Terrestrial Worlds: Interiors and Impact Cratering

1. There will be a review session next Tuesday in the lab at 12:00 PM. I will be available by appointment anytime between now and the final (December 16). Mr. Chang will also be available during his normal office hours and by appointment. We are also available by phone or email.

2. Inside the Terrestrial Worlds After they formed, the molten planets differentiated into three zones: - core - made of metals - mantle - made of dense rock - crust - made of less dense rock Most of Earth’s interior is rock - only narrow region of upper mantle is molten rock - where lava comes from Interior layers also categorized by strength of rock which depends on composition, temperature, and surrounding pressure. Weaker rock can slowly deform and flow over millions of years. Why asteroids are irregularly shaped - weak gravity unable to overcome rigidity of rock. Gravity of larger world can overcome strength of solid rock, shaping it into a sphere - will shape anything over about 500 km in diameter into a sphere in about 1 billion years Lithosphere - the rigid, outer layer of crust and part of the mantle which does not deform easily - “floats” on softer rock beneath.

3. Inside the Terrestrial Worlds

4. Comparison of Terrestrial World Interiors inactive geology - the cores of Mercury, mars and the Moon have long since cooled and solidified active geology- Earth and Venus(?) still have molten cores

5. Impact Cratering • objects hit planet at 10 – 70 km/s (30,000 - 250,000 km/hr) • solid rock is vaporized • a crater about 10 times the size of the object and one to two times as deep is excavated

6. matter is ejected in all directions - craters are circular large craters have a central peak - like the way water rebounds in the center when you drop a pebble in it Impact Cratering

7. Production of a Crater Animation

9. Counting Craters to find Surface Age Lunar highlands - crowded areas of cratering - rocks date to 4.4 billion years Lunar maria - huge impact basins filled in by lava flow - relatively few craters - rocks date to 3 - 3.9 billion years Heavy bombardment must have subsided very early in solar system history Cratering rate decreased as Solar Systems aged. The older the surface, the more craters are present.

10. History of Cratering Animation

11. Craters on Mars Crater shapes reveal surface conditions and history

12. Comet Shoemaker-Levy 9 Ripped apart by tidal forces to create “string of pearls”. Impacted Jupiter in 1994. Created massive fireballs of hot gas rising thousands of miles above impact sites. Scars larger than Earth lasted for months.

13. One by one, each fragment collided with Jupiter in July 1994. - infrared cameras observed hot plumes ejected from the planet - material from deep inside Jupiter was ejected, and fell… left dark spots Such impacts probably occur on Jupiter once every 1,000 years. Comet Shoemaker-Levy 9 This was a reminder to us that impacts still occur in the present!!

14. Something similar had happened to Callisto. This crater chain is evidence that a string of nuclei once impacted it. Comet Shoemaker-Levy 9

15. We know that larger objects have impacted Earth - Meteor Crater in northern Arizona - caused by a 50-meter asteroid - impact occurred 50,000 years ago Impacts and Mass Extinctions on Earth 65 million years ago, many species, including dinosaurs, disappeared from earth Sedimentary rock layer from that time shows: - Iridium, Osmium, Platinum - grains of “shocked quartz” - spherical rock droplets - soot from forest fires

16. Elements like Iridium, rare on Earth, are found in meteorites. Shocked quartz, found at Meteor Crater, forms in impacts. Rock droplets would form from molten rock “rain.” Forest fires would ensue from this hot rain. All this evidence would imply that Earth was struck by an asteroid 65 million years ago. Impacts and Mass Extinctions on Earth In 1991, a 65 million year old impact crater was found on the coast of Mexico. - 200 km in diameter - implies an asteroid size of about 10 km across - called the Chicxulub crater

17. Impacts and Mass Extinctions on Earth We have a plausible scenario of how the impact led to mass extinction. - debris in atmosphere blocks sunlight; plants die…animals starve - poisonous gases form in atmosphere

18. Could it happen again? This chart shows how frequently objects of various sizes will impact Earth. The odds of a large impact are small … but not zero!

