9/29/11 1 The Jovian Planets (Gas Giants) Jupiter Saturn Uranus Neptune (roughly to scale) Discoveries Jupiter and Saturn known to ancient astronomers. Uranus discovered in 1781 by Sir William Herschel (England). Neptune discovered in 1845 by Johann Galle (Germany). Predicted to exist by John Adams and Urbain Leverrier because of irregularities in Uranus' orbit. Almost discovered by Galileo in 1613 Remember that compared to Terrestrial planets, Jovian planets: are massive are less dense (0.7 – 1.3 g/cm3) are mostly gas (and liquid) rotate fast (9 - 17 hours rotation periods) have rings and many moons Jupiter Saturn Uranus Neptune 318 95 15 17 Mass (MEarth)!11 9.5 4 3.9 Radius (REarth)!Known Moons 63 61 27 13 (0.001 MSun)!Orbital Properties: Jupiter Saturn Uranus Neptune Distance from Sun (AU) 5.2 9.5 19.2 30.1 Orbital Period (years) 11.9 29.4 84 164 Major Missions: Voyager 1 Voyager 2 Galileo Cassini Launch Planets visited 1977 1977 1989 1997 Jupiter, Saturn Jupiter, Saturn, Uranus, Neptune Jupiter Jupiter, Saturn Jupiter's Atmosphere Composition: mostly H, some He, traces of other elements (true for all Jovians). Gravity strong enough to retain even light elements. Mostly molecular. Altitude 0 km defined as top of troposphere (cloud layer) Ammonium hydrosulfide (NH4SH) ice should form here, somehow giving red, yellow, brown colors. Ammonia (NH3) ice gives white colors. Water ice layer not seen due to higher layers. DEMO Optical – colors dictated by how molecules reflect sunlight Infrared - traces heat in atmosphere. So white colors from cooler, higher clouds, brown and red from warmer, lower clouds.9/29/11 2 On other Jovian planets, different chemistry causes different coloration. Saturn - predominant yellow color due to haze and ammonia ice layer Blue/green color of Uranus and blue color of Neptune due to methane. Colder than Jupiter and Saturn, their ammonia has frozen and sunk lower. Methane still in gas form. It absorbs red light and reflects blue. Jupiter's Bands Lighter-colored "zones" and darker-colored "belts". - Zones and belts are Jupiter's high and low pressure systems, respectively. - They mark a convection cycle. - - Zones were thought to be where warm gas rises and are higher than the belts, where cooled gas sinks. However, Cassini data indicates the opposite! Most upwelling occurs in belts, gas falls back down in zones. - Winds flow in opposite directions in zones vs. belts. Differences are 100s of km/hr. - Jupiter's rapid rotation stretches them horizontally around the entire planet. Clicker Question: Jupiter’s atmosphere is primarily made up of: A: hydrogen B: helium C: carbon dioxide D: ammonia Clicker Question: Which planets have we found water (liquid or frozen) on: A: Mercury, Venus and Earth B: Mercury, Earth, Mars and Jupiter C: Only the Earth D: Mars and Venus E: All terrestrial planets - Mercury, Venus, Earth and Mars. Clicker Question: It takes 8 minutes for light to travel 1 AU, how long does it take for light to travel from Earth to Jupiter at its closest point to Earth in its orbit? A: 1 minute B: 5 minutes C: 30 minutes D: 2 days E: 1 year9/29/11 3 Banded structure on other Jovian planets is present, but not as distinct due to more uniform cloud coverage. Neptune Saturn Uranus Storms on Jovian Planets Jupiter's Great Red Spot: A hurricane twice the size of Earth. Has persisted for at least 340 years. New storm “Oval BA” "white ovals" - may last decades "brown ovals" - only seen near 20° N latitude. Not known why. May last years or decades. Storms on Jovian Planets Lightning on Jupiter: Cassini captured images of lightning during a nighttime pass over the planet. Each stroke is about 10,000 times more energetic than one on Earth. Neptune's Great Dark Spot: Discovered by Voyager 2 in 1989. But had disappeared by 1994 Hubble observations. About Earth-sized. Why do storms on Jovian planets last so long? On Earth, land masses disrupt otherwise smooth flow patterns. Not a problem on Jovian planets. Jupiter's Internal Structure Can't observe directly. No seismic information. Must rely on physical reasoning and connection to observable phenomena. (acts like a liquid metal, conducts electricity) Core thought to be molten or partially molten rock, maybe 25 g/cm3, and of mass about 10-15 MEarth .!Other Jovians similar. Interior temperatures, pressures and densities less extreme.!9/29/11 4 Jupiter has a strong magnetic field detection of the magnetic belts around Jupiter synchrotron emission from energetic particles in magnetic fields Jupiter has a strong magnetic field detection of aurorae Impact of high-energy Particles at the poles. 1 10 102 103 104 105 106 Frequency (MHz) 107 106 105 104 103 102 10 LWA range VLA range Low Frequency Detection of Exo-Planets!● Below 40 MHz, Jupiter is the brightest object in the solar system.!● The LWA has a good chance of detecting emission from extra-solar “Jupiters”.!– Would provide independent verification of planetary systems.!– Would confirm presence of magnetic field – pre-requisite for life as shield against cosmic rays.!~40 MHz (LWA range) Interaction of Jupiter’s magnetosphere with the Solar Wind. Flux Density (Jy) Rapid rotation causes Jupiter and Saturn to bulge: Gravity without rotation with rotation Gravity Jupiter and Saturn rotate every ~10 hours. Radius at equator several % larger due to bulge. Clicker Question: Which gas giant has the lowest average density: A: Jupiter B: Saturn C: Uranus D: Neptune Clicker Question: The Great Red Spot is: A: A large basin on Mars B: A long-lived high-pressure storm in Jupiter’s atmosphere. C: The colored polar cap of Jupiter D: Clouds of dust swirling around Jupiter’s largest volcano9/29/11 5 Clicker Question: Saturn is less massive than Jupiter but almost the same size. Why is this? A: Saturn’s interior is hotter than that of Jupiter’s. B: Saturn is composed of lighter material than Jupiter. C: Saturn is rotating faster than Jupiter so the increased centrifugal force results in a larger size D: Saturn’s smaller mass provides less gravitational force to compress it. Differential Rotation Rotation period is shorter closer to the equator: Jupiter Saturn Uranus Near poles At equator 9h 56m 10h 40m 16h 30m 9h 50m 10h 14m 14h 12m How do we know? Differential Rotation Rotation period is shorter closer to the equator: Uranus' rotation