DOCSLIB.ORG

Earth, Venus and Planetary Diversity

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Earth, Venus and Planetary Diversity Earth, Venus and Planetary Diversity Dave Stevenson Caltech Comparave Tectonics & Geodynamics May 4 2015 Lissauer et al, 2014 • There are many rocky exoplanets – Typically somewhat more massive than Earth (“superEarths”) • Many (perhaps most) of these have atmospheres, almost certainly hydrogen rich – Even those that do not may have started out with such an atmosphere – Testament to rapid formaon (Earth & Venus formed slowly by comparison) • These planets are mostly not Earthlike! The atmosphere creates a basal T that can readily sustain a magma ocean • But perhaps a few of them have Earthlike or Venus-like atmospheres. What do all (most*) Planets have in Common *there will always be oddballs, for example pulsar planets, which were the first planets found outside our solar system • Use the same periodic table – But not necessarily the same radioac2vity! • Use the same physics (& obey the same rules of chemistry) • Have a broadly common genesis story? – Gravitaonal collapse; disk because of angular momentum – Accreon of solids followed (somemes ) by accre2on of gas Guidelines for the Diversity of Planets • STRONG CONSTRAINT: Dynamically stable (most of the mass sasfies hydrostac equilibrium); stable to “evaporaon” for ~109 yrs. • MODERATE CONSTRAINT: Consistent with cosmic abundances. • WEAK CONSTRAINT: Conceivable formaon scenario. (In prac2ce, theorists are very good at explaining any observaon, even when it’s wrong!) Why do Planets of Similar Mass & Composi;on have different Behaviors? • Distance from star plays a role (but not necessarily dominant) • Environment of formaon plays a role – Water – Carbon dioxide, or… • Planets (unlike stars) are made of thermodynamically complex materials – Complex mel2ng behavior (volcanism) – Effect of alloying (e.g., sulfur in iron) – Effect of water on rock rheology • Role of chance in formaon – Major aspect of Earth-Venus difference? • Life Main Proposi;ons • The planets in our solar system exhibit great diversity, in part because of the role of minor cons2tuents (e.g. water, sulfur) • The planets in our solar system are a small, unrepresentave sample of all planets. • The extrasolar planets found thus far are different but s2ll unrepresentave! • Most no2ons of the habitable zone are anthropocentric. The habitable zone may be almost everywhere in the Universe if we include all places with energy sources & liquid water. (False?) Concept of a Habitable Zone Goldilocks scenario • Locaon, locaon locaon! • Size maers • Also depends on availability of H2O. Very abundant in the Universe… very under-abundant on Earth • Adopts a narrow perspec2ve • Magellan was launched 26 years ago today. – Magellan died 21 years ago. – It was the last Venus mission designed to tell us something about the geophysics of Venus • Magellan was launched 26 years ago today. – Magellan died 21 years ago. – It was the last Venus mission designed to tell us something about the geophysics of Venus – But we know more about the atmosphere – And we have learned much more about Earth, including interes2ng ideas about the operang regime(s) for plate tectonics – But s5ll, we know lile about Venus Q. Why do we know so lifle about Venus? A. Because it is a hard place to get to know. Q. Why do we want to know about Venus? A. Because it is as important as Earth in understanding the formaon, evolu2on and nature of terrestrial planets. Earth Venus 6400 km radius 6050 km radius 5.53 g/cc mean density 5.24 g/cc Basal2c volcanism Surface consistent with basal2c crust; volcanic constructs Large Fe-rich Core Large Fe Core inferred ~100 bars equivalent of CO2 100 bars of CO2 Earth Venus Thin atmosphere& water Thick atmosphere and ocean much less water(?) Plate tectonics Stagnant lid (but previously mobile?) 