Helium is the second most abundant element in Saturn's atmosphere.
Saturn's upper atmosphere is dominated by molecular hydrogen, which makes up about 96.3% by volume. Helium follows at roughly 3.25%, while methane, ammonia and other compounds occur only in trace amounts. The percentages describe the atmosphere's outer layers, not Saturn's entire interior.
Saturn's helium abundance is notably lower than the Sun's. Scientists think helium has gradually separated from hydrogen inside the planet and “rained” downward as droplets, releasing heat as it sinks. This process helps explain Saturn's excess internal warmth and its brighter-than-expected infrared emission.
A common mix-up is confusing Saturn's atmospheric composition with Earth's: oxygen is abundant in Earth's atmosphere but is only a trace constituent at Saturn. Ammonia helps form Saturn's visible upper clouds, yet it is not the second-most abundant atmospheric element.