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Module 9: The Moon in Close-up Activity 1: Exploring the Lunar Surface Summary: In this Activity, we will investigate (a) Moon missions, (b) the Moon’s vital statistics, and (c) cratering. (a) Moon Missions The Moon is the only other Solar System body which has been visited by a manned mission from Earth. Grainy images of the Apollo landings on the Moon are part of history now ... Buzz Aldrin descending from the Apollo 11 lunar module ... however in 1969 the first landing, and views of Earthrise from the Moon, had a profound effect on the way many people think about the Earth and our place in the Solar System. As the Earth and Moon share a common orbit around the Sun, sending a spacecraft to the Moon is not particularly difficult. (We will see in later Activities that sending spacecraft to visit bodies orbiting closer or further from the Sun is significantly more difficult.) To send a spacecraft plus human occupants from Earth to the Moon and back required a significant payload. A spacecraft that massive would have been dangerously cumbersome to maneuver for landing and takeoff on the Moon. Instead the main spacecraft - the Command Module was used to travel between the Earth and Moon ... Command module, Apollo 11, photographed from the Lunar Module … connected to a light Lunar Module built for low weight and high maneuverability in the low lunar gravity and lack of an atmosphere. Lunar ascent module, Apollo 11, photographed from the Command Module The six Apollo missions which successfully landed on the Moon brought back 380 kg of soil and rock samples, and visited a range of sites, from flat low-lying plains to craters, the edge of a lunar mountain range, and a huge channel carved in the Moon’s surface by an ancient lava flow. Apollo 16 lunar rover leaves tracks on the Moon The USSR’s unmanned Luna missions also visited the Moon between 1965 and 1976, with 5 missions achieving soft landings on the Moon. Luna 9, the first soft lander on the Moon Luna missions between1972 and 1976 drilled for rock samples and then brought them back to Earth. Luna 17 & 21 were equipped with robotic lunar rovers, Lunokhod 1 & 2, which were controlled from Earth. Lunokhod 1 toured Mare Imbrium for 11 months; Lunokhod 2 covered 27 km of the Moon’s surface in 4 months. Artist’s conception of Lunokhod 1 To find out more about the USA and USSR missions to the Moon, visit the History page of the Lunar Prospector site at the following Internet site: http://lunar.arc.nasa.gov/history/index.html One advantage of using unmanned missions like the Luna series is the simplicity and cost savings due to the reduced payload because of the absence of a crew or life-support system. An advantage of using manned missions like Apollo is flexibility - for example, astronauts can assess a landing site and change plans to suit circumstances more easily than can a robotic mission. Although costly, the importance of manned Apollo missions from the public relations point of view for both NASA and the USA should not be underestimated. We will revisit these issues when we consider the Mars Pathfinder mission, planned to be the first of a new generation of unmanned landers. For now, we will investigate the main facts of what we know (and are still discovering) about the Moon and its evolution - largely as a result of space missions to the Moon over the last 30 years, both manned and unmanned. (b) The Moon’s vital statistics In the last Activity we investigated the space missions which have given us a close-up view of Luna, our Moon. In this Activity we’ll summarize some of the basic information which we have learnt about the Moon. When we look at the properties of other planets and natural satellites in our Solar System, we will use our Earth - the Solar System we know best - as a reference point. For this purpose, we use the symbol to refer to Earth, so that, for example, in the following tables M means the mass of the Earth, and D means the diameter of the Earth: Moon mantle Earth core mantle crust crust Core 0.27 D M = 0.01 M D M = M The Moon is much less dense than the Earth, which means that its core must be very small. Results from Lunar Prospector experiments confirmed the Moon’s small core in 1999, indicating that the core is less than 4% of the Moon’s total mass. The Earth’s core, for comparison, is about 30% of the Earth’s total mass. The different densities also have implications when we look at models for the Earth and Moons’ formation, as we will see later. Moon Earth 1 AU 1 AU Length of “Year” 1y 1y Length of solar day 29.5 d Av. Distance from Sun Inclination of axis to ecliptic 1.3° 1 d 23.5° Look back at the previous table, and decide whether you would expect the Moon to have seasons. Then go to the next slide to see if you are right... 