The 8 year long mission of the Mars rover Opportunity just
imaged this collection of blueberries, or Martian spherules.
These spheres don't show the iron/hematite abundance
of all the others.
http://www.jpl.nasa.gov/news/news.php?release=2012-290

Micro imager
http://marsrover.nasa.gov/gallery/al...ity_m3064.html

Panoramic imager
http://marsrover.nasa.gov/gallery/al...ity_p3064.html


NASA Mars Rover Opportunity Reveals Geological Mystery


Opportunity is investigating an outcrop called Kirkwood in the Cape York
segment
of the western rim of Endeavour Crater. The spheres measure as much as
one-eighth
of an inch (3 millimeters) in diameter. The analysis is still preliminary,
but it
indicates that these spheres do not have the high iron content of Martian
blueberries.

"This is one of the most extraordinary pictures from the whole mission,"
said
Opportunity's principal investigator, Steve Squyres of Cornell University in
Ithaca,
N.Y. "Kirkwood is chock full of a dense accumulation of these small
spherical objects.
Of course, we immediately thought of the blueberries, but this is something
different.
We never have seen such a dense accumulation of spherules in a rock outcrop
on Mars."

The Martian blueberries found elsewhere by Opportunity are concretions
formed by
action of mineral-laden water inside rocks, evidence of a wet environment on
early Mars.
Concretions result when minerals precipitate out of water to become hard
masses
inside sedimentary rocks. Many of the Kirkwood spheres are broken and eroded
by
the wind. Where wind has partially etched them away, a concentric structure
is evident.

Opportunity used the microscopic imager on its arm to look closely at
Kirkwood.
Researchers checked the spheres' composition by using an instrument called
the
Alpha Particle X-Ray Spectrometer on Opportunity's arm.

"They seem to be crunchy on the outside, and softer in the middle," Squyres
said.
"They are different in concentration. They are different in structure. They
are different
in composition. They are different in distribution. So, we have a wonderful
geological
puzzle in front of us. We have multiple working hypotheses, and we have no
favorite
hypothesis at this time. It's going to take a while to work this out, so the
thing to do
now is keep an open mind and let the rocks do the talking."

Just past Kirkwood lies another science target area for Opportunity. The
location
is an extensive pale-toned outcrop in an area of Cape York where
observations from o
rbit have detected signs of clay minerals. That may be the rover's next
study site
after Kirkwood. Four years ago, Opportunity departed Victoria Crater, which
it had
investigated for two years, to reach different types of geological evidence
at the rim
of the much larger Endeavour Crater.


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