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Online Astronomy 100! Lecture 16: Terrestrial Planets II NASA 1 The Terrestrial Planets, Part 2 Today, we will continue our discussion of the terrestrial planets, looking at the geological histories ASTR 100 Lecture 16 ‣ Topics for Today: ‣ Histories of the Terrestrials: Geological activity 2 The Terrestrials (& the Moon) Mercury Mars Venus Earth NASA/Wikimedia Commons Moon Why have the terrestrial planets turned out so differently, when they formed at the same time from the same materials? 3 Earth’s Moon ‣ A small, barren, airless world ‣ 25% Earth’s diameter ‣ 1.2% Earth’s mass ‣ Not geologically active today ‣ Heavily cratered surface of years old ‣ But, not all parts are equally cratered Luc Viatour / www.Lucnix.be ‣ Billions 4 The mass of the Moon is about 1/80th that of the Earth, and its diameter is about 1/4th that of the Earth. The orbit of the Moon is very nearly circular (eccentricity ~ 0.05) with a mean separation from the Earth of about 384,000 km, which is about 30 Earth diameters. Since the moon rotates once per orbit, Lunar day and Lunar night are each about 15 Earth days long. During the Lunar night the temperature drops to around -113℃, while during the Lunar day the temperature reaches 100℃. The temperature changes are very rapid since there is no atmosphere or surface water to store heat. One striking difference between the Lunar surface material and that of Earth concerns the most common kinds of rocks. On the Earth, the most common rocks are sedimentary, because of atmospheric and water erosion of the surface. On the Moon there is no atmosphere to speak of and little or no water, and the most common kind of rock is igneous ("fire-formed rocks"). The Moon’s Defining Feature: Impact Craters ‣ Everywhere on the moon, you will find craters: giant basins a few hundred km in size... ‣ To microscopic pits in moon rocks! NASA NASA ‣ From Tycho crater, 86 km wide Tiny microcrater on a moon rock 5 Because the Moon has no atmosphere, even the smallest impactor can create a crater in the surface rocks. Lunar regolith ‣ The Moon is covered with a gently rolling layer of powdery soil with scattered rocks that is called the regolith ‣ It is made from debris blasted out of the Lunar craters by the meteor impacts that created them Apollo 11 Astronaut Buzz Aldrin's bootprint 6 No weather or wind, so these footprints will persist for some time. Lunar Maria & Highlands ‣ Two types of regions on the Moon ‣ Highlands ‣ Lighter-colored hilly regions ‣ Covered in craters ‣ Maria smooth, lowlying plains ‣ Few craters ‣ Cover about 17% of the Moon’s surface NASA ‣ Darker, Note the contrast in crater abundance between the highlands and maria 7 The Moon’s surface can be divided into light areas called the Lunar Highlands and darker areas called Maria (literally, "seas"; the singular is Mare). The Maria are lower in altitude than the Highlands, but there are no bodies of water on the Moon so they are not literally seas. The maria cover about 17% of the lunar surface, and are found mostly on the near side of the Moon. The dark material filling the Maria is actually basalt rock - dark, solidified lava - from earlier periods of Lunar volcanism. Both the Maria and the Highlands exhibit large craters that are the result of meteor impacts. There are many more such impact craters in the Highlands. The Highlands rocks are largely Anorthosite, which is a kind of igneous rock that forms when lava cools more slowly than in the case of basalts. This implies that the rocks of the Maria and Highlands cooled at different rates from the molten state and so were formed under different conditions. Thought QuesLon The fact that there are fewer craters on the lunar maria than on the lunar highlands indicates that the A. B. C. D. highlands are younger than the maria. craters formed as a result of volcanic eruptions. maria formed more recently. maria are of volcanic origin. 