Lecture #7: Radiation and Spectra II How is light absorbed and emitted? Models of Atomic Structure. Formation of Spectral Lines. Doppler Shift. Applications in Solar System Studies Detecting gaseous phases (atmospheres, stars). Detecting solid phases (ices, minerals). The Main Point Astronomers use spectroscopy, the study of the interaction of light and matter, to determine what planetary surfaces and atmospheres are made of. Astro 102/104 1 Astro 102/104 2 How do light and matter interact? The interaction occurs at the atomic scale. Therefore, understanding the interaction means understanding the atom. But the realm of the atom is at a scale that is completely outside our intuitive experience. So scientists have devised models of the atom, to try to grasp what's going on. Subatomic Particles J.J. Thomson discovered the existence of electrons in 1897. He used a cathode ray tube to show that particles appeared to be carrying negative charge. Since atoms are neutral, these particles must represent only one part of the atom, whose charge must be balanced by something else. Wild! Controversial! "Could anything at first sight seem more impractical than a body which is so small that its mass is an insignificant fraction of the mass of an atom of hydrogen?" -- J.J. Thomson. Astro 102/104 3 Astro 102/104 4 1
Subatomic Particles Ernest Rutherford devised an experiment in 1911 and discovered the other fundamental part of the atom: the central nucleus of the atom, consisting of positively-charged protons. These and other experiments indicated that the nucleus appeared to be > 1000 times as massive as an electron. Rutherford and others modeled the electrons as orbiting the nucleus. Subatomic Particles In 1932, James Chadwick and others discovered that the nucleus must also contain neutrons--particles with no charge but roughly the same mass as protons. Completes a simplistic "solar system" analogy for atoms. Astro 102/104 5 protons, neutrons, and electrons define atomic properties. Astro 102/104 6 Size Scale: Ångstroms (10-10 m) Astro 102/104 7 The Bohr Model Danish physicist Niels Bohr developed a model of the atom in 1915 in which the electron orbits are quantized, or limited to certain discrete energy levels. Highly non-intuitive! Analogy: window washer on a ladder. Electrons moving from one level to another must either gain energy (to move to a higher energy level) or lose energy (to move to a lower level) in discrete amounts (recall, E = h c / λ). Astro 102/104 8 2
Absorption of specific wavelengths gives electrons more energy. Emission of specific wavelengths gives electrons less energy. Preferred (lowest energy) state: ground state. More energy absorbed: excited states. Enough energy absorbed to liberate electrons: ionized state. Astro 102/104 9 Astro 102/104 10 The energies of these jumps are different for different atoms. This provides a "fingerprinting tool" to identify composition! These energy level jumps are the source of dark (absorption) and bright (emission) lines in the spectra of stars, galaxies, planets,... E = h c / λ Lyman: λ in the UV. Balmer: λ in the Visible. ionized state Astro 102/104 11 Astro 102/104 12 3
Spectroscopy in Planetary Science Planetary Atmospheres (H 2 O, CO 2, CO, CH 4,...) Consist of molecules that exhibit absorption and emission lines of their constituent atoms, but also rotational and vibrational lines from the absorption of energies that excite the entire molecular structure. Just like atoms, different molecules exhibit different spectroscopic "fingerprints" from their rotational and vibrational frequencies. Astro 102/104 13 Spectroscopy in Planetary Science Planetary Surfaces: Consist of minerals, with lattices that vibrate and rotate at specific frequencies, absorbing or emitting radiation at diagnostic wavelengths. Surface absorption features are wide and shallow compared to atomic or molecular gas lines because of the overlap of many different absorptions (impurities, mixtures of rocks), and variations in surface temperatures (changes in bond lengths). But positions of features still diagnostic of composition. Mineral Lattice Astro 102/104 14 Interpretation of Spectra Examples of Real Spectra Broad absorption bands caused by minerals (solids) on the surface. Saturn Lots of effects/information tangled up in each measurement! Astro 102/104 15 Narrow absorption lines caused by molecules (gases) in the planet's atmosphere. Astro 102/104 16 4
The Doppler Shift Christian Doppler showed in 1842 that if an object is moving towards or away from an observer, light waves from that object will be compressed or spread out, respectively. This Doppler Shift makes the wavelength of absorption or emission lines different from those at the source. Δλ = λ v / c, where v = line-of-sight velocity of the source. Astro 102/104 17 Doppler Shift in Planetary Science Narrow absorption or emission lines shifted slightly from their "rest" wavelengths: e.g., Water vapor on Mars can only be detected when Mars H 2 O lines are shifted away from Earth's H 2 O lines. Narrow spacecraft transmitter frequencies are shifted slightly when the spacecraft's velocity is changed by a nearby planet or asteroid: Can use Doppler Shift to determine mass of objects! Planets cause stars to "wobble", inducing slight periodic Doppler shifts in their spectra. Astro 102/104 18 Summary Light and matter interact on the atomic scale. The properties of this interaction depend on the specific subatomic structure of atoms. Electrons absorb and emit energy only at specific energies. Patterns of absorbed or emitted wavelengths can be used to identify the presence of atomic, molecular, or even mineralogic materials in the Universe. This kind of remote sensing allows us to determine the composition and nature of planets, satellites, asteroids, and comets without having to visit or sample each one individually! Astro 102/104 19 Next Lecture... Astronomical Instruments: Optics: Lenses and Mirrors. Ground Based Telescopes: Optical, Infrared, and Radio. Space Based Telescopes. Spacecraft Missions. Reading: Chapter 6. Astro 102/104 20 5