The Spirit rover has crossed into a new martian frontier – an unexplored hilltop that might hold secrets from the planet’s earliest history. Since it arrived Jan. 4, Spirit has spent its time exploring rocks on top of a powdery surface of volcanic rock and ash – called an ejecta blanket – spewed out from an ancient meteorite impact, said Larry Crumpler, a Mars Exploration Rover team member and curator at the New Mexico Museum of Natural History and Science. The rocks there were interesting, but not nearly as cool or old as the rocks Spirit reached last week, when it got to Columbia Hills, named after the space shuttle that exploded in February 2003.