Part (3) Earth and Space Science Soe Htet Naing How old the Earth where we live? • It is estimated that Earth formed about 4.6 billion years ago. • Scientists use three major types of evidence when determining the age of Earth and of objects on Earth: • landforms, • fossils, and • radiometric dating. Landforms • Let’s say we are using a sand layering. Which layer will be first and which layer will be last? • One process scientists study is the formation of sedimentary rock. Over time, as sediment (weathered material) settles, it becomes compacted into layers of sedimentary rock. Look at the following diagram. Since new sediment is added to the top of sedimentary rock, the youngest layers are at the top and the oldest layers are at the bottom. This allows scientists to determine the relative age of the layers in a landform. Fossils Fossils are remains (shells or bones) or evidence (imprints) left behind by dead organisms. Fossils are most often found in sedimentary rock. As shown in the diagram, the age of a fossil can be determined by looking at the layer it is found in. The opposite is also true. Finding a fossil whose age is already known can provide information about the layer it is found in. While true, fossils are buried with plenty of clues that allow us to reconstruct their history. An array of absolute dating techniques has made it possible to establish the timescale of Earth’s history, including the age and origin of life, the timing of mass extinctions and the record of human evolution. Radiometric (or radioactive) dating Comparing rock layers and fossils provides information about the relative age of objects on Earth, Radiometric (or radioactive) dating provides the exact, or absolute, age of objects on Earth. The physical process of radioactive decay has provided Earth scientists, anthropologists, and evolutionary biologists with their most important method for determining the absolute age of rocks and other materials (Dalrymple 1991; Dickin 2005). Ötzi the iceman was discovered in 1991 frozen in the Italian Alps. Carbon-14 dating revealed that he died about 5,300 years ago. Photo courtesy South Tyrol Museum of Archaeology, www.iceman.it What is Radiometric (or radioactive) dating? Trace amounts of isotopes of radioactive elements, including carbon-14, uranium-238, and dozens of others, are all around us—in rocks, in water, and in the air (Table 1). These isotopes are unstable, so they gradually break apart or “decay.” Remember that an element has a specific number of protons in its nucleus. Some elements have radioactive isotopes, or versions that turn into another element by losing a proton. Carbon-14 and uranium-235 are radioactive isotopes used by scientists in radiometric dating. The amount of time it takes half of an isotope’s atoms to change into another element is called the half-life. If you have a collection of one million atoms of a radioactive isotope, half of them will decay over a span of time called the “half life.” Uranium-238, for example, has a half life of 4.468 billion years, so if you start with a million atoms and come back in 4.468 billion years, you’ll find only about 500,000 atoms of uranium-238 remaining. The rest of the uranium will have decayed to 500,000 atoms of other elements, ultimately to stable (i.e., nonradioactive) atoms of lead-206. Wait another 4.468 billion years and only about 250,000 atoms of uranium will remain . Day and Night The Earth take to rotate itself for 24 hours. During this period of time, it experience one daytime with the Sun and one night with the stars and moon. The earth takes to rotate to the Sun over its obit about 365.25 days. During this period, it experience the seasons. We, the Earth, are in a solar system. Our solar system is made up of the sun and all objects that orbit the sun. The sun is a star located at the center of our solar system. Remember that all objects exert gravitational force on each other. Since the sun is the largest object in the solar system, its gravity keeps all other objects orbiting around it. After the sun, the largest objects in the solar system are the round-shaped planets. Planets There are 8 planets in our solar Systems: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. We can divided into two groups : terrestrial planets (Inner planets) which is located closest to the sun all have rocky surfaces, and (gas giants (Outter Planets) which are farther from the sun all have gaseous surfaces. Others Natures in our solar system Recently, a new category of planet was created called dwarf planets or minor planets. One example of these objects is Pluto. Any natural object in space that orbits a planet is called a moon. You know that Earth has one moon. A planet may have no moons, like Mercury, or several moons, like Jupiter. Asteroids are another type of object in the solar system. An asteroid is a large, irregularly shaped chunk of rock. Although asteroids exist throughout the solar system, most are found in a band called the asteroid belt. The asteroid belt separates the gas giants from the terrestrial planets. Comets are made up of frozen gases and dust particles and are usually smaller than