MULT10011 Lecture Notes - Lecture 30: Circumstellar Habitable Zone, Escape Velocity, Runaway Greenhouse Effect
15 Jun 2018
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L.E.U – Lecture 30
Solar System planets
Terrestrial planets
• There are 4 planets in the inner part of the solar system – Mercury, Venus, Earth and
Mars
• Key properties
o Composition – earth-like → rocks and metals (e.g. iron)
o Size – comparable to earth
o Location – close to sun, therefore warm to hot
o Atmosphere? – on earth and Venus, traces on mars
o Moons – very few → our moon, and Deimos and Phobos around mars
• Mercury – barren hot rock, like the moon
• Venus – similar to earth, but with ~70x atmosphere → runaway greenhouse
• Earth – goldiloks plaet – just right conditions
• Mars – next most hospitable; potential for life
o Has it already been colonised? Did it die out?
Jovian planets
• There are 4 planets in the outer part of the solar system – Jupiter, Saturn, Uranus
and Neptune
• Key properties
o Composition – sun-like → hydrogen, helium and trace amounts of others
o Size – big → 40-300x Earth
o Location – far from the Sun → therefore cold
o Atmosphere – gaseous, no surface
o Moons – many → hundreds altogether, in miniature solar systems
• Jupiter – the red plaet, e a see stors i the atosphere
• Saturn – magnificent ring system
• Uranus – tipped on its side due to major collisions in formation stage
• Neptune – furthest out of the major planets
• No opportunity for life on these planets, but some of their moons are promising
Escape velocity: can a planet keep an atmosphere?
• There are 2 physical characteristics which determine whether a planet will hold into
a molecule in its atmosphere:
o The temperature of the gas
o The gravitational field at the surface
Larger velocity allows an object to escape
• Doest atter if its a roket or a oleule, eloity is sae –Earth EV is 11km/sec
• Temperature – all molecules at same temp have same kinetic energy (1/2mv2)
o For a fixed temp, lighter molecules have a higher velocity (heavier = lower)
o Kinetic energy and velocity increases with temperature
o Sall, hot plaets at keep light oleules (H), can keep heavy ones (CO2)
o Big, cool planets like Jupiter keep everything from hydrogen onwards
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