A section of Mars' Utopia Planitia covered with dark volcanic materials is slowly expanding across the Red Planet's surface. And experts are not sure why. (Image credit: ESA/DLR/FU Berlin) A massive dark patch lurking within a giant Martian crater has been creeping across the Red Planet's surface since the feature was first spotted 50 years ago, new photos reveal — and scientists are unsure exactly why this is happening. The shadowy structure is a patch of ground covered with ash and volcanic rocks, such as olivine and pyroxene, from ancient eruptions that occurred millions of years ago , before Mars was considered geologically dead. It is located in Utopia Planitia, a roughly 2,000-mile-wide (3,300 kilometers) plain in Mars' northern hemisphere. It is unclear how large the shadowy feature is. However, sections of the patch's southern boundary have moved farther south by at least 200 miles (320 km), based on the latest photos from ESA .…