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Got a HiWish request fulfilled today. Geology is interesting, but don't know what to make of it. Here's kind of the broad view - there is some darker material that is filling a valley network draining into Hellas. Looking closely, we can see embedded bright blocks. What are they?
My first thought might be that they were possibly a preserved evidence of a mudflow - large boulders plucked from the surrounding bright material. We can rule that out - small craters punched through the bright material, showing it's a thin layer on top of the dark channel fill.
However, looking at another contact between the bright and dark material, it's not so clear cut. The small crater at bottom right has a dark-toned rim and a light-toned floor, suggesting that the bright materials underlie the dark material at this location.
Here are some of the bright speckles we saw in the regional images. The lighting makes it difficult to tell whether they're on top or embedded in the dark material, but it's very clear they're big chunks of the bright material.
Looking at the wider view, we can see that these are probably big ejecta blocks from a fresh 1.6 km crater, as suggested by @acstadermann. We can see the ejecta sheet is light-toned material, and that some bright chunks form chains - similar to crater rays seen on the Moon.
This A+ diagram I just drew in paint shows a cross-section of what we're seeing from a top-down view.
Some other cool stuff from the HiRISE scene. 45 m bebe crater. Weird krinkly textures in the probable ejecta unit. Migrating sand snakes. Triangle.
Also want to fully credit @acstadermann here, who made a prediction that suddenly made everything I was seeing click. This thread was mostly just elaborating on what she said https://twitter.com/acstadermann/status/1300995832737460224?s=20
