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Tharsis Recharge: Analysis of Groundwater Flow to the Martian Outflow Channels
Abstract
The large Martian outflow channels terminating in Chryse Planitia are remnants of Hesperian flooding events involving the localized discharge of millions of cubic kilometers of subsurface water. Given reasonable crustal porosities, such volumes cannot be stored in the regional aquifer and recharge is required. Initial results of dynamic groundwater models [1] demonstrated that snowpack or glaciers on the Tharsis rise, formed during periods of high obliquity when ice was stable at low latitudes, may have provided an efficient source of recharge and hydraulic head for the circum-Chryse outflow channels. Comparison of model results with Martian South Pole recharge simulations (based on the hypotheses of Clifford and Parker [2]) show that Tharsis recharge produces up to four times more discharge through the circum-Chryse outflow channels in a given time period. The Medusae Fossae formation may support the past existence of large low-latitude ice sheets as early as the Hesperian [3]. Ground ice may have been involved in the formation of rootless cones, the Olympus Mons aureoles, and glacial features on volcanic edifices. The U.S. Geological Survey MODFLOW-2000 groundwater code, which we have modified to simulate spherical geometry, is used to explore further two key aspects of Tharsis recharge: infiltration area and initial water table. In previous models we assumed an infiltration area equal to that estimated by other authors for recharge over the Martian South Pole. We present new models with the infiltration area extended to regions of Tharsis above a range of threshold elevations. Early Hesperian Tharsis recharge also depends on initial water table elevations determined by hydrologic conditions in the late Noachian. In previous models, we assumed that Noachian precipitation rates maintained a shallow water table that could be approximated by the Martian topography. The transition from unconfined to confined conditions in the late Noachian may, however, have resulted in some relaxation of the water table before Tharsis recharge commenced. We thus present models with alternative initial water tables representing varying degrees of relaxation, including the extreme case of constant elevation. [1] Harrison and Grimm, GRL, 31, DOI:10.1029/2004GL020502, 2004. [2] Clifford and Parker, Icarus, 154, 40-79, 2001. [3] Head and Kreslavsky, 35th LPSC, 1635, 2004.
... Skinner and Tanaka, 2007) and (b) large troughs created by extensional tectonics and/or collapse (notably Valles Marineris), some of which may have once hosted lakes (e.g. Harrison and Grimm, 2004;Harrison and Chapman, 2008;Warner et al., 2013;Okubo, 2016). For example, Tewelde and Zuber (2013) proposed that 2-4 km of sediment fill accumulated in Acidalia Planitia and up to 5 km of sediments were deposited in Utopia Planitia over time. ...
Extensive fields of sub-kilometre- to kilometre-scale mounds, cones, domes, shields, and flow-like edifices cover large parts of the martian lowlands. These features have been compared to structures on Earth produced by sedimentary volcanism – a process that involves subsurface sediment/fluid mobilisation and commonly releases methane to the atmosphere. It was proposed that such processes might help to explain the presence of methane in the martian atmosphere and may also have produced habitable, subsurface settings of potential astrobiological relevance. However, it remains unclear if sedimentary volcanism on Earth and Mars share genetic similarities and hence if methane or other gases were released on Mars during this process. The aim of this review is to summarise the current knowledge about mud-volcano-like structures on Mars, address the critical aspects of this process, identify key open questions, and point to areas where further research is needed to understand this phenomenon and its importance for the Red Planet's geological evolution. We show here that after several decades of exploration, the amount of evidence supporting martian sedimentary volcanism has increased significantly, but as the critical ground truth is still lacking, alternative explanations cannot be ruled out. We also highlight that the lower gravity and temperatures on Mars compared to Earth control the dynamics of clastic eruptions and surface emplacement mechanisms and the resulting morphologies of erupted material. This implies that shapes and triggering mechanisms of mud-volcano-like structures may be different from those observed on Earth. Therefore, comparative studies should be done with caution. To provide a better understanding of the significance of these abundant features on Mars, we argue for follow-up studies targeting putative sedimentary volcanic features identified on the planet's surface and, if possible, for in situ investigations by landed missions such as that by the Zhurong rover.
... Alternatively, Amazonian valley networks and outflow channels [Mouginis-Mark, 1990;Berman and Hartmann, 2002;Plescia, 2003;Basilevsky et al., 2006] may have been formed due to tectonic and/or magmatic mechanisms because many of them start from faults in the Tharsis and Elysium regions and thus could have been released to the surface by tectonic activity [Andrews-Hanna et al., 2007] or dike emplacement events [Head et al., 2003b;Wilson and Head, 2007]. Finally, water could have been released by topographically higher aquifers (as in the Tharsis and Elysium regions, for example) as a result of the aquifers' geometry, even in the present epoch [Russell and Head, 2003;Harrison and Grimm, 2004;Russell and Head, 2007]. ...
