EART 164 PLANETARY ATMOSPHERES Francis Nimmo F Nimmo

EART 164: PLANETARY ATMOSPHERES Francis Nimmo F. Nimmo EART 164 Spring 11

Course Overview • How do we know about the gas envelopes of planetary bodies? Their structure, dynamics, composition and evolution. • Techniques to answer these questions – Remote sensing (mostly) – In situ sampling – Modelling • Case studies – examples from this Solar System (and exoplanets) F. Nimmo EART 164 Spring 11

Course Outline • • • Week 1 – Introduction, overview, basics Week 2 – Energy balance, temperature Week 3 – Composition and chemistry Week 4 – Clouds and dust Week 5 – Radiative Transfer; Midterm Week 6 – Dynamics 1 Week 7 – Dynamics 2 Week 8 – Exoplanets Week 9 – Climate change & Evolution Week 10 –Recap; Final F. Nimmo EART 164 Spring 11

Logistics • Website: http: //www. es. ucsc. edu/~fnimmo/eart 164 • Set text –F. W. Taylor, Planetary Atmospheres (2010) • Another good reference (higher level) is Lissauer & De. Pater, Planetary Sciences 2 nd ed. (2010), Chs. 3&4 • Prerequisites – some knowledge of calculus expected • Grading – based on weekly homeworks (~40%), midterm (~20%), final (~40%). • Homeworks due by 5 pm on Monday (10% penalty per day) • Location/Timing –MWF 2: 00 -3: 10 in E&MS D 236 • Office hours – MWF 3: 15 -4: 15 (A 219 E&MS) or by appointment (email: fnimmo@es. ucsc. edu) • Questions? - Yes please! F. Nimmo EART 164 Spring 11

Expectations • I’m going to assume some knowledge of calculus • Homework typically consists of 3 questions • If it’s taking you more than 1 hour per question on average, you’ve got a problem – come and see me • Midterm/finals consist of short (compulsory) and long (pick from a list) questions • Showing up and asking questions are usually routes to a good grade • Plagiarism – see website for policy. F. Nimmo EART 164 Spring 11

This Week • • • Introductory stuff Overview/Highlights (Taylor Ch. 1) How do planets form? (Taylor Ch. 2) Where do atmospheres come from? What observational constraints do we have on atmospheric properties? (Taylor Ch. 3) • Introduction to atmospheric structure F. Nimmo EART 164 Spring 11

Three classes of planetary bodies “Rock” 1 ME 300 GPa ~6000 K GJ 876 d “Rock”+ice ~0. 1 ME ~10 GPa ~1500 K Ice + H, He ~15 ME 800 GPa ~8000 K Other solar systems will certainly contain planets very different from ours (super-Earths, mini. Jupiters, iron planets. . . ) Mainly H, He ~300 ME 7000 GPa ~20, 000 K HD 149026 b F. Nimmo EART 164 Spring 11

Useful Data Venus Earth Solar constant (Wm-2) 2620 Obliquity (o) Titan Jupiter Saturn Uranus Neptune 1380 594 15. 6 50. 5 14. 9 3. 7 1. 5 177 23. 4 24. 0 (27) 3. 1 26. 7 98 28. 3 Orbital period (years) 0. 62 1 1. 88 (29. 4) 11. 9 29. 4 84 165 Rotation period (hours) 5832 24 24. 6 383 9. 9 10. 7 17. 2 16. 1 Bond albedo A 0. 76 0. 4 0. 15 0. 34 0. 3 0. 29 Molecular wt. m (g/mol) 43 29 2. 2 2. 1 2. 6 Tsurface or T 1 bar (K) 730 288 220 95 165 134 76 72 Surface pressure (bar) 92 1 . 007 1. 47 n/a n/a g (ms-2) 8. 9 9. 8 3. 7 1. 35 24. 2 10. 0 8. 8 11. 1 Teq (K) 229 245 217 83 113 84 60 48 Scale height H (km) 15 8. 5 12 23 27 60 28 20 Radius (km) 6052 6370 3390 2575 71, 500 60, 300 25, 000 24, 800 Mass (1024 kg) 4. 87 5. 97 1900 Data mostly from Taylor, Appendix A Mars 0. 64 0. 13 568 87 102 F. Nimmo EART 164 Spring 11

Units! • • SI in general but 1 bar = 105 Pa g/cc vs. kg/m 3 Per mol vs. per kg F. Nimmo EART 164 Spring 11

Overview/Highlights F. Nimmo EART 164 Spring 11

Venus • • Thick CO 2 atmosphere Hot (“runaway greenhouse”) Cloud-covered Lost a lot of water Slow rotator (retrograde), not tilted Fast winds (“superrotation”) Sulphur cycle (active volcanism) • Pioneer Venus, Venera & Vega probes (USSR), Magellan, Venus Express (ESA) F. Nimmo EART 164 Spring 11

