{"id":81,"date":"2017-01-23T16:35:15","date_gmt":"2017-01-23T16:35:15","guid":{"rendered":"https:\/\/pressbooks.ccconline.org\/introduction-to-oceanography\/chapter\/3-1-origin-of-earth-and-the-solar-system\/"},"modified":"2021-10-25T19:08:26","modified_gmt":"2021-10-25T19:08:26","slug":"3-1-origin-of-earth-and-the-solar-system","status":"publish","type":"chapter","link":"https:\/\/pressbooks.ccconline.org\/introduction-to-oceanography\/chapter\/3-1-origin-of-earth-and-the-solar-system\/","title":{"raw":"3.1 Origin of Earth and the Solar System","rendered":"3.1 Origin of Earth and the Solar System"},"content":{"raw":"
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\r\n\r\nAccording to the [pb_glossary id=\"588\"]Big Bang theory[\/pb_glossary]<\/strong>, the universe blinked violently into existence 13.77 billion years ago (Figure 3.1.1). The Big Bang is often described as an explosion, but imagining it as an enormous fireball isn\u2019t accurate. The Big Bang involved a sudden expansion of matter, energy, and space from a single point. The kind of Hollywood explosion that might come to mind involves expansion of matter and energy within<\/em> space, but during the big bang, space itself<\/em> was created.\r\n\r\n[caption id=\"attachment_77\" align=\"aligncenter\" width=\"600\"]\"An<\/a> Figure 3.1.1<\/strong> The Big Bang and development of the universe (Steven Earle, \"Physical Geology\").[\/caption]\r\n\r\nAt the start of the Big Bang, the universe was too hot and dense to be anything but a sizzle of particles smaller than atoms, but as it expanded, it also cooled. Eventually some of the particles collided and stuck together. Those collisions produced hydrogen and helium, the most common elements in the universe, along with a small amount of lithium. Gravity caused clouds of these early elements to coalesce into stars, and it was inside these stars that heavier elements were formed\r\n\r\nOur solar system began to form around 5 billion years ago, roughly 8.7 billion years after the Big Bang. A [pb_glossary id=\"1152\"]solar system[\/pb_glossary]<\/strong> consists of a collection of objects orbiting one or more central stars. All solar systems start out the same way. They begin in a cloud of gas and dust called a [pb_glossary id=\"962\"]nebula[\/pb_glossary]<\/strong>. Nebulae are some of the most beautiful objects that have been photographed in space, with vibrant colors from the gases and dust they contain, and brilliant twinkling from the many stars that have formed within them (Figure 3.1.2). The gas consists largely of hydrogen and helium, and the dust consists of tiny mineral grains, ice crystals, and organic particles.\r\n\r\n \r\n\r\n[caption id=\"attachment_78\" align=\"aligncenter\" width=\"600\"]<\/a> Figure 3.1.2<\/strong> Photograph of a nebula. The Pillars of Creation within the Eagle Nebula viewed in visible light (left) and near infrared light (right). Near infrared light captures heat from stars, and allows us to view stars that would otherwise be hidden by dust. This is why the picture on the right appears to have more stars than the picture on the left [NASA, ESA, and the Hubble Heritage Team (STScI\/AURA) http:\/\/bit.ly\/1Dm2X5a].[\/caption]<\/div>\r\n
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\r\n\r\nA solar system begins to form when a small patch within a nebula (small by the standards of the universe, that is) begins to collapse upon itself. Exactly how this starts isn\u2019t clear, although it might be triggered by the violent behavior of nearby stars as they progress through their life cycles. Energy and matter released by these stars might compress the gas and dust in nearby neighborhoods within the nebula. Once it is triggered, the collapse of gas and dust within that patch continues for two reasons. One of those reasons is that gravitational force pulls gas molecules and dust particles together. But early in the process, those particles are very small, so the gravitational force between them isn\u2019t strong. So how do they come together? The answer is that dust first accumulates in loose clumps for the same reason dust bunnies form under your bed: static electricity. As the small patch within a nebula condenses, a star begins to form from material drawn into the center of the patch, and the remaining dust and gas settle into a disk that rotates around the star. The disk is where planets eventually form, so it\u2019s called a [pb_glossary id=\"1058\"]protoplanetary disk[\/pb_glossary]<\/strong>. In Figure 3.1.3 the image in the upper left shows an artist\u2019s impression of a protoplanetary disk, and the image in the upper right shows an actual protoplanetary disk surrounding the star HL Tauri. Notice the dark rings in the protoplanetary disk. These are gaps where planets are beginning to form. The rings are there because incipient planets are beginning to collect the dust and gas in their orbits. There is an analogy for this in our own solar system, because the dark rings are akin to the gaps in the rings of Saturn (Fig. 3.1.3, lower left), where moons can be found (Fig. 3.1.3, lower right).