8 Answer C The maria have relatively few craters compared to the highlands, must be younger! (about 3 billion years old) How did the maria form? ‣ ~4 billion years ago, large (tens of km across) objects struck the lunar surface ‣ Formed giant basins hundreds of km in diameter ‣ Up to 10 km deep into Moon’s mantle! ‣ Molten material from young Moon’s interior flooded craters JAXA Illustration of mare formation 9 At the end of the heavy bombardment, the largest impacts formed giant basins hundreds of kilometers in diameter. After about 3.8 billion years ago, the cratering rate fell rapidly to the current low rate. The tremendous impacts cracked the crust as deep as 10 kilometers and led to flooding by lava. That formed the maria—from 3.8 to 3.2 billion years ago. Studies of the Apollo moon rocks show that the basins were flooded by lava flows of dark basalts. The maria are lava-filled impact basins! Mercury is also a barren airless world ‣ Surface similar to the Moon’s NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington ‣ Mercury ‣ Heavily cratered ‣ With regions of smooth plains ‣ Mercury is small ‣ Cooled rapidly ‣ Little geological activity Image of Mercury taken by MESSENGER 10 40% Earth’s diameter, 5.5% the mass of the Earth. Orbits the Sun at 0.4 AU, it has the most eccentric orbit of the 8 planets, e=0.21. Orbits the Sun in 88 Earth days. Each solar day lasts 176 Earth days. Lack of an atmosphere and long days means daytime temperatures reach a balmy 700 K (430 C), nighttime temperatures plunge to 100 K (-170 C). Mercury’s “Cracked” Surface exhibits long cliffs called scarps ‣ Many scarps are over 1 km high! Scarp 110 km NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington ‣ Mercury Extending from the upper left edge of this image diagonally toward the lower right corner is a long scarp (cliff) face. 11 This scarp runs through a large ancient crater right of center in the frame and was seen for the first time during MESSENGER's second Mercury flyby. Planetary geologists use the Latin term "rupes" for scarps on Mercury. Scarps such as this one have been identified all over the planet. Note: The large crater crosscut by the scarp is approximately 110 kilometers (70 miles) in diameter. How did Mercury’s scarps form? ‣ Mercury has large core for a small planet of its volume! ‣ Compared to 17% for Earth’s core Mercury’s iron core shrinks ‣ 42% ‣ As Mercury cooled, its core shrunk by 20 km ‣ Mercury’s crust “wrinkled” to compress down onto the smaller interior NASA Compression of the crust creates a scarp NASA/Arizona State University 12 Mercury has a very large iron core for such a small planet, 42% of its internal volume - Earth’s is only 17% of its volume. The large metal core cooled fast (little insulating rock to hold in the heat). The giant scarps are believed to have formed when Mercury's interior cooled and the entire planet contracted slightly as a result, causing the surface rocks to fracture and some blocks of crust to thrust over others along great faults. What is the geological history of Venus? ‣ Venus is a large terrestrial planet ‣ 95% the diameter of Earth ‣ We would expect similar levels of geological activity Craters? ‣ Volcanism? ‣ Tectonics? ‣ Erosion? NASA ‣ Impact A radar image of Venus’ surface from the Magellan spacecraft 13 Similar in structure and size to Earth, Venus' thick, toxic atmosphere traps heat at the Venusian surface. The scorched world has temperatures hot enough to melt lead. Glimpses below the clouds reveal volcanoes and deformed mountains. Venus spins slowly in the opposite direction of most planets. Impact craters on Venus ‣ Few craters ‣ Only ~900 identified ‣ Evenly spread across surface ‣ No small craters (under 2 km across) atmosphere burns up smaller meteoroids ‣ Craters are “rough” ‣ Rarely lava-filled ‣ No erosion NASA ‣ Dense Danilova, Aglaonice and Saskja craters on Venus 14 Low crater count indicates Venus has certainly been geologically active in the recent past. There are no craters smaller than 2 km, because of the effects of the dense atmosphere on incoming objects. On Venus, about 85% of craters are in pristine condition - no erosion! Venus Crater Map USGS Even crater distribuLon = global resurfacing event? Venus' craters are evenly scattered around the planet, indicating the surface is all about the same age (~500 million years old) 15 Venus has a surprisingly even crater distribution. No regions are significantly more or less heavily cratered than others. Compare this to craters on the Moon - the highlands are much more heavily cratered than the maria. We interpret the Moon’s crater