more inclined to the direction of illumination from the Sun and to the view of the observer. Wide open rings contribute about one magnitude of brightness to the disk alone. The icy particles that compose the rings also produce a strong opposition surge. Hubble Space Telescope and Cassini spacecraft images have been analyzed in an attempt to characterize the ring particles based on their phase curves. The Moon The phase curve of the Moon approximately resembles that of Mercury due to the similarities of the surfaces and the lack of an atmosphere on either body. Clementine spacecraft data analyzed by J. Hillier, B. Buratti and K. Hill indicate a lunar opposition surge. The Moon's apparent magnitude at full phase is −12.7 while at quarter phase it is 21 percent as bright. Planetary satellites The phase curves of many natural satellites of other planets have been observed and interpreted. The icy moons often exhibit opposition brightness surges. This behavior has been used to model their surfaces. Asteroids The phase curves of many asteroids have also been observed and they too may exhibit opposition surges. Asteroids can be physically classified in this way. The effects of rotation can be very large and have to be factored in before the phase curve is computed. An example of such a study is reported by R. Baker and colleagues. Exoplanets Programs for characterizing planets outside of the solar system depend largely on spectroscopy to identify atmospheric constituents and states, especially those that point to the presence of life forms or which could support life. However, brightness can be measured for very distant Earth-sized objects that are too faint for spectroscopic analysis. A. Mallama has demonstrated that phase curve analysis may be a useful tool for identifying planets that are Earth-like. Additionally, J. Bailey has pointed out that phase curve anomalies such as the brightness excess of Venus could be useful indicators of atmospheric constituents such as water, which might be essential to life in the universe. Criticisms on phase curve modelling Inferences about regoliths from phase curves are frequently based on Hapke parameterization. However, in a blind test M. Shepard and P. Helfenstein found no strong evidence that a particular set of Hapke parameters derived from photometric data could uniquely reveal the physical state of laboratory samples. These tests included modeling the three-term Henyey-Greenstein phase functions and the coherent backscatter opposition effect. This negative finding