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Chapter 2: Solar System Learning objectives: You will be able to… • Explain the solar nebula hypothesis (origin of the solar system) • Describe the Sun and how it works • Explain how Mercury, Venus and Mars differ form Earth • Characterize the gas giants • Describe dwarf planets, comets, and asteroids • Identify key events in the formation of Earth The Universe Universe is finite Universe is expanding (Doppler Effect) Expansion is accelerating Our solar System is not the center of the Universe Universe has no center nor an edge The Solar System consists of: The Sun Eight classical planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, & Neptune Five dwarf planets Pluto, Ceres, Haumea, Makemake, and Eris 240 known satellites (moons), including 162 orbiting the classical planets Millions of comments and asteroids Kuiper Belt and Oort Cloud Countless particles and interplanetary space Origin of Solar System Pick a theory, any theory, but it must be consistent with these facts: 1) Planets all revolve around the Sun in the same direction in nearly circular orbits. 2) The angle between the axis of rotation and the plane of orbit is small (except Uranus). 3) All planets (except Venus and Uranus) rotate in the same direction as their revolution; their moons do, too. Origin of solar system 4) Each planet is roughly twice as far as the next inner planet is from the Sun (the Titus-Bode rule). 5) 99.9 % of mass is in the Sun; 99 % of angular momentum is in the planets. 6) Planets in two groups: terrestrial (inner): Mercury, Venus, Earth, Mars & Mercury is mostly Fe ( = 5.4) Jovian (outer): Jupiter, Saturn, Uranus, & Neptune mostly gas and ice ( = 0.7) 7) Terrestrial planets are have a lot of Fe and O, Si & Mg. The Sun is almost entirely H & He (also important in Jovian planets). Collision hypothesis Portions of the Sun were torn off by a passing star: planetesimals then collided to form planets. Problems: gases coming from Sun would be too hot to condense; stellar collision exceedingly rare Protoplanet hypothesis Large gas cloud begins to condense. Most mass in the center, turbulence in outer parts. Turbulent eddies collect matter meters across; small chunks grow and collide, eventually becoming large aggregates of gas and solid chunks. Protoplanets, much bigger than present planets, eventually contracted due to their own gravity. Nebular hypothesis Most popular theory is called the nebular hypothesis Fell out of favor for a number of years, but now it is considered the definitive model Nebular hypothesis Primeval slowly rotating gas cloud (nebula) condensed into several discrete blobs. fits rotation mass doesn't fit angular momentum Solar Nebula Hypothesis The beginning of the Solar System 6.9 billion years ago – nebula formation An ancestral star – ended its life: Red Giant Explosion - Nebula Early Nebula Solar System began when part of a molecular cloud of interstellar gas filled with particles of ice, dust, rock, and other particles, collapsed Clouds collapsed caused it to heat up and eventually turn into a star Most of the cloud formed the Sun Other material from the cloud flattened around the Sun forming a planetary disc The material from the planetary disc went to form the planets and other objects in our Solar System A nebula is a cloud of gas and dust made by an exploding star. Planetary nebula: remaining mineral particles and gas after a star explodes Stars release energy and build elements through nuclear fusion. Nuclear fusion creates new elements. Stars “burn” hydrogen, becoming brighter. Eventually, stars become Red Giants and explode. Nebula Collapse and Condensation Rotation started by shockwaves from a nearby explosion (?) Because the solar nebula was rotating, it contracted into a disc, and the planets formed with orbits lying in nearly the same plane. Planetesimal accretion - ~5 to 4.6 billion yrs ago Planet Formation Some material formed solid objects Eventually, each object got large enough to attract more dust and ice with its gravitational influence Got larger as other particles collided with it and stuck together These balls then form the cores of the planets Astronomers believe that it took millions of years for the planets to form Asteroids and other planetesimals are “failed planets” – objects formed from the solar nebula that never got large enough to turn into planets Inner and Outer Planets Nebula was composed mostly the two simplest and lightest atoms hydrogen and helium, in gaseous form Also contained heavier atoms (mostly oxygen, carbon, silicon, iron), some of them in the form of dust particles As the Sun took shape in the center, matter coalesced in the outer regions of the disk Critical phase in development of the Solar System Earth and other planets would formed in an area of the disk that was enriched with heavier elements because it was much warmer The outer region of the disk cooled rapidly, with the result that some rock and frozen volatile elements condensed as tiny particles Particles grew by colliding with one another, forming larger masses that eventually developed gravitational fields Terrestrial Planets Gas Giants •Rocky particles and metallic compounds formed solids in the inner portion of the condensing solar nebula •It was too hot for hydrogen compounds to solidify •In the cooler outer region, hydrogen compounds, metals, and rocks condensed