homework #4 has been posted, due tuesday, oct. 13, 11 pm
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Homework #4 has been posted, due Tuesday, Oct. 13, 11 pm. 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!. - PowerPoint PPT PresentationTRANSCRIPT
Building the Planets. ICOLLAPSE 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!
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
Building the Planets. III Accretion
Accretion is growing by colliding and stickingThe 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
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
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).
To aid in our search for life and suitable environments, we will be examining various timescales of importance, e.g.,
How old is the Earth
How long did it take for the Earth to develop an oxygen atmosphere
How long did it take for life to form on Earth
Many others…
Recall that some isotopes of an element are unstable and will decay into another element.
Parent: the atoms of the original unstable isotope
Daughter: atoms of the element that results from the decay of the parent
Parent decays into daughter
Half-Life: the Time it Takes for Half of the Original (Parent) Atoms in a Sample to Decay to a "Daughter"
Product
Examples of radioactive isotopes useful in dating
Parent Daughter Half Change in...
Carbon-14 Nitrogen-14 5730 years
Uranium-235 Lead-207 704 million years
Uranium-238 Lead-206 4,470 million years
Potassium-40 Argon-40 1,280 million years
Thorium-232 Lead-208 14,010 million years
Rubidium-87 Strontium-87 48,800 million years
Life depends critically on environment. We will examine how life-friendly
environments can form in the universe.
TemperatureLiquids (particularly H2O) Sources of EnergyChemical environmentRadiation environment
Fundamentals:
What determines the environments of terrestrial-like planets? A look at:
(much of what follows also has applications to Jovian moons).
interiors surfaces
atmospheres
Terrestrial planets are mostly made of rocky materials (with some metals) that can deform and flow.
Likewise, the larger moons of the Jovian planets are made largely of icy materials (with some rocks and metals) that can deform and flow.
The ability to deform and flow leads every object exceeding approximately 500 km in diameter to become spherical under the influence of gravity.
Early in their existence, the Terrestrial planets and the large moons had an extended period when they were mostly molten.
The heating that led to this condition was caused by impacts, where the kinetic energy of the impacting material was converted to thermal energy.
Today, the interiors of planets are heated mainly by radioactive decay.
Differentiation – the process by which gravity separates materials according to their densities
Denser materials sink, less dense material “float” towards top
Terrestrial planets and many large moon had an extended period where their interiors were “molten”.
During this time, denser material sank towards center of planet while less dense material “floated” towards top
Terrestrial planets have metallic cores (which may or may not be molten) & rocky mantles
Earth (solid inner, molten outer core)
Mercury (solid core)Earth’s interior structure
The Lithosphere…
Layer of rigid rock (crust plus upper mantle) that floats on softer (mantle) rock below
While interior rock is mostly solid, at high pressures stresses can cause rock to deform and flow (think of silly putty)
This is why we have spherical planets/moons
The interiors of the terrestrial planets slowly cool as their heat escapes.
Interior cooling gradually makes the lithosphere thicker and moves molten rocks deeper.
Larger planets take longer to cool, and thus:
1) retain molten cores longer
2) have thinner (weaker) lithospheres
The stronger (thicker) the lithosphere, the less geological activity the planet exhibits.
Planets with cooler interiors have thicker lithospheres.
lithospheres of the Terrestrial planets:
Geological activity is driven by the thermal energy of the interior of the planet/moon
Earth has lots of geological activity today, as does Venus. Mars, Mercury and the Moon have little to no geological activity (today)
This has important repercussions for life:1) Outgassing produces atmosphere2) Magnetic fields (need molten cores)
protect planet surface from high energy particles from a stellar wind.
Larger planets stay hot longer.
Earth and Venus (larger) have continued to cool over the lifetime of the solar system thin lithosphere, lots of geological activity
Mercury, Mars and Moon (smaller) have cooled earlier thicker lithospheres, little to no geological activity
Initially, accretion provided the dominant source of heating.
Very early in a terrestrial planet’s life, it is largely molten (differentiation takes place).
Today, the high temperatures inside the planets are due to residual heat of formation and radioactive decay heating.
Stresses in the lithosphere lead to “geological activity” (e.g., volcanoes, mountains, earthquakes, rifts, …) and, through outgassing, leads to the formation and maintenance of atmospheres.
Cooling of planetary interiors (energy transported from the planetary interior to the surface) creates these stresses
Convection is the main cooling process for planets with warm interiors.
Convection - the transfer of thermal energy in which hot material expands and rises while cooler material contracts and falls (e.g., boiling water).
Side effect of hot interiors - global planetary magnetic fields
Requirements:
• Interior region of electrically conducting fluid (e.g., molten iron, salty water)
• Convection in this fluid layer
• “rapid” rotation of planet/moon
Earth fits requirements
Venus rotates too slowly
Mercury, Mars & the Moon lack molten metallic cores
Sun has strong field
Planetary Surfaces4 major processes affect planetary surfaces:
Impact cratering – from collisions with asteroids and comets
Volcanism – eruption of molten rocks
Tectonics – disruption of a planet's surface by internal stresses
Erosion – wearing down or building up geological feature by wind, water, ice, etc.
Impact Cratering: The most common geological process shaping the surfaces of rigid objects in the solar system (Terrestrial
planets, moon, asteroids)
Erosion: the breakdown and transport of rocks and soil by an atmosphere.
Wind, rain, rivers, glaciers contribute to erosion.
Erosion can build new formations: sand dunes, river deltas, deep valleys).
Erosion is significant only on planets with substantial atmospheres.
Tectonics: refers to the action of internal forces and stresses on the lithosphere to create surface features.
Tectonics can only occur on planets or moons with convection in the mantle
Earth & Venus Jupiter’s moons Europa & Ganymede?
Tectonics…
•raises mountains
•creates huge valleys (rifts) and cliffs
•creates new crust
•moves large segments of the lithosphere (plate tectonics)
divergent plate boundary (plates move away from each other).
Atlantic Ocean
Great Rift Valley in Africa
Valles Marineris (Mars)
convergent plate boundary with subduction : plates move towards each other & one slides beneath the other.
Nazca plate being subducted under the South American plate to form the Andes Mountain Chain.
Island arc system
convergent plate boundary without subduction : plates move towards each other and compress.
Formation of Himalayas.