24 hr day 243 day rotaon period (retrograde) Large Moon No moon (requires explanaon?) Dynamo No intrinsic magne2c field How to think about a Planet? • Could discuss provenance- the proper2es of an apple depend on the environment in which the tree grows • Or could discuss it as a machine (cf. Hero[n], 1st century AD) • Need to do both How Should we Compare Earth & Venus? • Provenance – The materials that were accreted – The stochas2c nature of that accre2on… do Earth & Venus differ in part because of chance? • Operang Condi2ons – Planets are heat engines whose evolu2on depends on the materials, the ini2al condi2ons, temperature ,etc. In current terrestrial accretion models, the material that goes into making Earth or Venus comes from many different regions… It is very unlikely that the Venus is substantially different from Earth but it is very likely that Venus started out somewhat different from Earth by chance Recent work on the Earth- Moon similarity (Mastrobuono-Basttisti et al, 2015) might suggest greater Results from Chambers, 2003 Earth-Venus similarity (but (Similar results from Morbidelli) not the same sequence of events) What do Planet Formaon Stories tell us about Venus-Earth differences? • It is not difficult to make models where Earth ~Venus in mass & composi2on • It is difficult to make models where Earth is different from Venus in bulk composi2on • It is difficult to make models where Venus receives much less water than Earth – But what do we make of the 36Ar puzzle? • Isotopic differences are diagnos2c – Oxygen isotopic difference, if any, would be of interest Where has All the Water Gone? Long 0me passing… • Delivery seems likely • Surface & crust are dry • “Runaway “ greenhouse likely but not assured – More correct to say that Earth “healed” and Venus did not? – 2D models needed? LeConte et al , 2013. • What about the interior? • Loss from the atmosphere – Water photolysis and hydrogen loss affected by presence of CO2 (e.g., Wordsworth &PierreHumbert, 2013) – Magne2c field may maer? (Did early Venus have a dynamo?) Origin of the Atmosphere • Presumably (as with Earth) part of primary accre2on – “late veneer” is a minor component for Earth; of interest to establish its presence for Venus – Comets also minor? • Ingassing of solar nebula of interest (e.g., Ne isotopes) • History of outgassing related to volcanism ; expressed in noble gases Stagnant lid vs. Plate Tectonics • We do not know why Earth has plate tectonics – Water, mel2ng, con2nents may all play a role • Stagnant lid is the “natural” form for convec2on when you have a very high viscosity near surface region – Venus has this despite the high surface temperature – Maybe this actually favored stagnant lid (Bercovici & Ricard, 2014) • Other things being equal, stagnant lid planets run hofer and have greater capability for magmasm – But pressure release mel2ng dominates on Earth Did Venus undergo a Change in Convec2ve Style over Geologic Time? • Catastrophic resurfacing at 500Ma to 1Ga ago? Compable with but not required by the surface age deduced from cratering record (e.g. McKinnon, Zahnle). • Convec2on models are permissive (i.e., can be tuned); some2mes show episodic behavior (e.g. ,Armann & Tackley, 2012) • May connect to evidence for a current thick lithosphere (possible low heat flow) compared to earth – Most of Earth’s heat flow is secular cooling, not radiogenic at present (Urey number problem) Changes in Convective Style (existence or extent of plate tectonics) can have big effects on Thermal History T No plates Mantle T does not have to decrease monotonically. This can also affect core cooling and the presence of plumes or extent of igneous With plates activity t “With plates” means any convective style that involves recycling of the surface layer Plate Tectonics & the Role of Water (Earth Perspec2ve) • Water lubricates the asthenosphere • Water defines the plates • Maintenance of water in the mantle depends on subduc2on; this