The Origin and Evolution of the Solar System

38 The structure of the Solar SystemFigure 1.20. A section through a chondritic meteorite.It has been suggested that these represent material ejected from the surface ofMars by a projectile. Mars could have been volcanically active in the sufficientlyrecent past to explain the SNC ages.1.6.3 Stony-ironsStony-irons consist of roughly equal proportions of stone and iron. A possiblescenario for their formation is within an interface region of a cooling solid bodyin which there had been separation of dense metal and less dense stone. However,one type of stony-iron, mesosiderites, contain minerals only stable at pressuresbelow 3 kbar suggesting that they are not directly derived from deep within amassive body.1.6.4 Iron meteoritesMost iron meteorites consist of an iron–nickel mixture that was originally in a liquidstate. There are, however, a few iron meteorites that look as though they havenever been completely molten. Within the metal two iron–nickel alloys form—