This subimage shows (near center) densely fractured light-toned rock in the vicinity of the Nili Fossae. The light-toned material is finely layered; these layers are visible in cross-section along a scarp face at the bottom of the image.
At full resolution, the light-toned layered materials resemble those seen in other HiRISE images of Nili Fossae and its surroundings, some of which have been identified on the basis of their infrared spectra (by OMEGA and CRISM) as containing phyllosilicates (clays), which require the presence of water to form. These layers likely formed very early in Martian history, but must have been rapidly buried due to the lack of overprinting impact craters.
Presently, the light-toned materials are being exhumed as the overlying material is eroded away by wind.
Additionally, the light-toned layers are overlain by a darker, densely pitted, rubbly layer. The areal extent of this darker layer, which has no apparent internal layering, is visible in the full image. The dark layer may represent lava flows, possibly extruded from the Nili Fossae fissures or from the Syrtis Major volcano, 1000 kilometers (620 miles) to the southwest.
In the full image, the large valleys cutting into the dark material and its underlying layers may have formed by groundwater seepage and erosion, or by tectonic processes related to the opening of the Nili Fossae fissure system, to which the valleys connect just southeast of this image.
South of the large area capped by dark material is a complex terrain of irregularly shaped pits and mesas, some of which are also capped by dark, pitted rock. The lighter, layered, densely fractured material is well exposed here. The pits are filled with relatively dark-toned, fine-grained material, and lighter wind-blown ripples are also present in some cases. Large boulder-sized fragments of light-toned rock are also visible in some pits, especially near the eroding scarp face highlighted in the sub-image above.
Photo Credit: NASA/JPL/University of Arizona