Morella Crater is an ancient impact scar 78 kilometers (49 miles) across that sits barely 5 km (3 mi) away from the southern edge of the large canyon, Ganges Chasma. Roughly 3 billion years ago, groundwater filled Morella Crater with as much water as Glacial Lake Missoula on Earth held (or about as much as Lakes Ontario and Erie combined). Finally, the water overtopped the crater’s rim and spilled out.
In less than a day, the spill became a flood as the pent-up water chopped a 5 kilometer (3 mile) wide breach through the crater rim. Racing eastward across the plateau, the water carved Elaver Vallis, a channel system that today extends for about 150 km (90 mi).
As Neil Coleman (University of Pittsburgh at Johnstown) reports in the Journal of Geophysical Research, the floodwaters eroded Elaver in about a week. How far the valley originally extended is unknown, he says, because Elaver Vallis abruptly ends at the rim of Ganges Chasma.
“After the flood, the Elaver channel was truncated by the southward expansion of Ganges Chasma,” Coleman explains.
The newly published work builds on previous studies by Coleman and others, and provides a detailed hydrologic analysis of the crater lake, the breach in its rim, and the resulting flood channels and their features.
The groundwater to fill the lake came from what is now Ganges Cavus, a depression in the floor of Morella Crater, although the cavus may not have been fully formed at the time.
“Groundwater was the only plausible source of the water that filled Morella Crater,” says Coleman, “because Elaver Vallis exits Morella at a single outlet and no inflow channels exist.” (In this it’s unlike Gusev Crater, which has both an outlet and an inflow channel.)
The subsurface water was pressurized enough to rise (“like water in a standpipe,” Coleman says) to cover Morella’s floor at an elevation of about 1,100 meters – and keep rising until it found a low spot in the rim at an elevation of about 1,800 meters. The filling could have taken several decades. The lake was certain to be ice-covered, though the ice was probably less than 20 meters (66 feet) thick, according to the paper’s estimates.
The breach, when it came, likely took less than a day to reach full size. This is because the volume of water impounded was much more than what could flow through the breach, Coleman says. “In such cases the breach fully forms before significant drawdown of the lake occurs.”
Running the numbers, Coleman calculates, “Approximately 95 percent of the drainable lake volume discharged in 6.4 to 7.5 days.” The rate of discharge from Morella was about two times greater than the peak for the Spokane (or Missoula) Flood during the last Ice Age on Earth.
Elaver Vallis shows features common to megaflood channels on both Earth and Mars. These include longitudinal ridges, streamlined buttes, cataracts, hanging valleys, and channels that divide and merge.
Coleman developed hydrographs to show how the flood discharge varied over time and compared them to flow calculations using traditional open channel methods. Results show that the northern Elaver Vallis channel we see today never flowed bank-full because the flows would have exceeded the peak breach discharges.
Close to the rim breach, the flood swept a scabland about 75 km (47 mi) wide. This then narrowed and divided into two channels around an elevated highland. The channels merged again into a single channel about 30 km (20 mi) wide. The channel then vanishes at the lip of Ganges Chasma.
Says Coleman, “The Elaver flood occurred before the formation of Ganges Chasma as we see it today. The chasma margin continued to grow southward after the fluvial episode.” Evidence, he says, lies in the abrupt truncations of ridges in the channel which sharply end at the canyon’s edge as the ground falls away.
Another piece of evidence for the flood predating today’s Ganges Chasma lies in the source of the groundwater. If the edge of Ganges Chasma were as close to Morella Crater at the time of the flood as it is now, Coleman explains the same pressure that pushed the groundwater up inside the crater would instead have burst out into the canyon itself.
He says, “High groundwater pressures would have been relieved by breakouts from the walls or floor of this chasma, rather than by groundwater discharges high on the plateau” inside Morella Crater.