Download Planetary Evolution - Lincoln-Sudbury Regional High School

Module 6:
Modelling the Formation
of the Solar System
Activity 2:
Planetary Evolution
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.
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.
(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.
• 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
Energy released by
accreting material
Deformation by big impacts
Radioactive decay
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 and the
radioactive decay of elements within the rocks also added
to the internal heat of the planet. 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 planet’s 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..
• 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
(this is not to
cooled gradually to
scale - the crust
would be
form brittle outer skins
much thinner
(crusts):
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 the Earth has a molten core, and we
see the heat released from the Earth’s 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 ...
– rocky bodies with diameters greater that 200 km
are roughly spherical:
– 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.
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.
The mantle and even the core of the planet can solidify given
enough time. In particular, smaller planets radiate their heat
much more quickly than large planets,and can cool down and
solidify relatively rapidly.
The process of planetary differentiation also
leads to outgassing, whereby internal gases
escape to the surface of the planet to form
an atmosphere and – in the case of the
Earth – oceans. The formation of the
atmospheres and the Earth’s oceans was
also aided by impacting icy comets in the
early history of the Solar System.
• Cratering
The early Solar System would have contained many
planetesimals 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, resulting
in the growth of planetesimals and protoplanets,
- other more energetic collisions would have broken
young planetesimals apart!
As we have seen, the planetesimals which managed to
grow large enough to differentiate will have 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 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. This
is because oceans cover much of the Earth’s surface, but
also because tectonic activity, weathering, extensive plant
life, and human activities such as agriculture effectively
erase the cratering
record on Earth.
Some spectacular
examples of craters
do remain,
however...
Barringer Meteor Crater, Arizona USA
Manicouagan Impact
Crater, Quebec,
Canada
• Basin Flooding
Cratering causes cracks in the planet’s crust which can
be filled up by lava (molten mantle material heated by
radioactive decay), as it wells 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 and forming the first oceans.
• Plate tectonics
Long after the planet’s crust forms, the mantle may still be
hot enough to undergo plastic flow
- that is, move in convective currents like those we see
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 can drag along
regions of crust by a few cm per year.
This is what we call plate tectonics, or continental drift,
on Earth.
• Volcanism
We have already seen that lava flows are likely to
occur if an impact cracks the planet’s thin crust while
the mantle is still molten.
As we will see when we investigate the Earth, plate
tectonics occurs when plates collide with each other,
crumpling the crust to form mountain chains and
forcing molten lava to the surface to erupt as volcanoes.
Aniachak Volcano,
Alaska USA
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 force molten lava up to the
surface over millions of years, forming huge volcanoes.
Olympus Mons,
Mars
• Weathering
Once a planet’s mantle cools enough to bring its volcanic
activity largely to an end, provided it has an atmosphere,
the planet will then largely settle down to a long period of
gradual weathering, from one or more of:
• dust storms,
• wind erosion
• water erosion (if the planet supports liquid water
and rain).
Which of these processes occur, the rate of occurrence and
to what degree, depends on the atmospheric conditions
and circulation patterns on the particular planet involved.
(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.
The ices - such as water, methane and ammonia ices are made of elements which were much more abundant
than those that went into forming the rocky planetesimals.
Therefore planets in the outer Solar System could grow
to be very large - much larger than the terrestrial
planets.
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.
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.
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.
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.
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, outer crusts and sometimes
atmospheres. The terrestrial planets 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.
Not just the 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.
We’ll return to the subject of their formation and
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.
Image Credits
Earth globe, Mercury globe, Venus globe, Mars globe, Callisto globe, Io globe,
Europa globe - NASA
http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery jupiter.html#satellites
Titan globe, Dione globe, Enceladus globe - NASA
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 - NASA
http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-asteroids.html#ida
Mathilde - NASA
http://nssdc.gsfc.nasa.gov/planetary/near_mathilde.html
Phobos and Diemos - NASA
http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-mars.html#satellites
http://nssdc.gsfc.nasa.gov/image/planetary/mars/f854a81-3.jpg
Almathea - NASA
http://nssdc.gsfc.nasa.gov/image/planetary/jupiter/amalthea.jpg
Image Credits
5 smaller satellites of Saturn - NASA
http://nssdc.gsfc.nasa.gov/image/planetary/saturn/saturn_small_satellites.jp
Proteus - NASA
http://nssdc.gsfc.nasa.gov/image/planetary/neptune/1989n1.jpg
Mercury - Mosaic of Colaris Basin & surrounding area - NASA
http://nssdc.gsfc.nasa.gov/image/planetary/mercury/caloris.jpg
Mimas - NASA
http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-saturn.html#satellites
Barringer Meteor Crater, Arizona - NASA
http://antwrp.gsfc.nasa.gov/apod/ap971117.html
Manicouagan Impact Crater, Quebec - NASA
http://cass.jsc.nasa.gov/images/sgeo/sgeo_S18.gif
Aniachak Volcano, Alaska - NASA
http://cass.jsc.nasa.gov/publications/slidesets/geology.html
Olympus Mons - NASA
http://nssdc.gsfc.nasa.gov/image/planetary/mars/olympus_mons.jpg
Image Credits
Europa - NASA
http://nssdc.gsfc.nasa.gov/image/planetary/jupiter/europa_close.jpg
Neptune - NASA
http://nssdc.gsfc.nasa.gov/image/planetary/neptune/neptune.jpg
Uranus - NASA
http://www.hawastsoc.org/solar/thumb/uranus/uranus.gif
Saturn - NASA
http://nssdc.gsfc.nasa.gov/image/planetary/saturn/saturn.jpg
Jupiter & Ganymede - NASA
http://nssdc.gsfc.nasa.gov/image/planetary/jupiter/jupiter_gany.jpg
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