Formation of the Solar System Comparative Planetology Chapter 5

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Formation of the Solar System Uncovering the origin of the Solar system Early days of the formation Building the planets and other stuff

Comparative Planetology Studying planets as worlds and compare them with each other is called comparative planetology Planetology is applied to any noticeably large object in the system (planets, moons, asteroids, comets) To start we need to seek clues to the origin of the Solar system

What does the solar system look like from far away? NASA Figure Sun, a star, at the center… Inner Planets (Mercury, Venus, Earth, Mars) ~ 1 AU They are all rocky planets… Asteroid Belt, ~ 3 AU Outer Planets (Jupiter, Saturn, Neptune, Uranus), ~ 5-40 AU They are all gaseous planets.. Pluto: odd ball planet, more like a comet… Kuiper Belt ~ 30 to 50 AU Oort Cloud ~ 50,000 AU Where comets come from…

Four Challenges Pattern of Motion All planets orbit the Sun in the same direction (counterclockwise as seen from the Earth’s North Pole) Planet orbits are nearly circular and co-planar Planets rotate in the same direction which they orbit (with one exception) Almost all moons orbit their planets in the direction of the planet rotation The Sun rotates in the direction planets orbit it Explain: Why is this order so good?

Four Challenges Different types of planets Two distinct groups of planets: Terrestrial planets (Mercury, Venus, Earth, Mars) Small, rocky, abundant in metals, few moons Jovian planets (Jupiter, Saturn, Uranus, Neptune) Large, gaseous (made of hydrogen and its compounds), no solid surfaces, have rings, a lot of moons (made of low-density ices and rocks) Explain: Why is the inner and outer Solar system divided so neatly?

Four Challenges Asteroids and Comets Asteroids are small, rocky bodies that orbit the Sun mostly between Mars and Jupiter (the asteroid belt) Almost 9,000 asteroids have been discovered Comets are small and icy bodies that spend most of their lives beyond the orbit of Pluto They occupy 2 regions: Kuiper belt and Oort cloud Explain: The existence and general properties of the large number of these small bodies

Four Challenges Exception to the Rules Mercury and Pluto have larger orbital eccentricities Uranus and Pluto have tilted rotational axes Venus rotates backwards (clockwise) Earth has a large moon Pluto has a moon almost as big as itself

Planetary Nebula or Close Encounter? Historically, two hypothesis were put forward to explain the formation of the solar system…. Gravitational Collapse of Planetary Nebula (Latin for “cloud”) Solar system formed form gravitational collapse of an interstellar cloud or gas Close Encounter (of the Sun with another star) Planets are formed from debris pulled out of the Sun during a close encounter with another star. But, it cannot account for The angular momentum distribution in the solar system, Probability for such encounter is small in our neighborhood…

The Nebular Theory The Solar system was formed from a giant, swirling interstellar cloud of gas and dust The hypothesis was originally suggested by Immanuel Kant (1755) and Pierre-Simon Laplas (~1790) A cloud is called nebula - nebular hypothesis The collapsed piece of cloud that formed our own solar system is called the solar nebula

The Nebular Theory* of Solar System Formation Interstellar Cloud (Nebula) *It is also called the ‘Protoplanet Theory’. Protoplanetary Disk Protosun Gravitational Collapse Metal, Rocks Condensation (gas to solid) Sun Gases, Ice Heating  Fusion Terrestrial Planets Accretion Nebular Capture Jovian Planets Asteroids Leftover Materials Comets Leftover Materials

Gravitational Collapse A Pictorial History Gravitational Collapse Interplanetary Cloud Condensation Accretion Nabular Capture

The Interstellar Clouds The primordial gas after the Big Bang has very low heavy metal content The interstellar clouds that the solar system was built from gas that has gone through several star-gas-star cycles.