Earth • • • Mostly N 2, O 2 Moderate greenhouse Hydrological cycle & oceans Weathering buffer Moderate rotator Tilted (seasons) Hadley cell Milankovitch cycles Biological activity F. Nimmo EART 164 Spring 11

Mars • • Thin CO 2 atmosphere Dust and polar caps important Massive climate change Moderate rotator Tilted (seasons) Global dust storms Orbital forcing important (Milankovitch cycles) • Mars Odyssey, Mars Express (ESA), Mars Exploration Rovers, Mars Science Laboratory, MAVEN F. Nimmo EART 164 Spring 11

Jupiter & Saturn • • Thick H/He atmospheres ~10 Earth mass rock/ice cores Internal energy sources Rapid rotators Saturn is tilted Banded winds + storms Multiple cloud layers • Voyagers, Cassini, Galileo, Juno (we hope) F. Nimmo EART 164 Spring 11

Uranus & Neptune • • Thin (relatively) H/He atmos. Massive rock/ice cores Rapid rotators Banded winds + storms Multiple cloud layers Uranus is tilted (seasons) Poorly understood • Voyagers F. Nimmo EART 164 Spring 11

Titan • • Moderate N 2 atmosphere “Hydrological” cycle (methane) Subsurface replenishment Moderate rotator Saturn tilted (seasons) Local clouds and storms Large atmospheric loss? • Voyager, Cassini/Huygens F. Nimmo EART 164 Spring 11

Thin Atmospheres F. Nimmo EART 164 Spring 11

Exoplanets Swain et al. 2008 F. Nimmo EART 164 Spring 11

1. How do planets form? F. Nimmo EART 164 Spring 11

Solar System Formation - Overview • Some event (e. g. supernova) triggers gravitational collapse of a cloud (nebula) of dust and gas • As the nebula collapses, it forms a spinning disk (due to conservation of angular momentum) • The collapse releases gravitational energy, which heats the centre • The central hot portion forms a star • The outer, cooler particles suffer repeated collisions, building planet-sized bodies from dust grains (accretion) • Young stellar activity (T-Tauri phase) blows off any remaining gas and leaves an embryonic solar system • These argument suggest that the planets and the Sun should all have (more or less) the same composition F. Nimmo EART 164 Spring 11

Sequence of events • 1. Nebular disk formation • 2. Initial coagulation (~10 km, ~105 yrs) • 3. Orderly growth (to Moon size, ~106 yrs) • 4. Runaway growth (to Mars size, ~107 yrs), gas blowoff • 5. Late-stage collisions (~107 -8 yrs) F. Nimmo EART 164 Spring 11

What is the nebular composition? • Why do we care? It will control what the planets (and their initial atmospheres) are made of! • How do we know? – Composition of the Sun (photosphere) – Primitive meteorites (see below) – (Remote sensing of other solar systems - not yet very useful) • An important result is that the solar photosphere and the primitive meteorites give very similar answers: this gives us confidence that our estimates of nebular composition are correct F. Nimmo EART 164 Spring 11

1. 4 million km Solar photosphere Note sunspots (roughly Earth-size) • Visible surface of the Sun • Assumed to represent the bulk solar composition (is this a good assumption? ) • Compositions are obtained by spectroscopy • Only source of information on the most volatile elements (which are depleted in meteorites): H, C, N, O F. Nimmo EART 164 Spring 11

Primitive Meteorites • Meteorites fall to Earth and can be analyzed • Radiometric dating techniques suggest that they formed during solar system formation (4. 55 Gyr B. P. ) • Carbonaceous (CI) chondrites contain chondrules and do not appear to have been significantly altered • They are also rich in volatile elements • Compositions are very similar to Comet Halley, also assumed to be ancient, unaltered and volatile-rich 1 cm chondrules F. Nimmo EART 164 Spring 11

Meteorites vs. Photosphere • This plot shows the striking similarity between meteoritic and photospheric compositions • Note that volatiles (N, C, O) are enriched in photosphere relative to meteorites • We can use this information to obtain a best-guess nebular composition Basaltic Volcanism Terrestrial Planets, 1981 F. Nimmo EART 164 Spring 11

Nebular Composition • Based on solar photosphere and chondrite compositions, we can come up with a best-guess at the nebular composition (here relative to 106 Si atoms): Element H He C N O Ne Mg Si Log 10 (No. 10. 44 9. 44 7. 00 Atoms) 6. 42 7. 32 6. 52 6. 0 Condens. Temp (K) 120 180 -- 78 -- -- 6. 0 S Ar 5. 65 5. 05 5. 95 1340 1529 674 40 Fe 1337 • Blue are volatile, red are refractory • We would expect planetary atmospheres to consist primarily of H, He, C, N, O, Ne, Ar and their compounds Data from Lodders and Fegley, Planetary Scientist’s Companion, CUP, 1998 This is for all elements with relative abundances > 105 atoms. F. Nimmo EART 164 Spring 11

