Varying electric and magnetic fields - either light waves or particles - photons.

RADIO IR VISIBLE UV XRAYS GAMMA RAYS IR = InfraRed UV = UltraViolet

Low Frequency         High Frequency

Large Wavelength     Small Wavelength
 

Wavelength of maximum emission is inversely proportional to temperature.
Hotter objects emit at shorter wavelengths / higher frequency

(7) Spectroscopy

Emission Lines - Figure 7.13

A hot gas emits light at specific characteristic wavelengths (in contrast to a continuous rainbow) .

Absorption Lines- Figure 7.5

Light comes to us through a gas ( atmosphere of star or planet) - the gas absorbs light at (the same) wavelengths.

Spectral "Fingerprints" (Figures 7.8, 7.9, 7.10)

Each atom (or molecule) emits / absorbs at characteristic wavelengths.
These spectral emission / absorption signatures indicate composition of the emitting / absorbing gas.
Molecules have many more lines than atoms.

(8) Inverse Square Law

The intensity received decreases with the square of the distance from the source (I 1/D²⁾

Key Words: Electromagnetic Spectrum, wavelength, frequency, speed of light, photon, nanometer, Kelvin, thermal radiation, intensity, lmax, T⁴, Spectroscopy, emission lines, absorption lines, composition, molecules

B - Planetary Atmospheres

Equilibrium Temperature

See Figure 10.3

 

Energy absorbed from VISIBLE sunlight Decreases away from the Sun Black/white objects absorb/reflect more sunlight

Energy radiated in IR thermal emission Radiates out to space Greenhouse gases (CO2, H2, O3, CH4) absorb IR as it tries to escape

Which solar system objects have atmospheres

• Moon, Mercury - no atmosphere to speak of
• Earth - yes, atmosphere of mostly N₂ and some O₂
• Venus - yes, thick atmosphere of mostly CO₂
• Mars - yes, tenuous atmosphere of mostly CO₂
• Other objects? Titan, Triton, Io
• What do these objects have in common?
  • Volcanoes - source of atmospheric gases
  • Larger objects - with enough gravity to hold gases

Summary - Climate Change

On the topic of the evolution of planetary atmospheres, we are left with lots of questions, rather than facts, to summarize:

• Could the change in the Earth's atmosphere - such as due to human-generated CO₂ - lead to a change in Earth's climate - such as send us to a runaway greenhouse effect, like Venus?

• Direct measurements of CO₂ and the global, average temperature show increases since the industrial revolution - suggesting they are due to human activity. What will happen as human activity is increasing exponentially?

• Measurements of CO₂ and temperature from Greenland ice cores going back hundreds and thousands of years, suggest that there have been much stronger variations in CO₂ and temperature in the past - due to changes in the Earth's orbit. Does this geological evidence of climate change in the past raise concerns or reassure us that the Earth's climate is stable? (That is change - but not become "runaway")

• There are several theories about climate evolution. Right now we do not know enough about our atmosphere (and its relationship to the ocean) or about the atmospheres of other planets to say which theory is correct.

• Climate change - theory 1: Subtle changes in temperature can send a planet towards either runaway greenhouse effect or towards a runaway icebox. This limits the "habitable zone" in a solar system to a very narrow region. This would mean that the earth's atmosphere is precariously balanced - it could tip catastrophically in either direction.

• Climate change - theory 2: The biosphere is self-stabilizing (Gaia hypothesis) - it reacts to changes to maintain habitable environment for life. This means there is a large range in conditions that could be habitable and there could be life all over the solar system. This also means that we do not need to worry about pollution...or does it?

C - Giant Planets
 

Importance of Rotation

•       Jupiter, Saturn, Uranus and Neptune are low-density "blobs" of gas/liquid (fluid) that are pulled into a spherical shape by gravity.
•       The 10-15 hour rotation periods mean that rapid rotation pulls their equatorial regions out, making the planets oblate.
•       Rotation is also a major factor in shaping the weather on the giant planets.

•       The white clouds on Jupiter and Saturn are fresh Ammonia ice clouds
•       The orange/red/brown clouds are produced by the same process as the photochemical smogs of big cities (e.g. Denver).
•       Sunlight (particularly UV) hitting simple hydrocarbons (e.g. methane) produce more complex hydrocarbons. These react with
•       molecules that contain sulfur and nitrogen to make dirty yellow/brown compounds.
•       In the atmospheres of Uranus and Neptune there are higher concentrations of methane which absorbs the RED part of sunlight, so
•       that only the BLUE part of the sunlight is reflected - making these planets look green/blue. Uranus and Neptune have high altitude,
•       white clouds which are condensed methane.

•       The striped pattern of alternating light and dark bands correspond to jets blowing in opposite directions, producing a strong WIND
•       SHEAR
•       The wind shear produces EDDIES - circulating storms, much like the cyclones and anti-cyclones on Earth.
•       The Great Red Spot on Jupiter is such an eddy. It has been observed since the first telescopes were used to look at Jupiter in the
•       1600s.
•       Saturn, Neptune and, to a lesser extend, Uranus, all show similar weather patterns:- strong east and west jets and eddies generated in
•       the wind shears.

•       The tilt of the planet's spin axis determines the strength of the seasons experienced by the planet.
•       Jupiter's equator is very close to the ecliptic (small spin axis tilt) so that it experiences very weak seasons
•       Saturn and Neptune have spin axis tilts very similar to Earth - and hence have similar seasonal effects.
•       Uranus is tipped on its side to that it has very strong seasons.