may not have persisted except on Earth • Plate tectonics helps the dynamo by promo2ng core cooling Water Plate tectonics Magnetic field These all influence… LIFE Neith - Venus’ Moon • “Discovered” by Cassini in 1672 • “Confirmed” by Lagrange in 1761 • Named aer Egyp2an goddess • Consistent with philosophy of the 2me wherein other planets should have Earthly aributes The Moon that Wasn’t Helge Kragh (2008) What would happen if you placed Earth-Moon at Venus orbit (4.5Ga ago)? • Moon stays well within Hill sphere of Venus –escape does not seem likely in any unperturbed story • Moon+Sun do not despin Venus to orbit period=spin period in less than 4.5Ga (unless 2dal Q is even smaller than for Earth, which is very unlikely) – It must be conceded , however, that our ignorance about 2dal Q’s is very large! (In general, not just in this case!) • But if you increase moon mass by factor of 2 (or decrease ini2al angular momentum budget by factor of 2) then moon would end up absorbed by Venus.. Incompable with observed low spin; might also leave a signature on surface Why Venus has no Moon? • Never had one (certainly a possibility) – BUT why then does Venus have such low spin? – Standard story for this is ques2onable • Had one and lost it early on – Escaped & subsequently crashed – Spiraled in & crashed • WORKING HYPOTHESIS: – Giant impacts that make moons are common – Earth-Moon differs from Venus-Neith in that it was the last major event in the terrestrial zone. This is suggested by isotopic evidence Origin of the moon is the most striking consequence of giant impacts, but it is singular only in being the last event of its kind. (Earlier moons could have existed) Sets the ini2al condi2on for Earth evolu2on A Story to Consider • Giant impact led to Neith. This impact occurred earlier than the Earth-Moon event (perhaps only ~10 Ma aer solar system formaon) • Moon
Load more

Recommended publications
  • Callisto: a Guide to the Origin of the Jupiter System
    A PAPER SUBMITTED TO THE DECADAL SURVEY ON PLANETARY SCIENCE AND ASTROBIOLOGY Callisto: A Guide to the Origin of the Jupiter System David E Smith 617-803-3377 Department of Earth, Atmospheric and PLanetary Sciences Massachusetts Institute of Technology, Cambridge MA 02139 [email protected] Co-authors: Francis Nimmo, UCSC, [email protected] Krishan Khurana, UCLA, [email protected] Catherine L. Johnson, PSI, [email protected] Mark Wieczorek, OCA, Fr, [email protected] Maria T. Zuber, MIT, [email protected] Carol Paty, University of Oregon, [email protected] Antonio Genova, Univ Rome, It, [email protected] Erwan Mazarico, NASA GSFC, [email protected] Louise Prockter, LPI, [email protected] Gregory A. Neumann, NASA GSFC Emeritus, [email protected] John E. Connerney, Adnet Systems Inc., [email protected] Edward B. Bierhaus, LMCO, [email protected] Sander J. Goossens, UMBC, [email protected] MichaeL K. Barker, NASA GSFC, [email protected] Peter B. James, Baylor, [email protected] James Head, Brown, [email protected] Jason Soderblom, MIT, [email protected] July 14, 2020 Introduction Among the GaLiLean moons of Jupiter, it is outermost CaLListo that appears to most fulLy preserve the record of its ancient past. With a surface aLmost devoid of signs of internaL geologic activity, and hints from spacecraft data that its interior has an ocean whiLe being only partiaLLy differentiated, CaLListo is the most paradoxicaL of the giant rock-ice worlds. How can a body with such a primordiaL surface harbor an ocean? If the interior was warm enough to form an ocean, how could a mixed rock and ice interior remain stable? What do the striking differences between geologicaLLy unmodified CaLListo and its sibling moon Ganymede teLL us about the formation of the GaLiLean moons and the primordiaL conditions at the time of the formation of CaLListo and the accretion of giant planet systems? The answers can be provided by a CaLListo orbitaL mission.
