The hydrogeology of Mars has continued to be an exciting and ground-breaking area of research since early telescopic observations made by astronomers such as Giovanni Schiaparelli, Christiaan Huygens, and Percival Lowell which propelled the curiosity about water on Mars deeper into the scientific community. The understanding of water and its history on this planet continues to improve through the compounding research made possible by NASA rovers, satellites, and numerous researchers but still leaves the scientific community with important questions. Although presently, water on Mars exists mainly as ice at the polar caps, there is evidence that it had a much more dynamic history. Planetary scientists analyze rock and mineralogical distributions as well as geomorphic features to reconstruct past hydrological cycles on Mars.
This paper considers water’s existence on Mars throughout different time periods, it’s resulting geomorphology, and the current state of water on Mars. The ongoing debate regarding a global ocean versus wide-spread ice sheets is taken into consideration as well as a look into recent hot topics such as recurring slope lineae and subglacial lakes.
The geologic history of Mars has been divided into three time periods. The Noachian (4.6-3.8 Ga), Hesperian (3.8-3.0), and the Amazonian (3.0-present). Because of current atmospheric conditions, the presence of surface water on Mars in unlikely able to form. In its early history though, Martian obliquity and atmosphere provided stable enough conditions for liquid surface water, ice, vapor, and ground water flow. During the early Noachian, Mars was hit with a bombardment of meteors. This is believed to coincided with a thicker and warmer atmosphere, as well as the first appearance of liquid water. Hydrological models reported in Andrews-Hanna et al. (2010) suggest that during this time period, the hydrological cycle was much more active than in the Heperian and Amazonian due to the Noachian’s higher total water inventory with a saturated near-surface zone and high rates of precipitation. The presence of abundant liquid water in the Amazonian is supported by the presence of phyllosilicates as well as river valley networks dated as middle to late Noachian in age (Andrews-Hanna et. al, 2011). The large deposits of these hydrated minerals associated with Noachian terrains are primary geologic evidence for long-term water weathering of Mars’ basaltic crust (Chevrier et. al, 2017). Although there are still many questions about early Mars environment, it is certain that liquid water played a role in shaping its geology.
There is one well-known feature on Mars that is not only an extremely recognizable feature, but also a cause of much debate in Martian paleoclimate and hydrogeology research. The Martian dichotomy is a feature of elevation difference set between the north and south hemispheres. It is accepted that the dichotomy most likely evolved during the early Noachian (Roberts et. al, 2006) but it’s genesis is still unclear and a number of formation mechanisms are still openly debated. These proposals include tectonic mantle convection (Roberts et. al, 2006) and giant impact origins in the northern hemisphere (Andrews-Hanna et. al, 2008). Nonetheless, this is a huge area of elevation contrast that has been the center of focus when it comes to global water on Mars.
Geologic features around the dichotomy have been used in a “global ocean” versus “global ice sheet” debate when considering the ancient climate and presence of water on Mars. The significantly lower elevation in the northern hemisphere, geomorphology resembling paleo-shorelines, deltas, and river valley networks have been proposed as evidence for a global ocean during the time when Mars had a much denser atmosphere. An analysis of the deltaic deposits along the margins of the northern lowlands may represent the past contact of a northern hemisphere ocean around 3.5 Ga (Di Achille et. al, 2010). A global ocean hypothesis would also support the idea of an active global hydrosphere and may explain river valley networks that have been considered either formed by groundwater or precipitation. This ancient ocean has been estimated to have covered at least 36% of the planet. The ‘Arabia shoreline’ is one of the proposed paleo-shorelines located along the dichotomy and has been dated to 3.8 Ga but has been rejected by some due to elevational inconsistencies. This interpretation of a global ocean has been mainly challenged by the uncertainty of global temperatures at this time. Past models of Mars’ CO2-HO2 atmosphere and a weak solar influence, suggest that a “wet and warm” climate might not have been sustained long enough to see a mass ocean with a groundwater system but a recent publication this year by has tied together these two main hypotheses by modeling a Noachian groundwater-fed ocean that eventually reach became a sub-freezing Martian climate (Palumbo et. al, 2019).
