An interesting article in Astronomy discusses the subject of crater clusters on Mars. Crater clusters are defined as groups of small craters, generally consisting of 10 or more craters less than 1- to 2-km-diameter, which overlap and thus are presumed to have formed simultaneously. N. G. Barlow suggested several years ago these were formed long ago by incoming meteors breaking up before impact in a then-thicker Martian atmosphere, and cited previous crater clusters imaged on Venus by Magellan as a prior example of such processes. Crater clusters happen on Earth like this, too. William Hartmann of the Planetary Science Institute originally agreed with this thick-Mars-atmosphere idea but has since suggested that these craters are created by martian rocks launched from the planet’s surface during impacts, as described in his recent paper “SNCs That Don’t Make It”. Those SNCs that DO make it land on Earth and constitute our only samples of Mars currently available for study.