1. Due: Noon, Wednesday, Oct. 13
Homework #4 is posted.
Due: Noon, Wednesday, Oct. 13

2. Kirkwood Observatory Open House every clear Wednesday thru Thanksgiving break.
check activities page Kirkwood link for times and dates

3. How do we account for what we see in the solar system?

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

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

6. Protostellar nebulae?

7. stellar/planetary systems
The struggle to form
stellar/planetary systems
Gravity:
Seeks to collapse the cloud
Gas Pressure:
Seeks to expand the cloud

8. 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!

9. Collapse of the Solar Nebula
If cloud size and density exceeds a critical value, nebula collapses:
Temperature increased: Conservation of energy
Rotation rate increased: Conservation of angular momentum
Rotating cloud flattened into a disk: ”protoplanetary” disk
Motions of material in disk became circularized (from collisions)
Top view
Side view

10. Images of protostellar disks
Material in the newly formed proto-planetary disk:
- similar orbital planes
- approximately circular orbits
Images of protostellar disks
Material in this disk will form planets orbiting in the same manner as the material from which they are formed.

11. 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

12. 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

13. Inner Solar System: Too hot for ices & carbon grains.
Outer Solar System: Carbon grains & ices form beyond frost line
Frost line inside orbit of Jupiter

14. 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

15. 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.

16. 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

17. 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

18. Building the Planets. VI. Period of Massive Bombardment

19. 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).

20. Brief
Summary

21. WE NOW UNDERSTAND HOW THE LOCATIONS AVAILABLE FOR LIFE WERE FORMED.
NOW, WHAT ARE THE REQUIREMENTS FOR LIFE?

22. Life depends critically on environment
Life depends critically on environment. We will examine how life-friendly environments can form in the universe.
Fundamentals:
Temperature Liquids (particularly H2O) Sources of Energy Chemical environment Radiation environment

23. interiors surfaces atmospheres
What determines the environments of terrestrial-like planets? A look at: (much of what follows also has applications to Jovian moons).
interiors surfaces atmospheres

24. What accounts for interior, surface, and atmospheric structures?

25. 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.

26. 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.

27. Differentiation – the process by which gravity separates materials according to their densities
Denser materials sink, less dense material “float” towards top

28. Layering of interiors by density due to differentiation

29. 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

30. 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

31. Earth (solid inner, molten outer core)
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

32. Differentiated Jovian moons have rocky cores & icy mantlesIo
Europa
Ganymeade
Callisto

33. 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

34. Larger planets take longer to cool, and thus:
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

35. lithospheres of the Terrestrial planets:
Geological activity is driven by the thermal energy of the interior of the planet/moon
The stronger (thicker) the lithosphere, the less geological activity the planet exhibits.
Planets with cooler interiors have thicker lithospheres.
lithospheres of the Terrestrial planets:

36. Earth has lots of geological activity today, as does Venus
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:
Outgassing produces atmosphere
Magnetic fields (need molten cores) protect planet surface from high energy particles from a stellar wind.

37. 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

38. 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.

39. Stresses in the lithosphere lead to “geological activity” (e. g
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.

40. Convection - the transfer of thermal energy in which hot material expands and rises while cooler material contracts and falls (e.g., boiling water).

41. Convection is the main cooling process for planets with warm interiors.

43. 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

44. Earth fits requirements
Venus rotates too slowly
Mercury, Mars & the Moon lack molten metallic cores
Sun has strong field