The magnitude and albedo of Mars

A comprehensive set of magnitudes obtained between 1954 and 2006 are analyzed. The martian brightness and its variations are characterized empirically at UBVRI wavelengths. Geometrical factors including phase angle, orbital longitude and rotation angle are distinguished from geophysical factors including dust storms and changing albedo features. The phase function indicates a brightness surge near opposition at all wavelengths except possibly in the U band. The color indices reveal increased reddening with phase angle. No significant brightness difference between morning and evening hemisphere observations is indicated with the possible exception of the I band. There is no conclusive evidence for inter-annual brightness variation during the years from 1991 to 2006 when abundant photometry is available. Major dust storms caused brightness excesses that were strongest in the R band at an average of 0.15 mag more luminous than the empirical model for dust-free conditions. The storm of 2001 produced a rapid increase at the onset followed by a slower decline, while the 2003–2004 event show a more gradual increase. The return to normal brightness was linear in magnitude for both storms. Brightness excesses at longer wavelengths were about 0.20 to 0.25 mags at the peak of the 2001 storm. The observed geometric albedo of Mars is 0.059±0.001 in U, 0.089±0.001 in B, 0.170±0.002 in V, 0.289±0.003 in R, and 0.330±0.003 in I. The corresponding albedo values for all five colors exceed those recorded in the literature, with larger percentage increases at shorter wavelengths.