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The Evolving Earth Activity: Modelling the Earth’s Evolution.

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1 The Evolving Earth Activity: Modelling the Earth’s Evolution

2 Summary: In this Activity, we will investigate (a) the evolution of the Earth’s interior and surface, (b) atmospheric evolution - the Earth’s primeval atmosphere - theories on the origin of water on Earth - the origin of the Earth’s atmospheric oxygen

3 In the Activities on Modelling the Formation of the Solar System, we discussed the basic steps in the evolution of a planet’s interior and its surface, according to the conventional model. In this Activity we will investigate what this model has to say about the Earth’s evolution.

4 The Evolution of the Earth’s interior and surface Condensation Accretion Differentiation Cratering Basin Flooding, Vulcanism, Plate Tectonics Weathering In the Activities on Modelling the Formation of the Solar System, we modelled the basic steps of planetary evolution as: Let’s now briefly investigate if these steps are consistent with the properties and dynamics of the Earth’s interior and surface.

5 The Earth is a terrestrial - that is, rocky - planet, made up of the sort of materials which the model claims were present in the early Solar System - for example, metal oxides & silicates, and iron oxide Temperature ( o K) Distance from Sun (AU) Iron oxide metal oxides, silicates

6 - in good agreement with the modelled processes of condensation and accretion.

7 As we saw in the Activity on The Earth as a Model Planet, the Earth’s interior is largely separated out into layers of increasing density as one approaches the core - as would be expected if the early Earth underwent differentiation, as predicted... crust mantle liquid outer core solid inner core

8 … and although evidence has largely been wiped out on Earth by vulcanism, weathering & biological effects (including land cultivation and settlement), some spectacular evidence for cratering still remains:

9 Basin flooding, vulcanism and plate tectonics still continue on Earth...

10 as of course, does weathering. NASA: Dust Storm, Red Sea and Saudi Arabia

11 It’s hardly surprising that there is plenty of evidence on Earth to support our model for the basic steps of planetary evolution. After all, our knowledge of Earth has formed the basis of the model! Our knowledge of Earth’s evolution can be used for comparison when the characteristics of other terrestrial planets are investigated.

12 To summarize the evolutionary steps from our model and how they apply to Earth:

13 Checklist for Earth: Steps in Planetary Evolution: Earth Condensation  Accretion  Differentiation  Cratering  Basin Flooding  (Vulcanism) Plate tectonics  Weathering  (Slow decline)

14 So far we have discussed the evolution of the Earth’s surface and interior only. Atmospheric Evolution The Earth’s atmosphere has evolved too - and studying its continuing evolution is of vital importance to us, and of great use when trying to model the atmospheric evolution of other planets.

15 The Earth’s Primeval Atmosphere the newly-formed Earth initial atmosphere of hydrogen & helium As the Earth differentiated and formed its crust, its gravity will have attracted nearby gases such as hydrogen and helium which dominated the Solar Nebula. However hydrogen and helium are light enough to escape the Earth’s atmosphere even at today’s temperatures - in the hot early inner Solar System they would have easily escaped the pull of Earth’s gravity. (Click here to see how mass, speed and temperature are related)Click here - easily escaped Earth’s gravity

16 This transitory atmosphere of hydrogen and helium will have been largely “blown away” by the strong solar wind of the newly-emerging Sun.

17 Hot, primeval Earth after differentiation Volcanic eruptions release gases..or is this mostly from comet impacts? Even after the primeval (young) Earth’s crust had formed, its core and mantle would have been very hot, continually venting lava onto the surface and releasing with it gases which had been trapped in the forming Earth.

18 Hot, primeval Earth after differentiation Volcanic eruptions release gases - but not oxygen, even though oxygen is now the second most abundant gas in the Earth’s atmosphere! Gases released in volcanic emissions included carbon dioxide, nitrogen, methane, ammonia and at least some water vapour - these would have formed the first long-term primeval atmosphere of Earth. e.g. carbon dioxide, nitrogen and water vapour (?), methane, ammonia

19 Hot, primeval Earth after differentiation Volcanic eruptions release gases - e.g. carbon dioxide, nitrogen and water vapour (?), methane and ammonia The conventional model states that Earth’s present supply of water (liquid, ice and vapour) came from volcanic emissions. Theories on the Origin of Water on Earth

20 Hot, primeval Earth after differentiation.. or is this mostly from comet impacts? However a recent, controversial challenge to this theory has suggested that a large amount of the water vapour in the primeval Earth’s atmosphere came from cometary impacts. Volcanic eruptions release gases - e.g. carbon dioxide, nitrogen and water vapour (?), methane and ammonia

21 The evidence for this new “small comet” theory is partly based on satellite photos of the Earth’s atmosphere showing what appears to be streaks from impacts by small cometary (ice and rock) fragments. (Real comet streak on simulated background) © Louis A. Frank, U. Iowa and © ARC Science Simulations reproduced with permission

22 If such fragments hit the Earth in great numbers, they may have released enough water vapour as they burned up in the Earth’s atmosphere to supply most of the Earth’s water.

23 © Louis A. Frank, U. Iowa reproduced with permission

24 This claim is hotly contested by other astronomers, and the jury is definitely still out. Therefore the conventional model is probably still winning the “popularity stakes”. The recent discovery of water ice at the Moon’s poles, presumably also due to cometary impacts, lends some weight to the small comet theory - but this ice may be due to ancient impacts, not recent ones. However, an isotopic analysis of the water in comets (obtained from their optical spectra) is not consistent with the isotopic makeup of water on Earth.

