Los Angeles ? The Opportunity rover used its robotic arm to inspect a freshly dug hole on Mars, while its twin, Spirit, rolled into a shallow depression halfway around the Red Planet, NASA said Wednesday.

Opportunity examined the 4-inch-deep trench it scooped out with one of its six wheels, measuring the composition of the clumpy soil and photographing it with a microscopic imager.

Scientists chose the trench site because earlier observations revealed it is rich in hematite, an iron-bearing mineral that typically forms in water.

On the other side of the planet, Spirit passed the halfway mark of its planned 90-day mission.

Spirit rolled into a shallow depression, nicknamed Laguna Hollow, after completing a 63-foot drive. The depression is probably an impact crater.