Module 6: Modelling the Formation of the Solar System

Activity 2:

Planetary Evolution

Summary

(a) the evolution of the terrestrial planets: accretion, differentiation, cratering, basin filling, plate tectonics, volcanism, weathering; and

(b) the evolution of the Jovian planets.

In this Activity, we will investigate:

We will also explore how the planets have evolved since their formation some 4.5 billion years ago.

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.

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.

(a) The Evolving Terrestrial Planets

The rocky planetesimals gradually accreted more material, again due to gravitational attraction:

• Accretion

As the planetesimal grows to planetary size, its interior heats up.

The heating is due to

Energy released by accreting materialDeformation 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..

Since gravity is directed towards the centre of a planet, the molten material tried to fall inwards ...

• Differentiation

Denser (iron-rich) material settles in the centre (core)

Lighter (silicon-rich) material rises towards

the surface (mantle)

… 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)

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 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!

Once solid crusts formed, more impacts with debris in the early Solar System caused extensive cratering:

As we have seen, the planetesimals which managed to grow large enough to differentiate will have gradually cooled and formed solid, brittle crusts.

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 crateringrecord on Earth.

Some spectacular examples of craters do remain, however...

Barringer Meteor Crater, Arizona USA

Manicouagan Impact Crater, Quebec, Canada

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.

• Basin Flooding

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.

• 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.

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.

• Volcanism

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

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:

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.

• dust storms,• wind erosion• water erosion (if the planet supports liquid water and rain).

• Weathering

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

(b) The Evolution of the Jovian Planets

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.

SummaryIn 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.

Earth globe, Mercury globe, Venus globe, Mars globe, Callisto globe, Io globe, Europa globe - NASAhttp://nssdc.gsfc.nasa.gov/photo_gallery/photogallery jupiter.html#satellites

Titan globe, Dione globe, Enceladus globe - NASAhttp://nssdc.gsfc.nasa.gov/photo_gallery/photogallery saturn.html#satellites

Galileo 3 colour filter image of Moonhttp://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-moon.html

Ida & Dactyl, Gaspra - NASAhttp://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-asteroids.html#ida

Mathilde - NASAhttp://nssdc.gsfc.nasa.gov/planetary/near_mathilde.html

Phobos and Diemos - NASAhttp://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-mars.html#satelliteshttp://nssdc.gsfc.nasa.gov/image/planetary/mars/f854a81-3.jpg

Almathea - NASAhttp://nssdc.gsfc.nasa.gov/image/planetary/jupiter/amalthea.jpg

Image Credits

5 smaller satellites of Saturn - NASAhttp://nssdc.gsfc.nasa.gov/image/planetary/saturn/saturn_small_satellites.jp

Proteus - NASAhttp://nssdc.gsfc.nasa.gov/image/planetary/neptune/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.html#satellites

Barringer Meteor Crater, Arizona - NASAhttp://antwrp.gsfc.nasa.gov/apod/ap971117.html

Manicouagan Impact Crater, Quebec - NASAhttp://cass.jsc.nasa.gov/images/sgeo/sgeo_S18.gif

Aniachak Volcano, Alaska - NASAhttp://cass.jsc.nasa.gov/publications/slidesets/geology.html

Olympus Mons - NASAhttp://nssdc.gsfc.nasa.gov/image/planetary/mars/olympus_mons.jpg

Image Credits

Europa - NASAhttp://nssdc.gsfc.nasa.gov/image/planetary/jupiter/europa_close.jpg

Neptune - NASAhttp://nssdc.gsfc.nasa.gov/image/planetary/neptune/neptune.jpg

Uranus - NASAhttp://www.hawastsoc.org/solar/thumb/uranus/uranus.gif

Saturn - NASAhttp://nssdc.gsfc.nasa.gov/image/planetary/saturn/saturn.jpg

Jupiter & Ganymede - NASAhttp://nssdc.gsfc.nasa.gov/image/planetary/jupiter/jupiter_gany.jpg

Image Credits

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