to a solid state •The transition zone between the two regions is known informally as the frost line. When the Solar wind “turned on”, volatiles were expelled from inner Solar System Earth’s Geomagnetic Field Blown into a streamlined shape by the Solar Wind. •Early in the history of the Solar System, the solar wind stripped the inner planets of their primitive atmospheres •In this image, the modern solar wind “ blows” the geomagnetic field into a streamlined shape with the blunt end facing into the solar wind and the tail extending downwind Our Sun: A Massive Hydrogen Bomb held together by gravity Solar core is site of nuclear fusion. H is converted to He, which has less mass 4 H atoms fuse together to form one He atom Mass differential is expelled as energy (light and heat). The Sun is getting “lighter” through time. Enough fuel to last another 4 to 5 billion years. •Everything in Solar System orbits around the Sun •Balances between Sun’s gravity with the centrifugal force •The Sun contains over 99% of the Solar System’s mass Mercury •Vertical axis (no seasons) •Probable molten (Fe) core •Silicate (SiO2) shell •Atmosphere created by solar wind •227oC to -137oC Venus •Axis spin opposite to other planets (upside down?) •Is core liquid or solid? - Unknown •Active volcanism? Probably •Atmosphere 96.5% (CO2) •460oC “runaway” Greenhouse Effect Greenhouse Worlds Why is Venus so much hotter than Earth? Although solar radiation 2x Earth, most is reflected but 96% of back radiation absorbed Energy Budget Earth’s temperature constant ~15C Energy loss must = incoming energy Heat loss called back radiation Earth is constantly receiving heat from Sun, therefore must lose equal amount of heat back to space Wavelengths in the infrared (long-wave radiation) Earth is a radiator of heat If T > 1K, radiator of heat Energy Budget Average Earth’s surface temperature ~15C Reasonable assumption Surface of Earth radiates heat with an average temperature of 15C However, satellite data indicate Earth radiating heat average temperature ~-16C Why the discrepancy? What accounts for the 31C heating? Energy Budget Greenhouse gases absorb 95% of the long-wave, back radiation emitted from Earth’s surface Trapped radiation reradiated down to Earth’s surface Accounts for the 31C heating Satellites don’t detect radiation Muffling effect from greenhouse gases Heat radiated back to space from elevation of about 5 km (top of clouds) average 240 W m-2 Keeps Earth’s temperature in balance Greenhouse Worlds Why is Venus so much hotter than Earth? Although solar radiation 2x Earth, most is reflected but 96% of back radiation absorbed What originally controlled C? During formation of solar system most carbon was CH4 Lost from Earth and Venus Earth captured 1 in 3000 carbon atoms 60 out of every million C atoms CaCO3 (limestone and dolostone) and organic residues (kerogen) Bulk of carbon in sediments on Earth Venus probably had similar early planetary history Tiny carbon fraction in the atmosphere as CO2 Most carbon is in atmosphere as CO2 Venus has conditions that would prevail on Earth All CO2 locked up in sediments were released to the atmosphere Earth and Venus Water balance different on Earth and Venus If Venus and Earth started with same components Venus should have either Sizable oceans Atmosphere dominated by steam H present initially as H2O escaped to space H2O transported "top" of the Venusian atmosphere Disassociated forming H and O atoms H escaped the atmosphere Oxygen stirred back to surface Reacted with iron forming iron oxide Planetary Evolution Similar Although Earth and Venus started with same components Earth evolved such that carbon buried safely in early sediments Avoiding runaway greenhouse effect Venus built up CO2 in the atmosphere Build-up led to high temperature High enough to kill all life If life ever did get a foothold Once hot, could not cool Why Runaway Greenhouse? Don't know why Venus climate went haywire Extra sunlight Venus receives? Life perhaps never got started? No sink for carbon in organic matter Was the initial component of water smaller than that on Earth? Mars •Most Earth-like of planets in Solar System •Iron core, partially liquid •Silicate (SiO2) mantle and crust •Active volcanism? Probably •Atmosphere 95.3% (CO2) •Past “flooding” and fluvial erosion of surface Water has flowed in the past. But is now locked up as ice in the ground and as polar ice caps. Drainage features due to short-lived melting events Mars Exploration Broad Goals of Mars Exploration Determine whether life ever arose on Mars Characterize the climate of Mars Characterize the geology of Mars Prepare for human exploration Mars Rover Curiosity Curiosity landed on Mars August 5, 2012 Mission – robotic exploration of Mars Rather than look for life directly, Curiosity was designed to see if Martian environments could ever have hosted life Curiosity carries the most advanced suite of instruments ever sent to the Martian surface Curiosity analyzes samples scooped from the soil and drilled from rocks The record of the planet's climate and geology is "written in the rocks and soil“ Curiosity's onboard laboratory will study rocks, soils, and the local geologic setting in order to detect the chemical building blocks of life (e.g., forms of carbon) on Mars and will assess what the Martian environment was like in the past Curiosity discoveries Mars is a suitable home for life