This is one of the strangest space images I've seen in a long time. This is a mosaic of four images taken with Opportunity's Microscopic Imager of an outcrop of rock facing the interior of Endeavour crater.
Now, Opportunity has seen spherical things embedded in rock before: the ubiquitous blueberries of Meridiani planum. But Opportunity is now looking at rocks more ancient than (because they're found underneath) the blueberry-bearing ones. Furthermore, these don't look like the blueberries. They're more resistant to erosion than blueberries, but they also have a tendency to crack in half, which blueberries generally don't. And the cracked-open ones seem to have rinds with different properties from their interiors. And there's so many of them, packed so closely together. What could they be?
I'm looking forward to finding out what the science team thinks!
In the meantime, I've been sadly remiss in featuring Stuart Atkinson's Road to Endeavour blog posts here. I will need to concatenate a few together to build a story about what Opportunity's been doing lately!