Earth as a Habitat

Question 1

Why is Earth habitable?

Answer 1

1. Not too hot, not too cold
2. Earth is protected from harmful solar radiation by its magnetic field
3. Has the right chemical ingredients for life, including water and carbon

Question 2

Not too hot, not too cold

Answer 2

• Distance from the sun

- Solar Radiation

Question 3

Distance from the sun:

Answer 3

• 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

Question 4

Solar Radiation

Answer 4

• 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

Question 5

Albedo

Answer 5

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

Question 6

Earth temperature is ~constant over time, therefore:

Answer 6

Energy IN = Energy OUT

- So 238W/m2 must be lost by Earth (back radiation)

Question 7

Energy travels through space at many wavelengths:

Answer 7

• Solar energy is mainly in visible spectrum and some UV radiation (shortwave)
• Energy that radiates back from Earth is infrared radiation (longwave)

Question 8

Greenhouse Gas (GHG)

Answer 8

Any gas that absorbs or emits infrared radiation

Question 9

Direct GHGs:

Answer 9

• H2O
• CO2
• Methane CH4
• Nitrous oxide N2O
• O-zone O3
• Chlorofluorocarbons (CFCs)

Question 10

CFCs

Answer 10

• 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

Question 11

Indirect GHGs

Answer 11

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)

Question 12

Sulphur dioxide (SO2):

Answer 12

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

Question 13

Composition of Atmosphere

Answer 13

• 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

Question 14

Earth’s Magnetic Field

Answer 14

• 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

Question 15

Earth’s magnetic field protects organisms on Earth from:

Answer 15

• 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

Question 16

Ingredients for life (as we know it):

Answer 16

All organisms are built from the same six essential elemental ingredients: 
- carbon
- hydrogen
- nitrogen
- oxygen
- phosphorous
- sulfur
(CHNOPS)

Question 17

Carbon bonds easily with other carbon atoms, which forms:

Answer 17

long chains that other atoms bond to, making them the perfect building blocks for large and complex organic molecules

Question 18

Nitrogen, hydrogen, and oxygen are abundant and bond with carbon to make:

Answer 18

amino acids, fats, lipids, and the nucleobases from which DNA and RNA are built

Question 19

Sulfur in sulfides and sulfates help:

Answer 19

Catalyze reactions

Question 20

Phosphate is vital to metabolism because:

Answer 20

molecules (i.e ATP) store a huge amount of energy in their chemical bonds. Breaking the bond releases its energy.

Question 21

Where do the ingredients come from?

Answer 21

• 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

Question 22

Can live exist on other planets?

Answer 22

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

Question 23

In our own solar system, it may be possible to find life on other planets or moons. Likely:

Answer 23

• 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