H205 Cosmic Origins

Today: Forming the Solar System

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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:

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

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

Hypothesis 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

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

Clouds also contain tiny dust particles called interstellar grainsGrain 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…

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

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 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

Condensation

Condensation occurs 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

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

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 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 PlanetsSimulations 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 Planets

Planet growth was especially fast in the outer solar nebula due to:Larger volume of

material to draw uponLarger objects (bigger

than Earth) could start gravitationally capturing gases like H and He

Craters Everywhere!Continued

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

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

Where did OUR Moon come from???

The Spongmonkies: We Like The Moon

Lunar Formation Before ApolloThree hypotheses for the Moon’s

origin:Co-Accretion TheoryCapture TheoryFission Theory

The Earth-Moon System

The Moon’s diameter is ¼ of the Earth’sThe Moon’s surface is very dark, like a

charcoal briquetThe Moon’s mass is 1/80th of the Earth’sThe Moon has no atmosphere

It’s the little dot inside!

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

Co-Accretion Theory

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

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

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

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 surprisingFor some elements, the composition was the

same, but for others, it was very differentNone of the three hypotheses could explain

these observations

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?

The Earth is made of many chemical elements

Iron, Silicon, Magnesium, Oxygen, Carbon

And water!Earth rocks

contain water!

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?

It’s not green

cheese!

What is it made of?

Where did it come from?

What’s Inside the Moon?

The Moon has a TINY iron core

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

Maybe there is a tiny iron core

inside

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

Does it work?It sounds like

science fiction!

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

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

Told through the art of William Hartmann

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

The material forms a disk around the Earth

Material in the disk coalesces to form the Moon

The early Moon is hot from the heat of accretion

The Large Impact SolutionThis “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?

The Earth

gobbled it!

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

The Early Moon

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

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

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

Tides were originally much greaterTides were originally much greater

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

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

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 Compositi

on

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

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

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

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 CometsMost 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

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

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:

giant planet orbits the Trojan Asteroids (both for Jupiter and

Neptune) the Kuiper belt the giant planet irregular satellites the “Late Heavy Bombardment”

http://www.psrd.hawaii.edu/Aug06/cataclysmDynamics.html

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 Extrasolar Planets

Origin of Elements