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Big bang Nuclear fusion in stars Supernova nucleosynthesis Planetary formation Current Solar System 4. Current Solar System Current Solar System • Oort cloud is a hypothetical spherical cloud. No direct observations. • 100 000AU; defines gravitational boundary of solar system • Source of long-period Comets Nearest Star (Proxima Centuri) at 4.2 light year = 1.3 pc = 2.6x105 AU • Terrestrial (or inner rocky) planets: Mercury, Venus, Earth, Mars • Asteroid belt: Rocky objects. Some asteroids around the outer edge may have ice deep inside the bodies. • Gas Giant Planets: Jupiter, Saturn, Uranus, Neptune • Trans-Neptunian Objects – Pluto is no longer defined as a planet. – Edgeworth-Kuiper Belt Objects: Icy bodies, located beyond the planets extending from the orbit of Neptune (at 30 AU) to approximately 55 AU from the Sun – Scattered Disk: Icy bodies: These objects have eccentric orbits, and perihelia greater than 30 AU. (e.g. Sedna) – Oort cloud: nearly spherical swarm of comets, originally formed in outer solar system. Terrestrial planets Gas planets Dwarf planets Diameter Mass Semi-major axis Rotational period Number of satellite Ring Atmosp here Mercury 0.38 0.06 0.39 59 0 no No substantial Venus 0.95 0.82 0.72 -243 0 no CO2, N2 Earth 1 1 1 1 1 no N2, O2 Mars 0.53 0.11 1.52 1.03 2 no CO2, N2 Jupiter 11.2 318 5.20 0.41 >63 yes H2, He Saturn 9.45 95 9.54 0.43 >60 yes H2, He Neptune 4.01 14.6 19.22 -0.72 27 yes H2, He Uranus 3.88 17.2 30.06 0.67 13 yes H2, He Ceres 0.08 0.0002 2.77 0.38 0 no - Pluto 0.19 0.002 39.48 -6.39 3 no No substantial Diameter Mass Semi-major axis Rotational period Sun Terrestrial planets Orbital period Mass density - Mercury 0.38 0.06 0.39 59 0.24 5.4 Venus 0.95 0.82 0.72 -243 0.62 5.3 Earth 1 1 1 1 1 5.5 Mars 0.53 0.11 1.52 1.03 1.9 3.9 Jupiter 11.2 318 5.20 0.41 12 1.3 Saturn 9.45 95 9.54 0.43 30 0.7 Uranus 4.01 14.6 19.22 -0.72 84 1.2 Neptune 3.88 17.2 30.06 0.67 165 1.7 Ceres 0.08 0.0002 2.77 0.38 4.6 2.1 Pluto 0.19 0.002 39.48 -6.39 248 2.0 Gas planets Dwarf planets Mass densities of Europe and Titan are 3.0 and 1.9 g/cm3. Why molecular abundance of terrestrial planets are significantly different? N2 (78 %) O2 (21 %) CO2 (96 %) N2 (3.5 %) CO2 (95 %) N2 (2.7 %) H2 (93 %) He (7%) Formation of Earth Atmosphere • The Earth is too small to retain atmospheric H & He. H and He are the most abundant elements in the Universe, yet they are very rare in the Earth's atmosphere, because H and He elements are small and light, and so moves very fast at a given atmospheric temperature. • After losing most of original H and He, the primordial atmosphere of the Earth was built up by outgassing of the crust by volcanoes: - Mostly H2O and CO2 - Small amounts of N2 • The primordial atmosphere had ~1000 times more CO2 than it does now. Where did it all go? - H2O condensed to form the oceans. - CO2 dissolved into the oceans and precipitated out as carbonates (limestone, 석회암). • By contrast, N2 is chemically inactive. It can remain in the atmosphere. • The second major constituent of the present-day atmosphere is O2. It comes primarily from photosynthesis in plants. • Ozone (O3) forms in stratosphere from O2 interacting with solar UV photons. • It blocks UV photons from reaching the ground. This makes land on the ground possible to live against the solar UV radiation. • The presence of O2 and O3 in our atmosphere is a sign of life. Greenhouse Effect • The atmosphere is transparent to visible light, but mostly opaque to infrared. • The Infrared "opacity" comes from absorption bands of H2O, CO2, CH4 and others molecules. - Photons are absorbed by the ground, heating it up - The warm ground radiates infrared photons • The atmosphere, however, is mostly opaque to infrared photons: Most of the infrared photons emitted by the warm ground get absorbed by the atmosphere on their way out, heating the atmosphere. • The Greenhouse Effect is responsible for making the Earth about 30~40 K warmer than it would be if there it had no atmosphere. • Therefore, without the Greenhouse Effect, there would be no liquid water on the Earth, only ice. Since life as we understand it requires liquid water, if there was no Greenhouse Effect, the Earth would be inhospitable to life. Atmospheric Composition (average) Gas Mole fraction Nitrogen (N2) 0.78 Oxygen (O2) 