Astronomy 340 Fall 2005 27 September 2005 Class #7

90 Minutes Doing Your Homework

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

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

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

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

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

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

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

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

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

Earth Topographic Map

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

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:

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

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

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

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