Although some researchers such as Baker et. al (1991) suggest that the formation of valley networks during Mars’ early history is evidence of a long-term hydrological cycle associated with the existence of an ocean, others give evidence that the valley networks are erosional features formed by subglacial channelized meltwater and represent drainage pathways of a Late Noachian Icy Highlands ice sheet (Galofre et. al, 2019). The 3.0 Ga “thumbprint terrain” feature has been argued to support basin-wide ice sheet coverage in Isidis Planitia crater by means of basal ice melt (Soucek et. al, 2015). Because of the difficulty modeling long term warm and wet climates on Mars, these deltas, and river valley networks have been attributed to widespread ice deposits, suggesting snow would have precipitated at low latitudes during periods of high obliquity (Carr et. al, 2003). Melting of the ice sheets could generate meltwater which, if large enough, could flow up to thousands of kilometers and create these branching valley networks (Carr et. al, 2003).
The results on a global ocean or global ice sheets is admittedly still unclear but geomorphology such as these river valley networks are huge indicators of Mars’ past hydrological cycle. Data has also been suggested to support precipitation and surface runoff mechanisms for the formation of the river valleys rather than groundwater processes (Hynek et. al, 2010). Liquid precipitation has also been supported by high resolution images and topographic data showing that drainage densities across Mars had been significantly high (Andrews-Hanna et. al, 2011).
The most recent models include both a global ocean and global ice sheet view (Palumbo and Head, 2019) and account for unanswered questions on both sides of the debate. In 2018, Seybold et. al published a comparison of terrestrial analogue river valley systems with those found on Mars. It was found that the branching angles of groundwater flow were much wider that surface runoff. The branching angles of the river valley networks on Mars support an active hydrologic cycle on Mars which produced overland flow erosion with only a minor role played by groundwater seepage (Seybold et. al, 2018).
The initial wet and warm environments on Mars that formed the deltas and river valleys still preserved today did not last but instead transitioned into drier and colder conditions seen in the Hesperian period. During the Hesperian, there were increases in tectonic and volcanic activity. Liquid water became acidic from interactions with volcanically released SO2 in the atmosphere. The strongly acidic weathering led to sulfate deposits in ancient lakebeds (Chervier et. al, 2017). Many craters, such as Jezero Crater (a proposed landing site for Mars 2020) became ideal spots for these lakes to pool up. Many are identifiable by areas of rich clay deposits in places and sulfate deposits.
Mars has a relatively unstable obliquity compared to Earth which has been stabilized by its large moon. Earth’s stability has given the planet a chance to sustain liquid water for a long period of time which in turn has helped life to evolve. Water on Mars is known to have been present but not in large, stable liquid quantities. Solar winds and a thinner atmosphere limit the chances of liquid surface water. A recent discovery of RSL’s (recurring slope lineae) by Lujendra Ojha, were thought to have been a new discovery of downhill flowing liquid brines in fine grained regolith near southern hemisphere crater rims which lengthened during Mars’ warm seasons (McEwen et. al, 2013). This was later rejected in 2017 when data by Mars Odyssey showed no signs of any hydronated sediment.
The lack of even liquid brines has left Mars with little to no evidence of liquid surface water. Currently, the state of water on Mars is found mostly frozen at the polar caps. Surface ice water is seen at the northern polar cap but is only under carbon dioxide ice at the southern cap. Radar evidence of Mars’ southern ice cap has also suggested possible liquid subglacial lakes which has been kept at a liquid state due to a combination of high salinity and increased pressure from ice above (Orosei et. al, 2018).
There are major implications to developing a better understanding of water on Mars throughout its history and today. Planetary geologists, biologists, and physicists, are just some of the researchers working in this interdisciplinary study of Mars. Terrestrial analogues are important and useful in the research and thanks to radar and rovers, better observations can be made. Although this can be an interesting and challenging area of research, it is worth the time, effort, and money for many reasons including the opportunity of revealing possible signs of life in this now arid and seemingly void planet.
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