Why do some meteorites contain gas from the Sun?
Grains that once lay on the surface of an airless asteroid were bombarded by the solar wind, which implanted solar gases into their outer layers.
The Sun constantly sends out a stream of charged particles called the solar wind, mostly protons and helium nuclei, with smaller amounts of heavier elements. On Earth, our magnetic field and atmosphere stop these particles. But on airless bodies such as asteroids and the Moon, the solar wind strikes surface grains directly.
When solar wind particles hit a grain, they penetrate only a very short distance, tens of nanometres, and become embedded. Over thousands to millions of years, grains at the surface accumulate significant amounts of solar hydrogen, helium, neon, argon and other elements.
Meteorites that contain these solar wind gases are usually regolith breccias, made of compacted surface soil. When scientists heat these meteorites in stages and measure the gases released, they find isotope ratios matching the Sun, measured directly by NASA's Genesis mission, which collected solar wind between 2001 and 2004.
These gases tell us several things. They show that a meteorite's grains once lay at the surface of their parent body. They reveal how long grains were exposed. They give information about the Sun's composition and, possibly, how the solar wind has changed over billions of years.
Solar wind can also create water. Hydrogen from the solar wind reacts with oxygen in minerals to form hydroxyl and water molecules. Studies of Itokawa grains in 2019 suggested this process could make water on asteroid surfaces, adding another possible source of Earth's water.
More about the science of meteorites
- What are short-lived radioactive isotopes and why do they matter?
- How long did it take to form the planets?
- What is the isotopic dichotomy in meteorites?
- Did Jupiter affect which meteorites reach Earth?
- What can meteorites tell us about the Moon's origin?
- Why are there no known meteorites from Mercury or Venus?