West of the immense equatorial gash of Valles Marineris lies a checkerboard region named Noctis Labyrinthus, the Labyrinth of Night. This feature's origins are not certain, but scientists think it began to develop when volcanic activity stirred in the adjoining region of Tharsis, stretching the martian crust and fracturing it.
As cracks and faults opened, ice and water in the subsurface escaped, making the ground collapse. The result today is a tangle of tablelands cut by canyons, troughs, and pits.
This false-color THEMIS mosaic focuses on one junction where canyons meet to form a depression 4 kilometers (13,000 feet) deep. The mosaic combines visible wavelength images made during daytime with nighttime infrared images. The nighttime view records the predawn temperature of the surface. This data can tell scientists about the nature of the materials on the ground.
As on Earth, debris that tumbles down canyon walls ranges in size from large boulders down to fine-grain material such as gravel, sand, and dust.
Here, dark streaks mark the paths of sand or dust avalanches. These may have begun as large slabs, perhaps only a few inches thick but hundreds of yards wide. The slabs soon broke into individual streams as they raced downhill. In their wake, they left a surface swept at least partly clean of light-colored dust. And where they came to rest, they left no discernable debris.
At the foot of the slope, however, lie the traces of older, more substantial avalanches that piled up rocks and large debris. As such rockslides slow their fall and halt, they pile up ramparts that resemble the "toes" seen on the canyon floor.
The heat-seeking eye of THEMIS can spot the coarser and rockier portions of a landslide's debris by their residual warmth, shown in redder tints in the image. Late at night, rocky debris on Mars is still radiating heat absorbed during daytime, just as asphalt pavement does on Earth. At the same time of night, however, patches of ground mantled in dust (shown in bluer tints) have long since cooled off.
In the canyon bottom lies a curious deposit whose origin is unclear. Measurements by THEMIS show this material has a temperature of –70° Celsius (–100° Fahrenheit). While this is extremely cold by Earth standards, in martian terms it's comparatively warm, especially in contrast to the dust on the canyon rim, which has a temperature of –115° C (–175° F).
Since the warm deposit lies where many landslide toes meet and merge, it's a good guess the material contains a lot of rocks. Yet this may not tell the complete story. In places, the deposit resembles wind-eroded features called yardangs, seen in many places on Mars.
Yardangs, which also occur in terrestrial desert regions, form as strong winds erode soft sediments. Typically, yardangs show as low, elongated bumps or hills that align with the prevailing wind direction.
Complicating the story is the fact that in the bottom of Valles Marineris and within dozens of martian craters and canyons, scientists see older material laid down in layers and now exposed by erosion. The deposits seen here partly resemble those layered sediments.
So is the warm, rock-rich material in the canyon bottom a mingling of landslide toes? Does it also embody wind-sculpted sediments? Or is this some unknown older material emerging as the canyon walls erode? No one can yet say - and its origin might even combine all these in some way. Stay tuned.