Temperature and Condensation Nebular conditions can be used to predict what components of the solar nebula will be present as gases or solids: Mid-plane Photosphere “Snow line” Earth Saturn (~300 K) (~50 K) Temperature profiles in a young (T Tauri) stellar nebula, D’Alessio et al. , A. J. 1998 Condensation behaviour of most abundant elements of solar nebula e. g. C is stable as CO above 1000 K, CH 4 above 60 K, and then condenses to CH 4. 6 H 2 O. From Lissauer and De. Pater, Planetary Sciences F. Nimmo EART 164 Spring 11

“Snow line” • Beyond the “snow line” (~180 K), water ice condenses • Ice is ~10 times more abundant (by mass) than rock in the solar nebula • So it is much easier to build big planets beyond the snow line • Gas giants need a big solid core to start accumulating H or He (see next slide) • Close-in exoplanets almost certainly formed beyond the snow line and then migrated F. Nimmo EART 164 Spring 11

Gas/ice giant formation • Once a solid planet gets to ~10 Earth masses, its gravity is large enough to trap H 2 and He present in the local nebula • J, S, U and N all have cores made of “high-Z” elements (rock+ice) • J, S have thick H/He envelopes; U, N have thin H/He envelopes • So the cores of J&S probably grew early enough to trap nebular H/He before it dissipated. U&N were too slow. Why? F. Nimmo EART 164 Spring 11

Migration (hot Jupiters) Gas disk (with density waves) planet • If the gas disk is still present, planets will migrate inwards • This migration can be very rapid (~104 -105 yrs) • Migration stops where the disk stops (e. g. due to stellar magnetic fields) • This is why there are so many “hot Jupiters” • But it apparently didn’t happen in our solar system F. Nimmo EART 164 Spring 11

Nice Model Early in solar system Ejected planetesimals (Oort cloud) “Hot” population J S U Initial edge of planetesimal swarm N 18 AU Present day J 30 AU “Hot” population Planetesimals transiently pushed out by Neptune 2: 1 resonance S U 48 AU N “Cold” population Neptune 3: 2 Neptune 2: 1 Neptune stops at resonance original edge resonance (Pluto) See Gomes, Icarus 2003 and Levison & Morbidelli Nature 2003 F. Nimmo EART 164 Spring 11

Planet Formation - Summary • Initial nebular composition is well-known • Planetary volatile abudance depends (mostly) on where the planet formed (temperature) • Timing of planet growth relative to nebular blowoff also important • The planets may have moved during or after the formation phase F. Nimmo EART 164 Spring 11

2. Where do atmospheres come from? F. Nimmo EART 164 Spring 11

Where do atmospheres come from? • Primary – directly accreted from nebula • Secondary – outgassed from planet • Tertiary – derived from comets, asteroids and/or solar wind • We’ll discuss more later in the quarter. Examples: – Does Earth’s hydrosphere come from comets or asteroids? (D/H ratio) – How much outgassing has there been on Earth, Venus, Mars, Titan? (40 Ar) – Did the gas giants acquire a solar composition? (C/H, H/He) F. Nimmo EART 164 Spring 11

Where do atmospheres go to? • Again, we’ll discuss more later, but there are several processes which can remove atmospheres • Loss to space – Thermal processes (Jeans escape) – Hydrodynamic escape – Sputtering & photodissociation – Impacts • Loss to surface/interior – Chemical reactions (e. g. carbonate formation) – “Ingassing” (e. g. plate tectonics) – Freeze-out (Mars, Pluto) F. Nimmo EART 164 Spring 11

3. Observational constraints (see Taylor ch. 3) F. Nimmo EART 164 Spring 11

Radiometry (Spectroscopy) • “Near” infra-red: 0. 7 -5 mm, reflected sunlight • “Thermal” IR: 5 -1000 mm, emission from atmosphere • Absorption/emission tells us what species are present, and where in the atmosphere they are • Background spectrum (~black body) tells us about temperature structure of atmosphere F. Nimmo EART 164 Spring 11

Radiometry (cont’d) Ha ze lay er • We can see to different depths within an atmosphere by using different wavelengths • By looking at emission from the limb, we can probe the vertical temperature and pressure structure F. Nimmo EART 164 Spring 11

Occultations observer • Atmospheric absorption of light/radio waves provides information on composition, pressure and temperature • Good for probing thin atmospheres (e. g. Pluto, Enceladus) Hansen et al. 2006 F. Nimmo EART 164 Spring 11

