planets

Earth and the Solar System - slides shown in class

The organization and motions of solar system are readily observable.

Aristarchos of Samos (310-230 BC) first proposed that Earth and the other planets move around the sun.

- The idea was not accepted until proposed again by Copernicus (1543)

- and established late 16th early 17th centuries by invention of the telescope by Galileo (1609) and work of Tycho Brahe (Danish nobleman and astronomical observer) working with J. Kepler (German mathematical astronomer)

Why did it take so long? Aristotelian philosophy and scholasticism dominated from about 2nd century AD through Middle Ages. In this period

- scientific questions addressed by just talking and thinking about them

- there was no attempt to check by experiment

- the tools for observation were not available

- and mathematics was for technicians, with no claim to philosophical relevance so it could not be expected to solve scientific questions.

Isaac Newton (1642-1727) Universal law of gravitation - permits an explanation of planetary motion

Gravitational force (F) between 2 masses m₁ and m₂ is proportional to the product of the two masses divided by the square of the distance between the two centers of mass

F_g = (G x m₁ x m₂)/(r x r)

G = Universal constant of gravitation = 6.6742 x 10^-11 m³/kg-sec². For Earth this reduces to F_g = M x g Where M is the mass of the object being attracted by Earth and g = acceleration due to gravity = 9.8 m/sec².

Hypotheses on origin of solar system

"catastrophic"
grazing collision or near miss of sun by passing star
explosion former companion to sun
passage sun through a dust cloud
planets are a normal by-product of star formation. All recent attempts to understand planet origin are along this line.

Normal star formation -

interstellar dust cloud contracts by gravitation
gravitational energy converted to heat
the cloud fragments into subunits
to conserve angular momentum, subunits spin faster as they contract and become flattened disks
contraction slows and ceases due build-up of internal heat
high internal heat promotes thermonuclear reactions.

More on the origin of the solar system

Observations - on sun and solar system

- Sun formed 4.7 billion years ago. Chemically not "primitive" . So comes from explosion of older star (supernova). - 90 % of mass scattered into space explosion made many unstable elements - Chemistry of solar system dictated by this explosion.

Conclude - initial dust cloud cold, strong magnetic fields, rotated as one 'rigid' body. Without all chartacteristics, the dust cloud core would not condense into a star.

Sun now:

- core = 170,000 km (24% of the radius of the sun). Temp15 million ^oK, density 160 g/cc (8.5x gold) 62% He, 38% H by mass

- radiative layer = 60% of the radius

-convective layer = 15%

both 72% H, 26% He, 2% heavier elements

-photosphere - surface layer we see. Av. T 5770 ^oK

Planets arise from the portion of the condensed dust cloud not incorporated in the central star by processes of

gravitational collapse
accretion
condensation

dust cloud - made of crystals of frozen gases and other solids.

Evidence of the composition of planets comes - from meteorites

Solids

metals (Fe, Ni)

metal oxides

Fe Þ FeO, FeS

Fe-Mg silicates

Frozen gases

O Þ H20, methane, ammonia

The early sun raised the temperature of the interior of solar system. This swept out inner solar system of volatile elements (easily vaporized) solids, dust particles remain

outer planets - colder part of cloud so >> volatile elements. Outer gas giants, 10X more massive than Earth. Big enough to gravitationally pick up and retain H and He from solar nebula.

inner planets - from hotter part of cloud so many fewer volatile elements. Inner rocky planets formed in regions solar nebula hot enough to boil away ice

Planet formation stopped when a protoplanet became so large its gravitational pull opened up a gap in the nebula - similar to gaps in Saturn's rings. When this happened there was no more material to attract.

Present circular orbits - result from the average motions of many planetesimals

Young planets consisted of chaotic mixture silicates, metals, trapped gases. Formed by interactions that included - direct collisions, non-capture deflections (near misses), and disintegrative partial or complete capture

Formation stage brief and turbulent - no longer than about 100 million years. Bodies grew to planetesimals as grains collided. When critical radius reached (~2400 km) get gravity fields strong enough to affect bodies to distance R. Greater gravity resulted in a greater likelihood of collisions.

- ~ 1 million years to get earth size object from planetesimals.

Early planetary atmosphere destruction by

hydrodynamic and thermal escape
solar wind erosion impact erosion
surface chemical reactions - carbonate formation? rust? -

smaller bodies (Mercury) quickly lost gases - small gravity fields so low escape velocity, also solar wind

Mars lost most of atmosphere.

Gas giants retained light gases. Atmospheres mostly H (85%) and He (15%)

During accretion get

warming due to loss of kinetic energy and due to radioactive decay unstable isotopes
Center each planet cold, but bulk hot because decay short-lived radioisotopes and much heat of accumulation was trapped inside (silicates poor conductors). T goes up over time. - planets hot enough to be plastic (maybe?? liquid - esp. metals). - denser metals sank, less dense silicates floated. Results in metals melting and sink to form core. Core separation may have started before initial accretion was complete. Separation of core from mantle may have been complete as early as 4.4 billion years ago.
expulsion of gases to form atmosphere. Sinking of metals raised T more, sinking faster. Get overturn, forced out gases Þ atmospheres. Atmospheres also possibly from infall volatile-rich meteorites.
On earth early whole-mantle convection would have resulted in partial melting in upper part leads to segregation of crust. - also surface bombardment effects? Created molten surface layers? We think this happened on the moon so possibly also on Earth.

By 2.8-2.5 billion years ago crust stabilized. Near surface T gradients decreased significantly.

Moon - origin unsolved. Chemically different from earth known to have moved away from earth over time

- theories:

binary system - formed at same time but separate
capture - formed elsewhere
fission - split off from Earth due to a major impact. This theory has many proponents and its favored at this time.
coagulation - from leftover materials

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