Using seismic data from NASA’s InSight Lander, researchers recently found evidence of a deep groundwater reservoir on Mars, potentially of global extent.
In 1993, Planetary Science Institute Senior Scientist Stephen Clifford was the first to propose the existence of such a global reservoir and consider its potential role in the hydrologic evolution of the planet.
“At the time, the idea was received with some skepticism,” Clifford said, “While it was understood that the catastrophic release of local reservoirs of groundwater confined beneath a thick layer of frozen ground played a role in the formation of the Martian outflow channels, there was no unambiguous evidence of a global groundwater system.”
In the late 1970s, the dominant thought was that the water inventory on Mars was very small – the equivalent of a global ocean about 10 to 100 meters deep. But by the mid-1980s, evidence acquired by NASA’s Viking Orbiters, suggested that the planetary inventory of water was up to 10 times greater.
As part of this transition in thinking, Clifford argued that if the planetary inventory of water was just a few percent greater than what could be stored in the planet’s frozen crust – a region known as the cryosphere – the result would have been a global groundwater system that may have played an important role in the long-term exchange of water between the Martian atmosphere, polar caps and deep subsurface. Then, in 2001, Clifford and Timothy Parker from the Jet Propulsion Laboratory outlined how the existence of such a global groundwater system could explain the origin and ultimate fate of an early northern ocean.
“As the planet’s early climate cooled, such an ocean would have frozen and eventually sublimed away, to be transported and deposited at the planet’s south pole, creating a massive ice sheet up to several kilometers thick, great enough to undergo geothermal melting at its base,” Clifford said.
They theorized that the resulting meltwater would have seeped into the crust, adding to the groundwater stored in the global aquifer.
“One consequence of this process would have been the rise of the global groundwater table over time – a prediction that appears consistent with the rise in the maximum elevation of the Martian outflow channels over 3 billion years ago,” Clifford said.
This groundwater would have been trapped beneath a thick layer of permafrost preventing it from reaching the surface. Any disruption of this barrier – by impact, earthquake or volcanic eruption – could have resulted in sudden catastrophic discharge of the groundwater, creating the outflow channels.
However, as the planet’s interior cooled, the Martian frozen ground would have thickened, causing the groundwater level to fall, a prediction consistent with both the declining elevation of outflow channels later in the planet’s history and with the present depth of groundwater deduced from InSight’s recent seismic investigations.
“The InSight discovery provided some reassurance that the theoretical work I’ve done over the past 40 years has some relationship to reality,” Clifford said.