Organic carbon was found on Mars Information necessary for a human mission Thick atmosphere with water [CH4] changed 10-fold! Radiation could be health risks for humans Raw ingredients necessary for life to get started Methane concentration is variable on Mars Environment suitable for living microbes Found S, N, O, P and C “Heavy” isotopes of H, C and Ar; “light” lost to space Evidence that there was once more water on Mars Ancient streambed found Water about knee-deep on flowed on Mars MERCURY VENUS EARTH Terrestrial planets are small and rocky, with thin atmospheres, silicate and metallic shells. O, Fe, Si, Mg, Ca, K, Na, Al MARS •Not all of the planetesimals ended up becoming planets •Some were made up primarily of rocky and metallic substances, and they became asteroids •Most asteroids reside in a belt of rocky debris between Earth and Jupiter that may be left over from the early solar system •The total mass of all the asteroids is less than that of our Moon. •Jupiter is the largest planet in Solar System – more than twice as massive as all other planets combined •Enormous size is due to its huge gaseous atmosphere •Holdover from the early days of the Solar System •While the terrestrial planets lost their early atmospheres to the heat of the evolving Sun, Jupiter did not Jupiter's Great Red Spot - A hurricane the size of Earth lasting several centuries Does Jupiter have a hard surface? Lack of hard surface may allow for different winds at different speeds – hence, banding 90% Hydrogen, 10% Helium •Saturn: 9 rings of rock and ice particles, 10,000 km wide and 200 km thick •Outer layer of frozen ammonia (NH3) 96% Hydrogen, 3.35% Helium •62 moons •Uranus: axis tilted completely on its side •Uranus north pole points toward the Sun for half of a Uranian year; its south pole points toward the Sun for the other half of the year •Results in extreme seasonal effects • The polar regions get the greatest amount of sunlight • Rest of the planet fails to experience the daily heating and cooling experienced by other planets •82.5% Hydrogen, 15.2% Helium, 2.3% Methane (CH4) •Neptune is the outermost classical planet in our Solar System •The inner two-thirds of the planet are thought to be composed of a mixture of molten rock, water, liquid ammonia, and methane •Neptune: highest winds in Solar system, 2000 km/hr •80% Hydrogen, 18.5% Helium, 1.5% Methane (CH4) Gas Giants are massive planets with thick atmospheres. He, H, CO2, H2O, N2, CO, NH3, CH4 Neptune Jupiter Uranus Saturn Dwarf Planet “an object in the Solar System that orbits the Sun and is not a satellite of a planet or other celestial body. It must be spherical (or nearly so) in shape.” •Dwarf planets do not resemble the inner, terrestrial planets or the outer, gaseous planets in their makeup •Closely resemble the ice moons of the outer planets •Researchers suspect that they are large icy chunks of debris left over from the formation of the Solar System. Formation of Earth Accretion from the nebular cloud as particles smashed into each other, forming planetesimals Earth’s mass and gravitational field grew causing it to compress into a smaller, denser spherical body Compression generated heat in Earth’s interior Heat also produced by Earth’s interior began to melt Decay of radioactive elements in Earth’s interior Impacts of thousands of extraterrestrial objects Iron is abundant and heaviest common element - droplets of liquid iron sank toward the planet’s center and condensed to form the core Melting iron moving through the planet generated friction that contributed more heat and raised Earth’s temperature Deep magma ocean formed on the planet surface Earth began to differentiate In time EARTH’S interior accumulated heat New atmosphere created by volcanic outgassing and delivery of gases and water by ice-covered comets. “Hadean Era” Formation of our Moon Capture hypothesis Earth’s gravity captured a passing planetesimal, which became the Moon Hypothesis predicts that Earth and Moon were formed at separate locations Chemistry of Moon rocks is similar to that of Earth rocks, suggesting that both developed either from a common source or at least at the same location in the planetary nebula Double planet hypothesis Earth and the Moon were formed concurrently from a local cloud of gas and dust Hypothesis fails to account for the unusual tilt of the Moon’s axis, melting of its surface rocks, and the fact that it is less than half as dense as Earth Formation of our Moon Fission hypothesis Centrifugal force associated with Earth’s spin caused a bulge of material to separate from Earth in the area of the equator Hypothesis requires that Earth rotate once every 2.5 hours in order to develop the necessary force. Impact hypothesis Most widely accepted explanation for the Moon’s formation During planetesimal accretion, Earth suffered a massive collision with a huge object the size of Mars, and this collision led to the formation of our Moon •The impact hypothesis suggests that Earth suffered a massive collision that led to formation of our Moon •This image shows Earth and a Mars-size object, each peppered by hundreds of smaller impacts, colliding with one another in the early Solar System Why worry about the beginning? The evolutionary course is significantly influenced by the initial state We know the state of the Earth today relatively well; knowing the beginning will help constrain the in between