    [Show full text]
  • Dwarf Planet Ceres
    Dwarf Planet Ceres drishtiias.com/printpdf/dwarf-planet-ceres Why in News As per the data collected by NASA’s Dawn spacecraft, dwarf planet Ceres reportedly has salty water underground. Dawn (2007-18) was a mission to the two most massive bodies in the main asteroid belt - Vesta and Ceres. Key Points 1/3 Latest Findings: The scientists have given Ceres the status of an “ocean world” as it has a big reservoir of salty water underneath its frigid surface. This has led to an increased interest of scientists that the dwarf planet was maybe habitable or has the potential to be. Ocean Worlds is a term for ‘Water in the Solar System and Beyond’. The salty water originated in a brine reservoir spread hundreds of miles and about 40 km beneath the surface of the Ceres. Further, there is an evidence that Ceres remains geologically active with cryovolcanism - volcanoes oozing icy material. Instead of molten rock, cryovolcanoes or salty-mud volcanoes release frigid, salty water sometimes mixed with mud. Subsurface Oceans on other Celestial Bodies: Jupiter’s moon Europa, Saturn’s moon Enceladus, Neptune’s moon Triton, and the dwarf planet Pluto. This provides scientists a means to understand the history of the solar system. Ceres: It is the largest object in the asteroid belt between Mars and Jupiter. It was the first member of the asteroid belt to be discovered when Giuseppe Piazzi spotted it in 1801. It is the only dwarf planet located in the inner solar system (includes planets Mercury, Venus, Earth and Mars). Scientists classified it as a dwarf planet in 2006.
    [Show full text]
  • THE PENNY MOON and QUARTER EARTH School Adapted from a Physics Forum Activity At
    ~ LPI EDUCATION/PUBLIC OUTREACH SCIENCE ACTIVITIES ~ Ages: 5th grade – high THE PENNY MOON AND QUARTER EARTH school Adapted from a Physics Forum activity at: http://www.phvsicsforums.com/ Duration: 10 minutes OVERVIEW — The students will use a penny and a quarter to model the Moon’s rotation on its axis and Materials: revolution around the Earth, and demonstrate that the Moon keeps the same face toward One penny and one the Earth. quarter per pair of students OBJECTIVE — Overhead projector, or The students will: elmo, or video Demonstrate the motion of the Moon’s rotation and revolution. projector Compare what we would see of the Moon if it did not rotate to what we see when its period of rotation is the same as its orbital period. Projected image of student overhead BEFORE YOU START: Do not introduce this topic along with the reason for lunar phases; students may become confused and assume that the Moon’s rotation is related to its phases. Prepare to show the student overhead projected for the class to see. ACTIVITY — 1. Ask your students to describe which parts of the Moon they see. Does the Moon turn? Can we see its far side? Allow time for your students to discuss this and share their opinions. 2. Hand out the pennies and quarters so that each pair of students has both. Tell the students that they will be creating a model of the Earth and Moon. Which object is Earth? [the quarter] Which one is the Moon? [the penny] 3. Turn on the projected student overhead.
    [Show full text]
  • Download Student Activities Objects from the Area Around Its Orbit, Called Its Orbital Zone; at Amnh.Org/Worlds-Beyond-Earth-Educators
    INSIDE Essential Questions Synopsis Missions Come Prepared Checklist Correlation to Standards Connections to Other Halls Glossary ONLINE Student Activities Additional Resources amnh.org/worlds-beyond-earth-educators EssentialEssential Questions Questions What is the solar system? In the 20th century, humans began leaving Earth. NASA’s Our solar system consists of our star—the Sun—and all the Apollo space program was the first to land humans on billions of objects that orbit it. These objects, which are bound another world, carrying 12 human astronauts to the Moon’s to the Sun by gravity, include the eight planets—Mercury, surface. Since then we’ve sent our proxies—robots—on Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune; missions near and far across our solar system. Flyby several dwarf planets, including Ceres and Pluto; hundreds missions allow limited glimpses; orbiters survey surfaces; of moons orbiting the planets and other bodies, including landers get a close-up understanding of their landing Jupiter’s four major moons and Saturn’s seven, and, of course, location; and rovers, like human explorers, set off across the Earth’s own moon, the Moon; thousands of comets; millions surface to see what they can find and analyze. of asteroids; and billions of icy objects beyond Neptune. The solar system is shaped like a gigantic disk with the Sun at The results of these explorations are often surprising. With its center. Everywhere we look throughout the universe we the Moon as our only reference, we expected other worlds see similar disk-shaped systems bound together by gravity. to be cold, dry, dead places, but exploration has revealed Examples include faraway galaxies, planetary systems astonishing variety in our solar system.
