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Astronomy 110: SURVEY OF ASTRONOMY 7. Cosmic Debris 1. The Asteroid Belt 2. Comets & Dwarf Planets 3. History of the Solar System “Look here brother / Who you jiving with that Cosmik Debris” — Frank Zappa Overview: Structure 100 AU S 20,000 AU 5 AU Inner system: terrestrial planets, asteroids. Outer system: giant planets and moons, “KBOs”. Oort Cloud: comets. In addition to the major planets, our solar system contains a variety of asteroids, “dwarf planets”, and other rubble. These objects continue to play an important role in the evolution of major planets; they also provide a window on conditions in the early solar system. 1. THE ASTEROID BELT a. Asteroid Properties b. Belt Structure c. Asteroid Impacts Asteroid Properties Comet Nuclei Asteroids and comets to scale Mission to Eros Eros: The Final Approach The Last Global Rotation Movie Most asteroids may be rubble piles: loose collections of fragmented rock held together by self-gravity. Giant Asteroids Ceres 975 km Large asteroids are complex objects which appear to have differentiated. Largest Asteroid May Be 'Mini Planet' Vesta Computer Model 530 km Hubble Reveals Huge Crater on the Surface of the Asteroid Vesta Meteors and Meteorites A meteor enters Earth’s atmosphere; a meteorite survives the fall. Fireball Meteor Over Groningen Samples From the Asteroid Belt Primitive meteorites are old (4.6 Gyr); they are relics of the solar system’s formation. Processed meteorites come from differentiated asteroids which were fragmented by collisions. Origin of Processed Meteorites Key Stages in the Evolution of the Asteroid Vesta Family Members Crust Magnesium-Sliicate Mantle (Olivine) Surface Lavas Iron-Nickle Core Stony Irons? As smaller bodies in the early Solar System fall together, the asteroid agglomerates. Heavier elements sink to the center as the asteroid heats. This forms a separate core, mantle, and outer crust. Lava from the interior flows to the surface. PR95-20 • ST ScI OPO • April 19, 1995 • B. Zellner (GA Southern Univ.), NASA A fragment of Vesta Hubble Maps the Ancient Surface of Vesta Occasional impacts with other bodies break off pieces of the crust, exposing the mantle. Hubble Maps the Ancient Surface of Vesta Origin of Processed Meteorites Key Stages in the Evolution of the Asteroid Vesta Family Members Crust Magnesium-Sliicate Mantle (Olivine) Surface Lavas Iron-Nickle Core Stony Irons? As smaller bodies in the early Solar System fall together, the asteroid agglomerates. Heavier elements sink to the center as the asteroid heats. This forms a separate core, mantle, and outer crust. Lava from the interior flows to the surface. PR95-20 • ST ScI OPO • April 19, 1995 • B. Zellner (GA Southern Univ.), NASA A fragment of Vesta Hubble Maps the Ancient Surface of Vesta Occasional impacts with other bodies break off pieces of the crust, exposing the mantle. Hubble Maps the Ancient Surface of Vesta Belt Structure 14 August 2006 Hildas Trojans Mars Jupiter Trojans Wikipedia: Asteroid belt Belt Structure Hildas Trojans Mars Inner Belt: a < 2.5 AU Mid Belt: 2.5 AU < a < 2.8 AU Outer Belt: a > 2.8 AU Jupiter Trojans Wikipedia: Asteroid belt Trojans Hildas Outer Middle Inner Resonances With Jupiter Resonances with Jupiter sort asteroids by orbital period; period determines semi-major axis (Kepler III: P2 = a3). Asteroid Families Many asteroids are members of families; they have similar orbits and compositions (indicated by colors). Asteroid Belt Populations Inner belt asteroids (left) and families (right). Origin of Families Hildas Trojans PO W Large Asteroid Breakup — Don Davis ! Mars Fragments are scattered on similar orbits. Jupiter Trojans Wikipedia: Asteroid belt A Suspected Asteroid Collision Suspected Asteroid Collision Leaves Odd X-Pattern of Trailing Debris Origin of Near-Earth Objects (NEOs) Mars Some fragments wind up on orbits which are resonant with Jupiter. Their orbits grow more elliptical, finally entering the inner solar system. Wikipedia: Asteroid belt Asteroid Impacts Asteroid Impacts Age: 49,000 yr 1.2 km Barringer Meteor Crater, Arizona Age: 212 Myr 70 km Manicouagan, Quebec, Canada Historical Impacts Tunguska (1908): impactor exploded in air with H-bomb force. Jupiter (1994): string of comets hit planet; visible from Earth Chicxulub (pronounced tʃikʃu'lub) Cretaceous–Tertiary extinction event Impact crater map Iridium-rich layer (65 Myr old) Chicxulub Impactor: A Possible Timeline 1. Baptistina parent body (170 km diameter) smashed ~160 Myr ago. Large Asteroid Breakup — Don Davis 2. Fragment hits Moon, forming Tycho crater (110 Myr ago). 3. Fragment hits Earth, forming Chixulub (65 Myr ago). K-T event Impact Threat Small impacts are more common than big ones. Millions of years ago Wikipedia: K-T extinction event The fossil record shows many mass extinctions over Earth’s history. Only K-T is associated with a definite crater. 2. COMETS AND DWARF PLANETS a. The Kuiper Belt b. Comets Largest Known Kuiper Belt Objects (and Satellites) Wikipedia: Kuiper Belt Pluto and Charon Double planet with 2 small moons; possibly formed by giant impact (similar to Earth-Moon system). • orbit mass: MPluto = 0.002 M⊕♁ 3 density: ~ 2 g/cm • rd rock, 2/3rd ice composition: 1/3 • • thin atmosphere: N2, CH4, CO Pluto has probably differentiated; Charon is too small to melt itself. Hubble Maps Pluto Pluto’s Orbit Pluto’s orbit is highly tilted (inclination i = 17°) to the rest of the solar system. Wikipedia: Pluto Pluto is in a 3:2 resonance with Neptune. This is a stable resonance — no real change can occur. Pluto’s Orbit KBO Orbits Classical: outside Neptune’s orbit Resonant: like Pluto’s orbit Scattered: highly elliptical Plan View of the Solar System KBOs and Comets Classical and resonant KBOs are safe from Neptune’s influence. KBOs above this line cross Neptune’s orbit 3:2 Scattered KBOs which cross Neptune’s orbit are easily perturbed. The Resonant KBOs These scattered KBOs may become comets. Comets Comets are icy objects which fall into the inner solar system. Warmed by the Sun, they may develop long tails. un To S Comets Com Mot et’s ion Comet Halley At other times, a comet is an inert lump of ice & dust. Changes in a Comet Comet Nuclei Asteroids and comets to scale Deep Impact: the Nucleus of Tempel 1 Evidence of Cometary Ice Analysis of collision debris suggests Tempel formed ~ 30 AU from the Sun. Tempel Alive With Light Origins of Comets After 1000 passages (or less) a comet nucleus disintegrates. Where do new comets come from? Short-period comets (P<200 yr): • stay close to plane of ecliptic • originate in Kuiper belt • scattered by Neptune (et al. ???) Long-period comets (P>200 yr): • arrive from all directions • originate in Oort cloud • scattered by passing stars Comets and Meteor Showers Comets shed dust, sand and gravel which slowly spread out as they move along the comet’s orbit. If the Earth encounters one of these trails, we get a meteor shower. Meteor Showers Comet Encke Perseid Meteor Shower Raining Perseids Major Meteor Showers Forty Thousand Meteor Origins Across the Sky 3. HISTORY OF THE SOLAR SYSTEM a. Four Facets of Formation Four Facets of Formation 1. Cloud collapse ordered motion of Solar System Orbits & spins are “fossils” of motion in early Solar System. 2. Frost line two types of planets Terrestrial planets form near Sun, jovian planets further away. 3. Impacts & encounters exceptions to the rules E.g., Earth’s big Moon, and some objects with unusual motions. 4. Planet migration rearrange outer Solar System Form Oort cloud & Kuiper belt; cause late heavy bombardment. Cloud Collapse 1. A gas cloud starts to collapse due to its own gravity. 2. It spins faster and heats up as it collapses. 3. Vertical motions die out, leaving a spinning disk. 4. The solar system still spins in the same direction. 