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Geology of the Terrestrial Planets © Sierra College Astronomy Department Terrestrial 1 Midterm! Part I (Take home exam, including 10 points from Mastering Astronomy, 50 pts) is available, due October 26th, noon This week, Part II (in class exam, 50 pts.) – Taken in 3rd hour (week of 10/22 to 10/25) – Bring SCANTRON (882 form) and #2 pencil – Based on “Review Questions” handout, available now! Also: 10 of the 25 extra credit points are due by October 26th, noon. Lecture 9: Terrestrial Geology Basics The Moon and Mercury Moon Mercury The Moon’s geology The Moon’s surface can be divided into two main landforms: lunar maria and highlands (mountainous and cratered) regions. Maria (plural of mare) are any of the lowlands of the Moon (some circled by mountains) that resemble a sea when viewed from Earth. Moon © Sierra College Astronomy Department Moon features 3 Nearly Full Moon The Far side of the Moon Lecture 9: Terrestrial Geology Basics The Moon’s Surface Moon Mare forms The Maria were caused (3 to 4 billion years ago, just after the Moon was formed) by large impacts cracking through the crust and the consequent magma flow from the Moon’s mantle. Asymmetry of maria between the two sides of Moon is caused by differences in crust thickness (which ranges in depth from 60-100 km and is thinner on Earth-facing side). This asymmetry also lead to the “locking” of one face of the Moon always towards the Earth (since the maria are made of denser materials). The interior of the Moon has cooled too much for this to occur again Micrometeorites, sand sized particles from space, remain as the only major erosion process © Sierra College Astronomy Department interior 6 Walking on the Moon Apollo 17 surface December 1972 © Sierra College Astronomy Department 7 Lecture 9: Terrestrial Geology Basics The Moon and Mercury Moon Mercury Mercury’s geology – extreme conditions Radar observations show that Mercury rotates once very 58.65 Earth days, which is precisely 2/3 of its orbital period. Mercury’s solar day is quite different from its sidereal day. The solar day is 176 Earth days long (two Mercurian years.) Only 2 longitudes on Mercury experience noon while the planet is at perihelion High temperatures on Mercury can reach 425°C (790°F), well above the melting point of lead (330°C or 626°F). On the night-side of Mercury, temperatures can fall to -150°C (-250°F). © Sierra College Astronomy Department 8 Lecture 9: Terrestrial Geology Basics Mercury and the Moon Mercury’s geology - Moon Comparison S1 Mariner 10 flew by Mercury in 1974 (and subsequently twice more), returning a total of 4,000 photographs for the three fly-bys. Mercury appears similar to our Moon; both are covered with many impact craters. Mercury’s craters are less prominent; the planet’s surface gravity is twice that of the Moon so loose material will not stack as steeply. Ray patterns are also less extensive on Mercury because of the higher gravity. Mosaic Map © Sierra College Astronomy Department 9 Lecture 9: Terrestrial Geology Basics Mercury’s Surface Mercury’s surface history is thought be: Shrink Merc – Mercury was hot and melted due to radioactive Intercrater decay and expanded in size plain – This fractured the crust and allowed lava to reach the surface to form the intercrater plains Smooth – Lava eruptions in impact basins formed the plains smooth plains – Then the interior cooled and the planet shrunk scarps cracking the surface forming the scarps – This probably happened in the first 700 million years after Mercury formed S2 Scarp © Sierra College Astronomy Department 6_a2MercScarp 10 Mercury Composite by Mark Robinson (Northwestern) New Merc From Sky & Telescope (September 2004) Lecture 9: Terrestrial Geology Basics Mercury and the Moon A large “bulls-eye” impact crater called S3 Caloris Basin is visible. Caloris The Moon has a similar impact region This impact was so intense that there is Caloris schematic broken terrain in the region opposite of the Caloris basin S4Moon © Sierra College Astronomy Department Caloris Basin 12 Lecture 9: The Terrestrial Planets Mars by Lowell Mars Historical Mars William and Caroline Herschel made first extensive observations of Mars Lowell v Photo – In 1784, W. Herschel spoke with confidence about “inhabitants” of Mars In 1879 Schiaparelli’s drawing of channels or canali on Mars was misinterpreted by the public to mean canals dug by a race of intelligent beings. – Schiaparelli may