    [Show full text]
  • Moon-Earth-Sun: the Oldest Three-Body Problem
    Moon-Earth-Sun: The oldest three-body problem Martin C. Gutzwiller IBM Research Center, Yorktown Heights, New York 10598 The daily motion of the Moon through the sky has many unusual features that a careful observer can discover without the help of instruments. The three different frequencies for the three degrees of freedom have been known very accurately for 3000 years, and the geometric explanation of the Greek astronomers was basically correct. Whereas Kepler’s laws are sufficient for describing the motion of the planets around the Sun, even the most obvious facts about the lunar motion cannot be understood without the gravitational attraction of both the Earth and the Sun. Newton discussed this problem at great length, and with mixed success; it was the only testing ground for his Universal Gravitation. This background for today’s many-body theory is discussed in some detail because all the guiding principles for our understanding can be traced to the earliest developments of astronomy. They are the oldest results of scientific inquiry, and they were the first ones to be confirmed by the great physicist-mathematicians of the 18th century. By a variety of methods, Laplace was able to claim complete agreement of celestial mechanics with the astronomical observations. Lagrange initiated a new trend wherein the mathematical problems of mechanics could all be solved by the same uniform process; canonical transformations eventually won the field. They were used for the first time on a large scale by Delaunay to find the ultimate solution of the lunar problem by perturbing the solution of the two-body Earth-Moon problem.
    [Show full text]
  • Geology 111 – Discovering Planet Earth
    Geology 111 – Discovering Planet Earth A1) Early History of the Earth The earth and the rest of the solar system were formed about 4.57 billion years ago from an enormous cloud of fragments of both icy and rocky material which was produced from the explosions (super novae) of one or more large stars - [see page 11]1. It is likely that the proportions of elements in this material were generally similar to those shown in the diagram below. Although most of the cloud was made of hydrogen and helium, the material that accumulated to form the earth also included a significant amount of the heavier elements, especially elements like carbon, oxygen, iron, aluminum, magnesium and silicon2. As the cloud started to contract, most of the mass accumulated towards the centre to become the sun. Once a critical mass had been reached the sun started to heat up through nuclear fusion of hydrogen into helium. In the region relatively close to the sun - within the orbit of what is now Mars - the heat was sufficient for most of the lighter elements to evaporate, and these were driven outward by the solar wind to the area of the orbits of Jupiter and the other gaseous planets. As a result, the four inner planets - Mercury, Venus, Earth and Mars are "rocky" in their composition, while the four major outer planets, Jupiter, Saturn, Neptune and Uranus are "gaseous". As the ball of fragments and dust that was to eventually become the earth grew, it began to heat up - firstly from the heat of colliding particles - but more importantly from the heat generated by radioactive decay (fission) of uranium, thorium, and potassium (figure below).
    [Show full text]
  • Origin and Evolution of Earth Research Questions for a Changing Planet
    Origin and Evolution of Earth Research Questions for a Changing Planet Questions about the origins and nature of Earth have long preoccupied human thought and the scientific endeavor. Deciphering the planet’s history and processes could improve the abil- ity to predict catastrophes like earthquakes and volcanoes, to manage Earth’s resources, and to anticipate changes in climate and geologic processes. This report captures, in a series of questions, the essential scientific challenges that constitute the frontier of Earth science at the start of the 21st century. arth is an active place. Earthquakes rip along plate boundaries, volcanoes spew fountains of Emolten lava, and mountain ranges and seabed are constantly created and destroyed. Earth scientists have long been concerned with deciphering the history—and predicting the future—of this active planet. Over the past four decades, Earth scientists have made great strides in understanding Earth’s workings. Scientists have ever-improving tools to understand how Earth’s internal processes shape the planet’s surface, how life can be sustained over billions of years, and how geological, biological, atmospheric, and oceanic NASA/NDGC processes interact to produce climate—and climatic change. At the request of the U.S. Department of Energy, Na- tional Aeronautics and Space Administration, National Science Foundation, and U.S. Geological Survey, the National Research Council assembled a committee to propose and explore grand ques- tions in Earth science. This report, which is the result of the committee’s deliberations and input solicited from the Earth science community, describes ten “big picture” Earth science issues being pursued today.
