ORIGIN AND AGE OF EARTH , SOLAR SYSTEM, AND UNIVERSE

To begin chronology at the beginning we have to look beyond the Earth to the context of the entire Solar System and even of the Universe; still fraught with speculation and lately much argument over age of the universe (due to discovery of dark matter and paradox that some massive stars are estimated to be older than estimates of Universe age based on red-shift)

We know that Universe is expanding; though stars and galaxies are not (balloon analogy); from study of spectra of stars all have spectral lines shifted toward red end of wavelength spectrum, the further away stars are, the more extreme the shift; this is the Doppler effect which infers that wavelengths of light  from objects that are moving away from the observer is stretched out or attenuated (the opposite would be blue shift: analogous to sound of approaching vs. receding train)

Can extrapolate from rates of dispersal of galaxies backward in time to bring all matter back to point in space; all matter and energy concentrated into a point of infinite density about 15-18 billion years ago.

THE MOST SIGNIFICANT EVENT IN HISTORY OF UNIVERSE (and by interpolation of Earth's history) is called "Big Bang"; during this event energy and matter were created and expanded outward creating most of the matter in a few milliseconds to a few million years (slide)

Galaxies and larger local groupings (of galaxies) were formed as matter concentrated via gravitational pull into local centers;

Solar System apparently developed from solar nebula a spinning discoidal mass of dust and gases that probably  was debris from a pre-existing exploded star; the abundance of denser elements  (e.g. O, Si, Al, Fe. Ca, Mg, etc)in solar system suggests that it is derived from recycled material because heavy elements are only produced by nuclear fusion reactions near centers of massive stars; Sun as a second cycle star

Astronomers observe various phases of nebula formation in space; begin with dark nebulae (clouds of dust and H gas); generally infer that these undergo gravitational contraction and increased rotational velocity through time;  gradually, center draws in most mass and begins to emit energy

Stars begin undergoing thermonuclear reactions that produce heat and light at a critical temperature (several million degrees C) and density (Hyashi phase) at which fusion begins; then go star goes through a stellar evolution that varies depending upon size but may involve:

a) increasing luminosity and temperature into the T-tauri phase due to gravitational contraction

b) a relatively cool, orange early phase;

c) gradual increase in luminosity  (by about 40%) in main sequence stage (then stable for a few billion years; e.g. Sun about half through its stable phase)

d) expansion into red giant phase once nuclear fuel largely expended; this giant will then cool, collapse and contract into a white dwarf and then black dwarf- after about 10 billion years

e) for more massive stars (> 5 x the Sun’s mass) more rapid evolution to red supergiant, then development of supernova explosion followed by collapse of part of the material to form a neutron star (pulsar), or, if very massive, collapse to a black hole;

Such an exploded star was probably the source of Solar System; astronomers believe Sun will follow the red giant pathway; also note that the Sun was probably cool in its early history and has gradually heated up: BUT Earth temperatures have stayed nearly similar. Why?  Probably because of compensatory CO2 loss- icehouse cooling (More later)

Age of Earth and Solar System: oldest rocks dated thus far on Earth about 3.9 billion years (Ga); but this is probably not the true age; inherited zircon crystals in ancient sedimentary rocks yield dates of 4.1 to 4.2 Ga; probably earliest evidence eradicated by tectonics and erosion;

What other evidence is available?

Meteorites provide samples of early solar system ; most  all give radiometric dates near 4.6 billion years

Moon Rocks yield similar ages, back to 4.6 Ga; because it is a "dead" planetary surface; probably give reasonable estimates for age of early solar system

Solar System Origins: Key Facts: nine planets in two groups: Terrestrial (inner); Jovian (outer); all except Pluto revolve in same plane, "ecliptic plane";  planets, except Venus rotate in same (prograde) direction as Earth on axes sub-perpendicular to ecliptic plane (except Uranus which rotates around a sub-horizontal axis); most of mass is at center; though slow rotation of Sun is anomalous: may be due to "braking" action of magnetic coupling;
Such observations provide evidence for Solar Nebula hypothesis:

Solar Nebula was a rotating mass of gas and dust derived from previous supernova; rotated more rapidly as it contracted flattened due to gravity, material separated into rings; planets began accreting in "snowball" fashion by eddies within the rings;

Meteorites and planetesimals: planets formed by aggregation of bodies (protoplanets or planetesimals)  that were probably similar to meteorites; sizes millimeters to asteroids several km in diameter; may be remnants of original materials from which planets accreted, or some may be remains of exploded planet(s); meteorites are of three types (similar to inner planetary layers):

a) iron-nickel (analogous to Earth's core)
b) stony (chondrites) and stony-iron (analogous to Earth's mantle)

