Nebular Theory of Planet Formation

• Several theories address planet formation relative to the Sun, but the current model is the Nebular Theory

Nebular Theory

• Explains how the Sun and planets formed from the gravitational collapse of a solar nebula (interstellar cloud of gas and dust)
• Has supporting evidence of similar composition across the solar system, ordered planetary motion and presence of disks around young stars

Galactic Recycling

• The universe initially consisted of hydrogen and helium
• Heavier elements were formed inside stars
• Elements dispersed via supernovae contributed gas/dust, leading to galactic recycling
• Our solar system originated from one such recycled cloud

Star Formation

• Stars formation occurs in clouds of gas and dust, visible in Hubble and ALMA images
• Young stars are embedded in disks of denser material, representing the early stages of solar systems
• Gaps in these disks often indicate planet formation

From Cloud to Disk

• The solar system started as a dense clump in a larger interstellar cloud that was ~1 light-year across
• Known as the solar nebula, it was spherical, cold, and low-density ~4.5 billion years ago
• Gravity caused this cloud to collapse inward

Key Physical Laws in Nebular Theory

• Conservation of Energy (Heating): Inward-falling gas molecules gain speed as molecules turn kinetic energy into heat, causing the solar nebula to heat up
• Conservation of Angular Momentum (Rotation): slight initial rotation increases dramatically as the cloud contracts, causing rapid spinning of the nebula
• Conservation of Momentum (Flattening): random gas molecule collisions average out, transforming the nebula into a flat, spinning disk to convert elliptical orbits into circular ones

Protoplanetary Disk

• This is the hot, spinning, flat disk of gas, (~200 AU across), that results from collapse
• Planets form from this disk, resulting in orbits in the same plane and direction, and are mostly circular

Material Composition & Temperature

• The ability for material to form solids depends on temperature
• Rock and metals condense in warmer, inner regions
• "Ices" (H₂O, CH₄, NH₃) condense in colder, outer regions beyond the Frost Line
• Hydrogen and helium never condense into solids
• The Frost Line divides the region where ices can and cannot condense

Solid Formation in the Disk

• Solids require low temperatures and high density to form: -The inner solar system has only rocks and metals that condense -The outer solar system has rocks, metals, and ices that condense beyond the frost line

Origin - Nebular Theory summary

• The solar system originated from the gravitational collapse of a gas cloud (solar nebula) ~4.5 billion years ago
• The composition was ~98% H/He and 2% other elements

Orderly Motion - Nebular Theory summary

• The solar nebula heated and rotated faster as it collapsed, then flattened into a disk
• Planetary orbits reflect the origin of movement, same direction and plane, circular paths

Composition

• Inside the frost line, only rocks and metals condense
• Beyond the frost line, rocks, metals, and ices condense, with ices being abundant
• Hydrogen and helium never condense into solids

Formation of Terrestrial Planets

• Begins with dust grains sticking together with static electricity
• Growth from ~1 meter to ~1 km is not well understood
• Planetesimals (>1 km) accrete more mass through collisions, where gravity becomes the dominant force
• Growth becomes more effective for larger bodies, "big get bigger" effect
• Results in few large rocky-metallic bodies (terrestrial planets)

Formation of Jovian Planets

• Rapid ice growth occurs beyond the frost line
• A protoplanet can capture hydrogen and helium gas once it exceeds ~10 Earth masses -Jupiter has ~300 Earth masses of H/He and only ~10 of rock/metal/ice
• In surrounding mini-nebulas, moons form of gas and dust around giant planets

The Sun

• Was was not yet shining during planet formation
• The center was too hot at to permit solids
• Fusion began when it reached ~10 Jupiter masses
• Solar wind and radiation cleared any remaining gas and dust to end planet formation
• Terrestrial planets were too small to capture gas

The Angular Momentum Problem

• The collapsing nebula should have sped up the Sun to rotate more rapidly but slows down over time
• Solution: The Sun's magnetic field and solar wind drag on its rotation to slow it down
• Charged particles (protons/electrons) flow outward and rotate with the Sun

Asteroids and Comets

• Both leftover planetesimals from solar system formation
• Inner solar system = leftover rocky planetesimals called asteroids
• Outer solar system = icy planetesimals = comets.
• Much of material was ejected or accreted, some remained in
• Asteroid Belt: located between Mars and Jupiter, and
• Kuiper Belt: is located beyond Neptune

Heavy Bombardment Period

• Early collisions formed craters and delivered water and organics to Earth.

