Transcript Document
Earth-rise on Moon The Moon Apollo and Luna Landing Sites A15 A17 L24 L20 A12 A14 A11 A16 L16 Lunar Surface – Sandy and Rocky The Moon: Some Factoids • Lunar exploration: Luna series (USSR), Surveyor (US) unmanned missions • Apollo manned missions explored geology • Density = 3.34 g/cc ; composition? • D (Earth, Mercury,Venus) = 5.52, 5.43, 5.24 g/cc Composition? • Basic surface and interior similar to rocky part of the Earth (little or no metallic core or magnetic field) Highlands and Marias • Highlands were heavily cratered during the early history, from formation to about 4 billion years ago • Marias (dried up lava oceans) formed later, about 3 billion years ago Cratered Highlands and Marias Highlands are much older than marias. Why? Evolution of highlands and maria Craters and chronology (age) • Highlands are highly cratered regions that must be older than smoother regions of surface since most cratering activity was at the time of the formation of the solar system • Marias are much younger, and evolved later at places where large crater impact basins were formed and filled with lava from the interior Atmospheres and Surfaces • Lack of atmosphere - results in extreme temperatures - exposes surface to meteoroid impact craters - lack of current geological activity and evolution (ceased long time ago) • Moon and Mercury lack atmosphere (too small and too close to the Sun) • Both have similar surface geology craters and marias Formation of a lunar sea: Impact of meteoroid Impact Basin Filled with Lava Maria A Mare Maria – Lightly cratered smooth surface Lava River Lunar Geology Formation of Moon • No generally accepted theory: three scenarios - Accretion - Fission (breakup) - Capture Possibly a combination Formation of Moon Earth Tides on Moon Tidal forces due to the Earth forced the Moon to rotate at the same rate as Its revolution; therefore it keeps the same face towards the Earth Another view of Moon ? And, Mercury ! Cratered highlands of Mercury Mercury – Basic Statistics • • • • • • • Mass ~ 1/20 Earth’s Diameter ~ 1/3 “ Density ~ 5.5 g/cc Year ~ 88 Earth days Day ~ 59 “ 0.4 AU from the Sun Surface T: Day 700 F, Night -300F Mariner Fly-By’s Several orbital reconnaissance satellites, but no space probes landed yet Mariner Fly-By Picture Mosaic Best Mercury Sighting at Maximum Western Elongation Eccentric orbit implies different maximum angles Greatest Elongations of Mercury Optimum Mercury Sighting at Maximum Elongation, and/or just after sunset or before sunrise NOT easily seen, like Venus as “evening star”: Mercury is too small, too close to the Sun, and lacks atmosphere and hence has small albedo Mercury – Basic Features • Metal ball (mostly iron) surrounded by heavily cratered rocky crust like Moon • But weak magnetic field, why? • No molten, moving core! • Only refractory elements; volatile elements evaporated • Geology like the Moon, e.g. Caloris basin resembles lunar maria – large crater impact basin flooded later with lava • No Plate Tectonics, why? Interiors of Earth and Mercury Dopper Radar measurement of rotation speed of Mercury Aracebo Radio Telescope(Puerto Rico) A “Day” on Mercury • A year is the period of revolution (orbit) around the Sun • Two kinds of “day” on a planet: Sidereal and Solar • Sidereal day is the actual period of rotation on its axis (viewed with respect to the stars, not the Sun) • Solar day is the period of rotation with respect to the Sun • On the earth: solar day = 24 hrs, longer than sidereal day by 4 mins. • Mercury’s sidereal day is 2/3 of a Mercury year Rotation and Revolution of Mercury: Day and Year • • • • Rotation period ~ 2/3 revolution period A solar day is twice as long as a year !! Solar Day (176 d) = 2 x Sidereal year (88 d) Wait two years to see the sun in the same place in the sky (on the earth it is just 1 solar day, or 1 sidereal day + 4 mins) • What’s going on? Rotation and Revolution of Mercury Moon’s rotation period equals orbital period: see only one side or face of Moon Strong Tides due to Sun Like the Moon Periods of Mercury A solar day on Mercury is twice as long as the sidereal year Hot Poles on the Equator ! • Perihelion of Mercury is 1.5 times closer than aphelion • Mercury receives 2.25 times more solar energy at perihelion than at aphelion • Two extremities of the equator pointing towards the sun constitute two hot ‘thermal’ poles • Largest variation in temperature between day/night, but maximum temperature ~700 F is still less hot than Venus (> 800 F)