Addendum: the day after this article was posted, an official report about these oddities was posted by NASA here. Interesting!
One of the most interesting discoveries made so far by the Opportunity rover on Mars has been the small round spherules or “blueberries” as they are commonly referred to, covering the ground at the rover’s landing site. Typically only a few millimetres across, some lie loose on the soil while others are imbedded in rock outcrops.
Analysis by Opportunity indicates that they are most likely a type of concretion, which are also found on Earth. These Martian concretions have been found to contain the mineral hematite, which explains its detection in this region from orbit, and one of the main reasons that the rover was sent to this location in Meridiani Planum in the first place. They are similar to the Moqui Marbles, iron-oxide concretions in the outcrops of Navajo Sanstone in Utah, which formed in groundwater.
Now, the rover (eight years later and still going!) has found what may be a different type of spherule. These ones generally resemble the previous ones, but are quite densely packed in an unusual rock outcrop that is on the eastern side of Cape York, the small island-like ledge on the rim of the huge Endeavour crater. With brittle-looking “fins” of material, the outcrop is an an area that from orbit has been identified as containing small clay deposits. There are also more substantial clay deposits farther south along Endeavour’s rim at the much larger Cape Tribulation, the next major destination of Opportunity.
See Universe Today for the full article.