Download Planetary Atmospheres

Theme 9 – Planetary Atmospheres
ASTR 101
Prof. Dave Hanes
What Characterizes an Atmosphere?
[many interrelated properties]
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Composition
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Interactions with light
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Constituent gases
Suspended particles (dust, etc)
Condensates (clouds of moisture, ices)
Biological importance / inferences
Colour
Transparency
Greenhouse effect
Temperature (and its dependence on altitude)
Pressure (and its dependence on altitude)
Circulation systems
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Heat flow
Climate and weather
Composition
The Jovian Planets:
H compounds, lots of He, and
deep clouds of complex molecules
Terrestrial planets:
[Mercury has no atmosphere]
Mars and Venus: Almost pure carbon dioxide
Earth: ~ 80% N2, 20% O2, water in the form of
vapour and clouds
The O2 is surprising! A clear sign of life!
The Clouds
of Jupiter
On Earth:
Blue Skies
“Rayleigh Scattering”
by gas molecules (O2 N2 H2O etc)
…Hence Red
Sunsets
Clouds
Water droplets (or ice crystals in cirrus clouds) are
big compared to the wavelength of light. No
colour dependence, so clouds are white or grey.
Suspended Particles
Aurora Borealis (and Australis)
- Northern (and Southern) Lights
Charged particles in
the solar wind are
deflected by the
Earth’s magnetic field,
directed towards the
Poles.
Collisions with
particles in the upper
atmosphere make the
gas fluoresce (glow).
Air Pressure Drops
Steadily with Altitude
Hence high-altitude
sickness, the need for
pressurized aircraft, and
so on
Mauna Kea: 60% of
sea level pressure
At ALMA: 50%
On Everest: 40%
By Contrast:
Air Temperature
Varies with Altitude
(note that the
variations are fairly
complex)
The Circulation
of Heat
Warm air rises, and cool air
from other latitudes moves
in. Large circulation cells
and weather patterns (the
trade winds, the jet
stream) result. In this
way, heat is redistributed
around the planet.
(There are similar effects
in the oceans: think of the
Gulf Stream.)
Not Just on Earth
We see coloured bands and complex
cloud motions in Jupiter’s thick
atmosphere, thanks to its rapid
rotation.
On Jupiter, the clouds differ
in composition. (On Earth, all
clouds are water.)
The Great Red Spot is like a
long-lasting hurricane, larger
than the Earth itself.
It is Cold on Mt Everest!
Why?
First, Let’s Remember
What Temperature Means
Temperature = a measure of the energy within a solid body or in
a substance (e.g. a fluid body or a cloud of gas)
This energy is contained in the random jiggling or moving about
of atoms within the body or substance (but not the overall
directed motion of the object! An asteroid moving quickly
through empty space is not ‘hot’ by virtue of that speed.)
But warm material should lose its heat and cool off: the particles
should gradually lose their energy of random motion
That lost energy is emitted as radiation.
Three ‘Hot’
Regions
The thermosphere (the
very outer parts of the
atmosphere)
The stratosphere (about
30-50 km up)
The troposphere (the
thickest air, near the
Earth’s surface)
Two Obvious Questions
You expect the warm parts of the atmosphere to
cool down over time. What keeps them warm?
Why is there such a complex temperature profile
in the atmosphere? Why does the temperature
differ so irregularly from one place to another?
What is The Source of
Atmospheric Heating?
In principle, energy could come from above (the
Sun) or from below (within the Earth itself),
but remember:
Consequently
[and perhaps surprisingly…]
The interior heat of the Earth is essentially irrelevant
All three of these regions are heated by energy from
the Sun, although, paradoxically, the troposphere
(the lowest zone) is heated from beneath.
This is thanks to the ‘greenhouse effect,’ as we will
see.
Start from
the Top
The thermosphere is heated by X-rays
(very energetic radiation) and cosmic
rays (fast-moving charged particles,
mostly from the Sun in the “solar
wind.”) They collide with particles at
the top of the atmosphere and kick
them up to high velocity - that is, they heat the gas. (But you
would not feel warm up there! The gas is too thin.)
Some fast-moving particles escape: consequently, the top of this
region is called the exosphere. Many of the particles are ionized
(their electrons are torn off) so this is also the ionosphere.
The Stratosphere
Ultraviolet light (the‘tanning
rays’) from the Sun penetrates
down to the stratosphere, which
is where we find an abundance
of ozone (O3).
Ozone preferentially absorbs UV light and its inflowing
energy. (Solar UV-B is 350 million times stronger at the
top of the atmosphere than at ground level! Only a trickle
gets through.)
The ‘Ozone Hole’
- depletion of O3 over large areas
The Troposphere
The energy is coming in from the Sun (at the top). So
why is it warmest at the bottom?
Heating from Below
The soup is heated from below, and the
hottest part of the soup is the bit closest to
the element (which is heated by an electric
current or by burning gas).
The troposphere is also heated
from below, but the fundamental
source of energy is the Sun! It
heats the ground, which in turn
heats the air immediately above it.
This happens through the greenhouse
effect and can lead to global warming.