1. Astronomy190 - Topics in Astronomy
Astronomy and Astrobiology
Lecture 6 : Atmospheric Science
Ty Robinson

2. 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?

3. Height of Atmosphere = 100km
Scale
Radius of Earth = 6,300km
Height of Atmosphere = 100km

4. Height 1 Pressure [bars] At depth in the ocean? >1,000 bars
Atmospheric Pressure
Height 1
Pressure [bars]
At depth in the ocean? >1,000 bars

5. 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
Answer = c

6. An atmosphere is required for a world to have liquids on its surface!
Water Phase Diagram
An atmosphere is required for a world to have liquids on its surface!
[bars]
water on the Moon

7. 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
Nitrogen fixing, diazotroph [di : two ; azot : Nitrogen ; troph : eat]
Dimethyl sulphide

8. Surface Pressure [bars]
Venus, Earth and Mars
Surface Pressure [bars]
0.01 1
100
Composition [%]
Nitrogen 3 72
Oxygen
0.1 21
Water
0.03
1 - 4
0.01
Carbon Dioxide 95
0.04 97
Methane ?
2 x 10-4
Ozone
7 x 10-6

9. Volcanic Gasses
Nitrogen
Water (> 60%)
CO2 (>10%)
SO2, H2S, H2 …

10. Earth’s Atmospheric Structure : Troposphere
Questions
Why is the
surface the
warmest part of
the troposphere?
Absorption of
solar radiation at
the surface
Lapse Rate
8º C per km
What happens
to water vapor
as you lift it higher
into the
atmosphere?
The gas cools and
condenses to
form clouds.

11. Earth’s Atmospheric Structure : Stratosphere
Question
Why does the
temperature
increase with
height in the
stratosphere?
UV light O3
(absorption) O2 O

12. Earth’s Atmospheric Structure : Mesosphere
How do the
structure and
composition of
the atmosphere
determine surface
conditions?

13. Transmission/Absorption of UV and Vis Light
In the UV
and visible…
(Sun)
(heating)
Ozone Layer
Cloud
(scattering)
Cloud
(scattering)
(heating)
Surface

14. Albedo
(Bolometric) Albedo = Reflected Light / Incident Light
Albedo
0.15
0.37
0.65
4πR2σTe4 = S(1-A) πR2
Effective
Temperature [ºC]
-55
-17
-43
Now for the infrared (IR)…

15. Transmittance : how well a gas transmits radiation
Greenhouse Gasses
Transmittance : how well
a gas transmits radiation
Low transmittance
implies the gas is a
good absorber
Important greenhouse
gasses (on Earth):
H2O
CO2 O3
CH4

16. In the infrared… Ozone Layer (absorption) Cloud (absorption)
Greenhouse Effect I
In the infrared…
Ozone Layer
(absorption)
Cloud
(absorption)
(absorption)
Cloud
(absorption)
(emission)
Surface

17. In the infrared… Ozone Layer (emission) Cloud (emission) (emission)
Greenhouse Effect II
In the infrared…
Ozone Layer
(emission)
Cloud
(emission)
(emission)
Cloud
(emission)
Surface
(absorption)

18. Effective Temperature [ºC] -55 -17 -43 Surface -50 15 470 Greenhouse
Surface Temperatures
Effective
Temperature [ºC]
-55
-17
-43
Surface
-50 15
470
Greenhouse
Warming
[ºC] 5 32
513

19. Surface Pressure [bars]
Atmospheric Escape
Why do worlds even have atmospheres?
Surface Pressure [bars]
0.01 1
100

20. 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
Answer = a

21. velocity gravity Earth’s magnetic field prevents the
Gas Escape I
Earth’s magnetic field prevents the
Solar wind from stripping away our
atmosphere.
velocity
Each molecule in Earth’s upper
atmosphere has a velocity (which
depends on its mass and surrounding
temperature). If this velocity is
large enough, a molecule can escape
Earth’s gravity.
gravity

22. Mercury: weak magnetic field, low gravity, high
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

23. Gas Escape III
H atoms
Earth
Features on the Earth’s disk (dayglow from the sunlit atmosphere, a northern auroral oval, and equatorial airglow) are due to the emission of atomic O at and nm and emission of molecular N2 ( nm). 93,000 metric tonnes of hydrogen escapes each year.
9.3x10^10 g/(3 x 10^7 s/yr) ~ 3x10^3 g/s = 3 kilograms per second

24. Why does Mars have such a thin atmosphere?
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.

25. 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?

26. 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?