A Flash In the Valley

Ramp

Whatever the trigger was, the resulting flow was impressive. The area seen at left lies at the east end of Aromatum and marks where Ravi Vallis proper begins.

This is a ramp — where the ground rises by about 1,500 meters (4,600 ft) from the bottom of Aromatum to a level that lies about 1,000 meters (3,300 ft) below the top of the Xanthe plateau. From here, over the remaining 200 km (130 mi) of Ravi's course, the valley descends about 300 meters (1,000 ft) before dropping over an edge into Hydraotes.

The deep scour marks — which rise uphil in the direction of the downstream flow — give evidence of the flood's speed, which the Lancaster University team estimates ranged as high as 90 km/hour (50 mph). The flow's depth ranged from 50 to 150 meters (150 to 460 ft).

The scientists also estimate that the flood was a single outburst event lasting 2 to 10 weeks. The edges of the Ravi channel show terraces caused by erosion when the waters were highest. Then as the flow slackened, the water narrowed and separated into a few individual channels that carved deeper grooves.

The flat, roughly circular area in the center — see the closeup at right — is likely the location of a tiny chaos area, just 2.5 km across. It must have contributed some water to the flow, then it was filled with channel-bottom sediments. Finally, only a single small bump was left to stick above the flat, sediment-covered surface.

Oxia

What caused chaos to erupt at Oxia? Scientists hypothesize that erosion in the channel bed likely removed enough overlying material that was keeping the groundwater penned up. If this material happened to be especially thin here, then scouring into the valley bed could be enough to trigger an outburst. 

The closeup image at right shows streamlined mesas and deep gouges cut into the channel bed by the flood — likely augmented by the eruption of water from Oxia itself. 

Edge

Did Ravi Vallis once extend farther to the east? Perhaps. But if so, there's no sign of it in the rubble that lies at the foot of the slope.

Which poses a puzzle, because the natural feature usually found at the end of a fluvial valley is a delta built from all the debris washed down the channel. The journey's-end seen here has no recognizable delta, at least nothing like those seen elsewhere, such as in Holden and Eberswalde craters.

Instead of a delta, the Xanthe plateau has a deeply notched edge. One possibility is that Ravi's floods did build a delta, but it was destroyed when the chaotic regions to the east collapsed. Or perhaps the delta was destroyed by the last of the Ravi floods, which appear to have left a small streambed deposit down the center of the groove (see closeup at right).

The Aromatum/Ravi Vallis system poses a final question. How much water did it take to do all this? Based on the amount of erosion, the Leask team estimated 10,000 to 16,000 cubic kilometers of water were needed. (For comparison, Lake Superior holds a volume of 11,600 cubic km.)

That much water is far too much to have come solely from the empty depression that is now Aromatum Chaos. That hole could contain only about 4,100 cubic km — and naturally most of that was rock, with water contributing less than 10 percent.

So where did all the necessary additional water come from? Scientists point to Ganges Chasma. There, they think, was a possible source for a large-scale aquifer underlying the Xanthe region. If it existed, then that could feed enough water into both Ravi and Shalbatana Vallis to produce what we see today.