The Athabasca Valles are a late Amazonian-period outflow channel system in the central Elysium Planitia region of Mars, located to the south of the Elysium Rise. They are part of a network of outflow channels in this region that are understood to emanate from large fissures in the Martian surface rather than the chaos terrains that source the circum-Chryse outflow channels. The Athabasca Valles in particular emanate from one of the Cerberus Fossae fissures and flow downstream to the southwest, constrained to the south by a wrinkle ridge for over 100 km, before debouching into the Cerberus Palus volcanic plain. The Athabasca Valles are widely understood to be the youngest outflow channel system on the planet.
Although researchers generally agree that the valley was formed by the catastrophic outpouring from the southernmost Cerberus Fossae fissure, the scientific community has not reached a consensus on the precise formation mechanism behind the Athabasca Valles – both in the nature of the fluids that tracked through the valley, and in terms of later geologic events that have since resurfaced the region. Researchers concurrently propose a floodwater origin (akin to the Missoula Floods that formed the Channeled Scablands of Washington state), a low-viscosity lava origin (similar to the pāhoehoe flows of Hawaii), a glacial origin, or some combination of the aforementioned mechanisms. The presence of pitted mounds on the valley floor has also been subject to debate and underpins the different hypotheses that have been proposed, and have variably been suggested to be pingoes and rootless cones. Polygonal terrains of varying scales observed in the Athabasca Valles and downstream in Cerberus Palus have been proposed to have both and/or either volcanic and periglacial features. Interpretations on these terrains differ strongly even with respect to in what order these features superpose other events in the valley.
Context
The Athabasca Valles system lies to the south of the Albor Tholus peak of the Elysium volcanic province, the second most significant volcanic province on the planet Mars. It lies within the southern Martian highlands in a diffuse part of the planet's crustal dichotomy. It is a valley that trends northeast–southwest at the southernmost end of the Elysium province. Cerberus Fossae exists uphill to the northeast of the easternmost part of the valley and score the terrain in a perpendicular direction to the Athabasca Valles' trend. Downstream to the southwest of the valley system lies the Cerberus Palus plain. The outflow channel's route during its formation likely followed a pre-existing southwest-trending pathway, as it is bounded to the south by a wrinkle ridge associated with compressive stresses emanating from the Elysium volcanic province. It emanates from its source at Cerberus Fossae in two channels that converge approximately 25 km southwest of the fissure; after a further 80 km, the valley becomes distributary, with some of its offshoots breaching the south-bounding wrinkle ridge. Geomorphic evidence of valley-affiliated deposits disappears at its southwestern end under recent lava flows. The materials forming the valley floor of the system are thought to be ultramafic or mafic in composition, characterized by an abundance of Fe and a relative dearth of K and Th based on data from the Gamma Ray Spectrometer (GRS). Some aeolian exhumation is observed to have resurfaced the floor. Furthermore, large-scale extension and compression are evident in the Athabasca Valles floor unit, which may have been associated with earlier regional tectonic events or the emptying of an underlying magma chamber. The volcanic unit proposed to compose the floor of the Athabasca Valles (among other terrains) is hypothesized by some researchers to be the youngest and largest flood-emplaced lava unit on Mars, and the only instance of a flood lava unit that displays morphological evidence of turbulent flow. In total, the areal extent of the debouched lava flows that formed the Athabasca Valles system have been mapped as covering a region reaching completely across Elysium Planitia to the south, indistinctly disappearing into the northern margin of Zephyria Planum and stretching across a wide swath of Cerberus Palus in the east–west sense, canvassing a region nearly as wide as the Elysium Rise. This flood lava unit is as large as Oregon and is of a greater areal extent than the largest of the large igneous provinces on Earth – the Deccan and Rajamundry Traps of southern India. A knobby terrain lies to the northwest of the debouchment of the Athabasca Valles and has been dated by crater counting to be the oldest extant geologic unit in the Athabasca Valles system, and is Noachian in age. Modern extensional near-source faulting associated with southern Cerberus Fossae has been found to postdate the formation of all features in the valley, and are likely the most geologically recent features of the Athabasca Valles system.
The Athabasca Valles are located within the broader Elysium Planitia region and cross-cuts a vast swath of plains land interpreted to be composed largely of flood basalts. The outflow channels of central Elysium Planitia are distinguished from those of circum-Chryse region (Kasei Valles, Ares Vallis, etc.) because they appear to emanate from volcanic fissures rather than chaos terrain. The Athabasca Valles are the westernmost of the outflow channels in Elysium Planitia and the only one of the channel systems in this region that flows westwards. The other major outflow channels in this region are (from west to east) the Grjotá Valles, the Rahway Valles, and Marte Vallis. Historically, some researchers have associated the outpouring of fluid from the Athabasca Valles with the downstream formations of Marte Vallis and the Grjotá Valles, but this perspective fell out of favor as higher-resolution MOC data became available, allowing updated crater counts (the age dates of each valley floor are asynchronous) and geomorphic interpretations (high-permeability fresh lava rock would have caused large-scale infiltration of errant floodwaters long before reaching the other valleys' heads).
