Ganges Mensa (also occasionally termed Gangis Mensa in literature) is a mesa and an interior layered deposit in Ganges Chasma, one of the peripheral valleys of Valles Marineris on Mars. The mesa rises up to from the floor of Ganges Chasma, nearly to the same elevation as the surrounding plateaux of Lunae Planum. Like Hebes Mensa, the mesa is completely separated from the surrounding canyon walls and has sustained significant erosion that has caused it to retreat in areal extent.
The mesa is composed of friable, thinly-layered units which decompose into fluted patternations, interpreted by most researchers as erosional aeolian features known as yardangs. It is capped by a more resistant layer that is interpreted by many researchers to be volcanic in origin. Although the mesa is understood to have formed through some combination of volcanism and sedimentary deposition, there is ongoing contention over whether the volcanism associated with the mesa occurred subglacially (into an ice megalaccolith) or subaqueously (into a paleolake). Those who favor the subglacial hypothesis believe that Ganges Mensa is a tuya that is extremely similar to analogues observed in the Azas Plateau of Tuva, Russia.
Context
Ganges Mensa is a mesa that sits in a deep upstream basin of the peripheral Ganges Chasma arm of the Valles Marineris valley network. It stretches for nearly from east to west and to the north and south between the walls of Ganges Chasma, bounded to the north by a sharp escarpment and gradually tapering towards the valley floor on the south. At its peak, the mesa rises up to from the Ganges Chasma floor, and its profile has an average elevation of from the valley floor. Ganges Mensa and Hebes Mensa are the only mesas of Valles Marineris that extend to the height of the surrounding plateau terrain. The mesa is separated from the canyon walls to the north by a moat-like stretch of valley floor, similar to Hebes and Capri Mensae and many of the mesa structures observed within Valles Marineris. Some researchers have interpreted Ganges Mensa to overlay chaos terrains in upstream Ganges Chasma.
The mesa is surrounded by the most extensive and densely-concentrated dune sea on Mars outside the polar regions of the planet. A broad sandy plain stretches out to the south of the mesa and is interspersed with knobby plains and smaller mesas up to across in size, gradually decreasing in size to the east. This region has been interpreted as a chaos terrain or a mantled, eroded remnant of volcanic edifices. The moat-like region to the north of the mesa province is dominated by landslide terrains from collapses in the Ganges Chasma canyon wall. Some of this area has since been mantled by sand dune cover.
Researchers have also reported evidence for sulfate signatures manifesting in light-toned mounds that can be observed across the Ganges Chasma floor. Some researchers have interpreted these landforms as originating in the erosion of sulfate-bearing layers present within units of Ganges Mensa.
Ganges Mensa is in the Coprates quadrangle of Mars, centered near the equator in the western hemisphere at 7.2° S and 48.8° W. The landform was named after a classical albedo feature that was published in a 1930 manuscript called La Planéte Mars authored by the French-Greek astronomer Eugène Michel Antoniadi. The International Astronomical Union officially approved Ganges Mensa's name in 2006.
Geology
Researchers have identified two, three, four, or five stratigraphic units on the mesa that are most conspicuously defined by the presence, distribution, and length of long erosional flutes (interpreted by many researchers as yardangs and, historically, possibly as the result of groundwater sapping), and thematically consistent features of differing albedo. Yardangs are fleets of elongated linear features cut from bedrock by winds that blow in a sustained direction, surficially covering vast swaths of the mesa. These landforms are generally hundreds of meters long and tens of meters high, but they have been observed to become much larger on the northern face of Ganges Mensa. The mesa's southwestern face is steeper and contains the most well-incised examples of yardangs on the landform, suggesting that wind erosion was most severe on this part of the mesa. These units were deposited in at least two depositional events, interspersed by at least one or possibly two extended periods of erosion, depending on specific interpretations about the presence of angular conformities observed in the stratigraphy.
The basal stratigraphic units of Gange Mensa demonstrate spectral signatures of polyhydrated sulfates like kieserite, which are visible up to elevations of uphill on the mesa. These sulfate signatures are not consistently represented elsewhere in the Valles Marineris network but are present within Eos Chasma and Capri Chasma and particularly on Capri Mensa, which lies to the south. Some of these units are scored by what have been interpreted by some researchers to be foreset beds of dark aeolian (wind-formed) materials. These dark beds lack any spectral signatures of common mafic minerals such as pyroxenes, olivine, or iron oxides (like hematite). Against fluted terrains the beds have been observed to dip between 15° and 25°. These thick and friable units, at their greatest extents, reach up to in thickness.
They are capped by a highly resistant unit interpreted by some researchers to be volcanic in origin, but which has been undermined by the erosion of the underlying weak rock. The cap rock of Ganges Mensa does not display this dipping and instead appears to horizontally stack atop these layers. These layers have been tentatively dated to the Mid-to-Late Amazonian period, concurrent with the formation of some outflow channels debouching out into Chryse Planitia. Dark domes and ridges on the cap of the material have been historically associated with volcanic modification but have more recently been proposed to be the intrusive feeding source of this cap rock. Because of this susceptibility to erosion and collapse, Ganges Mensa was likely once far more extensive than it appears to currently be. An exhumed crater apron nearly to the east of the mesa appears to cross-cut a structure that has been interpreted by some researchers to be a now-buried part of the mesa. Large-scale mass wasting of these weaker stratigraphic units (most notably, in the landslide of two blocks down the southern face of the mesa) has been conspicuously observed on the mesa.
