Planetary Evolution

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Module 6 Modelling the
Formation of the Solar System
Activity 2 Planetary Evolution
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Summary
In this Activity, we will investigate
(a) the evolution of the terrestrial planets
accretion, differentiation, cratering, basin
filling, plate tectonics, volcanism,
weathering and (b) the evolution of the Jovian
planets.
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Introduction
In the previous Activity we learned how
protoplanets formed in the Solar Nebula. In this
Activity we will have a look in some detail at
how they evolved to form the terrestrial
(rocky) planets in the inner Solar System, and
the Jovian (gas giant) planets of the outer
Solar System.
We will also explore how the planets have evolved
since their formation some 4.5 billion years ago.
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(a) The Evolving Terrestrial Planets

• 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 -
Mercury, Venus, Earth and Mars - and the natural
satellites (or moons) of the inner Solar System.
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• Accretion

The rocky planetesimals gradually accreted more
material, again due to gravitational attraction
As the planetesimal grows to planetary size, its
interior heats up.
The heating is due to
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The continued impact of planetesimals kept the
terrestrial protoplanets in a near molten state.
As they continued to grow in size, the rocks in
the interior of the planets were compressed due
to the increase in gravity. The radioactive decay
of elements within the rocks also added to their
internal heat. If the rate of heating due to
these three processes was faster than the rate of
cooling, then the planet would heat up.
In the first billion years of the terrestrial
planets life, its interior is hot enough to melt
iron. The dense molten iron sinks to the centre
of the planet, and the lighter materials begin to
rise towards the surface. This process is called
planetary differentiation..
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• Differentiation

Since gravity is directed towards the centre of a
planet, the molten material tried to fall
inwards ...
and so the planets took roughly spherical
shapes, then cooled gradually to form brittle
outer skins (crusts)
(this is not toscale - 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)
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The idea that planet-sized rocky objects can
melt due to their own internal energy is pretty
surprising, until we remember that the Earth has
a molten core, and we see the heat released from
the Earths still hot mantle in volcanic
activity.
There is another particularly clear piece of
evidence for this
if we take a census of Solar System objects, we
find that ...
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rocky bodies with diameters greater that 200 km
are roughly spherical
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whereas bodies with diameters less than 130 km
are usually irregular
which agrees quite well with calculations
of how large an accreting object can become
before it differentiates.
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Once the planet has differentiated, the interior
then gradually cools (though radioactive decay
still acts as an internal heating source in the
terrestrial planets) and the upper crust
solidifies.
In some planets, the mantle and even the core
slowly solidifies. The smaller the planet, the
quicker it can radiate its internal heat, cool
down and finally solidify.
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• Cratering

The early Solar System would have contained
manyplanetesimals 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
beenquite common - some would have caused more
accretion, resulting in the growth of
planetesimals and protoplanets, - other more
energetic collisions would have broken young
planetesimals apart!
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As we have seen, the planetesimals which managed
to grow large enough to differentiate will 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
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Evidence of cratering
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Cratering evidence exists on all the 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, the oceans
covering much of the surface, and human
activities such as agriculture.
Some spectacular examples of craters do however
remain...

• Barringer Meteor Crater, Arizona USA

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• Manicouagan Impact Crater, Quebec, Canada

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• Basin Flooding

The cratering caused cracks in the planets
crustwhich could be filled up by lava (molten
mantle material), heated by radioactive decay,
as it welled up through the cracks.
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If there was significant liquid water on the
young planetit was likely to be present firstly
as water vapour.
As the atmosphere cooled, the water would have
condensedas rain, filling craters and forming
the first oceans.
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• Plate tectonics

Long after the crust on a planets 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
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If the planets interior does not cool down too
quickly,the convection currents in the mantle
could drag along regions of crust by a few cm
per year.
This is what we call plate tectonics, or
continental drift, on Earth.
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• Volcanism

• We have already seen that lava flows are likely
  to occur as a result of cracking in the planets
  thin crust due to cratering impacts, while the
  mantle is still molten.

Where plate tectonics occur, as we will see when
weinvestigate the Earth, plates can collide with
each other,crumpling the crust to form mountain
chains and pushing up molten lava to erupt as
volcanoes.
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• Aniachak Volcano, Alaska USA

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• Where convection currents in the mantle do not
  exist(such as Mars and Venus - see later
  Modules), local hot spots in the mantle can still
  squirt molten lava up over millions of years,
  forming huge volcanoes.

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Olympus Mons,Mars
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• Weathering

Once a planets 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
• water erosion (if the planet supported liquid
  water and rain).

Which of these happen, and the rate degree,
depended on the atmospheric conditions
circulation patterns on theparticular planet
involved.
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(b) The Evolution of the Jovian Planets

• Like the terrestrial planets, the outer gas giant
  planets - the Jovians - Jupiter, Saturn, Uranus
  and Neptune were initially formed by the
  accretion of planetesimals.

However, the outer Solar System was cold enough
for ices to condense out.
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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 planets in the outer Solar System could
  grow to be very large - much larger than the
  terrestrial planets.

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• However, as can be easily seen, the giant gas
  planets are not just rock and ice they are
  largely made up of gases.

