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
Albedo feature regio. On bodies with a very thick atmosphere like Venus or Titan, permanent albedo features cannot be seen using ordinary optical telescopes because the surface is not visible, and only clouds and other transient atmospheric phenomena are seen. The "Cassini–Huygens" probe observed multiple albedo features on Titan after its arrival in Saturn's orbit in 2004. The very first albedo feature ever seen on another planet was Syrtis Major Planum on Mars in the 17th century. Today, thanks to space probes, very high-resolution images of surface features on Mars and Mercury are available, and the classical nomenclature based on albedo features