  Recent laboratory efforts and telescopic observations of Europa have shown
the relevance of a yellow colouration of sodium chloride (NaCl) caused by
crystal defects generated by irradiation. We further investigate this process
by irradiating (with energetic electrons) different types of analogues where
NaCl is associated in different ways to water ice. We produce two types of icy
analogues: compact slabs and granular particles where we investigate two
particle sizes (5 and 70 $\mu$m). We perform electron irradiation at cryogenic
temperatures (100 K) and under high vacuum (10-7 mbar) conditions, with
energies of 1 and 5 keV. We observe the formation of two different types of
colour centres. The so-called F-centres (460 nm) were formed in every sample,
but the intensity of the absorption band within the compact slabs surpassed any
other icy analogues and was comparable to the intensity of the absorption band
within pure NaCl grains. M-centres (720 nm) have not been detected at the
surface of Europa so far, and were close to the detection limit during our
irradiation of compact slabs. The slabs could be good analogues for Europa_s
surface as they produce mainly F-centres. Other notable differences have been
observed between compact slabs and granular samples, such as the presence of an
absorption band at 580 nm attributed to colloids of Na, exclusively within
granular samples. Such absorptions have not been reported in previous studies.
