Transcript Planetary formation.pps

Modelling the Formation
of the Solar System
Activity:
Planetary Evolution
Summary:
In this Activity, we will investigate
(a) the evolution of the terrestrial planets:
forming planetesimals, accretion, differentiation,
cratering, basin filling, plate tectonics, volcanism,
weathering, and
(b) the evolution of the Jovian planets
In this Activity we will look at an overview of how the
model suggests that the planetesimals grew to form the
terrestrial (“rocky”) planets in the inner Solar System,
and the Jovian (“gas giant”) planets in the outer Solar
System.
The Evolution of the Terrestrial Planets
Forming Planetesimals
As compounds began to condense out in the cooling
Solar Nebula, regions of slightly higher density would
have accumulated more of the surrounding material by
gravitational attraction.
As we have just seen, close to the Sun this material would
have consisted mostly of metal oxides, iron & nickel
compounds and silicates - the materials which form the
basis of the present day rocky (or “terrestrial”) planets and
natural satellites (“moons”) of the inner Solar System.
So small rocky lumps of accumulated material would have
formed gradually - called planetesimals.
Planetesimals started to “condense out”:
- roughly 4.6 billion years ago
Accretion
According to the model, these rocky planetesimals gradually
accreted more material, again due to gravitational attraction:
As the planetesimal grows to planetary size,
its interior heats up:
Energy released by
accreting material
The heating is due to
Deformation by big impacts
Radioactive decay
Differentiation
If the planetesimal grew to be big enough, it became hot
enough to literally “melt”! This process is called differentiation.
As gravity is directed towards the centre of a planet, the
molten (“melted”) material tried to fall inwards ...
Denser material settled
in the centre (core)
Lighter material floated
towards the surface (mantle)
… and so the planets took roughly spherical shapes, then
cooled gradually to form brittle outer skins (crusts):
(this is not to
scale - the crust
would be
much thinner
than shown here)
Denser (iron-rich)
material settles
in the centre (core)
Lighter (silicon-rich)
material rises towards
the surface (mantle)
The idea that planet-sized rocky objects can “melt” due to
their own internal energy is pretty surprising, until we
remember that geologists tell us that the Earth has a
molten core, and we see the heat released from the
Earth’s still hot crust in volcanic activity.
When
There is another particularly clear piece of evidence for this:
When we take a census of Solar System objects, we
find that ...
- rocky bodies with diameters  200 km are roughly
spherical
- whereas bodies with diameters < 130 km are usually
irregular
- which agree quite well with calculations of how
large an accreting object can become before it
differentiates.
Cratering
The early Solar System would have contained many
planetesimals and copious amounts of gas & dust
left over from the Solar Nebula.
The planets and natural satellites that we see today in
the inner Solar System only represent a fraction
of the number of planetesimals and general debris
which would initially have been present.
With all this debris around, collisions must have been
quite common:
- some would have caused more accretion, causing
the planetesimals to grow
- other more energetic collisions would have broken
young planetesimals apart.
As we have seen, the planetesimals which managed
to grow large enough to differentiate would have then
gradually cooled and formed solid, brittle crusts.
Once solid crusts formed, more impacts with debris in the
early Solar System caused extensive cratering:
Evidence of cratering:
Cratering evidence exists on all the rocky (or terrestrial)
planets, and on all the natural satellites with ancient
surfaces.
However we do not see signs of cratering on natural
satellites with active (volcanic) or icy surfaces,
and we only see limited signs of cratering on Earth
- due to volcanic activity, weathering, extensive plant life,
and human activities such as agriculture.
Some spectacular examples do remain to be seen ...
Wolf Creek Crater, Western Australia
Barringer Meteor Crater, Arizona USA
Manicouagan Impact
Crater, Quebec,
Canada
Basin Flooding
The cratering caused cracks in the planet’s crust
which could be filled up by lava (molten mantle material),
heated by radioactive decay, as it welled up through the
cracks.
If there was significant liquid water on the young planet
it was likely to be present firstly as water vapour.
As the atmosphere cooled, the water would have condensed
as rain, filling craters & forming the first oceans.
Plate tectonics
Long after the crust on a planet’s surface has formed, the
mantle may still be hot enough to undergo plastic flow
- that is, move in convective currents like those in
water heated in a saucepan on a stove.
crust
mantle
If the planet’s interior does not cool down too quickly,
the convection currents in the mantle could drag along
regions of crust by a few cms per year
- this is what we call plate tectonics, or continental drift,
on Earth.
Vulcanism
We have already seen that lava flows are likely to
occur as a result of cracking in the planet’s thin crust
due to cratering impacts, while the mantle is still molten.
