When the Opportunity rover first landed on Mars in 2004, one of the first discoveries it made was that the ground was covered by tiny, round spherules up to a few millimetres in diameter. Also found imbedded in rock outcrops, they were an unexpected and fascinating surprise. What were they?
After extensive analysis by the rover, it was concluded by the scientists involved that they were concretions – little bb-like iron-oxide spherules similar to those found on Earth, notably those in the Navajo Sanstone deposits in Utah. The Martian ones also contain the mineral hematite, explaining the hematite signature seen in this region from orbit by the Mars Reconnaissance Orbiter spacecraft.
See Examiner.com for the full article.