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Modelling the Formation of the Solar System Activity: Planetary Evolution.

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1 Modelling the Formation of the Solar System Activity: Planetary Evolution

2 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

3 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.

4 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. The Evolution of the Terrestrial Planets

5 Planetesimals started to “condense out”: - roughly 4.6 billion years ago So small rocky lumps of accumulated material would have formed gradually - called planetesimals.

6 According to the model, these rocky planetesimals gradually accreted more material, again due to gravitational attraction: Accretion

7 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

8 If the planetesimal grew to be big enough, it became hot enough to literally “melt”! This process is called differentiation. Denser material settled in the centre (core) Lighter material floated towards the surface (mantle) As gravity is directed towards the centre of a planet, the molten (“melted”) material tried to fall inwards... Differentiation

9 … and so the planets took roughly spherical shapes, then cooled gradually to form brittle outer skins (crusts): Denser (iron-rich) material settles in the centre (core) Lighter (silicon-rich) material rises towards the surface (mantle) (this is not to scale - the crust would be much thinner than shown here)

10 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...

11 - rocky bodies with diameters  200 km are roughly spherical

12 - whereas bodies with diameters < 130 km are usually irregular

13 - which agree quite well with calculations of how large an accreting object can become before it differentiates.

14 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.

15 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.

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

17 Evidence of cratering:

18 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...

19 Wolf Creek Crater, Western Australia

20 Barringer Meteor Crater, Arizona USA

21 Manicouagan Impact Crater, Quebec, Canada

22 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. Basin Flooding

23 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.

24 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

25 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.

26 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. Vulcanism 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.

27 Aniachak Volcano, Alaska USA

28 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.

29 Olympus Mons, Mars

30 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: Which of these happened, and the rate & degree, depended on the atmospheric conditions & circulation patterns on the particular planet involved. dust storms, wind erosion, and even water erosion, if the planet supported liquid water & rain. Weathering

31 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. So far we have looked at the evolution of the rocky - terrestrial - planets. The Evolution of the Jovian Planets

32 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.

33 However the giant gas planets are self- evidently not just rock and ice - they are largely made up of gases.

34 The average speed of gas atoms and molecules depends on the temperature of the gas. 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. 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.

35 So the four Jovian planets are modeled as having rock and ice cores surrounded by a huge hydrogen-rich atmosphere.

36 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.

37 NASA: Earth globe, Mercury globe, Venus globe, Mars globe, Callisto globe, Io globe, Europa globe jupiter.html#satellites jupiter.html#satellites Titan globe, Dione globe, Enceladus globe saturn.html#satellites Galileo 3 colour filter image of Moon Ida & Dactyl, Gaspra Mathilde Phobos and Diemos Image Credits

38 NASA: Almathea 5 smaller satellites of Saturn Proteus Mercury - Mosaic of Colaris Basin & surrounding area Mimas Barringer Meteor Crater, Arizona Wolf Creek crater © V.L. Sharpton (used with permission NASA: Manicouagan Impact Crater, Quebec

39 NASA: Aniachak Volcano, Alaska Olympus Mons Europa Neptune Uranus Saturn Jupiter & Ganymede

40 Hit the Esc key (escape) to return to the Index Page

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