Martian oceans

This view began to change around 1990 when a few scientists argued that a multitude of enigmatic features in the northern plains could be explained as evidences of past oceans within the northern lowlands (Figure 7.5). Parker etal. (1989)

Figure 7.5 Geologic analysis combined with MOLA topography and roughness have led to the hypothesis of a large ocean covering much of the martian northern plains in the Hesperian to Amazonian period. The extent of Oceanus Borealis is shown in lighter tone in this image superposed on a MOLA-derived shaded relief map. The Tharsis volcanoes and Valles Marineris are seen near the bottom. (NASA/JPL/GSFC/MOLA team/Brown University.)

Figure 7.5 Geologic analysis combined with MOLA topography and roughness have led to the hypothesis of a large ocean covering much of the martian northern plains in the Hesperian to Amazonian period. The extent of Oceanus Borealis is shown in lighter tone in this image superposed on a MOLA-derived shaded relief map. The Tharsis volcanoes and Valles Marineris are seen near the bottom. (NASA/JPL/GSFC/MOLA team/Brown University.)

cited the gradational nature of the dichotomy boundary in western Deuteronilus Mensae as evidence of sedimentary deposition and proposed that this deposition occurred in a sea produced during outflow channel flooding of the northern plains. Later work identified two possible shorelines which were largely continuous around the entire northern plains, leading to the proposal of one or more oceans forming in this area during Hesperian to Amazonian times (Parker et al., 1993). Baker et al. (1991) and Kargel and Strom (1992) mapped the distribution of possible flood, permafrost, and periglacial features across Mars and argued that the hydrologic cycle associated with outflow channel formation could produce short-lived northern oceans and southern glaciers into the Amazonian period.

Models of the hydrologic cycle producing oceans in the northern plains invoke high internal heat flow to release water from subsurface ice reservoirs or aquifers. Clifford and Parker (2001) argue that this process first produced a northern plains ocean shortly after formation of the dichotomy boundary. Recharge of the deep subsurface aquifer by polar basal melting permitted subsequent outbursts through the outflow channels into more recent times, producing episodic flooding of the northern lowlands. As the geothermal flux declines with time and the permafrost layer thickens, outbursts become rarer and ocean episodes diminish. An alternate model (Baker et al., 1991; Gulick et al., 1997; Baker, 2001) also invokes internal heat to trigger the outflow channel flooding through melting of subsurface ground ice and CO2 clathrate (Figure 7.6). The release of the CO2 from the clathrates as well as volcanism leads to greenhouse warming and rainfall. The northern ocean (Oceanus Borealis) produced by the outflow channel flooding begins to evaporate

Figure 7.6 One possible scenario for how northern oceans could be produced on Mars is shown in this diagram. This model (Baker, 2001) suggests long-term cold conditions which are interspersed by short-lived cataclysmic episodes of volatile release. (Reprinted by permission from Macmillan Publishers Ltd: Nature, Baker [2001], Copyright 2001.)

Figure 7.6 One possible scenario for how northern oceans could be produced on Mars is shown in this diagram. This model (Baker, 2001) suggests long-term cold conditions which are interspersed by short-lived cataclysmic episodes of volatile release. (Reprinted by permission from Macmillan Publishers Ltd: Nature, Baker [2001], Copyright 2001.)

and the moisture is transferred to the southern hemisphere where it is precipitated as snow to produce glaciers. Carbon dioxide is removed from the atmosphere by dissolving in the ocean water and through silicate weathering. Within 103 to 105 years, cold temperatures return and the CO2 is sequestered into CO2 clathrates underneath the ground ice resulting from infiltration of the ocean water. Another pulse of high heat flow can cause the cycle to repeat.

MOLA analysis reveals that one of Parker et al.'s (1993) proposed shorelines corresponds with an equipotential surface continuous around the northern plains (Head etal., 1998, 1999). Roughness studies of the northern plains indicate they are extremely smooth, similar to sediment-covered ocean floors on Earth (Figure 4.3) (Kreslavsky and Head, 2000; Smith et al., 2001a). However, MOC imagery does not reveal features which would be expected along shorelines (Figure 7.7) (Malin and Edgett, 1999) and the mineralogic data do not reveal evidence of hydrated minerals within the proposed ocean boundaries (Bibring et al., 2005, 2006). Olivine, which alters quickly in the presence of water, has been detected in Hesperian-aged outcrops and argues against a recent hydrologic cycle involving liquid water on the surface. An ice-covered ocean might alleviate some of these concerns,

Water in the post-Noachian period 197

Water in the post-Noachian period 197

Figure 7.7 MOC has imaged the locations of proposed shorelines from Oceanus Borealis. (a) This context image shows the location of one of the proposed shorelines northwest of Olympus Mons. (b) This MOC image corresponds to the square identified as SPO2-428/03 on the context image. Although this image lies along the proposed shoreline, no evidence of such a feature is seen. (MOC images MOC2-180a and MOC2-180c, NASA/JPL/MSSS.)

Figure 7.7 MOC has imaged the locations of proposed shorelines from Oceanus Borealis. (a) This context image shows the location of one of the proposed shorelines northwest of Olympus Mons. (b) This MOC image corresponds to the square identified as SPO2-428/03 on the context image. Although this image lies along the proposed shoreline, no evidence of such a feature is seen. (MOC images MOC2-180a and MOC2-180c, NASA/JPL/MSSS.)

but at the present time there is still considerable debate as to whether Hesperian-to-Amazonian-aged oceans have existed on Mars.

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