2000 February 8, SPS 1020 (Introduction to Space Sciences) - Read TNSS Ch. 12 and especially 13 (Comparative interiors and atmospheres) and PBD Ch. 10 & 14 by Thursday, February 10. - Mid-Term Exam 1 will be Thursday, February 17. The exam will cover: TNSS 1-2, 6-13, PBD Intro, 1, 10-12, 14 and The Basics of Space Flight Handbook, Chapters 1-17. - Tuesday, Feb 15 will be a discussion and review session for the exam. Please think up questions, so we can go over them! ------------------------------------------------------------------------- Give out "Essentials to Study," practice questions ------------------------------------------------------------------------- MERCURY (TNSS Chapter 7): ------------------------ The "forgotten planet." Hard to observe from ground: never farther than 27 degrees from the Sun. (One hour on a clock is how many degrees?) Hubble Space Telescope is not allowed to observe it: spacecraft heating! Visited by only 1 spacecraft, Mariner 10, which made 3 flybys in 1974-75. (There have been over 40 missions to the Moon; 15 to Mars; 20 to Venus) Innermost and smallest of the terrestrial planets: Porb = 88 days Rotation and revolution (orbit) are _not_ the same, as thought for many years. Radar measurements => rotation = 58.6 days, exactly 2/3 of the orbital period This is an example of spin-orbit coupling, a _resonance_. => Hot poles, weird sunrise/set/rise sequence. Surface temperature can reach 740 K at hot poles, 525 K elsewhere (night 90 K, like Earth's Moon: almost no atmosphere to retain heat) Density: above linear trend expected for terrestrial planets Magnetic field: a surprise! A dynamo, generated by circulation in Earth's core, generates its magnetic field. This is much like an electric generator. Faraday's Law: any moving conductor will make a magnetic field. => Fe core, 75% of radius: much larger, in proportion to planet's radius, than any other planet. Why? - Perhaps high T environment, in Solar Nebula - Perhaps giant collision (like Earth's Moon) => Implies 70% of planet is Fe, 30% is silicate. Yet little Fe at surface (from Earth-based spectra): diffusion? Terrain superficially similar to the Earth's Moon's: =>Careful! Less than 45% of Mercury's surface was mapped by Mariner 10. Remember what happened with Mars... (Discovered Tharsis volcanoes, Valles Marineris, runoff and outflow channels, etc.) Still, Mariner 10 found: - Highlands: craters - but relatively fewer than in lunar highlands Crater rims half height of Moon's (6-10 km), because of 2x greater gravity. - Intercrater plains: smooth to gently rolling hills, lots of small craters => Major resurfacing event, 4.2 - 4.0 b. y. a. (during LHB) - Lowland plains: lava plains, like lunar Maria? Or are there undiscovered volcanoes? Or impact melt? - Caloris basin: 1300 km diameter - Novel hummocky terrain, on opposite side of Caloris basin => from waves from giant impact, propagating through the (partially molten) planet? - Lobate scarps, or cliffs from shrinkage of crust from cooling, e.g. Discovery Rupes. Also seen on Earth's Moon (e.g. Rupes Recti, "The Straight Wall"), but more common on Mercury Atmosphere: almost none (like Earth's Moon's: not enough surface gravity to hold onto an atmosphere), but the trace gases are still interesting - H and He accrete from the Solar wind (stream of outflowing charged particles from Sun), but is also continually lost Unexpected volatiles, including O (most common gas), Na, K - ??? From sputtering (atoms knocked off surface, by micrometeorites or solar wind) or residual outgassing (like _maybe_ Transient Lunar Phenomena)? An unexpected radar discovery: ice at poles! Mercury's spin axis is almost perpendicular to its orbital plane. Ice therefore may have accreted from passing comets and stayed in permanently shaded, deep craters, for aeons (billions of years) => Sign of stability? MESSENGER orbiter: approved 1999 VENUS: ----------- Many successful scientific missions, since Mariner 2 in 1962 (Sagan was on team). Called