\r\n\r\n \r\n\r\n[caption id=\"attachment_79\" align=\"aligncenter\" width=\"600\"]\"Protoplanetary<\/a> Figure 3.1.3<\/strong> Protoplanetary disks and Saturn\u2019s rings. Upper left: An artists impression of a protoplanetary disk containing gas and dust, surrounding a new star. [NASA\/ JPL-Caltech, http:\/\/1.usa.gov\/1E5tFJR] Upper right: A photograph of the protoplanetary disk surrounding HL Tauri. The dark rings within the disk are thought to be gaps where newly forming planets are sweeping up dust and gas. [ALMA (ESO\/NAOJ\/NRAO) http:\/\/bit.ly\/1KNCq0e]. Lower left: A photograph of Saturn showing similar gaps within its rings. The bright spot at the bottom is an aurora, similar to the northern lights on Earth. [NASA, ESA, J. Clarke (Boston University), and Z. Levay (STScI) http:\/\/bit.ly\/1IfSCX5] Lower right: a close-up view of a gap in Saturn\u2019s rings showing a small moon as a white dot. [NASA\/JPL\/Space Science Institute, http:\/\/1.usa.gov\/1g2EeYw].[\/caption]<\/div>\r\n
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In general, planets can be classified into three categories based on what they are made of (Fig. 3.1.4). [pb_glossary id=\"1214\"]Terrestrial planets[\/pb_glossary]<\/strong> are those planets like Earth, Mercury, Venus, and Mars that have a core of metal surrounded by rock. [pb_glossary id=\"880\"]Jovian planets[\/pb_glossary]<\/strong> (also called [pb_glossary id=\"772\"]gas giants[\/pb_glossary]<\/strong>) are those planets like Jupiter and Saturn that consist predominantly of hydrogen and helium. [pb_glossary id=\"858\"]Ice giants[\/pb_glossary]<\/strong> are planets such as Uranus and Neptune that consist largely of water ice, methane (CH4<\/sub>) ice, and ammonia (NH3<\/sub>) ice, and have rocky cores. Often, the ice giant planets Uranus and Neptune are grouped with Jupiter and Saturn as gas giants; however, Uranus and Neptune are very different from Jupiter and Saturn.[caption id=\"attachment_80\" align=\"aligncenter\" width=\"700\"]\"Illustrations<\/a> Figure 3.1.4<\/strong> Three types of planets. Jovian (or gas giant) planets such as Jupiter consist mostly of hydrogen and helium. They are the largest of the three types. Ice giant planets such as Uranus are the next largest. They contain water, ammonia, and methane ice. Terrestrial planets such as Earth are the smallest, and they have metal cores covered by rocky mantles. [KP, after public domain images by Francesco A, Wolfman SF (http:\/\/bit.ly\/1eP75P4), and NASA (http:\/\/1.usa.gov\/1gFVsf6, http:\/\/1.usa.gov\/1M89jI3)].[\/caption]<\/div>\r\n
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\r\n\r\nThese three types of planets are not mixed together randomly within our solar system. Instead they occur in a systematic way, with terrestrial planets closest to the sun, followed by the Jovian planets and then the ice giants. Part of the reason for this arrangement is the [pb_glossary id=\"766\"]frost line[\/pb_glossary]<\/strong> (also referred to as the [pb_glossary id=\"1148\"]snow line[\/pb_glossary]<\/strong>). The frost line separated the inner part of the protoplanetary disk closer to the sun, where it was too hot to permit anything but silicate minerals and metal to crystallize, from the outer part of the disk farther from the Sun, where it was cool enough to allow ice to form. As a result, the objects that formed in the inner part of the protoplanetary disk consist largely of rock and metal, while the objects that formed in the outer part consist largely of gas and ice. The young sun also blasted the solar system with raging [pb_glossary id=\"1154\"]solar winds[\/pb_glossary]<\/strong> (winds made up of energetic particles), which helped to drive lighter molecules toward the outer part of the protoplanetary disk.\r\n\r\nThe objects in our solar system formed by [pb_glossary id=\"530\"]accretion[\/pb_glossary]<\/strong>. Early in this process, mineral and rock particles collected in fluffy clumps because of static electricity. As the mass of the clumps increased, gravity became more important, pulling material from farther away and growing these solid masses into larger and larger bodies. Eventually the mass of the objects became large enough that their gravity was strong enough to hang onto gas molecules, because gas molecules are very light.\r\n\r\nOur Earth formed though this process of [pb_glossary id=\"530\"]accretion [\/pb_glossary] about 4.6 billion years ago. The early Earth was very hot and had a molten, fluid composition, with lost of geological and volcanic activity on the surface. The Earth\u2019s heat came from a variety of processes:\r\n
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