Earth differs greatly from its two closest neighboring planets, Venus and Mars. The Venusian and Martian atmospheres are composed almost entirely of carbon dioxide,while Earth's atmosphere contains very little. Tha dominant meterial in our atmosphere is nitrogen (77percent). The other major component of Earth's atmosphere is oxygen (21percent), a gas that is almost nonexistent on Venus and Mars. Our planet has an abundance of water, which covers about 70 percent of Earth's surface and supports life on our planet. In contrast, Venus and Mars are extremely dry planets and incapable of supporting life.