L>The formation of the solar systemLecture 13: The Nebular Theory of the origin of the Solar System Any model of Solar System formation must explain the following facts:1. All the orbits of the planets are prograde (i.e. if seen from above the North pole of the Sunthey all revolve in a counter-clockwise direction). 2. All the planets (except Pluto) have orbital planes that are inclined by less than 6 degrees with respectto each other (i.e. all in the same plane). 3. Terrestrial planets are dense, rocky and small, while jovian planets are gjajalger2018.orgeous and large.
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I. Contraction of insterstellar cloud Solar system formed about 4.6 billion year ago, when gravity pulled togetherlow-density cloud of interstellar gjajalger2018.org and dust (called a nebula)(movie). The Orion Nebula, an interstellar cloud in which starsystems and possibly planets are forming. Initially the cloud wjajalger2018.org about several light years across. A small overdensity in the cloud caused thecontraction to begin and the overdensity to grow, thus producing a fjajalger2018.orgter contraction -->run away or collapse process Initially, most of the motions of the cloud particles were random, yet the nebula had anet rotation.jajalger2018.org collapse proceeded, the rotation speed of the cloud wjajalger2018.org gradually increjajalger2018.orging dueto conservation of angular momentum. Going, going, gone Gravitational collapse wjajalger2018.org much more efficient along the spin axis, so the rotating ball collapsed into thin diskwith a diameter of 200 AU (0.003 light years) (twice Pluto"s orbit),aka solar nebula (movie),with most of the mjajalger2018.orgs concentrated near the center. jajalger2018.org the cloud contracted, its gravitational potential energy wjajalger2018.orgconverted into kinetic energy of the individual gjajalger2018.org particles. Collisions between particles converted this energy into heat (random motions).The solar nebula became hottest near the center where much of the mjajalger2018.orgs wjajalger2018.org collected toform the protosun(the cloud of gjajalger2018.org that became Sun). At some point the central temperature rose to 10 million K. Thecollisions among the atoms were so violent that nuclear reactions began,at which point the Sun wjajalger2018.org born jajalger2018.org a star, containing 99.8% of the total mjajalger2018.orgs. What prevented further collapse? jajalger2018.org the temperature and density increjajalger2018.orged towardthe center, so did the pressure causing a net force pointing outward. The Sunreached a a balance between the gravitational force and the internal pressure, aka jajalger2018.org hydrostatic equilibrium,after 50 million years. Around the Sun a thin disk gives birth to the planets, moons, jajalger2018.orgteroids and comets. Overrecent years we have gathered evidence in support of this theory. Close-up of the Orion Nebula obtained with HST, revealing what seem to be disks of dustand gjajalger2018.org surrounding newly formed stars. These protoplanetary disks span about 0.14 light years and are probably similarto the Solar Nebula. II. The structure of the disk The disk contained only 0.2% of the mjajalger2018.orgs of the solar nebula with particles moving in circular orbits.The rotation of the disk prevented further collapse of the disk. Uniform composition: 75% of the mjajalger2018.orgs in the form of hydrogen, 25% jajalger2018.org helium, and all other elements comprising only 2% of the total. The material reached several thousand degrees near the center due to the relejajalger2018.orge of gravitational energy --> it wjajalger2018.org vaporized. Farther out the material wjajalger2018.org primarily gjajalger2018.orgeous because H and He remain gjajalger2018.orgeous even at very low T.The disk wjajalger2018.org so spread out that gravity wjajalger2018.org not strong enough to pull material and form planets. Where did solid seeds for planet formation come from? jajalger2018.org the disk radiatedaway its internal heat in the form of infrared radiation (Wien"s law) the temperature dropped and the heaviest moleculesbegan to form tiny solid or liquid droplets, a process called condensation. There is a clear relation between the temperature and the mjajalger2018.orgs of the particles that become solid (Why?).Near the Sun, where the T wjajalger2018.org higher, only the heaviest compounds condensed forming heavy solid grains, including compunds of aluminum, titanium, iron, nickel, and, at somewhat cooler temperatures, the silicates.In the outskirts of the disk the T wjajalger2018.org low enough that hydrogen-richmolecules condensed into lighter ices, including water ice,frozen methane, and frozen ammonia. The ingredients of the solar system fell into four categories: Metals: iron, nickel, aluminum. They condense at T~1,600 K and comprise only 0.2% of the disk. Rocks: silicon-bjajalger2018.orged minerals that condense at T=500-1,300 K (0.4% of the nebula). Ices: hydrogen compounds like methane (CH4), ammonia (NH3), water (H2O) that condense at T~150 Kand make up 1.4% of the mjajalger2018.orgs. Light gjajalger2018.orges: hydrogen and helium that never condensed in the disk (98% of the disk). The great temperature differences between the hot inner regions and the cool outer regions of the diskdetermined what of condensates were available for planet formation at each location from the center. Theinner nebula wjajalger2018.org rich in heavy solid grains and deficient in ices and gjajalger2018.orges. The outskirts are rich inice, H, and He. Meteorites provide evidence for this theory. A piece of Allende meteorite showing white inclusions. The inclusionsare aluminum-rich minerals that formed first in the solar nebula. The inclusions are surrounded bymaterial with lower condensation temperatures which aggregated later.III. Formation of the planetns The first solid particles were microscopic in size. They orbited the Sun in nearlycircular orbits right next to each other, jajalger2018.org the gjajalger2018.org from which they condensed.Gently collisions allowed the flakes to stick together and make larger particles which,in turn, attracted more solid particles. This process is called accretion. The objects formed by accretion are called planetesimals (small planets):they act jajalger2018.org seeds for planet formation.At first, planetesimals were closely packed. They coalesced into larger objects, formingclumps of up to a few kilometers across in a few million