Planetary Geology: Early Solar System History

Early Solar System History

• Origin & Composition of the Universe

• Birth & Life Cycle of Stars

• Formation of Solar/Planetary Systems

• Distribution of rocky vs. gas/ice bodies

Main Concepts

The Big BangAt the Big Bang, all matter and energy in the universe was compressed into an extremely small space, a singularity.

As soon as it existed, that point exploded (the ‘bang’) and released all of the matter/energy and began to expand.

(singularity)

(time)

After ~380 thousand years the first atoms begin to form (mostly hydrogen).

After a few minutes, the first deuterium and helium nuclei form.

After 10-6 seconds, neutrons and protons start to form.

The Big BangThe universe is 13.82 billion years old.

The oldest galaxy may have formed 600 million years after the Big Bang.

The universe is expanding (and will continue to expand?)

We can measure the changes in the intensity of the radiation to get a better understanding of what the universe looked like billions of years ago.

The intensity of the radiation is proportional to the density of the place from where it originated.

Radiation from the Big Bang is traveling through space, but it is not uniform.

The universe is made of:

- 4.9% Matter (Atoms)

- 26.8% Dark Matter

- 68.3% Dark Energy

Wilkinson Microwave Anisotropy Probe (WMAP) & Planck mission measured the Cosmic Microwave Background (CMB).

The Big Bang

These are maps of microwave radiation acquired by WMAP & Planck.

They provide a picture of the universe ~380 thousand years after the Big Bang.

It is observing the thermal radiation left over from the Big Bang (Cosmic Microwave Background).

WMAP estimate of age is 13.73 billion years, newer Planck estimate is 13.82 billion years.

The Big BangWMAP (NASA)

Planck mission (ESA)

The Universe Has Many Galaxies

Looking into the PastThe Hubble Space Telescope has several instruments that can measure infrared light.

Coincidence?

Hubble Telescope

The next generation space telescope will be the James Webb Space Telescope, launching in 2018.

Light from very old, distant objects (such as other galaxies) has been shifted to longer wavelengths due to expansion of the Universe, and Hubble can measure this infrared light.

Is the Expansion of the Universe Speeding Up?

Home Sweet Home: The Milky Way Galaxy

http://home.arcor-online.de/axel.mellinger/

This is a mosaic of how our galaxy looks from Earth, providing a full panoramic view of the Milky Way.

For more information on how this image was made:

Dark regions aren’t starless, they are regions of dense molecular clouds.

Solar System Formation: The Nebular HypothesisMaximum rate of star formation was ~8-10 billion years ago; the rate has slowed since then.

How exactly do stars form??

Eagle Nebula

Galaxies contain interstellar medium (ISM), which is composed of gas and dust.

The cloud breaks up and forms fragments as it collapses, and these fragments condense to form protostars.

If the pressure of the gas can’t balance the forces of gravity, then the cloud experiences gravitational collapse.

The nebula, or molecular clouds, contain mostly H2.

High density regions of the interstellar medium form nebulae, and this is where stars are ‘born’. However, these regions are cold (10-20K)!

Once a ‘clump’ has formed by fragmentation of the molecular cloud, it has its own gravity and can become a protostar.

The mass of the star is now fixed and it can be considered a ‘young’ star.

Fusion begins in the core of the protostar after several million years,then a stellar wind is created and this stellar wind stops new mass from falling into the star.

As the cloud is collapsing to form a star, it begins to spin to conserve angular momentum.The spinning and collapsing of the cloud flattens it into a rotating disk, the accretionary disk.

Solar System Formation: The Nebular Hypothesis

Life of a Star: Hertzsprung-Russell DiagramThe Hertzsprung-Russell diagram (or HRD) plots the luminosity of a star versus its temperature.

The groups and trends observed in this type of diagram were a major step forward in understanding the evolution of stars (that is, the life cycle of a star).

There are distinct classes or groups of stars and there are clear trendlines.

When many stars are plotted in this way we start to see a pattern emerge.

Our Sunis in here.

Life of a Star: Hertzsprung-Russell Diagram

Life of a Star

Solar System Formation: Origin of ElementsStellar evolution provides a process by which we can explain the creation of all naturally occurring elements in the Universe.

Cosmic Abundance of Elements

Fusion immediately after the Big Bang was able to produce light elements: H, He, Li, Be, and B.

Slightly heavier elements, from C to Fe, are formed by fusion reactions in the cores of stars.

However, fusion to create elements greater than a mass of 60 uses up more energy than is produced by the reaction, so these reactions can’t be sources of fuel for stars, and we don’t see elements heavier than Fe produced in stars.

Solar System Formation: Origin of Elements

In this process, elements gain neutrons (increasing their mass) to form isotopes of the original element. Many of these new isotopes are unstable, and they experience radioactive decay.This radioactive decay forms new heavy elements with higher atomic numbers.

Slow neutron capture works as long as the decay time is longer than the neutron capture time. This can lead to formation of elements up to Bismuth (atomic # 83).

Elements heavier than Fe require neutron capture to form.

Rapid neutron capture occurs in an environment that is very dense with neutrons....so dense that the unstable isotopes don’t have time to decay. This type of environment is only found in a supernova explosion.

