Some of this I haven't read before and some is old news but more info in this article than most.

Blueberry Fields
Take, for instance, the Martian "blueberries," as some scientists are playfully calling them, small, iron-bearing balls found scattered all over inside the small crater on Meridiani Planum where Opportunity landed. There are at least three ways such spherules get formed on Earth, all of which are candidates for Mars as well.

The first is that they are the frozen drops of molten material flung out by asteroid impacts, explained Arvidson. Since Mars is covered with impact craters, this is perfectly plausible.

The second hypothesis is that the spherules were created during a volcanic eruption when ash particles in the air collected dust and dropped out of the eruption cloud. Tiny balls like this are found all over the Island of Hawaii, he said.

Finally, they could be chemical "concretions," formed by groundwater water passing through soils and causing some substances, like iron-bearing minerals, to cement grains of soils in spherical clumps that become harder than surrounding soil. When the surrounding soil erodes away, the strange rock balls remain. Concretions are found in a variety of places all over the Earth.

A closer look with the rover's mineral and chemical analysis instruments will help, but the spherules are too small to see closely with the rover's microscopic imager, Rover scientist Robert Sullivan of Cornel University told Discovery News. First they have to find a dense enough collection of the tiny spherules so that the instruments can fill their views with the spherules and nothing else.

Sticky Clods
Then there are the clods that the trenching maneuvers have turned up. After the rovers a dig a trench with their wheels, scientists are trying to see if the surface of Mars is any different than the soils a few inches down. While digging such a trench with Spirit in the Hawaii-sized Gusev Crater, they noticed something odd happening to the sand they were piling up alongside the trench: it was clumping.

Clods of soil might seem unremarkable, since they are so common on Earth. But on Mars they are intriguing because there is no apparent water to make the soil sticky, so what's holding the sand together? The two most likely answers are salts and electricity, Arvidson said.

Salts in water that could have leached through the soil during possible rare warm periods in Mars' history could have carried dissolved. When the ground dried, the salts remained, crystallizing like weak glue, sticking sand grains together to form clods.

The electrical possibility, on the other hand, requires no water at all, since it's akin to the gentle "static electric" force that makes dog hair stick to clothing on Earth.

Which hypothesis is correct? "I think it's really a work in progress," said Arvidson.

Naked Rocks
One sign that there hasn't been much water present for a very long time on Mars' surface are the rocks that have been studied by Spirit. In Gusev Crater Spirit used its Rock Abrasion Tool (a.k.a., RAT) to brush away dust on a rock called Adirondack and found that the rock was a lot like a basalt rock you might find near a low-lying extinct volcano in the Mojave Desert. Only something was missing.

"It's completely different then what you find in the Mojave Desert," Arvidson said. In the Mojave, seasonal moisture helps create a thin, weathered "varnish" layer on the surface of rocks. Adirondack shows no sign of a varnish layer, which would seem to indicate that Mars, said Arvidson, "has been in a cold deep freeze for a long time."

Layers of Debate
Perhaps the most intriguing mystery comes from the layered rocks found in the light-colored outcrop along the side of the small crater Opportunity is exploring. The texture of the rocks shows layering. Some scientists think they might even see what's called cross bedding in the rock.

On Earth, cross bedding is seen in rocks made of material that was being moved by water or air along ripples or dunes. If cross bedding is found in Mars rocks, it could be nothing more than windblown volcanic ash. Or it could be shallow lake sediments. "We're still debating that," said Arvidson.