Geology of the Terrestrial Worlds

Geology of theTerrestrial Worlds © Sierra College Astronomy Department

Geology of the Terrestrial WorldsThe Moon and 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. © Sierra College Astronomy Department

Geology of the Terrestrial WorldsThe Moon’s Surface • 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

Geology of the Terrestrial WorldsThe Moon and 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

Geology of the Terrestrial WorldsMercury and the Moon Mercury’s geology - Moon Comparison • Mariner 10 flew by Mercury in 1974 (and subsequently twice more), returning a total of 4,000 photographs for the three fly-bys. • MESSENGERhas recently flown by twice and will eventually orbit Mercury in 2011 • 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. © Sierra College Astronomy Department

Geology of the Terrestrial WorldsMercury’s Surface • Mercury’s surface history is thought be: • Mercury was hot and melted due to radioactive decay and expanded in size • This fractured the crust and allowed lava to reach the surface to form the intercrater plains • Lava eruptions in impact basins formed thesmooth plains • Then the interior cooled and the planet shrunk cracking the surface forming thescarps • This probably happened in the first 700 million years after Mercury formed © Sierra College Astronomy Department

Geology of the Terrestrial WorldsMercury and the Moon • A large “bulls-eye” impact crater called Caloris Basin is visible. • The Moon has a similar impact region • This impact was so intense that there is broken terrain in the region opposite of the Caloris basin © Sierra College Astronomy Department

Geology of the Terrestrial WorldsMars Historical Mars • William and Caroline Herschel made first extensive observations of Mars • 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

Geology of the Terrestrial WorldsMars 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

Geology of the Terrestrial WorldsMars • 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

Geology of the Terrestrial WorldsMars 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 • Other special points on Mars’s (or any other outer planet’s) orbit: • Conjunction, eastern and western quadrature, opposition • Mars can exhibit a significant gibbous phase near quadrature © Sierra College Astronomy Department

Geology of the Terrestrial WorldsMars 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

Geology of the Terrestrial WorldsMars • 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 • 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

Geology of the Terrestrial WorldsMars • 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 Earth-like 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 © Sierra College Astronomy Department

Geology of the Terrestrial WorldsMars Ancient Water on Mars • 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

Geology of the Terrestrial WorldsMars 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

Geology of the Terrestrial WorldsCurrent and Upcoming Mars Missions • Currently there: • Mars Global SurveyorandOdyssey(Orbiters;Relays) • Spirit and Opportunity(Mars Exploration Missions: MER-A and MER-B; Rovers) • Mars Express • Beagle 2rover 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 • Even higher resolution of surface, subsurface, atmosphere • Phoenix lander(2008) – mission ended • Digger arms, oven and portable laboratory • Landed in polar regions, discovered ice just under the surface © Sierra College Astronomy Department

Geology of the Terrestrial WorldsCurrent and Upcoming Mars Missions • Mars Science Laboratory (Landed Aug 2012) • Called “Curiosity” • Bigger and better rover with many cameras and instruments Up Next: • Maven (Mars Atmospheric and Volatile EvolutioN; launch 2013) • Exploring the Martian atmosphere and how it is changing today and in the past • ExoMars/Trace Gas Orbiter (2016 by ESA) • Includes “demonstration” lander • Orbiting spacecraft that will help with telecommunications • Orbiter looking for trace gases • ExoMars/Trace Gas Rover- (2018 by ESA) • Looking for organic materials on Mars. • Prelude to return sample mission. © Sierra College Astronomy Department

Geology of the Terrestrial WorldsVenus 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

Geology of the Terrestrial WorldsVenus Venus’s Motions • 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

Geology of the Terrestrial WorldsVenus 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

Geology of the Terrestrial WorldsVenus 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. © Sierra College Astronomy Department

Geology of the Terrestrial WorldsVenus 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

Geology of the Terrestrial WorldsThe 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

Geology of the Terrestrial WorldsThe 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

Geology of the Terrestrial WorldsPlate Tectonics Continental Motion • 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 to move continents was not known. © Sierra College Astronomy Department

Geology of the Terrestrial WorldsPlate Tectonics Plate Tectonics • In the mid-1950s began to observe evidence for continental motion: mid-ocean ridges • Mantle material erupts onto the ocean floor, pushing apart the existing seafloor on the either side. This is referred to as seafloor spreading © Sierra College Astronomy Department

Geology of the Terrestrial WorldsPlate Tectonics • 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 and so the seafloor crustis 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. © Sierra College Astronomy Department

Geology of the Terrestrial WorldsPlate Tectonics 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 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

Geology of the Terrestrial WorldsPlate Tectonics 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 © Sierra College Astronomy Department

Geology of the Terrestrial WorldsPlate 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

Geology of the Terrestrial WorldsPlate 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 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

Geology of the Terrestrial Worlds