Picture a 40 hour nonstop drive from New York to California along the edge of a single cliff whose bottom disappears into haze. That is the scale of Valles Marineris, the Martian canyon system over 4,000 kilometers long, 200 kilometers wide and up to seven kilometers deep, stretching across nearly a quarter of the planet’s circumference. This episode recounts its 1971 discovery by Mariner 9 as a global dust storm cleared, then explains the leading theory: a tectonic rift fault created when magma upwelling and the enormous weight of the Tharsis volcanic plateau overloaded a stagnant lid crust that, unlike Earth’s, has no plate tectonics to relieve the stress.

The hosts tour the canyon west to east. Noctis Labyrinthus, the labyrinth of the night, formed by downward block faulting and is now an astrobiology target after the 2024 identification of the eroded volcano Noctis Mons with glacier deposits at its base. The central chasmata show groundwater sapping valleys like those on the Colorado Plateau and country-sized landslides likely triggered by impact shock waves during the Late Heavy Bombardment. Coprates Chasma plunges 11 kilometers, could have held a kilometer-deep lake, and hosts over a hundred volcanic cones crater-dated to just 200 to 400 million years old. The eastern outflow into Chryse Planitia shows teardrop islands and scoured bedrock compared to the Missoula floods of the Channeled Scablands. The episode closes on the canyon’s likely localized weather and what Earth’s own erased scars might have looked like.

• The bookshelf analogy for how Tharsis volcanism snapped the Martian crust
• Stagnant lid planets versus plate tectonics, and why Olympus Mons grew so large
• Noctis Mons, glaciers and the extremophile case for life in the labyrinth
• Crater counting and the surprisingly young volcanism on the canyon floor
• Catastrophic outflow floods and the Washington state Scablands comparison