Earth and Mars formed from the same stuff swirling around our infant Sun. These planetary siblings may have been alike in their early days. But their different positions in the Solar System family and different histories led one to become a water world smothered in life and the other a cold, dry desert. Earth’s geology is our template to understand—and sometimes misunderstand—our planetary sibling. This book unpacks decades of scientific exploration of Earth and Mars to tell the story of how we figured out what we know about Mars. It sets the stage to consider the biggest question of all: Is there or was there ever life on Mars? The book explores Mars through the lens of planetary geology with chapters on: Solar System formation Minerals Plate tectonics Sedimentary rocks and environments Magma and volcanoes Marsquakes Mars' internal structure Ice Water Atmosphere and weather The possibility of life All of these discoveries come together to reveal a history for Mars, including humankind's attempts to unlock its secrets.