Moa Valles is a well-preserved, likely Amazonian (youngest than 2 Ga old), paleodrainage system that is nearly 300-km-long and carved into ancient highland terrains west of Idaeus Fossae. The fluvial system apparently originated from fluidized ejecta blankets, and it consists of a series of dam-breach paleolakes with associated fan-shaped sedimentary deposits. The paleolakes are interconnected and drain eastward into Liberta crater, forming a complex and multilobate deltaic deposit exhibiting a well-developed channelized distributary pattern with evidence of switching on the delta plain. A breach area, consisting of three spillover channels, is present in the eastern part of the crater rim. These channels connect the Liberta crater to the eastward portion of the valley system, continuing toward Moa Valles with a complex pattern of anabranching channels that is more than 180-km-long. Based on hydrological calculations of infilling and spillover discharges of the Liberta crater-lake, the formation of the whole fluvial system is compatible with short to medium (<1000 yrs.) timescales, although the length and morphology of the observed fluvial-lacustrine features suggest long-term periods of activity based on terrestrial analogs. Water for the 300-km-long fluvial system may have been primarily sourced by the melting of shallow ice due to the thermal anomaly produced by impact craters. The occurrence of relatively recent (likely Amazonian) hydrological activity, which could have been primarily supported by groundwater replenishment, supports the hypothesis that hydrological activity could have been possible after the Noachian-Hesperian boundary, which is commonly considered as the onset epoch of the present cold-dry climate.
... The groundwater source was likely to have been west of the valley due to the orientation of most valleys towards that direction. A possible source of groundwater flow from the west could be recharge in the Tharsis region (Harrison and Grimm, 2004). Seepage of groundwater likely took place before the formation of a global confining cryosphere, which is considered a requirement for aquifer pressurization for Martian outflow channels in the Hesperian (Clifford, 1993;Marra et al., 2014b). ...
Valleys with theater-shaped heads can form due to the seepage of groundwater
and as a result of knickpoint (waterfall) erosion generated by overland
flow. This ambiguity in the mechanism of formation hampers the interpretation
of such valleys on Mars, particularly since there is limited knowledge of
material properties. Moreover, the hydrological implications of a groundwater
or surface water origin are important for our understanding of the evolution
of surface features on Mars, and a quantification of valley morphologies at
the landscape scale may provide diagnostic insights on the formative
hydrological conditions. However, flow patterns and the resulting landscapes
produced by different sources of groundwater are poorly understood. We aim to
improve the understanding of the formation of entire valley landscapes
through seepage processes from different groundwater sources that will
provide a framework of landscape metrics for the interpretation of such
systems. We study groundwater seepage from a distant source of groundwater
and from infiltration of local precipitation in a series of sandbox
experiments and combine our results with previous experiments and
observations of the Martian surface. Key results are that groundwater flow
piracy acts on valleys fed by a distant groundwater source and results in a
sparsely dissected landscape of many small and a few large valleys. In
contrast, valleys fed by a local groundwater source, i.e., nearby
infiltration, result in a densely dissected landscape. In addition, valleys
fed by a distant groundwater source grow towards that source, while valleys
with a local source grow in a broad range of directions and have a strong
tendency to bifurcate, particularly on flatter surfaces. We consider these
results with respect to two Martian cases: Louros Valles shows properties of
seepage by a local source of groundwater and Nirgal Vallis shows evidence of
a distant source, which we interpret as groundwater flow from Tharsis.
... Climatic shift is a possible mechanism for both deposition of water on the surface and recharging of aquifers. For example, snowpacks or glaciers on the Tharsis rise, formed during periods of high obliquity when precipitation occurs at low latitudes (e.g., Jakosky and Carr, 1985;Abe et al., 2005) have been suggested to be an efficient source of recharge and may have provided hydraulic head for the circum-Chryse outflow channels (Harrison and Grimm, 2004;Russell and Head, 2007). This mechanism could have stored crustal water necessary also for the hydrological activities observed in Aurorae and Ophir Plana . ...
The plains of Aurorae and Ophir in the equatorial region of Mars display geomorphic evidence indicative of extensive but generally short-lived paleohydrological processes. Elaver Vallis in Aurorae Planum south of Ganges Chasma is an outflow channel system >180 km long, and here inferred to have formed by cataclysmic spillover flooding from a paleolake(s) contained in the Morella crater basin. Ganges Cavus is an enormous 5-km-deep depression of probable collapse origin located in the Morella basin. The fluid responsible for the infilling of the Morella basin likely emerged at least partially through Ganges Cavus or its incipient depression, and it may have been supplied also from small-scale springs in the basin. Similar paleohydrological processes are inferred also in Ophir Planum. It is reasonable to assume that water, sometimes sediment-laden and/or mixed with gases, was the responsible fluid for these phenomena although some of the observed features could be explained by non-aqueous processes such as volcanism. Water emergence may have occurred as consequences of ground ice melting or breaching of cryosphere to release water from the underlying hydrosphere. Dike intrusion is considered to be an important cause of formation for the cavi and smaller depressions in Aurorae and Ophir Plana, explaining also melting of ground ice or breaching of cryosphere. Alternatively, the depressions and crater basins may have been filled by regional groundwater table rising during the period(s) when cryosphere was absent or considerably thin. The large quantities of water necessary for explaining the paleohydrological processes in Aurorae and Ophir Plana could have been derived through crustal migration from the crust of higher plains in western Ophir Planum where water existed in confined aquifers or was produced by melting of ground ice due to magmatic heating or climatic shift, or from a paleolake in Candor Chasma further west.
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