Starship Asterisk*

http://www.planetary.org/blog/article/00003214/ wrote:
Notes from Day 5 of the EPSC/DPS meeting
The Planetary Society Blog, By Emily Lakdawalla | Oct. 7, 2011

[img3="On January 9, 2011, Mars Express passed Mars' moon Phobos at a distance of only 111 kilometers. This photo shows the south side of Phobos at a resolution of 4.1 meters per pixel. The south pole is marked with a dot and an "S" inside the large crater near the center. Stickney crater, which is positioned at the center of Phobos' leading, Mars-facing quadrant, makes the bite out of the upper right of the globe.
Credit: ESA / DLR / FU Berlin (G. Neukum)"]http://www.planetary.org/image/image2-4 ... _50pct.jpg[/img3]

<<The first [Phobos] talk [was] by T.P. Andert [concerning Phobos's] gravity field; the conclusion was that it is likely highly porous, with local density anomalies (that is, in some places it's denser than it is in others, and it's not neatly stratified in density). So it's likely it formed by re-accretion of a disrupted body – it's a rubble pile.

Then I saw John Murray give a talk on the origin of Phobos' grooves. He outlined a set of observations about the grooves based on hundreds of Mars Express images:

• Each groove traces a plane through Phobos. Even ones that appear irregular actually cut a plane straight through it, if viewed from within the plane.
  
  There are several (10 or 15) distinct families of parallel grooves.
  
  For each groove family, the parallel plane passing through the center of Phobos also passes through Phobos' leading apex
  
  A corollary of this is that all grooves become parallel along sub- and anti-Mars meridians
  
  Each groove family extends over no more than one half of Phobos, fading out at tips and failing to appear over other half
  
  At the back end of Phobos, all grooves suddenly end, and there are none within 20 to 30 degrees of the trailing end of Phobos.
  
  Grooves are interrupted by topography near the edge of their hemisphere
  
  The grooves are not radial to Stickney crater; they actually cross Stickney both radially and tangentially; there is no geometric relation
  
  Each family of grooves is of a different age
  
  All grooves are younger than Stickney
  
  When viewed from low sun angles, it's evident that all grooves are crater chains and they have raised rims

There is only one origin hypothesis, he argued, that is consistent with all of these observations: that they are secondary impacts, chains of craters, made by strings of melt that solidified into beads from impact craters from Mars. It was a fairly convincing case, and he did well at refuting what is apparently a fashionable interpretation right now, that the grooves represent the tracks of rolling boulders.>>