The observed layers described above—termed interior layered deposits by researchers—are of considerable interest to researchers studying the possibility of past life on Mars due to the spectrally-inferred presence of kieserite, a polyhydrated magnesium sulfate mineral. Such a mineral only forms in sufficient quantities in acidic aqueous environments, suggesting the longstanding presence of water in areas where these materials are found. Ganges Mensa is the westernmost of the regions where these kinds of layers have been found in the greater Valles Marineris region. In terrestrial situations, polyhydrated sulfates are almost always found together with hematite (an iron oxide mineral that also is known to form diagenetically in neutral aqueous environments), but no spectral signatures of hematite have been found with kieserite in Ganges Mensa. Researchers have speculated that hematite in Ganges Mensa has since weathered out and is no longer present within the ILDs there in significant quantities. Hematite has been found elsewhere in interior layered deposits downstream, suggesting that the aqueous environments of Valles Marineris including Ganges Mensa grew less and less acidic into the late Hesperian and early Amazonian.
Formation mechanism interpretations
Subglacial volcanism hypothesis
The Ganges Mensa province and other interior layered deposits in Valles Marineris have been strongly analogized to the Azas Plateau, a region in the Tuva Republic (a federal subject of Russia) near Lake Baikal and the Mongolian border. The Azas Plateau volcanic field is thought to have formed through subglacial volcanism; that mechanism is used accordingly to interpret the geomorphology of Ganges Mensa, which has been analogized to a terrestrial landform known as a tuya. It has been proposed by some researchers that Ganges Mensa might be an extremely eroded version of one. Historically, other researchers have alternatively proposed that the modern shape of Ganges Mensa was a depositional and not erosional effect. The volcanic activity that formed Ganges Mensa could have occurred into a gigantic pingo-like laccolith composed of ice, or into a thoroughly-frozen lake within Ganges Chasma.
Supporters of the subglacial volcanism origin hypothesis note that the Ganges Mensa's horizontally-oriented cap rock is morphologically consistent with the cap rock seen in tuyas in the Azas Plateau. Various domes and arcuate ridges have been observed across the cap rock of Ganges Mensa, leading some to speculate that these likely volcanic landforms are actually evidences of magmatic dikes, volcanic vents, or the erosional remnants of volcanic necks that may have intruded underlying layers to deposit the cap rock.
The presence of extensive thin layering in the thick friable stratigraphic units underneath Ganges Mensa's cap rock could correspond closely to hyaloclastites, which are volcanic breccias that are formed when lavas are erupted directly into water or ice and then quenched. The proposed hyaloclastite facies have been analogized to those comprising tuyas in Iceland. Other authors have proposed that these layered terrains could constitute alternating mafic flows and tuffs made of palagonite, as has been observed in some Icelandic tuyas.
Researchers have noted, however, that the studied mesas of Valles Marineris, including Ganges Mensa, are surrounded by an extensive dune sea that has never been observed in the vicinity of terrestrial tuyas. On the mesa itself, foreset beds are typical of terrestrial tuyas, but the dips of the bedding observed within Ganges Mensa are far steeper than those that have been observed in the tuyas of British Columbia.
Subaqueous volcanism hypothesis
Those who favor a subaqueous hypothesis for Ganges Mensa's formation argue that the mesa developed due to a combination of volcanic and sedimentary factors underneath a persistent kilometres-deep paleolake. Supporters of this hypothesis tend to favor a low-energy depositional explanation to the formation of the mesa rather than a subglacial one, with the basal sulfate-enriched IDLs deposited during the Hesperian, when the circum-Chryse outflow channels were forming.
A subaqueous formation could explain the thinly-layered friable deposits within the walls of Ganges Mensa if they were turbidites, but because the sedimentary particles may be variable in composition and in distance traveled from source, the traditional Bouma sequence characteristic of terrestrial turbidites may not necessarily be observed.
Critics of this hypothesis for Ganges Mensa note that there are no barriers in the chasmata that appear to confine a putative paleolake downstream towards the outflow channels that they usually source (such as how Ganges Chasma opens into the uplands of Chryse Planitia), and how there are no observed morphologies on Ganges Mensa or on any of the other mesas of Valles Marineris that could conceivably correspond to paleoshoreline benches. More generally, Hebes Mensa also extends to a higher elevation than the rim of Hebes Chasma, which has implications on the broader formational mechanism of the valley mesas that are unlikely to be explained by a lacustrine mechanism. However, it is entirely possible that resurfacing could have eradicated any sign of such paleoshorelines, and that the geometry of Ganges Chasma might have been different enough in the past to have supported the presence of a deep paleolake.