Planets.

Planets

Solar System Formation Terrestrial Planets
Terrestrial Planet Atmospheres Terrestrial Planet Characteristics Jovian Planets Trans-Neptunian Objects

Solar System Formation

Solar system – consists of six basic categories:
1. Sun (typical small star) 99.9% of the solar system’s mass 2. Planets and their moons (~ 0.1% of the Sun’s mass) 3. Planetesimals (remainder of the planet-forming debris) Asteroids Comets 4. Small debris (includes meteoroids and micrometeoroids) 5. Dust (small particles) 6. Gas Dominated by hydrogen and helium

Solar system composition
Solar system composition is the contents of gas cloud that collapsed to form the Sun and other nearby stars 70% H 25% He 5% other stuff Ices Dust grains Gases

Early universe composition consisted of only gases
Solar System Early universe composition consisted of only gases 75% H 25% He <0.001% other stuff 3He 2D (deuterium, or heavy hydrogen) 4Li

Solar System Planet formation sequence is actually the sequence f the formation of a star and its disk of gas and dust 1. Gas cloud collapse and rotation 2. Condensation and crystallization 3. Pre-solar heating 4. Planetary core formation 5. Gas dispersion and transparent disk 6. Planet stabilization 7. Outer solar system remnants

Solar system (star) formation
Star (Sun) forms early but requires time to stabilize Rotating disk around star shapes the position of the planets Planets form from debris in disk Planet formation halted by several mechanisms Some planets change position and energy by dynamical interactions Remnants (leftovers) still orbiting the Sun

Solar system segregation (as star forms and stabilizes)
Star isolated from planets that begin forming due to intense energy, gravitational inflow, and particle wind and radiation outflow Small and large clumps that will make up larger planetesimals are created in roughly three zones Interior – hot, dense materials remain Exterior – cold ices and gas remain Distant – bound by gravity but relatively unaffected by solar heat Planets build (accrete) from smaller planetesimal material Interior – rock and metal planets (terrestrial) Exterior – giant gas and ice planets (Jovian) Distant – ice bodies and comets

Final solar system structure
Star dominates entire solar system in mass and energy Planets are created in violent collisions between small and large bodies Segregated due to star/Sun heating Natural accretion of disk material in rotating star formation Planets are common Chaos produces varying size and composition High temperature materials near star Ices and gases beyond “ice line” located between Mars and Jupiter

What was responsible for creating the three planetary zones?
Solar System What was responsible for creating the three planetary zones? The Sun’s T-Tauri stage created a hot zone nearest the star High radiation and high-velocity outward winds swept most of the inner solar system clear of material The remainder fell inward, or was left behind as dense, high-temperature (refractory) debris T-Tauri phase named after the 19th brightest star (Tau) in the constellation Taurus (Tauri) now going through a similar phase as our Sun passed through 4.6 billion years ago This created solid planets in a temperate region of the stable Sun Venus Earth Mars

Solar System Outside this region was the “ice line” sometimes referred to as the frost line or freeze line Temperatures are well below water ice temperature and ices accumulate on the solid bodies Located in asteroid belt between Mars and Jupiter Asteroids inside the ice line have a number of different surface characteristics than those beyond Outside the Jovian planets are even colder objects that have had only minor heating from the Sun over the 4.6 billion year history of the solar system

Surroundings were not depleted by the Sun’s strong T-Tauri winds
Planets Jovian or Giant planets formed with large cores and huge gas atmospheres Surroundings were not depleted by the Sun’s strong T-Tauri winds Composed mostly of hydrogen and helium Inner two (Jupiter, Saturn) are closest to the Sun and composed of 80-95% gas Outer two (Uranus, Neptune) are rich in ices

Planets Body Rock/metal % Ices % Gas %
Terrestrial planets (approximate) 70 30 Asteroids (approximate) Comets, Pluto 15 85 Jupiter (gas planet) 2 5 93 Saturn (gas planet) 6 14 80 Uranus (ice planet) 25 58 17 Neptune (ice planet) 27 62 11

Terrestrial Planets

Terrestrial Planets Inner solar system Terrestrial means “Earth-like”
Composed of rock and metals These are refractory (high-temperature) materials Similar orbit planes with slight orbital eccentricity Smaller rock-metal bodies include many of the asteroids and our Moon

