Astronomy 340 Fall 2005

1. Astronomy 340Fall 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 • 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 • 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 • 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 • 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 t½ = 1.4 x 109 yrs • 87Rb  87Sr t½ = 6 x1010 yrs • 238 U  206 Pb t½ = 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 300-500 Myr (Price & Supper 1994 Nature 372 756) • No global ridge system and a lack of significant upwellings (Solomon et al. Science 252 297) • 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 ActivityConnerney 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 ActivityConnerney 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