About 4.6 billion years ago, the sun and planets of our solar system began to form. This involved the collision of various rocks and asteroids that melted together and gradually built the planets. Astronomers call this process "hot accretion".

The heavy metals such as iron would sink towards the centre of each molten world while the lighter rocks would float to the surface and cool into the landforms.

As the planets grew in size, the collisions between them tended to become more violent. Sometimes, the nascent worlds would be shattered rather than melt together.

Iron meteorites come from the deep, core regions of these doomed planets.

According to the statement from US space agency Nasa that accompanied an image of the Lebanon meteorite, the cavities visible on its surface may once have been filled with the mineral olivine.

If so, that could indicate it hailed from a place near the core-mantle boundary in its parent world. This was where the iron core gave way to the rocks and minerals of the mantle region.

Although this is the first meteorite found by Curiosity, its predecessor rovers Spirit and Opportunity both found numerous smaller examples. The first meteorite on Mars was discovered by Opportunity in 2005.

Meteorites may be common on Mars because there is little atmosphere to weather them away. On earth, meteorites survive for a few million years before being eroded away. On Mars, the weathering rate is probably a thousand times slower, so meteorites could last for billions of years. They would accumulate on the surface of the planet like flies on fly-paper.

Curiosity is almost two years into its mission. It is still en route to Mount Sharp, a shallow mountain it will attempt to ascend. Progress has been much slower than anticipated. This is partly because the mission's geologists keep wanting to stop and investigate interesting-looking rocks.