Geometric albedo node of their orbit, and hence lie on the ecliptic. In practice, measurements at small nonzero phase angles are used to derive the parameters which characterize the directional reflectance properties for the body (Hapke parameters). The reflectance function described by these can then be extrapolated to zero phase angle to obtain an estimate of the geometric albedo. For very bright, solid, airless objects such as Saturn's moons Enceladus and Tethys, whose total reflectance (Bond albedo) is close to one, a strong opposition effect combines with the high Bond albedo to give them a geometric albedo above unity (1.4 in the
Classical albedo features on Mars their borders change in response to windstorms on the Martian surface that move the dust around, widening or narrowing the features. The dust-storms themselves also appear as light patches, can cover vast areas and sometimes last for many weeks; when Mariner 9 arrived in Martian orbit in November 1971 the entire planet was covered by a single enormous dust-storm, with only the peaks of the four or five highest mountains showing above it. This variability may explain many of the differences between telescopic observations over the years. The first map of Martian albedo features was published in 1840 by Johann