PHOBOS GROOVES

PHOBOS GROOVESPHOBOS GROOVESA LUNAR ANALOGYA LUNAR ANALOGY

Thomas Duxbury, GMUGerhard Neukum, Freier Univ, and the MEX HRSC Team

Mark Robinson, ASU, and the LRO LROC Team

12 OCT 2010 11st Moscow Solar System Symposium

PHOBOS GROOVES

PHOBOS GROOVESPHOBOS GROOVES

12 OCT 2010 1st Moscow Solar System Symposium 2

MEX SRC ImageG. Neukum, PI Freie Univ

PHOBOS GROOVES

PHOBOS GROOVE NETWORKPHOBOS GROOVE NETWORK

12 OCT 2010 1st Moscow Solar System Symposium 3

Thomas, P. et al. (1979) JGR, 84, B14,. 8457–8477Murray, J. et al. (2006) LPS XXXVII 2195

ANTI-STICKNEYSTICKNEY

PHOBOS GROOVES

POSSIBLE GROOVE ORIGINSPOSSIBLE GROOVE ORIGINS

• Related to surface fractures of the interior generated by Mars tidal forces while undergoing large impacts

• Tidal stresses during capture into Mars orbit (if Phobos was an asteroid) • Secondary craters chains from Phobos crater ejecta • Crater ejecta rolling on the surface • Secondary crater chains from Mars crater ejecta

Thomas P, et al., Nature, 273, 1978 Head J, and M Cintala, PGP Report, 1979 Thomas P, et al., JGR, 84, B14, 1979 Murchie S, et al., LPS, XXXIX, 1434, 2008 Wilson L and J Head., LPS, XX, 1212, 1989Murray J, et al., LPS, XXXVII, 2195, 2006

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PHOBOS GROOVES

LUNAR ANALOGYLUNAR ANALOGY

12 OCT 2010 1st Moscow Solar System Symposium 5

LRO LROC NAC IMAGE: M. ROBINSON, PI, ASU

http://lroc.sese.asu.edu/news/index.php?/archives/227-Hole-in-One!.html

Boulder trail

M122597190L

10 m Boulder

PHOBOS GROOVES

LUNAR “GROOVES”LUNAR “GROOVES”KING CRATERKING CRATER

12 OCT 2010 1st Moscow Solar System Symposium 6

M103717945R

PHOBOS GROOVES

LUNAR “GROOVES”LUNAR “GROOVES”KING CRATERKING CRATER

12 OCT 2010 1st Moscow Solar System Symposium 7

M103717945R

PHOBOS GROOVES

LUNAR “GROOVES”LUNAR “GROOVES”Near Apollo 15 (25.65 N, 3.53 E)Near Apollo 15 (25.65 N, 3.53 E)

12 OCT 2010 1st Moscow Solar System Symposium 8

~1 km

#2 #1

#2

#2

#1

#1

M111571816R

80 m diameter crater

5 - 10 m diameter boulders

PHOBOS GROOVES

LUNAR “GROOVES”LUNAR “GROOVES”

• Caused by Rolling Boulders from Crater Ejecta– Rolling boulder leaving trails seem to be more prevalent with oblique impacts, being

ejected downstream in the direction of the more prominent ejecta blanket– Boulder’s longest (rotation) axis tend to stay normal to groove after reaching some

level of rolling stability– Groove width ~ 60% of boulder length– Some boulders constantly stay in contact with surface – some hop

• Dependent on topography and axis of rotation– The lunar boulders travel downhill and their paths are effected by local topography

and their rotation axis• Can flip and hop on surface when long axes make surface contact

12 OCT 2010 1st Moscow Solar System Symposium 9

PHOBOS GROOVES

GROOVE ORIGINGROOVE ORIGIN• Many Phobos grooves, the ones that follow the surface topography and cross other

grooves, are possibly caused by rolling boulders ejected from Stickney impact at oblique angle from the west into regolith-covered rubble pile (if Phobos was accreted from Mars crater ejecta)

– Eastern rim of Stickney is more pronounced with evidence of ejecta blanket causing color variation (CRISM, Murchie, et al., 2009,HiRISE, Thomas, et al., 2010)

• Boulders have significant rotational and local horizontal velocity and eventually escape the surface of Phobos near the Stickney antipodal point (only a few m/s required to escape)

– Phobos orientation and rotation state at Stickney impact unknown• Width of grooves proportional to length of boulders (most stable axis for rolling)• Depth of groove related to regolith thickness / compaction, boulder rotational speed,

friction between regolith and boulder, etc.

12 OCT 2010 1st Moscow Solar System Symposium 10

NASA MRO HiRISE image of Phobos,McEwen, A., PI, U of AZ,http://hirise.lpl.arizona.edu/phobos.php

• The LRO LROC NAC and MEX SRC images provide an excellent dataset for comparative planetology studies on the possible origin of the Phobos grooves

PHOBOS GROOVES

Comparative “Moon”ologyComparative “Moon”ology

12 OCT 2010 1st Moscow Solar System Symposium 11

Phobos groove network

Lunar boulder trails