Introduction to the Solar System Chapter 6

The Solar System Ingredients?

The Solar System Ingredients? 8 Planets + a few “minor planets” 1 Star: the Sun 8 Planets + a few “minor planets” 126 moons around these planets Asteroids, meteoroids, comets A lot of nearly empty space

Questions What percentage of the total mass of the solar system does the Sun contribute? How is the solar system laid out in space? Spacing between planets? Orbital directions?

Mass in Solar System Sun 99.8% Jupiter 0.1% Comets 0.05% All Other Planets 0.04% Earth 0.0003%

Sun, Planets and Moon to scale Sun accounts for 99.9% of solar system mass!

Solar System Temperatures Planet Distance Temperature (top of atmosphere) Mercury 0.38 AU 450 K Venus 0.72 AU 330 K Earth 1.00 AU 280 K Mars 1.52 AU 230 K Jupiter 5.20 AU 120 K Saturn 9.54 AU 90 K Uranus 19.22 AU 60 K Neptune 30.06 AU 50 K Pluto 39.5 AU 40 K 350 F 45 F -390 F

Comparative Planetology Categorize planets by properties Compare similarities and differences Ask: What physical processes can explain these properties?

6.2 Planetary Properties 63 60 13

6.2 Planetary Properties Distance from Sun known by Kepler’s laws Orbital period can be observed Radius known from angular size Masses from Newton’s laws Rotation period from observations Density can be calculated knowing radius and mass

6.3 The Overall Layout of the Solar System All orbits paths are close to the ecliptic plane Pluto’s orbit does not (17° tilt)

Planet Orbits

Planet Orbits Orbits aligned in same plane (the ecliptic) Explains why planets always found in Zodiac Pluto’s orbit tipped the most (17 degrees) All planets orbit Sun counter-clockwise Planets rotate counter-clockwise except Venus Rotation axis roughly perpendicular to orbit except Uranus and Pluto

The Terrestrial Planets

Terrestrial Planets Terrestrial = Earth-like Small, low mass Mercury Venus Earth (and Moon) Mars Small, low mass No large moons (except Earth) Mars has two small ones… Close to Sun

Terrestrial Planets Rocky Surface High density (3-5 gm/cm3) (water = 1 gm/cm3) Geologic Activity (volcanoes, continental drift) Present on larger planets (Earth and Venus) Absent on smaller planets (Moon, Mercury, and Mars) Atmosphere Little hydrogen and helium Mostly carbon dioxide (Venus and Mars) or nitrogen (Earth) Smaller planets have no atmosphere (Mercury, Moon)

The Kuiper Belt Objects Origin of Pluto Large member of a class of objects in the outer reaches of the Solar System: The Kuiper Belt Objects 100's found since 1992. Orbits tend to be more tilted, like Pluto's. Leftover planetesimals from Solar System formation?

Asteroids Mars The Asteroid Belt

Asteroids Small rocky bodies Asteroid Belt High density (3-5 gm/cm3) Usually not round Primitive composition (oldest bodies in solar system) Asteroid Belt Found between Mars and Jupiter Probably a failed planet?

Asteroid Belt Perhaps a planet was going to form there. But Jupiter's strong gravity disrupted the planetesimals' orbits, ejecting them out of Solar System. The Belt is the few left behind. And Finally . . . Remaining gas swept out by intense period of solar wind activity.

The Jovian Planets

Jovian Planets Jovian = Jupiter-like Large, massive Many moons Saturn Uranus Neptune Large, massive Many moons Far from Sun

Jovian Planets Low density (1 gm/cm3) No obvious surface Atmosphere Mostly hydrogen and helium Other gases (methane, ammonia) may form ices

The Outer Solar System Comets Kuiper Belt and Oort Cloud

The Outer Solar System Pluto Kuiper Belt Oort Cloud Small, icy Low density Little or no atmosphere Kuiper Belt Small icy bodies Is Pluto really the largest Kuiper Belt object? Distributed in disk-like geometry (in plane of solar system) Distance: 100 - 10,000 AU Oort Cloud Distributed spherically around solar system Distance: 10,000 - 100,000 AU

Let’s consider a scale model of the Solar System!

6.4 Terrestrial and Jovian Planets Relative sizes of the Sun & Planets It would take 109 Earths to span the Sun!

6.4 Terrestrial and Jovian Planets Terrestrial planets: Mercury, Venus, Earth, Mars Jovian planets: Jupiter, Saturn, Uranus, Neptune Pluto is neither but a new class called the Dwarf planets

6.4 Terrestrial and Jovian Planets Differences (Comparative Planetology) between the terrestrial planets: Atmospheres and surface conditions are very dissimilar Only Earth has oxygen in atmosphere and liquid water on surface Earth and Mars rotate at about the same rate; Venus and Mercury are much slower, and Venus rotates in the opposite direction Earth and Mars have moons; Mercury and Venus don’t Earth and Mercury have magnetic fields; Venus and Mars don’t

