Brief history of the universe

Atoms

Atoms – consist of a dense nucleus of positively charged protons and uncharged neutrons surrounded by a cloud of negatively charged electrons.

Atoms typically have equal numbers of protons and electrons, but may lose electrons to become ionized.

The atom

Elements and isotopes

The element is determined by the number of protons in the nucleus

The isotope is determined by the number of neutrons

Opposite charges attract

Positively charged protons attract negatively charged electrons due to the electric force.

Think of static cling.

Like charges repel

Two electrons or two protons will exert a repulsive force, pushing each other away.

Discussion

If positive charges repel each other, how can all those positively charged protons get packed so tightly in the nucleus of an atom? Why don’t atoms just blow themselves up?

The Strong Nuclear force

The nucleus is held together against the repulsive electric force of the protons by the strong nuclear force.

The strong nuclear force is exerted only over short distances between protons and neutrons.

Discussion

If the strong force is the same between two protons as between a proton and a neutron, are protons more strongly attracted to other protons, or neutrons, or are they equally attracted to both?

Protons bind more tightly with the electrically neutral neutrons, because they do not need to overcome the repulsive force between to protons.

Thus the more protons you stuff into an atomic nucleus, the more neutrons are needed to keep it stable.

Neutrons – decay into a proton and electron

Neutrons – proton/electron pair bound together

Neutrons, Protons and electrons

Temperature

Nucleosynthesis

Hydrogen and Helium

The early universe contained about 75% hydrogen, 25% helium and trace amounts of lithium and beryllium.

opaque transparent

CMBR temperature map

Dark matter simulation

The nebular hypothesis

A star, like the Sun and planets, formed from the gravitational collapse of a single, spherical, slowly rotating cloud of cold interstellar gas and dust.

Star Formation

Consequence

Planet formation is a natural outcome of star formation.

Planetary systems should be common.

Discussion

Why do you think a collapsing rotating cloud of gas forms a disk?

The Planetesimal Hypothesis

Fluffy dust grains condensing out of the solar nebula stick together as a result of low-speed collisions, building up to small bodies called planetesimals.

Cassiopeia A

Did a supernova trigger the collapse of the Solar nebula?

Xenon-129 is found in some meteorites

Gaseous even at extremely low temperaturesCould only get there by decay of iodine-129 with a half life of 17 million years

Iodine-129 created in a supernova explosion was injected into solar system within a few tens of millions of year before its formation.

Protoplanets

As the protoplanets grow by accretion of planetesimals, their gravity increases spurring more accretion.

Two classes of planets

Terrestrial – mostly silicates and iron, smaller in mass, have solid surfaces, close to Sun

Jovian – mostly made of lighter elements such as hydrogen and helium, higher mass, far from Sun

Discussion

What’s a silicate? Give and example of a silicate.

Solar nebula composition

We expect that the solar nebula from which the Sun formed, had the same composition as the current solar surface.

98% hydrogen and helium1.4% hydrogen compounds – CH4, NH3, H2O0.4% silicate rocks0.2% metals

Discussion

Given the composition of the solar nebula, why do you think all the terrestrial planets have smaller masses than the Jovian planets?

Discussion

Why do you think the Jovian planets which are rich in hydrogen compounds, formed far the Sun, while the rocky terrestrial planets formed close to the Sun?

Why do you think Uranus and Neptune didn’t get as big as Jupiter and Saturn?

Discussion

Discussion

Can the Earth hold hydrogen and helium gas in its atmosphere? Explain.

Discussion

Can any of the other terrestrial planets hold hydrogen and helium gas? Explain.

Discussion

The Jovian planets all appear to have cores of rock and icy materials with a mass of about 10 times that of the Earth. What happens when a protoplanet gets to be about 10 times the mass of the Earth?

Terrestrial Planets

All the terrestrial planets are more or less differentiated, i.e. the densest materials have sunk to the core and the lighter materials have floated to the surface.

Discussion

If the planets grew by being bombarded with planetesimals, why do you think the most dense material ended up in the cores of the terrestrial planets?

Discussion

Why do you think all the terrestrial planets were so hot in the past? Isn’t space rather cold?

How the planets got hot

1) Heat of accretion

2) Heat of differentiation

3) Heat from radioactive decay

Terrestrial planets interior structure

Core – highest density material, mostly iron and nickel

Mantle – high density silicate rocks

Crust – lower density silicate rocks, granite and basalt.

Heat and planets

All the terrestrial planets started out hot and have been losing heat over time by radiating it into space from their surfaces.

Planetary size and heat loss

Larger planets lose heat more slowly than do smaller planets.

Discussion

Don’t larger planets have larger surface areas, and with a larger surface area shouldn’t larger planets be able to radiate more energy into space? So shouldn’t larger planets cool faster? Why doesn’t this work?

Volume and heat

The greater the surface area, the faster heat will be radiated. But, it is the volume that stores the heat. The greater the volume of a planet the more internal heat it can retain.

Also, more massive planets have more radiative material.

It’s geometry

The surface area increases as the square of the radius. But the volume increases as the cube of the radius. Thus, a larger sphere has less square miles to radiate the heat per cubic mile of material.

This is why people get fat!

Geologic activity

Internal heat drives geologic activity on the planets’ surfaces.

Discussion

How does the heat from the interior of the planet get to the surface?

Lithosphere

The convective cells in the planets do not make it to the surface, but are stopped at the base of the lithosphere. The lithosphere includes the crust and the upper mantel region of cooler, stronger rock which does not flow as easily as the warmer, lower mantel rock.

Plate Tectonics