Meteorites show anomalous abundances of rare isotopes that form by decay of short-lived parent isotopes that would only have been around for short time after formation in previous supernova: proves that condensation of the nebula was probably completed within about 50-100 million years after formation of nebula

Great Meteor Shower: Must have been intense meteorite bombardment during early phases of solar system; continued after accretion of planets up to about 4 billion years ago; would have melted surface repeatedly; evidence is present in form of craters and mares on Moon and other Terrestrial Planets ?except Earth

Earth Accretion:  Earth formed during this accretionary process, but fundamental questions remain: was it hot (inhomogenous) accretion or cold accretion??: How did Earth attain its layered construction?

a) inhomogenous accretion; solar nebula hot, cooled down to form various components: Fe-Ni meteorites formed at highest temperatures;  accreted to form core of Earth; later condensed chondritic meteorites accreted to form mantle; some differentiation of crust

b) homogenous accretion: all of Earth accreted from cool, homogenous material; later, heating and partial melting due to a 1) high heat flow (lots more decay of radioactive isotopes); 2) gravitational collapse; 3) asteroid impacts
heavy elements separate, sink to center to form core ("iron catastrophe"); lighter silicates remained high to form mantle

 it is still unclear which of these two alternatives is more likely-though most texts only present the latter

Origin of the Moon: Moons surface consolidated by 4.5 Ga;; mostly covered by feldspar rich anorthosite; small metallic core  but where did moon come from:

Double planet?      Fission?
Capture?       Large body impact on Earth?

Most  hypotheses eliminated.  The large body impact on Earth is presently accepted; body of Mars size (1/10 Earth) impacted in first few million years after Earth accreted; moon depleted in iron, magnesium may have formed by reworking of mantle of impactor (probably not a chunk of Earth despite some isotopic similarities; core may have joined Earth’s core after melting
Also explains Earth’s fast rotation- was accelerated by impact

Comparative Planetology:

Comparisons with other planets sheds light on Earth’s unique features

Nine planets divided into two basic groups separated by the asteroid belt:

Terrestrial planets: (Mercury, Venus, Earth-Moon, Mars) inner, small, dense; few stony moons

Jovian planets: (Jupiter, Saturn, Uranus, Neptune), outer, huge, gaseous, low density; large number of moons

plus: Pluto: oddball
Oort Clouds: areas of comet formation and "storage"
Terrestrial-Jovian differences reflect proximity to sun ; volatile elements expelled from inner part of solar system;

Contrast Earth with other Terrestrial Planets

:Mercury very small, very dense, VERY hot, cratered surface

Venus: twin to Earth in size but  somewhat too close to Sun; no liquid water to dissolve CO2;  intense runaway greenhouse; VERY HOT; atmosphere of CO2, sulfuric acid;
some evidence of tectonics; no moons; slow retrograde rotation indicates that Venus was hit hard by object that knocked it out of normal rotation

Mars: 1/10 size of Earth;  similar day length; cratered, but some evidence of former liquid water; thin atmosphere with CO2, little oxygen; giant shield volcanoes but no plate tectonics; life ??

Earth: has a number of unique features: a) Few craters; b) Plate tectonics very active; mobile Lithosphere; c) Liquid water ("blue planet"): a Hydrosphere; ; d)  Less CO2, oxygenic Atmosphere;  e) Life: a Biosphere!!

Key explanations for "Goldilocks" Planet

a) Right Size: Earth was just massive enough that it s interior cooled more slowly than other inner planets (A/V issue!!) this means it continues to have volcanism, mantle plumes, and even mantle convection cells; smaller Moon, Mercury , and Mars were effectively "dead" after about 4.0 to 3.0 billion years ago.  Venus may still have some volcanism but mantle convection probably ceased ~ 2.0 Ma

b) Right Distance: Earth is "just right" distance from Sun to maintain liquid water on surface; Moon is waterless, Mars may once have had some liquid water (gorge erosion);

c) Greenhouse-Icehouse Balances: Venus closer to Sun; probably started with surface temperatures of about  60 C; too warm to allow CO2 to be dissolved in water, less weathering than Earth; CO2 built up in the atmosphere to "runaway greenhouse" conditions; very hot surface with thick atmosphere;  Earth cooler CO2 soluble in water  carbonic acid acts on rocks in weathering; "soaks" up CO2 from atmosphere; also,

c) Life evolved on Earth  production of organic matter and limestone (CaCO3) serve as CO2  sinks; without limestone we would also have "runaway greenhouse effect, with  100X higher CO2 and surface temps. ~ 400 C; ALSO photosynthesis generates oxygen as byproduct!

HADEAN EVENTS:

Although oldest rocks on Earth are ~ 3.9 Ga,  we can be sure that several events took place prior to this time during the first half billion  years of Earth history; these include:

Formation of  Core and Mantle: Either primary or secondary differentiation; hints of paleomagnetism in some of the most ancient rocks indicates core was present.
 

