Things That Fall From the Sky

1. Things That Fall From the Sky Comets & Meteors

2. Questions from Before • What is Jupiter made of? • Gas (hydrogen-rich gasses and helium), but in a liquid form. • Can rocks fall from the sky? • Yup.

3. Questions for Today • What is a meteor? • How old is the Earth? • Will we all be killed by a giant asteroid?

4. Meteors: Things That Fall From the Sky

5. Meteors • There are many small chunks of matter orbiting the Sun. • A piece that is in space is a meteoroid. • A piece that burns up in the Earth’s atmosphere is a meteor (a bright streak of light). • A piece that lands on Earth is a meteorite.

6. Meteors • Many “meteor showers” are associated with comets.

8. Dust from Comets • The dust tail contains small particles evaporated from the comet. • These particles remain in orbit about the Sun. • If the Earth passes through the “dust cloud”, then several meteors may be seen.

9. Meteor Showers • During periods of high meteor activity, most of the events appear to come from one spot on the sky.

10. Meteor Showers • During periods of high meteor activity, most of the events appear to come from one spot on the sky. • This point is roughly where the comet’s tail was.

11. Meteor Showers • During periods of high meteor activity, most of the events appear to come from one spot on the sky. • This point is roughly where the comet’s tail was. Dust particles enter the atmosphere and burn up, causing a streak of light.

12. Rocks from Space • Some early cultures were aware that rocks sometimes fell from the sky. These items had great religious value, e.g. the Black Stone of Ka’aba.

13. Rocks from Space • Some early cultures were aware that rocks sometimes fell from the sky. These items had great religious value, e.g. the Black Stone of Ka’aba. • “Enlightened” scientists in the 18th and 19th centuries declared that stones cannot possibly fall from space. It was all primitive superstition.

14. Rocks from Space • Thomas Jefferson said: “It is easier to believe that two Yankee professors [Profs. Silliman and Kingsley of Yale] would lie than that stones would fall from the sky.”

15. Rocks from Space • Thomas Jefferson said: “It is easier to believe that two Yankee professors [Profs. Silliman and Kingsley of Yale] would lie than that stones would fall from the sky.” • Jefferson was wrong: stones do fall from the sky.

16. Rocks from Space • Evidence that rocks fall from space:

17. Rocks from Space • Evidence that rocks fall from space: • There have been eyewitness accounts of impacts.

18. Rocks from Space • Evidence that rocks fall from space: • There have been eyewitness accounts of impacts. • In many cases, the mineral composition of samples indicates the material cannot be native to Earth.

19. Rocks from Space • Evidence that rocks fall from space: • There have been eyewitness accounts of impacts. • In many cases, the mineral composition of samples indicates the material cannot be native to Earth. • Most older samples are iron, most “fresh” samples are stony material.

20. Rocks from Space

21. Where to Find Meteorites • Antarctica is one of the best places to find meteorites on Earth, owing to the high contrast (black rocks on white snow). http://www-curator.jsc.nasa.gov/curator/antmet/program.htm

22. Where to Find Meteorites • Over time, meteorites tend to get concentrated in certain areas because of large-scale ice flows. http://www-curator.jsc.nasa.gov/curator/antmet/program.htm

23. Meteorites • Most older samples are iron.

24. Meteorites • Most older samples are iron. • Iron is dense and not easily weathered.

25. Meteorites • Most older samples are iron. • Iron is dense and not easily weathered. • Most “fresh” samples are composed of stony materials.

26. Meteorites • Most older samples are iron. • Iron is dense and not easily weathered. • Most “fresh” samples are composed of stony materials. • This material is easily weathered and does not last long on the Earth’s surface.

27. Rocks from Space • Why is are meteorites useful?

28. Rocks from Space • Why is are meteorites useful? • They are material samples from outside the Earth that can be analyzed in the laboratory.

29. Rocks from Space • Why is are meteorites useful? • They are material samples from outside the Earth that can be analyzed in the laboratory. • We can measure the age of the solar system by studying meteorites.

30. Radioactive Decay • A chemical element is uniquely determined by the number of protons its nucleus has. For example, hydrogen has 1 proton, carbon has 6 protons, etc.

