1. Theme 9 – Planetary Atmospheres ASTR 101 Prof. Dave Hanes

2. What Characterizes an Atmosphere? [many interrelated properties] Composition Composition Constituent gases Constituent gases Suspended particles (dust, etc) Suspended particles (dust, etc) Condensates (clouds of moisture, ices) Condensates (clouds of moisture, ices) Biological importance / inferences Biological importance / inferences Interactions with light Interactions with light Colour Colour Transparency Transparency Greenhouse effect Greenhouse effect Temperature (and its dependence on altitude) Temperature (and its dependence on altitude) Pressure (and its dependence on altitude) Pressure (and its dependence on altitude) Circulation systems Circulation systems Heat flow Heat flow Climate and weather Climate and weather

3. Composition The Jovian Planets: H compounds, lots of He, and H compounds, lots of He, and deep clouds of complex molecules deep clouds of complex molecules Terrestrial planets: [Mercury has no atmosphere] Mars and Venus: Almost pure carbon dioxide Earth: ~ 80% N 2, 20% O 2, water in the form of vapour and clouds vapour and clouds The O 2 is surprising! A clear sign of life! The O 2 is surprising! A clear sign of life!

4. The Clouds of Jupiter

5. On Earth: Blue Skies

6. “Rayleigh Scattering” by gas molecules (O 2 N 2 H 2 O etc) Blue light is scattered (bounced) more than red light Blue light is scattered (bounced) more than red light

7. …Hence Red Sunsets

8. 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.

9. Suspended Particles

10. 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).

11. 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%

12. By Contrast: Air Temperature Varies with Altitude (note that the variations are fairly complex)

13. 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.)

14. 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.

15. It is Cold on Mt Everest! Why? It is Cold on Mt Everest! Why?

16. 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.

17. 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)

18. 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?

19. 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:

20. 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.

21. 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.

22. The Stratosphere Ultraviolet light (the‘tanning rays’) from the Sun penetrates down to the stratosphere, which is where we find an abundance of ozone (O 3 ). 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.)

23. The ‘Ozone Hole’ - depletion of O 3 over large areas

24. The Troposphere The energy is coming in from the Sun (at the top). So why is it warmest at the bottom?

25. 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.