    [Show full text]
  • Earth/Mars Comparison of Geological Features
    Earth/Mars Comparison of Geological Features Objective(s): • Students will use satellite images to observe and compare various geological features between the planets Earth and Mars. • Students will work in groups to simulate and reproduce one of the geological features that they observed in the satellite images. • Students will use observational data from the experiment to conclude that some of the geological features on Mars could have been formed by flowing water. National Science Education Standards: • Content Standard D Grades 5 – 8: Earth’s History • Content Standard F Grades 5 – 8: Science and Technology in Society • Content Standard G Grades 5 – 8: Science as a Human Endeavor and Nature of Science Background: In this lesson, students explore the landforms of both Earth and Mars by carefully examining satellite images. The role of the student is to compare these landforms and share their observations with the class. Noting the similarities and the differences, the teacher then has the students focus on particular landforms on Mars – the channels and valley features. After discussing possible explanations for these features, the teacher introduces the stream tray investigation and the students conduct the experiment. Afterward, the students compare their data to several more images taken of Mars and conclude that these landforms could be the result of water having once flowed across A the planet’s surface. 2.5 km (1.6 mi) NAS “Did water once flow across the surface of Mars?” is a question Figure 1. This image taken from that scientists are still struggling to answer. A great deal of the Mars Global Surveyor using the evidence leads many to believe that water did indeed exist on Mars Orbiter Camera reveals a 9.8 Mars.
    [Show full text]
  • Where Did the Earth Come From?
    Where did the Earth come from? Earth formed from debris orbiting around our sun about 4 ½ billion years ago. That is also the approximate age of the sun, but it is not the beginning of our story. The story really begins with the Big Bang nearly 14 billion years ago, which spewed hydrogen atoms throughout the universe. The hydrogen atoms are not perfectly distributed, leaving some areas that are denser and others that are more rarified. The denser areas tend to become more dense as gravity brings atoms together. The atoms fall toward each other. Falling objects convert potential energy, the energy of height, into kinetic energy, the energy of motion. When atoms coalesce in the local center of mass, their speed is converted again, this time into heat. Eventually, about 200 million years later, so much heat forms that the atoms start to fuse, forming helium, releasing energy, and a star is born. Fusion is a process that makes heavier elements out of lighter ones. It has a limit, though, at iron. Iron is happy just the way it is, and it will not fuse. When a star builds up enough iron, the iron interferes, or poisons the star’s fusion, and the star dies. A large enough dying star forms a nova, or even a supernova. These are huge explosions that scatter the matter of the star into space. Iron, with its 26 protons, is less than a third of the way down the natural periodic table of elements. The rest of the natural elements, from cobalt to uranium, are formed during the explosion.
