Dark meets light on Mars
Bright and dark regions on Mars can actually be distinguished from Earth by telescope. They differ by composition and particle size: bright regions are mostly covered by fine dust from silicate minerals, whereas dark regions are dominated by mafic minerals found for instance in basalt, which represents the most common volcanic rock found on the Red Planet. Layers of basaltic sands, as seen in the northern (right) part of the color image, probably originated from volcanic ashes and elsewhere form imposing dune fields in various places on Mars. Five large volcanic provinces and countless small, scattered volcanoes have distributed ash layers all over Mars, later concealed beneath other rocky material. When exposed at slopes, such as in impact craters, the ash can be carried out by wind and deposited over wide areas.
The largest impact crater in the image has a diameter of 25 kilometers, but is only about 300 meters deep, probably because it has been filled by volcanic or sedimentary deposits. Mesas south of the impact crater and valleys in the near vicinity are remnants of a former widespread fluvial system in the Terra Cimmeria region. Recent studies point to surface runoff of ice or snow melt, probably during several episodic glacial-related events, as the source of water. The cap rock forming the mesas might originate from eolian deposits, comparable to airfall deposits in China forming loess up to several hundred meters thick. Numerous valley networks locally crosscut these deposits, as seen for instance north (right) of the impact crater. They most likely dewatered into the putative ancient Eridania sea, connecting the lower-elevation provinces north of the presented region.
In the southern (left) part of the image, numerous dark dust devil tracks are identified. They form when eddies remove the uppermost dust layer which has a slightly different color than the now- exposed surface. Dust devil tracks are the short - lived witnesses of the ongoing geologic and atmospheric activity on Mars, which consists mainly of the transport of dust by wind.