Astronomy 1400 – 001:
Solar System Astronomy
Lecture 31: Planetary Atmospheres II
Monday, November 10, 2014
Saturday, over Texas
Review of last lecture:
3 ways to add to atmosphere
Outgassing
Sublimation
Impacts
4 Ways to Remove Atmosphere
Rule of thumb:
For example: Oxidation — e.g.,
oxygen combines with metals (iron
—rust, aluminum, magnesium)
The Moon and Mercury
Tenuous Exospheres Only Through Surface Ejection
Mars: A Cold, Desolate, Inhospitable Place
•  Pressure = 1% of Earth’s
•  Atmospheric content
-  But the density is so low, the greenhouse effect is small
➡  You couldn’t breathe
•  No ozone to protect from UV
Mars: A Planet of Extremes
Remember, tilt and eccentricity can cause seasons (HW#2):
Tilt: 25.2°
(Earth: 23.5°)
Eccentricity: 0.093
(Earth: 0.017)
Southern Hemisphere
has long, cold
winters
Southern Hemisphere
has short, hot
summers
But tilt ranges 0° – 60°
over millions of years
1.  Jupiter pulls on it
2.  No stabilizing Moon
Extreme Seasons Means
Extreme Changes
1.  Some CO2 at poles melts
2.  Increased atmospheric pressure
circulates it toward other pole
3.  Drives planet-wide dust storms!
⇐
If all of the known water at the
poles melted, it would make an
ocean ~10 meters deep (over the
whole planet)
Why is the sky on Mars red?
•  Red dust in the air!
•  Wind, dust devils, and dust storms keep lots of red dust in the
air
Size Matters!
Mars probably did have an ocean billions of years ago, but what happened to it?
If the interior is too cool:
1.  No more outgassing to replenish atmosphere
2.  No magnetic field to protect atmosphere from solar wind
Runaway Cooling Effect
•  Surface T depends more on Sun than
internal heat (directly). •  Cooling interior  lessened magnetic field
•  Less magnetic field  More CO blown
2
away by solar wind
•  Less CO  Cooler atmosphere
•  Cooler atmosphere  CO condensing
•  Less CO  Cooler atmosphere
2
2
2
What happened to Mars’ Water?
•  Some frozen, but most is gone
•  No statosphere to absorb UV
•  UV breaks up water molecule, hydrogen floats off into
space
Venus
Another Even More
Extreme World
•  Pressure = 90× Earth
-  Walking there is like walking
~1 km below the ocean!
•  Very dull atmosphere
-  Little wind
-  Uniform temperatures
•  Content
-  Mostly CO
2
Venus without greenhouse
effects would be colder than Earth
because its clouds are so
reflective
Why is it so hot?
200,000x as much CO2 in
atmosphere as Earth!
Where did it come from?
Not different outgassing –
similar compositions at birth
Earth actually has
similar amount of
CO2, but it’s locked
up in solids!
What would happen to Earth’s
surface temperature if we
moved it to Venus’ orbit?
a) Nothing, distance to the Sun does not affect
temperature, the size of the planet does.
b) A higher surface temp would evaporate
oceans, put greenhouse gases into air, raising
surface temp even more
c) Solar wind would remove Earth’s atmosphere
and greenhouse gases, lowering surface temp
What would happen to Earth’s
surface temperature if we
moved it to Venus’ orbit?
a) Nothing, distance to the Sun does not affect
temperature, the size of the planet does.
b) A higher surface temp would evaporate
oceans, put greenhouse gases into air, raising
surface temp even more
c) Solar wind would remove Earth’s atmosphere
and greenhouse gases, lowering surface temp
Venus
A Runaway Greenhouse Effect
• No magnetic field (slow rotation)
➡  Water evaporated and then UV destroyed water
and oxygen/hydrogen escaped/pushed out by solar
wind
But the amount of CO2 is
about the same as Earth!
What did Earth do with
all of its CO2?…
Venus either never formed oceans,
or they evaporated quickly
•  No rain  –  CO left in atmosphere
–  Water vapor destroyed by UV
2
(because Venus has no magnetic field
either).
•  Venus was not as reflective then as it is
now – it’s atmosphere has changed
•  Brightening Sun could play some role as
well
Why Didn’t Earth Suffer the
Same Fate as Venus?
I. The CO2 Cycle
•  There is ~170,000× as much CO
2
rocks than in atmosphere!
•  Another thermostat…
in
But the CO2 Cycle Works Over Hundreds of
Thousands of Years
What if it can’t adjust fast enough?
(small changes in tilt and eccentricity work faster)
⇓
Ice ages – “Snowball Earth” – “Hot house Earth”
“Cambrian Explosion”
What About the Harmful UV?
1.  Magnetic field
protects water in
atmosphere
2.  Life makes
oxygen
3.  O2 + UV = O3!
4.  Life is why we
have Oxygen
and a
stratosphere
•  Continue Reading Chapter 10
•  Homework 8 due Friday