Terraforming of Mars incoming sunlight in terms of heat absorption. This could be done by spreading dark dust from Mars's moons, Phobos and Deimos, which are among the blackest bodies in the Solar System; or by introducing dark extremophile microbial life forms such as lichens, algae and bacteria. The ground would then absorb more sunlight, warming the atmosphere. However, Mars is already the second darkest planet in the solar system, absorbing over 70% of incoming sunlight so the scope for darkening it further is small. If algae or other green life were established, it would also contribute a small amount of oxygen to
Solar System moons. The centaurs and many short-period comets also orbit in this region. Due to their greater distance from the Sun, the solid objects in the outer Solar System contain a higher proportion of volatiles, such as water, ammonia, and methane than those of the inner Solar System because the lower temperatures allow these compounds to remain solid. The four outer planets, or giant planets (sometimes called Jovian planets), collectively make up 99% of the mass known to orbit the Sun. Jupiter and Saturn are together more than 400 times the mass of Earth and consist overwhelmingly of hydrogen and helium.