NASA on the Nodules:
"This is wild looking stuff," said Steven Squyres, principal investigator for the Mars Exploration Rover (MER) project from Cornell University,
referring to microscopic images revealing the rocks layers and embedded spherical grains.
At a press conference today, Squyres said the layers of the rock are made of fine material, probably either dust or sediment, and not sandstone. Each
layer is just a few millimeters thick.
The tiny spherules, as they are called, are embedded in the layers "like blueberries in a muffin," he said.
The spherules are clearly made of a different material than the rock's primary layers, or matrix, Squyres said. The matrix is tan or buff in color,
he said, and the spherules are very gray.
"That's a hint that they may be different in composition," he said.
Millions of years of sandblasting in the harsh Martian environment have exposed many of the spherules. Some have dropped out of the rock and others
are hanging on like a child's loose tooth. This makes them ripe for observation.
The spherules are tougher material than the rock matrix, Squyres said. Some are "strung like beads" along a crack in the rock. But his team does not
yet know what they're made of or how they formed.
There are three leading hypotheses:
The spherules might have formed when ash from a volcanic eruption was suspended in the air, stuck together, and fell from the sky. That idea is
rapidly falling from favor, Squyres said, because it would tend to produce spherules made of the same material as the rock's main matrix.
They might have formed when molten rock -- either from a volcano or a meteor impact -- froze in mid-air into glass beads.
Most interesting, they might be "concretions," which form when a fluid, possibly water, carrying dissolved minerals flows through a rock and
"precipitates" a grain that typically grows into a sphere.