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The Search for a Lunar Dynamo

The Search for a Lunar Dynamo. Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010. The utility of planetary magnetism. Earth. Mars. Mercury. Ganymede. Asteroids. Moon. (in order of decreasing radius). The utility of planetary magnetism. Earth. Mars.

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The Search for a Lunar Dynamo

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  1. The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010

  2. The utility of planetary magnetism Earth Mars Mercury Ganymede Asteroids Moon (in order of decreasing radius)

  3. The utility of planetary magnetism Earth Mars Mercury Ganymede Asteroids Moon (in order of decreasing radius)

  4. What is the structure of the Moon? Rich in heat producing elements Core evidence: seismic, moment of inertia, magnetic induction, and wobble.

  5. Early views of the Moon Pre-Apollo era: Hot Moon vs. Cold Moon Credit: Bill Hartmann Others (e.g Shoemaker): experienced melting Harold Urey: primitive chondritic object

  6. In Search of a Lunar Dynamo S. Dolginov Magnetometer Principal Investigator Luna 1, January 2, 1959 Luna 2 September 12, 1959 Result: lunar dipole field at least ~10,000 weaker than the Earth’s

  7. Hot Moon • Surveyor 5 spacecraft (1967) detected basalt. • Apollo missions directly sampled and confirmed the volcanic origin for the lunar mare. • The Moon had experienced at least some melting. Surveyor 3

  8. Crustal magnetism discovered Apollo 15 and 16 subsatellites Russell et al. 1974

  9. Crustal magnetism discovered Apollo 12 magnetometer Apollo 16 magnetometer

  10. Does crustal magnetism = dynamo? From Mark Wieczorek’s 2009 AGU Talk

  11. The lunar rock magnetic record Modern Earth field (~ 50 μT) Wieczorek, et al. (2006) & Cisowski and Fuller (1987)

  12. The lunar rock magnetic record ? Modern Earth field (~ 50 μT) Wieczorek, et al. (2006) & Cisowski and Fuller (1987)

  13. What we know and don’t know • It is clear that fields existed on Moon: • Crustal remanence. • Paleomagnetic record. • It is not clear whether the fields are from a dynamo or impact processes. • Doell et al. (1970): transient impact-generated fields could magnetize rocks as a shock wave passes through them: “shock magnetized.”

  14. Rock magnetic approach • We seek rocks with ages > 4.0 Ga. • But we also carefully select a rock with favorable petrologic history.

  15. We started looking at a lot of old rocks

  16. 76535 – Pristine Troctolite 1 mm • Age: 4.2-4.3 Ga • Argon age • Plutonic • No shock effects

  17. Why the troctolite is so important • 1) Lack of detectable shock features: remanence is less likely due to shock effects • Restricts impact related processes. • 2) Cooling history is well constrained: slow cooling history implies any remanence is from long-lived fields • Further restricts impact related processes. • 3) It is very old. It is somewhat easier to accept a core dynamo at early times.

  18. Measurements • Thermal demagnetization is the gold standard, but: • It is destructive, rocks frequently alter (Lawrenceet al. 2008). • Our approach: first perform nondestructive AF demagnetization to understand the samples, and then if desirable, perform thermal.

  19. Alternating Field Demagnetization z Magnetization vector Demag. Step 1 y Demag. Step 2 Sample x Ideally, trends to the origin

  20. Alternating Field Demagnetization z y Magnetization y x Sample x

  21. Alternating Field Demagnetization z z y y Sample x

  22. Display of demagnetization Both Projections y,z z y = + x,y y x

  23. Two Samples Magnet-like overprints: IRMs

  24. Easily reproduced/removed

  25. Once removed, first sample:

  26. Two samples:

  27. Second component decays to origin

  28. Second component decays to origin

  29. Second component decays to origin HC HC MC MC

  30. Two Magnetization Components z 2 Net y 1 x

  31. Two Magnetization Components z 2 Net y x

  32. Two Magnetization Components z 2 Net y x

  33. Two Magnetization Components z Net y x

  34. Two Magnetization Components z y x

  35. Two Magnetization Components z y x

  36. Two Magnetization Components z y x

  37. Two Magnetization Components z MC y 142°-149° HC x Four of our best samples show these two components: HC to MC: 142-149° apart (~10° error).

  38. 145° HC Mutually Oriented Samples MC MC MC 145° 145° ? HC HC

  39. 2/3 Mutually Oriented Samples

  40. 3 Components of 3 Samples Best fit directions

  41. 3 Components of 3 Samples MC-HC distances: 147° 123° 81° Compared with: 142-149° previously Best fit directions

  42. The rock is unshocked, so what thermal (cooling) events could have permitted its magnetization? Focus on the timescales for cooling events – compare with timescales for impact-generated fields.

  43. Thermal History of 76535 4.2 Ga (multiple chronometers)

  44. Thermal History of 76535 4.2 Ga (multiple chronometers)

  45. Thermal History of 76535 First Magnetization 4.2 Ga

  46. Thermal History of 76535 4.2 Ga

  47. Thermal History of 76535 4.2 Ga

  48. Thermal History of 76535 4.2 Ga

  49. Thermal History of 76535 4.2 Ga

  50. Thermal History of 76535 Post 4.2 Ga? 4.2 Ga

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