Once the mass of the rocky and icy protoplanetary
cores reached about ten Earth masses, they had
enough gravity to begin attracting the
surrounding nebula gas, and a gaseous envelope
began to form around the cores.
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The average speed of gas atoms and molecules
depends on the temperature of the gas. In the
outer Solar System, even while it was still
forming, the temperatures were so low that gas
atoms moved very slowly and were easily captured.
Both the core and envelope of the giant planets
continued to grow, and as they became more
massive, so too did their attractive
gravitational force. At some stage, the envelope
mass began to increase more rapidly than the core
mass and a runaway accretion process
followed. The atmospheres of the giant planets
accumulated very rapidly, attracting more and
more gas mostly hydrogen from the surrounding
Solar Nebula.
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• In the meantime, the Sun had become a full-grown
  star at the centre of the Solar System.

After about a million years the solar wind began
to blow, clearing out most of the remaining gas
and dust from the Solar Nebula and thereby
halting any further growth of the planets.
This means that both the cores and atmospheres of
the giant planets must have formed within that
time.
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It seems likely that more massive Jupiter and
Saturn attracted much of the gas in the outer
Solar Nebula, leaving less material available to
go into producing the atmospheres of the smaller
giants Uranus and Neptune.

• So the four Jovian planets are modelled as having
  rock and ice cores surrounded by huge
  hydrogen-rich atmospheres.

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Summary
In this Activity we have investigated how
protoplanets become planets. The internal heat of
the rocky terrestrial protoplanets leads to
differentiation, resulting in stratified planets
with iron cores, silicate mantles and upper
crusts, and sometimes atmospheres. The
terrestrial can continue to evolve due to
cratering, volcanism and weathering. The cores of
the giant planets are much larger, being built up
of the icy materials from the outer Solar System.
They become so massive that they accumulate the
surrounding gas of the Solar Nebula, resulting in
giant gaseous atmospheres.
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• Not just planets (and natural satellites) were
  formed from the Solar Nebula. We have yet to see
  how the model explains the existence of Pluto,
  the asteroids, comets and other debris in the
  Solar System.

Well return to the subject of their evolution,
but first we will spend several Modules studying
the planets in more detail, both for their own
intrinsic interest and to see what evidence they
provide for the model we have been outlining in
this Module.
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Image Credits
Earth globe, Mercury globe, Venus globe, Mars
globe, Callisto globe, Io globe, Europa globe -
NASAhttp//nssdc.gsfc.nasa.gov/photo_gallery/phot
ogallery jupiter.htmlsatellites Titan globe,
Dione globe, Enceladus globe - NASAhttp//nssdc.g
sfc.nasa.gov/photo_gallery/photogallery
saturn.htmlsatellites Galileo 3 colour filter
image of Moonhttp//nssdc.gsfc.nasa.gov/photo_gal
lery/photogallery-moon.html Ida Dactyl, Gaspra
- NASAhttp//nssdc.gsfc.nasa.gov/photo_gallery/ph
otogallery-asteroids.htmlida Mathilde -
NASAhttp//nssdc.gsfc.nasa.gov/planetary/near_mat
hilde.html Phobos and Diemos - NASAhttp//nssdc.g
sfc.nasa.gov/photo_gallery/photogallery-mars.html
satelliteshttp//nssdc.gsfc.nasa.gov/image/planet
ary/mars/f854a81-3.jpg Almathea -
NASAhttp//nssdc.gsfc.nasa.gov/image/planetary/ju
piter/amalthea.jpg
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Image Credits
5 smaller satellites of Saturn -
NASAhttp//nssdc.gsfc.nasa.gov/image/planetary/sa
turn/saturn_small_satellites.jp Proteus -
NASAhttp//nssdc.gsfc.nasa.gov/image/planetary/ne
ptune/1989n1.jpg Mercury - Mosaic of Colaris
Basin surrounding area - NASAhttp//nssdc.gsfc.
nasa.gov/image/planetary/mercury/caloris.jpg Mimas
- NASA http//nssdc.gsfc.nasa.gov/photo_gallery/
photogallery-saturn.htmlsatellites Barringer
Meteor Crater, Arizona - NASAhttp//antwrp.gsfc.n
asa.gov/apod/ap971117.html Manicouagan Impact
Crater, Quebec - NASAhttp//cass.jsc.nasa.gov/ima
ges/sgeo/sgeo_S18.gif Aniachak Volcano, Alaska -
NASAhttp//cass.jsc.nasa.gov/publications/slidese
ts/geology.html Olympus Mons - NASAhttp//nssdc.g
sfc.nasa.gov/image/planetary/mars/olympus_mons.jpg
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Image Credits
Europa - NASAhttp//nssdc.gsfc.nasa.gov/image/pla
netary/jupiter/europa_close.jpg Neptune -
NASAhttp//nssdc.gsfc.nasa.gov/image/planetary/ne
ptune/neptune.jpg Uranus - NASAhttp//www.hawasts
oc.org/solar/thumb/uranus/uranus.gif Saturn -
NASAhttp//nssdc.gsfc.nasa.gov/image/planetary/sa
turn/saturn.jpg Jupiter Ganymede -
NASAhttp//nssdc.gsfc.nasa.gov/image/planetary/ju
piter/jupiter_gany.jpg
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• Now return to the Module home page, and read more
  about models of the evolution of planets in the
  Textbook Readings.

Hit the Esc key (escape) to return to the Module
6 Home Page