Chapter 8: The Giant Planets- All giant planets but Neptune can be seen with the naked eye- 1781, William Herschel cataloged the sky, found Uranus, it seemed to be straying from predicted pattern in the sky, turns out it was because of gravitational pull from Neptune, discov-ered by Johann Gottfried Galle at Berlin Observatory8.1 Giant Planets Are Large, Cold, and Massive- Jupiter more than 5AU from Sun, Sun is 1/27 as bright on Jupiter as on Earth, Daytime on Neptune is like twilight on Earth- Jupiter is largest planet of all, more than 1/10 size of the Sun, 11 times the size of Earth, twice as massive as all the other planets combined, 318 times as massive as Earth and 3 1/2 times as massive as Saturn, but still only a thousandth of the Sun- Saturn is slightly smaller, 9.5 Earths- Uranus and Neptune about 4 Earth diameters across, Neptune slightly smaller- most accurate measurements taken from observing length of time it takes a planet to eclipse or occult a star, called stellar occultations- not counting sun, giant planets make up 99.5 percent of all mass in SS- Uranus and Neptune are most lightweight among giants, but 15 more times massive than Earth- before space age, scientists measured a planet’s mass by observing the motions of its moons, we can use Newton’s law of gravity and Kepler’s third law, if we know moon motion we can find planet’s mass, now we can measure mass by how much the planet’s gravity deflectsa spacecraft- Jupiter and Saturn are gas giants, Uranus and Neptune are ice giants (more water and ice)- on giant planets, atmosphere merges with deep ocean, which merges into denser liquid or solid core, atmospheres much thicker than those of terrestrial planets, only highest levels are vsible to us, strong scattering of sunlight prevents us from seeing lower cloud layers- Jupiter’s Chemical Composition is much like the sun’s- on terrestrial planets, the masses of atmospheres and oceans are insignificant compared with total planetary mass, densest objects in Solar System- giant planets have lower densities, made of materials like hydrogen, helium, water, among them Neptune has highest density (1.6 of water), Saturn has lowest (0.7 times of water)- chemical compositions of giant planets differ, solar abundance is term for relative amounts of the elements in the Sun as a standard reference, hydrogen most abundant, helium next, jupiter has a dozen hydrogen atoms for every helium, typical of the Sun, only 2 percent ofmass is heavy elements, atoms combine to form water, methane, ammonia, hydrogen sulfide- helium, neon, and argon are inert gases that don’t combine to make molecules in the at-mosphere- Jupiter’s liquid core has iron silicate and water left over from original rocky planetesimal- principle difference in giants is abundances of hydrogen and helium, Saturn has less, more of heavier elements, Jupiter has tons, Uranus and Neptune’s major components are heavy elements - giant planets rotate rapidly, days short, Jupiter (10 hours), Saturn, a little longer, Neptune (16) and Uranus (17), this distorts their shape, normally they’d be perfect spheres, but because they’re fluid, they oblate, bulge at equators like accretion disk, Saturn is very oblate- intensity of seasons is determined by obliquity, inclination of equatorial plane to orbital plane, Jupiter’s is only 3 degrees, almost no seasons at all, Saturn and Neptune have a little more, Uranus spins almost perpendicular, seasons are extreme, each polar region gets 42 years of sunshine, 42 years of darkness, thought to have been knocked over by a huge planetesimal near end of accretion phase, obliquity of 98 degrees, large than 90, so it rotates in a clockwise direction when seen from above orbital plane, same way with Venus, Pluto, Charon (Pluto moon), and Triton (Neptune moon)8.2 A View of the Cloud Tops-Jupiter most colorful giant, orange to red, to bluish gray, brown, pink, pale yellow, darker bandsare called belts, lighter are called zones, small clouds appear along the edges of or within the belts, Great Red Spot could hold two Earths- Great Red Spot: been circulating for 300 years, seen just after telescope invented, spins counterclockwise with a period of about a week, rotates opposite of a terrestrial hurricane, anti-cyclonic, shows “cloud cannibalism”, Alaska-sized clouds seen carried around the vortex and then ejected, other swallowed- Saturn has rings, unmatched by anything else in SS, farther away than Jupiter and smaller, appears less than half as large as Jupiter from Earth, Saturn also has atmospheric bandsbut they tend to be wider and colors less subdued compared to Jupiter, narrow band in mid-northen latitudes circles planet, similar to our jet stream, individual clouds are rare, they erupt in the tropics spread out in longitude, fade away in a few months, largest clouds are bigger thanUS, immense lightning storms have been seen in Southern hemisphere, “storm alley” - from Earth, Uranus and Neptunen look like tiny, pale-bluish-green disks, infrared shows clouds and belts, strong absorption of reflected sunlight by methane causes atmospheres of Uranus and Neptune to appear dark in near infrared, once a large dark oval found in Neptune’s Southern Hemi was thought to be like Jupiter’s, called it the Great Dark Spot, but it was gray and quickly changed length and shape, disappeared8.3 A Journey into the Clouds- atmospheric temp, density, and pressure all increase with decreasing altitude, although temperature sometimes reverses (like Earth’s thermosphere)- a thin haze above clouds is visible in profile above the limbs (outer edge of disk) of the planets, may be smog from UV light acting on hydrocarbon gases like methane- giant planets can have clouds made of other things besides water, beneath theses are dense layers of cloud separated by regions of clear atmosphere, different layers caused by each volatile’s condensing temperature and pressure point, convection carries volatiles up with other gases, when it reaches an altitude with its condensation temperature, it condenses and separates from other gases- the farther from the sun, the colder the troposphere will be, distance from Sun also deter-mines the altitude at which a volatile will condense- the highest clouds in the frigid atmosphere of Uranus and Neptune are crystals of meth-ane ice, highest clouds on Jupiter and Saturn are made up of ammonia ice (methane can’t make ice in these atmospheres)-
View Full Document
Unlocking...