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

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

Each groove traces a plane through Phobos

Several “families” of parallel grooves

Each family of grooves is of a different age

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

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

Each groove family extends over no more than one half of Phobos

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

All grooves are younger than Stickney

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

Grooves are not radial to Stickney crater

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

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

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

Stickney re-opening of tidal fractures 1.Stickney impact: no sign of radial outward compression: 2.Radial outward movement of laboratory hypervelocity impact into sand: (Oberbeck et al 1977)

Stickney re-opening of tidal fractures 1.Stickney 10 km: km diameter Aorounga impact crater, Chad:

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

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

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

If Phobos is a captured asteroid, then it has twice lost its regolith 1. During capture (Thomas, Veverka, Bloom & Duxbury 1979, JGR) 2. During Stickney impact (Horstman & Melosh 1989, JGR) If Phobos is a captured asteroid, then it has twice lost its regolith 1. During capture (Thomas, Veverka, Bloom & Duxbury 1979, JGR) 2. During Stickney impact (Horstman & Melosh 1989, JGR)

Grooves cannot be faults or fractures of any kind. 1.Propagation through voids 2.Detached slices would be unsupported: could not remain open for regolith drainage

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

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

Escape velocity: <11 m sec -1 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 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 Moon Phobos Moon Phobos

Secondary impact chains from Mars craters

Formation of ejecta strings

Tracing the groove families back to Mars 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 1. LAUNCH from MARS. Several different launch latitudes were chosen, from which the ejecta was launched at an angle of 49 o +3 o, the mean launch angle of ejecta in 45 o impacts, the most likely impact angle.

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

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

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

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

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 49 o +3 o, the mean launch angle of ejecta in 45 o impacts, the most likely impact angle

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

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)

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

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

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

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 and 4.5 x deg. year Lyot is probably the source impact basin for groove family A

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

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

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 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