Secrets of the Valles Marineris

The first and only time I ever saw the Grand Canyon was when I was eight years old, traveling through the southwestern United States on a family road trip. I knew roughly what it looked like, of course, but nothing had prepared me for the sheer scale of that thing. It was a sculpted, dizzyingly complex landscape, a hundred rocky slopes extending far out to the horizon. To each side, the flat Arizona ground just dropped away, and kept dropping, level after level, down into shadowy depths inhabited by an unassuming blue river. While the geological processes that had made it were beyond my understanding at the time, I knew I was gazing upon something wondrous.

I like to think a similar view is in store for future astronauts or colonists, on Mars, when they crest that last hill and look out over the Grand Canyon’s big brother: Valles Marineris. The maximum depth of the Grand Canyon is an impressive 1.8 kilometers, its length 446 kilometers. The figures for Valles Marineris? Seven kilometers, and four thousand. For a world as small as Mars, only half of Earth’s radius, its canyon system has achieved staggering proportions, expanded and molded over the aeons in ways not yet fully understood—today, let us peer into its mysteries, and see what we can discover about this great natural wonder of the Solar System.

The key difference between the two canyons, besides size, is the method of formation. The Grand Canyon was carved by the Colorado River over a period of about seventeen million years; the Valles Marineris dates back billions of years, and is believed to have been formed by the tumultuous stretching apart of the Martian crust. A more appropriate terrestrial comparison might be the East African Rift. The end result is that the Valles Marineris is far larger than any river canyon could ever be, and also exhibits greater complexity, with numerous offshoots, areas of labyrinthine “chaos terrain,” and several smaller canyons running in parallel. Planetary scientists have not yet developed an airtight explanation for this dizzying variety of terrain features. And since we haven’t yet sent any rovers or astronauts to explore on the ground, all we know is what we can see from orbit, along with a general knowledge of some of the factors at play.

For most of its length, the Valles Marineris comprises a remarkably straight main channel, with the Ius Chasma running east until it broadens out into the wide, flat-bottomed Melas Chasma, only to narrow again and form the large Coprates Chasma leading towards the complex of smaller structures comprising the terminus. There are also parallel valleys such as the Tithonium Chasma, as well as whatever is going on in the Noctis Labyrinthus out west. The parallel channels are generally smaller, shallower versions of the main one, with flat bottoms and steep, jagged sides, but the Noctis Labyrinthus is an entirely different beast—it is a giant maze of crisscrossing fissures, like cracked soil on Earth, seemingly pasted to the end of a relatively orderly canyon system.

The overall structure of the Valles Marineris is ascribed to large-scale tectonic forces; to simplify a complex and mysterious topic, the scientific consensus appears to be that, billions of years ago, volcanism on the newborn Mars created a large plateau called the Tharsis Bulge, and as this plateau expanded outwards, driven either by rising layers of lava or pressure from within the mantle, it ripped apart the crust to form a titanic fissure. Liquid water helped shape the Valles Marineris in the form of mudslides and river erosion, but did not drive the process.

That is believed to be the origin of the main chasms. The Noctis Labyrinthus, meanwhile, is more mysterious. It is composed of huge blocks of rock, primarily volcanic, with smoother basaltic rock in the bottoms of canyons towards the western side. Canyons in the east tend to have rougher floors, choked with debris from the surrounding walls. Scientists have spent considerable energy attempting to decipher the origin of this strange landscape; Noctis Labyrinthus can’t be any sort of river system, since rivers definitely do not flow in spiderweb patterns, but there are geological explanations that could fit the bill. It is generally agreed that the region’s canyons are what geologists classify as “graben” features: blocks of terrain displaced downwards and bordered by parallel faults. Examples on Earth include the East African Rift and Lake Tahoe in the United States. On Mars, these graben formations are thought to be the result of collapsing magma tubes, or perhaps—though this is less accepted—subterranean caverns hollowed out by acidic rainwater. The degree to which these processes interacted with the broader Valles Marineris is unclear.

While the overall contours of the Valles Marineris were set in place by large-scale tectonic upheavals, occurring during the planet’s tumultuous primordial era, it has been shaped over the billions of years since by the dynamic activity of water and wind—it is to these forces that we now turn our attention. Much of the geography of the canyon walls was indeed shaped by mudslides. We see this in any number of smooth, sandy slopes, and in piles of detritus at the bottom of Coprates Chasma and others. Some valley floors are also host to wind-blown dunes. Most prominent, however, are the sinuous valleys leading into the sides of the larger fissures—particularly Ius Chasma, pictured below—which were created by groundwater sapping, as liquid water steadily eroded the Martian landscape from below. The process is ongoing; groundwater activity has been observed on Mars in the present day, with the Mars Reconnaissance Orbiter picking up evidence of seasonal brine flows just beneath the surface.

Close-up exploration of the Valles Marineris will have to wait for a future rover mission, or perhaps a visit by astronauts. Its sheer size already boggles the mind when viewed from space, and the effect will doubtless be even greater for someone standing on the ground, looking out over a cliff at an expanse of jagged terrain, or perhaps rappelling down steep canyon walls. The scientific possibilities are immense, of course. Sample collection and high-resolution photography could advance our understanding of Martian geology by leaps and bounds, perhaps offering some conclusions to the mystery surrounding the Valles Marineris’ origin. There are also opportunities here for enterprising colonists; the frenetic geological activity that formed the canyons may have left behind rich metal ores, as well as lava tubes that could serve as sheltered colony sites. Looking even further out, to a future where we attempt to terraform Mars, low-lying sites like the Valles Marineris will be the first to gain breathable air pressure. And, perhaps, one day, wide-eyed little Martians will stop there on family road trips, and marvel at the sweeping vista of the Valles Marineris National Park.

Thanks for reading this week’s entry—I’ll catch y’all next time!

Further Reading:

• https://en.wikipedia.org/wiki/Valles_Marineris
• https://science.nasa.gov/valles-marineris-grand-canyon-mars