1 / 23

Surface Chronology of Phobos – The Age of Phobos and its Largest Crater Stickney

Surface Chronology of Phobos – The Age of Phobos and its Largest Crater Stickney. 3MS³ Symposium, Moscow, 08.-12.10.2012.

treva
Télécharger la présentation

Surface Chronology of Phobos – The Age of Phobos and its Largest Crater Stickney

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Surface Chronology of Phobos – The Age of Phobos and its Largest Crater Stickney 3MS³ Symposium, Moscow, 08.-12.10.2012 N. Schmedemann1, G. Michael1, B. A. Ivanov2, J. Murray3 and G. Neukum1,1Institute of Geological Sciences, FreieUniversität Berlin, Berlin, Germany; 2Institute of Dynamics of Geospheres, Moscow, Russia; 3Department of Earth Sciences, Open University, Milton Keynes, UK.

  2. Characteristics & Origin of Phobos • shape, spectral characteristics and density similar to primitive (C), D or T-type asteroids (Jones et al., 1990; Giuranna et al., 2011) • Inside Mars synchronous orbit • Instable orbit and potential disintegration/crash within 30-50 Ma (Burns, 1978) • Tidal interactions may have lowered the orbit to current state  higher orbit in the past but likely always inside Mars synchronous orbit • Theories on Phobos’ origin include: • capture of an asteroid • in-situ formation with Mars • coalesced debris from Martian ejected material • Irregular shape indicates major collision(s) • Several sets of grooves with yet unexplained origin

  3. Two Cases of Phobos’ Chronology • End-member cases of Phobos’ history • Case A: Phobos was in its current orbit since its formation • Average projectile impact velocities are converted form Mars to Phobos’ orbit • Average impact rate equals Martian impact rate – corrected for different crater scaling • Case B: Phobos is a recently captured Main Belt asteroid • Average projectile impact velocities equals average Main Belt impact velocities • Average impact rate equals average Main Belt impact rates

  4. The Lunar Chronology • Lunar Chronology Function • derived from radioisotopic measurements of lunar rock samples and measurements of the cratering record at the Apollo landing sites Neukum (1983)

  5. Scaling Laws – Conversion of Projectile to Crater Diameters Ivanov (2001; updated 2011) If D < Dsimple to complex transition then Dt ~ D If D > Dsimple to complex transition then = D D – observedcraterdiameter Dt – transient craterdiameter DP – impactordiameter G – gravityaccelerationoftargetbody δ – projectiledensity ρ– targetdensity v – impactvelocity α – impact angle Dsg – strengthtogravitytransitioncraterdiameter (Dt>>Dsg -> gravityregime; Dt<<Dsg -> stregthregime)

  6. Scaling Laws – Conversion of Projectile to Crater Diameters

  7. Summary Production & Chronology Functions Resulting production and chronology functions for cases A and B

  8. Measurement Areas HRSC Basemap: Wählisch et al. (2010)

  9. Measurement Areas • Average Surface to the West of Stickney: N-S grooves stratigraphically above E-W grooves

  10. Measurement Areas • Area S1: Interior of Stickney

  11. Measurement Areas • Area S2: SRC image of Interior of Stickney; N-S grooves stratigraphicallybelow solitary E-W groove

  12. Randomness Analysis • Analysis according to Michael et al. (2012)

  13. Surface Ages Age of Phobos • Cumulative crater plots of average area west of Stickney • Age of Phobos equals last global resurfacing event (break-up of parent body)

  14. Surface Ages Age of Stickney • Cumulative crater plots of S1 area inside Stickney

  15. Surface Ages • Cumulative crater plots of S2 area inside Stickney

  16. Surface Ages • Comparison of cumulative crater plots of average and S1 area Stratigraphic relations suggest a formation age of E-W grooves of 3.8 – 3.85 Ga

  17. Apex-/Antapex Asymmetry Large (old) craters show apex-/antapex ratio of ~1.5  Phobos is not a recently captured object. Form recent orbit a factor 4 is expected according to Morota et al. (2008).

  18. Conclusion • Production and chronology function were derived for two end-member cases of Phobos’ evolution • Case A: Phobos was always in its current orbit • Case B: Phobos is a recently recently captured MB asteroid • Oldest surface age 4.3-4.4 Ga/ ~3.7-3.8 Ga last global resurfacing/break-up of Phobos parent • Age of Stickney: ~4.2 Ga/ ~3.5 Ga • Surface ages show multiple resurfacing events, probably connected to groove formation • Groove formation appears to be ancient but very young ages can’t be resolved based on current imaging data • Stratigraphic relationships indicate similar formation age of E-W striking grooves in at least two areas around 3.8-3.85 Ga/ 2.9-3.1 Ga • Apex-/antapex asymmetry of large/old craters indicate long cratering history in orbit about Mars • Ratio (1.5) is more than a factor of two less than the expected value (4.1) from current orbit  e.g. reorientation event(s with more frequent current position)

  19. Questions

  20. Crater Production Function • Lunar production function is used as base line, because the main impactor source is the same on the Moon, Phobos and Main Belt asteroids • Case A • vimpM= 9.4 km/s (Ivanov, 2008) • vescM= 5 km/s • vescP= 3 km/s • vimpP = 8.5 km/s • Case B • Average impact velocities among • Main Belt asteroids are calculated • following (Bottke et al., 1994) • vimpP~ 5 km/s Velocity distribution of 682 Main Belt asteroids D>50 km Bottke et al. (1994)

  21. Chronology Function • Lunar chronology is used as base line, because the main impactor source is the same on the Moon, Phobos and Main Belt asteroids • Case A • Impact probability of Mars (Ivanov, 2001)  0.45 x lunar impact rate • Correction for different crater scaling between Mars and Phobos 0.84 x lunar impact rate (same projectile is forming larger craters on Phobos than on Mars or the same crater size is achieved by smaller projectiles on Phobos)

  22. Chronology Function • Lunar chronology is used as base line, because the main impactor source is the same on the Moon, Phobos and Main Belt asteroids • Case A • Impact rate at Mars (Ivanov, 2001)  0.45 x lunar impact rate • Correction for different crater scaling between Mars and Phobos 0.84 x lunar impact rate (same projectile is forming larger craters on Phobos than on Mars or the same crater size is achieved by smaller projectiles on Phobos) 1 Ga Isochrones 1 Ga isochrones for Phobos and Mars

  23. Chronology Function • Lunar chronology is used as base line, because the main impactor source is the same on the Moon, Phobos and Main Belt asteroids • Case B • Average impact probabilities among Main Belt Asteroids are calculated following (Bottke et al., 1994)  Pi ~ 2.9*10-18 km-2/a  2.9*10-9 km-2/Ga • Conversion from intrinsic impact probability to chronology: • f=Pinir2mean(O’Brien and Greenberg, 2005) • f: impact frequency forming craters ≥ 1 km/Ga • Pi: intrinsic impact probability • ni: number of projectiles forming craters ≥ 1 km • observed number of Main Belt asteroids ≥ 10 km (obs. limit): 9554 • crater size on Phobos as average Main Belt asteroid from 10 km projectiles: 104.5 km • correction factor for frequency of 104.5 km craters to 1 km craters based on Phobosproduction function as MBA: 4*103 •  ni: 3.8*107 • r: mean radius of target body scaled to unit area = (11 km)²/4π*(11 km)² • f ~ 9*10-3 • (Neukum, 1983)

More Related