Is the white stuff in the Martian soil ice or salt? That's the question bedeviling scientists in the three weeks since the Phoenix lander began digging into Mars' north pole region to study whether the arctic could be habitable. Shallow trenches excavated by the lander's backhoe-like robotic arm have turned up specks and at times even stripes of mysterious white material mixed in with the clumpy, reddish dirt. Phoenix merged two previously dug trenches over the weekend into a single pit measuring a little over a foot long and 3 inches deep. The new trench was excavated at the edge of a polygonshaped pattern in the ground that may have been formed by the seasonal melting of underground ice. New photos showed the exposed bright substance present only in the top part of the trench, suggesting it's not uniform throughout the excavation site. Phoenix will take images of the trench dubbed "Dodo-Goldilocks' over the next few days. If it's ice, experts expect it to sublimate