have had an eye defect which made some details appear as channels Lowell, who opened his observatory in Flagstaff, AZ, in 1894, reported he saw many canals. Other astronomers could not confirm his findings. Changes in the dark areas on Mars led to speculation that there is vegetation on the planet that changes color in response to seasonal growth. © Sierra College Astronomy Department 13 Mars’s Canals? Lecture 9: Terrestrial Geology Basics Mars Mars’s Basics Mars orbits the Sun at an average of 1.524 AU (about 228 million km). Mars’ orbit is more eccentric than Earth’s, so Mars’ distance from the Sun varies from 210 million km to 250 million km. Mars takes 1.88 Earth years to complete its orbit around the Sun. Polar caps of water-ice and carbon dioxide can be seen © Sierra College Astronomy Department 15 Lecture 9: Terrestrial Geology Basics Mars Mars Rotation Mars’ sidereal period is 24h37m; its solar day is 24h40m long, very similar to that of Earth. Mars’ equator is tilted 25.2° with respect to its orbital plane, close to Earth’s 23.4°. We see seasons on Mars as we do on Earth. – The polar caps grow and shrink accordingly © Sierra College Astronomy Department Mars Seasons 16 Lecture 9: The Terrestrial Planets Mars Mars as Seen from Earth Mars is best seen at opposition, once every every 2.2 years (= 780 days = synodic period) – All oppositions are not equal due to the significantly elliptical orbit of Mars, so every 15 to 17 years Mars has a much closer than average opposition S1 Other special points on Mars’s (or any other outer planet’s) orbit: – Conjunction, eastern and western quadrature, opposition Configs Mars can exhibit a significant gibbous phase Mars at near quadrature Quadrature © Sierra College Astronomy Department 17 Oppositions of Mars (1997 – 2010) 2007 2005 P 2010 0.37 A 1997 A 1999 2001 2003 P Mars during its opposition in 2003 MarsObs Lecture 9: Terrestrial Geology Basics Mars Geology of Mars Besides the polar caps, Mars has other remarkable features The southern hemisphere has most of the higher elevation and the great impact region called Hellas Basin and most of the impact craters The northern hemisphere has the lower elevation, few impact craters and most of the volcanoes © Sierra College Astronomy Department 20 Altitude Maps of Mars from the Mars Global Surveyor (MGS) Lecture 9: Terrestrial Geology Basics Mars Olympus Mons The largest volcano is Olympus Mons, who height of 24 km (15 mi) is twice that of Earth’s largest mountain. – Several other large volcanoes can be found in the surrounding Tharsis Region S3,4 One reason Mars can “grow” larger volcanoes than Earth is because they lack Earth-like tectonic plates. Formed over a hot spot of lava that wells up from within a planet, a volcano can grow to enormous size if it does not move off the hot spot. © Sierra College Astronomy Department 22 Lecture 9: Terrestrial Geology Basics Mars S5 There were some tectonic activities in Mars’ past: Valles Marineris is an enormous canyon on Mars that stretches nearly 4,800 km (3,000 mi). – However, it was not carved out by a river nor a result of Earthlike plate tectonics – Instead it is a split in the crust which caused the Tharsis Region to bulge outward – There do appear to be runoff channels on the edges of the canyon which may have been formed by the outpouring of subsurface water There may be current geologic actively, though Mars will “die” in the next few billion years S9,10 © Sierra College Astronomy Department 23 Valles Marineris Outflow channels Lecture 9: Terrestrial Geology Basics Mars blueberries Ancient Water on Mars blueberries2 Could Mars have been water filled in its past? – Outflow channels seem to imply that water flowed 2-3 billion years ago (based on crater counts) Rovers Spirit and Opportunity (Mars Exploration Missions: MER-A and MER-B; Rovers) landed on Mars in 2004 looking for evidence of ancient water – Opportunity found rocks that must have been soaking in water at some time: Jarosite and the “blueberries” containing hematite © Sierra College Astronomy Department 25 Lecture 9: Terrestrial Geology Basics Mars Channels 1.5km Present Water on Mars Under the current conditions, free flowing water is unlikely to exist on Mars since the pressure and temperature are too low. – Water will only exist as a gas or solid on Mars – However, there is evidence of “gullies” which seemed to have running water in the recent past However, water or water-ice may exist just underneath the surface of