Lecture 12 - Solar System and its Formation

Interplanetary Matter

  • Vast space between planets isn't entirely empty.
  • Countless chunks of rocks and ice - cosmic debris from the formation of the Solar System.
  • Sizes range from asteroids to dust grains.
  • Interplanetary space is a good vacuum, though not by interstellar or intergalactic standards.

Asteroids and Meteoroids

  • Have rocky composition (similar to outer layers of terrestrial planets).
  • Meteoroids: less than 100 m across.
  • Asteroids: more than 100 m across.
    • Example: Asteroid Vesta is 500 km across.
  • Most asteroids orbit between Mars and Jupiter in an Asteroid Belt.
  • Total mass of all asteroids/meteoroids is less than the mass of our Moon.
  • They don't play an important role in the dynamics of our Solar System.
  • They are nearly unchanged since the Solar System formed.
  • Provide crucial information about the origin and history of the Solar System.
  • Meteoroids sometimes enter Earth’s atmosphere.
    • They burn up and produce a meteor (streak of light).
    • If they are large enough to not disintegrate, their remnants are called meteorites.
    • Meteorites allow us to study material from the early Solar System directly on Earth.

Kuiper Belt

  • The outer asteroid belt
  • Range of sizes:
    • Less than 10 m to over 1000 km
  • Pluto is the closest large Kuiper Belt object to the Sun
  • Information from the Kuiper Belt will help us understand more about the history of our Solar System

Comets

  • Are icy, with some rocky parts.
  • Similar composition as the moons of the Jovian planets.
  • Originate further than the Kuiper Belt in the Oort Cloud, which extends out to about 1 light-year from the Sun.
  • Have highly elliptical orbits and take a long time to orbit the Sun.
  • Most comets are composed largely of ice and thus tend to be relatively fragile.
  • The ice gets vaporized by the Sun’s radiation as it gets close, creating a long tail pointing away from the Sun.

Spacecraft Exploration of the Solar System

  • Started in the 1960s (first artificial satellite Sputnik: 1957).
  • Many missions types:
    • Flyby: satellite passes by a planet without orbiting.
    • Orbit: satellite orbits a planet or moon.
    • Lander: spacecraft lands on surface, may use probes or a rover that moves around on surface.
    • Manned: humans land on surface.

Spacecraft Exploration of the Moon

  • 12 humans have walked on the surface of the Moon.
  • Apollo 11 (1969) was the first manned mission.
  • Apollo 17 (1972) was the most recent.
  • Brought back rock samples that were crucial in understanding the Moon’s formation.
  • NASA’s Artemis mission plans to return to the moon in 2024.

Spacecraft Exploration of Mercury

  • Mariner 10
    • Launched 1973 (stopped returning data 1975)
    • Orbit passed by Mercury every six months
    • 4000 photographs mapping 45% of surface
  • Messenger mission
    • Orbited Mercury 2011-2015
    • Detailed maps of the surface
    • Studied composition
    • Found Water ice in shadows inside craters!

Spacecraft Exploration of Venus

  • Venus is the most visited planet: 20 spacecraft
  • Soviet Venera Program
    • Late 60s – Early 80s
    • Launched probes on the surface (only lander on Venus)
  • Pioneer Mission (US, 1978)
    • Radar mapping of planet’s surface
  • Magellan orbiter
    • Most recent Venus expedition from the US (1990–1994)
    • High-resolution radar mapping (120 m scale) of 98% of the surface
  • Venus Express (2005-2014)
    • European Space Agency (ESA) mission to study atmosphere

Spacecraft Exploration of Mars

  • Exploration of Mars started in the 1960s (US & USSR)
  • Mariner Program
    • Mariner 4: First flyby, July 1965
    • Mariner 9: included an orbiter, early 1970s
    • Entire surface mapped: Evidence the liquid water existed!
  • Viking Mission, included orbiter and landers
    • Landers arrived in 1976
    • sampled surface chemistry, transmitted data for several years
  • Many missions since Viking:
    • Mars Global Surveyor
    • Mars Pathfinder (Sojourner minirover)
    • Mars Odyssey
    • Mars Express (ESA)
    • Mars Exploration Rover (Spirit and Opportunity).
    • Mars 2020, NASA Perseverance rover
  • Mars 2020 landed on 18 February 2021 on Mars
    • Perseverance Rover searching for signs of ancient microbial life, exploring the past habitability of Mars
  • Ongoing missions to Mars:
    • Mars Reconnaissance Orbiter
      • Launched in 2005, still in orbit
      • Studied climate, geology
    • Curiosity Rover
      • Landed on Mars in 2012, still operational
      • Studied climate, searched for water
    • Mars 2020 Rover:
      • Launch in Summer 2020, arrived in February 2021
      • Will search for signs of habitable conditions on Mars in the distant past, and for signs of past microbial life itself
    • InSight Lander:
      • Landed on Mars in 2018
      • Studying Mars’ interior and sending results back to Earth

Exploration of the Outer Planets

  • Most exploration of outer planets was done by:
    • Pioneer 10, Pioneer 11 (1972 & 1973)
    • Voyager 1, Voyager 2 (1977)
  • Pioneer Mission:
    • Flyby of Jupiter
    • Scientific mission and scout for Voyager mission
    • Pioneer 10 has left the Solar System!
  • Voyager 1: flyby of Jupiter
  • Voyager 2: grand tour of the outer Solar System
    • Flybys of Jupiter, Saturn, Uranus and Neptune
    • Voyager 1 & 2 are the most distant man-made objects from Earth (147 AU and 120 AU)
    • They are still sending and receiving radio signals!