23.5° The seasons on Earth are due to the sizeable (23.5°) tilt of the Earth’s rotation axis. plane of the ecliptic (To remind yourself of why the axis tilt causes seasons on Earth, revisit the Activity on Earth’s Seasons) 1.3° 23 The tilt of the Moon’s axis is very small - only 1.3° to the ecliptic - so we would not expect the Moon to experience significant seasons, and indeed it does not. Note that the plane of the Moon’s orbit around the Earth is tilted 5° to the plane of the ecliptic, but it is the tilt of the rotation axis to the ecliptic that determines whether a planet (or satellite) will experience seasons. 23 Sun-Earth-Moon system av. albedo Moon Earth 0.10 0.39 Note the Moon’s low albedo. The Moon looks bright in our sky not because it is highly reflective, but rather because it is so close to us. The albedo of the Moon (like the Earth) varies depending on the terrain, varying from 5-10% for the maria* to between 12-16% for the highlands.* * these will be discussed in the next Activity. av. albedo acceleration due to gravity Moon Earth 0.10 0.39 0.17g g The Moon’s low mass and density means that the effects of gravity are much weaker on the Moon than on Earth. av. albedo Moon Earth 0.10 0.39 acceleration due to gravity 0.17g atmosphere 78% N2, 21% O2 Almost none 0.03% CO2, ~2% H2O g Because of the low lunar gravity, gases can easily escape. The Moon has only the slightest trace of an atmosphere made up from gases “baked out” (outgassed) from the rocky surface, with a pressure of about 1/100 000 000 000 000 th of p , atmospheric pressure on Earth! av. albedo Moon Earth 0.10 0.39 acceleration due to gravity 0.17g atmosphere 78% N2, 21% O2 Almost none 0.03% CO2, ~2% H2O surface temperature -170 °C +130 °C g - 50 °C + 50 °C The insulating effect of our atmosphere helps to “smooth out” temperature variations here on Earth - with essentially no atmosphere, temperature variations on the Moon are much more extreme. av. albedo Moon Earth 0.10 0.39 acceleration due to gravity 0.17g atmosphere Almost 78% N2, 21% O2 none 0.03% CO2, ~2% H2O surface temperature surface geology -170 °C +130 °C Heavy cratering, ancient volcanoes & lava flows g - 50 °C + 50 °C Cratering largely obliterated by active surface (volcanoes & plate tectonics), weathering and biological activity (c) Cratering Heavy cratering, mostly caused by impacts with Solar System debris over the history of the Moon, forms the most prominent feature of the Moon’s surface. Surface features on the Moon like craters stand out best when viewed near the terminator - the boundary between sunlight and shadow on the Moon. The largest craters are named after philosophers, mathematicians, and scientists. For example, the crater Copernicus dominates a region called the Ocean of Storms. Many craters, including Copernicus, exhibit patterns called rays, caused by debris thrown out by the initial impact. This debris, called ejecta, can in turn create secondary craters when it falls back to the surface. The low surface gravity of the Moon means that some ejecta can escape entirely - some meteorites found on Earth turn out to have been ejected long ago from the Moon. When a meteorite strikes, it penetrates into the lunar surface, deforming the surface layers and vaporizing itself and surrounding rock. The crater starts to form. Surface material (“ejecta”) is thrown out by the shock wave, and the outer edges of the crater collapse and overturn. The surface may partly rebound to form a central peak in the crater, and the walls of the crater may partly fall back in to create a terraced effect, as can be seen in this crater from the far side of the Moon. Modelling the formation of a crater by a meteorite impact: Two of the largest impact craters, Mare Imbrium and Mare Orientale, are the results