.

Geology and life. Part 1

(Page 99-123)

January 28

Contents

Assignment 1. (Handing out) Due date in two weeks from now.

Textbook: Pages 99-123

• Geology and life

• Type of rocks and their analysis

• Radiometric dating

• Fossil formation

• Geological time scale

• The age of Earth

• The Hadean Earth and dawn of life

• The heavy bombardment

• What is Earth like on the inside

• Differentiation and internal heat

Acknowledgments: Images from NASA, Apollo 16 crew

Geology and life

The Earth owes its habitability primarily to a combination of its size and its distance

from the Sun, in addition to other factors.

Geology – describes processes and features that shape worlds.

Aspects of Earth’s Geology Important to Life

1. Outgassing by volcanoes on the Earth’ssurface - important source of theEarth’s atmosphere.

2. Plate tectonics - largely responsible forthe long-term climate stability that hasallowed life to evolve.

3. Global magnetism - shield the Earth’satmosphere from energetic particles ofsolar wind from the Sun which wouldhave gradually stripped it away intospace.

Geological record = rocks from earlierperiods of the Earth's history.

Fossil record = remains of ancientorganisms preserved in rocks.

The fossil record is part of the geologicalrecord.

Reconstructing the history of Earth and life

Types of rocks

Igneous – molten lava that solidifies(basalt, granite)

Metamorphic – structurally or chemicallytransformed by high pressure or heatnot enough to melt it.

Sedimentary – gradual compression ofsediments at the bottom of seas andswamps (sandstone, limestone).

Sedimentary strata

Important in the study of Earth’s history

1) because it may contain intact fossils

2) it forms in a way that tends to produce a record of time

Erosion of land -> carried by rivers -> floodplains or oceans seafloor -> weight of upperlayers compresses layers into rocks + fossils buried along with sediments

Sediments deposited at different times – look different – different rates of sedimentation,composition, grain size, type or organisms fossilized.

Sedimentary rocks are marked by distinct layers = strata.

Sedimentary strata

Detailed geological record – comparingsedimentary strata from many sitesaround the world (looking for layerswith similar fossils)

Fossils of dinosaurs appear in layersolder than primates fossils - >dinosaurs lived before primates

Example: A fossil found in a layer ofstrata below one layer dated at 1million years must be older than 1million years.

Rock analysis:

Mineralogical – identify the minerals(temperature and pressure conditions)

Chemical – elemental and molecularcomposition (what the rock is madeof)

Isotopic – ratio of different isotopes of aparticular element – (when the rockwas formed)

More than 500 million years

How do we learn the age of a rock or fossil?

Radiometric dating = measurement of proportions of various atoms and isotopes.

A radioactive isotope is an unstable nucleus that spontaneously breaks apart.

Alpha decay - a helium nucleus is ejected.

Beta decay – emits an electron & a neutron turns into a proton.

Electron capture – absorbs an electron & a proton turns into a neutron

Radiometric dating

Because of the probabilistic nature of a radio decay, we can determine the rate at whichlarge numbers of radioactive nuclei decay.

Half-life = the time for half the number of radioactive nuclei to decay

Example: rock that contains equal amounts of potassium-40 and argon-40 -> the rockis 1.25 billion years old.

Accurate – verified on Egyptian artifacts, tree ring data, detailed study of the Sun andother stars (4.5 billion years old solar system)

What does the geological record show?

What does the geological record show?

The geological record contains fewer older rocks than younger

rocks because older rocks have been destroyed by

geological processes like plate tectonics and erosion

Fossils typically contain little or no organic matter because

over time inorganic minerals have gradually replaced the

organic matter

Only a tiny fraction of living organisms leave behind any kind

of fossil remnant because the vast majority of dead

organisms decay long before any mineral replacement can

occur.

Fossils of early life are very difficult to identify in very old

rocks because early life was very simple and microscopic

in size.

The geological time scale

The geological time scale

The Earth’s geological time scale is divided into four main eons:

1) Hadean – first 500 million years on Earth (hellish conditions – Hades –underworld; early bombardment )

2) Archean (ancient life - the earliest evidence of life )

3) Proterozoic (earlier life – fossils of single-celled organisms)

4) Phanerozoic (visible life fossils visible with naked eye)

• The most recent eon – Phanerozoic – is divided in 3 major eras: Paleozoic,Mesozoic, Cenozoic (old, middle, and recent life). There eras aresubdivided into periods.

