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New survey of Phobos’ grooves Further evidence for groove origin

John Murray. CEPSAR Centre for Earth, Planetary, Space & Astronomical Research. The Open University. New survey of Phobos’ grooves Further evidence for groove origin. New map of Phobos’ grooves from HRSC, HiRISE and Viking images.

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New survey of Phobos’ grooves Further evidence for groove origin

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  1. John Murray CEPSAR Centre for Earth, Planetary, Space & Astronomical Research The Open University New survey of Phobos’ grooves Further evidence for groove origin

  2. New map of Phobos’ grooves from HRSC, HiRISE and Viking images. -different from all other planetary and satellite lineaments

  3. Each groove traces a plane through Phobos

  4. Each groove traces a plane through Phobos

  5. Several “families” of parallel grooves

  6. Each family of grooves is of a different age

  7. For each groove family, the plane passing through the centre of Phobos also passes through its leading apex Leading apex

  8. All grooves become parallel along the sub- & anti-Mars meridian Sub-Mars meridian

  9. 1 2 3 3 1 2 Each groove family extends over no more than one half of Phobos

  10. Zone of avoidance at trailing apex of Phobos No grooves

  11. All grooves are younger than Stickney

  12. Groove families are obstructed by topography near the edge of their hemisphere

  13. Grooves are not radial to Stickney crater

  14. Grooves are crater chains with raised rims, with apparent deposition in places

  15. Proposed origins of parallel grooves opened by Stickney impact Fractures: caused by tidal forces caused by drag forces during capture tidal fractures re-opened by Stickney impact from Stickney crater Secondary impacts: from rolling boulders from Stickney from impacts on Mars

  16. Direction of impact Stickney impact fractures? Analogue experiments Impact at 4 km sec into aluminium sphere From Nakamura & Fujiwara (1991) Map of polygonal fractures formed from above impact. No straight or parallel grooves seen

  17. Stickney re-opening of tidal fractures • Stickney impact: no sign of radial outward compression: • Radial outward movement of laboratory hypervelocity impact into sand: • (Oberbeck et al 1977)

  18. Stickney re-opening of tidal fractures • Stickney 10 km: • 12.6 km diameter Aorounga impact crater, Chad:

  19. Problems with all fracture hypotheses: No sign of lateral movement that would occur if grooves were fractures Upper limit of c.20 metres horizontal fracture opening Phobos Ganymede

  20. Problems with all fracture hypotheses: No sign of lateral movement that would occur if grooves were fractures Upper limit of c.20 metres horizontal fracture opening No sign of lateral movement that would occur if grooves were fractures Upper limit of c.20 metres horizontal fracture opening Phobos Ganymede

  21. 200m En echelon faulting 20m maximum width Moon Hyginus rille Mars Faults not straight or planar Pit craters over fissures Always associated with faulting Fracture models require a very thick regolith - 100-400 m

  22. If Phobos is a captured asteroid, then it has twice lost its regolith • During capture • (Thomas, Veverka, Bloom & Duxbury 1979, JGR) • 2. During Stickney impact • (Horstman & Melosh 1989, JGR)

  23. Grooves cannot be faults or fractures of any kind. • Propagation through voids • Detached slices would be unsupported: • could not remain open for regolith drainage

  24. 38 km 6 km 18 km 4 km Secondary impact hypotheses Mercury Phobos Moon Phobos Grooves have raised rims, and appear similar to secondary impact craters

  25. Secondary impact • hypotheses • 1. From Stickney Crater: • - Velocities too low to form craters Escape velocity: <11 m sec-1

  26. Secondary impact hypotheses 2. Rolling ejecta: - No boulders at end of grooves - Grooves do not run downhill - No repeated pattern - Boulders do not roll around obstacles Escape velocity: <11 m sec-1 Moon Phobos

  27. Secondary impact chains from Mars craters

  28. Formation of ejecta strings

  29. Tracing the groove families back to Mars 1. LAUNCH from MARS. Several different launch latitudes were chosen, from which the ejecta was launched at an angle of 49o+3o, the mean launch angle of ejecta in 45o impacts, the most likely impact angle. 2. ARRIVAL at PHOBOS. For each ejecta batch, the orientation and velocity of the ejecta strings impacting Phobos was calculated. MOST EJECTA ARRIVES AT A VELOCITY OF 4km sec-1

  30. family A (oldest) family B family C family D family E

  31. 2ndry impact Model with 12 groove families included. HRSC map of Phobos grooves

  32. STICKNEY EJECTA (after Thomas 1988) TIDAL STRESS (Dobrovolskis 1982) STICKNEY ROLLING BOULDERS (Head & Wilson) SECONDARY IMPACTS FROM MARS (Murray 1994) STICKNEY FRACTURING (Fujiwara & Asada 1983)MAP OF PHOBOS’ GROOVES

  33. THE END

  34. Early ejecta travelling at ~4 km sec-1 49o Tracing the grooves back to Mars craters: Experimental laboratory impacts in vacuum Similar results from recent numerical modelling

  35. Tracing the groove families back to Mars 1. The centre of the grooved hemisphere indicates the direction from whence the ejecta came, but not its velocity 2. By varying the velocity, we can find the latitude on Mars from which the ejecta was launched at an angle of 49o+3o, the mean launch angle of ejecta in 45o impacts, the most likely impact angle

  36. At what distance do we place Phobos? Phobos was further from Mars in the past

  37. We have to increase Phobos’ orbit to 14,000 km to get groove family A to trace back to Mars At 49° launch, it traces back to a crater at +37° latitude (± a lot) Splat ! ! !

  38. What age is family A ? Easy to detect craters older than the grooves Age of groove family A can be determined from crater counting

  39. Pre-groove: 4.3 Gy Post-groove: 3.3 Gy The age of groove family A is 3.3 Gy

  40. There is only one Mars basin as young as 3.3 Gy: the basin Lyot. It is at latitude +52o latitude = 52° Lower ejection angles Model at ejection angle 49° latitude = 37°

  41. 1. Phobos has been in synchronous orbit around Mars since at least 3.3 Gy. 2. Phobos mean secular acceleration during this time has been between 3 x 10-5 and 4.5 x 10-5 deg. year -1 3. Lyot is probably the source impact basin for groove family A

  42. What is Phobos’ regolith thickness? Method of Quaide & Oberbeck 1968 Normal Central mound Regolith Solid rock Flat-bottomed Concentric craters

  43. Mean regolith thickness = 20 metres Extremes are 8m to 42m Concentric double craters on Phobos

  44. Will Mars rocks be found on Phobos? • At secondary impact speeds of 4 km sec-1 most will be ejected at >11 m s-1 : • Look for Mars rocks near a • groove within a topographically-protected hollow

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