Exceptions to the Rules

• Earth’s large Moon
• Uranus’s sideways tilt
• Venus’s retrograde (backwards) rotation
• Irregular moon orbits around Jovian planets

Giant Impact Hypothesis (Earth’s Moon)

• A mars-sized object collided with young Earth
• Impact ejected debris; some reaccreted into the Moon and: -Explains size, Moon is too big compared to Earth -Explains composition, less metal, less water -Explains orbital tilt, close to Earth’s orbital plane, not its equator

Other Possible Giant Impacts

• The Tilt of Uranus may be due to massive impact
• Venus could be rotating backwards from a similar collision
• Pluto/Charon and Mercury’s core may also be results of giant impacts

Capture of Small Moons

• Some moons have irregular orbits likely formed by gravitational capture: -Passing planetesimals lost speed due to friction in a planet’s extended mini-nebula

Destiny of Solar Systems

• Restarting star formation would have, -basic structure (inner rocky planets, outer gas giants) highly similar but, -details (number, size, spacing, giant impacts) would differ significantly

Planet Types

• Terrestrial: form in hot regions from rock/metal
• Jovian: form in cold regions, grow large, capture gas

Solar System Age

• ~4.5 billion years ago, relative to: -Milky Way galaxy, being ~13 billion years old and -Universe, at ~14 billion years old
• Planet formation occurred within 50 million years from first solid grains forming planets

Basics of Radioactivity

• Stable nuclei stay the same
• Unstable nuclei transform, via radioactive decay, from one element into another Example: -A neutron (n) can decay into a proton (p) and electron (e) increasing atomic number -A proton can combine with an electron to become a neutron decreasing atomic number
• Overall charge is conserved, atom stays neutral

Parent and Daughter Isotopes

• Original isotope is the parent (e.g., Carbon-14)
• New isotope (after decay) is the daughter (e.g., Nitrogen-14)
• Electrons adjust atom to maintain electrical neutrality

Radiometric Dating

• The measurement of the amount of the parent isotope remaining vs the daughter isotope in a rock
• Concept of "half-life" is relied on -This is the time it takes for half the atoms in a sample to decay -Rate is predictable and constant for each isotope

Half-Life Details

• Individual atom is random and unpredictable
• Large atom numbers have predictable decay (specific time)
• Carbon-14 will decay in 6000 years and is used for organic material and not very old rocks

Potassium-40 Example

• ⁴⁰K decays to Argon-40 (⁴⁰Ar) -⁴⁰K has 19 protons, 21 neutrons -⁴⁰Ar has 18 protons, 22 neutrons
• Potassium is common in rocks but argon is not -Half-life: 1.25 billion years -Present argon must solidify from radioactive decay Radioactive material is useful for dating ancient rocks and meteorites

Radiometric Dating Revelation

• Indicates a solar system age of ~4.5 billion years
• Points to -Oldest grains in meteorites being earliest solid materials and planets that formed quickly after grains appeared

Key Points about Radiometric Dating

• It’s based on measured isotope ratios and known half-lives
• Different isotopes are used for dating materials of variable ages
• Works best on igneous rocks with solidification giving a clear "start time"

How Ages Are Measured (summary)

• Uses the predictable decay of radioactive isotopes (radiometric dating)
• The ratio of parent to daughter reveals the time since rock’s solidification date
• It has shown that the solar system formed about 4.5 billion years ago

Sources of Interior Heat in Terrestrial Planets

• Long-lived isotopes (uranium, thorium, potassium) that emit heat as they decay, are a radiative heat source with gamma rays that heat elements
• Falling Planetismials fall into proto-planets converting gravitational potential energy into thermal energy
• Differentiation of molten young planets with sinking dense metals into a core and lighter rock rising to form the mantle and crust

Terrestrial Worlds’ Internal Structure

• Common for Mercury, Venus, Earth, Mars, and Earth’s Moon Three primary layers (by density and composition): Core: Mostly iron and nickel (densest) Mantle: Rocky, rich in silicates and radioactive elements Crust: Rocky, similar elements to mantle but lower density

Lithosphere Basics

• Consists of a cool, rigid outer layer (includes crust and upper mantle)
• Fractures are due to compressed or stretched
• Material below is slow “flowing” under pressure (not liquid, but ductile)

Big Worlds

• Small bodies (asteroids) are have rigid lithosphere and are irregular shapes
• Gravity overcomes rock strength when hot in Larger worlds making their shapes spherically

Cooling Mechanisms in Terrestrial Planets

• Convection (most effective): Hot material rises, cool material sinks (if mantle is mobile)
• Conduction: Passes heat through solid material without movement (dominates in rigid lithosphere)
• Radiation: Heat is radiated as infrared light from the surface into space (least efficient)

Planetary Size Relation

• Heat content ∝ volume (~r³)
• Heat loss ∝ surface area (~r²)
• Cooling time ∝ radius
• Smaller planets lose heat faster being geologically active less over time
• Larger planets cool more slowly being active longer.

