Complex sentence: Conditions on the Venusian surface differ radically from those on Earth because its dense carbon dioxide atmosphere is 96.5% carbon dioxide, with most of the remaining 3.5% being nitrogen.
Split into simpler sentences: Conditions on the Venusian surface differ radically from those on Earth because of its dense carbon dioxide atmosphere. The mass of the atmosphere of Venus is 96.5% carbon dioxide, with most of the remaining 3.5% being nitrogen.