• The divisions in geological scale are determined not by duration, but byspecific changes in the geological record.

The age of Earth

• The oldest intact rocks found on Earth date

back to about 4.0 billion years ago.

• Tiny grains of zirconium silicate (zircons)

found embedded in sedimentary rocks have

been radiometrically dated to 4.4 billion

years ago.

• Moon rocks brought by Apollo astronauts –

are older than any Earth rocks – volcanism

and other geological processes4.4 billion

years ago.

• The Earth’s crust appears to have already

differentiated from the interior

approximately 4.5 billion years ago.

• Isotopic analysis of meteorites suggests the

Earth and the rest of the solar system

formed 4.57 billion years ago.

The Hadean Earth and the dawn of life

• Models of planetary formation – Earth formedfrom local planetesimals of rock and metal andsome from farther out in the solar systemcontaining ice and rocks bound with water andgases

• The gas became trapped within Earth (carbonatedbeverage in a pressurized bottle)

• The main source of the Earth’s oceans andatmosphere was outgassing by volcanoes.

• The composition of the early atmosphere wasdominated by CO and had no molecular oxygen.

• Today atmosphere 98% N2,<0.1% CO2, 21% O2.The present oxygen atmosphere is a result ofphotosynthesis by plants.

• Evidence from the analysis of zircon crystalssuggests that the Earth may have been habitable aslittle as 100 million years after its formation.

Present-day volcanoes release

water vapors, carbon dioxide,

nitrogen, sulfur-bearing gases,

and hydrogen.

Hadean - The heavy bombardment

Hadean - The heavy bombardment

• Most craters on Moon’s highlands

formed during the period of early

bombardment (first few hundred

million years of the Earth’s history).

• Geological processes (plate tectonics,

volcanism, and erosion) have erased

the carters from Earth surface.

• The dark maria on the moon are huge

impact basins filled in by lava flows.

• Analysis of rocks returned from the

lunar maria during the Apollo program

suggest they formed between 3.9 and

3.0 billion years ago.

The heavy bombardment

All bodies in the solar system should have

suffered equally from collisions during

the period of early bombardment.

A body with relatively few craters on its

surface ->younger surface due to

geological activity or other processes.

Planetary scientists use crater counts to

estimate the ages of planetary surfaces. Moon- the far size

Io

Possible sterilizing impacts

A sterilizing impact is one during which

all life on a planet is destroyed.

During the Hadean Eon, life may have

existed but probably could not have

survived for long periods due to

sterilizing impacts.

• Probably happened 5-10 times during

Earth’s first few hundreds million

years of existence

The amount of energy released by an

impact with an asteroid more than

250-400 km – vaporize Earth’s

oceans and raise the global surface

temperature to 2,000oC.

What is Earth on the inside

Surface rocks have a lower density than overall density of the planet -> Earthhas a dense central core.

Information on Earth internal structure from seismic waves.

Earth`s interior structure:

1) CORE – highest density material; solid inner core made of Ni and Fe,surrounded by a molten outer core.

2) Mantle – rocky materials with moderate density - silicate minerals (Si, O)

3) Crust – lowest density rocks – igneous rocks

Earth differentiation and internal heat

• Geological processes on Earth’s surface - related to internal heat.

• Transfer of heat in the Earth’s mantle is dominated by convection.

• Differentiation = process by which denser materials (Fe) iron sank to the center of

the Earth while less dense materials (rock) rose to the surface (e.g. oil & water).

• Sources of heat that helped differentiate the Earth: impacts, gravitational potential

energy, radioactive decay.

• The amount of geological activity occurring on a terrestrial planet is related to its

size and the amount of internal radioactive heating.

The impact that created the moon occurred

after differentiation.

Differentiation occurred quite early.

Differentiation and internal heat

• All worlds in our solar system went differentiation.

• Since then they have been cooling with time.

Cooling rate depends on

1) the size of the world

2) The existence of an ongoing heat source

- radioactive decay, long half-time – heat for billions of years

- Tidal heating for moons of jovian planets

Moon

Next lecture

• Plate tectonics and Earth`s magnetic field