Planetesimals are asteroids and comets
Terrestrial Planets Formation of terrestrials was by violent collisional heating from impacting planetesimals Planetesimals are asteroids and comets Collisional accretion and impact heating produced: Liquid metal cores Liquid rock mantle After major formation events that included the heavy bombardment phase, cooling created a thin, solid crust

Terrestrial Planets In the early formation stages, the crust was still very hot, but allowed gas and liquids (atmosphere and oceans) to begin accumulating Cooling increased from accumulating water primarily from cometary impacts on the surface Liquid mantle remained molten because of its high temperature, and the insulating properties of the light, thin, solid crust

Terrestrial Planets Materials that remained closest to the hot Sun were the most dense and lowest in abundance → a simple hypothesis Hypothesis test: Terrestrial planets farther from the Sun should be larger (closest should be smallest) Terrestrial planets farther from the Sun should be lower in density (closest should be the most dense) Terrestrial planets farther from the Sun should have a smaller core/mantle ratio (closest should be largest)

Mercury Venus Earth Mars Terrestrial Planets
Highest density (except for Earth which is gravitationally compressed) → supports hypothesis Smallest terrestrial planet → supports hypothesis Venus Lower density than Mercury → supports hypothesis Larger than Mercury → supports hypothesis Earth Highest density, but because of gravitational compression Larger than Venus → supports hypothesis Mars Lowest density → supports hypothesis Smaller than Earth x does not support hypothesis

Molten planets differentiate in density
Terrestrial Planets Liquid rock and metal interior allowed segregation of the materials by density (differentiation) Heat from impacts and gravitational compression melts rocky planets and larger moons Molten planets differentiate in density Gravity produces buoyancy that floats lighter materials in the liquid state Liquid metal core is hot and dense, but circulates in a large rotating planet because of low viscosity (low resistance to flow)

Geodynamo magnetic field theory:
Terrestrial Planets Geodynamo magnetic field theory: Circulating, conductive liquid metal core generates magnetic fields First initiated by surrounding magnetic field of Sun Induced electric current in conductive metal core generated by a weak external magnetic field strengthens planet’s internal magnetic field Increased magnetic field further increases current Strong geomagnetic field generated in what is known as the geodynamo process

Terrestrial Planets Magnetic fields Liquid metal core must be significant in size (mass larger than the Moon) Planet must rotate rapidly enough to circulate conductive liquid metal core The exception to this simple geodynamo theory is Mercury which has a very small mass (6% of the Earth's mass) and a slow rotation rate of 59 days Mercury’s small residual magnetic field is most likely induced by the nearby strong magnetic field of the Sun, with a core kept liquid by the Sun's tidal flexing

Terrestrial Planets Crust Thicker crust-to-radius ratio for smaller planets because of their faster cooling Earth's crust about 0.5% of radius Mars' crust 1% of radius Moon's crust about 5% of radius Thin crust can fracture on larger terrestrial planets if the rotation is fast enough to circulate the liquid mantle that supports the thin crust

Terrestrial Planets Crust Hypothesis: Fast rotation and the exchange of heat/mass create a continuous and active surface geology, including: Quakes Volcanoes Mountain and elevated plateaus Crustal motion in broad fragments, or plates (plate tectonics)

Geological activity from rapid rotation and thin crust
Terrestrial Planets Geological activity from rapid rotation and thin crust Earth – yes, fast 24 hr rotation period → supports hypothesis Venus – no, too slow (240 day rotation period) Large mountains and plateaus formed by different mechanisms → almost supports hypothesis Mars - no (rapid rotation, but mass is too small) Mercury - no (slow rotation and too small) → supports hypothesis

Terrestrial Planets Mantle Between the light, thin crust and the dense metal core of the terrestrial planets lies the molten rock mantle Composed of various types of rock under tremendous pressure from the overlying mass Tremendous pressure can force the liquid mantle (called lava when it reaches the surface) through cracks or fissures in the crust

Terrestrial Planets Mantle For rapidly rotating planets, the movement of the dense mantle can stretch and/or fracture the thin overlying crust Any break or weakness in the crust allows upward flow of the liquid rock mantle to the surface = volcanoes