    [Show full text]
  • Circulating Transportation Orbits Between Earth and Mars
    CIRCULATING TRANSPORTATION ORBITS BETWEEN EARTH AND MRS 1 2 Alan L. Friedlander and John C. Niehoff . Science Applications International Corporation Schaumburg, Illinois 3 Dennis V. Byrnes and James M. Longuski . Jet Propulsion Laboratory Pasadena, Cal iforni a Abstract supplies in support of a sustained Manned Mars Sur- This paper describes the basic characteristics face Base sometime during the second quarter of the of circulating (cyclical ) orbit design as applied next century. to round-trip transportation of crew and materials between Earth and Mars in support of a sustained A circulating orbit may be defined as a multi- manned Mars Surface Base. The two main types of ple-arc trajectory between two or more planets non-stopover circulating trajectories are the so- which: (1) does not stop at the terminals but called VISIT orbits and the Up/Down Escalator rather involves gravity-assist swingbys of each orbits. Access to the large transportation facili- planet; (2) is repeatable or periodic in some ties placed in these orbits is by way of taxi vehi- sense; and (3) is continuous, i.e., the process can cles using hyperbolic rendezvous techniques during be carried on indefinitely. Different types of the successive encounters with Earth and Mars. circulating orbits between Earth and Mars were Specific examples of real trajectory data are pre- identified in the 1960s 1iterat~re.l'*'~'~ Re- sented in explanation of flight times, encounter examination of this interesting problem during the frequency, hyperbol ic velocities, closest approach past year has produced new solution^.^'^'^ Some distances, and AV maneuver requirements in both involve only these two bodies while others incor- interplanetary and planetocentric space.
    [Show full text]
  • Unit 5: Earth-Sun-Moon Key Concepts
    Unit 5: Earth-Sun-Moon Key Concepts Earth – Sun System The earth revolves around the sun over 365.25 days Earth rotates on its axis every 24 hours; giving daylight in areas facing the sun and night in areas facing away from the sun The North Pole of the Earth’s axis always points to the North Star (Polaris) Seasons: The seasons occur because there are different amounts of direct sunlight and hours of daylight as the Earth revolves around the sun. If the Earth’s axis was not tilted, then the seasons would not occur o Summer - the North Pole is angled towards the sun, and the northern hemisphere experiences warmer weather and longer days. June 20-21 is the summer solstice, the longest day of the year in the northern hemisphere. On this day, the sun is directly overhead at the Tropic of Cancer and areas above the Arctic Circle have 24 hours per day of daylight . The northern hemisphere’s summer occurs as the southern hemisphere has its winter o Winter – the North Pole is angled away from the sun, and the northern hemisphere experiences colder weather and shorter days . December 21-22 is the winter solstice, the shortest day of the year in the northern hemisphere. On this day, the sun is directly overhead at the Tropic of Capricorn and areas above the Arctic Circle receive 24 hours per day of darkness . The northern hemisphere’s winter occurs as the southern hemisphere has its summer o Spring and Autumn Equinox – during the equinox, the days and nights are of approximately equal length, and the sun is directly over the Equator Unit 5: Earth-Sun-Moon Key Concepts Earth – Moon System The Moon revolves around the earth leading to the phases of the Moon cycle which occur over ≈ 28 days, as the Moon completes one revolution around the Earth The same phase of the Moon cycle will be viewed by all places on earth as the earth rotates through a 24 hour day.
    [Show full text]
  • Analyzing an Earth-Sun Model to Explain the Stars Seen in The
    EXPLORATION 1 Analyzing an Earth-Sun Model to Explain the Stars Seen in the Sky A group of stars that form a pattern is called a constellation. The constellation Ursa Major also has an inner pattern known as the Big Dipper. But if you look for the Big Dipper, you will see that it is not always in the same position in the night sky. In addition to the movement of the stars across the sky every night, their rising times change throughout the year. So the constellations visible in the early evening are different in winter, spring, summer, and fall. 3. Discuss When you look at the night sky, what do you see? The Big Dipper is one of the most familiar star patterns in the northern sky. It looks Changes in Stars Seen in the Night Sky like a ladle used to scoop water. Just as the sun appears to move in a path across the sky, stars in the night sky also appear to move. They seem to change locations nightly. 4. Why do the stars appear to move across the sky every night? A. Because Earth orbits the sun once a year. B. Because Earth tilts on its axis. C. Because Earth rotates once every 24 hours. 5. Act With your class, investigate why star patterns change yearly. • Several students form a large circle. They represent the stars in the night sky. One student represents the sun and stands in the center. One student represents Earth and stands between the stars and the sun. The head of the student who is Earth represents the North Pole of Earth.
    [Show full text]