Astronomy 1010 Planetary Astronomy Fall_2015 Day-33 Course Announcements • • SW-chapter 9 due: Wed. Nov. 18 SW-chapter 10 due: Mon. Nov. 23 • I will collect the L-T books on Monday, Nov. 23 • 1st Quarter Obs. (last one of semester): Thurs. Nov. 19 Reports Due: Monday Nov. 23 – AT CLASS TIME! • You-tube Water Cycle CONNECTIONS 9.2 Convection along with Earth’s water cycle creates violent weather patterns on Earth, including thunderstorms, tornadoes, and hurricanes. Dust devils can also be created on Mars. Lightning on Venus and the giant planets. Parts of Earth are heated differently. Convection distributes surface heating. Hadley circulation transports heat between equator and poles. Global winds carry heat from hot to cool regions. On Earth, Venus, and Mars, the circulation depends on heating pattern and rotation period. The Coriolis effect breaks the larger convection cells into smaller ones. Climate is the average state of an atmosphere. Can be affected by astronomical, geological, and biological mechanisms. Astronomical: changes in the Sun’s energy output, change in a planet’s obliquity, and more. Geological: volcanic and tectonic activity. Biological: photosynthesis by bacteria and plants, methane byproducts, human activity. Paleoclimatology has yielded much information about Earth’s climate change. Has found temperature and climate cycles, including ice ages. Earth’s climate is very sensitive to changes in temperature. CONNECTIONS 9.1 The temperature of a gas is a measure of the average kinetic energy of its molecules. The motion of the gas molecules in an atmosphere creates its pressure. MATH TOOLS 9.1 If the speed of a gas molecule is less than a planet’s escape speed, it will be more easily retained. The speed is represented by the temperature. If the gas speed is less than 1/6 of the escape speed, the planet will keep its atmosphere. PROCESS OF SCIENCE When presented with complex science (such as climate change), it is important to analyze the issue inclusively from different angles. Jupiter, Saturn, Uranus, and Neptune are the giant planets. Jupiter and Saturn: mainly hydrogen and helium. 5.2 AU from Sun, and 9.6 AU from Sun. Called gas giants. Uranus and Neptune: smaller, have much more water, water ice, and other ices. 19.2 AU from Sun, and 30.0 AU from Sun. Called ice giants. Jupiter and Saturn were known to the ancients. Uranus was too faint to be discerned from the other stars. Discovered in 1781 by Herschel by accident (at first thought it was a comet). Neptune was found because Uranus was straying from its predicted orbit. Gravity of Neptune was tugging on Uranus. Found in 1846 by Galle after being mathematically predicted by Le Verrier and Adams. Called giant planets because of their mass— from 14.5 Earth masses (Uranus) to 318 (Jupiter)—and also, their physical size. No solid surfaces: We just see the cloud layers in the atmospheres. More Giant Characteristics We see atmospheres (some very cloudy, some not), not surfaces. They are less dense than the terrestrial planets; in fact, Saturn would float in a large enough tub of water. Jupiter’s chemistry is like the Sun: mostly hydrogen and helium. Saturn has some more heavy elements, but is like Jupiter; Uranus and Neptune have much more heavy elements. Planetary diameters are found by observing how long it takes for a planet to pass over a star: stellar occultation. Planetary masses are found by observing the motions of a planet’s moons and effects of gravity. i_Clicker Question Jovian Planets: Jovian Transit Jovian Planets: Jupiter Formation All giants have rapid rotation and therefore different amounts of oblateness. They also have different obliquities. • Jupiter: 3°. • Uranus: 98°, which results in extreme seasons. i_Clicker Question Jovian Planets: Oblateness Strong dark and light bands. A long-lasting giant storm (Great Red Spot). Many smaller storms. Colors indicate complex chemistry. Saturn has a similar band structure to Jupiter, but less pronounced. It has violent storms and a feature similar to Earth’s jet stream. Infrared observations let us see details of structure on Uranus. Weak banding on both Uranus and Neptune. Small, scattered bright or dark clouds. Transient large storms (Great Dark Spot on Neptune). The gas giants have similar cloud layers. Temperature, pressure increase downward. Different heights of cloud layers. Clouds on Jupiter: • Ammonia (NH3) at T = 133 K. • Ammonium hydrosulfide (NH4SH) at T = 193 K. Under the Clouds: Uranus/Neptune Unlike Jupiter and Saturn, the highest clouds on Uranus and Neptune are methane ice. Bluish because of scattering of light by the methane. Clouds on Jupiter and Saturn are colored by impurities. Helium Rain In Jupiter and Saturn, Temperature and Pressure meet the correct conditions to condense Helium in the lower atmosphere. In essence, this cause liquid Helium drops to form and fall down as “rain”. This contributes to the internal heating of both planets. Mixed in the Helium drops is Neon, which helps explain the low levels of neon in the atmospheres of both planets. i_Clicker Question Jovian Planets: Helium Rain Rapid planetary rotation results in strong Coriolis forces. This causes storm rotation. Most extreme equatorial winds are in Saturn’s and Neptune’s atmospheres, with maximum speeds up to 2,000 km/hr. Alternating east–west winds make banded clouds on Jupiter. Circulation pattern differs from planet to planet in ways not understood. MATH TOOLS 10.1 Wind speeds on gas giants can be measured by observing the movement of clouds above an assumed “surface.” Using