Videos

• Two videos on layers of the Earth, similar coverage with slightly different styles:
  • Amit Sengupta
  • Moo Moo Math and Science
• Video on the basics of the carbon cycle

What makes a planet habitable?

• The primary factors that appear to make Earth habitable are its relatively large size and its distance from the Sun.
  • Distance from the Sun, along with the greenhouse effect, determines surface temperature. For habitability, surface temperature must be in a range within which liquid water can exist on the surface.
  • The greater the size, the longer a planet can retain enough internal heat to drive various geological processes (e.g., volcanic outgassing) which build up and maintain a dynamic hydrosphere. A large size (high gravity) also helps a planet retain atmospheric gases and slow down their escape into space.
  • A magnetosphere protects the atmosphere from being stripped away by solar wind (charged particles). The requirements for a magnetosphere to exist are liquid metal in the core and a sufficiently fast rotation. Small planets cool too fast to retain a sufficiently molten core.
  • Both factors, size and distance, have contributed to secondary factors which maintain climate stability on Earth.
  • Once started, living organisms themselves can have a stabilizing effect on the environment. Check out this tutorial on Daisyworld, a hypothetical world in which daisies can regulate temperature.
• Earth's relatively large size has allowed it to retain much of its internal heat to drive a very active geology.
  • Plate tectonics allows carbon (and other elements) trapped at the surface to be recycled back into the hydrosphere. The greenhouse gas carbon dioxide is the major temperature regulator on Earth.
  • If the Earth were as big as a Jovian planet, it would retain H and He in its atmosphere. This would result in an atmosphere that is too hot for for liquid water to exist.
  • Thus, to be hospitable to life, a planet cannot be too hot or too cold, or too big or too small. A rough estimate is within the range of 0.2-10 Earth masses. Conditions have to be "just right", hence the label Goldilocks principle.
• Mercury and the Moon, similar in size and appearance, are both geologically dead because of their relatively small size.
• Mars is bigger than Mercury but not big enough to have retained enough geological activity to continuously replenish the atmosphere, which is constantly lost to space.
• Venus, only slightly smaller than Earth, is geologically active. However, excessive exposure to solar radiation due its proximity to the Sun has pushed it into a runaway greenhouse effect.

Processes shaping the Earth's surface

• Virtually all geological features originate from the following surface-shaping processes:
  • Impact cratering
    • Craters are typically about 10 times as wide as the impactor and about 10-20% as deep as they are wide.
    • Impactors typically have an impact speed in the range 40,000-250,000 km/h.
  • Volcanism
    • The old view was that Earth's atmosphere and oceans were produced by volcanic outgassing.
    • Recent studies suggest that the atmosphere and oceans possibly originated primarily from a late bombardment by comets, which deposited materials rich in gas and water.
  • Tectonics
  • Erosion

