Atmospheres of the Planets

1. Atmospheres of the Planets By Danielle Stroup

2. Introduction-Definitions • Atmosphere consists of molecules and atoms moving at various speeds • Temperature of gas is the measure of the average kinetic energy of particles, K=½mv² kT => larger mass => smaller speed at given temp. • Atmospheric escape: gravity holds down any atmosphere of a celestial body • Thin layers of the atmosphere, far fewer collisions occur; if escape speed reached here, the particles speed into outer space • Main constituent of lunar atmosphere? Ne-very massive

3. Our Moon • Moon escape speed is only 2.4 km/s • Most gases have escaped the moon since its formation • Some material from solar wind stays around briefly, but this does not amount to much • Moon has no shield from lethal X-rays and ultraviolet radiation from the sun and from other particles in space

4. Mercury • Long hot solar days and low escape speed: 4.3 km/s • Escape speed makes it unlikely for Mercury to have an atmosphere; but a helium and hydrogen atmosphere has been detected, which was probably picked up by solar wind • Na and K vapor exists in the atmosphere on the day side • No atmosphere? No insulation from space; noon to midnight temperatures are severe

5. Venus-Atmosphere Statistics • Atmosphere: 60% CO2, 3% N2, some Ar and traces of water vapor • Surface pressure: 90 atm • Surface temperature: 740 K; probably results from the effective trapping of surface heat by CO2 and water vapor • Temperatures vary about 10 K or less from day to night • Has to be a good insulator to result in the high temperatures recorded

6. Venus – Clouds and Wind • Yellowish-white clouds conceal Venus’s surface: flow at 100 m/s with the upper atmosphere in patterns similar to the jet streams of the earth • 90% Sulfuric acid, H2SO4 mixed with water • Wind blows from the Equator to the poles in large cyclones that culminate in two giant vortices that cap the polar regions • What drives the wind? Solar heating (not unlike Earth) • The wind flows carry heat which helps to keep temperatures fairly constant

7. Mars - Statistics • Thin atmosphere • 95% CO2, 0.1–0.4% O2, 2-3% N2, 1-2% Ar • Very similar composition to Venus • Very dry planet • Water vapor in atmosphere is found in the greatest amounts in high northern latitudes in the summer • Low density of atmosphere, even though it contains CO2, limits greenhouse effect • Surface temperature remains below the freezing point of water both day and night • Temp. difference between day and night? 100 K

8. Mars - Atmosphere • Cannot rain because of low surface pressure, about 0.005 times the Earth’s • Only in canyons could liquid water exist on the surface • Water may exist in a permafrost layer beneath the surface • A layer of water ice coats the rocks and soil in the winter is extremely thin, less than a mm

9. Jupiter • Visible disk of Jupiter is the upper atmosphere • Has alternating strips of light and dark regions (zones and belts) running parallel to the equator • Light and dark implies that zones are higher than the belts because temperature in planet’s atmosphere decreases with altitude

10. Jupiter-Atmosphere • Convective atmospheric flow transports energy out to space from the planet’s interior; indicates hot interior • Jupiter’s upper atmosphere, by mass contains 82% H2, 18% He, and traces of other elements; essentially the same composition as the Sun • Clouds in zones are probably ammonia crystals • Entire atmosphere? 1000 km thick • There is no distinct boundary between atmosphere and interior

11. Jupiter – Differential rotation • Indicates Jupiter acts like a fluid • Jupiter spins in 9h 50 min at its equator and 9h 55 min at the poles • Solid body like the Earth will rotate so each point in the surface has same rotational period • Rapid rotation and large radius produces an equatorial speed of 43,000 km/s; makes planet fairly oblate • Rotation drives the circulation in Jupiter’s atmosphere • Wind speeds are about 100 m/s

12. Saturn • Resembles Jupiter’s atmosphere • Belts running parallel to equator, driven by rapid rotation • Rotational period: 10h 14 min at the equator and varies with latitude • Also shows differential rotation • Composition: mostly H2 and He • Also has methane, water vapor, and ammonia

13. Saturn’s clouds • Appear far less colorful than those of Jupiter (mostly a faint yellow and orange) • Lie lower in atmosphere than Jupiter • Wind speeds are up to 500 m/s near the equator

14. Uranus • Upper atmosphere very cold: 58 K • Atmosphere consists of 15% H2 and He, 60% icy materials (water, methane, and ammonia) and 25% earthy materials (silicates and iron) • Ammonia clouds • Low bulk density; implies mostly lightweight elements exist

15. Neptune • Great Dark Spot: storm 30,000 km across, rotating counter clockwise in a few days; lacks the typical atmospheric methane • Bright cirrus-like clouds accompany the Dark spot • Most of the clouds change size or shape from one rotation from the next • Atmosphere is likely driven by the outflow of Neptune’s internal heat

16. Pluto • Atmosphere stretches over 600 km from the planet’s surface • Probably consists of N2, CO, and methane gas that has been released from the ice on the surface as the planet is heated • Surface pressure of a mere 10-8 atm

17. Conclusion • Temperature, clouds, and composition of the atmosphere differs from planet to planet • Escape speed determines whether a planet will be able to keep in the atmospheric elements that are present • Rotational speed and internal heat can drive the atmospheric circulation