The journey from the asteroid belt
Most meteorites begin as fragments of asteroids between Mars and Jupiter. This lesson follows the long, slow route that brings a few of them to Earth.
- Describe the asteroid belt and asteroid families.
- Explain how resonances and the Yarkovsky effect move fragments towards Earth.
- Explain what cosmic-ray exposure ages tell us.
Links to: Foundations: orbits and delivery.
The asteroid belt
The asteroid belt holds millions of bodies, from Ceres, about 940 km across, down to boulders and dust. Together they contain only a few per cent of the mass of the Moon. Jupiter's gravity prevented them from assembling into a planet.
Collisions are frequent over long timescales. A large collision shatters an asteroid into a family of fragments on similar orbits, many of which can still be recognised today. Several meteorite groups have been linked to particular families or to the break-up events that formed them.
Escape routes
Fragments drift slowly because of the Yarkovsky effect: a spinning body absorbs sunlight and re-radiates heat from its warmer afternoon side, giving a tiny thrust that changes its orbit over millions of years. Eventually a fragment drifts into a resonance, a zone where its orbital period is a simple fraction of Jupiter's. There, regular tugs from Jupiter stretch the orbit within about a million years until it crosses the paths of Mars and Earth.
Exposure ages
Once a fragment is only metres across, cosmic rays penetrate it and produce rare isotopes such as helium-3 and neon-21. Measuring them gives a cosmic-ray exposure age, the time spent as a small body in space. Most stony meteorites have exposure ages of a few million to a few tens of millions of years; iron meteorites, being tougher, often hundreds of millions. Clusters of similar ages within a group point to single collisions that released many fragments at once.
Visit the asteroids section of this site and look up Vesta and one asteroid of your choice. Note where each orbits and how big it is. Then write a few sentences describing the journey of a fragment from Vesta to Earth, using the words collision, Yarkovsky effect, resonance and exposure age.
Quiz
5 questions. Choose an answer to see whether you are right and why. Finish the quiz to complete the lesson.
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Question 1Why did no planet form in the asteroid belt?
Jupiter's gravity stirred the planetesimals so that they collided too violently to grow into a planet.
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Question 2What is an asteroid family?
A collision produces a group of fragments that share similar orbits.
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Question 3What does the Yarkovsky effect do?
Thermal re-emission gives a tiny but persistent thrust that shifts a small body's orbit over millions of years.
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Question 4What is a cosmic-ray exposure age?
Isotopes made by cosmic rays build up only once a body is small, so they measure its travel time.
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Question 5Why do iron meteorites often have longer exposure ages than stones?
Metal resists break-up and erosion, so iron meteoroids last longer in space.