Origin of Earth’s Crust: Must have been some solid crust by 3.9 Ga.  See igneous, metasedimentary rocks by this time; probably mainly mafic crust at first, very thin, mobile; felsic (sialic) crust began accumulating to form small cratons; may have been associated with magma differentiation near oceanic hotspots; initial cratons small, steep-sided; not until about 2.8 Ga did larger cratons begin to buildup; massive increase around 2.5 Ga

Origin of (Primary) Atmosphere: Formed by outgassing of Earth, frequent volcanic eruptions releasing gasses; probably a mixture of N, CO, CO2, NH3 (ammonia)?; H2O vapor; these are the sorts of gases produced by volcanism;
Initial atmosphere had little oxygen; some O2 accumulated due to photolysis; breakdown of water molecules in upper atmosphere due to impact of UV radiation; subsequently, much more O2 was added by photosynthesis; evidence for chlorophyll and for cyanobacteria in early Archean; however, despite these two sources of O2 there may have been little net buildup of atmospheric oxygen before about 2.0 Ga; because of oxygen "sinks", especially Fe and organic matter;

There is good evidence for anoxic Archean atmosphere; detrital pyrite and uraninite in terrestrial sediments (these minerals are unstable and readily disintegrate in the presence of oxygen

Also, redbeds absent in terrestrial sediments up to about 2 Ga; during the same time see peak of development of BIFs in marine sediments (MORE SOON)

Origin of Hydrosphere: probably a high proportion from condensation of water vapor; but also maybe a significant fraction added by cometary impacts: comets carry a  lot of  water ice; evidence for liquid water by 3.8 Ga: water laid sedimentary rocks present

Origin of Life: Attributes of life: self-replication and self- regulation, sustaining internal chemical reactions that allow for growth.
Unique to Earth ? Still no firm evidence of life on Mars; but seems very probable that life does exist elsewhere in the Universe.
:
Origin is still debated
Seeding of Earth with essential building blocks or live prokaryotic life is possible, but it is also plausible that essential building blocks of life were synthesized and assembled abiotically  in anaerobic settings on Earth

Miller and Urey’s  famous 1953 experiment already showed that amino acids could be generated in an anaerobic atmosphere of H2O vapor; H2, N, NH3, CH4 (methane); later it was also found that amino acids could be formed in CO, CO2 rich atmosphere, also needed a spark or UV radiation to trigger reactions; subsequently, nucleic acids, chlorophyll, and other key compounds generated;

 Problem: how to concentrate these compounds in order to polymerize large compounds such as proteins: some silicates such as surfaces of clay minerals with unbalanced charges might have served as templates; also some fatty acids form spherules-micelles that might have been precursors of cell membranes

A critical finding was that the  biochemical synthesis could only take place under anaerobic conditions; little O2 at first but there MAY have been too much generated by photolysis to allow reactions to proceed,

Newer hypothesis is that life originated in hot waters associated with deep sea vents or rift zones: have a) hot water; b) anoxic conditions; c) abundant nutrient elements;  d) necessary energy sources for metabolism, and e) clays to template biosynthesis; some very primitive bacteria still live in this

Earliest life replication may have been based on RNA
; simpler to produce, more versatile; may have triggered protein synthesis AND eventually, DNA formation; later DNA took over and directed RNA synthesis
 
 
 
 

Early life functions:
Earliest life forms may have been heterotrophs; obtained energy by metabolizing abiotically generated organic compounds:
 

Heterotrophic Fermentation:

C6H12O6 --- alcohols + water and energy

Critical steps was ability of some organism to tap radiant or chemical energy  to produce  simple sugars that could be "burned" to yield energy
Autotrophy"

Chemosynthesis; utilize chemical energy to produce simple sugars;

Photosynthesis: utilize chlorophyll to tap Sun light for chemical reaction that stores energy in sugars; complex Krebs cycle, but net reaction is:

 H2O + CO2  (in sunlight) -----  C6H12O6 + O2

(Note some primitive, anaerobic bacteria use H2S instead of water, as a hydrogen donor; produce S as by product instead of O2)

Photosynthesis may have been coupled with evolution of aerobic metabolism
 

 C6H12O6 + O2---- CO2 + H2O  plus energy;

much bigger yield than with fermentation; oxygen produced as byproduct in photosynthesis can be harnessed (and rendered harmless to cells) to "burn" sugars; however, some sugars are modified to form building materials; these store CO2 and enable free O2 to evade to the atmosphere: both photosynthesis and aerobic metabolism were critical breakthroughs for oxygenic atmosphere;

Chemical fossils suggest existence of photosynthesis in some of most ancient rocks 3.8 Ga  Isua Group of Greenland show carbon with very low isotopic ratios of C13/C12; typical of fractionation that occurs during photosynthesis