Geological Time Scale & Global Properties

ASTR 4: Life in the Universe

Outline

• Radiometric Dating • Global Properties • Geologic Time Scale & Evolution of Life • Tree of Life

Radiometric Dating

• Isotopes which are unstable are said to be

radioactive

.

• They spontaneously change in to another isotope in a process called

radioactive decay

.

– protons convert to neutrons – neutrons convert to protons • The time it takes half the amount of a radioactive isotope to decay is called its

half life

.

• By knowing rock chemistry, we chose a stable isotope which does not form with the rock…its presence is due solely to decay.

• Measuring the relative amounts of the two isotopes and knowing the half life of the radioactive isotope tells us the age of the rock.

The Age of our Solar System

• Radiometric dating can only measure the age of a rock

since it solidified

.

• Geologic processes on Earth cause rock to melt and resolidify.

 Earth rocks can’t be used to measure the Solar System’s age.

• We must find rocks which have not melted or vaporized since the condensed from the Solar nebula.

– meteorites imply an age of 4.6 billion years for Solar System • Radioactive isotopes are formed in stars & supernovae – suggests that Solar System formation was triggered by supernova – short half lives suggest the supernova was nearby

Inside the Terrestrial Worlds

• • After they have formed, the molten planets differentiate into three zones: • • • core - made of metals mantle - made of dense rock crust - made of less dense rock Lithosphere - the rigid, outer layer of crust & part of the mantle which does not deform easily

Inside the Terrestrial Worlds

active geology inactive geology

Heating the Terrestrial Worlds • Planetary interiors heat up through:

• • accretion differentiation Supplies all the heat at the beginning • radioactivity Supplies heat throughout the planet’s life

Cooling the Terrestrial Worlds

• Planets cool off through: • conduction - heat flowing on the microscopic level • convection - heat flowing on the macroscopic level (bulk motions) • eruptions - hot lava bursts through crust • the

larger

the planet, the

longer

to cool off!

it takes

Cooling the Terrestrial Worlds

Magnetic Fields

• Electric charges moving via convection in a molten iron core and spinning acts like an electromagnet  magnetic field • Earth has a magnetic field • Venus, Mars, & the Moon do not • Mercury surprisingly has a weak magnetic field ??

Shaping Planetary Surfaces

• Major geological processes that shape planetary surfaces: • impact cratering : excavation of surface by asteroids or comets striking the planet • • volcanism : eruption of lava from interior tectonics : disruption of lithosphere by internal stresses • erosion : wearing down by wind, water, ice

Impact Cratering

• objects hit planet at 10 – 70 km/s • solid rock is vaporized • a

crater

is excavated • matter is ejected in all directions • craters are circular – large craters have a central peak

Counting Craters to find Surface Age

• Cratering rate decreased as Solar Systems aged.

• The older the surface, the more craters are present.

Volcanism

• Underground, molten rock, called

magma

, breaks through crack in the lithosphere.

• Trapped gases are released: • H 2 O, CO 2 , N 2 • Viscosity of

lava

(typically basalt) determines type of volcano

Tectonics & Erosion

• convection cells in the mantle causes both: • compression in lithosphere • mountains are produced • extension in lithosphere • valleys are produced • mountains & valleys appear on the surface • movement of rock by ice, liquid, or gas • valleys shaped by glaciers • canyons carved by rivers • sand blown by wind • erosion not only wears down features, it also builds them: • sand dunes • river deltas • sedimentary rock

Atmosphere

• • A layer of gas which surrounds a world is called an

atmosphere

.

• they are usually very thin compared to planet radius

Pressure

is created by atomic & molecular collisions in an atmosphere.

• heating a gas in a confined space increases pressure • number of collisions increase • unit of measure: 1 bar = 14.7 lbs/inch 2 = Earth’s atmospheric pressure at sea level • Pressure balances gravity in an atmosphere.

Effects of an Atmosphere on a Planet

• greenhouse effect • makes the planetary surface warmer than it would be otherwise • scattering and absorption of light • absorb high-energy radiation from the Sun • scattering of optical light brightens the daytime sky • creates pressure • can allow water to exist as a liquid (at the right temperature) • creates wind and weather • promotes erosion of the planetary surface • creates auroras • interaction with the Solar wind when magnetic fields are present

Planetary Energy Balance

• Solar energy received by a planet must balance the energy it returns to space • planet can either reflect or emit the energy as radiation • this is necessary for the planet to have a stable temperature

What Determines a Planet’s Surface Temperature?

• Greenhouse Effect cannot change incoming Sunlight, so it cannot change the total energy returned to space.

• it increases the energy (heat) in lower atmosphere • it works like a blanket • In the absence of the Greenhouse Effect, what would determine a planet’s surface temperature?

• the planet's distance from the Sun • the planet’s overall reflectivity • the higher the albedo, the less light absorbed, planet cooler • Earth’s average temperature would be –17º C (–1º F) without the Greenhouse Effect

Magnetospheres

• The Sun ejects a stream of charged particles, called the

solar wind

.

• it is mostly electrons, protons, and Helium nuclei • Earth’s magnetic field attracts and diverts these charged particles to its magnetic poles.

• the particles spiral along magnetic field lines and emit light • this causes the

aurora

(aka northern & southern lights) • this protective “bubble” is called the

magnetosphere

Earth’s Magnetosphere

Weather and Climate

weather

– short-term changes in wind, clouds, temperature, and pressure in an atmosphere at a given location

climate

– long-term average of the weather at a given location • These are Earth’s

global wind patterns

or circulation • local weather systems move along with them • weather moves from W to E at mid-latitudes in N hemisphere • Two factors cause these patterns • atmospheric heating • planetary rotation

Four Major Factors which affect Long term Climate Change

Gain/Loss Processes of Atmospheric Gas

• Unlike the Jovian planets, the terrestrials were too small to capture significant gas from the Solar nebula.

• Sources of atmospheric gas: •

outgassing

– release of gas trapped in interior rock by volcanism • • what gas they did capture was H & He, and it escaped • present-day atmospheres must have formed at a later time

evaporation/sublimation

– surface liquids or ices turn to gas when heated •

bombardment

– micrometeorites, Solar wind particles, or high energy photons blast atoms/molecules out of surface rock • occurs only if the planet has no substantial atmosphere already

Gain/Loss Processes of Atmospheric Gas

• Ways to lose atmospheric gas: • • • • •

condensation

– gas turns into liquids or ices on the surface when cooled

chemical reactions

– gas is bound into surface rocks or liquids

stripping

– gas is knocked out of the upper atmosphere by Solar wind particles

impacts

– a comet/asteroid collision with a planet can blast atmospheric gas into space

thermal escape

– lightweight gas molecules are lost to space when they achieve escape velocity gas is lost forever!

Tree of Life

• Three Domains of Life –

Prokaryotes (without nucleus)

• •

Archaea Bacteria

–

Eukaryotes (with nucleus)

•

Eucarya

• Phylogenetic Tree of Life – Carl R. Woese, 1977 – 16S

ribosomal RNA