the circumference of the planet, you can find how much a spot travels in a given time. All but Uranus have significant internal heat. Heat flows from the hot interior outward. Heat has a big effect on global circulation patterns. MATH TOOLS 10.2 Jupiter, Saturn, and Neptune radiate away more energy than they get from the Sun. A small increase in internal temperature leads to a large increase in emitted energy. Something has to be increasing the temperature. It is believed that the planets are still shrinking, with gravitational energy being converted into heat during that process. For Jupiter: Jupiter/Saturn: At depths of a few 1,000 km, gases are compressed so much they liquefy. At higher pressure and temperature, this liquid hydrogen can act like a metal. Cores are a liquid mixture of water, rock, and metals. Uranus and Neptune are smaller and have less pressure than the gas giants. They have more water and ices (ammonia, methane). Deep oceans containing dissolved gases and salts are present. Jupiter and Saturn formed from the protoplanetary accretion disk while hydrogen and helium were still present. Solar wind later blew out these gases. Uranus and Neptune formed later, by the merger of icy smaller bodies. All four possess a dense liquid core containing rocky materials. Many details are still not understood. Magnetic fields are generated by the motion of the electrically conducting liquids. Their orientation is at an angle to the rotation axis. Like a bar magnet. Magnetospheres are huge (Jupiter’s is 6 AU). They interact with the solar wind. Auroras (“Northern lights” on Earth). Produce strong radio waves/synchrotron radiation. i_Clicker Question Jovian Planets: Magnetic Fields Strong magnetospheres concentrate charged particles in radiation belts, including the plasma torus created by particles from Jupiter’s moon Io. Powerful flux tubes create bright auroras. CONNECTIONS 10.1 Accelerating charged particles emit photons. When moving, charged particles are forced in circles by magnetic fields—acceleration! Occurs often with magnetic fields and electrons around planets and stars. Synchrotron radiation. “Hot Jupiters” are seen orbiting close to their stars in extrasolar planetary systems. Computer simulations show that the giant planets may not have formed where they exist now, but rather could have migrated to their positions due to gravitational influences. PROCESS OF SCIENCE Theories make predictions that serve as opportunities to falsify them or strengthen confidence in the theory. These predictions are essential to scientific progress. Dozens of “worlds” of rock and ice exist in our Solar System; some large, some small. Liquid water under some surfaces is possible. The moons are made of rock, ice, or both. Some were formed by accretion and differentiation. They have many diverse properties, only partially understood. Most of the larger moons formed with their planets through the processes of accretion and differentiation. These are called regular moons. They revolve around their planets in the same direction that they rotate. Almost all are tidally locked, meaning one hemisphere always faces the planet the moon is orbiting. Some moons are objects that formed apart from a planet, but were later gravitationally captured by one. These are called irregular moons. They are usually on retrograde (“backward”) orbits. Largest: Triton, moon of Neptune. Many are only a few kilometers across. The giant planets have several large moons, and many are as large as Earth’s Moon. Some are geologically active, while others used to be. Surface markings, craters, and bright/dark areas reveal geological activity. Categorized as active now, possibly active, active in the past, and never active. For a moon to be geologically active, it must have internal heat. Tidal stretching by a planet heats the moon’s interior. Analogy: flexing a paper clip. Example: Jupiter’s moon Io. Io is the most volcanically active object in the Solar System. Eruptions of silicate magmas. Has no craters and a very young surface. For a moon to be geologically active, it must have internal heat. Tidal stretching by a planet heats the moon’s interior. Analogy: flexing a paper clip. Example: Jupiter’s moon Io. Io is the most volcanically active object in the Solar System. Eruptions of silicate magmas. Has no craters and a very young surface. Enceladus (Saturn): partially young surface. Experiences cryovolcanism, in which the “magma” is water. Thermal energy melts ice and drives it up to the surface. Enceladus’s low gravity cannot hold onto the icy particles once they are ejected. This is the source of material for Saturn’s faint E Ring. Triton is an irregular moon of Neptune with a retrograde orbit. Cantaloupe-like surface is a clue to its activity. Cryovolcanic activity: geysers of nitrogen. Thin atmosphere. Europa (Jupiter) is possibly active. Jupiter’s tidal heating should be too low for volcanism, but should allow for subsurface liquid, perhaps as underground lakes. Broken slabs of ice that appear to have floated and collided suggest geologic activity. Titan is Saturn’s largest moon. It has a thick, dense, nitrogen-rich atmosphere. Huygens lander revealed icy “rocks” and a soil rich with organic compounds. Possibly active. Possibly Active: Titan Methane appears to experience a cycle like rain on Earth, involving methane lakes and clouds. Methane in Titan’s atmosphere is most likely renewed by active geology. Definitions & Terms -1 • • Photon: A particle of light. Spectrum: The flux of an object as a function of wavelength.