Complex sentence: Before the age of space probes, several astronomers created maps of Mars in which they gave names to the albedo features that they could see, the most popular system of nomenclature being devised by Giovanni Schiaparelli, who used names from classical antiquity.
Split into simpler sentences: Before the age of space probes, several astronomers created maps of Mars on which they gave names to the features they could see. The most popular system of nomenclature was devised by Giovanni Schiaparelli, who used names from classical antiquity.