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

• radioactivity

Supplies all the heat at the beginning

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 it takes to cool off!

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:

• H2O, CO2, N2

• 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/inch2 = 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

• 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

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

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.• what gas they did capture was H & He, and it escaped• present-day atmospheres must have formed at a later time

• Sources of atmospheric gas:• outgassing – release of gas trapped in interior rock by volcanism• 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