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48 Why was Steve so thrilled to see rock layers? On Earth and on Mars,
rock layers tell us about the geologic, weather, and climate conditions
that prevailed during the years and decades each layer was formed. It’s
like rings inside a tree stump: some rings are thick and others are thin,
reflecting the weather conditions that accompanied the tree’s growth.
Steve and the other scientists looked at rock layers the rover
photographed and wondered whether they were brought about by
volcanic eruptions, blowing dust, flowing water, or all three.

49 Over the coming days and weeks, Steve and his team studied the
images that filled their screens, and debated what they meant. Many of
Opportunity’s photos featured blueberry-shaped pebbles strewn across
the soil, like beads spilled from a broken necklace. “They were the
strangest-looking things I’d ever seen on Mars,” Steve said.
50 Some team members thought the pebbles looked like volcanic
hailstones. Others thought they might be droplets of lava that had cooled
quickly.

51 Like detectives intent on understanding a clue, the team considered
other possibilities. Maybe the rocks had rolled around in water, which
smoothed them. Maybe material dissolved in water had dried out and
solidified layer by layer to make the round forms. Was what they were

seeing evidence that water had once existed on Mars?
52 To find out, Opportunity took close-up photos, tried RATing a
blueberry, and took measurements. He discovered some salts!

This magnified photo of Eagle Crater, taken by the
Microscopic Imager on Opportunity’s arm, shows coarse
grains that scientists nicknamed blueberries. The examined
patch of soil is 1.2 inches (3 cm) across, and the largest
blueberry shown is about the size of a sunflower seed.
This high-resolution image captured by Opportunity’s
panoramic camera shows the rock outcrop on the rim of Eagle
Crater, where the rover landed. These layered rocks measure
only 4 inches (10 cm) high. Data from the panoramic camera’s
near-infrared blue and green filters were combined to create
this approximate, true color image.

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