31. Radioactive Decay • A chemical element is uniquely determined by the number of protons its nucleus has. For example, hydrogen has 1 proton, carbon has 6 protons, etc. • Different isotopes of the same element differ only in their number of neutrons. For example 12C has 6 protons and 6 neutrons and 14C has 8 neutrons and 6 protons.

32. Radioactive Decay • Different isotopes of the same element differ only in their number of neutrons. For example 12C has 6 protons and 6 neutrons and 14C has 8 neutrons and 6 protons. • A radioactive isotope is an isotope prone to spontaneous change.

33. Radioactive Decay • Different isotopes of the same element differ only in their number of neutrons. For example 12C has 6 protons and 6 neutrons and 14C has 8 neutrons and 6 protons. • A radioactive isotope is an isotope prone to spontaneous change. • 14C changes into 14N • 40K changes into 40Ar

34. Radioactive Decay • A radioactive isotope is an isotope prone to spontaneous change. • 14C changes into 14N • 40K changes into 40Ar • The decay rate for a given isotope is fixed and can be measured in the laboratory. The rate is usually given as a “half life”, which is the amount of time required for half of a given sample to decay.

35. Radioactive Decay • The decay rate for a given isotope is fixed and can be measured in the laboratory. The rate is usually given as a “half life”, which is the amount of time required for half of a given sample to decay. • The half life can be as short as a fraction of a second or as long as billions of years.

36. Radioactive Decay • For a given atom, there is a certain probability that it will decay.

37. Radioactive Decay • For a given atom, there is a certain probability that it will decay. • For a large collection of atoms, a well-determined half life emerges from the statistics of a large number of events. Image from Nick Strobel (http://www.astronomynotes.com)

38. Radioactive Decay • Example: the half life of 40K is 1.25 billion years. Suppose we start with 1 kg. • In 1.25 billion years, we have 1/2 kg of 40K and 1/2 kg of 40Ar.

39. Radioactive Decay • Example: the half life of 40K is 1.25 billion years. Suppose we start with 1 kg. • In 1.25 billion years, we have 1/2 kg of 40K and 1/2 kg of 40Ar. • In 2.50 billion years, we have 1/4 kg of 40K and 3/4 kg of 40Ar.

40. Radioactive Decay • Example: the half life of 40K is 1.25 billion years. Suppose we start with 1 kg. • In 1.25 billion years, we have 1/2 kg of 40K and 1/2 kg of 40Ar. • In 2.50 billion years, we have 1/4 kg of 40K and 3/4 kg of 40Ar. • In 3.75 billion years, we have 1/8 kg of 40K and 7/8 kg of 40Ar.

41. Radioactive Decay • My measuring the relative amounts of the radioactive “parent” isotope to the resulting “daughter” isotope in a rock, one can measure the amount of time since the rock sample solidified.

42. Radioactive Decay • My measuring the relative amounts of the radioactive “parent” isotope to the resulting “daughter” isotope in a rock, one can measure the amount of time since the rock sample solidified. • In practice one looks at many parent/daughter combinations, and also looks at stable isotopes of the parent and/or daughter.

43. Radioactive Decay • The oldest rocks on the Earth were solidified about 4 billion years ago.

44. Radioactive Decay • The oldest rocks on the Earth were solidified about 4 billion years ago. • The oldest rocks from the Moon were solidified 4.4 billion years ago.

45. Radioactive Decay • The oldest rocks on the Earth were solidified about 4 billion years ago. • The oldest rocks from the Moon were solidified 4.4 billion years ago. • The oldest meteorites solidified 4.55 billion years ago.

46. Radioactive Decay • The oldest rocks on the Earth were solidified about 4 billion years ago. • The oldest rocks from the Moon were solidified 4.4 billion years ago. • The oldest meteorites solidified 4.55 billion years ago. The Sun and the solar system are about 4.6 billion years old.

47. Rocks from Space • Why is are meteorites useful? • They are material samples from outside the Earth that can be analyzed in the laboratory. • We can measure the age of the solar system by studying meteorites.

48. Next:Minor Planets or Asteroids

49. Minor Planets or Asteroids • The region between between Mars and Jupiter is populated by thousands of small rocky bodies called minor planets or asteroids.

50. Minor Planets or Asteroids • The region between between Mars and Jupiter is populated by thousands of small rocky bodies called minor planets or asteroids. • Ceres, the largest one with a diameter of 1000 km, was discovered in 1801.