Terrestrial Planets Mantle Semi liquid (plastic) or liquid molten rock between core and crust Transfers heat from core to the surface Thin crust on larger planets allows volcanic activity Thicker crust and/or slow rotation allows broad, long-term volcanic buildup because the crust has little or no motion

Relatively rapid crustal motion and active fracturing
Terrestrial Planets Volcanic shields and plateaus found on both Mars and Venus, but for different reasons Mars - rapid rotation Relatively rapid crustal motion and active fracturing Produced volcanoes on Mars in its early history (1st billion years) Volcanoes are still active on the Earth Venus - slow rotation Large volcanic buildup possible such as the large plateaus on Venus (e.g., Maxwell Montes) and volcanic shields

Terrestrial Planets Lithosphere Part of the Earth's solid crust is connected to the upper portion of the mantle, forming the major surface plates that move in an irregular fashion known as plate tectonics Defining the crust, lithosphere, and mantle is not by the structure but by the type of flow Upper mantle which includes the lithosphere deforms until fracturing Layer below the lithosphere known as the asthenosphere has a semi-liquid (plastic) deformation mode

Terrestrial Planets Earth’s internal structure

Terrestrial Planet Atmospheres

Atmospheres Primary Created during the planets’ formation
Terrestrial Planets Atmospheres Primary Created during the planets’ formation H, He, plus some other atomic gases Dispersed by heat since H and He have low escape velocity Secondary added by: Volcanism, impact deposition from comets, asteroids Comets – N2, O2, CH2 Volcanoes - CO2, SO2, H2S, H2O Long-term atmospheric gas buildup and retention Lighter gases retained according to planetary mass (escape velocity) and planet temperature

Terrestrial Planets Atmospheres Secondary atmospheres remained permanently for Venus and Mars because of their surface inactivity (Mars is small, Venus has slow rotation) H2O lost in Venus' hot atmosphere and from its lower mass (gravity) H2O lost from Mars because of its small mass (gravity)

Terrestrial Planets Atmospheres CO2 absorbed in Earth's rock crust, mantle, and oceans O2 was generated by some H2O breakdown, and early biological life Results of the evolution of the three terrestrial planet atmospheres: Venus 96% CO2, 4% N2 Mars 95% CO2, 3% N2 Earth 78% N2, 21% O2, 0.04% CO2

Terrestrial Planets – Retained Gases
While the four Giant planets can retain any gas in their atmospheres including hydrogen and helium, the Moon and Mercury cannot retain even the heaviest common gas CO2 Gases retained for a specific planet or moon are represented by the diagonal lines that appear below the planet/moon

Terrestrial Planet Characteristics

Mercury

Mercury Mercury which is the closest planet to the Sun is also the smallest terrestrial planet Mass x1023 kg ( MEarth) Radius 2,439 km Mean density g/cm3 Orbital eccentricity Orbit inclination o Semimajor axis km (0.387 AU)

Magnetic field - 0.0033 gauss (1% of Earth)
Mercury Orbit period days Rotation period days (3:2 orbit to rotation resonance period due to nearby Sun) Magnetic field gauss (1% of Earth) Albedo % (Earth = 37%, Moon = 12%) Atmosphere - trace (approx. 1,000 kg total, composed of K, Na, Ar, O, O2, He and other trace gases)

Magnetic field - Not significant (~10-5 x Earth)
Venus Venus is the next planet from the Sun and often called the sister planet of the Earth since it is only 5% smaller Mass x1024 kg (81% of Earth) Radius - 6,052 km (95% of Earth) Mean density g/cm3 Semimajor axis AU Orbit period days Rotation period days Magnetic field - Not significant (~10-5 x Earth) Albedo % (Earth = 37%) Atmosphere bar (92 Earth atmospheres) 96.5% CO2, 3.5% N2

Venus The surface geology of Venus includes volcanic features that include large elevated plateaus, large and small volcanic cones and shields, and pancake-shaped formations of lava Features on the surface of Venus indicate an age of only million years This relatively new surface is thought to be from the periodic remelting of the lower crust Magma would then flow through the cracks, fissures, and craters to cover much of the lower elevations