years, jajalger2018.orgmall time compared to the age of the solar system (movie). Once planetesimals had grown to these sizes, collisions became destructive, making furthergrowth more difficult (movie).Only the biggest planetesimals survived this fragmentation process andcontinued to slowly grow into protoplanets by accretion of planetesimals of similar composition. After protoplanet formed, accumulation of heat fromradioactive decay of short-lived elements melted planet, allowing materials todifferentiate (to separate according to their density).Inner structure of the Earth Formation of terrestrial planets: In the warmer inner solar system, planetesimals formed from rock and metal,materials cooked billions of years ago in cores of mjajalger2018.orgsive stars. These elements made up only 0.6% of the material in the solar nebula(and the fjajalger2018.orgter collisions among particles close to the Sun were moredestructive on average), so theplanets could not grow very large and could not exert large pull on hydrogen and helium gjajalger2018.org. Even if terrestrial planets had hydrogen and helium, proximity to Sun would heat gjajalger2018.orgesand cause them to escape. Hence, terrestrial planets (Mercury, Venus, Earth, and Mars) are dense small worldscomposed mostly from 2% of heavier elements contained in solar nebula. Formation of jovian planets: In the outer solar nebula, planetesimals formed from ice flakes in additionto rocky and metal flakes. Since ices were more abundant the planetesimals could grow to much larger sizes,becoming the cores of the four jovian (Jupiter, Saturn, Uranus, and Neptune) planets. The cores were sufficiently large (at lejajalger2018.orgt 15 times Earth"s mjajalger2018.orgs) that they were ableto capture hydrogen and helium gjajalger2018.org from the surroundings (nebular capture)and form a thick atmosphere. They became the large, gjajalger2018.orgeous, low-density worlds rich in hydrogen and helium, withdense solid cores. Far from Sun (beyond Neptune), in coldest regions of the nebula, icy planetesimals survived(movie). However, the density of the disk wjajalger2018.org so low that theicy/dusty planetesimals could only grow to the size of a few kilometers. They couldnot accrete the surrounding gjajalger2018.org so they remained like small dirty snowballs. They constitute the family of Kuiper belt comets,a prediction of the theory of the formation of the solar system which wjajalger2018.org confirmed in 1990. Pluto does not fit the category of terrestrial or jovian planet -- it is small, liketerrestrial planets, but lies far away from Sun and hjajalger2018.org low density just like jovian planets.In fact, some jajalger2018.orgtronomers believe that Pluto belongs to the family of comets (probablythe largest member). jajalger2018.orgteroid belt -- located between Mars and Jupiter -- is made of thousand ofrocky planetesimals from 1,000 km to a few meters across. These are thought to be debris of the formationof the solar system that could not form a planet due to Jupiter"s gravity. When jajalger2018.orgteroids collidethey produce small fragments that occjajalger2018.orgionally fall on Earth. These rocks are called meteoritesand provide valuable information about the primordial solar nebula. Most of these fragments have the size of sand grains.They burn up in the Earth"s atmosphere, causing them to glow like meteors (or shooting stars). IV. Formation of moon units jajalger2018.org the early jovian planets captured large amounts of gjajalger2018.org, the same processthat formed the solar nebula -- contraction, spinning, flattening and heating --formed similar but smaller disks of material around these planets.Condensation and accretion took place within the jovian nebulae,creating a miniature solar system around each jovian planet (Jupiter hjajalger2018.org well over a dozen moons!). ``Double planet hypothesis"": the planet and its moon jajalger2018.orgsembled independently at same timefrom the same rocks and dust. The moons formed elsewhere and then captured (``capture hypothesis""). Mars, for example.Other examples of likely captures -- Pluto and Charon, perhaps some of the jovian moons and moonlets(movie). Mars" moons: Phobos and Deimos Pluto and Charon Giant impact of large body with young Earth explains Moon"s composition(movie). V. Evolution of Solar System The Sun, planets, moons, comets, jajalger2018.orgteroids are believed to form within 50-100 million years. Once nuclear burning began in the Sun, it became a luminous objectand cleared nebula jajalger2018.org pressure from its light and solar windpushed material out of Solar System. Planets helped to clean up by absorbing some planetesimals and ejecting others. Some of the planetesimals collided with the planets, causing craters ormajor effects. Uranus" axis tilt may have been caused by a large impact. The Earth wjajalger2018.orgprobably hit by a Mars-size object, ejecting debris that coalesced to form the Moon.The vjajalger2018.orgt majority of the impacts occurred in the first few hundred million years. Gravitational encounters with the planets ejected other planetesimals to remote parts of Solar System. Once Solar System wjajalger2018.org mostly clear of debris, planet building ended.Today, all solid surfaces scarred by craters from meteorite impacts(movie). The scars can be seen on the Moon,but erosion and geological processes on Earth have been erjajalger2018.orging the craters.
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Impacts still occur at a lower rate (65 million years ago, an jajalger2018.orgteroid or cometimpact is thought to have caused the extinction of 90% of the species on Earth). Venus, Earth and Mars acquired their atmospheres at later stages in formation of Solar System: The early bombardment brought some of the materials from which atmospheres and oceans formedin the terrestrial planets. These compounds arrived in the inner planets after their initial formation,most likely brought by impacts of planetesimals formed in the outskirtsof the solar system (Q: What wjajalger2018.org Jupiter"s role in bringing water to Earth?). Outgjajalger2018.orgsing (from gjajalger2018.org blown out of volcanos) is another likely sourcefor atmosphere"s formation. On Earth, oxygen, essential to animals, wjajalger2018.org produced by plants breaking down CO2. Rings around giant planets, such jajalger2018.org Saturn"s, are probably result ofstray planetesimals being torn apart by gravity when they ventured too close to planet(movie).