Several kinds of scientific evidence are used to determine where meteorites come from. These include the crystallization age of the meteorite, its chemical and physical composition, the effects that cosmic rays have had on it, and the composition and concentrations of gases trapped long ago in tiny fissures and bubbles in the meteorite. From this evidence it was determined that ALH84001 originated on Mars. So when features were found in the meteorite that resembled fossilized microorganisms, it sparked a worldwide sensation that ancient life may have existed on Mars.

Gas giant planets are so named because they are much larger than the terrestrial planets, and they have atmospheres so thick that the gas is a dominant part of the planets’ structures.