New survey of Phobos’ grooves

Further evidence for groove origin

John MurrayJohn Murray

CEPSARCentre for Earth, Planetary, Space & Astronomical ResearchT

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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

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

Leadingapex

Leadingapex

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

meridianS

ub

-Mars m

eridian

Su

b-M

ars merid

ian

1 2

3

1 2 3

Each groove family extends

over no more than one half of Phobos

Zone of avoidanceat trailing apex ofPhobos

Zone of avoidanceat trailing apex ofPhobos No grooves

All grooves are younger than

Stickney

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

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

Grooves are not radial to Stickney crater

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

Directio

n of

impact

Stickney impact fractures? Analogue experimentsImpact at 4 km sec into aluminium sphereFrom 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:

2. 12.6 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 GanymedePhobos Ganymede

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

Upper limit of c.20 metres horizontal fracture opening

Phobos GanymedePhobos 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 fissuresPit craters over fissuresFracture models require a very thick regolith - 100-400 mFracture models require a very thick regolith - 100-400 m

200m

20m maximum width

En echelonfaulting

Always associated

with 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 voids2. Detached slices would be unsupported:

could not remain open for regolith drainage

Mercury Phobos Moon PhobosMercury Phobos Moon Phobos

Secondary impact hypotheses

Secondary impact hypotheses

Grooves have raised rims, and appear similar to secondary impact cratersGrooves have raised rims, and appear similar to secondary impact craters

38 km 6 km 18 km 4 km

Escape velocity: <11 m sec-1Secondary 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-1Secondary impact hypotheses2. Rolling ejecta:

- No boulders at end of grooves- Grooves do not run downhill- No repeated pattern- Boulders do not roll around obstacles

Secondary impact hypotheses2. 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 impactchains from Mars craters

Formation of ejecta strings

Formation of ejecta strings

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

family A (oldest)

family B

family C

family D

family E

2ndry impactModel with 12 groove families included.

HRSC map of Phobos grooves

2ndry impactModel 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 ENDTHE END

Tracing the grooves back to Mars craters:

Experimental laboratory impacts in vacuum

Similar results from recent numerical modelling

49oEarly

eje

cta

trave

lling

at ~

4 km

sec

-1

Tracing thegroove familiesback to Mars1. 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?

Phobos was furtherfrom Mars in the past

At what distance do we place Phobos?

Phobos was furtherfrom 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 as3.3 Gy: the basin Lyot. It is at latitude +52o

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 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?

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 metresExtremes are 8m to 42m

Mean regolith thickness = 20 metresExtremes are 8m to 42m

Concentric double craters on PhobosConcentric double craters on Phobos

0

1

2

3

4

5

0 10 20 30 40 50

Regolith Depth in metres

Phobos regolith depth from crater

geometry

0

10

20

30

40

50

-90 -60 -30 0 30 60 90 120 150 180 210 240 270

Longitude

Phobos regolith

depthRe

go

lith

de

pth

in

me

tre

sLeading Trailingapex apex

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