Asteroids, Comets, and Dwarf Planets: Notes

Asteroids, Comets, and Dwarf Planets

Asteroids and Meteorites

  • Goals:
    • Understand why there is an asteroid belt.
    • Explain how meteorites are related to asteroids.

Asteroid Facts

  • Approximately 150,000 asteroids are listed in catalogs.
  • Estimated over a million asteroids exist with a diameter greater than 1 km.
  • Small asteroids are more common than large ones.
  • The combined mass of all asteroids would not equal even a small terrestrial planet.
  • The largest asteroid is Ceres, with a diameter of approximately 1000 km, comprising about 1/3 of the main asteroid belt's mass.

Asteroid Appearance

  • Asteroids are generally cratered and not spherical.

Asteroids with Moons

  • Some large asteroids possess their own moons.
  • Example: Asteroid Ida has a small moon named Dactyl.

Asteroid Orbits

  • Most asteroids orbit within the main asteroid belt, located between Mars and Jupiter.
  • Some asteroids, like the Trojans, share Jupiter’s orbit.
  • Near-Earth asteroids have orbits that cross Earth’s orbit.

Orbital Resonances

  • Asteroids in orbital resonance with Jupiter experience periodic gravitational nudges.
  • These nudges can cause asteroids to be ejected from resonant orbits, creating gaps in the asteroid belt.

Origin of the Asteroid Belt

  • Rocky planetesimals between Mars and Jupiter failed to accrete into a planet.
  • Jupiter's gravity, particularly through orbital resonances, disrupted asteroid orbits and prevented accretion.

Meteor Terminology

  • Meteorite: A rock from space that survives passage through Earth's atmosphere and reaches the ground.
  • Meteor: The visible bright trail produced by a meteorite passing through the atmosphere.

Relationship between Meteorites and Asteroids

  • Most meteorites are fragments of asteroids.

What is a meteor?

  • A "shooting star": the glowing trail of hot gas and vaporized debris left by friction as an object moves through the Earth's atmosphere

Plausible Explanation for the Asteroid Belt

  • The belt is where all the asteroids happened to survive.

Comets

  • Goals:
    • Describe how comets develop tails.
    • Identify the origin of comets.

Comet Facts

  • Comets formed beyond the frost line, making them icy counterparts to asteroids.
  • A comet's nucleus is often described as a "dirty snowball."
  • Most comets do not have tails.
  • The majority of comets remain frozen in the outer solar system.
  • Comets only grow tails when they enter the inner solar system.

Comet Anatomy

  • Nucleus: The solid, icy core of a comet.
  • Coma: An atmosphere that forms around the nucleus as it's heated.
  • Plasma Tail: Ionized gas escaping from the coma, pushed by the solar wind.
  • Dust Tail: Small solid particles pushed away from the comet by photons (sunlight).

Growth of Tail

  • As a comet approaches the Sun, the nucleus warms and begins to sublimate.
  • A gas coma starts forming around the nucleus at approximately 5 AU from the Sun.
  • Tails form around 1 AU from the Sun and point away from it due to solar wind and radiation.
  • Solar heating diminishes between 3 and 5 AU from the Sun, causing the coma and tail to disappear.

Meteor Showers

  • Comets eject small particles that follow their orbit, causing meteor showers when Earth passes through the comet's orbit.
  • Meteors in a shower appear to originate from the same area of the sky due to Earth's motion through space.

Comet Origins

  • Kuiper Belt: Comets with orderly orbits between 30 to 100 AU in the plane of the solar system.
  • Oort Cloud: Comets on random orbits extending up to 50,000 AU.
  • Only a small fraction of comets enter the inner solar system.

Formation and Distribution

  • Kuiper belt comets formed within the Kuiper belt, orbiting in the same plane and direction as the planets.
  • Oort cloud comets originated closer to the Sun but were ejected to greater distances by gravitational interactions with Jovian planets, resulting in a spherical distribution.

Deep Impact Mission

  • A mission to study the nucleus of Comet Tempel 1.
  • A projectile impacted the surface on July 4, 2005, and the aftermath was studied by numerous telescopes.
    • Another mission → ESA Rosseta mission

Why are there two tails to some comets?

  • The first tail, called the "gas or plasma tail," is pushed out by the solar wind. The second tail, called the "dust tail," is pushed out by the solar radiation.

Pluto: Lone Dog No More

  • Goals:
    • Estimate how large a comet can be.
    • Describe Pluto and other large Kuiper belt objects.

Size Comparison

  • Relative sizes of Pluto, Eris, Makemake, Haumea, Sedna, Quaoar, and their moons (Charon, Dysnomia).