Where plate tectonics occur, as we will see when we
investigate Earth, plates can collide with each other,
crumpling the crust to form mountain chains and
pushing up molten lava to erupt as volcanoes.
Aniachak Volcano,
Alaska USA
Where convection currents in the mantle do not exist
(such as Mars and Venus), local hot spots in
the mantle can still squirt molten lava up over millions of
years, forming huge volcanoes.
Olympus Mons,
Mars
Weathering
Once a planet’s mantle cooled enough to bring its volcanic
activity largely to an end, if the planet had an atmosphere,
it would then have largely settled down to a long period of
gradual weathering, from one or more of:
• dust storms,
• wind erosion, and even
• water erosion, if the planet supported liquid water
& rain.
Which of these happened, and the rate & degree, depended
on the atmospheric conditions & circulation patterns on the
particular planet involved.
The Evolution of the Jovian Planets
So far we have looked at the evolution of the rocky terrestrial - planets.
Like the terrestrial planets, the outer gas giant (Jovian)
planets - Jupiter, Saturn, Uranus and Neptune - may
have begun to form by accretion of planetesimals.
However, as we saw in the last Activity, in the outer
Solar System it was cold enough for ices to condense
out.
The ices - such as water, methane and ammonia ices are made of elements which were much more abundant
than the elements which formed the rocky planetesimals.
Therefore planetesimals in the outer Solar System
could accrete ices as well as rocky material, growing to
be much larger than the terrestrial planets.
However the giant
gas planets are selfevidently not just rock
and ice - they are
largely made up of
gases.
The average speed of gas atoms and molecules depends
on the temperature of the gas.
The temperatures in the outer Solar System, even while
it was still forming, were so low that gas atoms moved
very slowly and were easily captured.
The ice and rock cores would have accreted gas atoms
in this way, and as the growing planets became more
massive, the attractive gravitational force increased too.
The planets would have accreted more and more gas mostly hydrogen - in runaway growth until all available
nearby gas was used up.
So the four Jovian
planets are modeled
as having rock and ice
cores surrounded by a
huge hydrogen-rich
atmosphere.
In the meantime, the Sun
had become a full-grown
star at the centre of the
Solar System.
In the process, it would
have ejected bursts of its
outer material into space
at high speed, clearing out
most of the remaining gas
and dust from the Solar
Nebula and thereby
halting the further growth
of the planets.
Image Credits
NASA:
Earth globe, Mercury globe, Venus globe, Mars globe, Callisto globe, Io globe,
Europa globe
http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery jupiter.html#satellites
Titan globe, Dione globe, Enceladus globe
http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery saturn.html#satellites
Galileo 3 colour filter image of Moon
http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-moon.html
Ida & Dactyl, Gaspra
http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-asteroids.html#ida
Mathilde
http://nssdc.gsfc.nasa.gov/planetary/near_mathilde.html
Phobos and Diemos
http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-mars.html#satellites
http://nssdc.gsfc.nasa.gov/image/planetary/mars/f854a81-3.jpg
NASA:
Almathea
http://nssdc.gsfc.nasa.gov/image/planetary/jupiter/amalthea.jpg
5 smaller satellites of Saturn
http://nssdc.gsfc.nasa.gov/image/planetary/saturn/saturn_small_satellites.jpg
Proteus
http://nssdc.gsfc.nasa.gov/image/planetary/neptune/1989n1.jpg
Mercury - Mosaic of Colaris Basin & surrounding area
http://nssdc.gsfc.nasa.gov/image/planetary/mercury/caloris.jpg
Mimas
http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-saturn.html#satellites
Barringer Meteor Crater, Arizona
http://antwrp.gsfc.nasa.gov/apod/ap971117.html
Wolf Creek crater © V.L. Sharpton (used with permission
http://cass.jsc.nasa.gov/images/simp/simp_S07.gif
NASA: Manicouagan Impact Crater, Quebec
http://cass.jsc.nasa.gov/images/sgeo/sgeo_S18.gif
NASA:
Aniachak Volcano, Alaska
http://cass.jsc.nasa.gov/publications/slidesets/geology.html
Olympus Mons
http://nssdc.gsfc.nasa.gov/image/planetary/mars/olympus_mons.jpg
Europa
http://nssdc.gsfc.nasa.gov/image/planetary/jupiter/europa_close.jpg
Neptune
http://nssdc.gsfc.nasa.gov/image/planetary/neptune/neptune.jpg
Uranus
http://learn.jpl.nasa.giv/projectspacef/UR_1.jpg
Saturn
http://nssdc.gsfc.nasa.gov/image/planetary/saturn/saturn.jpg
Jupiter & Ganymede
http://nssdc.gsfc.nasa.gov/image/planetary/jupiter/jupiter_gany.jpg
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