25 Investigate the debate yourself - follow this link to find a list of Internet sites to get you started!follow this link In the process you’ll see a particularly interesting case of the evolution of not just the Earth, but of scientific theory. It’s interesting to see that a field like planetary science, which many would think is rather well-trodden scientific ground, has given rise to two of the hottest debates in recent times - the “small comet” theory, and the “ancient life on Mars” theory.

26 As we have seen, the model for the Earth’s primeval atmosphere does not include any oxygen. The Origin of the Earth’s atmospheric oxygen The other terrestrial planets do not have any significant amount of oxygen in their atmospheres either.

27 Volcanic eruptions release gases - e.g. carbon dioxide, nitrogen and water vapour (?), methane and ammonia Let’s see how our model pictures the Earth’s atmosphere as evolving from a primeval atmosphere rich in carbon dioxide and nitrogen, plus methane, ammonia and water vapour,

28 to a modern-day atmosphere with 78% molecular nitrogen 21% molecular oxygen only 0.03% carbon dioxide, and up to 2% water vapour.

29 As the Earth cooled, the water vapour in the atmosphere was able partly to condense as clouds and then rain. Basin flooding by rain occurred and lakes & oceans formed:

30 carbon dioxide … therefore the atmosphere was left rich in nitrogen. Carbon dioxide is highly soluble in water, and so much of the carbon dioxide from the carbon dioxide-rich atmosphere dissolved easily in water...

31 The carbon dioxide dissolved in the water reacted with other dissolved compounds, producing limestones and other minerals which precipitated out. Carbon dioxide in solution is highly reactive.

32 Possibly stimulated by the energy from lightning strikes, life may have started in ocean’s “primeval soup” approx. 3.6 billion years ago or more - or perhaps it began, even earlier, deep within the Earth’s surface? This topic is important to the discussion of the possibility of ancient life on Mars, and life elsewhere in our Solar System. But for now, we’ll concentrate on the effects that the evolution of life on Earth had on its early environment:

33 When primitive living organisms formed, they included plant life which absorbed more carbon dioxide from the air.

34 … and coal, oil & natural gas deposits were formed from decayed organisms. (When humans burn these now, some carbon dioxide is returned to the atmosphere.)

35 Photosynthesis, the process where plants take in carbon dioxide and release oxygen into the atmosphere, probably started ~ 3.3 billion years ago. carbon dioxide is absorbed oxygen is released into Earth’s atmosphere At first the oxygen, which is very highly reactive, will have reacted with other compounds and formed oxides - but eventually the levels of oxygen in the atmosphere will have gradually built up till they reached their present levels.

36 So the evolution of life on Earth has had a profound effect on its surface - with land clearing, agriculture, mining, pollution and so on - but an even greater effect on Earth’s atmosphere. The atmosphere has been changed by the presence of life from being carbon-dioxide rich to being composed of only 0.03% carbon dioxide and about 21% oxygen. In science fiction terminology, lifeforms have terraformed the Earth.

37 NASA: The Western Himalayas (from the Shuttle Atlantis) ex.html NASA: Barringer Meteor Crater, Arizona NASA: Dust Storm, Red Sea and Saudi Arabia Comet Streak, © Louis A. Frank, U. Iowa and © ARC Science Simulations Inc. ( reproduced with permission Small Comets Graphic, © Louis A. Frank, U. Iowa, reproduced with permission Image Credits

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


40 Small Comets Theory Links NASA Press Release: : POLAR SPACECRAFT IMAGES SUPPORT THEORY OF INTERPLANETARY SNOWBALLS SPRAYING EARTH'S UPPER ATMOSPHERE Comet rain debate continues (Dec ) Other Views of the Small Comets Debate Small Comets - original discovery papers The Small Comets Web Site Cosmic Snowballs Detected Pelting Earth’s Atmosphere - May 29, 1997

41 Back to the Small Comets controversy!


43 Speed, Mass and Temperature For millions of years, humans (and other creatures) have been able to sense the average kinetic energy in a whole bunch of particles. We call it temperature. Two objects touching each other will always tend to “compare notes” about how much kinetic energy their particles have, and will come to some kind of agreement (called equilibrium) about whether energy should flow from one to the other to even things up, or not. Boy, my particles are flying today! How are you? A bit cold … my particles are almost at a standstill Is that better? Terrific! Thanks!

44 Hotter and colder If the average kinetic energy of the particles of something is high, we call it hot. But if the average kinetic energy is low, we say that the object is cold. Just call me Mr Average... Cool...Hot!

45 So if the masses of the particles in two gases are about the same, the one with particles moving faster (on average) will be at a higher temperature. Cool...Hot!

46 But kinetic energy depends on particle mass as well as velocity. Less massive... If both the containers below contain gases at the same temperature, the more massive particles will be moving more slowly on average. More massive!

47 So in a gas at a particular temperature, the least massive particles are moving fastest, and the most massive particles are moving slowest. Which is why, for example, light gases like hydrogen and helium are moving fast enough on average to escape from the Earth’s atmosphere, but heavy gases like nitrogen and oxygen are trapped by it.

48 Back to the Earth’s Primeval Atmosphere


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