I've just looked at the Sol 27 panoramics for Opportunity, and it is
very clear to me that these are indeed sedimentary rocks. A simple
explanation would be that the spheres are the fossils of some simple
organism, that upon dying, would drop to the bottom and be buried in the
same manner that fossils here on Earth can be trapped in layers of sediment.
You can clearly see that the spheres are trapped in the layers, and that
they are eroding out. The least hypothesis is that the spheres were
deposited in the layers as the layers were forming. They are more durable
than the layers themselves, which is easy to see in the weathering images
from the microscope.
Furthermore, the spheres themselves show some characteristics that are
otherwise extremely difficult to explain using standard geological
processes. As a long time amateur rockhound, I have seen and collected many
specimens, both abiotic and fossils. I could easily understand a sphere
with consistent layering throughout, but not many spheres with similar
markings such as parallel grooves or chevrons.
I have compiled the microscopic images and performed some contrast and
image processing to extract features that are otherwise faint or difficult
to discern, and there are definitely common features on many of the spheres.
In my (perhaps flawed but experienced) opinion, we are seeing fossils.
There, it's been said.
I have a good reason to take this position- in 1992, I wrote an article
about the possibility of life on Mars (which was published in Astrolog
magazine), and used some of the reasoning of Thomas Gold about petroleum
formation and organisms that metabolize petroleum.
In it, I predicted that organisms could still be extant in the rock of
Mars if it consumed petroleum as many such organisms here on Earth do.
Also, I predicted that in that case, we should look for fine grained
magnetite, which is a metabolic byproduct of the digestion process of those
sorts of organisms.
This was four years before the flap about ALH84001 (1996) so in a sense
I beat them to press with at least two good predictions that matched what
they found.
At this point, I am very encouraged by what we are seeing that life did
indeed exist on Mars, and that if we were to bore deeply into the rocks
where petroleum might exist, we would discover that deep inside the planet,
there are still organisms that are alive and well.
After all, a loss of atmosphere here on Earth would not destroy those
organisms that live within the rock.
I intend to post these processed images on my website shortly, as this
is a very interesting development and it is good, reasonable support for
this idea.
Once again, I am not a geologist, but I am a scientist and have been a
rock collector for about 40 years. My opinions could be completely wrong,
but I believe that Opportunity has succeeded in finding remnants of extinct
Martian organisms.

Cheers!

Chip Shults