the planet. – Odyssey and Mars Express orbiter both saw evidence for subsurface water © Sierra College Astronomy Department Odyssey subH20 26 Midterm! Part I (Take home exam, including 10 points from Mastering Astronomy, 50 pts) is available, due October 26th, noon This week, Part II (in class exam, 50 pts.) – Taken in 3rd hour (week of 10/22 to 10/25) – Bring SCANTRON (882 form) and #2 pencil – Based on “Review Questions” handout, available now! Also: 10 of the 25 extra credit points are due by October 26th, noon. Lecture 9: Terrestrial Geology Basics Current and Upcoming Mars Missions Currently there: – Mars Global Surveyor and Odyssey (Orbiters;Relays) – Spirit and Opportunity (Mars Exploration Missions: MER-A and MER-B; Rovers) – Mars Express Beagle 2 rover crashed on surface, but orbiter is working fine and it is taking some of the highest resolution pictures of the Martian surface ever from orbit – Mars Reconnaissance Orbiter (Launched: 12 Aug 2005) Even higher resolution of surface, subsurface, atmosphere (Inserted in Martian orbit on 10 March 2006, aerobraking has put into proper low orbit) © Sierra College Astronomy Department MRO 28 Lecture 9: Terrestrial Geology Basics Current and Upcoming Mars Missions Up next: – Phoenix lander (2007) Digger arms, oven and portable laboratory – Mars Science Laboratory (2009) Bigger and better rover © Sierra College Astronomy Department 29 Lecture 9: Terrestrial Geology Basics Venus Venus’s Motions Venus is easily seen in the sky with a maximum elongation of 47 degrees – (Ancient Greek names: Hesperus (evening) and Phosphorus (morning)) Special points on Venus’s (or Mercury’s orbit): – Inferior and superior conjunction – Greatest western (morning) and eastern (evening) elongation Each of these is repeated every 584 days (Synodic period) © Sierra College Astronomy Department Orbit diagram 30 Venus at Crescent Lecture 8a: The Terrestrial Planets Venus Venus’s Motions Starry Venus can be seen high in the sky around maximum elongation setting up to 3-4 hours after sunset (or rising 3-4 hours before sunrise) Venus can sparkle so brilliantly that it is often mistaken for an airplane (or UFO) and in a dark site can even cast a shadow (!) Venus can be seen in the daytime under clear sky conditions, if you know where to look Like Mercury, Venus can transit the Sun, but is far rarer © Sierra College Astronomy Department 32 Lecture 9: Terrestrial Geology Basics Venus Venus What are the Major Geological Features of Venus? Since Venus is only 5% smaller than the Earth, we expect it to be geologically active Orbiting probes Pioneer Venus 1 (1978), Venera 15 and 16 (1983-84), and Magellan (1990-93) have produced detailed radar maps of Venus’s surface. About two-thirds of Venus’s surface is covered with rolling hills. Highlands occupy <10% of the surface, with lower-lying areas making up the rest. Venus has about 1,000 craters that are larger than a few kilometers in diameter © Sierra College Astronomy Department Venus Radar Globe Venus Radar Map S5-S9 Skip? 33 Venus as seen in the UV Pictures taken 5 hours apart Clouds take about 4-5 days to circle planet Lecture 9: Terrestrial Geology Basics Venus Venusian features What are the Major Geological Features of Venus? While it has volcanoes and a lithosphere contorted by tectonics, Venus has some unique features, such as coronae, probably made of hot rising plumes of mantle rock. Volcanoes are still active (erupting in the last 100 million years) since the atmosphere contains sulfuric acid There is the lack of erosion on Venus: the winds are very weak. Venus surface © Sierra College Astronomy Department 35 Lecture 9: Terrestrial Geology Basics Venus What are the Major Geological Features of Venus? Venus has a lack of Earth-like plate tectonics: no super high mountain ranges Crater counts are uniform across the planet, suggesting an uniform age for the planet’s surface which is estimated to be 750 million years old. The uniformity of this age suggest that the entire planet “repaved” itself at that time. Since Venus should be a warm underneath the lithosphere as the Earth, the lithosphere of Venus must be thicker than that of the Earth and resists fracturing into pieces – No direct proof of this – May have come about from higher temperature surface © Sierra College Astronomy Department 36 Lecture 9: Terrestrial Geology Basics The Unique Geology of Earth The Earth is the most active of the terrestrial worlds – The Earth’s size explains the abundance