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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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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.

-different from all other planetary and satellite lineaments




Several “families” images.

of parallel grooves



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

Leading

apex



1 meridian

2

3

3

1

2

Each groove

family extends

over no more than one half of Phobos


Zone of avoidance meridian

at trailing apex of

Phobos

No grooves





Grooves are crater chains with raised rims, of their hemisphere

with apparent deposition in places


Proposed origins of parallel grooves of their hemisphere

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 of their hemisphere

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 of their hemisphere

  • Stickney impact: no sign of radial outward compression:

  • Radial outward movement of laboratory hypervelocity impact into sand:

  • (Oberbeck et al 1977)



Problems with all fracture hypotheses: of their hemisphere

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: of their hemisphere

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


200m of their hemisphere

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




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


  • - Velocities too low to form craters

  • Escape velocity: <11 m sec-1


    Secondary impact hypotheses 4 km

    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


    Secondary impact 4 km

    chains from

    Mars craters



    Tracing the 4 km

    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


    family A 4 km

    (oldest)

    family B

    family C

    family D

    family E


    2ndry impact 4 km

    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


    THE END (Dobrovolskis 1982)


    Early ejecta travelling at ~4 km sec (Dobrovolskis 1982)-1

    49o

    Tracing the grooves back to Mars craters:

    Experimental laboratory impacts in vacuum

    Similar results from recent numerical modelling


    Tracing the (Dobrovolskis 1982)

    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


    At what distance do we place Phobos? (Dobrovolskis 1982)

    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)

    Splat ! ! !


    What age is family A ? groove family A to trace back to Mars

    Easy to detect craters older than the grooves

    Age of groove family A can be determined from crater counting


    Pre-groove: 4.3 Gy Post-groove: 3.3 Gy

    The age of groove family A is 3.3 Gy


    There is only one Mars basin as young as Post-groove: 3.3 Gy

    3.3 Gy: the basin Lyot. It is at latitude +52o

    latitude = 52°

    Lower ejection angles

    Model at ejection angle 49° latitude = 37°


    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


    What is Phobos’ regolith thickness? least 3.3 Gy.

    Method of

    Quaide & Oberbeck 1968

    Normal Central mound

    Regolith

    Solid rock

    Flat-bottomed Concentric craters


    Mean regolith thickness = 20 least 3.3 Gy. metres

    Extremes are 8m to 42m

    Concentric double craters on Phobos


    • Will Mars rocks be found on Phobos? least 3.3 Gy.

    • 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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