Week 3 - Habitable Zone

Question 1

What is the habitable zone?

Answer 1

“Habitable zone” is the distance from a star at which liquid water could exist on orbiting planets’ surfaces

Question 2

Explain which conditions we need to make planet habitable in general

Answer 2

1. Access to elements that make up living systems (carbon, oxygen, hydrogen and nitrogen)
2. Access to a liquid solvent (e.g. water).
3. An energy source.
4. A stable climate.
5. A long-lived host star.

Question 3

Explain why we think water is crucial for habitability of a planet

Answer 3

It functions as a solvent. It is capable of dissolving substances and enabling key chemical reactions in animal, plant and microbial cells. Its chemical and physical properties allow it to dissolve more substances than most other liquids

Question 4

Identify the factors which determine the surface temperature of a planet

Answer 4

1. The presence or absence of an atmosphere
2. The composition of such an atmosphere
3. The rotation rate of the planet
4. The albedo of the planet
5. The obliquity of the planet
6. The eccentricity of the planet’s orbit

Question 5

Explain how greenhouse effect increases the surface temperature of a planet

Answer 5

Solar energy absorbed at Earth’s surface is radiated back into the atmosphere as heat. As the heat makes its way through the atmosphere and back out to space, greenhouse gases absorb much of it. Greenhouse gases radiate the heat back to the Earth’s surface, to another greenhouse gas molecule, or out to space.

Question 6

Explain how long-term carbon cycle regulates Earth’s temperature

Answer 6

1. CO2 from the atmosphere dissolves in the oceans due to effect of rain
2. Rainfall also erodes silicate rocks and carries minerals to the oceans
3. Silicates react with the dissolved CO2 to form carbonate minerals which sink to the ocean floor
4. Plate tectonics carries the carbonates to subduction zones where they are taken into the Earth’s mantle.
5. Some rocks melt and their carbon is returned to the atmosphere in the form of CO2 through volcanoes.

Question 7

Describe and explain the differences between Earth, Mars and Venus now and in the past

Answer 7

Mars - not massive enough to retain its atmosphere, gases were lost to space.

Venus - Venus, being closer to the Sun and hotter than Earth, had a higher proportion of its water in the atmosphere, and was exposed to a higher UV flux to destroy the water. Over time the water disappears until only a tiny fraction of it is left

Earth - the cooler temperatures resulted in a lower fraction of water vapour in the atmosphere, and the greater distance to the Sun meant less UV to dissociate the atmospheric water molecules, allowing them to condense into rain and fall back onto the surface to form oceans.

Question 8

Determine the habitable zone for a star given parameters such as luminosity, lifetime and/or mass

Answer 8

Question 9

Explain why the location and width of the habitable zone around a star vary with time

Answer 9

Stellar luminosities also vary as stars age. The Sun’s energy output has almost doubled since it was born. This means that the location of a habitable zone will also vary as the star ages.

Question 10

Explain how stellar mass determines the length its planet will spend in a habitable zone

Answer 10

Stars that are smaller, cooler and lower mass than the Sun have their habitable zone much closer to the star than the Sun. Stars that are larger, hotter and more massive than the Sun have their habitable zone much farther out from the star.