Astronomy190 - Topics in Astronomy

Astronomy and Astrobiology

Lecture 6 : Atmospheric Science

Ty Robinson

Questions of the Day• How do atmospheres help make a world

habitable?

• Why/how do the compositions of the atmospheres of Mars, Venus and Earth differ?

• How does the greenhouse effect work to warm a world’s surface?

• What factors affect the ability of a world to maintain its atmosphere?

Scale

Radius of Earth = 6,300km Height of Atmosphere = 100km

Atmospheric Pressure

Pressure [bars]

Height

10

Questions?What would happen to Earth’s oceans if we did not have an atmosphere?

our oceans would sublimate

the oceans will remain liquid, but expand

our oceans would freeze through

nothing

Water Phase Diagram

An atmosphere is required for a world to have liquids on its surface!

[bars]

Earth’s Atmospheric Composition

Gas Amount [%]Nitrogen 78

Oxygen 21

Water 1 - 4

Carbon Dioxide 0.04

Methane 2 x 10-4

Ozone 7 x 10-6

Venus, Earth and Mars

Surface Pressure [bars] 0.01 1 100

Composition [%]

Nitrogen 3 72 3

Oxygen 0.1 21 0

Water 0.03 1 - 4 0.01

Carbon Dioxide 95 0.04 97

Methane ? 2 x 10-4 0

Ozone 0 7 x 10-6 0

Volcanic Gasses

Water (> 60%)CO2 (>10%)

Nitrogen

SO2, H2S, H2 …

Earth’s Atmospheric Structure : Troposphere

Lapse Rate8º C per km

Questions

What happens to water vapor

as you lift it higherinto the

atmosphere?

Why is the surface the

warmest part of the troposphere?

Absorption of solar radiation at

the surface

The gas cools andcondenses toform clouds.

Earth’s Atmospheric Structure : Stratosphere

QuestionWhy does the temperature

increase with height in the

stratosphere?

UV light

O3

O2O

(absorption)

Earth’s Atmospheric Structure : Mesosphere

How do thestructure and composition of

the atmosphere determine surface

conditions?

Transmission/Absorption of UV and Vis Light

(Sun)

Ozone Layer

Cloud

Cloud

Surface

In the UV and visible…

(heating)

(heating)

(scattering)

(scattering)

Albedo

(Bolometric) Albedo = Reflected Light / Incident Light

Albedo 0.15 0.37 0.65

Now for the infrared (IR)…

Effective

Temperature [ºC]

-55 -17 -43

4πR2σTe4 = S(1-A) πR2

Greenhouse Gasses

Transmittance : how wella gas transmits radiation

Low transmittanceimplies the gas is a

good absorber

Important greenhousegasses (on Earth):

H2OCO2

O3

CH4

Greenhouse Effect I

In the infrared…

Ozone Layer

Cloud

Cloud

Surface

(absorption)

(absorption)

(absorption)

(absorption)

(emission)

Greenhouse Effect II

In the infrared…

Ozone Layer

Cloud

Cloud

Surface

(emission)

(emission)

(emission)

(absorption)

(emission)

Surface Temperatures

Effective

Temperature [ºC]

-55 -17 -43

Surface

Temperature [ºC]

-50 15 470

Greenhouse

Warming

[ºC]

5 32 513

Atmospheric Escape

Surface Pressure [bars] 0.01 1 100

Why do worlds even have atmospheres?

Questions?What factor(s) determine a world’s ability to retain an atmosphere?

temperature, gravity, magnetic field and atmospheric composition

planetary composition

gravity and atmospheric composition

magnetic field

Gas Escape I

gravity

velocityEach molecule in Earth’s upper atmosphere has a velocity (whichdepends on its mass and surroundingtemperature). If this velocity is large enough, a molecule can escape Earth’s gravity.

Earth’s magnetic field prevents theSolar wind from stripping away our atmosphere.

Gas Escape II

Mercury: weak magnetic field, low gravity, high temperature : no atmosphere

Venus: no magnetic field, high gravity, relatively high temperature : thick atmosphere even after a great deal of gas has escaped

Moon: no magnetic field, low gravity, same distance from the Sun as Earth : no atmosphere

Gas Escape III

Earth

H atoms

Gas Escape IV

Question

Why does Mars have such a thin atmosphere?

Massive impacts can also drive off atmospheric gasses, especially on worlds with low gravity.

Questions of the Day• How do atmospheres help make a world

habitable?

• Why/how do the compositions of the atmospheres of Mars, Venus and Earth differ?

• How does the greenhouse effect work to warm a world’s surface?

• What factors affect the ability of a world to maintain its atmosphere?

Quiz

1 - Consider two stacked sedimentary layers. The lower layer has been determined to be 1.5 Gyrs old. Most likely, how was this age determined? What can we deduce about the upper layer?

2 - Describe the different processes that can affect visible radiation as it passes through Earth’s atmosphere.

3 - What is one thing you did not understand from today’s lecture?