of internal heat – The erosion from wind and water is explained by the Earth’s distance from the Sun and the rotation rate – The Earth’s plate tectonics is unique among the terrestrial worlds. © Sierra College Astronomy Department 37 Lecture 9: Terrestrial Geology Basics The Earth’s surface in motion Plate tectonics is the movement of fractured pieces of the lithosphere or plates. The plates of the Earth “float” on the mantle as convection moves the plate about the surface. The plates move at a rate of a few centimeters per year – about the rate of fingernails on a human hand © Sierra College Astronomy Department Drift Plates 38 Lecture 9: Terrestrial Geology Basics Plate Tectonics Drift Continental Motion Plates2 Alfred Wegener is credited with first developing the idea of continental drift the gradual motion of the continents relative to one another. He noticed that the coasts of South America and Africa seem to fit together and that the continents shared similar fossils Not initially accepted because a mechanism SA Afr to move continents was not known. © Sierra College Astronomy Department 39 Lecture 9: Terrestrial Geology Basics Plate Tectonics Plate Tectonics In the mid-1950s began to observe evidence for continental motion: midocean ridges Drift Plates2 Mantle material erupts onto the Rift ocean floor, pushing apart the Subduc existing seafloor on the either side. This is referred to as seafloor tectonics spreading © Sierra College Astronomy Department 40 Lecture 9: Terrestrial Geology Basics Plate Tectonics Plates2 The Earth’s surface has two very different types of crust: seafloor and continental – The seafloor crust is thinner, denser, and younger It’s typically 5-10 km think These plates get renewed in a process called subduction 2 crusts and so the seafloor crust is never more the 200 million years old. As a result, the continents have been spreading away from each other for 200 million years. – The continental crust is thicker, less dense, and older Typically between 20 and 70 km in thickness though its weight pushes it down so that it only sticks out slight higher than the seafloor. Can be as old as 4 billion years. subduction © Sierra College Astronomy Department 41 Lecture 9: Terrestrial Geology Basics Plate Tectonics Plates2 subduction Building up continents Unlike seafloor crust, which get recycled, continental crusts are gradually growing with time Volcanism shapes west North America as volcanic islands have merged into the rest of North America Erosion played a big role the Great Plains and Midwest Some mountains were formed when one plate Himalayas subducted under another When two continent-bearing plates collided with each other they also produced mountains. The Appalachians formed from several collisions: two from South America and one from western Africa. © Sierra College Astronomy Department Major geol. features 42 Lecture 9: Terrestrial Geology Basics Plate Tectonics Rift valley Rifts, Faults, and Earthquakes When continental plate are pulling apart, we can get a rift valley like the one in East Africa Plates that slide sideways are called faults – – San Andreas is famous example and will eventually bring LA and SF together in 20 million years When a fault moves it can move at several meters in a few seconds Earthquake Faults © Sierra College Astronomy Department 43 Lecture 9: Terrestrial Geology Basics Plate Tectonics Hot Spots Some volcanoes are created away from plate boundaries under places known as hot spots. The heat pushes up mantle (forming an island if in the middle of the ocean) The Hawaiian islands are the great example of this – – Main island of Hawaii is currently under a hot spot which is moving southeast Other islands “behind” the hot spot include Oahu (3 million years ago), Kauai (5 Mya), Midway (27 Mya) © Sierra College Astronomy Department Hawaii 44 Lecture 9: Terrestrial Geology Basics Plate Tectonics Plate Tectonics Through Time Since we know how the continents are drifting at the present we can predict where they have been and where they are going About 200 million years ago all continents are together as one called Pangea Before that the continents have moved all around: a billion years ago Africa was located at the South Pole drift and Antarctica was near the Equator Earth’s activity is certainly a function of its size and distance from the Sun and its rotation rate © Sierra College Astronomy Department History Terrestrial45 The End © Sierra College Astronomy Department 46