7. Cosmic Debris

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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

20,000 AU 5 AU

Inner system: Outer system: Oort Cloud: terrestrial planets, giant planets and comets. asteroids. moons, “KBOs”. 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 Largest Asteroid May Be 'Mini Planet' objects which Vesta Computer Model appear to have 530 km differentiated.

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 Key Stages in the Evolution of the Asteroid Vesta

Processed Family Members Crust Surface Magnesium-Sliicate Lavas Meteorites Mantle (Olivine)

Iron-Nickle Core

Stony Irons?

As smaller bodies in the early Solar System Heavier elements sink to the Occasional impacts with other bodies fall together, the asteroid agglomerates. center as the asteroid heats. break off pieces of the crust, exposing This forms a separate core, the mantle. mantle, and outer crust. Lava from the interior flows to the surface. Hubble Maps the Ancient Surface of Vesta

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 Origin of Key Stages in the Evolution of the Asteroid Vesta

Processed Family Members Crust Surface Magnesium-Sliicate Lavas Meteorites Mantle (Olivine)

Iron-Nickle Core

Stony Irons?

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 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 Resonances With Jupiter dle ojans r Inner Hildas Outer Mid T

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 Large Asteroid Breakup — Don Davis W!

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 Jupiter (1994): string of comets air with H-bomb force. 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). Impact Threat ent Small impacts are more v K-T e 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⊕ • density: ~ 2 g/cm3 • composition: 1/3rd rock, 2/3rd ice

• thin atmosphere: N2, CH4, CO

Pluto has probably differentiated; Hubble Maps Pluto Charon is too small to melt itself. 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 KBOs above this line cross Neptune’s orbit

Classical and resonant KBOs are safe from Neptune’s inﬂuence. 3:2

Scattered KBOs which cross Neptune’s orbit are easily perturbed.

The Resonant KBOs

These scattered KBOs may become comets. Comets Comets o Sun T Comet’ Motion s Comets are icy objects which fall into the inner solar system.

Comet Halley Warmed by the Sun, they may develop long tails.

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 Outside, hydrogen compounds rocks & metals can condense. 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 2. Jupiter & Saturn 3. Planetesimals are born closer to Sun; migrate into 2:1 scattered outward, icy planetesimals resonance; Uranus populating Kuiper orbit in outer disk. & Neptune switch. 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