densities to indicate that the highlands are older than the maria. This says, in effect, that the present Venusian surface is nearly uniformly young geologically. This rather surprising outcome needs explanation., but that explanation is still being debated. Various models have been proposed. Most 1) reject any plate tectonics activity, and 2) hold that volcanism has been active during this time period and hence the surface has been largely repaved. Venusian Volcanism 1,600 major volcanoes on Venus ‣ Most are probably long extinct ‣ 80% of surface is covered in lava flows ‣ Clouds prevent direct observation of volcanic eruptions ‣ There is circumstantial evidence for ongoing volcanism NASA ‣ Over Venus's highest volcano Maat Mons 16 The Magellan probe revealed evidence for comparatively recent volcanic activity at Venus's highest volcano Maat Mons (8 km high), in the form of ash flows near the summit and on the northern flank. Sulfur dioxide is put into the atmosphere via volcanism on the Earth. It is possible that large observed fluctuations in sulfur dioxide in Venus’ atmosphere are due to current volcanism on Venus The Pioneer and Venera probes detected bursts of radio emission in the atmosphere of Venus similar to types seen from lightning strikes around the plumes of Earth’s volcanoes. USGS ‣ Distribution Smithsonian Institution Venus Volcano Map of volcanoes around Venus’ surface is random ‣ Earth’s volcanoes are concentrated along plate boundaries ‣ Indicates that Venus’ crust lacks plate tectonics Earth Volcano Map 17 Distribution of volcanoes around Venus’ surface is random, unlike Earth, where volcanic activity is concentrated on plate boundaries - indicates that Venus’ crust lacks plates. Venus has tectonics, but not plate tectonics. Erosion on Venus? ‣ Venus’ surface is too hot for liquid water or ice ‣ At the surface, wind is only a slow breeze no gusts or storms ‣ Photos of rocks taken by landers sent to Venus show no evidence of erosion Photo taken by the Venera-13 lander on Venus in 1981 and reprocessed by Don P. Mitchell. 18 Thought QuesLon ‣ The maps below show representative regions of three terrestrial worlds. Which world’s surface is uniformly young, like Venus? University of Nebraska Lincoln, Astronomy Education Group 19 Answer A A has the least number of craters. What are the major geological features of Mars? Image Credit: National Geographic Society, MOLA Science Team, MSS, JPL, NASA The Martian terrain includes broad towering volcanoes, vast windswept plains, and enormous canyons. 20 Mars’ two-‐faced geology Southern hemisphere is heavily cratered highlands, Northern hemisphere is volcanic lowland plains 21 Martian geology: Mars’ surface has two distinct regions. Southern region is mostly heavily cratered highlands. Northern region is largely volcanic lowland plains, with several large volcanoes. Altitude relief map shows that the southern hemisphere is generally higher elevation than the north and shows the extensive cratering in the south. Image Courtesy NASA/JPL-Caltech Image Credit: National Geographic Society, MOLA Science Team, MSS, JPL, NASA Olympus Mons is the largest volcano in the Solar System. As big as Arizona and three times taller than Everest! 22 Olympus Mons - a huge volcano. About 25km high, 624 km in diameter (for comparison, the outline of Arizona is shown). For comparison: Mauna Loa (largest volcano on Earth) - 10 km high and 120 km across. The volume of Olympus Mons is about 100 times larger than that of Mauna Loa. But, there are impact craters on the side of Olympus Mons. Indicates that Olympus Mons is probably extinct. Image Courtesy NASA/JPL-Caltech Image Credit: National Geographic Society, MOLA Science Team, MSS, JPL, NASA Valles Marineris is a vast canyon stretching over about one-fifth the circumference of Mars! 23 Valles Marineris is a vast canyon stretching over about one-fifth the circumference of Mars! It is 10 times longer, 5 times deeper, and 20 times wider than the Grand Canyon. Comparing Valles Marineris to North America. it would stretch from New York City to Los Angeles, and it is nearly as deep as Mount Everest is high! Valles Marineris is not a river canyon, although it does show some water erosion features (so water likely flowed through it, or parts of it, at one