0.21 Water (H2O) 0.04 to < 5x10-3; 4x10-6 -strat Argon (Ar) 0.0093 Carbon Dioxide (CO2) 370x10-6 (date: 2000) Neon (Ne) 18.2x10-6 Ozone (O3) 0.02x10-6 to 10x10 –6 Helium (He) 5.2x10-6 Methane (CH4) 1.7x10-6 Krypton (Kr) 1.1x10-6 Hydrogen (H2) 0.55x10-6 Nitrous Oxide (N2O) 0.32x10-6 Carbon Monoxide (CO) 0.03x10-6 to 0.3x10-6 Chlorofluorocarbons 3.0x10-9 Carbonyl Sulfide (COS) 0.1x10-9 Gas giant planets: Jupiter & Saturn • • • • Dominant composition: Hydrogen + Helium, like the sun Surface clouds: ammonia ice, water ice.... Deep in interior: liquid metallic hydrogen Even deeper: rocky core of ~ 10~15 x (Earth mass) Jupiter • 5th planet from the Sun, at least 63 known moons. Largest planet in the solar system. • The planet is composed of mostly hydrogen and helium just like the Sun. • Colorful bands parallel to the Equator. Lot's of swirls and spots: storms. One storm, the "Great Red Spot" is 2 times the size of Earth, and has lasted for at least 300 years (Galileo saw it in his telescope). It's a gigantic hurricane. • The planet appears oblate (squashed). This is due to rapid rotation (once every 10 hours!). • Jupiter has very thin rings seen for the first time by the Voyager spacecraft • The Galilean Satellites: the 4 largest moons of Jupiter – Ganymede: The largest moon in the Solar System, it has an old, cratered surface. – Callisto: Heavily cratered like Ganymede, lots of ice on the surface – Europa: Smooth surface with narrow, dark stripes running across, few craters. The surface seems to be fractured ice (water ice). Recent information from the Galileo spacecraft suggests that there is world-wide ocean of liquid water beneath the frozen surface. Life? – Io: Erupting volcanos! The interior is molten, this is due to tidal forces. • The other known moons are all rather small; many are captured asteroids or comets. Some have retrograde orbits. Io Europa Ganymede Callisto Europe (Moon of Jupiter) Surface temperature Surface pressure Size 50-125 K 0.1 micro Pa 0.245 Earth radius John, S. 1987 Saturn • 6th planet. Saturn has a remarkably low density (<1 g/cm3). • Like Jupiter and the Sun it is mostly composed of Hydrogen and Helium, with an Earth-like core. • It's most outstanding feature are its magnificent rings. The rings are made of chunks of rock and ice. It is material that failed to coalesce into a moon. • Saturn has more than 60 moons. The largest of the Moons, Titan has a thick Nitrogen atmosphere like Earth! It's possible that there are methane lakes and a rain of organic materials onto the surface. Life? Titan (Moon of Saturn) False color image of the cloud on the pole Surface temperature Surface pressure Atmospheric composition 94 K 147 kPa (Earth 100 kPa) N2 98.4 % Titan is - the largest moon of Saturn - the only natural satellite known to have a dense atmosphere - and the only object other than Earth for clear evidence of surface liquid Based on the density of 1.9 g/cm³, the bulk composition is half water ice and half rocky material. On January 14, 2005, the Huygens probe landed on the surface of Titan. The probe photographed pale hills with dark "rivers" running down to a dark plain. Current understanding is that the hills are composed mainly of water ice. Dark organic compounds, created in the upper atmosphere by the ultraviolet radiation of the Sun, may rain from Titan's atmosphere. They are washed down the hills with the methane rain and are deposited on the plains over geological time scales. http://saturn.jpl.nasa.gov/multimedia/flash/Titan/index.html Ice giant planets: Uranus & Neptune • Dominant composition: – Water + Ammonia + Methane ices – Only atmosphere contains H, He (in total only minor) • Uranus: – 25% Iron + Silicates – 60% Methane + Water + Ammonia – 15% Hydrogen + Helium • Neptune: – 20% Iron + Silicates – 70% Methane + Water + Ammonia – 10% Hydrogen + Helium Uranus (Near-Infrared Images by Keck Telescope) Neptune from Voyager 2 Terrestrial vs. gaseous planets Surface temperature on equator So many craters like moon 700K (day), 100K (night) Surface temperature Surface pressure Atmospheric composition 735K 9.3 MPa CO2 96.5% N2 3.5% SO2 0.015% Surface temperature Surface pressure Atmospheric composition 180-330K (average 287K) 100 kPa N2 78% O2 21% H2O about 1% Surface temperature Surface pressure Atmospheric composition 180-270 K 0.4-0.9 kPa CO2 95.3% N2 2.7 %