Of the outflow channels on Mars, the Athabasca Valles have been of particular interest to the Martian planetary geological community as crater age estimates suggest that the outflow channel might have formed as recently as 20 Ma – the youngest-known of its kind on Mars – assuming the embaying lava units (upon which the crater dating was performed) were deposited contemporaneously with the outflow channel's formation. Explanations of its formation would allow researchers to better constrain the hydrological conditions in this region of Mars well into the late Amazonian, long after most hydrological activity on the Martian surface is canonically thought to have ceased. The most recent flood to pass through the Athabasca Valles may have done so as recently as 2–8 Ma.
Around 80% of the craters in the Athabasca Valles are secondary craters from the impact that created Zunil crater, which is the youngest-known +10 km-diameter rayed crater on the Martian surface and a candidate source of the shergottite meteorites that have been found on Earth. The presence of these modern secondaries was initially thought to have skewed the very modern age dates based on crater counts on the Athabasca Valles floor. Zunil Crater is located due east of the Athabasca Valles network, extending along the southeastern trend beyond the Cerberus Fossae fissures. Secondaries from nearby Corinto crater, another very young large rayed crater in Zunil's neighborhood, are also suspected to superpose the Athabasca Valles valley floor, but the morphologies of these secondaries are uncertain and their alignment with the rays of Corinto might be coincidental.
The Athabasca Valles are named for the Athabasca River, which runs through Jasper National Park in the Canadian province of Alberta. It was initially named "Athabasca Vallis" (singular form). The International Astronomical Union officially approved the feature's name in 1997.
Geographic features
Formation
There are competing interpretations regarding the formation of the Athabasca Valles system. The different hypotheses and supporting and competing evidences are described below.
Megaflooding formation hypothesis
The Athabasca Valles are the youngest of the outflow channel systems on Mars, and has historically been understood to have formed as the result of megaflooding. Distinctive streamlined teardrop-shaped landforms, branching channels, and transverse ripple dunes (interpreted to have formed under water) are all found within the valley system, and are morphologically similar to those found in the Channeled Scablands on Earth in eastern Washington state. The Channeled Scablands were formed during the catastrophic Missoula Floods, a series of megafloods sourced from sudden breaches in ice dams buttressing the Pleistocene-aged glacial Lake Missoula. According to this interpretation, these streamlined landforms were created when passing floodwaters deposited sediment against protruding bedrock outcroppings, such as crater rims or bedrock mesas. (In the case of the Athabasca Valles, the vast majority of such streamlined forms arose around relict bedrock mesas.) The floodwater from the event that formed the Athabasca Valles is thought to have come from Cerberus Fossae at 10°N and 157°E, where groundwater may have been trapped under an ice layer that was fractured when the fossae were created. Because evidence of fluvial erosion is present on both sides of the fissure, some authors have proposed that the outflow of floodwater from Cerberus Fossae was violent, forming a fountain akin to Old Faithful in Yellowstone National Park, which is a geyser in the U.S. state of Wyoming. Some researchers noted as early as on relatively low-resolution data from the Mars Global Surveyor mission that the flooding events thought to have formed the Athabasca Valles were interspersed with the formation of the plains units from lava in certain parts of the outflow channel, with some researchers believing that the floodwater could have been accommodated by significant permeability in the freshly-formed lava rock of Cerberus Palus. The interplay of fresh lavas and floodwaters could be responsible for rootless cones observed near the proposed sink region of the Athabasca Valles within the Cerberus Palus region.
Some researchers have proposed that the formation of the streamlined forms in the Athabasca Valles may have been a result of bedrock obstacles (such as crater rims) persisting in areas of low elevation, where hydrological modeling suggests floodwaters might have ponded. The resulting deposition around these bedrock obstacles would have then been carved again in subsequent megaflooding events, with the only surviving sections of these sedimentary deposits sitting in the regions behind the bedrock obstacles. For some of the upstream streamlined forms of the Athabasca Valles, however, modern topography is not suggestive of a ponding event. Some researchers have proposed that they were regions where this ponding event was possible in the past, but later eruptions of lava from the fissure (by the same mechanisms as the floodwaters) may have shallowed out the topographic profile of the valley. As seen on Viking and MOC imagery, the streamlined forms of the Athabasca Valles often have up to ten distinct layers exposed by later catastrophic erosion, with each layer having a thickness of up to 10m. They are often paralleled by grooves that are up to 10m tall, fading out from the streamlined forms within a few hundreds of meters. These grooves are interpreted to be depositional, and are dimensionally consistent with similar features observed within the Channeled Scablands of Washington State.