Atmospheres of the Planets By Danielle Stroup

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

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

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

Venus-Atmosphere Statistics Atmosphere: 60% CO 2, 3% N 2, 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 CO 2 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

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, H 2 SO 4 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

Mars - Statistics Thin atmosphere 95% CO 2, 0.1–0.4% O 2, 2-3% N 2, 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 CO 2, limits greenhouse effect Surface temperature remains below the freezing point of water both day and night Temp. difference between day and night? 100 K

Mars - Atmosphere Cannot rain because of low surface pressure, about 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

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

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% H 2, 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

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

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 H 2 and He Also has methane, water vapor, and ammonia

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

Uranus Upper atmosphere very cold: 58 K Atmosphere consists of 15% H 2 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

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

Pluto Atmosphere stretches over 600 km from the planet’s surface Probably consists of N 2, CO, and methane gas that has been released from the ice on the surface as the planet is heated Surface pressure of a mere atm

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