Homework 1 Due Thurs 1/14 Observing Habitable Environments Light & Radiation Given what we know about the origin of life on Earth, how would you recognize life on another world? Would this require a physical visit? Write a short paragraph on this topic of ~1/3 of a page. Karen J. Meech, Astronomer Institute for Astronomy ElectroMagnetic Spectrum Fundamentals Oscillating electric & magnetic fields Characterized by "λ region Scale γ 10-10 m X-ray 10-9-10-8 m UV 10-8-10-7 m Vis: 0.3-0.9 µm IR: 1.0-100 µm Microwave Radio Light has wave properties Travels at c = 3 x 105 km/s Wavelength, λ" "[m]" Frequency, f [Hz] c=λf Light carries energy, E Energy E = hf h is called Planck s constant Structure of the Atom Units Review m cm 10-2 m mm 10-3 m µm 10-6 m nm 10-9 m o A 10-10 m km 103 m AU 1.5x1011 m pc 206265 AU = 3.26 LY Nucleus Small rocks Sand grains Bacteria Atomic scale Surrounded by electrons in orbits Earth-Sun Distance 4.2 LY nearest star Similar / dissimilar to planet orbits " Quantized # e- = # of protons defines chemical identity Ion some of the e- are missing Naming Conventions: ANameZ Protons (+), Neutrons (0) Atomic number (Z) = # protons Atomic weight (A) = protons + neutrons (in units of the mass of H) mm ~m
Structure of the Atom Periodic Table of Elements Tabular display of elements ordered on basis of atomic number and similar chemical properties Isotopes Variants of a chemical element have a different number of neutrons in the nucleus Elements can have up to 10 isotopes chemically identical different nuclear properties, different mass Many are radioactive Life interacts differently with different isotopes The Astronomer s Periodic Table..... 12C, 13C, 14C Useful for understanding environmental conditions useful for dating Black Body Radiation EM Interaction with Atoms Quantization e.g. specific orbits Spacing of orbits is unique for each element / molecule Changing orbits can be done via addition or release of energy, E Emission spectrum fluorescence E from light used up when moving e- away from nucleus Solids & dense gases e- can fall to an orbit closer to the nucleus, releasing E as light There is an attractive force between the nucleus and ee- orbits vs. planetary orbits Representation of e- orbits unwrapped Temperature Physical Definition A measure of how much atoms are moving (oscillating) Absolute zero when classical motion stops F H 2O Freeze 32 H 2O Boil 212 C 0 100 K 273 373 Scale Temperature scales Relates color to temperature Stefan-Boltzmann L = A σ T4 (σ is a constant) Relates total brightness to size and temperature Inverse Square Law Description of how the brightness or amount of energy drops off with distance. K = C + 273 C = (F 32) x 5/9 defined 0 at the H2O freezing point, 100 at the H2O boiling point EM radiation expands out from source in all directions Brightness = amount of energy per unit area Area increases as the square of distance, r Brightness varies as 1/r2 defined 0 as absolute zero (same degree sizes) Fahrenheit λmax = 0.0029 / T Temperature Size of object " Luminosity (total E output / sec) Kelvin At equilibrium with surroundings Closely packed atoms No individual e- orbits Celsius Wein s Law Spectrum has a characteristic shape Spectrum depends only on Absorption spectrum Temperature Scales Two useful laws: defined the freezing point of brine as 0 Why this is important for remote sensing The more distant an object is, the more difficult to observe
Kirchoff s Laws Visual & Graphical Representation of Spectra Description of wavelength specific emission and absorption of radiation from a body Graphical Continuous spectrum: emission from solid, or hot dense gas Emission: diffuse gas, electrons fall to lower energy states Absorption: electrons raised to higher orbits Absorption lines The laws Continuum (BB) looking at a blackbody through gas Looking at a cool solid Visual Astronomical Sources Continuous spectrum: hot black bodies (stars) Emission: diffuse clouds of gas Absorption: Star behind a gas cloud Starlight reflected off the surface Absorption lines Compositional Fingerprints Hydrogen Helium Lithium Carbon Calcium Aluminum Iron Nitrogen Oxygen Earth s Atmospheric Transmission Stellar Spectra Combination spectrum Blackbody Radiation Absorption spectra Cooler outer envelope Formation of molecules in outer stellar atmosphere # Transmission 0% Where does radiation get in from space 100% # Wavelength 2 windows: optical, radio Why is transmission low in infrared? Water absorption # Wavelength (infrared) CO2 absorption # Wavelength (infrared)
Depth in Atm to which EM Radiation Penetrates Practical Examples Real Spectra Are a complex mixture of blackbody radiation, absorption and emission Can often get Why Mauna Kea is a great site: Airglow from MKO Daytime sunlight is absorbed by molecules in atmosphere Atoms emit light, but process takes time so this occurs over many hours Emission at night is seen as airglow Temperature of body Idea of Composition but this gets complex and needs lab data on spectra, and modeling High Altitude " above most H2O vapor Minimal city lights Dark Sky (low airglow) Middle of ocean " smooth airflow " good seeing Applications: Aurorae Solar wind Charged particles from sun Charged particles can t cross magnetic field lines " flow around a planet Some particles get trapped and spiral around field lines Collisions with atm gases " give atoms E Eventually they lose E and emit light Blue N2+ Green [O] Red [O] 4278A 5577A 6300A Emitted Radiation Aurorae Other Worlds
Comet Spectra Icy Satellites Skylab Spectrum Kohoutek (1974) Jupiter s Moon Europa Strong emission from CN fluorescence where s H2O? Saturn s Moon Iapetus Infrared region OH + H = H2O (OH visible in UV) Comet Holmes 2007 outburst Green: C2 molecule, Yellow: C2, NH2 Spectrum: composition is mostly water ice Lab experiments Spectral changes vs Temperature Dark side very different composition from bright side Bright: H2O + CO2 Dark: organics Deep Impact Experiment Space dust similarity Organics in the Solar system & ISM Detecting Life on Earth Source of lab PAHs Infra red image Spectrum of Asteroid/comet Pholus (above) Mixture of: CH3OH, H2O, and organic goo Broad, non-unique absorptions (hard to ID) CH2 and CH3 groups (C-H stretching) IR feature seen everywhere in space Spectrum matches PAH organics Complex molecules are *everywhere* Optical wavelength image A Biomarker Chlorophyll has a strong wavelength response to light Several missions are now looking at Earthshine Summary of the Lecture Detecting Life on Earth A Biomarker The red edge is a subtle feature in a real spectrum This is an *easy* spectrum to get of Earth. What about an extra solar planet? Light s wave properties characterized by c = λ f Wavelength, λ (γ to radio)" Frequency, f [Hz] Travels at c = 3 x 105 km/s EM Interaction with matter Applications Light carries energy, E=hf Atomic properties Terrestrial Planet Finder Long integrations Poor spectral resolution gets: Nucleus, protons, neutrons, electrons, isotopes # protons " chemistry e- orbits quantized Absorption, emission, continuous (BB) spectrum Most spectra are a complex combination of types Spectra give T, and possibly composition