2000 February 10, SPS 1020 (Introduction to Space Sciences)

- Mid-Term Exam 1 will be Thursday, February 17.
The exam will cover: TNSS 1-2, 6-13, PBD Intro, 1, 10-12, 14, and 
The Basics of Space Flight Handbook, Chapters 1-17.

- Tuesday, Feb 15 will be a discussion and review session for the exam.
Please think up questions, so we can go over them!

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Terrestrial Planet Atmospheres (TNSS Ch. 13):

Origin and Evolution -

1) Primary: formed by accretion of volatile elements in Solar Nebula
(H/He) Lost completely from all terrestrial planets during Sun's late T
Tauri wind phase.

2) Secondary: Outgassing from Volcanoes
NO free O_2 yet, on Earth or any other planet

~ 85% H_2 O vapor - precipitated out during cooling
~ 10% CO_2, CO    - chemically bound now in Earth's crust (limestone
                    rocks), still around on Venus, Mars
~  5% H_2, CH_4, NH_3, H_2 SO_4: Primordial "soup"
		more later, when we do pre-biotic chemical evolution,
		although life *couldn't* have started in Earth's present
		oxidizing atmosphere

3) Tertiary, on Earth: anaerobic life forms (e.g. blue-green algae)  
"poisoned" atmosphere over 1.5 Gyr.  Gave it its present composition:

Earth (by mass) 78% N_2
                21% O_2
             ~0.03% CO_2
	     0.1-3% H_2 O

Contrast this with Venus and Mars (both reducing atmospheres):

Venus           96% CO_2
       	       3.5% N_2

Mars            95% CO_2
                 3% N_2

In all, this result is from the interplay of volcanism, water, the
planet's crust, and especially for Earth, LIFE.


Present structure:

Why Earth's sky is blue: Rayleigh *scattering* ~1/lamba^4 
	(lambda = wavelength)
Violet light is scattered (7000 A/4000 A)^4 = 9x more than red

Why Earth's sunsets are red: dust extinction (*absorbs* blue light more)


Mars: sky is red, from dust: atm not dense enough for Rayleigh scattering,
except at sunset/rise, which are blue




Hydrostatic equilibrium:





=> P = Po exp(-h/H), 

(i.e. pressure drops off exponentially)

P = atmospheric pressure
Po = atmospheric pressure at Sea Level (= 1 bar)
h = altitude above Sea Level
H = scale height of the atmosphere
  = RT/mg,
	R = gas constant from ideal gas law PV = nRT
	T = temperature
	m = mass of one mole of atmospheric gas
	g = acceleration of gravity


density Venus atm ~ 100 density Earth atm ~ 100 density Mars atm

Mercury and Earth's Moon can't hold onto significant atmospheres: 






Classification of layers

- Exosphere: Essentially just the solar wind
	h > 250 km

- Thermosphere (also called the Ionosphere): T increases to 2000 K.
	h > 90 km
	"Upper layer": Short-wave radio waves reflect off here, propagate
	around Earth.  See Figure 2, p. 177 of TNSS: it gets hot here!

- Mesosphere:
	50 < h(km) < 90
	T ~ minimum (180 K) at 80-85 km
	Most meteors here.

- Stratosphere: 
	40 < h(km) < 50
	T ~ const; Jet streams
	O_3 (ozone) layer and growing *polar hole*

	Summary of ozone problem:
	Chlorofluorocarbons (CFCs) react with O_3, deplete it.
	Atm circulation concentrates O_3 at poles on ice crystals
	=> Antarctic hole in spring
	Discovered in 1980s - grown seasonally by ~ 1/3 over last decade

- Troposphere:
        6 < h(km) < 10
	"Weather layer": convective, circulating air transfers heat from
	ground (airplane turbulence, astronomical "seeing").
	=> All clouds

Hadley cells between poles and equator:


Venus					Earth
























Mars has global dust storms, near perihelion



Earth's CO_2 problem:
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CO_2 is a greenhouse gas: it increases atmospheric T by absorbing heat and
	re-radiating it only after much molecular scattering.

CO_2 content of Earth's atmosphere has increased by ~10% since 1958.
	(***handout from Sagan, Billions and Billions)

=> Global temperature has risen by ~ 0.7 C since 1880s.
	1990s are warmest decade on record.

Effects of global warming:
	- More severe storms
	- Rising sea level => coastal erosion
	- Expanding deserts
	- Northward shift of agricultural areas
	- Economic impact