of impacts so large (it has been estimated that Mare Imbrium was caused by a meteorite 65 km in diameter) that seismic shock waves caused by the impacts would have travelled right around the Moon. This image, centred on Mare Orientale, has been image enhanced to bring out fine details. In particular, the locations on the Moon directly opposite these two huge impact craters are regions called jumbled terrain - areas which could have been disturbed by the concentrated shock waves shaking the surface up and down by up to 10 m. Not all craters are huge: they extend in size all the way down to microcraters in the surface of rocks - tiny crater pits which are less than 1 mm across. (Why do you think we don’t see microcratering on Earth?) The bedrock of the Moon’s surface is covered by the regolith, a layer of loose soil and rocks produced by meteorite impacts over the long history of the Moon. Footprint left in the Moon’s regolith The regolith absorbs most of the light which falls on it, which explains the Moon’s low albedo (0.10). Much of the rocks on the Moon’s surface are made up of fragments of older rocks which have been broken up by meteorite impacts, then fused together by heat and pressure of subsequent impacts. These rocks are called breccias. Rate of Cratering Geologists have used radioactive dating to estimate the ages of rocks collected from the Moon’s surface during the Apollo program. All are igneous (formed from lava). The oldest rocks collected are 4.4 billion years old - the youngest 3.1 billion years old. By comparing the ages of the rocks with the amount of cratering at their original locations, geologists have concluded that the Moon underwent intense bombardment from at least 4.6 billion years ago (which is when its surface solidified into a crust), until 3.8 billion years ago. Cratering rate The first 0.8 billion years of the Moon’s history was dominated by heavy bombardment, including some giant impacts - perhaps by planetesimals. By 3 billion years ago, the cratering rate had dropped to about 1 millionth of its initial value. 4 3 2 1 0 Time before the present (billions of years) Presumably the intense early bombardment was due to planetesimals and smaller debris in the early Solar System as it emerged from the Solar Nebula. As the debris was gradually swept up into planet making, deflected by gravitational interactions with the large planets like Jupiter and ejected into the outer Solar System, or gradually driven outwards by the young Sun’s strong solar wind, the rate of cratering dropped quickly to its present relatively low value. In the next Activity we will investigate other prominent features of the lunar surface and its evolution. Image Credits NASA: Three-filter color image of the Moon (Galileo) http://nssdc.gsfc.nasa.gov/image/planetary/moon/gal_moon_color.jpg Apollo 17 astronaut Harrison Schmitt standing next to boulder at Taurus-Littrow during third EVA http://nssdc.gsfc.nasa.gov/image/planetary/moon/apollo17_schmitt_boulder.jpg Apollo 11, Buzz Aldrin descending LM ladder http://images.jsc.nasa.gov/images/pao/AS11/10075261.jpg Apollo 11, Earth rising over Moon’s surface http://images.jsc.nasa.gov/images/pao/AS11/10075248.jpg Apollo 11, Command/Service modules photographed from Lunar Module in orbit, http://images.jsc.nasa.gov/images/pao/AS11/10075257.jpg Apollo 11, Lunar Module ascent stage photographed from Command Module, http://images.jsc.nasa.gov/images/pao/AS11/10075285.jpg Luna 9 http://antwrp.gsfc.nasa.gov/apod/image/luna_9_unk.gif Image Credits NASA: View of the Mid-Pacific Ocean http://nssdc.gsfc.nasa.gov/image/planetary/earth/gal_mid-pacific.jpg Lunar cratering, Clementine mission http://nssdc.gsfc.nasa.gov/image/planetary/moon/clem_strtrk.jpg Taurus-Littrow http://pds.jpl.nasa.gov/planets/welcome/thumb/taurus.gif Copernicus Crater http://images.jsc.nasa.gov/images/pao/AS17/10075987.gif Mare Imbrium & Orientale Basin http://images.jsc.nasa.gov/images/pao/STS34/10063796.gif Cratering on the Moon's far side http://images.jsc.nasa.gov/images/pao/AS11/10075255.gif Highland breccia http://pds.jpl.nasa.gov/planets/welcome/thumb/breccia.gif Now return to the Module home page, and read more about exploring the lunar surface in the Textbook Readings. Hit the Esc key (escape) to return to the Module 9 Home Page