1. Astronomy 340 Fall 2005
27 September 2005
Class #7

2. 90 Minutes Doing Your Homework

3. Review
CO molecule – Rayleigh-Jeans approximation  substitute temperature for intensity in radiative transfer eqn.
Tb = (λ2/2k)Bλ
Tb(s) = Tb(0)e-τ(s)+T(1-e-τ(s))
Planetary Surfaces
Processes at work: impact, weathering, atmosphere, geology (tectonics/volcanic
Mercury: heavily cratered, no tectonics
Venus: global resurfacing 300 Myr ago, no tectonics
Mars: water, older volcanoes
Earth: tectonics, water, weather

4. Surface Composition Reflection spectroscopy  derivation on board.
What is the surface made of? Rocks, mostly igneous
Minerals = solid chemical compounds with specific atomic structure

5. Common Minerals Silicates Various Oxides
Si is produced via He-burning in stellar interiors, released via SNe.
O is produced in massive and intermediate mass stars
Si, O bind easily  SiO4, SiO3 bind with lots of other things (Mg, Al, Fe) and form a solid at high temperature
SiO2 = quartz
(Fe,Mg)2SiO4 = olivine (most common)
CaAl2Si2O8 = feldspar  60% of surface rocks on Earth
Various Oxides
Fe2O3 = hematite  generally formed from a reaction between Fe, O, and H2O  has been found in Martian samples

6. Common Minerals Silicon 3rd most abundant element (after O, Fe)
Cosmically as abundant as Fe, Mg
Less abundant than C,N,O
Chemically between metals and non-metals
Can survive as solid in interstellar/circumstellar environment

7. Common Minerals Silicon Silicates
3rd most abundant element (after O, Fe)
Cosmically as abundant as Fe, Mg
Less abundant than C,N,O
Chemically between metals and non-metals
Can survive as solid in interstellar/circumstellar environment
Silicates
“lithophiles” = silicates and things that tend to attach themselves to silicates  low density minerals, reside in the crust

8. Common Minerals Silicon Silicates Igneous rocks
3rd most abundant element (after O, Fe)
Cosmically as abundant as Fe, Mg
Less abundant than C,N,O
Chemically between metals and non-metals
Can survive as solid in interstellar/circumstellar environment
Silicates
“lithophiles” = silicates and things that tend to attach themselves to silicates  low density minerals, reside in the crust
Igneous rocks
40-75% SiO2
O:Si ratio is high at high temperature crystallization and you get more olivine; low at low T and you get more quartz

9. Common Minerals Silicon Silicates Igneous rocks
3rd most abundant element (after O, Fe)
Cosmically as abundant as Fe, Mg
Less abundant than C,N,O
Chemically between metals and non-metals
Can survive as solid in interstellar/circumstellar environment
Silicates
“lithophiles” = silicates and things that tend to attach themselves to silicates  low density minerals, reside in the crust
Igneous rocks
40-75% SiO2
O:Si ratio is high at high temperature crystallization and you get more olivine; low at low T and you get more quartz
Differentiation  absence of “siderophiles” in crust is evidence of differentiation

10. Tectonics  What Separates the Earth from Others
Convection  means of transporting heat  driven by internal heat (radioactive decay?)
Crustal plates are cold upper lid on convective cells  “subsolidus” convection in mantle (3000 km thick)
Consequences
Volcanic activity, mountain chains
Mid-ocean ridges
Continental drift, earthquakes

11. Tectonics
1st evidence  mapping magnetic field in Indian Ocean floor  detection of distinct linear features interpreted as “sea-floor spreading”
Puzzle-piece like nature of continents
Youngest rocks near mid-ocean ridges

12. Earth Topographic Map

13. Dating: Radionuclide Chronometry
Processes ( Most Important Cases)
40K  40Ar t ½ = 1.4 x 109 yrs
87Rb  87Sr t ½ = 6 x1010 yrs
238 U  206 Pb t ½ = 5 x 109 yrs
Lunar Results
Oldest Highland Anorthosite tsolid. = 4.2 x 109 yrs
Youngest Mare Basalts tsolid. = 3.1 x 109 yrs
Terrestrial Results

14. Radioactive Decay Consider a number density, n, of atoms
Which decays at an average rate, l
The solution to which is:
Now n = ½ n0 at time t = t1/2 so
Or,
And finally:

15. Venus’ Tectonic Activity? Smrekar & Stefan 1997 Science 277, 1289
Venus’ past
Crater distribution is even & young  no resurfacing over past Myr (Price & Supper 1994 Nature )
No global ridge system and a lack of significant upwellings (Solomon et al. Science )
Why such a big difference compared with Earth?
Catastrophic loss of H2O from mantle?  no convection
“coronae” are unique to Venus
rising plumes of magma exert pressure on lithosphere
less dense lithosphere deforms under pressure
deformation of crust without tectonics

16. Martian Tectonic Activity Connerney et al ’99 Science 284 794
Mars Global Surveyor
Detected E-W linear magnetization in southern highlands
“quasi-parallel linear features with alternating polarity”
Note: Earth’s global B-field is so much stronger it makes crustal sources hard to detect

17. Martian Tectonic Activity Connerney et al ’99 Science 284 794
Mars Global Surveyor
Detected E-W linear magnetization in southern highlands
“quasi-parallel linear features with alternating polarity”
Note: Earth’s global B-field is so much stronger it makes crustal sources hard to detect
Mars has no global field so crustal field must be remnant (“frozen in time”) from crystallization

18. Martian Crustal Magnetization
Working model
Collection of strips 200 km wide, 30 km deep
Variation in polarization every few 100 km
3-5 reversals every 106 years (like seafloor spreading on Earth)
Some evidence for plate tectonics…but crust is rigid 
Earth’s crust appears to be the only one that participates in convection