1. Surface Chronology of Phobos – The Age of Phobos and its Largest Crater Stickney 1 N. Schmedemann 1, G. Michael 1, B. A. Ivanov 2, J. Murray 3 and G. Neukum 1, 1 Institute of Geological Sciences, Freie Universität Berlin, Berlin, Germany; 2 Institute of Dynamics of Geospheres, Moscow, Russia; 3 Department of Earth Sciences, Open University, Milton Keynes, UK. 3MS³ Symposium, Moscow, 08.-12.10.2012

2. 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. 3 Two Cases of Phobos’ Chronology End-member cases of Phobos’ history  Case A: Phobos was in its current orbit since its formation o Average projectile impact velocities are converted form Mars to Phobos’ orbit o Average impact rate equals Martian impact rate – corrected for different crater scaling  Case B: Phobos is a recently captured Main Belt asteroid o Average projectile impact velocities equals average Main Belt impact velocities o Average impact rate equals average Main Belt impact rates

4. 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. 5 Scaling Laws – Conversion of Projectile to Crater Diameters Ivanov (2001; updated 2011) If D < D simple to complex transition then D t ~ D If D > D simple to complex transition then D– observed crater diameter D t – transient crater diameter D P – impactor diameter G– gravity acceleration of target body δ – projectile density ρ – target density v – impact velocity α – impact angle D sg – strength to gravity transition crater diameter (Dt>>D sg -> gravity regime; Dt stregth regime)

6. 6 Scaling Laws – Conversion of Projectile to Crater Diameters MoonPhobos (Case A) Phobos Asteroid Case (Case B) Target Density (g/cm³) 2.5 (est. surface regolith) 1.9 (Willner et al., 2010) 1.9 (Willner et al., 2010) Projectile Density (g/cm³)2.5 Impact Velocity (km/s)188.5 5 Impact Angle (most probable case after Gilbert, 1893) 45 Surface Gravity (m/s²)1.62 6*10 -3 (Willner et al., 2010) 6*10 -3 (Willner et al., 2010) Diameter Strength to Gravity Transition (km) 0.3 1000 (all craters are in the strength regime) Diameter Simple to Complex (km)15 1000 (all craters are simple)

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

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

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

10. 10 Measurement Areas Area S1: Interior of Stickney

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

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

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

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

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

16. 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. 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. 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. 19 Questions

20. 20 Crater Production Function Bottke et al. (1994) Velocity distribution of 682 Main Belt asteroids D>50 km

21. 21 Chronology Function 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) Lunar chronology is used as base line, because the main impactor source is the same on the Moon, Phobos and Main Belt asteroids

22. 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 for Phobos and Mars 1 Ga Isochrones

23. 23 Chronology Function