How a rock leaves Mars and reaches Earth

Several hundred meteorites found on Earth are pieces of Mars. No spacecraft brought them: each was blasted off the planet by an asteroid impact, drifted around the Sun for up to tens of millions of years, and finally fell to Earth. Here is how a rock makes that journey, and the evidence that proves where it began.

1. Step 1 of 7

   An asteroid strikes Mars

   Mars is smaller than Earth, so its gravity is weaker: anything launched faster than about 5 kilometres per second escapes into space for good, compared with 11.2 km/s from Earth. Its thin atmosphere barely slows an incoming asteroid, so impacts there keep almost all their speed.

   Even so, throwing solid rock off a planet is difficult. The impact must be large, probably making a crater at least a few kilometres across, and it must launch some rock fast without crushing it to powder or melting it completely. For a long time scientists doubted that this was possible at all.

2. Step 2 of 7

   Launched from the spall zone

   The answer lies in a thin layer of rock just below the surface around the impact point, called the spall zone. Here the shock wave travelling through the ground meets its own reflection from the free surface above. The two partly cancel, so the pressure stays moderate, yet the rock is given a huge upward kick.

   Fragments from this zone can leave faster than escape speed while staying solid. Martian meteorites carry the scars: many have been shocked to tens of gigapascals, enough to turn their feldspar into glass called maskelynite, but they are still recognisable volcanic and mantle rocks, not melted blobs.

3. Step 3 of 7

   Into orbit around the Sun

   Once free of Mars, each fragment is a tiny asteroid in its own right, travelling around the Sun. At first its orbit is close to Mars's own, but every fragment leaves at a slightly different speed and direction, so the swarm spreads out.

   Only a small fraction of all the rocks thrown off Mars will ever reach Earth. Some fall back to Mars, many are eventually thrown out of the inner Solar System or fall into the Sun, and others spend tens of millions of years wandering before anything happens to them.

4. Step 4 of 7

   Years in space, counted by cosmic rays

   In space, a small rock is exposed to cosmic rays, very fast particles from beyond the Solar System. They penetrate about a metre into rock and smash atomic nuclei apart, slowly building up rare isotopes such as helium-3, neon-21 and argon-38. On a planet's surface, under its rock and air, this barely happens, so the amount of these isotopes measures how long a meteorite has spent as a small body in space: its cosmic-ray exposure age.

   For Martian meteorites the clocks give from well under a million to about 20 million years. Many shergottites travelled for no more than about 5 million years, the nakhlites and chassignites for about 11 million, and ALH 84001 for about 15 million. Because these ages cluster in groups, scientists think all the known Martian meteorites came from only around ten separate impacts.

5. Step 5 of 7

   Crossing Earth's path

   A fragment from Mars does not head straight for Earth. Over thousands to millions of orbits, small gravitational tugs from Mars, Earth and Jupiter, together with close encounters, gradually stretch and tilt its path around the Sun.

   Eventually, for a lucky few, the orbit crosses Earth's at a point that Earth reaches at the same time. Then the rock enters our atmosphere like any other meteoroid, typically at more than 11 kilometres per second, and falls as a fireball.

6. Step 6 of 7

   Falling to Earth

   Martian meteorites arrive like any other, as fireballs that drop stones with a black fusion crust. Only a handful have been seen to fall, such as Chassigny in France in 1815, Shergotty in India in 1865, Nakhla in Egypt in 1911, Zagami in Nigeria in 1962 and Tissint in Morocco in 2011.

   Most have been found later, in the hot deserts of north Africa and Oman and on the ice of Antarctica, where dark stones are easy to spot and weathering is slow. Many finds are paired pieces of the same fall, so the few hundred known Martian meteorites represent fewer separate stones than the total suggests.

7. Step 7 of 7

   Proving it came from Mars

   The decisive evidence came in the early 1980s. Pockets of glass, melted by the shock of launch, inside the Martian meteorite Elephant Moraine 79001 were found to hold trapped gases whose amounts and isotopes match the Martian atmosphere as measured by the Viking landers in 1976. No other source fits.

   Other clues agree. Many Martian meteorites crystallised from lava only 150 to 600 million years ago, far too recently for a small asteroid, which would have frozen solid within the first few hundred million years; they need a large planet that was still volcanically active. Their oxygen isotopes form a distinct family, and their chemistry matches rocks analysed on Mars by rovers.

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