time). The rising of the Tharsis Bulge, a volcanic rise with three towering volcanoes, at the western end of the canyon (left in the image) opened a crack in Mars’ crust! Therefore Valles Marineris is a tectonic feature. Mars shows no evidence of plate tectonics. Thought QuesLon Which hemisphere of Mars has an older surface? A. The southern hemisphere B. The northern hemisphere 24 Answer A How do we know? Higher crater density in the southern hemisphere. A dried drainage channel on Semeykin Crater NASA/JPL/ASU NASA/JPL/ASU Did liquid water once flow on Mars’ surface? Water erosion features in Ares Vallis 25 Left images: This impressive drainage system empties into Semeykin Crater on the northern margin of Arabia Terra. Right image: Impacts, floods, and tectonic forces have all left traces on Ares Vallis, a Martian outflow channel. Outflow channels formed when massive floods of water poured out of the ground early in Martian history when the planet likely had a thicker atmosphere and a warmer climate. Scientists estimate the floods had peak volumes many times today's Mississippi River. Teardrop mesas extend like pennants behind impact craters, where the raised rocky rims diverted the floods and protected the ground from erosion. While Ares Vallis is not the widest or deepest outflow channel, it extends for nearly 1,600 kilometers (1,000 miles). Mars Global Surveyor found hemaLte on Mars iron mineral hematite lies on the surface of parts of the Meridiani Planum region of Mars ‣ Hematite often forms in the presence of liquid water ‣ Discovery led to the Opportunity rover being sent here NASA/JPL ‣ The Hematite abundances on the Meridiani Planum 26 The iron mineral hematite lies on the surface of parts of Meridiani Planum. Mapped from orbit by the TES instrument on Mars Global Surveyor, hematite abundances range from 5% (blue) to 20% (red). Because hematite needs liquid water to form, its presence indicates Meridiani has seen much liquid water in the past. The background image is a THEMIS daytime infrared mosaic. Image credit: NASA/JPL/Cornell Credit: Brenda Beitler, University of Utah Rovers found hemaLte “blueberries” "Blueberries": Hematite concretions found on the Martian surface at Meridiani Planum (left). Hematite marbles found in Utah (right). 27 Marble-shaped pebbles ("Blueberries", Fig. 1b) were also discovered on Mars by the Mars Exploration Rover Opportunity. Hematite (Fe2O3) is a mineral that in most cases requires water for its formation. Therefore hematite indicates wetter and warmer conditions on Mars in time of hematite formation. Image credit: NASA/JPL/Cornell Sediments at Erebus Crater 28 A rover view of a stack of fine layers exposed on a ledge in "Erebus Crater" shows a diverse range of primary and secondary sedimentary textures formed billions of years ago. These structures likely result from an interplay between windblown and water-involved processes. Image credit: NASA/JPL/Cornell Bright Salty Soil ‣ The rover Spirit's wheels have churned up light-toned, subsurface soil deposits on Mars ‣ Sulfur-rich salts - possibly left behind by water evaporation 29 Spirit’s right front wheel became stuck, which slowed its movements, but also churned up the largest amount of bright soil discovered so far in the mission. Spirit's instruments confirmed that those soils had a salty chemistry dominated by ironbearing sulfates. The salts may record the past presence of water, as they are easily mobilized and concentrated in liquid solution. What have we learned? ‣ What geological processes shaped our Moon? ‣ Early cratering still present ‣ Maria resulted from volcanism ‣ What geological processes shaped Mercury? ‣ Cratering and volcanism similar to Moon ‣ Tectonic features indicate early shrinkage ‣ What are the major geological features of Venus? ‣ Venus has few craters - young surface ‣ Dominated by volcanism ‣ Also has tectonics but little or no erosion 30 What have we learned? ‣ What are the major geological features of Mars? ‣ Differences in cratering across surface ‣ Giant shield volcanoes ‣ Evidence of tectonic activity ‣ What geological evidence tells us that water once flowed on Mars? ‣ Features that look like dry riverbeds ‣ Some craters appear to be eroded ‣ Rovers have found rocks that appear to have formed in water 31