GROOVES OF PHOBOS AS SEEN ON THE MEX HRSC RECTIFIED IMAGES

AND COMPARISONS WITH PLANETERY ANALOGS

A.T. Basilevsky1, J. Oberst2,3, K. Willner3, M. Waehlisch3, G. Neukum4

1Vernadsky Institute, Moscow, Russia, 2DLR Berlin, Germany, 3TU Berlin Germany, 4Free University Berlin, Germany

Contact: atbas@geokhi.ru or alexander_basilevsky@brown.edu

The Second Moscow Solar System SymposiumSpace Research Institute

Moscow, Russia10-14 October 2011

Grooves on Phobos,What is their origin?

There are considered several hypotheses:

Fractures from crater-forming impacts

e.g., Thomas et al., …1978

The younger fractures typically abut against the older ones.

Elysium, Mars, HIRISE image PSP_010348_1870.

SW of crater Drunlo, observed in HRSC images of the orbits: 6906 (a, b), 4847 (c, d), and 2813 (e, f), reprojected for central longitude 102oW.

No abuttings, No lateral offsets at intersections of grooves of different trends (different ages) are seen.

Phobos groove intersections

Collapsed pits in Cyane Fossae. Pits have no raised rims as somePhobos grooves have. HIRISE image PSP_010345_2150.

Martian potential analogs - pits and graben

Graben near Alba Patera.They have no raised rims. THEMIS image V20111005.

Martian potential analogs - graben

Martian potential analogs - graben

Intersecting graben near Alba Patera. No abuttings, no lateral offsets of the younger faults at intersections

are seen. THEMIS image V20111005.

Youngfaults

Older faults

Scars formed by ejecta from Stickney and other craters

e.g., Head and Wilson, …2010, Duxbury, 2010

The HRSC images of craters Drunlo (lower left, D = 4.2 km) and Clustril (upper right, D = 3.4 km). All images are reprojected for central longitude 31oW.

Plates a to f are from images obtained at the orbits 0756 (image a), 2780 (b, f), 2813 (c), 3310 (d) and 5851 (e).

Grooves crossing the craters show no gaps at the crater rims in contrast to trails of bouncing boulders seen on slopes of lunar relief (see the next slides).

Boulder trails on the wall of 15 kmcrater. LROC image M107985155LE

Downrange bright marks (white arrows) formed by a bouncing boulder (black arrow). LROC image M110383422R.

Lunar analogs

Boulder trail crossing the 100 m crater on the wall of 15 km unnamed crater. LROC image M107985155LE.

Details of boulder trail crossing the crater. LROC image M107985155LE

Lunar analogs Long jump after crater rim

Boulders reached the crater floor. LROC image M107985155LE.

Boulders reaching the floor traveled only 50-100 m. LROC image M107985155LE.

Lunar analogs

Inertia is not a powerful mover

Secondary impact craters chains. produced by ejecta from Mars craters

e.g., Murrey, …2010

Grooves morphology along 90o angular distance (left-right). Fragment of image taken during orbit 0756 (central longitude 31oW). Craters Drunlo and Clustril are in the image right. If grooves were formed by secondary impacts from Mars craters, one would expect a change in groove-forming crater morphology due to change in impact angle.

100 km asteroid Lutetia: No regular source of external secondaries Image taken by the OSIRIS camera. Rosetta, ESA.

Lutetian potential analogs - grooves

Grooves and crater chains similar to those observed on Phobos(upper red box on synoptic image)

Grooves on Lutetia

Grooves and crater chains similar to those observed on Phobos (lower red box on the synoptic image).

Grooves on Lutetia

500 km asteroid Vesta: No regular external source of secondaries (Dawn image 571329main_pia14313-full_full.jpg)

Its surface shows many grooves, some very similar to grooves of Phobos.

Vestian potential analogs - grooves

Grooves of Vesta looking similar to those observed on Phobos.

Phobos grooves

20 km stone is neededto produce this groove

Conclusions:

• All suggested hypotheses of formation of grooves on Phobos have their strong and weak sides.

• Further analysis is needed to resolve the problem of the groove formation.

• Currently it looks that hypotheses of scars from rolling stones and chains of secondary craters produced by ejecta from Mars contradict observations on lunar & asteroidal analogs.

• Further analysis is needed to resolve the problem of absence of abutting and lateral offset in the fracturing mechanism of the groove formation.

Thank you

for your attention