How does the composition of cosmic rays compare with the composition of solar-system matter?

The composition of cosmic rays generally resembles that of solar-system matter, suggesting that overall galactic abundances are similar. However, there are notable differences in their respective makeups:12

• Light Elements: Cosmic rays contain significantly higher proportions of lithium, beryllium, and boron than are found in solar-system matter. These are produced when heavier cosmic-ray nuclei, such as carbon and oxygen, collide with interstellar gas and undergo spallation.234
• Hydrogen and Helium: Cosmic rays are deficient in hydrogen and helium compared to solar-system abundances.2
• Heavy Elements and Dust: Elements that preferentially form dust grains are found in enhanced abundances within cosmic rays. Additionally, cosmic rays are proportionately richer in heavy elements than stars.23
• Isotopic Differences: Significant differences exist between cosmic rays and solar-system matter regarding isotopes of neon and iron.2

Scientists believe cosmic rays represent a mixture of material, with approximately 80 percent reflecting solar-system composition and 20 percent originating from massive, evolved stars such as supernovas.2