  Europa's leading hemisphere chaos regions have a spectral feature at 450 nm
that has been attributed to absorption by crystal defects in irradiated sodium
chloride, known as F-centers. Some discrepancies exist between the laboratory
data of irradiated sodium chloride and the observations, including a $\sim$10
nm shift in central wavelength of the F-center band and the lack of the
prominent 720 nm absorption on Europa from M-centers, which result from the
coalescence of pairs of F-centers. Here, we perform irradiation experiments on
sodium chloride in an attempt to understand these discrepancies. We show that
careful control of the temperature of the sample at a temperature of 120 K
yields F-centers with an absorption wavelength comparable to that of Europa. In
addition, we measure the effect of photobleaching -- the destruction of
F-centers by photons -- and show that at the energetic particle and photon flux
on Europa, an equilibrium will be reached where only a modest F-center
absorption develops. The density of F-centers never reaches high enough values
for the creation of secondary M-centers. Our experiments predict that F-centers
grow during the night on Europa in the absence of photobleaching and then
partially decay during the daytime. We show observations from the Hubble Space
Telescope consistent with this prediction. All observations of the 450 nm
F-center on Europa are now consistent with laboratory measurements of sodium
chloride, confirming the presence of this salt on Europa.