Atmospheres of the terrestrial planets

• About two-thirds of the air in our atmosphere lies within 10 km of the surface.
• The atmosphere protects us from harmful, short-wavelength radiation (UV, x-rays, gamma rays, etc.).
• Greenhouse gases (CO₂, H₂O, CH₄) in the atmosphere keep the Earth relatively warm.
  • Without the so-called greenhouse effect, the average surface temperature on Earth would be around -16 °C, well below the freezing temperature of water.
  • With the greenhouse gases, the average surface temperature on Earth is around 15 °C.
  • The warming effect of the greenhouse gases is known as the greenhouse effect.
• Mercury, like the Moon, is too small to retain any atmosphere. Furthermore, Mercury is geologically dead, so no new atmosphere is being produced.
• Mars has a very thin atmosphere (atmospheric pressure on Mars is less than 1% that on Earth) consisting mostly of CO₂.
  • Originally, Mars was warmer and wetter and had a much denser atmosphere.
  • Mars underwent a major and permanent climate change about 3 billion years ago.
  • The loss of the Martian atmosphere was probably accelerated by a weakening magnetosphere, as Mars cooled and its core solidified.
• Venusian atmosphere is 96% carbon dioxide and atmospheric pressure at the surface of Venus is 90 greater than that on Earth.
• Earth is the only planet in our solar system known to have a significant amount of free oxygen (O₂) in its atmosphere.
  • The oxygen in Earth's atmosphere and oceans primarily comes from photosynthesis, a metabolic process carried out by plants, algae, and some types of bacteria.
  • Photosynthesis is fundamental to life on Earth, as it provides the oxygen needed for aerobic respiration, which is used by most living organisms to generate energy.
  • Earth originally started accumulating oxygen in the atmosphere during the so-called Great Oxygenation Event that occurred around 2.5 billion years ago (about 2 billion years after the formation of Earth). During this event, cyanobacteria, also known as blue-green algae, began releasing oxygen as a byproduct of photosynthesis, which gradually led to the oxygenation of the atmosphere and oceans. This ultimately paved the way for the development of aerobic organisms and the complex ecosystems we see today.
  • Cyanobacteria, which are believed to be the earliest photosynthetic organisms, are still present today and are found in a wide range of environments, including freshwater and marine ecosystems, as well as soil and symbiotic relationships with other organisms. They are considered to be one of the most important groups of microorganisms in terms of their impact on the Earth's atmosphere and the evolution of life on our planet.

Magnetosphere

• The Earth has a strong magnetosphere which deflects most of the charged particles from the Sun (solar wind), as discussed in this short video.
  • In the absence of a protective magnetosphere, solar wind can strip a planet of its atmospheric gasses.
• The magnetospheres of the other terrestrial planets are much weaker than that of the Earth.
• The magnetosphere appears to result from the combined effects of liquid metal in the core, enough internal heat to drive convection, and a relatively rapid rate of rotation.
  • The Jovian planets have strong magnetospheres because, like Earth, they spin fast (with a period of 10-17 hours) and are still molten in their cores.
• Mercury has a large iron core but it is a relatively small planet (not much bigger than our Moon) and has cooled to the point of being geologically dead. It also has a slow 59-day-long rotation. The combined effect is a magnetic field about 1% as strong as the Earth’s.
• Due its very slow 243-day-long rotation, Venus has virtually no magnetosphere.
• Mars, intermediate in size between Mercury and Earth, has cooled significantly but probably retains some internal heat, but not enough for a significant magnetosphere.

Layers of the Earth

• Crust--thin outer layer of the Earth
• Mantle--just below the crust, magma (molten rock) can erupt from here and reach the surface as lava
  • Lithosphere--solid, outer part of the Earth, consisting of the brittle upper portion of the mantle and the crust; lithosphere is 'broken' into tectonic plates
  • Asthenosphere--molten, slushlike upper layer of the mantle just below the lithosphere; tectonic plates move on top of the asthenosphere
    • This video explains the movement of the tectonic plates.
  • The big difference between the lithosphere and asthenosphere is how the two layers respond to stress. The lithosphere remains rigid for very long periods of geologic time in which it deforms elastically and through brittle failure, while the asthenosphere deforms viscously and accommodates strain through plastic deformation.
• Outer core--liquid, with lots of iron and nickel
• Inner core--solid, with lots of iron and nickel
  • Here is a cute video about what a journey to the Earth's core might look like.
• There are three main sources of heat in the deep earth:
  • radiogenic heat from the decay of radioactive elements (Potassium 40, Uranium 238, Uranium 235, and Thorium 232) contained within the mantle; this is probably the biggest contributor
  • promordial heat left over from the formation of the Earth (accretion)
  • frictional heating, caused by denser core material sinking to the center of the planet (differentiation)
• These videos describe the layers and sources of heat.