2 methods

Solar System Formation: The Nebular HypothesisThe rotating disk surrounding a protostar consists of:

- gases (primarily H and He)- heavier elements (Si, Fe, C, Ni, etc.....)- ices (water, methane, ammonia; covers dust particles)

Some regions become more concentrated as materials collide and stick together. This forms planetesimals.

This process is often referred to as planetary accretion.

Planetesimals eventually become large enough to exert gravitational pull on each other. They begin to collide and grow rapidly in size. They can then become protoplanets.

The rotating disk surrounding a protostar consists of:

- gases (primarily H and He)- heavier elements (Si, Fe, C, Ni, etc.....)- ices (water, methane, ammonia; covers dust particles)

Some regions become more concentrated as materials collide and stick together. This forms planetesimals.

This process is often referred to as planetary accretion.

Planetesimals eventually become large enough to exert gravitational pull on each other. They begin to collide and grow rapidly in size. They can then become protoplanets.

Now things get interesting.....how do we get terrestrial

planets, gas giants, and icy planets/moons?

Solar System Formation: The Nebular Hypothesis

The inner parts of the disk are warmer.Light, volatile materials like ice and gases are not as stable as heavier elements that form ‘rocky’ materials.‘Rocky’ material dominates this region.

Eventually the Sun ‘turned on’ (fusion started) and blew much of the gas out of the Solar System. Planets beyond Saturn were growing slower and were not able to become larger after this point.

Water ice is made of H and O, which are quite abundant in the disk. When these elements are stable as ice there is a lot of material to make a core. The rock/ice cores of Jupiter and Saturn grew quickly; as they grew their gravity increased and they were able to hold onto a lot of gas to grow even larger.

Farther from the protostar, the temperatures are cold enough that ices are stable (called the ‘frost line’; occurs at about 2.7 AU for our Solar System). Objects beyond this point can be rock and ice.

Solar System Formation: The Nebular Hypothesis

Solar System Formation: The Nebular Hypothesis

The mass of disks ranges from 0.001 to 0.1 times the mass of our Sun.

Some disks are much, much larger than our Solar System (over 1000 AU!).

More recently, astronomers have discovered numerous planets around other stars (often referred to as ‘exoplanets’).

- Many are as massive or more massive than Jupiter

- Some have orbits closer to their star than Mercury is to our Sun

- Orbits can be highly elliptical

Solar System Formation: The Nebular Hypothesis

The Solar System ‘Frost Line’

The interior part of the accretion disk is hot and hydrogen remains vaporized, but rock and metal can condense to form terrestrial planets.

In contrast, the outer portion is cooler and ices can begin to form, leading to the rock/ice cores of gas giants and to the formation of icy moons.

Meteorites and the Early Solar System

This has been determined by age-dating meteorites, which we believe are representative of the earliest period in the Solar System.

Some meteorites are very primitive, meaning they formed very early in the Solar System and contain material that tells us a lot about the conditions and chemistry of interstellar space.

A specific group of meteorites, known as the carbonaceous chondrites, have a chemical composition that is nearly identical to the photosphere of the Sun.

These meteorites also contain Calcium-Aluminum Inclusions (CAIs), and these CAIs are the oldest known material in our Solar System.

Allende Meteorite Round ‘clasts’are chondrules

Our Solar System is 4.56 Gyr (Gyr = billion years old).

Sol

ar p

hoto

sphe

re

(ato

ms

Si =

106

)

CI carbonaceous chondrite (atoms Si = 106)

The Sun’s photosphere emits radiation, and we can measure this radiation using spectroscopic techniques.

When we plot the chemistry of the carbonaceous chondrite meteorites against the composition of the solar photosphere we see that they are very similar.

Therefore, these meteorites are believed to be representative of the chemical composition of the original material from which the Solar System formed.

Meteorites and the Early Solar System

The Cosmochemist’s Periodic Table

Behavior of Elements in Planetary Bodies

https://pubs.usgs.gov/fs/2002/fs087-02/

‘Godfather’ of geochemistry Viktor Goldschmidt classified elements by their behavior.Lithophile elements combine easily with oxygen and stay near surface in crust/mantle.Siderophile elements are high density and dissolve easily in iron, thus sink to the core.

‘Rarest’ or most depleted elements in the crust are not the heaviest ones, but rather the ones that are siderophile.

Lithophile

Siderophile

Planetary Composition & Differentiation

The planets in our Solar System exhibit a dramatic change from rocky planets with thin or absent atmospheres (terrestrial planets) to large planets dominated by gas and liquid (gas giants).

This is clear when comparing the average density of the planets.

The terrestrial planets have a higher bulk density because of metallic cores (Fe, Ni, S) overlain by silicate mantle and crust.

In contrast, the gas giants have rather small silicate/iron cores that are overlain by much lighter elements (liquid and gaseous H).

Planetary Composition & DifferentiationAs planets undergo accretion, the heavy elements (Fe, Ni, S) will tend to sink to the center to form a metallic core.

This is called planetary differentiation, and there are still many unanswered questions about the specifics of how this process occurred.

The lighter elements (Mg, Ca, Al, etc.) will remain outside the center to form the mantle. The outermost layer of the mantle is exposed to the atmosphere/space and will cool rapidly to form the initial crust.