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H205 Cosmic Origins. Today: Forming the Solar System Finish EP6. APOD. The Origin of the Solar System. All objects in the Solar System seem to have formed at nearly the same time, out of the same original cloud of gas and dust

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H205 Cosmic Origins

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H205 cosmic origins

H205Cosmic Origins

  • Today: Forming the Solar System

  • Finish EP6


The origin of the solar system

The Origin of the Solar System

  • All objects in the Solar System seem to have formed at nearly the same time, out of the same original cloud of gas and dust

  • Radioactive dating of rocks from the Earth, Moon, and some asteroids suggests an age of about 4.5 billion yrs

  • A similar age is found for the Sun based on current observations and nuclear reaction rates

A theory of the solar system s formation must account for what we observe

A theory of the Solar System’s formation must account for what we observe:

  • Planets orbit in the same direction and in the same plane

  • Two families of planets: terrestrial & Jovian

  • Compositions of planets

  • Ages 4.5 billion years (or less)

  • Other details – structure of asteroids, cratering of planetary surfaces, detailed chemical composition of surface rocks and atmospheres, etc.

The solar nebula hypothesis

The Solar Nebula Hypothesis

  • Derived from 18th century ideas of Laplace and Kant

  • Proposes that Solar System evolved from a rotating, flattened disk of gas and dust (an interstellar cloud), the outer part of the disk becoming the planets and the inner part becoming the Sun

The solar nebula hypothesis1

Explains the Solar System’s flatness and the common direction of motion of the planets around the Sun

Interstellar clouds are common between the stars in our galaxy and this suggests that most stars may have planets around them

The Solar Nebula Hypothesis

Interstellar clouds

Interstellar Clouds

  • The cloud that formed Solar System was probably a few light years in diameter and 2 solar masses

  • Typical clouds are 71% hydrogen, 27% helium, and traces of the other elements

Interstellar dust

Interstellar Dust

  • Clouds also contain tiny dust particles called interstellar grains

    • Grain size from large molecules to a few micrometers

    • They are a mixture of silicates, iron and carbon compounds, and water ice

Dust grains are very small - about 4000 could fit across a sucker stick

In the beginning

In the Beginning…

  • Triggered by a collision with another cloud or a nearby exploding star, rotation forces clouds to gravitationally collapse into a rotating disk

The solar nebula

The Solar Nebula

  • Over a few million years the cloud collapses into a rotating disk with a bulge in the center

  • This disk, about 200 AU across and 10 AU thick, is called the solar nebula

  • The bulge becomes the Sun and the disk condenses into planets

Temperatures in the solar nebula

Temperatures in the Solar Nebula

  • Before the planets formed, the inner part of the disk was hot, heated by gas falling onto the disk and a young Sun – the outer disk was colder than the freezing point of water

Astronomers have observed many gas/dust disks where planets may be forming


Condensationoccurs when gas cools below a critical temperature at a given gas pressure

Gas molecules bind together to form liquid or solid particles

On Earth, water vapor condenses to form clouds

Steam condenses on the bathroom mirror


Condensation in the solar nebula

Condensation in the Solar Nebula

  • Iron vapor condenses at 1300 K, silicates condense at 1200 K, and water vapor condenses at room temperature

  • In a mixture of gases, materials with the highest vaporization temperature condense first

  • The Sun kept the inner solar nebula (out to almost Jupiter’s orbit) too hot for anything but iron and silicate materials to condense

  • The outer solar nebula cold enough for ice to condense

The formation of the planets

The Formation of the Planets

Grains stick together

Grains stick together

  • Next step is for the tiny particles to stick together, perhaps by electrical forces, into bigger pieces in a process called accretion

  • As long as collisions are not too violent, accretion leads to objects, called planetesimals, ranging in size from millimeters to kilometers



  • Planetesimals in the inner solar nebula were rocky-iron composites, while planetesimals in the outer solar nebula were icy-rocky-iron composites

  • Planets formed from “gentle” collisions of the planetesimals, which dominated over more violent shattering collisions

Formation of the planets

Formation of the Planets

  • Simulations show that planetesimal collisions gradually lead to approximately circular planetary orbits

  • As planetesimals grew in size and mass their increased gravitational attraction helped them grow faster into clumps and rings surrounding the Sun

Formation of the planets1

Formation of the Planets

  • Planet growth was especially fast in the outer solar nebula due to:

    • Larger volume of material to draw upon

    • Larger objects (bigger than Earth) could start gravitationally capturing gases like H and He

Craters everywhere

Craters Everywhere!

