What is a mesosiderite?
A mesosiderite is a stony-iron meteorite made of roughly equal parts metal and broken basaltic rock, a mixture thought to result from a violent collision.
Mesosiderites are stony-iron meteorites made of iron-nickel metal mixed with silicate rock in roughly equal amounts. The name comes from Greek words meaning 'half iron'. Unlike pallasites, which contain olivine crystals in metal, mesosiderites contain fragments of basalt, gabbro and pyroxenite, similar to the HED meteorites from Vesta.
This mixture is a puzzle. The metal looks like molten core material, while the rock fragments look like crust. How did core and crust mix? The most widely accepted idea is that a collision between two bodies mixed the molten core of one with the crust of another, or with its own surface, perhaps in a huge impact.
Their metal cooled extraordinarily slowly at low temperatures, at well under 1 °C per million years, among the slowest cooling rates known. This suggests that after mixing, the material was buried deep inside a large body.
The silicates in mesosiderites have oxygen isotopes very similar to the HED meteorites, and some scientists think mesosiderites come from Vesta itself. Others think they come from a similar but separate body. Dating suggests the mixing event happened roughly 4.5 billion years ago.
Mesosiderites are often found as large masses. Famous examples include Vaca Muerta in Chile, Estherville in Iowa, which fell in 1879, and Mount Padbury in Australia. On a polished slice, they show a striking pattern of shiny metal and dark rock fragments.