Lecture 34 - (Moons) Asteroids & Comets

Saturn’s Moons

• Second largest moon in the solar system with an atmosphere - Titan

  • Titan is the only moon in the solar system with an atmosphere

  • Based off gravity and gases

  • Titan is large enough to gravitationally maintain an atmosphere if gas existed around it

  • Very cold

  • Slower gases move

  • Easier for something with weak gravity to hold onto

• Titan has methane gas to be held onto

• Ganymede has no atmosphere

Enceladus

• Medium-sized moon on Saturn

• Iapetus is largest of medium moons

• Active breaks in the ice sheet that’s actually releasing some of the water from beneath the surface

• Shows some tectonic activity

Medium Moons of Saturn

Ice fountains of Enceladus suggest it may have a subsurface ocean 

• Stuff is being emitted outside of Enceladus, beyond the surface

• Result of what we call a volcano

• Volcano of rock 

• Crack in the ice sheet of Enceladus 

• Bring water beneath the surface up to the surface 

• On the Earth, where there is water, there is ice 

• Enceladus does the work for us - send a spacecraft and fly thru plumes and collect up some of the water 

• Liquid water underneath the surface thru cryovolcanos

Moons of Uranus

• Uranus has tens of moons

• Biggest ones - Miranda, Ariel, Umbriel, Titania, Oberon

• Named after Shakespeare characters

• Uranus discovered by William Herschel

The Largest Moon of Uranus

Titania - smaller than Earth’s moon 

Moons of Neptune

Triton - largest moon

• Neptune is a CAPTURED object

• Triton is captured Kuiper Belt object

• Orbits Neptune in the wrong direction of rotation

• Made of icy bodies

• Similar to Pluto

• Evidence of past geological activity

• Triton is different from most major moons

Rocky Planets Versus Icy Moons

• Rocky planets are driven from geological activity - hot, rocks melt at some certain temperature

  • Harder to maintain geological activity

• Enceladus is geologically active because it has an external source of heat

• Ice heating drives a little bit more of the geological activity

Space Rocks are Heavier

• Space Rocks are heavier

• densest material sank towards the center

Asteroid Facts

• Rocky leftover planetesimals in the early solar system

• Asteroids haven’t changed that much because not geologically active

• Ceres is largest asteroid - dwarf planet

• First 8th planet 

• All the asteroids exist between Mars and Jupiter - handful called Trojan before and after Jupiter

• Hundreds and thousands of them 

• Harder to hit an asteroid than miss it - average distance between asteroids is huge 

• Wouldn’t be anywhere close to the mass of a planet 

• Many of the biggest asteroids - Ceres and Vesta are spherical 

• Majority are lumpy potatoes. 

• Aren’t big enough to have become spherical in the solar system 

Some asteroids have moons 

• Can measure the density of asteroid to know what they are made of 

• Asteroid Ida has a tiny moon named Dactyl

• Moons can be used to calculate density 

DART MISSION

• Have to be near the asteroid in order to view the details

• Send a spacecraft

Which explanation for the belt seems the most plausible?

• The belt is where all the asteroids happened to survive

• Asteroids formed everywhere beyond the frost line - don’t sum up to the size of the planet

Orbital Resonances

• Asteroids in orbital resonances with Jupiter experience periodic nudges

• Nudges move asteroids out of resonant orbits, leaving gaps in the belt

• Jupiter’s gravity stirred up asteroid orbits and prevented their accretion into a planet 

• Jupiter create gravitational barriers that you can’t go in between these regions - asteroids didn’t build up 

• Get rid of Jupiter, no asteroid belt 

Why are there very few asteroids beyond Jupiter’s orbit?

• Ice could form in the outer solar system

• Rock accessed iced —> comets (ice forms in the outer solar system)

Comet Facts

• Formed beyond frost line

• Icy counterparts

• Dirty snowball

• No tails 

• Perpetually frozen 

• Only comets in inner solar system get tails 

• Brahe first measured them 

Halley’s Comet

• Edmund Halley, in 1705, predicted the reappearance of this comet in 1758

  • Predicted a period of about 75.3 years based on Kepler’s laws and Newton’s new understanding of gravity

• Next appearance: 2061

• Comets can reappear again

• Periodic comet

• Once-in-a-lifetime comet

• Period of 76 years

If the period of Halley’s comet is 76 years, what is its average distance from the Sun? 

• P² = a³

• 18 AU 

Sun-Grazing Comet

• Smallest ones can get gravitationally destroyed by the sun

• Comets in the sky - big size - Halley’s comet - not destroyed by the sun

Nucleus of Comet

• Icy and rocky - “dirty snowball”

• Sources of material for comet’s tail

• Characteristic of sun that creates this tail

• Couple of comets we’ve explored

Watching a Comet Become Active 

• Rosetta mission to Comet 67P

• Comets stuck together - not dense objects, gravity is not strong 

What determines the direction of a comet’s tail?

• Tail extends AWAY from the direction of the Sun

• Sunlight or energy from the sun - charged particles, sun smacks into them, pushing it away from the sun - ENTIRELY away from the sun as a result

• Appearance of comets in the sky - no different from any other object

• Rise in the east, set in the west

• Tail always extends away from the sun