Earth as a Habitat Flashcards
Why is Earth habitable?
- Not too hot, not too cold
- Earth is protected from harmful solar radiation by its magnetic field
- Has the right chemical ingredients for life, including water and carbon
Not too hot, not too cold
- Distance from the sun
- Solar Radiation
Distance from the sun:
- Earth is within the “habitable zone” -> orbits the sun at a distance that allows liquid water to exist
- Mars is the only other planet within the habitable zone, but is too cold to have liquid water on its surface
Solar Radiation
- 340W/m2 of solar radiation hits Earth’s atmosphere
- ~30% is reflected and scattered back to space
- ~70% enters climate system (238W/m2)
- Of the 238, 1/3 is absorbed by clouds and 2/3 by Earth’s surface
- Absorbed radiations causes atoms and molecules to vibrate, heating up Earth’s surface and lower atmosphere
Albedo
Describes how reflective a surface is
- Dark surfaces are less reflective and have lower albedos (high absorption)
- Lighter surfaces are more reflective and have higher albedos (low absorption)
- Some surfaces are more reflective than others i.e fresh snow/ice > old snow > clouds > desert > sand > soil
Earth temperature is ~constant over time, therefore:
Energy IN = Energy OUT
- So 238W/m2 must be lost by Earth (back radiation)
Energy travels through space at many wavelengths:
- Solar energy is mainly in visible spectrum and some UV radiation (shortwave)
- Energy that radiates back from Earth is infrared radiation (longwave)
Greenhouse Gas (GHG)
Any gas that absorbs or emits infrared radiation
Direct GHGs:
- H2O
- CO2
- Methane CH4
- Nitrous oxide N2O
- O-zone O3
- Chlorofluorocarbons (CFCs)
CFCs
- Were used in refrigeration, ACs, as propellants in aerosols and as industrial degreasers for dry cleaning up until late 1970s
- Now heavily regulated because they break down the Earth’s ozone layer (which shields us from radiation) and are a very powerful GHG
Indirect GHGs
Not greenhouse gases, but contribute to the greenhouse effect in other ways
Examples that break down in the atmosphere to product another greenhouse gas (CO2 or O3):
- Nitrogen oxides (NOx)
- Carbon monoxide (CO)
- Non-methane volatile organic compounds (NMVOC)
Sulphur dioxide (SO2):
Contributes to aerosol formation - fine solid particles/droplets of liquid in air, which can either warm (through absorption of solar radiation on dark particles) or cool (from forming cloud droplets and reflecting radiation) the atmosphere
Composition of Atmosphere
- 78% N2, 21% O2, 1-3% H2O, 0.9% Ar
- All other gases 0.04%
- Total GHG: <3.04%
- Without GHGs, the mean air temperature on the surface of Earth would drop from 15C to -18C, but too much GHG results in overheating
Earth’s Magnetic Field
- Convection in the liquid outer core is thought to create Earth’s magnetic field
- High energy charged particles produced by the sun (fusion) rain on the Earth from all directions
- Solar wind distorts the field lines, compressing them into a tail that points away from the sun
Earth’s magnetic field protects organisms on Earth from:
- Heavy bombardment: charged particles spiral around the magnetic field lines rather than slamming directing into Earth
- UV radiation: prevents ozone layer from being stripped away
- Around the magnetic poles, magnetic field lines point directly into the ground and intersect the atmosphere
- Particles from space can escape the magnetosphere and collide with air molecules, producing an aurora
Ingredients for life (as we know it):
All organisms are built from the same six essential elemental ingredients: - carbon - hydrogen - nitrogen - oxygen - phosphorous - sulfur (CHNOPS)
Carbon bonds easily with other carbon atoms, which forms:
long chains that other atoms bond to, making them the perfect building blocks for large and complex organic molecules
Nitrogen, hydrogen, and oxygen are abundant and bond with carbon to make:
amino acids, fats, lipids, and the nucleobases from which DNA and RNA are built
Sulfur in sulfides and sulfates help:
Catalyze reactions
Phosphate is vital to metabolism because:
molecules (i.e ATP) store a huge amount of energy in their chemical bonds. Breaking the bond releases its energy.
Where do the ingredients come from?
- Solvents dissolve solids (i.e rocks) and allow the ingredients for life to move and interact with each other to carry out the reactions needed for life to happen
- Liquid water is an excellent solvent, capable of dissolving many substances
- Other liquids are also good solvents, but water on Earth is abundant and readily available
Can live exist on other planets?
Maybe
- Researchers have discovered a few Earth-like planets in the habitable region of other stars
- Astronomer estimate that there are ~40bil planets within the habitable zones of stars in the Milky Way Galaxy
- Because of the complexity of how life evolved on Earth, Earth-like planets are the best candidates for finding extraterrestrial life
In our own solar system, it may be possible to find life on other planets or moons. Likely:
- Microorganisms
- Extremophiles
- Live in subsurface oceans beneath a frozen crust, warmed by hydrothermal vents (where water in heated in the crust and ejected back into the sea. Generally near spreading centers/hotspots)
- Still carbon-based, but may rely on a different solvent
