Kirkwood Observatory Open House Out-of-class Activity: Every clear Wednesday evening for the rest of the semester. Check website for details.

Homework #7 due Monday, March 29, 2:30 pm

Review session Monday, March 29 Exam #2 Wednesday, March 31 Review session Monday, March 29 7:30 –9:30 pm Morrison Hall 007

Two “flavors” of planets Terrestrial Jovian   Two “flavors” of planets

Terrestrial Planets Jovian Planets Size: small large Location: closer to Sun distant Composition: rocky/metallic gaseous/icy Temperature: hotter cold Rings: none ubiquitous Rotation rate: slow rapid Surface: solid not solid Atmosphere: minimal substantial Moons: few to none many

Planetary orbits: 1) Prograde 2) approximately coplanar 3) approximately circular Rotation: 1) Mostly Prograde 2) Includes sun 3) Includes large moons

Surface features of solid objects in solar system

Craters are ubiquitous

There are lots of smaller objects in the Solar System, some are rocky and some are icy

Asteroids small Rocky Odd-shapes nearly circular orbits orbit planes are near Ecliptic Plane orbits in inner part of solar system

The “asteroid belt”

Asteroids

Mars’ moons and the asteroid Gaspra Deimos Phobos

Comets small nucleus very large tails “dirty snow ball” highly eccentric orbits all orbit inclinations

Comet Wild Halley’s Comet

Comets are found mainly in two regions of the solar system

Kuiper Belt Objects UB313 (1500 miles)

So how do we account for what we see in the solar system?

The Nebular Theory The Solar system was formed from a giant, swirling interstellar cloud of gas and dust (the solar nebula)

The Solar nebula may have been part of a much larger nebula

Protostellar nebulae?

Recipe for solar system formation Start with a giant, swirling interstellar cloud of gas and dust (nebula)

Recipe for solar system formation Start with a giant, swirling interstellar cloud of gas and dust (nebula) Perturb cloud to begin its collapse Sit back and let physics take over

Important physics in forming stars stars & planets Gravity Gas pressure Conservation of Angular Momentum Conservation of Energy Phases of matter

stellar/planetary systems The struggle to form stellar/planetary systems Gravity vs Gas Pressure

Protosolar nebula System initially in pressure balance – no collapse Slowly rotating System initially in pressure balance – no collapse

Gravity seeks to collapse cloud System initially in pressure balance – no collapse

Gas pressure seeks to expand cloud System initially in pressure balance – no collapse

System initially in balance – no collapse gas pressure gravity System initially in balance – no collapse

gas pressure gravity Now, whack the cloud

Perturbation triggers collapse – gravity is winning As collapse proceeds, rotation rate increases

As collapse continues, the rotation rate increases while nebula flattens

Most of this was in gaseous form! Building the Planets. I COLLAPSE OF PROTOSTELLAR CLOUD INTO A ROTATING DISK Composition of disk: 98% hydrogen and helium 2% heavier elements (carbon, nitrogen, oxygen, silicon, iron, etc.). Most of this was in gaseous form!

Collapse of the Solar Nebula As the solar nebula collapsed to a diameter of 200 A.U. (1 LY = 63, 240 AU), the following happened: The temperature increased as it collapsed (conservation of energy; gravitational potential energy becomes thermal energy) The rotation rate increased (conservation of angular momentum) The nebula flattened into a disk (protoplanetary disk) Motions of material in the disk became circularized

Material in the newly formed proto-planetary disk is: Orbiting in approximately the same plane Orbiting in approximately circular orbits This is the situation with the orbits of planets, so now we have the material in the proper location and moving in the proper manner.

Examples of proto-planetary disks

Building the Planets. II There was a range of temperatures in the proto-solar disk, decreasing outwards Condensation: the formation of solid or liquid particles from a cloud of gas (from gas to solid or liquid phase) Different kinds of planets and satellites were formed out of different condensates

Ingredients of the Solar Nebula Metals : Condense into solid form at 1000 – 1600 K iron, nickel, aluminum, etc. ; 0.2% of the solar nebula’s mass Rocks : Condense at 500 – 1300 K primarily silicon-based minerals; 0.4% of the mass Hydrogen compounds : condense into ices below ~ 150 K water (H2O), methane (CH4), ammonia (NH3), along with carbon dioxide (CO2), 1.4% of the mass Light gases (H & He): Never condense in solar nebula hydrogen and helium.; 98% of the mass

The "Frost Line” - Situated near Jupiter Rock & Metals can form anywhere it is cooler than about 1300 K. Carbon grains & ices can only form where the gas is cooler than 300 K. Inner Solar System: Too hot for ices & carbon grains. Outer Solar System: Carbon grains & ice grains form beyond the "frost line".

Building the Planets. III Accretion Accretion is growing by colliding and sticking The growing objects formed by accretion – planetesimals (“pieces of planets”) Small planetesimals came in a variety of shapes, reflected in many small asteroids Large planetesimals (>100 km across) became spherical due to the force of gravity

In the inner solar system (interior to the frost line), planetesimals grew by accretion into the Terrestrial planets. In the outer solar system (exterior to the frost line), accretion was not the final mechanism for planet building – nebular capture followed once accretion of planetesimals built a sufficiently massive protoplanet.

Building the Planets. IV. Nebular Capture Nebular capture – growth of icy planetesimals by capturing larger amounts of hydrogen and helium. Led to the formation of the Jovian planets Numerous moons were formed by the same processes that formed the proto-planetary disk Condensation and accretion created “mini-solar systems” around each Jovian planet

Building the Planets. V. Expulsion of remaining gas

The Solar wind is a flow of charged particles ejected by the Sun in all directions. It was stronger when the Sun was young. The wind swept out a lot of the remaining gas

Building the Planets. VI. Period of Massive Bombardment

Planetesimals remaining after the clearing of the solar nebula became comets and asteroids Rocky leftovers became asteroids Icy leftovers became comets Many of them impacted on objects within the solar system during first few 100 million years (period of massive bombardment - creation of ubiquitous craters).

Brief Summary