GENS4001The Solar System1 GENS4001 Astronomy Part 1: The Solar System Dr Michael Burton Department of Astrophysics School of Physics, UNSW

GENS4001The Solar System2 Overview 1 Star (the Sun) Planets –Mercury, Venus, Earth, Mars –Jupiter, Saturn, Uranus, Neptune –Pluto / Charon ≥ 61 Moons 100,000 (?) Asteroids, 100 million (?) Comets, Solar Wind

Formation of the Solar System Collapse of cloud of gas, dust & ice –4.6 billion years ago, –Swirling, disk-shaped, –Sun formed at centre of ‘Solar Nebula’. Inner planets form through accretion of dust particles to planitesimals into protoplanets. –Collisions & cratering dominate for 150 Myrs. Outer planets form through accretion of gas onto rocky protoplanetary cores.

GENS4001The Solar System4 Our Star, the Sun Giant ball of plasma undergoing thermonuclear fusion of hydrogen in centre! Photosphere - visible surface at 6000°C Chromosphere –Jets of gas (spicules) rise along along boundaries of granules. Corona - tenuous, hot (2,000,000°C) gas –Blends into Solar Wind

The Sun (continued) Surface features vary with 11-year cycle: –Sunspots: cooler with strong magnetic field, –Solar flare: eruption from sunspot group, –Convection cells, transporting energy outwards. Energy produced by thermonuclear fusion of 4 H-atoms into He-atom at 8 million °C. Solar Model well understood: –Fusion in core about 1/4 solar radius in size, –Neutrinos - 1/4 predicted number?

GENS4001The Solar System6 Earth / Moon Double planet system, tidally interacting. Plate Tectonics produces continents, oceans, mountains & volcanoes. Iron rich core producing magnetic field. Atmosphere of 80% nitrogen, 20% oxygen –Troposphere, stratosphere, mesosphere, thermosphere Supports life on land, oceans & atmosphere.

GENS4001The Solar System7 Earth / Moon (continued) Magnetosphere surrounding Earth, protecting atmosphere from Solar Wind. Collision-ejection with giant asteroid, with debris coalescing to form Moon. Weathering has erased asteroid impacts on Earth but past history still visible on Moon. –Cratered highlands, –Smooth-surfaced Maria, from lava flows.

Earth / Moon Phenomena Phases of the Moon –Orbit about Earth, and Solar illumination. Eclipses when Sun/Earth/Moon in line: –Lunar eclipses –Solar eclipses (inc. annular) Tides –Differential gravitational pull of near and far sides of Earth by Moon Asteroid collisions and mass extinctions?

GENS4001The Solar System9 What if the Moon didn’t exist? Effect on culture (romances)? No eclipses or phases - dark skies! Tides only 1/3 current size (from Sun). –12 hour cycle, constant level. Day would still be 6 hours long –Enormous tides helped form soup for life? –More powerful winds, mountains eroded.

GENS4001The Solar System10 The Inner and Outer Planets Small (<13,000 km) Rocky (iron core) Thin atmospheres Slow rotation Short years Few moons Warm No Rings Large (>50,000 km) Gaseous (H, He) Thick atmospheres Rapid rotation Long years Many moons Cold Rings

Mars No canals, but ancient river channels! No plate tectonics, resulting in giant shield volcanoes over hot spots, plus canyons. A few impact craters. Thin carbon dioxide atmosphere & red dust. Water must once have flowed - flash floods. Could life have once existed? 2 tiny moons (Phobos, & Deimos) are captured asteroids.

GENS4001The Solar System12 Jupiter The Giant of the planets (not a failed star). Rapid, differential rotation. Belts & Zones, methane & ammonia clouds. Cyclones interface: Red Spot & white ovals. Strong magnetic fields, huge aurorae. Gas / Liquid Metallic H / Rocky Core. Thin, transient dust ring - meteor impacts.

GENS4001The Solar System13 Moons of Jupiter 16 Moons with 4 giants (Galilean satellites). Io: Volcanic, sulphur-covered, kept molten through tidal heating. Europa: covered in ice with intricate pattern of cracks –tidal heating supports oceans, life?? Ganymede: Rock & Ice, past tectonics. Callisto: Rock, cratered terrain (impacts).

Comets and Asteroids Debris from formation of Solar System. Asteroids: lumps of rock a few km in size –Most in belt between Mars & Jupiter. Comets: dirty snowballs of ices and rocks –Primordial, but transient, –Highly elliptical orbits, from Kuiper Belt ( AU) or Oort Cloud (to 50,000AU), –Tails only when near Sun (vaporised ice), blown out by the solar wind & sunlight.