asteroids. Some comets orbit the sun continuously. Others orbit the sun once and then travel off into space. Halley’s Comet is a famous comet that continuously orbits the sun. Its orbit passes by Earth every 76 years. Interactions Between Earth and the Solar System Earth orbits, or revolves, around the sun. Earth completes one revolution around the sun every 365¼ days. We call one revolution of Earth a year. Earth also spins, or rotates, on an axis. Earth completes one rotation every 24 hours. We call one rotation of Earth a day. Interactions Between Earth and the Solar System-Continuous Slides Interactions between Earth, the moon, and the sun cause Earth to experience eclipses and tides. An eclipse is caused by the positions of the sun and moon in relation to Earth. An eclipse happens when one object in space blocks light from reaching another object. During a solar eclipse, the moon blocks sunlight from reaching Earth. In a lunar eclipse, Earth blocks sunlight from reaching the moon. Interactions Between Earth and the Solar System-Continuous Slides The daily rises and falls of ocean levels are called tides. A high tide occurs when the ocean reaches its highest point on the shore. A low tide occurs when the ocean reaches its lowest point on the shore. Shorelines experience two high tides and two low tides every day. Tides are caused by the pull of the moon’s gravity as Earth rotates on its axis. Look at the following tides diagram. As the moon orbits Earth, its gravity pulls on the water in the ocean. This causes the ocean to bulge on the sides closest to and opposite of the moon. Shores within the bulge areas experience high tide. Shores not located in the bulge areas experience low tide. As Earth rotates, the bulges stay in line with the moon, causing differentlocations to experience high tide at different times throughout a day. Interactions Between Earth and the Solar System-Continuous Slides The sun’s gravitational force also affects tides. When the sun is in line with the moon, they both pull the ocean in the same direction. This causes extra high and extra low tides called the spring tides. When the sun and moon are perpendicular to each other, they pull the ocean in different directions. This causes only minor differences in high and low tides, a condition referred to as the neap tides The Universe The total of all matter and energy that exists is called the universe. The different objects that exist in our solar system are found throughout the universe. The universe contains many other solar systems, as well as other types of objects. A star is a ball of gas that produces its own light and heat. Recall the Sun is also is a star. A constellation is a recognized pattern of stars in the night sky. Well-known constellations include Ursa Major (the big dipper) and Orion. Due to Earth’s orbit, different constellations are visible at different times of the year. Scientists can identify the area of space that is visible by the constellations present. A galaxy is a massive group of stars, gas, dust, and dark matter held together by gravity. Our solar system is part of the Milky Way Galaxy, along with about 200 billion other stars. The universe is estimated to contain at least a billion other galaxies. Constellation and How we can see A collection of stars that makes a pattern is called a constellation If you live in the northern hemisphere you will see different stars from someone who lives in the southern hemisphere. Cassiopeia is a constellation that you can see from the northern hemisphere, while you can see Crux from the southern hemisphere. The Sun and it is start the universe Stars vary in composition, size, and age. All stars, however, follow the same basic life cycle. A star forms in a cloud of gas and dust called a nebula. Gravity causes some gas and dust to pull inward, forming a protostar. A protostar then turns into a main sequence star. A star spends most of its life as a main sequence star. The fusion of hydrogen (H) atoms into helium (He) atoms causes main sequence stars to release heat and light. A star dies when it runs out of hydrogen. A main sequence star that has run out of fuel will eventually stop glowing and become a black dwarf. White dwarfs are stars that are close to dying out. Sometimes a protostar is too massive to become a main sequence star. These supergiant stars die in a sudden explosion called a supernova. The death of a supergiant can result in a black hole. How will sun die? Eventually all of the hydrogen gas will be used up and it will go through the final stages of its life. It will become a red giant and will swallow up Mercury, Venus, and possibly Earth as well. It will become far too hot for humans to survive on Earth. At this point the outer layers of the Sun will be thrown out into space to form clouds of gas. After that it will shrink back and become a hot white dwarf, and then cool down to become an invisible black dwarf star. How will stars die? The life cycle for a particular star depends on its size. The diagram shows the life cycles of stars that are: •about the same size as the Sun (left hand path) •far greater than the Sun in size (right hand path)
