Dust Storms Mars and Earth

Transcription

1 Dust Storms Mars and Earth

2 Bit of Administration. Reading BSNV Chaps. 10, week exam! Tonight, 7:15-8:30 pm, 3425 Sterling Hall Mathieu office hours for pre-test questions All day today

3 Nuclear Fusion - A Digression Thermonuclear Hydrogen Fusion 4 H => He Hydrogen Why Thermonuclear? Bringing two positive charges together Requires high velocity = high temperature Deuterium Positron Neutrino Helium 3 Helium 4

4 Nuclear Fusion - A Digression Thermonuclear Hydrogen Fusion D + T => He + n

5 Nuclear Fusion - A Digression Thermonuclear Hydrogen Fusion D + T => He + n

6 Nuclear Fusion - A Digression Thermonuclear Hydrogen Fusion D + T => He + n

7 Nuclear Fusion - A Digression Thermonuclear Hydrogen Fusion D + T => He + n Advantages Fuel source - deuterium, tritium from water - tritium also from reactors No risk of nuclear accident No air pollution Minimal radioactivity?

8 The Big Idea of Solar System Evolution The Second Law of Thermodynamics Heat can never pass spontaneously from a colder to a hotter body. As a result of this fact, natural processes that involve energy transfer must have one direction, and all natural processes are irreversible.

9 Profound Consequence: Equilibrium requires an Energy Source

10 Planetary Atmospheres Earth 77% N 2, 21% O 2 15 o C 1 atmosphere Mercury Trace of He, Na, O Venus 96% CO o C 90 atmospheres Mars 95% CO 2-50 o C 0.01 atmospheres Moon Trace of He, Na, Ar

11 Planetary Atmospheres Retention of Atmosphere Atmospheric particles (atoms, molecules, dust) respond to gravity in same way as any body Atmosphere will escape if velocities of atmospheric particles (THERMAL VELOCITY) are greater than ESCAPE VELOCITY of planet

12 Planetary Atmospheres Escape Velocity - a threshold velocity Escape Velocity - Minimum speed at which two objects will not be held together by their mutual gravity V escape = 2GM R M R

13 Planetary Atmospheres Thermal Velocities of Atmospheric Particles Values for speeds are just for example - depends on temperature

14 Planetary Atmospheres Peak Thermal Velocity Note V thermal increases with higher T Note V thermal decreases with higher m particle V thermal = 2kT m particle m particle = mass of atom or molecule T = temperature in o K k = Boltzmann constant = 1.4 x joules/k

15 Planetary Atmospheres Peak Thermal Velocity Same mass particles Low temperature Number of particles Medium temperature High temperature 0 Velocity of particles

16 Planetary Atmospheres Peak Thermal Velocity Same temperature High mass particles (e.g., CO 2 ) Number of particles Medium mass particles (e.g., H 2 O) Low mass particles (e.g., H) 0 Velocity of particles

17 Planetary Atmospheres Peak Thermal Velocity

18 Planetary Atmospheres Retention of Atmosphere Rule of Thumb - if V esc > 5 x V thermal, then retain an atmosphere Surface Escape Tension Velocity of drop of Water Low temperature Medium temperature Number of particles High temperature Escape Evaporation from Planet 0 Velocity of particles

19 Planetary Atmospheres Retention of Atmosphere Example - consider Earth and Moon - T 300 K H = 1.7 x kg O = 2.7 x kg H 2 O = 3.0 x kg For H, V thermal = 2 x 1.4x10-23 x x = 2300 m/sec = 2.3 km/sec For H 2 O, V thermal = 2 x 1.4x10-23 x x = 500 m/sec = 0.5 km/sec

20 Planetary Atmospheres Retention of Atmosphere Example - consider Earth and Moon - T 300 K V escape, Earth V escape, Moon, Earth = 11.2 km/sec No H, retains H 2 0 escape, Moon = 2.4 km/sec No H, no H 2 0 For H, V thermal = 2.3 km/sec For H 2 O, V thermal = 0.5 km/sec

21 ConcepTest! The escape velocity of Mercury is 4.3 km/sec. The escape velocity of Mars is 5.0 km/sec. Mercury does not have a substantial atmosphere; Mars has an atmosphere. This is primarily because A) Mars is further from the Sun so any gases are much colder B) Mars has a 20% higher escape velocity C) CO 2 has more mass on Mars than on Mercury D) Mars is a dynamically active planet with active volcanoes

22 Greenhouse Effect Planetary Atmospheres 90 atmospheres!

23 Greenhouse Effect Planetary Atmospheres 240 watts/m 2 in 50% reflected by clouds and surface Small fraction escapes into space 240 watts/m 2 out CO 2 H 2 0

24 Greenhouse Effect Planetary Atmospheres Energy In Energy Out 200 energy units 100 energy units stored 100 energy units in equlibrium 100% gate 50% gate

25 Greenhouse Effect Planetary Atmospheres Energy In Energy Out 10, 000 energy units 100 energy units stored 100 energy units in equlibrium 100% gate 1% gate

26 Greenhouse Effect Planetary Atmospheres