Tangled Terrain in Acheron Fossae
More than a thousand kilometers north of the giant volcano Olympus Mons lies a geologically jumbled zone. In Acheron Fossae, part of the volcanic province of Tharsis, dozens of faults curve like shards of window glass. These mark where tensions in the martian crust stretched and broke the surface, and buckled it up and down.
The pattern of cracks shows stresses from two directions. The major faults here run northwest to southeast (upper life to lower right), but a secondary pattern cuts this almost at right angles. Each down-dropped block - called a graben by geologists - lies nearly a kilometer (half a mile) below the adjoining surface.
According to planetary scientists, Acheron Fossae is a piece of very old terrain that has been surrounded by more recent Tharsis lava flows. Such flows can be glimpsed in the extreme lower right corner of the image as a smooth surface. It appears to flood the end of the grabens seen there as the ocean flows into fjords.
Looking down from orbit, the THEMIS camera imaged this terrain by day and night at infrared wavelengths. The results, shown in false color, give scientists clues about the surface: where do fine-grain materials cover the ground? How rocky is the surface?
In the image, areas in blue tints are cooler at night, which means they are covered with loose, fine-grain debris such as dust and sand. Areas with more solid materials - gravels, rocks, and outcrops - show in redder tints because they are warmer at night, having held onto daytime heat better.
Restless Ground
This impact crater, 8.2 km (5.1 miles) wide, shows evidence that the faulting in Acheron continued over many years. The meteorite making the crater struck fault-shattered ground and destroyed all traces of grabens and ridges where it hit.
But then the fault activity continued for one block that underlies the crater. By the time the tectonic activity ceased, it had uplifted a segment of the crater's floor, leaving a clear trace across it.
THEMIS' false-color view also shows the crater's interior is physically different from the ground outside the crater. In the false-color image, the yellowish crater interior shows that it is lined with more cohesive material than the cool, dusty surface surrounding the crater. And on the crater's northwest rim, touches of red suggest more cohesive material, larger particles, or bare rock is exposed there.
Fires Below
These bumps lie less than 100 kilometers (62 miles) northeast of Diacria Patera, a broad shield volcano superimposed on part of Acheron Fossae. (Diacria lies outside the image field of view.) The larger bump has a diameter of 5.7 km (3.5 miles) and a height of about 1,300 meters (4,300 feet); the smaller one rises less than 100 meters (330 feet).
Orbital images by THEMIS at visible wavelengths suggest these are more like small shield volcanos or volcanic domes than cinder cones, which typically have steep flanks. Seen from the ground, these would resemble wide, low blisters.
A close look at them suggests that, like the crater described above, faulting continued, either while the eruptions were in progress or after they stopped. The evidence lies in the fault-generated cracks that pass right through the small dome and go at least halfway through the larger one.
Interweaving effects such as these remind us that the history of Acheron Fossae, like that of Mars itself, is a complicated sequence of events unfolding over a long time.