1. What would happen if the gas cloud had no rotation whatsoever to begin with? A. The cloud would collapse more before forming a disk. B. The cloud would collapse less before forming a disk. C. The cloud would fly apart instead of collapsing. D. The cloud would fall straight in and not form a disk. 2. Which of the following is not explained by the idea of cloud collapse? A. All planets orbit in nearly the same plane. B. All planets orbit in nearly the same direction. C. Most planets spin in roughly the same direction. D. The square of a planet’s orbital period is proportional to the cube of its semi-major axis. The Frost Line The disk was hot at the center, and cool further out. Inside the frost line, only rocks & metals can condense. Outside, hydrogen compounds can also condense. The frost line was between the present orbits of Mars and Jupiter — roughly 4 AU from the Sun. The Frost Line: Jovian Planets 1. Outside the frost line, icy planetesimals were very common, forming planets about 10 times the mass of Earth. 2. These planets attracted nearby gas, building up giant planets composed mostly of H and He. 3. The disks around these planets produced moons. 3. What would have happened if our solar system formed with no oxygen (hence, no H2O)? A. Only terrestrial planets (small, rocky) would form. B. Only jovian planets (giant, gassy) would form. C. Jovian planets might form beyond the CH4 and NH3 frost lines. D. No planets of any kind would form. 4. Which of the following is not explained by the frost line idea? A. Terrestrial planets are much smaller than jovian planets. B. Jupiter and Saturn are composed of the same mix of elements as the Sun itself. C. Jovian planets have large satellites which orbit in the same direction as the planet spins. D. All jovian planets have rings. Impacts & Encounters 1. Giant impacts in early solar system: — explain rotation of Uranus,Venus — form Moon from collision debris 2. Satellite capture after near-miss: — moons of Mars captured from asteroid belt — Triton captured from Kuiper belt Impacts & Rotation If proto-planets side-swipe and merge, the angular momentum of their initial orbit is transformed into rotation of the merged planet. Stellar Collisions If the initial orbit is tilted with respect to the solar system, the merged planet’s spin will also be tilted. 5. Venus spins backward and slower than any other planet, taking 243 Earth days to rotate once. How might this have come about? A. Venus was side-swiped by a large asteroid, which reversed its rotation. B. Venus formed in a head-on collision, which left it with almost no angular momentum. C. Venus suffered a glancing collision with another planet, without actually merging. Formation of the Moon Moon-forming impact Mars-sized planet (Thea) hits Earth about 4.5 Gyr ago. Moon forms from debris: This explains why Moon is poor in metals and volatiles. 6. Which of the following facts does the giant impact hypothesis explain? A. The Moon’s surface composition is similar to Earth’s outer layers. B. The Moon has a very small iron core for its size. C. The Moon is poor in easily vaporized substances. D. The Moon orbits the Earth in the same direction as the Earth spins. E. All of the above. Planet Migration A planet embedded in a disk around a star can excite spiral waves — this process robs the planet of angular momentum, causing it to spiral inward. Planet Migration: The Nice Model Migration is expected whenever planets interact with disks; did this happen in our Solar System? Wikipedia: Nice Model 1. Giant planets born closer to Sun; icy planetesimals orbit in outer disk. 2. Jupiter & Saturn migrate into 2:1 resonance; Uranus & Neptune switch. 3. Planetesimals are scattered outward, populating Kuiper belt & Oort cloud. Outcome of the Nice Model 1. Kuiper belt drastically thinned and moved outward to present position. — many objects in resonances with Neptune 2. Majority of icy planetesimals scattered by Jupiter into extremely elliptical orbits, forming Oort cloud. — can’t form in place; density much too low 3. Some planetesimals scattered inward, explaining the Late Heavy Bombardment. — can match history of impacts on Moon’s surface