Radius - 6,378 km (equatorial) Mean density - 5.52 g/cm3
Earth Mass x1024 kg (1/333,000 MSun) Radius - 6,378 km (equatorial) Mean density g/cm3 Semimajor axis AU Orbit period days Rotation period hr 56 min (sidereal) Magnetic field Gauss Albedo % Atmosphere bar (1 atmosphere) % N2, 20.9% O2, 0.9% Ar

Mars Mars has a small CO2 atmosphere, but a distant past that had similar characteristics to Earth including a magnetic field, liquid surface water, and active volcanoes A thick crust and solidification of the interior of the planet halted most of its geological activity after the first billion years as a planet Mars does contain permanent polar ice caps of water and CO2 ice and subsurface ice discovered on recent exploration missions

Radius - 3,397km (equatorial) (0.53 REarth) Mean density - 3.93 g/cm3
Mars Mass x1023 kg (0.11 MEarth) Radius ,397km (equatorial) (0.53 REarth) Mean density g/cm3 Semimajor axis AU Orbit period yr Rotation period hr Magnetic field - No dipolar field, but weak, localized magnetization of the iron-rich crust Albedo % Atmosphere bar % CO2, 2.7% N2, 1.6% Ar

Jovian Planets

A number of common features exist in the Jovian planets, including:
Jovian planets are the giant gas and ice planets formed beyond the terrestrial region A number of common features exist in the Jovian planets, including: Large mass Innermost rock and metal core Outer envelope of hydrogen and helium gas Rapid rotation Strong magnetic fields

Jovian planets began as large ice and rock (planetesimal) cores
Large, dense gas regions accelerated the process These cores were large planets in themselves, and could attract ice and gas unavailable to the terrestrial planets This allowed them to rapidly gain mass because there was more gas and ices in their neighborhood

Jovian Planets

Jovian Planets A comparison of the interiors of the Jovian planets shows a distinct difference between the two largest, Jupiter and Saturn, and the two smallest, Uranus and Neptune The largest two, Jupiter and Saturn, are called giant gas planets because of their principal gas composition, although little is actually in the gas state The two smallest are the giant ice planets Uranus and Neptune Both Uranus and Neptune do contain a significant hydrogen and helium atmosphere, but they are dominated by ices in liquid form The term "ice planet” should not be confused with the dwarf ice bodies and planets similar to Pluto, its moon Charon, and a host of other comet-like planets much smaller These have a distinctly different composition than Uranus and Neptune

Jovian Planets Jupiter

Jovian Planets Jupiter One of the brightest planets in the night sky because of its tremendous size (11 x REarth) and light-colored upper cloud layers (high albedo) First to observe and document Jupiter's features and four nearby moons was Galileo, for which the four Galilean moons are named Rapid rotation (9.93 hr) and high mass (318 x MEarth) generates a huge magnetic field

Jupiter Largest magnetic field of the planets (11-14 Gauss)
Jovian Planets Jupiter Largest magnetic field of the planets (11-14 Gauss) Atmosphere – 90% H, 10% He Interior composed of liquid H, He, and H compounds Deep interior included hydrogen under tremendous pressure and temperature that forces the atoms into a degenerate state – metallic hydrogen

Jovian Planets Jupiter Metallic hydrogen mantle is superconducting which is thought to circulate magnetodynamo currents to flow without resistance and generate Jupiter’s enormous magnetic field Stretches beyond the orbit of Saturn 5 AU away Orbit 11.86 yr 5.2 AU Rotation period – hr (fastest of all planets)

Jupiter Jovian Planets Mass – 318 x MEarth Albedo (reflectance) – 52%
Moons – 63 (as of 2007) Exploration spacecraft Pioneer 10 Pioneer 11 Voyager I Voyager II Ulysses (gravity assist to Sun’s poles) New Horizons (gravity assist to Pluto) Galileo (dedicated orbiter)

Closest, and undergoes highest tidal forces
Jovian Planets Jupiter Four largest moons called the Galilean moons (named after Galileo Galilei) who discovered the moons Io Closest, and undergoes highest tidal forces Tidal flexing heats the interior to a molten state and generates the most geologically active celestial body in the solar system

