In all the fuss about Curiosity, the new kid on the Martian block, it's been easy to forget about the Opportunity rover, and the work it has been doing for eight long years. Well, it seems Opportunity has made an interesting discovery worth looking at — small, round spherules which scientists are calling "blueberries."
So, just what are these things? Only a few millimeters across, some appear to be embedded in the rock while others are loose on the soil. The perplexing little orbs are believed to be concretion — a phenomenon also found on Earth. They are hard, compact masses of rock formed by mineral cement found within layers of sedimentary strata. They usually form before the rest of the sediment is hardened and buried in time.
Opportunity had previously discovered orbs in the Meridiani Planum, which have been found to contain hematite. One of the reasons the rover went there in the first place was that they resemble iron-oxide concretions in Utah known as the Moqui Marbles, which formed in groundwater. Could water have formed these, too?
But the Martian "blueberries" concretions are different. It's an unusually dense field of little spheres found near Opportunity's landing site on the eastern side of Cape York — the island-like shelf on the edge of the Endeavour Crater. With material that looks like "fins," it was identified from orbit as an area of interest for study in the Opportunity mission for its small clay deposits.
The rover is currently examining the area, so whether it actually has clay isn't clear yet. Interestingly, some of the "blueberry" spherules have broken open, allowing the Microscopic Imager (MI) on the rover to snap shots of their insides.
The unusually dense and physical nature of the outcrop is of interest because scientists are interested as to whether they are related to the clay deposits. Given how different they appear, are these different from the concretions found earlier? No hematite signature was detected while in orbit, so everyone is curious as to what might have made this "blueberry" field so different.
One of the tasks assigned to Opportunity was to specifically look for clay deposits, as that could indicate the berry-like formations were made in non-acidic water as clay often does on Earth. Though there is no absolute explanation for Martian clays, the Opportunity has found white gypsum-like veins seen near Cape York, which also indicates liquid water was likely in this location in the distant past.
As we wait to find out what is inside these strange "blueberries," Opportunity will be exploring the clay deposits along Endeavour's rim at Cape Tribulation.