  • Continued planetesimal bombardment and internal radioactivity melted the planets and led to the density differentiation of planetary interiors

Formation of moons

Formation of Moons

  • Moons of the outer planets were probably formed from planetesimals orbiting the growing planets

  • Not large enough to capture H or He, the outer moons are mainly rock and ice giving them solid surfaces

H205 cosmic origins

Where did OUR Moon come from???

Lunar Formation Before Apollo

  • Three hypotheses for the Moon’s origin:

    • Co-Accretion Theory

    • Capture Theory

    • Fission Theory

The Spongmonkies: We Like The Moon

The earth moon system

The Earth-Moon System

It’s the little dot inside!

  • The Moon’s diameter is ¼ of the Earth’s

  • The Moon’s surface is very dark, like a charcoal briquet

  • The Moon’s mass is 1/80th of the Earth’s

  • The Moon has no atmosphere

Something s different

Something’s Different…

  • Unlike most of the other moons in the solar system, the Moon is very large relative to its central planet

  • These oddities indicate that the Moon formed differently from the other solar system moons!

Earth and moon by richard swarts

Earth and Moon by Richard Swarts

Co accretion theory

Co-Accretion Theory

  • Earth and Moon were twins, forming side by side from a common cloud of gas and dust

Capture theory

Capture Theory

  • Moon was originally a small planet orbiting the Sun and was subsequently captured by Earth’s gravity during a close approach

Fission theory

Fission Theory

  • The Moon spun out of a very fast rotating Earth in the early days of the Solar System

Moon rocks

One small step for man, one giant leap for mankind

Moon Rocks!

Hypotheses failed

Hypotheses Failed!

  • Each of these hypotheses gave different predictions about Moon’s composition:

    • In capture theory, the Moon and Earth would be very different in composition, while twin theory would require they have the same composition

    • In fission theory, the Moon’s composition should be close to the Earth’s crust

  • Moon rock samples proved surprising

    • For some elements, the composition was the same, but for others, it was very different

    • None of the three hypotheses could explain these observations

So where did the moon come from

So Where Did the Moon Come From?

  • Did the Earth and Moon form together as a binary system?

    • NO…

  • Did the Earth capture the Moon after both were already formed?

    • NO…

  • Did the early Earth break apart into the Earth-Moon system?

    • NO…

  • What do the Moon rocks tell us?

    • The Moon was formed from the Earth’s crust

What is the earth made of

What Is the Earth Made of?

  • The Earth is made of many chemical elements

  • Iron, Silicon, Magnesium, Oxygen, Carbon

  • And water!

Earth rocks contain water!

What s inside the earth

What’s Inside the Earth?

  • The chemical elements on Earth are separated (Earth is differentiated!)

  • Iron and nickel in the core

  • Lighter elements in the mantle and crust

What about the moon

What about the MOON?

  • What is it made of?

  • Where did it come from?

It’s not green cheese!

What s inside the moon

What’s Inside the Moon?

Maybe there is a tiny iron core inside

  • The Moon has a TINY iron core

  • The Moon does not appear to be as differentiated as the Earth

What have we learned from moon rocks

What Have We Learned from Moon Rocks?

  • The composition of moon rocks is very similar to the composition of the Earth’s crust

But, unlike the Earth, the Moon has very little iron

And Moon rocks are dry!

Giant impact theory

Giant Impact Theory

  • Does it work?

  • It sounds like science fiction!

The large impact hypothesis

The Large Impact Hypothesis

  • Moon formed from debris blasted out of the Earth by the impact of a Mars-sized body

  • Age of lunar rocks and lack of impact site on Earth suggests collision occurred at least 4.5 billion years ago

Impact theory

Impact Theory

  • A large body, about the size of Mars impacts the early Earth

Told through the art of William Hartmann

H205 cosmic origins

  • The collision blasts material from the Earth’s crust into space

H205 cosmic origins

The material forms a disk around the Earth

H205 cosmic origins

  • Material in the disk coalesces to form the Moon

H205 cosmic origins

  • The early Moon is hot from the heat of accretion

The large impact solution

The Large Impact Solution

  • This “large impact” idea explains:

    • The impact would vaporize low-melting-point materials (e.g., water) and disperse them explaining their lack in the Moon

    • Only surface rock blasted out of Earth leaving Earth’s core intact and little iron in the Moon

    • Easily explains composition similarities and differences with Earth

    • The splashed-out rocks that would make the Moon would more naturally lie near the ecliptic than the Earth’s equatorial plane – explains why the plane of the Moon’s orbit is not the Earth’s equatorial plane

    • Explains Earth’s tilted rotation axis

What happened to the moon s iron

What Happened to the Moon’s Iron?

