evolution of earth

8/4/2019 Evolution of Earth

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Evolution

of theEarth



Origin and Early Evolution of Earth

• Age of universe is ~ 14.5 By, about 10 By older than Earth

• Early universe had only protons & helium nuclei as condensed particles we arefamiliar with, rest was elementary particles & radiation

• First stars formed from hydrogen and helium, the rest of the elements formed in

protostars by nucleosynthesis

• Stars of a certain critical size exploded as supernovae, scattering hydrogen, He &newly formed elements as intergalactic “dust”. Other stars became “black holes”, brown dwarfs, etc.

• Inhomogeneities in dust clouds led to formation of secondary stars, similar to our

sun, but now could contain orbiting debris formed from elements in 1st generationstars.

• Inherited angular momentum caused debris to orbit main condensation center, andeventually gave rise to orbiting planets



“Hadean” is name given to Eon in

which Earth formed by accretion and

meteorite bombardment.

It was truly “hell on earth” as constantmeteorite bombardment and high

interior heat flow combined to keep

early Earth surface in nearly constant

molten state.

Atmosphere of early Earth likely

reducing (i.e. no oxygen) and similar

to present Jupiter atmosphere (?),mostly:

methane (CH4),

ammonia (NH3),

hydrogen (H2) and

helium (He)

with some traces of noble gases like

neon (Ne)

http://www.carleton.ca/%7Etpatters/teaching/intro/intro.html



Fig. 6.3

Stages in Planetary Evolution

1. Planetesimals… small bodies formed

from dust and gas

eddies

2. Protoplanets

9 or 10 formed from

planetesimals

3. Planets

formed by combining

protoplanets swept up by gravitational

attraction.



Broadly, four stages can be identified in the process of planetary formation.

1. The gravitational collapse of a star leads to the formation of a core to the gas cloud and the formation of ahuge rotating disc of gas and dust, which develops around the gas core. A star such as Beta Pictoris shows acentral core of this type, with a disc of matter rotating around the core. Beta Pictoris is thought to be a

young star showing the early stages of planetary formation.

2. The condensation of the gas cloud and the formation of chondrules. Chondrules are small roundedobjects found in some meteorites.. The presence of chondrules gives rise to a special class of meteoritesknown as chondrites. For example, the Allende meteorite is chondrule-rich and contains minerals rich in theelements Ca and Al, and Ti and Al, minerals which are unlike terrestrial minerals. It also include metallic

blobs of Os, Re, Zr. The chemistry of these unusual minerals suggest that they are early solar systemcondensates.

3. The accretion of gas and dust to form small bodies between 1-10 km in diameter . These bodies are knownas planetesimals. They form initially from small fragments of solar dust and chondrules by the processes of cohesion (sticking together by weak electrostatic forces) and by gravitational instability. Cohesion formsfragments up to about 1 cm in diameter. Larger bodies form by collisions at low speed which cause thematerial to stick together by gravitational attraction. Support for this view of the process of accretion comesfrom a region on the edge of the solar system known as the Kuiper Belt, where it is thought that theaccretionary 'mopping up' has failed to take place.

4. More violent and rapid impact accretion. The final stage of accretion has been described as 'runawayaccretion'. Planetesimals are swept up into well defined zones around the sun which approximate to the

present orbits of the terrestrial planets. The process leads eventually to a small number of large planetary bodies. Evidence for this impacting process can be seen in the early impact craters found on planetarysurfaces An explanation of the type given above for the origin of the planets in the solar system is supported

by mathematical simulations which show how accretion works by the progressive gathering together of smaller particles into large. It also provides an explanation of the differences between planetary bodies inthe solar system and explains the differences between the heavier terrestrial planets close to the sun, and thelighter, more gaseous planets situated at a greater distance.

http://www2.glos.ac.uk/gdn/origins/earth/ch3_2.htm



Beta Pictoris – a solar system in the making?

This new and very detailed image of the

famous circumstellar disk around the southern

star Beta Pictoris. It shows (in false colours)

the scattered light at wavelength 1.25 micron(J band) and is one of the best images of this

interesting feature obtained so far.

It has a direct bearing on the current search for

extra-solar planetary systems, one of the most

challenging astronomical activities. While

spectroscopic, astrometric and photometric

studies may only provide indirect evidence for

planets around other stars, coronographicimages like this one in principle enable

astronomers to detect dusty disks directly.

This is very important for our understanding

of the physics of planetary formation and

evolution.

The disk around Beta Pictoris is probably

connected with a planetary system. In particular, various independent observations

have led to the conclusion that comets are

present around this star, and variability of its

intensity has been tentatively attributed to the

occultation (partial eclipse) by an orbiting

planet.



Fig. 6.4

. From (A) a homogeneous, low-density protoplanet to (B) a dense,

differentiated planet

Stages in Formation of Early Earth



Fig. 6.5

Cross section through a spinning disk-shaped nebular cloud

illustrating formation of planets by condensation of planetesimals.