Jovian Planets Europa Next moon out is covered with an icy crust that overlays a liquid water interior Conditions may be sufficient to support primitive life Weak magnetic field measured by the Galileo spacecraft Indicative of conductive water ocean beneath icy surface Two possible structures shown on the right

Jovian Planets Ganymede Largest moon in the solar system
Weak magnetic field may be made possible by a conductive water layer beneath the icy crust Callisto 3rd largest moon in the solar system Interior 40% ice and 60% rock/metal Also has a conductive water ocean below the icy surface that influences Jupiter’s magnetic field

Jovian Planets Saturn

Saturn Second largest planet in the solar system
Jovian Planets Saturn Second largest planet in the solar system Lowest density planet in the solar system (0.69 g/cm3, water is 1.00 g/cm3) Rapid rotation (10.78 hr) and high mass (95x MEarth) generates a large magnetic field, but significantly smaller than Jupiter’s Orbit 24.86 yr 10.8 AU

Saturn Allbedo – 47% Also contains a metallic hydrogen mantle
Jovian Planets Saturn Allbedo – 47% Also contains a metallic hydrogen mantle Moons – 60 (as of 2007) Largest ring system of the planets Exploration spacecraft Pioneer 11 Voyager I Voyager II Cassini (dedicated orbiter)

Jovian Planets Saturn Hydrogen and helium atmosphere with trace amounts of methane, water, ammonia, and hydrogen sulfide Most prominent feature is its ring system Extends approximately 67,000 km to 480,000 km Saturn's rings composed primarily of small ice particles that have been charged Allows the orientation and position of the particles to change slightly when the Saturnian magnetic field passes through

Saturn and its rings in UV

Saturn’s rings

Saturn’s surroundings

Titan Saturn Saturn’s largest moon is Titan
Larger than both Pluto and Mercury Only moon with a significant atmosphere found in the solar system Similar to several of Jupiter's moons Rocky core overlayed with a mantle composed of water and ammonia ice layers Roughly 2/3 of the moon is rock/silicate/metal that remains warm, or perhaps hot Likely creates conditions for a liquid water layer in the upper ice mantle

Saturn Titan High atmospheric pressure (1.5 x Earth’s) and cold temperature (94 K, -290oF) creates a triple-point environment for methane and ethane on Titan’s surface Solid, liquid, and gas, like water on Earth Ammonia’s low abundance and variable concentration is also like water in Earth’s atmosphere

Located near poles because of seasons
Saturn Titan Rain composed of ethane and methane erode Titan’s surface which form large lakes Located near poles because of seasons Identified by imaging radar on Cassini (see image on right) Lakes are small in comparison to Earth’s

Saturn Titan Alcohol and ammonia rains create river channels and broad, flat basins that appear to be ocean beds Methane is not stable and not retained on Titan Must be replenished, probably from porous sands in “ocean” beds Small, cold volcanoes (cryovolcanoes) possible source of methane & ethane Thick haze that prevents direct observation is due to solar energy producing hydrocarbons from ethane and methane

Saturn Titan Images of Titan’s surface show rocks littering flat plateau at the landing site Origin of rocks likely from cryovolcanoes releasing liquid water that freeze into hard clumps that persist for a very long time Water ice is too cold to sublimate at 94 K Liquid water interior also responsible for a different rotation of its crust compared to its interior (measured accurately during Cassini flybys) Close similarity between Titan’s and Earth’s geology and atmosphere

Saturn Enceladus Smaller and more distant than Titan is Enceladus which has a tortured, icy outer surface Located in the dense region of the Saturn's diffuse outer E-ring Two different surface characteristics in northern and southern hemispheres Sparsely-cratered northern region Young, uncratered southerly zone

Saturn Enceladus Tidal heating of the interior by Saturn generates liquid cryovolcanoes near the south polar region which spew frozen particles into Saturn’s E-ring Light-colored surface indicative of active ice formation (celestial snow) Water ice from Enceladus contributes significantly to Saturn’s E-ring

