Complex sentence: The Hesperian is an intermediate and transitional period of Martian history during which Mars changed from the wetter and perhaps warmer world of the Noachian to the dry, cold, and dusty planet seen today.
Split into simpler sentences: The Hesperian is an intermediate and transitional period of Martian history. During the Hesperian Period, Mars changed from the wetter and perhaps warmer world of the Noachian to the dry, cold, and dusty planet seen today.