Hydrosphere

• Earth is the only planet with liquid water on the surface.
  • Water vapor outgassed from volcanoes rained down on the surface to make the oceans.
  • Moderate greenhouse effect and distance from the Sun kept the water in the liquid state.
  • The characteristics which appear to have made Earth special are size and a "comfortable" distance from the Sun.
• Mercury is often compared to our Moon because the two share a very simple history. Because both are considerably smaller than Venus, Earth, or Mars, they cooled much more rapidly than the other terrestrial worlds and are now geologically dead. No mechanism (i.e., volcanic outgassing) exists to replenish the atmosphere, which was lost to space long ago.
• Venus is only 5% smaller than the Earth and is geologically active. Nevertheless, it has no water on or around its surface and has a very high surface temperature.
  • Although Venus is only 30% closer to the Sun than Earth, the difference was apparently critical to its very different evolution.
  • Early in its evolution, the atmosphere of Venus was filled with water vapor, carbon dioxide, and other products of volcanic outgassing, not fundamentally different from the picture on early Earth. However, the greater intensity of sunlight on Venus did not allow the formation of oceans. Without a large reservoir of liquid water to dissolve and trap carbon dioxide, the atmosphere soon became overrun by carbon dioxide, which ultimately lead to a runaway greenhouse effect.
• Mars contains ice in the polar regions and evidence of water erosion suggests that liquid water may have flowed on the surface of Mars a billion or more years ago.
  • Early geological activity on Mars probably generated an atmosphere consisting of water vapor, carbon dioxide, and other gases.
  • Due to its relatively small size, however, Mars cooled quickly. As it cooled, it lost much of its geological activity, and was thus not able to replenish gasses lost to space. In addition, a cooling interior also resulted in a weakening magnetic field. Without a strong magnetosphere, water was left vulnerable to attack by solar wind, which split water molecules into hydrogen and oxygen. Hydrogen is a light gas and quickly escaped into space. Oxygen rusted the Martian rocks, giving the "Red Planet" its distinctive color.

