The Cataracts of Kasei

The southern Kasei channel divides around a tableland (Lunae Mensa, at lower left in the big mosaic). Just downstream from it lies a smooth oval bump in the channel floor. The bump rises about 1,200 m (3,900 ft), but its smooth contours suggest that Kasei's floods completely engulfed it during at least part of valley's history.

Just to the north lies a stretch of eroded, grooved channel floor with a rough appearance and a horseshoe-shaped edge. The scientists who first identified this feature as a waterfall searched for a terrestrial analog feature, finally settling on the 120-meter Dry Falls cataract in Grand Coulee, part of the Channeled Scablands of eastern Washington State.

On Mars, however, the cataract scale is several times larger. The Kasei dry falls have a vertical drop of 400 to 500 meters (1,300 to 1,600 ft) and the lip of the falls can be traced for more than 100 kilometers. As another, more familiar comparison, Niagara Falls stands about 50 meters high and the combined length of the American and Canadian Falls adds up to just 1.1 km (3,700 ft).

For all the jokes it sometimes occasions, Niagara Falls, as experienced at close range, is a powerful and impressive natural feature. And to think of seeing Kasei in full flood! The Martian cataracts would have been one of the most spectacular sights on any planet anywhere.

 

Floodwaters also carved their way along the northern side of the big mesa, creating Lobo Vallis. Lobo makes almost a full half-circle around the mesa before it rejoins Kasei South near where the whole valley empties into Chryse.

This part of the valley lacks large cataracts, but shows an abundance of streamlined islands and ridges, all aligned with the flow. From the lack of abrupt features, scientists think the flow here was less than to the south, and perhaps also the bedrock shaped by the flow was softer and easier to erode.

Here we see an impact that left a crater in the channel. Its rim and outer flanks acted as an anchor and shield, behind which the channel surface was partly protected from erosion by floodwaters. It's also likely that some debris carried downstream could have drifted into the lee of the crater, where the water flowed more slowly. This would have given the entrained material an opportunity to drop out of the flow and contribute to the pennant-like ridge growing behind the crater.
. Flowlines of a different kind run here: airflow, or wind. Behind these small impact craters extend "tails" that looks dark in both day and night infrared images. Yet the tails appear bright in visible light (see the close-up image at right). What's going on?

The tails are made of light-toned dust, the same as that falling out of the air all the time all over Mars. They created by easterly winds blowing up-canyon. As the wind flowed over the craters, their raised rims make the wind flow slower close to the ground, leaving sand particles in place. With the sand left undisturbed, the dust also remained in place. Elsewhere, however, the winds agitated the sand, lifting the dust particles so the wind could carry them away.

To THEMIS' infrared eyes, darkness indicates cool temperatures while brightness denotes warmth. The dust tails look dark in daytime infrared images because the dust's higher reflectivity prevents it from absorbing as much heat from sunlight. This keeps the dust a few degrees cooler, thus darker, than the surrounding rocky ground.

In nighttime infrared images, two effects cause the dust tails to look dark as well. First, being light in color the dust streaks never became as warm as the ground during daytime. Second, in a desert climate — which Mars has in spades — dust cools off quickly compared to rockier material. Thus by the time THEMIS flies over the streaks deep in the wee hours before sunrise, the dust has shed its heat and become cold and dark
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