Earth's "sister planet": mass, radius, density, surface gravity about same. Can't observe much, from Earth: featureless white appearance, goes through phases. (Does show some features at ultraviolet wavelengths.) Inferior planet (yo-yo motion): Superior planet (retrograde loop): Conjunction vs. opposition: Inferior planet: Superior planet: Radio observations in 1950s => blackbody temperatures > 800 C! Mariner 2 confirmed this. Soviet progress: many Venera orbiters, landers (1967-1986) Venera 14 (1982): survived at, imaged surface for 60 minutes Also radar mapping, most recently with Magellan orbiter (1992-1994). 98% mapped to 120- to 300-m resolution Rotation retrograde and very slow: 243 d period: Year is 225 d (orbital period)! Venus is HOT! Hotter than Mercury: 750 K (= 900 F) everywhere => This is hot enough for surface rocks to GLOW! Little temperature difference, either on sub-solar point, or between day and night side => heat retained by atmosphere. Clouds, mostly sulfuric acid, never reveal surface (and never show Sun, from surface). Lightning present (known from radio "whistlers", but come from under cloud deck, unlike Earth). Dense atmosphere, 96% C0_2, at 90 bars (Earth's atmospheric pressure is 1 bar), about same as pressure at Earth's ocean bottom (3 km). => Sagan's Ph.D. thesis (1957): the Greenhouse Effect Venus: runaway greenhouse (a positive feedback effect): ------> Higher T => water evaporated => | Less CO_2 dissolved in water, more in atmosphere -------- => T increases => water dissociation In other words, the hotter it gets, the drier it gets, and the drier it gets, the hotter it gets. Positive feedback = "a vicious cycle." Greenhouse here is really a misnomer. It's not like a greenhouse, which lets in light and gets hot by suppressing air circulation (convection). CO_2 absorbs heat, doesn't re-radiate: more like a blanket Venus is very dry: all water probably dissociated, from high T, and the H escaped from the planet, aeons ago. => dense CO_2 atmosphere? Earth's CO_2 is mainly in limestone rocks. If all Earth's limestone rocks vaporized, atmosphere would be like Venus. Also, water on Earth might have lubricated early surface, gotten plate tectonics going... The four primary geological processes: 1) Impact craters present, but rare. Hardly eroded at all, though. Show that whole surface had an episode of global melting about 500 million years ago. (Unknown why.) 2) Tectonism: all one plate! Although a kind of jumbled, disrupted folding mountains (tesserae) are present. 3) Volcanism: surface dominated by lava flows. High T important: less 4) Gradation: from lava flows. Also wind erosion: atmosphere is dense! Much slower than on Earth or even Mars, though. No water erosion. Venusian (Cytherian) Terrain: - Mostly flat. Many spectacular images from Magellan had their vertical relief exaggerated! 60% of surface < 1 km elevation, vs. Highlands (3-5 km elevation) < 15 % of surface, vs. Earth's continents, > 30% of surface. - Plains: have wrinkle ridges, as on maria on Earth's Moon => volcanic flooding - Shield volcanoes: Maxwell Mons (12 km, vs. Mauna Loa, Everest, 8.5 km), Sif Mons, Gula Mons - Maxwell Mons is on Ishtar Terra, a "continent", or uprise about the size of N. America (Not usually considered a proper continent, floating on tectonic plates on Earth). Also Aphrodite Terra, smaller Beta Regio. - Canyons, from faulting: Diana Chasma, Dali Chasma - Landslides - Sinuous lava channels, > 5000 km long - Over 1000 volcanic features known, including several types unique to Venus: - Coronae- domes with concentrically fractured centers, from upwelling mantle material - Arachnoids: a type of coronae - "Pancakes" - volcanic flows unknown on Earth, from more viscous lava than shield volcanoes - Evidence for ongoing activity: variable SO_2 in atmosphere Interior: almost completely unknown. No seismic data, magnetic field, but then little rotation. Heat transferred by volcanism: mantle upwelling & downwelling, not lateral plate motions.