Gravitational Slingshots

  • Gravitational "slingshots" can change the direction of a spacecraft and accelerate it.

Grand Tour of Voyager 2

  • The Grand Tour of the Voyager spacecraft was one of the greatest accomplishments of the Space Age.

Spacecraft Exploration of Jupiter

  • Galileo Mission:
    • Orbited Jupiter, 1989-2003
    • Studied atmosphere
    • Studied Jupiter’s moons (evidence for liquid ocean under Europa’s icy surface)
    • Saw a comet collide with Jupiter in 1994
  • Juno Mission
    • Orbiting Jupiter since 2016

Spacecraft Exploration of Saturn

  • Cassini mission arrived at Saturn in 2004
    • Mission ended in 2017
    • Studied its rings and moons (sent a lander to largest moon, Titan)
    • Has returned many spectacular images

Spacecraft Exploration of the Kuiper Belt

  • New Horizons Mission
    • Probe sent to Pluto and the Kuiper Belt
    • Arrived in 2015, has since moved on to study other Kuiper Belt objects
  • All these observations, satellites, and probes since the late 20th century have greatly improved our understanding of the present-day Solar System
  • They provide a foundation for understanding the origin and history of the solar system

How Did the Solar System Form?

Nebular Contraction Theory:

  • Old idea: 17th century philosopher Rene Descartes
  • Pierre Simon de Laplace: used mathematics (Newton’s Laws) to show that a collapsing gas flattens into a pancake, solar nebula
    1. Cloud of gas and dust contracts due to gravity
      • The collapse is triggered by a perturbation (i.e. a nearby Supernova or collision with another gas cloud)
    2. Conservation of angular momentum means the gas cloud spins faster and faster as its size decreases
      • Conservation of angular momentum says that the product of radius and rotation rate must be constant
        • This is also why figure skaters spin faster as they bring their arms in
        • This is also why planets move faster when at perihelion (from Kepler’s 2nd Law)
    3. Eventually, a small part of the cloud destined to become the solar system came to resemble a gigantic pancake.
      • The large blob at the center ultimately became the Sun.
    4. The planets that formed from the nebula inherited its rotation and flattened shape.
      • The Nebular Contraction Theory doesn’t explain the differences in composition between the planets

Planetary Condensation Theory

  • Variant of nebular theory, favored by most astronomers
  • Sun forms in the centre – temperature varies across solar nebula
  • Dust grains cause material to condense to form the planets (smaller pieces “stick together” like snowflakes condensing in a snowstorm)
  • Temperature at different locations in the solar nebula determines the types of grains that were able to condense
    • hot near sun – only rocky and metallic material can condense (terrestrial planets)
    • cooler regions further out allow gas giants to form
  • Nebular contraction is followed by condensation around dust grains, known to exist in interstellar clouds
  • Accretion then leads to larger and larger clumps
  • Finally, gravitational attraction takes over and planets form

Formation of Terrestrial Planets

  • Terrestrial (rocky) planets formed near Sun, due to high temperature – nothing else could condense there
  • Initially in the inner solar system, many moon-sized planetesimals orbited the Sun
  • Gradually, they collided and coalesced, forming a few large planets in roughly circular orbits.

Formation of Jovian Planets

  • After a few million years, the Sun was in a phase of its evolution called a T Tauri star
    • Strong solar winds from the still-forming Sun expelled the nebular gas
    • Some massive planetesimals in the outer solar system had already accreted gas from the nebula. These became the Jovian planets.
  • Jovian planets:
    • Once they were large enough, may have captured gas from the contracting nebula
    • Or may not have formed from accretion at all, but directly from instabilities in the outer, cool regions of the nebula (Jovian condensation)
    • Detailed information about the cores of Jovian planets should help us distinguish between the two possibilities

Asteroid Belt

  • Orbits mostly between Mars and Jupiter
  • Jupiter’s gravity kept them from condensing into a planet, or accreting onto an existing one
  • Fragments left over from the initial formation of the solar system

Kuiper Belt and Oort Cloud

  • Icy planetesimals far from the Sun were ejected into distant orbits by gravitational interaction with the Jovian planets

Planetesimal Ejection

  • After the giant planets had formed, leftover planetesimals were found throughout the solar system
  • Interactions with Jupiter and Saturn kicked planetesimals out to very large distances (the Kuiper Belt and the Oort cloud)
  • Interactions with Uranus and especially Neptune kicked planetesimals outward, but also deflected many inward to interact with Jupiter and Saturn
  • By the time the planetesimals inside Neptune’s orbit had been ejected (after 100s of millions of years), the orbits of all four giant planets were significantly modified
    • Neptune was affected most and may have moved outward by as much as 10 AU

Timeline of Solar System Formation

  • Jovian planets form by instabilities
  • Rocky grains form
  • Accretion and fragmentation
  • Asteroid belt
  • Terrestrial planets formed
  • Sun forms
  • T Tauri phase
  • Nebular gas ejected
  • Solar nebula