The Parallel Grooves of Phobos

The parallel grooves of Phobos

New evidence from HRSC Mars Express

John Murray, Jan-Peter Muller, University College London Gerhard Neukum & the Mars Express HRSC Team Several “families” of parallel grooves with different ages and morphologies Recent grooves have raised rims, and appear similar to secondary impact craters Shape-from-shading topography also shows raised rims ZoneZone ofof avoidanceavoidance atat trailingtrailing apexapex ofof PhobosPhobos Pre-MarsPre-Mars ExpressExpress mapmap ofof Phobos’Phobos’ groovesgrooves

Poor Coverage

Leading Sub-Mars Trailing edge meridian edge Stickney crater Deeply entrenched in Phobos mythology as fundamental to groove origin, either as: 1.Cause of fracture system 2.Source of secondary craters

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i D Analogue experiments From Nakamura & Fujiwara (1991) Impact at 4 km sec into aluminium sphere Fracturing Model

Phobos Stickney Secondary craters from Stickney? -1

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Ejecta from Stickney

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m 3 PhobosPhobos Stickney

StickneyStickney secondarysecondary Stickney modelmodel Secondary impact chains from Mars craters Secondary impacts on Phobos (above) & The Moon (left) Lunar secondary Crater chain

Primary impact

Ejecta strings

Family A K

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Many families J combined

Mars secondary impact model

Poor PhobosPhobos Coverage

MarsMars SecondariesSecondaries modelmodel Leading edge: 1. more grooves? 2. Random cross-cutting orientations? 3. Families cut off?

Groove topography: raised rims? impact profiles?

Groove ages: much later Further than Stickney? evidence? …enter Mars Express: six passes so far, more planned Phobos’ leading edge

Previously unmapped zone Same picture at full resolution, contrast enhanced

Stickney Crater Leading apex PoorPoor (vertical)(vertical) lightinglighting

Orbit 715 Leading apex CrossingCrossing grooves,grooves, -- mmanyany visiblevisible despitedespite poorpoor lightinglighting

Orbit 748 Poor Pre-Pre- MarsMars ExpressExpress Coverage

WithWith MarsMars ExpressExpress coveragecoverage Poor Coverage

Leading Trailing edge edge PhobosPhobos

MarsMars SecondariesSecondaries modelmodel THETHE ENDEND