Massive floods are not only common on Earth but 3.5 billion years ago they raged across the Martian landscape. One of the most dramatic issued from a breach in a rift called Mangala Fossae. ‘Mangala’ is the Sanskrit word for Mars and ‘fossae’ is Latin for ditch or trough. This striking linear crack is located 16 degrees south of the Martian equator north and east of the edge of the vast volcanic plateau called the Tharis bulge.
What’s now a wide valley began as a narrow fault in the crust. Scientists believe that a surge of molten rock (magma) beneath the fault caused it to widen into a long, linear valley seen in the photo below. Surges of magma that squeezed between cracks in earlier lava flows are called volcanic dikes. Many can be seen along the rivers and rock outcroppings of Lake Superior’s North Shore.
But there’s more, much more. The cracking released a vast reserve of subsurface water that had been capped and contained either by frozen ground or thick layers of rock. Once the surface was broken, water gushed forth in a fashion similar to shaking a pop bottle and then untwisting the cap. After filling the trough, it breached the wall and flowed with cataclysmic fury across hundreds of miles of desert Martian landscape scouring everything in its path. What a terrifying sight to have seen if eyes had been around back then. The floodplain is named Mangala Valles (valley).
The breach measures five miles wide and 1,600 feet deep. Think of the power it must have taken to tear through all that rock. By counting craters, scientists have estimated that the first and largest flow happened 3.5 billion years ago. Crater-counting has been used across the solar system to get a handle on a body’s age. The more craters, the more ancient the landscape; the fewer the younger. Judging by the terraces in the downstream walls of rock, smaller floods recurred 1 billion, 500 million and as recently as 200 million years ago, each leaving its own ‘water level mark’ on the hillsides.
Even more evidence for floods are found in the quilted patterns of mesas and valleys in what’s termed ‘chaotic terrain’. The crazy shapes are believe to form when powerful floodwaters scrape away much of the overlying rock in the floodplain. Without the confinement of that upper layer or rock (or ice), water may have bubbled out all over the place creating this strange scene of jumbled valleys and hilltops.
The Mangala flood tells us that despite current conditions, where liquid water can’t exist on Mars for long because of bitter cold temperatures and low atmospheric pressure, it once did pour across the landscape, creating vast floodplains as well as narrow, branching riverbeds. Since a growing crack in its surface was enough to liberate a flood, I wonder if a well-placed large meteorite strike might someday do the same. Will some future amateur astronomer catch an impact flash on Mars followed by a darkening of the surface as pressurized water once again seeks the light?