Astronomy 1010-H Planetary Astronomy Fall_2015 Day-37.

Astronomy 1010-H Planetary Astronomy Fall_2015 Day-37 Course Announcements SW-chapter 11, 12 due: Wed. Dec. 9 Lab this week is Comparative Planetology this one you can do on your own and turn in. Presentations are next week, during the normal lab time. 10 minutes, + 5 for questions 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 Earths 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 moons interior. Analogy: flexing a paper clip. Example: Jupiters 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. Enceladuss low gravity cannot hold onto the icy particles once they are ejected. This is the source of material for Saturns 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. Jupiters 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 Saturns 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 Titans atmosphere is most likely renewed by active geology. Formerly active Ganymede (Jupiter) is the largest moon in the Solar System. Signs of gradually filled-in craters. Bright terrain from some unknown past tectonic processes. Some moons of Saturn and Uranus also appear this way. Other moons show signs on the surface of being formerly active, including bright and dark areas and tectonic fracturing. Examples: Mimas and Iapetus of Saturn; Miranda of Uranus. Callisto (Jupiter) shows no sign of early geologic activity. Dark, heavily cratered surfaces. Bright regions where subsurface ice has been exposed after impacts. Others include Umbriel (Uranus). Concept QuizInternal Heat You discover a moon of Jupiter. It orbits very far from the planet, but it has many volcanoes. Is this a surprise? Why? A. No, any moon can have volcanoes. B. Yes, Jupiter is very far from Earths Moon. C. Yes, tidal forces are less for distant moons. Ring Systems All four gas giants have ring systems. Saturns rings are the largest and brightest. The fainter rings were discovered by stellar occultation methods. A very complicated system, composed of thousands of ringlets, each made up of tiny orbiting particles that obey Keplers laws. There are bright and dark rings, gaps, and divisions. The moons obey Keplers laws as they orbit the planet: For a particular planet, the left-hand side will be a constant for most of its moons. For Jupiter, Ganymede, Europa, and Io are in an orbital resonance of 1:2:4. Can estimate relative orbital distance. MATH TOOLS 11.1 Concept QuizRing Velocities If you could measure the velocities of ring particles at each distance from Saturn, you would find that A. inner particles orbit at slower speeds. B. inner particles orbit at faster speeds. C. orbital speed is the same at all distances. Gaps are not empty. Brightness/darkness reflects the amount of material in each ring. Though wide, the ring system is extremely thin. If Saturn were a basketball, a piece of paper is >1,000 times too thick. Diffuse rings are fainter and have no defined boundaries. Saturns largest ring is a diffuse dust ring, discovered in E Ring and G Ring are also diffuse. The rings of the other giant planets are mostly narrow and diffuse. Backlighting brings them into view. Neptune has denser sections known as ring arcs. Ring particles are from disrupted moons or from volcanic activity on moons. Saturn: bright rings because they are made of water ice. The total mass of Saturns bright rings is about the same as a small icy moon. Uranus and Neptune: dark rings from organic material (darker than coal). Jupiter: not as dark as the ice giants, nor as bright as Saturns; most likely composed of dark silicates.