Saturn Enceladus Tidal heating by Saturn generates continual cryovolcanism Discovered during Cassini flybys with backlighting from the Sun Source of tidal heating? Tidal flexing requires slightly elliptical orbit, but tides gradually reduce eccentricity (Enceladus’ orbit should be circular by now) Enceladus’ eccentric orbit due to perturbation from 2:1 orbital resonance with Saturn’s moon Dione

Jovian Planets Uranus

Uranus One of two Jovian giant ice planets
Jovian Planets Uranus One of two Jovian giant ice planets Discovered in 1781 by William Herschel First planet discovered by a telescope Previous discoveries were of large asteroids (minor planets Ceres and Vesta were the two largest) Rotational axis is offset 98o from the normal

Uranus Jovian Planets Structural model consists of three layers
Rocky core Small in comparison to giant gas planets Ice mantle Contains the majority of the mass Consists of water, ammonia and other volatiles in liquid form Outer gaseous hydrogen/helium envelope is a dull green-blue due to methane

Aligned with equator (nearly vertical)
Jovian Planets Uranus Mass – 14.5 MEarth Orbit 84.32 yr 19.23 AU Moons – 27 (as of 2007) Aligned with equator (nearly vertical) Exploration spacecraft Voyager II

Small ring system discovered in Voyager II data
Jovian Planets Uranus Small ring system discovered in Voyager II data False-color image from Voyager on the right shows circulation bands and the storm regions near the polar band

Jovian Planets Miranda One of the most unusual moons in the solar system is Uranus’ moon Miranda Chaotic features include deep, long canyons 10 times deeper than the Grand Canyon Violent collision between moon during Uranus catastrophic formation likely broke up the original moon and reformed it with jumbled surface features

Jovian Planets Neptune

The largest Jovian ice planet
Jovian Planets Neptune The largest Jovian ice planet Discovered 1846 by French astronomer Urbain Le Verrier Similar to Uranus, the structural model consists of three layers Rocky core Ice mantle Outer gaseous hydrogen/helium envelope

Neptune Mass – 17.2 MEarth Orbit Moons – 13 (as of 2007)
Jovian Planets Neptune Mass – 17.2 MEarth Orbit yr 30.10 AU Moons – 13 (as of 2007) Small ring system discovered in Voyager II data Exploration spacecraft Voyager II

Neptune Lowest temperatures in upper atmosphere of all the planets
Jovian Planets Neptune Lowest temperatures in upper atmosphere of all the planets Highest wind speeds measured at approx. 1,500 mph Circulation bands are visible, along with giant dark spot similar to Jupiter’s and high-altitude clouds travelling at extremely high speeds

Neptune’s largest moon is Triton
Triton’s diameter is 78% of the Earth's moon, but only 28% of its mass Density 2.05 g/cm3 (lunar density is 2.05 g/cm3 Surface shows complex features with few craters which indicates a relatively young surface

Neptune Triton Cryovolcanoes of liquid nitrogen from beneath the ice crust indicate tidal heating and subsurface liquid nitrogen and possibly water oceans Spectral imaging data shows Triton's surface covered with nitrogen ice, water ice, carbon dioxide ice, methane ice, and ammonia ice

Trans-Neptunian Objects

Beyond Neptune Trans-Neptunian objects are small icy bodies scattered by the massive planet Neptune which itself was scattered outward by Jupiter in its early history

Pluto Pluto, the most distant of the original nine planets has the largest moon-planet mass ratio in the solar system Discovered in 1930 by Clyde Tombaugh Pluto’s largest moon Charon was discovered in 1978 The large mass of Charon compared to Pluto makes them more of a binary pair than a planet-moon system (both orbit around a center of mass outside Pluto’s surface)

Pluto Pluto’s two other moons Nix and Hydra are 2 and 3 times the distance of Charon from Pluto, but much smaller (also orbit around the Pluto-Charon barycenter) Exploration spacecraft New Horizons (2006 launch for a July 2015 flyby)

Pluto Mass ×1022 kg (0.2% MEarth) Radius 1,195 km Mean density g/cm3 Orbital eccentricity Orbit inclination o Semimajor axis AU (5.91x109 km) Orbit period yr Rotation period 6d 9h 17m 36s Rotational axis tilt o Magnetic field Unknown Albedo –0.66 Atmosphere nitrogen, methane, and carbon monoxide (surface ice sublimation)

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