The Earth System: Our Views of Planet Earth

Transcription

1 The Earth System: Our Views of Planet Earth The Earth: Our Home Earth is a very small part of a vast universe, but it is our one and only home--it is the only environment most humans have ever experienced. Our home is so integral to our very existence that we often take it for granted. The unique qualities of Earth which sustain our lives are seldom appreciated. Like Goldilocks, on Earth we enjoy just the right combination of conditions and ingredients necessary to maintain our form of life and to support our activities. Also like Goldilocks, we are often careless in our approach to the environment assuming that everything that is in it is there for our use/consumption.

2 Earth: Goldilocks of the Solar System Earth is a unique planet in our solar system As far as we know, Earth is the only body in the solar system that hosts life of any kind. Why is Earth so unique? Perhaps the most important factor is the distance of Earth from the Sun, and the nature of the Sun itself. The Habitable Zone Life as we know it occurs only within planetary atmospheres, and requires the presence of at least minimal quantities of liquid water in the crust or suspended in the atmosphere. Liquid water only exists within a limited range of temperature (T) and pressure (P) conditions, and consequently, where predominant conditions are outside this range, life is unlikely to exist. There are also more limited P and T ranges within which complex organic molecules and their building blocks (e.g. amino acids, proteins, DNA, RNA) will remain intact and in which chemical rxns necessary for life will proceed. Many other factors to consider even for Earth-type life. Concept has been criticized as being too geocentric: Assumes life elsewhere would be based on same elements and preconditions as life on Earth. Concept is entirely conjectural. No adequate empirical evidence exists to support it.

3 Our Planetary Neighbourhood To appreciate the uniqueness of the Earth, we must compare our home planet with its neighbours. In general, we can think of planets in our solar system as belonging to one of two basic forms: Jovian Planets: Large, Jupiter-like, gaseous planets that have very thick atmospheres and are relatively far from the Sun. (Jupiter, Saturn, Uranus, Neptune) Terrestrial Planets: small, Earth-like, rocky planets that have thin atmospheres and are relatively close to the Sun. (Mercury, Venus, Earth, Mars) Why are these groups of planets so different in their characteristics? The Nine Eight? Planets The odd man out SUN Jupiter Earth Venus Mercury Mars Pluto Dwarf planet Neptune Jovian Uranus planets Saturn (Rocky core, liquid mantle, no crust, most of their terrestrial planets mass is (solid to liquid metallic gasses) core, solid high density silicate mantle, solid low density silicate crust). Light gasses burned off

4 Earth Within the Terrestrial Planet Family Earth s characteristics are more similar to its neighbouring planets Venus and Mars, than to the Jovian planets. But even within the family of terrestrial planets, Earth is unique. Earth is in a very precarious position! This is illustrated by the apparent absence of complex life on Venus and Mars. Distance from Sun To illustrate the importance of Earth s position relative to the Sun, we can think about what would happen to Earth if it was moved to the position of either of its two planetary neighbours- Venus and Mars

5 What if Earth Occupied the Orbit of Venus? If Earth were to occupy the orbit of Venus, the increased solar radiation would raise the average temperature of the planet. Raising global temperature would put more water vapour into the atmosphere and, through the greenhouse effect, make the temperature even higher. At extremely high temperatures, photosynthesizing organisms could not survive, leading to the accumulation of volcano-sourced carbon dioxide in the atmosphere, and a further increase in temperature. The planet s surface would be dry, hot and barren and generally unsuitable for Earth-type life. What if Earth Occupied the Orbit of Mars? If Earth were to move to the orbit of Mars, the decrease in incoming solar energy would cool the oceans, increase the size of the polar caps and lead to less water vapour in the atmosphere. Heat retention of the planet would decrease, thus reducing the temperature further, most likely below the threshold of tolerance for most multicellular organisms. In the extreme, the Earth would probably be covered with ice. Mars is currently not extensively glaciated as the supply of water on the surface is minimal compared to that on Earth. However, large quantities of subterranean water and ice may exist there.

