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 Direction of impact Impact at 4 km sec into aluminium sphere Analogue experiments From Nakamura & Fujiwara (1991) Fracturing Model Phobos Stickney Secondary craters from Stickney? 4 >3m s -1 m s - 1 3m s -1 Ejecta from Stickney escape velocity = ~3m s -1 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 H N 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.