Pluto's Orbit

  • Pluto's orbit is both tilted and significantly elliptical.
  • Pluto has a 3:2 orbital resonance with Neptune, preventing collisions.
  • {T{neptune}} = \frac{3}{2} T{pluto}

Is Pluto a Planet?

  • Pluto is significantly smaller than the eight major planets.
  • It is not a gas giant.
  • Pluto has an icy composition, similar to comets.
  • It has a highly elliptical and inclined orbit.
  • Overall, Pluto shares more characteristics with comets than with the major planets.

Discovering Large Iceballs

  • In 2005, Eris, an iceball approximately the size of Pluto, was discovered.
  • Eris also has a moon, Dysnomia.

Other Icy Bodies

  • Numerous icy objects similar to Pluto exist on elliptical, inclined orbits beyond Neptune.
  • The largest of these are comparable in size to Earth's Moon.

Kuiper Belt Objects

  • These large, icy objects have orbits similar to the smaller objects in the Kuiper belt, which become short-period comets.
  • This raises the question of whether they should be classified as very large comets or very small planets.

Dwarf Planet Definition

  • In 2006, the International Astronomical Union (IAU) classified Pluto and similar objects as "dwarf planets."
  • To be classified as a planet, an object must:
    1. Orbit the Sun.
    2. Be massive enough for its gravity to pull it into a spherical shape.
    3. Have cleared its neighborhood of smaller objects or be gravitationally dominant.
  • Dwarf planets only meet the first two criteria.

Pluto's Characteristics

  • Pluto's largest moon, Charon, is nearly as large as Pluto itself, likely formed by a major impact.
  • Pluto is extremely cold, with a surface temperature of about 40 K.
  • Pluto has a thin nitrogen atmosphere that refreezes onto the surface as it moves farther from the Sun.

New Horizons Mission

  • NASA's New Horizons mission studied Pluto and other Kuiper Belt Objects through a planned flyby.

Arguments for why Pluto is not a planet

  • Astronomers have discovered other objects in the Kuiper belt that are similar in size or even larger than Pluto
  • Pluto has an icy composition, more like comets

What does Pluto most resemble?

  • a comet

Cosmic Collisions: Small Bodies Versus the Planets

  • Goals:
    • Determine if a major impact has ever been witnessed.
    • Assess whether an impact caused the extinction of the dinosaurs.
    • Evaluate if the impact threat is a real danger or media hype.
    • Explain how other planets influence impact rates and life on Earth.

Witnessed Major Impact

  • Comet Shoemaker-Levy 9 (SL9) impacted Jupiter in 1994, serving as a reminder of ongoing catastrophic collisions.
  • Tidal forces broke the comet apart during a previous encounter with Jupiter.

Impact Site

  • Crater chain on Callisto, likely from a comet fragmented by tidal forces.

Mass Extinctions

  • The fossil record indicates occasional significant drops in species diversity, known as mass extinctions.
  • The most recent mass extinction occurred 65 million years ago, marking the end of the dinosaurs.

Iridium Layer

  • Iridium is rare in Earth's surface rocks but common in meteorites.
  • Luis and Walter Alvarez discovered a worldwide iridium layer dating back 65 million years ago, likely from a meteorite impact.
  • Dinosaur fossils are found below this layer.

Consequences of an Impact

  • A 10 km meteorite impact would eject large amounts of debris into the atmosphere.
  • This debris would reduce the amount of sunlight reaching Earth's surface, potentially leading to climate change and mass extinction.

Likely Impact Site

  • Geologists identified a large subsurface crater in Mexico, approximately 65 million years old, as a potential impact site.

Evidence of an Impact and role in the extinction of the dinosaurs

  • Scientists found a worldwide layer containing iridium, laid down 65 million years ago. All dinosaur fossils lie below this layer.

Facts About Impacts

  • Asteroids and comets have impacted Earth.
  • A major impact is inevitable.
  • Major impacts are rare.
    • Extinction-level events occur on million-year timescales.
    • Major damage events occur on tens to hundreds of years timescales.

Tunguska Event

  • On June 30, 1908, a ~40-meter object disintegrated and exploded in the atmosphere over Tunguska, Siberia.

Meteor Crater

  • A 50-meter object created Meteor Crater in Arizona 50,000 years ago.

Frequency of Impacts

  • Small impacts occur almost daily.
  • Impacts large enough to cause mass extinctions occur millions of years apart.

Asteroid Deflection

  • Deflecting an asteroid is more feasible with years of advance warning.
  • Controlled deflection is crucial because breaking a large asteroid into smaller ones is not helpful.
  • Killer comets provide less advance warning.
  • NASA DART mission (Double Asteroid Redirection Test)

Influence of Jovian Planets

  • Jupiter has directed some comets toward Earth but has ejected many more into the Oort cloud.
  • The gravity of Jovian planets, especially Jupiter, can redirect comets.