Theme 9 – Planetary Atmospheres

ASTR 101Prof. Dave Hanes

What Characterizes an Atmosphere?

[many interrelated properties] Composition

Constituent gases Suspended particles (dust, etc) Condensates (clouds of moisture, ices) Biological importance / inferences

Interactions with light Colour Transparency Greenhouse effect

Temperature (and its dependence on altitude) Pressure (and its dependence on altitude) Circulation systems

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)

Blue light is scattered (bounced) more than red light

…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 AltitudeHence high-altitude sickness, the need for pressurized aircraft, and so on

Mauna Kea: 60% of sea level pressureAt ALMA: 50%On Everest: 40%

By Contrast:Air Temperature Varies with Altitude

(note that the variations are fairly complex)

The Circulation of HeatWarm 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 EarthWe see coloured bands and complex cloud motions in Jupiter’s thick atmosphere, thanks to its rapidrotation.

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 (thevery outer parts of theatmosphere)

The stratosphere (about

30-50 km up)

The troposphere (thethickest air, near theEarth’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 cosmicrays (fast-moving charged particles,mostly from the Sun in the “solarwind.”) They collide with particles atthe top of the atmosphere and kickthem up to high velocity - that is, they heat the gas.

(But youwould 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, whichis 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 turnheats the air immediately above it.

This happens through the greenhouse effect and can lead to global warming.