The Earth gobbled it!

Forming the moon we see today

Forming the Moon We See Today

  • As Moon’s surface solidified, stray fragments from original collision created craters that blanket highlands

  • A few of the larger fragments created the large basins for the maria to form

  • By the time the maria filled with molten material and solidified, little material was left for further lunar bombardment – thus the smooth nature of the maria

Highlands and mare

Highlands and Mare

The early moon

The Early Moon

  • When the Moon formed it was much closer to the Earth

  • The Moon is moving away from the Earth at about 2 inches per year

  • The day and the month were originally much shorter than they are today

  • Tides were originally much greater

Leftovers of the solar system

Leftovers of the Solar System

  • Asteroids and comets are remnants of the formation of the Solar System

    • Some may be planetesimals

    • Best source of information about the Solar System’s early years

  • Asteroids and comets play a central role in planetary impact and in particular can have a large influence on Earth’s biological life



  • Asteroids are small, generally rocky bodies that orbit Sun

  • Most asteroids (thousands) lie in the asteroid belt, a region between the orbits of Mars and Jupiter

  • The combined mass of all the asteroids is probably less than 1/1000 the mass of the Earth

Asteroid Gaspara, image from Galileo spacecraft

The asteroid belt

The Asteroid Belt

Size and shape of asteroids

Size and Shape of Asteroids

  • Asteroids are small, so their sizes are best determined from infrared measurements: bigger bodies emit more IR than smaller ones at the same temperature

  • Asteroids range in size from 1000 km across (Ceres) down to kilometer-sized objects and even smaller

Asteroid Ceres from Hubble images

Asteroid composition

Asteroid Itokawa was visited by a Japanese spacecraft in 2005. The spacecraft will return to Earth in 2010.

Asteroid Composition

  • Reflection spectra show that asteroids belong to three main compositional groups: carbonaceous bodies, silicate bodies, and metallic iron-nickel bodies

  • Inner-belt asteroids tend to be silicate-rich and outer-belt asteroids tend to be carbon-rich

  • Some asteroids are loose lumps of material held together by gravity

Origin of the asteroids

Origin of the Asteroids

Asteroid belt structure

Regions of the asteroid belt seemingly empty of asteroids are called Kirkwood Gaps

The gaps are caused by the same resonance process that causes the gaps in Saturn’s rings

Trojan asteroids are two loose swarms located along Jupiter’s orbit, 60° ahead and 60° behind

Asteroid Belt Structure

Composition of comets

Composition of Comets

  • Spectra of coma and tail show comets are rich in water, CO2, CO

  • Solar UV radiation dissociates H2O, creating a large hydrogen cloud around the comet

  • After many close solar passes, comets’ gas is exhausted

  • Density ~ 0.2 g/cm3 - comets are “fluffy,” not compacted ices

Origin of comets


  • Most comets come from the Oort Cloud, the spherical shell of trillions of icy bodies believed to lie far beyond Pluto’s orbit to a distance of about 150,000 AU

The oort cloud the kuiper belt

The Oort Cloud & the Kuiper Belt

  • Comets originally orbited among the giant planets as planetesimals, then were tossed into the Oort cloud by those planets

  • The shape of the Oort cloud is determined from observations of comet orbits

    • Some comet orbits seem to come from a flatter, less remote region – the Kuiper belt, which extends from Neptune’s orbit out to some unknown distance – may contain much more mass than the asteroid belt

  • Comets in the Oort cloud are a frigid 3 K and only warm up enough to emit gas when they enter the inner Solar System

Late bombardment the kuiper belt and the oort cloud

Late Bombardment, the Kuiper Belt, and the Oort Cloud

  • Evidence suggests that the Solar System was bombarded with impacts about 3.8 million years ago

  • Probably resulted from dynamical rearrangement of the Solar System due to the interaction of the giant planets and the comet swarm

This model explains

This model explains:

  • giant planet orbits

  • the Trojan Asteroids (both for Jupiter and Neptune)

  • the Kuiper belt

  • the giant planet irregular satellites

  • the “Late Heavy Bombardment”


Asteroids around other stars

Asteroids around Other Stars

  • Dust comes from collisions of asteroids

  • We can detect dust with Spitzer

  • Observed 24 micron IR excess due to dust

  • Dust implies asteroids and planets

For next week

For Next Week

Extrasolar Planets

Origin of Elements

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