It weighs only 1.9 kilograms and is an estimated 4.5 billion years old. Yet the ALH84001 meteorite is probably the most argued-about rock in the universe (or at least in our solar system.) The anonymous member of an Antarctic geological field survey who found the rock in 1984 wrote in the field ticket's margins, "Yawza yawza!" Twenty years later, scientists are still making exclamations about the rock's innards. Some suspect that the ghostly forms in this scanning electron micrograph are merely mineralogical.[1] David McKay of the Lyndon B. Johnson Space Center famously asserted in 1996 that they are fossilized Martian microbes[2] – the first official alien encounter.