The reason for this lies in the fact that if light illuminates a surface at an oblique angle, a large proportion of it is reflected multiple times by the surface unevenness before either being reflected back into space with a small loss in brightness or partially absorbed by cavities from which it can no longer radiate. In the case of opposition, however, the entire amount of light that hits the surface vertically and is not absorbed by the surface material is reflected back into space. This leads to a significant brightening of the surface and partially reveals structures that are not visible under oblique illumination, such as the radiating ejecta patterns around impact craters. For scientists, studying the exact reflectivity of a planetary surface at different phase angles – deriving its phase curve – allows them to make statements about its material properties, such as the degree of weathering of the regolith, which darkens over many millions of years due to bombardment by micrometeorites.

Planetary surfaces reflect sunlight in very different ways. At full moon, the Moon is actually reflecting only 12 percent of the incident sunlight. Scientists specify this value – referred to as the geometric albedo – as the proportion of sunlight reflected at a phase angle of zero degrees. The lunar disc illuminated by the Sun is in sharp contrast to the black of space. In reality, the ‘mirror’ of the lunar disc provides us with only one millionth of the sunlight that reaches Earth directly from the Sun. Earth’s average albedo is 36.7 percent. On Mars it is 15 percent and on the Martian moon Phobos it is just seven percent. Scientists have been pondering the cause of this for decades. Perhaps the difference in brightness is an indication that Phobos is not debris from a gigantic impact on Mars, but that the misshapen body is an asteroid captured by the planet’s gravity.