19. The Tunguska Asteroid In !908, an asteroid estimated to be about 40 m across exploded over Tunguska, Russia - atmospheric friction caused it to explode before it hit the ground • Entire forests were flattened and set of fire • Knocked over people, tents, furniture over 200 km away • - Seismic disturbances recorded 100 km away • - Atmospheric pressure fluctuations detected 400 km away • - Energy equivalent of several atomic bombs released

20. Underground, molten rock, called magma, breaks through crack in the lithosphere. Trapped gases are released (outgassing): - H2O, CO2, N2 Viscosity (thickness) of lava (typically basalt) determines type of volcano Volcanism Low viscosity - flat lava plains (maria on the moon) High viscosity - tall, steep stratovolcanoes Medium viscosity - shallow-sloped shield volcanoes - can be tall but not very steep

21. Tectonics - the action of internal forces and stresses on lithosphere Convection cells in the mantle cause both: - compression in lithosphere - mountains are produced - extension in lithosphere - valleys are produced Tectonics

22. Plate Tectonics Continuing stress of mantle convection fractured Earth’s lithosphere into more than a dozen plates Plate tectonics - process in which these plates move over, under, and around each other - unique to Earth Plate Tectonics on Earth Animation

23. movement of rock by ice, liquid, or gas - valleys shaped by glaciers - Yosemite Valley - canyons carved by rivers - Grand Canyon, Rio Grande - sand blown by wind - Painted Desert of Arizona erosion not only wears down features, it also builds them: - sand dunes - river deltas - sedimentary rock - layers of piled sediments from erosion Erosion

24. Erosion

25. Erosion

26. Erosion

27. impact cratering - # of impacts same for all planets - larger planets erase more craters volcanism and tectonics - requires interior heat - retained longer by large planets erosion - requires an atmosphere - large size for volcanic outgassing - moderate distance from Sun - fast rotation needed for wind How Planetary Properties Affect each Process

28. The Moon highlands older surface more craters mare younger surface 3 – 4 billion yrs fewer craters dark basalt heavily cratered, no atmosphere, geologically inactive

29. Formation of the Maria Several large impacts made huge crater basins. - left cracks in lithosphere below - mantle was not molten at this time - heat from differentiation and acccretion had already leaked away - at a later time, radioactivity heated up interior and molten basalt leaked through the cracks (heat from radioactivity has been lost - the Moon now has a cold interior) This “runny” lava filled in the basins.

30. Tidal Heating On Jupiter’s Moon’s Gravitational tidal heating keeps the interiors of the three inner moons of Jupiter hot Moons have elliptical orbit and synchonous rotation • as Ganymede completes one orbit, Europa completes exactly two orbits, and Io completes exactly four orbits - moons periodically line up -causes orbital ellipticity. • tidal bulges are constantly being flexed in different directions - generates friction inside

31. Io Jupiter’s tidal forces flex Io like a ball of silly putty. - friction generates heat - interior of Io is molten Volcanoes erupt frequently. - sulfur in the lava accounts for yellow color - surface ice vaporizes and jets away Evidence of tectonics and impact cratering is covered.

32. Volcanic Plumes

33. Lava fountain - active lava hot enough to cause "bleeding" in Galileo's camera - overloading of camera by the brightness of the target Newly erupted hot lava flow. Dark, "L"-shaped lava flow marks the location of the November 1999 eruption.

34. Gas and Dust Plume A broad plume of gas and dust about 80 km high above a lava flow

35. “Movie” (two frames) of the Tvashtar volcano on Io taken by New Horizon’s spacecraft on the way to Pluto

36. Europa Metallic core, rocky mantle, and a crust made of H2O ice Its fractured surface tells a tale of tectonics. - few impact craters seen - double-ridged cracks - jumbled icebergs These provide photographic evidence of a subsurface ocean. Europa has a magnetic field. - implies liquid salt water beneath the icy crust Where liquid water exists, there could be life!

38. Evidence of a Subsurface ocean Jumbled crust with icebergs and surface cracks with double-ridged pattern - caused by tidal flexing of thick layer of ice on top of liquid ocean of water.

39. Europa Ice Rafts • Thin, disrupted, ice crust in the Conamara region of Europa • - white and blue colors outline areas blanketed by a fine dust of ice particles ejected at the time of formation of the large (26 kilometer in diameter) crater Pwyll 1000 kilometers to the south. • - a few small craters - less than 500 meters in diameter were probably formed at the same time as the blanketing occurred by large, intact, blocks of ice thrown up in the impact explosion that formed Pwyll.

40. Ganymede Largest moon in the Solar System Its surface has 2 types of terrain: - heavily cratered, implies old - long grooves, few craters, implies young like Europa It also has a magnetic field. Could it have subsurface ocean? - case not as strong as Europa’s - tidal heating would be weaker - would need additional heating from radioactive decay

41. Geyser-like eruptions of ice particles and water vapor shoot from the south pole of Saturn's moon, Enceladus