Relationship Between Geological Activity and Interior Heat

• Hot interiors:Thin lithosphere with mantle convection making the world geologically active
• Cold interiors:Thick lithosphere with inactive mantle making the world geologically dead

Processes That Shape Planetary Surfaces

• Impact Cratering: Bowl-shaped craters from asteroid/comet collisions preserved better on bodies with with little erosion (most common in early solar system)
• Volcanism: Molten rock (lava) emerges from interior Different viscosities produce different landforms (Thick lava = steep sided volcanoes)

Surface Deformation

• Tectonics = surface deformation from internal stress driven by mantle convection Creates: earthquakes, rifts, and mountain formations Erosion: Breakdown of surface material Wind: forms dunes Water: forms rivers, valleys, deltas Ice: forms glacial valleys

Surface Feature Changes:

• Can both erode features and build new ones (sedimentary rock)
• Active on Earth and Mars , limited on Venus (thick atmosphere, high temperatures)

Geologic Activity Relationship

• The more geologic activity, the larger the planet because its interior heat is driven by radioactive decay

Surface Shaping

• Impact Cratering: is shaped by External impacts
• Volcanism: shaped by Internal lava flow
• Tectonics: is shaped by Internal stress and movement
• Erosion: is shaped by surface change by wind, water, ice

Dating Surfaces

• Geologic age is the time since major surface features had formed
• Impact and lava events can alter how surface features are viewed
• More craters = older the surface and/or less craters = younger surface

Planet Study via Radiometric

• Apollo moon rocks mission allows correlation between crater count and known age
• Assumes that cratering rates are similar across the solar system -Accuracy is about ±500 million years

Factors Affecting Geological Activity and its Impacts

• Impact cratering impact forming craters from asteroids/comets or impact creating random, heavy early creation and impacts being destructed by volcanism Small planets and heat:

Volcanism and Plate Tectonics Relationship

• Erode activity ends early on in planets with less heat because of being smaller
• The more heat the long the volcanic tectonics will remain

Weathering Effects

• Large planets retain heat more allowing erosion due to retaining atmosphere
• Smaller planets cant retains atmosphere due to high/low heat, losing atmosphere slowing erosion

Moon Surface

• Lunar Highlands: Bright, cratered, ancient
• Lunar Maria: Dark, smooth plains formed by lava
• Heavy 4.5–4.2 billion years ago bombardment Created most craters
• Lava ~3–4 years ago Outflows triggered by internal radioactive heating Escaped from fractures produced dark (iron-rich), flat, circular lava

Inactivity of Moon Surface

• Significant activity has ended
• Little erosion and tectonic activity has occurred

Mercury

• Heavily cratered, some lava, but less surface area
• Iron core bigger, cooled and contracted, made: km long, hundred of km high cliffs

Age From Craters

• More craters makes objects old
• Radiometric dating helps relates lunar samples to age across planets

Different Geological Histories

• Planet size is key, larger = hotter = longer active planet in volcanic/tectonic region
• Atmosphere of planet effects erosion for planet to lose surface change
• Moon history is old, created, and highlands, younger lava with surface fractures and major tectonic periods
• Mercury has cractering and volcanism like moon with giant cliffs and global shrinkage due to core contraction

Geological Feature Factors

• Mars: Surface features tallest volcano. largest canyon, and surface features/erosion via past water use
• Venus: Active tectonic region is few and have heat for slow wind
• Earths lithosphere : are constantly reshaped by moverments via erosion

Planet Characteristics

• Def: layers of gas around the planet
• Terrestrial planets atmospherics are sized thin
• The gravity from moon and mercury are too weak to retain the gases and
• Effects of atmosphere is the regulation of surface pressure and temperature
• It includes presence of water and the absorption of sun light
• The atmosphere is not falling down due to gravity of air molecule, the molecular collisions

Altitude of Earth and Gas

• Density decreases with altitude
• Earth: every 10 km of altitude is 3 factors of dropped density

Greenhouse Effect, Global Heat

• Sunlight is infrared balanced out
• Earth and math predict 25K with average = 288K
• High discrepancy = greenhouse effect

Gases Effects

• Gases (CO2 and H20) Absorb IR radiation Emit in all directions for warming Mars, Venus, atmospheric effect for gas

Atmospheric Properties at Altitude

• Balance with sun for what gases will be on the surface
• Thermosphere, middle and lower range with greenhouse gas

Sky Light

• Air molecules for blue light and what planet they are
• Sky is blue
• Set at the sky is reflected on the sun
• Moon is black and atmosphere
• thin Layer for what planet is like -gas that effect the pressure and temperature

Weather

• Effect are from gasses
• Driven by radiation
• Heat is from angular momentum and air

How Planets Gas Move

• Moves faster to ear side
• Atmosphere the divided into 3 hemisphere that are hot, cloudy, rainy
• Clouds form rising

Causes Effect from

• Radiation ( 30% sun than b4
• Greenhouse that change water and weather and dust
• Effect Seasonal change
• Planets need atmosphere due to change and volcano due to gas

Outgassing

• Gas from CO2 and rock can be reversible with space activity