Orbiting hundreds of miles above the rusty plains of the Red Planet, high resolution cameras aboard the European Space Agency’s Mars Express spacecraft and NASA’s Mars Reconnaissance Orbiter have captured breathtaking views of a geological feature that looks as if a cosmic blade slashed straight across the face of the planet. Stretching across the Martian equator for more than two thousand five hundred miles, Valles Marineris—alongside smaller neighboring tectonic fractures like Aganippe Fossa and Tiu Valles—forms a colossal scar on the surface of Mars that dwarfs every canyon system on Earth and preserves the violent volcanic and aquatic history of our planetary neighbor.
A Canyon That Would Span the Entire United States
To grasp the sheer scale of the great Martian scar, terrestrial comparisons almost fall short. While Arizona’s Grand Canyon is an awe inspiring natural wonder on Earth, measuring roughly two hundred and seventy seven miles long, eighteen miles wide, and one mile deep, Valles Marineris on Mars is nearly ten times longer, twenty times wider, and five times deeper. Measuring four thousand kilometers (two thousand five hundred miles) from end to end, up to two hundred kilometers (one hundred and twenty miles) wide, and plunging seven kilometers (more than four miles) straight down into the Martian crust, the canyon system would stretch all the way from New York City on the Atlantic coast to Los Angeles on the Pacific coast if placed across North America, or reach from the northern tip of Norway down to the island of Sicily in Europe.
Named in honor of the Mariner 9 spacecraft—the NASA probe that first revealed the canyon through clearing global dust storms between 1971 and 1972—this monumental trench is so wide in its central sections, such as Melas Chasma and Candor Chasma, that an astronaut standing on the northern rim would look out across a red abyss where the opposite southern rim lay hidden entirely below the curvature of the Martian horizon.
Tectonic Tearing Rather Than River Erosion
On Earth, nearly all great canyons are carved slowly over millions of years by the persistent abrasive flow of a river cutting through rising rock plateaus, as the Colorado River carved the Grand Canyon. On Mars, however, planetary geologists quickly realized that rushing water alone could never have excavated a chasm four miles deep and two thousand miles long. Instead, the primary architect of Mars’s great scar was monstrous volcanic swelling to the west in the Tharsis region.
During the first billion years of Martian history, a titanic plume of magma welled up beneath the Tharsis bulge—the volcanic plateau home to Olympus Mons, the largest volcano in the solar system, standing three times the height of Mount Everest. Because Mars lacks the moving tectonic plates that relieve underground pressure on Earth, this massive magma dome pushed the Martian crust upward by several miles. Under the unbearable stretching tension of that rising volcanic bulge, the rigid planetary crust to the east literally cracked and split open along huge fault lines, causing vast rectangular blocks of crust tens of miles wide to drop miles downward into the planet’s interior, creating what geologists call a tectonic graben or rift valley similar in origin to the East African Rift Valley on Earth.
Catastrophic Mega-Floods and Ancient Deep Lakes
Although tectonic cracking ripped the original scar open around three point five billion years ago, liquid water played a dramatic secondary role in sculpting the jagged walls, terraced plateaus, and chaotic floors seen in modern satellite images. As the deep faults fractured the crust, they sliced straight through vast underground aquifers and subterranean ice tables trapped beneath the Martian permafrost. Released suddenly under immense pressure, billions of gallons of groundwater burst out of the canyon walls in catastrophic mega floods that roared eastward through outflow channels and emptied into the northern lowlands of Chryse Planitia.
Spectroscopic data collected by orbital instruments have detected thick, horizontal layers of water-altered sulfate salts and clay minerals high up along the interior cliff faces of Valles Marineris, proving that for extended periods in ancient Martian history, parts of the giant scar were filled to the brim with deep, standing lakes. Today, as massive landslides occasionally send billions of tons of rock crashing down the four-mile-high cliffs and morning clouds of water ice mist gather at the bottom of the trenches, the great scar of Mars stands as the ultimate time capsule for future human explorers seeking evidence of ancient microbial life.