The Formation of the Solar System

The Formation of the Solar System Interstellar Clouds By Mass 73% Molecular Hydrogen 25%Atomic Helium 2% Dust (Metals)

ConcepTest! What process heated the early solar nebula as it slowly condensed towards a central protosun? Atoms collided with increasing speed as they fell towards the center of the nebula, creating heat. Heat was released by the formation of molecules such as CO2 and NH3 from atoms, and the condensation of ices from gases such as H2O. Thermonuclear fusion in the protosun was followed by radiative heating of the material in the nebula

The Formation of the Solar System Dense Molecular Cores (“Bok Globules”) ≈ 1 Mo ≈ 50,000 AU ≈ 10 oK

The Formation of the Solar System

The Formation of the Solar System Protoplanetary Disks ≈ 0.01 Mo ≈ 100 AU ≈ 3000 -> 10oK

The Formation of the Solar System

The Formation of the Solar System Condensation Sequence Condensation Temperature Temperature at which Solid Gas T > 50 oK T > 200 oK T > 1000 oK Hydrogen (H) Helium (He) H20, Methane (CH4) CO2, Ammonia (NH3) Iron (Fe), Silicon (Si) Metal Compounds Gas Gas Gas Ice Gas Gas Rock Rock Gas

The Formation of the Solar System Condensation Sequence M V E M J 1000 K 200 K Rock Grains Rock, Ice Grains No Grains

The Formation of the Solar System

The Formation of the Solar System Grain Collisions  Planetesimals (100 km) random 100 km

The Formation of the Solar System Planetesimal Accretion  Rocky Planets and Jovian Cores gravity

The Formation of the Solar System Gas Accumulation ==> H and He onto Jovian Cores gravity Protomoons

ConcepTest! Most comets have orbits that take them well beyond Jupiter. You would expect their composition to be: A) Rocks and heavy elements only B) Rocks and ices only C) Rocks, ices, and hydrogen and helium

The Formation of the Solar System

The Formation of the Solar System Dispersal of Hydrogen and Helium Gas Solar Wind? Jets?

Collapse of the Solar Nebula Three important processes gave form to our system, when it collapsed to a diameter of 200 A.U. The temperature increased as it collapsed The rotation rate increased The nebula flattened into a disk (protoplanetary disk)

Building the Planets Initial composition: 98% hydrogen and helium, and 2% heavier elements (carbon, nitrogen, oxygen, silicon, iron) Condensation: the formation of solid or liquid particles from a cloud of gas Different kinds of planets and satellites were formed out of different condensates

Ingredients of the Solar System Metals : iron, nickel, aluminum, etc. Condense into solid form at 1000 – 1600 K 0.2% of the solar nebula’s mass Rocks : primarily silicon-based minerals Condense at 500 – 1300 K, 0.4% of the mass Hydrogen compounds : methane (CH4), ammonia (HN3), water (H2O) Condense into ices below 150 K, 1.4% of the mass Light gases: hydrogen and helium Never condense in solar nebula; 98% of the mass

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 Inner solar system: only rocks and metals condensed and only small bodies formed

Accretion

Nebular Capture Nebular capture – growth of icy planetesimals by capturing larger amounts of hydrogen and helium It led to the formation of the Jovian planets Numerous moons were formed by the same processes that formed the protoplanetary disk Condensation and accretion created mini solar systems around each Jovian planet

Formation of Gaseous Planets

Leftover Planetesimals Planetesimals remained from the clearing became comets and asteroids They were tugged by the strong gravity of the jovian planets and got more elliptical orbits Rocky leftovers became asteroids Icy leftovers became comets

Asteroids and Comets

Planetary Evolution - Geological Internal heating leads to geological activity: volcanism, tectonics As core cools and solidifies, activity slows, and eventually stops (Moon) Earth and Venus are large enough to be active

Planet Activity

Summary: Collapse of the Solar Nebula Gravitational Collapse Denser region in a interstellar cloud, maybe compressed by shock waves from an exploding supernova, triggers the gravitational collapse. Heating  Prototsun  Sun In-falling materials loses gravitational potential energy, which were converted into kinetic energy. The dense materials collides with each other, causing the gas to heat up. Once the temperature and density gets high enough for nuclear fusion to start, a star is born. Spinning  Smoothing of the random motions Conservation of angular momentum causes the in-falling material to spin faster and faster as they get closer to the center of the collapsing cloud.  demonstration Flattening  Protoplanetary disk. The solar nebular flattened into a flat disk. Collision between clumps of material turns the random, chaotic motion into a orderly rotating disk. This process explains the orderly motion of most of the solar system objects!