Temperatures refer to conditions at initial condensation.



Orion Nebula is part

of a large gas and dustcloud located in the

Orion Constellation. It

is one of the closest

stellar nurseries to us

at about 1,500 lightyears. The whole

cloud easily spans

over several hundred

light years.

Here you can see

recently formed stars

as they blink on in the

interior of the dust

cloud.

Orion Nebula, Star Nursery ?



This slide shows

the interaction

between the earth’s

magnetosphere andthe solar wind.

Early in the Earth’s

formation the solar

wind blew the light

gases, H an He tothe farther reaches

of the solar system.



Fig. 6.6

Planet Jupiter showing

moons Io (crossing at

equator) and Europa.



Fig. 6.7

The earth’s interior.

1. Crust

2. Mantle3. Outer core (liquid)

4. Inner core (solid)

Note density

discontinuity at core-

mantle boundary



Divisions of the Earth's interior

Cross section of

Earth showing in arudimentary way

the relation of the

upper mantle to

subduction zonesand midocean

ridges.

Note also the

region where

basaltic magma is

thought to form.



3-D image of the crust

3-D image of the crust beneath the

San Francisco Bay area developed

from monitoring the paths that

earthquake waves pass through it.

Colors correspond with differentchunks of the Earth's crust that have

been pushed together along the San

Andreas and Hayward faults.

Earthquakes are shown as yellow

dots.



The East African Rift – Surface Expression of a Mantle Hot Spot

ETOPO 30

DEM Model



Fig. 6.8

Structure of upper 300 km of Earth. The moho (M) was previouslytaken to be the boundary between the crust and upper mantle. It is

basically a seismic anomaly, but it is not as profound as the seismic

low-velocity zone. The zones shown here are based on analysis of

seismic velocities from earthquakes.



Fig. 6.9

Schematic diagram

illustrating Elsassar’smodel for the Earth’s

magnetic field. The solid

mantle rotates at a different

rate from the liquid outer

core, which is molten Fe

and Ni sulfides.

The magnetic field is

important for the evolution

of complex life on Earth

since it shields organismsfrom cosmic radiation (the

same high-energy particles

that form C-14 in the upper

atmosphere.

i



Fig. 6.10

Change in the Earth’s Heat

Flux through Time.

Although the diagram looks

complicated, there are only 4

radioactive isotopes that heat the

planet and 2 are uranium. The

other 2 are Th (thorium) and K-40 (potassium 40).

Note that the Earth's present-day

heat flux is only about 20% of

what it was originally.



Differentiation of Chemical Elements in Earth

Present distribution of major elements and U, Th, He and Ar

in the Earth’s atmosphere, crust and in seawater. (Elements

listed in order of abundance.

Fi 6 12



Fig. 6.12a

Zircon grain from the Acasta Gneiss, Slave Province, NWTerritories, Canada. The crystal has been etched with acid to

highlight the growth zones. These zircons have been dated to 4.03

By.

Fi 6 12b



Fig. 6.12b

The Acasta Gneiss. Great Slave Province, NW Territories,

Canada. One of the oldest (4.03 Bya) dated rocks on Earth.

This must have been one of the first crustal rocks to form

either at Late Hadean or shortly thereafter.

Fi 6 13



Fig. 6.13

Note the density stratification

with regard to the gases

(lightest farthest out, heaviest

closer to Earth surface).

Also note that vertical scale is

logarithmic.

Atmospheric Stratification and Important Types of Radiation and Radiation Shields.

Fi 6 14



Fig. 6.14

Evolution of Earth’s atmosphere from early Hadean (5 Bya) to present. Note the

changes from Stage I to Stage II, particularly the evolution of nitrogen, (N) the

virtual disappearance of hydrogen (H) and methane (CH4).

The important change from Stage II to Stage III was the rise in oxygen (due to

evolution of photosynthetic algae). Note the presence of the noble gases, Ar, Ne,

He and Kr. Most likely from the degassing upper mantle which continues to

today.

Fig 6 15



Fig. 6.15

The Global Chemostat.

This diagram shows the important flows for two elements, O and C (though

not reduced C). Other important elements, such as N, P, S, Na, Ca, and K

follow similar cycles. (Chemostat = hold chemistry constant or change slowly).

Start analyzing the cycle with the algae (as prime movers) and follow the

chain. Algae actually started the chemostat over 4 Bya. This chemostat is one

of the hallmarks of a planet with advanced life forms and it probably very rare

in the universe.

Fig 6 16



Fig. 6.16

The global thermostat. Shallow water is heated by the sun to form the Earth’s

most important heat reservoir. The photic zone above the thermocline is the

habitat of algae and phytoplankton which from the base of the aquatic food

chain.

Below the thermocline the water is cooler and less agitated, hence less

oxygenated. These waters may even become stagnant and reducing. When they

do they constitute the first step in the preservation of organic matter, which

eventually leads to gas and oil deposits.

evolution of earth

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