The biggest dust storms in 30 years have been raging since June on Mars, obscuring the planet’s surface, heating the upper reaches of the thin Martian atmosphere by up to 80 degrees and cooling the surface layers 10 degrees below normal, astronomers reported yesterday. From an earthly perspective, the global impact of the storms is roughly equivalent to the impact of the 1991 Mount Pinatubo volcanic eruption in the Philippines that carried ash and dust around the world.