Ganges Cavus is a deep, asymmetric, steep walled pit located on the southern edge of Morella crater (Fig. 1). The southern rim of the cavus has an elevation of 2000 m, which is 1000 m higher than the northern rim of the cavus. The hummocky, irregular floor of the cavus is tilted toward the north where it reaches its greatest depth at -4000 m elevation. A conservative estimate of the current volume of the depression is > 2100 km3. The floor units are composed largely of light-toned, smooth, hummocky deposits, but other parts of the floor contain irregular boulders that span an array of colours in HiRISE false colour data. Thermal inertia values of materials in the lower slopes of the cavus are ~ 500 and values for the hummocky floor deposits are ~ 800, suggesting the presence of rocky material (Fergason et al., 2006). The elevation of the floor of the Ganges Cavus (-4000 m) is approximately the same as the elevation of the floor of Ganges Chasma, the southern margin of which is located < 10 km from the northern rim of Morella Crater (Fig. 1).
The ~ 180 km-long Elaver Vallis outflow channel seemingly originates at a breach in the eastern wall of Morella Crater (Figs. 1 and 2a). It consists of two compound channels, a main northern channel and shorter southern channel. In longitudinal cross section, note that the maximum depth of the northern channel is ~ 400 m and the maximum depth of the southern channel is ~ 250 m. However, one critical point that has never been addressed is the fact that a cross section along the channel centre, in both cases, shows that the topographic high point of the channel occurs near the midpoint of the channel along its length. In other words, the elevation of the channel floor of the midpoint of Elaver Vallis is > 250 meters higher than both the origin and terminus of the channel.
Elaver Vallis is disrupted by chaos terrain, which composes the topographically higher ground near the midpoint of the channel (Fig. 2). The chaos occurs in multiple distinct patches (Fig. 2), with a total area of approximately 2000 km2 and depths below the surrounding plains of ~ 500 m. The chaos blocks rise to ~ 100–500 m above the floor of the chaos unit.
The plains surrounding Elaver Vallis are mapped as Middle to Late Noachian undifferentiated units by Tanaka et al. (2014). Higher resolution views show that they are volcanic plains containing NE-SW trending fractures (Fig. 2). A ~ 11 km-diameter, 700 m-deep irregularly shaped, flat-floored depression might be a volcanic vent (See Supplementary Materials). The depth/diameter ratio for this feature is high for any crater, and extremely high for anything but a youthful crater unmodified by erosion (Michalski and Bleacher, 2013). But the depression is likely Hesperian, and therefore it is unlikely to be an impact crater based on morphometrics.
Smaller channels are also observed within Morella crater (Fig. 3a). These consist of a few nearly straight (low sinuosity) channels up to ~ 35 m length that flow into Ganges Cavus. The channels occur within gently sloping valleys, but the channels themselves occur in positive relief approximately 5–15 m above the adjacent terrain.
2.2. Surface mineralogy
Both thermal infrared emission (THEMIS and TES) and near infrared/short-wave infrared reflectance (CRISM and OMEGA) data detect olivine within crater floor deposits (Fig. 3b-c). THEMIS daytime DCS images colour stretched with bands 8, 7, and 5 as R, G, and B, respectively show olivine occurrences as purple 28, and provide a reliable way to map olivine occurrence on Mars in the thermal infrared (Fig. 3b). OMEGA global olivine index maps 29 and CRISM multispectral index maps 26 indicate the presence of olivine in the same locations (Fig. 3c). The OMEGA spectral indices suggest that the olivine is Mg-rich or fine-grained, based on comparison to laboratory spectra. High thermal inertia values (> 600) are not consistent with the fine-grained scenario, indicating the olivine is indeed Mg-rich. The precise Fo# of the Mg-rich olivine mapped with OMEGA and CRISM is difficult to constrain without detailed gaussian modelling 30, but the olivine standard used to produce the maps of Martian olivine by Ody et al. (2013) is nearly pure forsterite.
The spectral detections of olivine-bearing materials are further constrained with TES data 31. Using data of a single orbit (called “ick” in this case), which minimizes difference potentially attributable to dynamic atmospheric conditions, the detection of olivine in Morella crater fill deposits is clear (See supplementary data). A simple approach to constraining the olivine mineralogy is to plot the position of (Mg,Fe)-O-Si absorptions observed in the mid-infrared as a function of Mg# (Mg-content), as measured in the lab; the results here suggest that the Mg# of the olivine is ≥Fo68 (See supplementary data). In addition, the wavelength position of the major Si-O surface can be used to estimate the SiO2 content of the rocks. In this case, the wavenumber of the Si-O stretching in the olivine-bearing rocks is ~ 930 cm−1, which corresponds to an approximate composition of the volcanic rocks ~ 44% SiO2 (See supplementary data). In other words, the rocks are olivine-rich and relatively silica-poor, near the boundary of mafic-ultramafic composition.
The collapse of Ganges Cavus has resulted in exceptional exposures of the Morella Crater floor units. The south-facing, northern wall of Ganges Cavus reveals a > 1 km-thick succession of erosion-resistant rocks capped by a relatively thin (~ 100m) covering of re-worked materials (Fig. 4a). CRISM infrared data draped onto HiRISE image data show that the olivine-bearing deposits are continuous and > 1 km-thick (Fig. 4b); they are not just a surface veneer. These olivine-rich deposits represent a widespread crater-fill unit that is exposed throughout Morella crater. It is potentially thinnest in this location, which is located near what was once the southern wall of Morella crater and might be substantially thicker elsewhere in the basin. Even if a conservative 1-km-thick average is assumed, it still suggests that a lower estimate of ~ 3200 km3 of olivine-rich volcanic material fills the basin.
THEMIS thermal inertia data of the same south-facing, northern wall of Ganges Cavus show variation in TI within the olivine-bearing unit. Layers are observed, with TI values that vary by 60–100 TI units from layer to layer (Fig. 5). This observation is important for two reasons: 1) because it shows that multiple olivine-rich units are present and 2) because it shows that the volcanic processes that produced the olivine-rich unit were cyclical and potentially therefore sustained for some period of time. In this scenario, the lower TI values might correspond to fractured, eroded flow tops or lava lake surfaces, or potentially interbedded volcaniclastics.
Olivine-bearing materials within the floor of Morella crater (Fig. 6a) are composed of olivine-rich (Fig. 6b), fractured blocks of bedrock (Fig. 6c). As noted by (Leverington, 2009), this material is morphologically similar to rock exposures within the floor of Syrtis Major caldera (Wray et al., 2013). THEMIS thermal inertia data indicate that these materials have values of ~ 325–475, consistent with a mixture of bedrock and sandy particulates.
The interior of Morella Crater contains several arcuate scarps, which form terraces with flat surfaces several km-wide (Fig. 7). The terraces contain olivine-rich deposits of similar morphology and composition to those on the floor of the basin, but the terraces are perched approximately 200 m above the floor. The olivine-rich deposits have high thermal inertia (500–700) indicative of rocky materials. It appears that this unit must have either been deposited effusively, and the adjacent floor was later structurally down-dropped by hundreds of meters, or that the material was emplaced through air-fall.
The floor of Ganges Chasma, shows enrichment in olivine, including within a sedimentary apron at the mouth of Elaver Vallis (See supplementary materials). Of course, volcanic materials would be mobilized in the event of a catastrophic flood by water, but it is notable that the olivine in relatively concentrated in those deposits.