Albedo

The albedo of a planet is the ratio between the amount of incident light on that planet to how much light is reflected.

$\alpha =\frac{I}{R}$, where $\alpha$ is albedo, $I$ is the measure of incident light and $R$ is the measure of reflected light.

The albedo is a strong factor in the regulation of a planet's atmosphere. Albedo may be increased by surface features (i.e. ice or snow in polar regions) or atmospherics (i.e. clouds), so any changes in climate strongly influences albedo. In the case of an ice age, much of a planet's surface may be covered in snow, increasing the reflectivity, hindering the greenhouse effect acting on the atmosphere. This would therefore lead to further cooling. Likewise, if all the ice melts, the albedo will decrease, enhancing the greenhouse effect, amplifying global warming.

The albedo of Mars is .16 or 16%.