The Hellas basin on Mars has just been reclassified as a volcanic hyper-caldera. Previously it was thought to be a gigantic impact crater. The basin is 1700 km wide and 8 km deep. It has long been a mystery how it could have formed.
Today, scientists reported to have found a tephra layer extending across the entire planet. The layer is 3.73 billion years old. The layer lacks iridium, which rules out an impact origin, and proves that it was caused by a volcanic eruption.
The size of the eruption is estimated at 20 million km3 (DRE), or three times more by volume of tephra, which in the scheme used for volcanic eruptions makes it a high VEI 12. It is the largest eruption known in the Solar System, exceeding even the famous eruptions of Yellowstone, Toba and Mt St Helens.
The scientists propose that the shock waves of the explosion propelled the core of Mars away from Hellas and so formed Olympus Mons, on the other side of the planet.