The image at right shows a picture of the Sun. The dark spots located on this image are sunspots. How does the size of Earth compare to the size of the sunspot that is identified on the right side of the image of Sun? A) Earth and the sunspot are about the same size. B) The sunspot is much larger than Earth. C) The sunspot is much smaller than Earth. Sunspot

If you were constructing a scale model of the solar system that used a Sun that was the size of a basketball (approximately 12 inches in diameter), which of the following lengths would most closely approximate the scaled distance between Earth and the Sun? A) 3 feet (length of an outstretched arm) B) 10 feet (height of a basketball goal) C) 100 feet (height of an 10 story building) D) 300 feet (length of a football field)

Questions What are some of the smaller objects (or debris) found in the solar system? What information do they contain that the planets and moons do not? (Hint: What effects do erosion, geological activity, vulcanism, etc. have on a planet?)

Questions What are some of the smaller objects (or debris) found in the solar system? Comets, asteroids, meteoroids What information do they contain that the planets and moons do not? Solar system debris is unevolved => gives direct evidence of conditions during solar system formation!

Solar System Debris Comets Short Period Comets Long Period Comets Comet Halley (1986) Comet Hale-Bopp (1997) Short Period Comets Long Period Comets 50-200 year orbits Orbits prograde, close to plane of Solar System Originate in Kuiper Belt Few times 105 or 106 year orbits Orbits have random orientations and large ellipticities Originate in Oort Cloud

Oort Cloud is a huge, roughly spherical reservoir of comets surrounding the Solar System. ~108 objects? A passing star may redirect Oort cloud objects, creating long period comets. Kuiper Belt object can be redirected by Neptune, creating a short-period comet.

Question What causes the tail of a comet? (Hint: The tail always points directly away from the sun.)

Comet Structure Nucleus: ~10 km ball of ice, dust Coma: cloud of gas and dust around nucleus (~106 km across) Tail: Always points away from Sun. Coma and tail due to gas and dust removed from nucleus by the Solar Wind. Far from Sun, comet is a nucleus only.

Comet Trajectory

Meteor Showers Comets break up when near Sun - solar wind, evaporation, tidal force. e.g. Halley loses 10 tons/sec when near Sun. Will be destroyed in 40,000 years. Debris spreads out along comet orbit. Intersection of orbits => meteor shower

How did the Solar System Form? What must be explained? Solar system is very flat. Planetary orbits are nearly circular. Almost all moons and planets (and Sun) rotate and revolve in the same direction. Planets are isolated in space. Terrestrial - Jovian planet distinction. Leftover junk (comets and asteroids).

Solar Nebula Start with rotating cloud of gas and dust Collapses because of gravity spins faster flattens into disk-shape gets hotter Sun forms in center Temperature decreases outward As nebula cools, gas condenses Forms solid particles (dust grains)

Nebular Theory Nebula: Cloud of interstellar dust and gas about a light- year across Condensing cloud heats up - star forms at center But why is solar system flat? Conservation of Angular Momentum! Ang. Mom. = mass x rotation speed x “size”

Conservation of angular momentum (Demo)

So, as nebula contracted it rotated faster. It became a flattened disk, like a pizza crust. (Centrifugal hoops demo)

But, clumps in rotating gas tend to disperse. Need modified theory. Solar Nebula: 98% of mass is gas 2% in dust grains Condensation theory: 1) Dust grains act as "condensation nuclei. Also radiate heat => help to cool gas => faster gravitational collapse. 2) Accretion: Clumps collide and stick 3) Gravity-enhanced accretion: objects now have significant gravity => faster growth

Forming Planets Dust grains stick together Grow into planetesimals form rocks Grow into planetesimals some still survive today asteroids comets Larger planetesimals attract smaller ones (gravity) Planetesimals accrete form protoplanets / planet cores initially cold Collisions become violent heating melts protoplanet differentiation occurs

Forming Jovian Planets Snow line Location beyond which ices form Building blocks (solids) both silicates and ices Protoplanets / planet core grew larger gravity captured hydrogen & helium composition similar to Sun gaseous accretion disk forms around planet Moons form in disk around planet

Evolution of the Solar System Collisions dominate early-on produces early heavy bombardment comets collide with terrestrial planets Deposit volatiles that form atmosphere (water, carbon dioxide, etc.) Planets sweep up / throw out remaining planetesimals Ones thrown out: Oort cloud Ones that remain: Comets (Kuiper belt) Asteroids (asteroid belt)

Planetary Ejection

Planetary Evolution - Geological Internal heating leads to geologic activity volcanism, tectonics active worlds As core cools & solidifies, activity slows, eventually stops e.g. Moon Earth, Venus large enough to still be active

Planetary Evolution - Atmosphere Atmosphere formed by gases escaping from interior impacts of comets (volatile-rich debris) Fate of water depended on temperature (distance from Sun) Atmospheres changed chemically over time Life on Earth substantially changed the atmosphere