Carbon dioxide cycle

• Carbon (C), the fourth most abundant element in the Universe, after hydrogen (H), helium (He), and oxygen (O), is the building block of life. It’s the element that anchors all organic substances, from fossil fuels to DNA.
• Carbon is stored on our planet in the following major sinks:
  1. as organic molecules in living and dead organisms found in the biosphere
  2. organic matter in soils
  3. atmospheric carbon dioxide
  4. lithosphere, which includes fossil fuels and sedimentary rock deposits such as limestone, dolomite, and chalk
  5. oceans, which include dissolved atmospheric carbon dioxide and calcium carbonate shells in marine organisms.
• On Earth, carbon constantly cycles through the hydrosphere, biosphere, land, and the Earth’s interior. This global carbon cycle can be divided into two categories: the geological, which operates over large time scales (millions of years), and the biological/physical, which operates over shorter time scales (days to thousands of years).
• The geological carbon cycle can be summarized as follows:
  • Atmospheric CO₂ dissolves in rainwater, which becomes mildly acidic.
  • The mildly acidic rainwater erodes rocks and ultimately carries broken-down minerals to the oceans.
  • In the oceans, the eroded minerals combine with dissolved CO₂ and fall to the ocean floor in the form of carbonate rocks, such as limestone.
  • Over millions of years, tectonic activity carries the trapped CO₂ into the mantle, from where CO₂ is eventually released through volcanic outgassing.
• The balance between weathering, subduction, and volcanism controls atmospheric carbon dioxide concentrations over time periods of hundreds of millions of years. Because of its significant contribution to the greenhouse effect, the Earth's CO₂ cycle acts as a long-term thermostat on Earth.
  • With mild temperature variations, the higher the temperature, the higher the rate at which CO₂ is removed from the atmosphere via the carbon dioxide cycle.
  • A slight increase in temperature results in more evaporation and rain. More rain results in more efficient deposition of carbonate rock in the ocean floors. Less atmospheric CO₂ implies less greenhouse effect and thus a cooling.
  • A slight decrease in temperature results in less evaporation and rain. Less rain results in less efficient deposition of carbonate rock in the ocean floors. This allows outgassed CO₂ to build up in the atmosphere, thereby strengthening the greenhouse effect.
• Biology also plays an important role in the movement of carbon into and out of the land and ocean through the processes of photosynthesis and respiration.
  • Nearly all forms of life on Earth depend on the production of sugars from solar energy and carbon dioxide (photosynthesis) and the metabolism (respiration) of those sugars to produce the chemical energy that facilitates growth and reproduction.
  •  Photosynthesis removes carbon dioxide from the atmosphere while respiration returns carbon dioxide to the atmosphere. The amount of carbon taken up by photosynthesis and released back to the atmosphere by respiration each year is roughly 1,000 times greater than the amount of carbon that moves through the geological cycle during the same period.
  • Photosynthesis and respiration also play an important role in the long-term geological cycling of carbon. The presence of land vegetation enhances the weathering of soil, leading to the long-term—but slow—uptake of carbon dioxide from the atmosphere. In the oceans, some of the carbon taken up by phytoplankton (microscopic marine plants that form the basis of the marine food chain) to make shells of calcium carbonate (CaCO₃) settles to the bottom (after they die) to form sediments. All of these biologically mediated processes represent a removal of carbon dioxide from the atmosphere and storage of carbon in geologic sediments.
• In addition to the natural fluxes of carbon through the Earth system, anthropogenic (human) activities, particularly fossil fuel burning and deforestation, are also releasing carbon dioxide into the atmosphere.
  • In burning fossil fuels, which formed over millions of years, we are effectively moving carbon much more rapidly into the atmosphere than is being removed naturally through the sedimentation of carbon.
  • Also, by clearing forests to support agriculture, we are transferring carbon from living biomass into the atmosphere (dry wood is about 50% carbon). The result is that humans are adding ever-increasing amounts of extra carbon dioxide into the atmosphere.
  • About 25% of the anthropogenic CO₂ is absorbed by the oceans, 25% by trees and vegetation, and the rest stays in the atmosphere, possibly for thousands of years.
  • Since the beginning of the industrial revolution around 1850, human activity has resulted in an increase of about 50% in atmospheric CO₂.
  • While natural processes can absorb some of the 6 billion metric tons (measured in carbon equivalent terms) of anthropogenic carbon dioxide emissions produced each year, roughly half is still added to the atmosphere annually. The Earth’s positive imbalance between emissions and absorption results in the continuing growth in greenhouse gases in the atmosphere.

Climate Change/Global Warming

• Global average temperature on Earth has risen about 0.8 C° in the past century.
• The current atmospheric concentration of carbon dioxide is rising rapidly and is currently about 30% higher than it has been at any time during the past half-million years or longer.
• The evidence is almost unanimous in the scientific community that at least part of the increase is surely anthropogenic (i.e., caused by human activities):
  • Burning fossil fuels (coal, oil, natural gas), which returns to the atmosphere carbon that has been locked within the earth for millions of years.
  • Clearing and burning of forests, especially in the tropics. In recent decades, large areas of the Amazon rain forest have been cleared for agriculture and cattle grazing.
  • Historical data from ice cores and modern data collected from the Mauna Loa observatory in Hawaii support the the notion that atmospheric CO₂ has been rising since the beginning of the Industrial Revolution
    •  Atmospheric CO₂ has risen by about 50% since the beginning of the Industrial Revolution (~1850).
  • According to careful measurements, atmospheric concentration of carbon dioxide and global average temperature over the past 400,000 years appear to be intimately linked. More carbon dioxide goes with higher temperature, and vice versa.
• Consequences of global warming may include the following:
  • Flooding of coastal regions.
  • Alteration of oceanic currents.
  • Increased incidence and severity of storms and hurricanes.
  • More severe winter conditions.
• According to current estimates (based on computer simulations), a doubling of the CO₂ concentration (expected by the end of this century) will cause the Earth to warm by an amount somewhere in the range of 2.5–3.5 C°.

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