to 49.2% of the Earth's crust as part of oxide compounds like silicon dioxide and other compounds like carbonates. It is also the main part of the Earth's oceans, making up 88.8% by mass. Oxygen gas is the second most common part of the atmosphere, making up 20.95% of its volume and 23.1% of its volume. Earth is strange compared to other planets, as a large amount of its atmosphere is oxygen gas. Mars has only 0.1% by volume, with the other planets having less than that.