Download Earth 110 – Exploration of the Solar System Assignment 4

Name:____________________________
Earth 110 – Exploration of the Solar System
Assignment 4: Planetary Atmospheres
Section:____________________
Due in class Tuesday, Feb. 17, 2015
An atmosphere is a thin layer of gas surrounding a planet. The amount and type of gas an
atmosphere contains depends largely on the size of the planet it surrounds. In other words, it
depends on gravity. What is so important about this gas layer? First, it provides a surface
pressure, which pushes in all directions and holds up the atmosphere. Second, it can warm a
surface through the greenhouse effect. The combination of pressure and temperature governs
what molecules are stable as solids, liquids, and gases. Third, atmospheres scatter and absorb
light. Scattering is why we have a beautiful blue sky in the daytime and absorbing keeps harmful
radiation from hitting the surface (thank you ozone!). Fourth, atmospheres create weather at the
surface and play a huge role in climate change. Lastly, gases in the atmosphere interact with a
planetary magnetic field to form a magnetosphere, which protects the planet from harmful solar
wind particles. An atmosphere is one of the main reasons why life happily exists on Earth and
studying atmospheres of other planets helps us learn about our own (discovering the cause of
Earth’s ozone hole relied on studies of Venus’ atmosphere).
This homework is based off of Chapter 10 in the textbook. While completing this assignment,
it’s best to keep in mind how important an atmosphere is to our existence, what positive effects it
has for us, and what Earth might be like if the atmosphere changed (by comparing to other planet
atmospheres). Also think about the fundamental links between atmospheres and planet properties
(size, distance, rotation), interior heat (gain/loss), and surface processes. If Earth’s atmospheric
composition changes, it has ways of regulating surface temperature but the main question is:
How fast can it respond? As humans wrestle with climate change, this is one of the uncertainties
we have to keep in mind.
Earth’s atmosphere from space (Credit: NASA)
1 Planets gain atmospheres through three processes: outgassing, evaporation/sublimation, and
surface ejection. Explain, in your own words, how these processes release gas.
Outgassing:
Evaporation/Sublimation (make sure to describe the difference between the two):
Surface Ejection:
Which process(es) is (are) most important for Venus’ atmosphere? Earth’s? Mars’? Mercury’s?
The Moon’s?
2 Planets lose atmospheres by condensation, chemical reactions, solar wind stripping, and thermal
escape.
Compare condensation and chemical reactions.
What are the differences between solar wind stripping and thermal escape?
What is (are) the main process(es) that have contributed to atmospheric loss for Mercury?
Venus? Earth? The Moon? Mars?
3 A planet’s global average surface temperature, in the absence of the greenhouse effect, depends
on two things: (1) distance from the Sun and (2) overall reflectivity (albedo):
1
T"no _ greenhouse"
€
⎛ 1 − reflectivity ⎞ 4
= 280 × ⎜
⎟ where T is in Kelvin.
⎝
⎠
d2
If planetary distance is kept constant (d = 1 AU), calculate the “no greenhouse” temperature for a
planet with 25% reflectivity and a planet with 50% reflectivity.
Now let’s keep reflectivity constant at 35% and vary planet distance. Calculate the “no
greenhouse” temperature for a planet at 0.25 AU and 0.5 AU.
What variable is more important, reflectivity or distance? Explain using your above answers.
4 Now let’s focus on the greenhouse effect.
Explain, in your own words, how the greenhouse effect affects planet surface temperature.
What are some important greenhouse gases?
Explain the idea behind a “runaway” greenhouse effect (positive feedback loop).
Venus’ atmosphere contains 96% carbon dioxide (CO2), resulting a large greenhouse effect
which results in surface warming of 510ºC. Mars also has an atmosphere composed mainly of
CO2 (95%), but its greenhouse effect only results in a surface warming of 6ºC. Why this
difference?
Without the greenhouse effect, Earth’s average surface temperature would be -16ºC. What would
this mean for life?
How could adding more greenhouse gases into Earth’s atmosphere trigger a runaway greenhouse
effect?
5 Four factors cause long-term climate change: solar brightening, changes in axis tilt, changes in
reflectivity, and changes in greenhouse gas abundance.
The Sun’s brightness has increased by about 30% since it formed. This gradual increase has
noticeable effects over tens of millions of years or more. Gravitational tugs from other celestial
bodies cause Earth’s axis tilt to change over time (known as Milankovich Cycles), with
noticeable effects over tens to hundreds of thousands of years. Changes in reflectivity affect how
much sunlight gets absorbed and can have noticeable effects very quickly (e.g., volcanic
eruptions). Changes in greenhouse gas abundance strengthens or weakens the greenhouse effect
over decades or more.
The below graph shows how Earth’s tilt has changed over the past 750,000 years. What trends do
you notice?
6 The above graphs show changes in temperature for solar irradiance (solar energy per area,
affected by brightness), volcanic aerosols (particles in the atmosphere), and human-caused
forcings (changes in greenhouse gas abundance, reflectivity, and aerosols) from 1890 to today.
What trends do you observe in each of the graphs? Does each of these forces contribute to
temperature increase? Explain your reasoning.
7 Scientists have analyzed carbon dioxide in air bubbles that were trapped in an Antarctic ice core
extending back 800,000 years. Over this period, natural factors caused fluctuations in CO2
concentration which played a role in global climate (see below graph).
What do you notice about the rate of change of CO2 over the past ~250 years as compared to the
past 800,000 years? How would you expect Earth’s temperature to respond?
On Earth, atmospheric CO2 dissolves in rainwater which erodes rocks into constituent minerals.
These minerals get carried to the ocean where they combine with dissolved CO2, get buried, and
are recycled into the mantle through plate tectonics. Is this an example of positive or negative
feedback? Explain. What would happen to Earth’s temperature without the CO2 cycle?
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