Tag Archive | Al-Bakri crater

Ejecta Blocking Boulder

On the floor of Al-Bakri crater lies an approximately 100 foot diameter crater with a very unusual feature that may have interfered with its impact ejecta pattern.

ACT-REACT map NAC photo strip

You can see the open wedge area (no ejecta) caused from a strategically placed boulder near its rim.  Here is what Moon Zoo Team member Irene Antonenko ( IreneAnt ) has to say:

” This is really interesting because the boulder really shouldn’t affect the placement of the ejecta all that much. When the crater is being formed, ejecta is being thrown out from closer to the centre, so the boulder would be too far away to affect the ejection of material. And, when the material lands, it is thought to land from above (not sideways), so again, the boulder shouldn’t affect the emplacement of ejecta material. “

There may be some tells as one looks around the crater floor for clues that may help solve this mystery.  Our ejecta blocking boulder appears to have been sitting at that location for quite some time. There does not appear to be any boulder tracks like the one below from a boulder that probably rolled down from the rim of Al-Bakri crater south of our mystery boulder.

If you look at the other boulders in the area you will see that they appear to be white (high albedo). The ejecta blocking boulder is darker and appears to be the black sheep of the boulder community. This anomaly may be resolved if one considers that the darker color of the boulder may be due to it being covered with a dusting of fine ejecta particles from the impact.

It appears that the major force of the impact, as Irene stated above, moved up and over the boulder not affecting or having little affect on its placement. However, there does seem to be a lateral surface flow of ejecta that hit the boulder. The boulder then acted like a nozzle/guide that created the unique ejecta- free wedge design out from the crater. There is also a possibility that crater excavation moved our mystery boulder up to its present position while blocking the ejecta along the way.

The old boulder may have staked its claim to that spot first and was not willing to give it up even with the more recent arrival of a claim jumping impactor.  What are your thoughts?


Tom128 is a regular contributor on the Moon Zoo forum.