Big bangs

Really large meteorites produce impact scars big enough to be called basins. Although not defined by a particular size, basins differ from smaller craters in that the floor of a basin follows the planet’s curvature. New calculations published in the February 2011 Icarus by a team of scientists led by Margarita Marinova (NASA Ames Research Center) show that basin-size impacts throw debris globally and produce no rim or only a low one. The impacts also inject a lot of heat deep in the target planet and do not melt large areas of surface rocks. Slow-speed impacts add material to the planet, but collisions faster than 20 kilometers (12 miles) per second erode the planet that’s struck. Finally, big impacts in the closing stages of a planet’s formation could spin up its rotation.