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

N. Schmedemann1, G. Michael1, B. A. Ivanov2, J. Murray3 and G. Neukum1,

1Institute of Geological Sciences, Freie Universität Berlin, Berlin, Germany; 2Institute of Dynamics of Geospheres, Moscow, Russia; 3Department of Earth

Sciences, Open University, Milton Keynes, UK.

3MS³ Symposium,Moscow, 08.-12.10.2012

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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

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Two Cases of Phobos’ Chronology

• End-member cases of Phobos’ history

- Case A: Phobos was in its current orbit since its formationo Average projectile impact velocities are converted form Mars to Phobos’ orbito Average impact rate equals Martian impact rate – corrected for different

crater scaling

- Case B: Phobos is a recently captured Main Belt asteroido Average projectile impact velocities equals average Main Belt impact velocitieso Average impact rate equals average Main Belt impact rates

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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)

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Scaling Laws – Conversion of Projectile to Crater Diameters

𝐷𝑡

𝐷𝑃( 𝛿𝜌 )0.43

(𝑣𝑠𝑖𝑛𝛼)0.55= 1.21

[ (𝐷𝑠𝑔+𝐷𝑡 )𝑔 ]0.28 Ivanov (2001; updated 2011)

= D

If D < Dsimple to complex transition then Dt ~ D

If D > Dsimple to complex transition then

D – observed crater diameterDt – transient crater diameterDP – impactor diameterG – gravity acceleration of target bodyδ – projectile density ρ – target densityv – impact velocityα – impact angleDsg – strength to gravity transition crater diameter

(Dt>>Dsg -> gravity regime; Dt<<Dsg -> stregth regime)

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Scaling Laws – Conversion of Projectile to Crater Diameters

Moon Phobos (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 2.5 2.5

Impact Velocity (km/s) 18 8.5 5

Impact Angle (most probable case after Gilbert, 1893) 45 45 45

Surface Gravity (m/s²) 1.626*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)

1000 (all craters are in the strength

regime)

Diameter Simple to Complex (km) 15 1000 (all craters are simple)

1000 (all craters are simple)

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Summary Production & Chronology Functions

Resulting production and chronology functions for cases A and B

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Measurement Areas

HRSC Basemap: Wählisch et al. (2010)

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Measurement Areas

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

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Measurement Areas

Area S1: Interior of Stickney

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Measurement Areas

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

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Randomness Analysis

Analysis according to Michael et al. (2012)

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Surface Ages

Cumulative crater plots of average area west of StickneyAge of Phobos equals last global resurfacing event (break-up of parent body)

Age of Phobos

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Surface Ages

Cumulative crater plots of S1 area inside Stickney

Age of Stickney

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Surface Ages

Cumulative crater plots of S2 area inside Stickney

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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

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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).

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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)

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Questions

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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

Bottke et al. (1994)

Velocity distribution of 682 Main Belt asteroidsD>50 km

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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

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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 GaIsochrones

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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/GaPi: intrinsic impact probabilityni: number of projectiles forming craters ≥ 1 km

o observed number of Main Belt asteroids ≥ 10 km (obs. limit): 9554o crater size on Phobos as average Main Belt asteroid from 10 km projectiles: 104.5 kmo correction factor for frequency of 104.5 km craters to 1 km craters based on Phobos production

function as MBA: 4*103

o 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)