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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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Presentation Transcript
slide1

John Murray

CEPSAR

Centre for Earth, Planetary, Space & Astronomical Research

The Open

University

New survey of

Phobos’ grooves

Further evidence for

groove origin

slide2

New map of Phobos’ grooves from HRSC, HiRISE and Viking images.

-different from all other planetary and satellite lineaments

slide5

Several “families”

of parallel grooves

slide7

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

Leading

apex

slide9

1

2

3

3

1

2

Each groove

family extends

over no more than one half of Phobos

slide10

Zone of avoidance

at trailing apex of

Phobos

No grooves

slide15

Grooves are crater chains with raised rims,

with apparent deposition in places

slide16

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

slide17

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

slide18

Stickney re-opening of tidal fractures

  • Stickney impact: no sign of radial outward compression:
  • Radial outward movement of laboratory hypervelocity impact into sand:
  • (Oberbeck et al 1977)
slide19

Stickney re-opening of tidal fractures

  • Stickney 10 km:
  • 12.6 km diameter Aorounga impact crater, Chad:
slide20

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

slide21

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

No sign of lateral movement that would occur if grooves were fractures

Upper limit of c.20 metres horizontal fracture opening

Phobos Ganymede

slide22

200m

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

slide23

If Phobos is a captured asteroid, then it has twice lost its regolith

  • During capture
  • (Thomas, Veverka, Bloom & Duxbury 1979, JGR)
  • 2. During Stickney impact
  • (Horstman & Melosh 1989, JGR)
slide24

Grooves cannot be faults or fractures of any kind.

  • Propagation through voids
  • Detached slices would be unsupported:
  • could not remain open for regolith drainage
slide26

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

slide27

Secondary impact

  • hypotheses
          • 1. From Stickney Crater:
  • - Velocities too low to form craters

Escape velocity: <11 m sec-1

slide28

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

Escape velocity: <11 m sec-1

Moon

Phobos

slide29

Secondary impact

chains from

Mars craters

slide32

Tracing the

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

slide33

family A

(oldest)

family B

family C

family D

family E

slide34

2ndry impact

Model with 12 groove families included.

HRSC map of Phobos grooves

slide35

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

slide39

Early ejecta travelling at ~4 km sec-1

49o

Tracing the grooves back to Mars craters:

Experimental laboratory impacts in vacuum

Similar results from recent numerical modelling

slide40

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

slide41

At what distance do we place Phobos?

Phobos was further

from Mars

in the past

slide42

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 ! ! !

slide43

What age is family A ?

Easy to detect craters older than the grooves

Age of groove family A can be determined from crater counting

slide44

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

The age of groove family A is 3.3 Gy

slide45

There is only one Mars basin as young as

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

latitude = 52°

Lower ejection angles

Model at ejection angle 49° latitude = 37°

slide46

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

slide47

What is Phobos’ regolith thickness?

Method of

Quaide & Oberbeck 1968

Normal Central mound

Regolith

Solid rock

Flat-bottomed Concentric craters

slide48

Mean regolith thickness = 20 metres

Extremes are 8m to 42m

Concentric double craters on Phobos

slide50

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