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Astronomy 340 Fall 2007. 25 September 2007 Class # 6-#7. Review. Physical basis of spectroscopy Einstein A,B coefficients probabilities of transistions Absorption/emission coefficients are functions of ρ , N, quantum mechanical factors, temperature Molecular spectroscopy
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Astronomy 340Fall 2007 25 September 2007 Class #6-#7
Review • Physical basis of spectroscopy • Einstein A,B coefficients probabilities of transistions • Absorption/emission coefficients are functions of ρ, N, quantum mechanical factors, temperature • Molecular spectroscopy • More available quantum states – rotational, vibrational • Low energy transitions IR, radio part of the spectrum (hν << kT) • Examples • CaI in the atmosphere of Mercury linewidth = Δλ = Δv (1/2)mv2 = (3/2)nkT
Quantum mechanics • Principle quantum # (n) energy • Angular momentum, l • Spin, s • Multi-electron atoms have many filled orbitals (constrained by exclusion principle) e.g. electron with n=2 could have l=1 or l=0, and if its l=1 it could have s=1/2 or -1/2 many orbitals, many transitions, many spectral lines http://physlab2.nist.gov/PhysRefData/ASD/lines_form.html
Molecules • Nuclei act as single nucleus with common potential • Multiple nuclei generate other quantum states • Electronic • Rotational • Vibrational low energy radio/NIR part of the spectrum • Most surface and atmospheric components are molecular
CO • Main product of stellar evolution • Transitions easily excited rotational modes • J = 1 0 (2.7mm, 115.3 GHz) • J = 2 1 (1.3mm) • Observations radiotelescopes • Measure “brightness temperature”, Tb • Optically thick vs optically thin
Example: Mercury • What does the spectrum of Mercury look like? • Planetary reflectance spectrum • Terrestrial emission and absorption • Narrow source emission lines wavelength shifted via Doppler • Process • What do you actually measure? • Linewidths? • Wavelength?
Spencer et al. 2000Science 288, 1208 • Io is the most geological activity of anything in solar system volcanoes discovered during Voyager flyby in ’79 • What’s coming out of that volcano?
Spencer et al. 2000Science 288 1208 • Use transit of Io across Jupiter to observe plumes from volcanoes why? • Scattered light dust scatters photons effectively so you get a “non-thermal” continuum effect is to fill in absorption line • Identify S2 and SO2 lines in 240.0-300.0nm range -> fit linewidths • T ~ 300 K • N(SO2) ~ 7 x 1016 cm-2 • N(S2) ~ 1 x 1016 cm-2 • Pure SO2 suggests a lack of Fe since Fe will bind with SO2 if available
CO molecule • C,O main products of stellar evolution, particularly intermediate mass stars • 3He 12C or 12C + 4He 16O • On terrestrial planets CO comes from CO2 + uv photons CO + O • Transitions • J = principle rotational quantum number • J=10 (2.7mm, 115.3 GHz) • J=21 (1.3mm), J=32 (0.87mm) • J=0 is ground state, but get to J=1 if there’s ambient thermal bath with T~5.5K it’ll get excited to J=1 level
CO molecule • Photons too dang weak for CCDs, so you need a radio telescope • Characterize intensity with a “brightness temperature” if line is optically thick the observed brightness temperature really is the thermal temperature • Tb = (λ2/2k)Bλ • Rewrite radiative transfer as: • (dTb(s)/dτλ) = Tb(s) – T(s) • Tb(s) = Tb(0)e-τ(s) + T(1-e-τ(s)) • Tb = τT (τ << 1) • Tb = T (τ >> 1)
Venus Images in J=1-0 Line • Observations • 2.7mm continuum, J=1-0 CO line • 3-element interferometer • Continuum results • 10% increase in Tb from day side to night side a change in atmospheric conditions? • CO line results • Line shape varies broad, shallow lines on dayside; deep, narrow lines on night side
Note on Conductivity • Specific heat units are J mole-1 K-1 function of temperature for most minerals • Example: feldspar (KAlSi3O8)
Transition Slide…. • Radiative transfer tells us how radiation is affected travelling through some substance (gas) • In Rayleigh-Jeans approximation we can substitute a temperature (Tb) for the radiation intensity • Now onto some fun stuff – planetary surfaces…. • Relevant reading: • Chapter 5
Processes at Work • Impact cratering • Weathering/erosion • Conditions of the atmosphere • Geological activity • Volcanic activity • Tectonics
Geological activity - Earth • Volcanism • Shield volcanoes • Formed via a single plume • Hawaii – crustal plate moving over a hot spot • “cone” volcanoes • Formed over subduction zones • Cascade mountains, Mount Etna • Earthquakes • At plate boundaries • Plate tectonics • Mid-ocean ridges, mountain chains, moving continents, earthquakes, “ring of fire”, global resurfacing
Mercury • Heavily cratered • No volcanoes, no mountain chains, no plate boundaries, no continents no recent tectonics • Shrinking? • Weak magnetic field • Conclusion: one plate planet with no activity over the past several billion years; surface is shaped by impacts
Luna, near side The far side LUNA Earth Facing Side
Moon from Galileo Spacecraft Apollo 15 Apollo 14 Apollo 17 Apollo 12 Apollo 11 Apollo 16
Venus • Lots of volcanic activity in the recent past • Characteristic feature is a “coronae” which is a circular structure like the caldera of a volcano but without the mountain to go with it • Global resurfacing about 300 Myr ago • Crater density (number per km2) • We call this a “young” surface • A couple of continent-like features • No obvious plate boundaries
Terrestrial Planet Surface Morphology (4) • Mars • Massive Shield Volcanoes • Huge Erosion Channels • Much Cratering, much eroded • Polar Caps
