CAPE CANAVERAL, Fla.—NASA is blaming unusually soft rock for last week’s sampling fiasco on Mars. The Perseverance rover came up empty after attempting to collect its first core sample on the red planet for eventual return to Earth. Data beamed back last Friday showed that the rover drilled to the proper depth of nearly 3 inches (8 centimeters), and pictures of the borehole looked good. But it quickly became clear the sample tube was empty. Since then, engineers have determined the rock was not strong enough to produce a core sample, and the small, powdery fragments remained in the hole or ended up in the cuttings pile—or both. So the rover is moving on to the next sampling site in its quest for signs of ancient Martian life; it should arrive there by early next month. Images taken by the rover and its companion helicopter, Ingenuity, show the sedimentary rock …