What’s the oldest thing you’ve ever seen? An Egyptian mummy? They tend to be between about 6,500 and 5,000 years old—I think we can do better than that. Maybe a dinosaur bone? That’s getting better, the last dinosaurs went extinct about 66 million years ago.
But they both pale into insignificance compared to meteorites, the oldest of which are 4.576 billion years old—that’s the age of our Sun and our Solar System.
A meteorite is a rock that has come from space and landed on the surface of Earth (or another planet). If you’ve ever seem a shooting star or fireball in the sky that’s a meteor, we only call it a meteorite when it has landed on Earth and we can pick it up.
We currently have a collection of over 56,000 meteorites on Earth. Most of these originated from the asteroid belt between Mars and Jupiter, which comprises thousands, perhaps millions, of objects ranging from a few hundred kilometres to a few meters across. The largest asteroid, Ceres, is actually classified as a dwarf planet. It is 950 km in diameter, less than 1/10th of the diameter of Earth. About 300 of the meteorites now on Earth are from the Moon, and almost 200 from Mars. All these space rocks are scientifically important as they preserve a record of the origin and evolution of our Solar System, including our own planet, Earth.
Earth is geologically active—we have earthquakes and volcanoes, rocks get hot, are subjected to huge pressures, or eroded by water and wind. The surface of Earth is ever-changing, and the geological record from when Earth first formed has been overwritten by four and a half billion years of processing.
Different asteroids have different chemical compositions and physical properties, and we can see the distinctive properties of the individual bodies, whereas the individual characteristics of the smaller bodies that contributed towards forming our planet are now all mixed up together in the earth.
Some asteroids have barely changed since they first formed four and a half billion years ago, and are thought to retain chemical compositions very similar to the starting material of the solar system. Other asteroids grew large enough in the early beginnings of the Solar System that they melted (heated by radioactive decay) and underwent a process known as differentiation – the heavy materials like metals sank to the middle of the asteroid, and the lighter, rocky materials rose to the surface. This process also happened on Earth—Earth has a metal core, and we live on the rocky outer surface which contains very little metal.
So by looking at asteroids we can learn about the chemical building blocks of the Solar System, and the types of physical processes taking place in the first few millions of years of its history. We could go to asteroids, and bring samples back to Earth – the Japanese Aerospace Exploration Agency’s (JAXA) Hayabusa mission brought back material from an asteroid called Itokawa in 2010, and Hayabusa 2 and NASA’s OSIRIX REx missions are due to return asteroid samples in 2020 and 2023 respectively. But these missions are very expensive (billions of dollars) and so only a few have taken place. In comparison, as meteorites come to Earth by themselves, they are easily accessible samples of our planetary neighbours.
Scientists from the Isotope Geochemistry and Cosmochemistry Group in the School of Earth and Environmental Sciences at the University of Manchester use meteorites and other extraterrestrial materials to understand the chemical and physical evolution of the Solar System. Come and meet them at the Science Spectacular, Pi: Meteorite Hunt and Science @ Central during the Science Festival to learn more about meteorites, the Solar System and their research. And you can even see and hold some of those 4.567 billion year old rocks!