Outflow Channel in Kasei Valles

Lying east of the enormous volcanic region of Tharsis, Kasei Valles forms perhaps the largest outflow channel on Mars. Like other such channels, it carved by liquid water - and in Kasei's case, probably during gigantic floods that originated in volcanic and tectonic activity in Tharsis.

Kasei runs for 2,400 kilometers (1,500 miles) across the lava plains of Lunae Planum. It begins in Echus Chasma, near Valles Marineris, and empties into Chryse Planitia, not far from where the robotic probe Viking 1 landed in 1976. Studying Kasei and its surroundings, scientists see evidence for several episodes of flooding and possibly also glacial activity.
The Thermal Emission Imaging System (THEMIS), an instrument aboard the Mars Odyssey orbiter, took this false-color photo. It shows a portion of Kasei lying north of a large tableland, Sacra Mensa, which divides Kasei into a northern and southern channel. In places, Kasei Valles spreads as much as 300 kilometers (200 miles) across, although this stretch of the northern channel averages only 11 kilometers (7 miles) wide.
The THEMIS image combines a visible-light daytime view with an infrared nighttime one. The result shows landscape details as small as 18 meters (59 feet) in size, while the false colors provide clues to the nature of the ground surface.
Looking down from orbit late at night, THEMIS senses residual warmth left from the previous day's solar heating. Coarse-grained material - ranging from gravel, cobbles, and boulders, to rock outcrops - holds heat better than fine-grain particles such as dust and sand. The false colors map the temperatures detected by THEMIS, with warmer surfaces showing in reds and yellows, while cooler and dustier areas appear green and blue-green.
Lava Mesa
The greenish color of the mesa top indicates that it is covered with fine material that cools quickly after dark. But large grooves, as well as a sprinkling of tiny craters - the smallest visible being 100 meters (330 feet) wide - indicate that dust does not blanket the mesa top too deeply.
The grooves mark places where the rock that caps the mesa has cracked and eroded, probably by the escape of groundwater. Elsewhere on Sacra Mensa, joints and fissures opened as groundwater burst out, pressurized by stresses from Tharsis as it grew, roughly 3 billion years ago.
The ungrooved 'shelf' lies about 900 meters (3,000 feet) lower than the mesa surface. It may have collapsed and become covered by debris hides its original surface. Its outer edge shows a characteristic spur-and-gully erosion pattern.
Valley Floor
According to scientists, Kasei Valles has seen floods of different sizes. Some may have been deep enough to cover even the mesa top, while lesser flows remained in the channels, deepening them. The water flowed from left to right through the image.
The channel floor lies 2,500 meters (8,200 feet) below the south mesa top, and 1,600 meters (1 mile) below the shelf. In contrast, the northern rim of the channel rises 800 meters (2,600 feet) above the valley floor.
Unlike much of Kasei, the floor in this part of the channel is flat and almost featureless. Nonetheless, a close look reveals that where the floor lies near the southern wall, it is both smoother and has fewer exposed rocks (as its color indicates) than the other side.
It's no surprise the channel floor is rockier on the northern side, given its curving shape. Water flows faster on the outside of river bends and erodes more deeply there. Conversely, a slower flow on the inside of a bend allows particles to drop out of suspension and accumulate on the bottom.
Another kind of flow, however, created the tiny white flecks along the foot of the northern shore. Driven by the wind, these are small drifts of sand dunes, which stand out by their cooler temperatures compared to the rockier banks.
Warm Red Walls
This unnamed crater spans a diameter of 11 kilometers (7 miles), and has a floor that descends 250 meters (820 feet) below the channel. Its channel-side rim, however, rises only about 60 meters (200 feet) above the valley floor. While the crater formed after the main channel was carved, later flows through the valley appear to have removed or buried the apron of debris from the crater's impact.
Many portions of the south-facing walls of both the channel and the impact crater show as redder, hence warmer, than other parts of the scene. Two effects are probably combining to produce this result.
The first is solar heating. This part of Kasei Valles lies in mid-northern latitudes. Compared to the south side of the channel, the slopes on the northern side face the Sun more directly and become warmer each day.
The second effect is that the south-facing walls appear rockier as well. Many areas show a layer of red along the cliffs, and at the foot of the crater's channel-side rim lies a thin red line marking rocky debris. Away the edge of the channel lie numerous slopes that also face generally south, but they are not as warm, showing yellows and greens. Subjected to the same solar heating, these are probably blanketed more thickly with finer-grain materials.
Finally, if any water or ice still lingers in the ground throughout the region, the extra warmth found on the slopes may have driven it away, leaving dry, bare rock exposed. 


Vital Statistics

26.7°N, 293.6°E
Image Size: 

1794x2850 pixels

18m (59 ft)