In situ sampling • Galileo probe (Jupiter) • Huygens (Titan) • Venera/Vega probes/balloons • Viking landers • Cassini INMS Le. Breton et al. Nature 2005 • Very useful! Ground truth for pressure, wind, temperature etc. Sensitive to trace gases (GCMS). • Generally limited duration (e. g. Venus) • Point measurement – what happens if you land in an anomalous region? (Galileo probe) F. Nimmo EART 164 Spring 11

4. Atmospheric structure F. Nimmo EART 164 Spring 11

Typical structure z stratosphere tropopause troposphere T Temperature structure of stratosphere in reality can be more complicated because of photochemistry (e. g. ozone) Lower atmosphere consists of a thick part (troposphere) where convection dominates, and a thinner part above (stratosphere) where radiation dominates F. Nimmo EART 164 Spring 11

Ideal Gas Equation P=pressure, r=density, R=gas constant, T=temperature (in K), m=molar mass (in kg) What is density of air at Earth’s surface? What is the column mass of Earth’s atmosphere? (kg/m 2) m m Venus (CO 2) 0. 04 Jupiter (H, He) 0. 0022 Earth (N 2, O 2) 0. 03 Saturn (H, He) 0. 0021 Mars (CO 2) 0. 04 Uranus (H, He) 0. 0026 Titan (N 2) 0. 03 Neptune (H, He) 0. 0026 F. Nimmo EART 164 Spring 11

Atmospheric Structure (1) • Atmosphere is hydrostatic: • Gas law gives us: • Combining these two (and neglecting latent heat): Here R is the gas constant, m is the mass of one mole, and RT/gm is the pressure scale height of the (isothermal) atmosphere (~10 km) which tells you how rapidly pressure decreases with height e. g. what is the pressure at the top of Mt Everest? Most scale heights are in the range 10 -30 km F. Nimmo EART 164 Spring 11

Exobase and mean free path • The exobase is the place where the mean free path of molecules exceeds scale height. This is where molecules can start to escape efficiently (if travelling fast enough) • You can think of the exobase as the effective “top” of the atmosphere • For planets with thin atmospheres, the exobase may be at the surface! What’s the mean free path at the surface of the Earth? prmol 2 l rmol is typically 1 Angstrom=10 -10 m F. Nimmo EART 164 Spring 11

Key concepts • • • Snow line Migration Troposphere/stratosphere Primary/secondary/tertiary atmosphere Emission/absorption Occultation First homework Scale height due next Hydrostatic equilibrium Monday! Exobase Mean free path F. Nimmo EART 164 Spring 11

End of lecture F. Nimmo EART 164 Spring 11

An Artist’s Impression The young Sun gas/dust nebula solid planetesimals F. Nimmo EART 164 Spring 11

Observations (1) • Early stages of solar system formation can be imaged directly – dust disks have large surface area, radiate effectively in the infra-red • Unfortunately, once planets form, the IR signal disappears, so until very recently we couldn’t detect planets (now we know of ~400) • Timescale of clearing of nebula (~1 -10 Myr) is known because young stellar ages are easy to determine from mass/luminosity relationship. Thick disk This is a Hubble image of a young solar system. You can see the vertical green plasma jet which is guided by the star’s magnetic field. The white zones are gas and dust, being illuminated from inside by the young star. The dark central zone is where the dust is so optically thick that the light is not being transmitted. F. Nimmo EART 164 Spring 11

Observations (2) • We can use the presentday observed planetary masses and compositions to reconstruct how much mass was there initially – the minimum mass solar nebula • This gives us a constraint on the initial nebula conditions e. g. how rapidly did its density fall off with distance? • The picture gets more complicated if the planets have moved. . . • The observed change in planetary compositions with distance gives us another clue – silicates and iron close to the Sun, volatile elements more common further out F. Nimmo EART 164 Spring 11

Cartoon of Nebular Processes Disk cools by radiation Polar jets Dust grains Infalling material Hot, high r Nebula disk (dust/gas) Cold, low r Stellar magnetic field (sweeps innermost disk clear, reduces stellar spin rate) • Scale height increases radially (why? ) • Temperatures decrease radially – consequence of lower irradiation, and lower surface density and optical depth leading to more efficient cooling F. Nimmo EART 164 Spring 11

Planetary Compositions • Which elements actually condense will depend on the local nebular conditions (temperature) • E. g. volatile species will only be stable beyond a “snow line”. This is why the inner planets are rock-rich and the outer planets gas- and ice-rich • The compounds formed from the elements will be determined by temperature (see next slide) • The rates at which reactions occur are also governed by temperature. In the outer solar system, reaction rates may be so slow that the equilibrium condensation compounds are not produced • The mass of a planet determines the mass and composition of its atmosphere F. Nimmo EART 164 Spring 11