6 The Earth System The Earth itself is a complex system of interacting components. In the most basic sense, we can think of the Earth as consisting of four major domains or spheres : Geosphere: comprises the solid Earth and includes both Earth s surface and the various layers of the Earth's interior. Atmosphere: gaseous envelope that surrounds the Earth and constitutes the transition between its surface and the vacuum of space. Hydrosphere: includes all water on Earth (including surface water and groundwater). Biosphere: the life zone of the Earth and includes all living organisms, and all organic matter that has not yet decomposed. The Earth s Four Spheres All of these domains connected with one another to form an integrated whole

7 Earth: A Closed System We regard Earth as a closed system: Energy is exchanged between Earth and the surrounding universe, but matter is not! A View of Home (From an Artist s Perspective) Most landscape paintings and photographs feature the four spheres (either explicitly or implicitly). Can you identify elements of the four spheres represented in this depiction of landscape (Algonquin Lake by Tom Thomson)?

8 This course will focus mainly on aspects of the geosphere, so it is important to consider some of the fundamental aspects of this domain of the Earth system. Because most of the geosphere (with the exception of what we can see at or near its surface) is invisible to us, details of how the geosphere functions must be inferred either from surficial features or from effects on energy transmitted through the various internal layers. Up until relatively recently, the little was known about the structure and behaviour of Earth s interior. The very concept of hell conjures images of a fiery underworld deep beneath the Earth s surface (also layered according to several sources e.g. Dante s The Divine Comedy -14th Century). The Underworld Centuries after the formulation of ancient underworld myths, many people continued to believe that Earth was hollow. For example, A Journey to the Centre of the Earth by Jules Verne (1864) detailed subterranean tunnels that led to a world filled with prehistoric creatures.

9 The Underworld From a Scientific Perspective Earth scientists view the Earth in a very different way! Based on the behaviour of seismic waves in the Earth s interior (and other lines of geological evidence) we are pretty much sure that the Earth is not hollow (we will look at such evidence in greater detail in upcoming lectures). However, this interpretation does not make Earth s interior any less interesting! Structure of the Solid Earth The solid Earth consists of a series of shells with differing characteristics. We can classify Earth s shells according to chemical composition or physical properties.

10 km below surface crust blith: bast: mantle core Overall Structure of the Earth Mainly iron and nickel Chemical Composition Similar to mantle but with lighter metals (more Aluminum, less Magnesium) Silica (SiO2) plus Magnesium, Iron, Aluminum, etc. lithosphere asthenosphere mesosphere outer core inner core Brittle, solid Near-liquid state, ductile flow ( jelly between bread ) Solid due to high P, but flows like a fluid over long periods of time (Middle Mantle) Lower Mantle? Liquid (lower T&P) Solid (higher T&P) Physical (Mechanical) Properties Another Depiction Note that the lithosphere contains the crust plus the uppermost (brittle) part of the mantle.

11 So Why is Earth Compositionally Zoned? Differentiation of the core, mantle and crust probably occurred during the early stages of planetary development when Earth was in a semi-molten state. Largely due to gravity, heavier/more dense elements (e.g. iron and nickel) and materials were concentrated at the centre of the planet (more mass=greater gravitational attraction), whereas lighter elements remained in the mantle. Still lighter elements (least gravitational attraction) were concentrated in the crust and atmosphere when they finally formed. Importance of the Lithosphere The lithosphere (the upper, brittle layer of Earth) is broken into plates (tectonic plates) that float on the asthenosphere. When these plates spread apart, bump into one another, and slide past one another, they form surface irregularities (more on this later).

12 And Onward to the Crust The crust (comprising the uppermost part of the lithosphere) is the part of the geosphere on which we reside and is the source of most of the geological materials that we will be investigating within a cultural context. In the next few lectures, we will begin to examine the building blocks of the crust. END OF LECTURE