Chapter 9: Small Bodies of the Solar System•When the sun was becoming star, tiny grains of primitive material stuck together to form swarms of small bodies called planetesimals: those in hotter, inner part of SS were made of rock and metal, those in the cooler, outer parts were made of ice, organic compounds, and rock•some consumed by larger bodies to become planets and moons, others ejected from the SS by gravitational encounters with larger bodies, some still around9.1 Dwarf Planets---Pluto and Others•planets orbit the sun and are large enough to pull themselves into a round shape and clear the area around their orbits; dwarf planets are round but don’t clear the area•5 recognized dwarf planets: Pluto, Eris, Haumea, Makemake, and Ceres (especially large object in the main asteroid belt, all others in outer SS)•Pluto’s orbit is 248 Earth years long, very elliptical, periodically bring the dwarf planet inside Neptune’s nearly circular orbit (like it was from 1979-99)•Pluto has four known moons, largest is Charon, half the size of Pluto, which is 2/3 as large as our moon•total mass of the Pluto-Charon system is 1/400 that of Earth•Pluto and Charon have a rocky core structure, surrounded by water-ice mantle, Pluto’s surface is icy mixture of frozen water, CO2, nitrogen, methane, and carbon monoxide, but Charon’s is dirty water ice•Pluto has thin atmosphere of nitrogen, methane, and carbon monoxide which freeze out of the atmosphere when Pluto is more distant from the Sun and cooler•P and C are tidally locked: each has one hemisphere that always faces the other, we knowlittle to nothing about surfaces and geological features (maybe more through New Horizons in 2015)•Eris, about same size as Pluto but more massive, large moon called Dysnomia•Eris has highly eccentric orbit that carries it 37.8 AU to 97.6 AU, orbit of 562 years•it was found near most distant point in its orbit, currently most remote known object in the SS, highly reflective, coating of pristine ice, spectra indicate the ice is mostly methane, which will form an atmosphere when Eris come closest to the sun in 2256•Haumea and Makemake both smaller and have slightly larger orbits than Pluto, Haumeahas two known moons, Hi’iaka and Namaka, Haumea spins so fast its distorted into a flattened ellipsoid with an equatorial radius approx. twice its polar radius (most distorted shape of any known planet)•no moons discovered around Makemake, we know the least about this planet•Ceres, diameter of 975km, larger than most moons, smaller than any planet, about a thirdof the total mass in the main asteroid belt, rotates on axis in 9 hours, survived largely intact, probably underwent differentiation•surface contains hydrated minerals like clays/carbonates, significant amounts of interior water, probably a quarter of its mass exists in the form of a water-ice mantle that surrounds a rocky inner core (Dawn will reach it in 2015)•once a large asteroid, now a dwarf planet due to spherical shape, has a bright spot on it9.2 Moons as Small Worlds•some moons have been frozen in time since they were formed, others more geologically active than Earth•four categories of geological activity: definitely active today, maybe active today, active in the past but not today, not active anytime since formation like Jupiter’s Callisto, Uranus’ Umbriel, and irregular moons (highly inclined, very elliptical, revolve opposite to planet), usu-ally heavily crateredGeologically Active Moons: Io, Enceladus, Triton•volcanic features on Io are incredible, Jupiter’s gravitational pull flexes Io’s crust and generates enough thermal energy to melt parts of it, most active volcanism known•lava and ask bury craters, none observed on surface, 150 active volcanoes, many simulta-neous•mixtures of sulfur, sulfur dioxide frost, and sulfurous salts of sodium and potassium cause colors on Io, bright patches may be sulfur dioxide snow, there’s evidence of plains, irreg-ular craters, and flows, due to eruption of silicate magmas on the surface, tall mountains (2x mt. everest), structures with multiple calderas (summit craters of volcanoes), long history of re-peated eruptions followed by collapse of partially emptied magma chambers, volcanoes spread out randomly, implies lack of plate tectonics, Io’s mass has turned inside out more than once, led to chemical differentiation, water and CO2 escaped into space a long time ago and heavier material sank to interior, sulfur constantly being recycled•far out in SS is Cryovolcanism-driven by subsurface low-temp volatiles rather than molten rock, Enceladus, Saturn’s icy moon, shows ridges, faults, smooth plains, evidence of tectonic processes that are unexpected for such a small body, some craters softened by viscous flow of ice (like in bottom layers of glaciers), parts have no craters, indicates recent resurfac-ing, South Pole is cracked and twisted, cracks are warmer than their surroundings, implies that tidal heating and radioactive decay in moon’s rocky core heat ice and drive it to the surface--active cryovolcanic plumes energetic enough to overcome low gravity, sending tiny ice crystalsinto space to replace particles lost in Saturn’s E Ring•mystery why Enceladus is so active, Mimas, neighbor appears dead•Cryovolcanism also on Neptune’s largest moon, Triton, it orbits Neptune opposite to Neptune’s spin, this means it was captured after Neptune’s formation (conservation of angular momentum requires that moons formed at the same time orbit in the same direction as theirplanet), Triton’s circular, synchronous orbit experienced tidal stresses from Neptune, generateslarge amounts of thermal energy, interior may have melted allowing Triton to become chemi-cally differentiated•Triton: thin atmosphere, surface of ices/frosts of methane and nitrogen, not many craters---young, some irregular pits and hills may be from slushy ice emerging from interior, veinlike features could include grooves/ridges from ice-oozing fractures, rest of Triton coveredwith smooth volcanic plains, 200 km wide depressions caused by mixtures of water, methane, nitrogen ice melted in interior and then erupted as rocky magmas that filled impact basins •clear nitrogen ice creates localized greenhouse effect, solar energy trapped beneath ice raises its temp, small increase in temp vaporizes nitrogen ice, expanding vapor causes pressure,ice ruptures vents gas into low-density
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