Back Into The Light Of Day


At the edge of the highlands in northwestern Arabia Terra on Mars, the land slopes away into the lowlands of Cydonia. But in the transition zone, the terrain shows a tangle of features that testify to a long geological history and much change.

Looking down from NASA's Mars Odyssey orbiter, the Thermal Emission Imaging System (THEMIS) took this portrait of a section of Arabia Terra that adjoins Cydonia. THEMIS is a multi-wavelength camera that sees in 5 visible and 10 infrared "colors." By combining visual and infrared views from THEMIS, scientists can make false-color images that reveal both landscape features and the nature of the ground surface.
In this false-color image, areas that are covered in sand and dust show up in bluer and cooler tints, while exposures of rock and hardened sediments show in warm tints.
Hard Layer
Looking a little like burnt oatmeal in the bottom of a kitchen pot, this reddish-brown area spans a width of about 4 kilometers (2.5 miles).
The color shows that the ground is made of a material that is relatively tough and rocky, while the bluish area surrounding it is covered in sand or dust. The hard layer is in the bottom of a depression about 400 meters (1,300 feet) deep, and it appears that at least one other hardened layer lies above it. This depression, then, offers a small look into the Martian past.
The tough deposit has obviously cracked and broken. Without looking closer it's hard to say if the cracks, which must stretch dozens of meters (yards) wide, occurred when the layer formed - or if they were produced as the layer weathered after being uncovered.
While it would not be easy terrain to land on or drive across, this deposit would make an interesting target for the Mars Science Laboratory. This is NASA's next-generation Mars rover, due for launch in 2009. Its mission is to probe into the Mars of long ago, looking for geological deposits that might record traces of ancient life.
Another multi-level window into the Martian past lies open in this double depression. Fragments of hard sediments line the floor of the main hole and the smaller, inner one, while sand and dust has accumulated between the larger pieces.
In addition, the rim of the depression shows a reddish-brown layer just under the surface. This is likely an outcrop of tougher and rockier material.
Is the double depression an impact crater? It's possible. The depression might have begun as an impact crater - even a doubled one - and then grown larger.
This could occur if the ground in this region originally contained a lot of subsurface water and ice. Once exposed by the initial crater, the hole could have grown bigger as the groundwater escaped and the surface collapsed.
Water seems to have played a big role in the development of this circular depression, following its birth as an impact crater. About 11 km (7 mi) across, it has a broad, low mound about 200 m (650 ft) high in the center of the floor. Reddish colors show some rockier areas, while fine sand and dust coat much of the remaining interior.
A large channel enters the crater at about the 9 o'clock position, and a small channel exits at the 3 o'clock position. At "noon," a wide trough cuts through the rim, with a thin brown line crossing the trough to show where exposed rock marks the trough's high point. This point's elevation corresponds to that of a threshold in the 9 o'clock channel.
The history of this crater remains largely unknown, although the channels at 9 and 3 o'clock clearly appear cut by flowing water. The trough in the northern rim, however, shows no obvious flow features, and its origin may have been due to faulting. If part of the rim collapsed between geological faults slicing through the rim, the result could look like this.
Here and elsewhere in Arabia, spacecraft cameras record countless scenes where glimpses of an older landscape show through later deposits. The sequence of deposits, both in this place and others, will doubtless prove extremely tricky to unravel. Martian geological history extends fully as long as Earth's, and scientists have just begun to understand it.

Vital Statistics

30.8°N, 345.5°E
Image Size: 

29.2 x 50.0 km
18.2 x 31.0 mi
1624 x 2776 pixels

18m (59 ft)