Pools of liquid water may even now exist just a few meters below the Martian surface, according to new research. The finding hints that humans may one day be able to tap into Mars's watery bounty.
Although the surface of Mars is too frigid for liquid water to be stable, pockets of water underground could be kept warm enough by an insulating blanket of porous sediment, an international team writes in the November issue of the journal Icarus.
(Related: "Liquid Water Recently Seen on Mars?")
The new theory is based on studies of Mars's biggest outflow channels, which stretch for hundreds of kilometers across the southern circum-Chryse region.
The ancient channels were likely carved by gushing torrents of water, each hundreds to thousands of times larger than the Mississippi River. These massive rivers are believed to have erupted from underground sources, suggesting that shallow groundwater reservoirs were once relatively common in Mars's upper crust.
In fact, collapsed terrain on the floors of the outflow channels could be showing where some now empty reservoirs once existed. (Related: "Ancient Mars Had Vast Ocean, New Evidence Shows.")
Thermal modeling then revealed an intriguing possibility, said study leader J. Alexis Palmero Rodriguez, of the Planetary Science Institute in Arizona.
"If you have a few tens of meters of porous, fine-grain sediments, dunes, or alluvial deposits, you can create a thermal anomaly that would produce melting or perhaps stabilize existing water at shallow depths," he said.
"At present, Martian heat flow"—heat moving from the planet's interior toward its surface—"is four times lower [than when the channels were formed some three billion years ago], so you need four times the thickness, maybe 120 meters [395 feet] of porous sediments," he continued.
"But that's still very shallow and may be accessible to humans."
Mars Groundwater Would Be Easy to Tap
If they exist, today's Martian groundwater reservoirs might be similar to the aquifers that lie beneath icy permafrost in Earth's northern regions, said study co-author Jeffrey Kargel, a research scientist at the University of Arizona in Tucson.
That means Martian reservoirs might be found by searching for deep dunes overlying known water-rich areas, such as salt beds or glacial ice. (See "Mars Water Discovered, 'Tasted' by Lander—A First.")
And because the water would lie below unconsolidated sediments, not solid rock, drilling for water could be relatively easy and inexpensive—creating a valuable resource for any future humans exploring the planet.
"Water is fundamental to sustaining long-term human settlements on Mars," study leader Rodriguez said. "In addition, hydrogen extracted from water can be used as fuel."
The study also has bearing on the search for extraterrestrial life: Subsurface aquatic environments have been suggested as likely spots for microbes to develop on Mars.
"From a biological perspective, the presence of liquid water is the only constraint for life development on Mars, given the presence of different energy sources and alternative radiation-protection mechanisms," said co-author Alberto Fairén of NASA's Ames Research Center in California.
For example, molecular adaptations could have enabled bacteria or other microorganisms to survive in the Martian underground without energy from the sun, just as some microbes thrive in icy subsurface areas on Earth, Fairén added.
Still in Search of Liquid Evidence
Of course, debates about liquid water on Mars will likely continue until liquid evidence emerges, said Jeff Andrews-Hanna, a geophysicist at the Colorado School of Mines who is unaffiliated with the new research.
"This shallow groundwater would be a prime target for future Mars missions, if it does indeed exist," he said. "But the distribution of water beneath the surface of Mars today remains a controversial topic."
(Related: "Lava, Not Water, Made Mars 'Riverbed.'")
For example, other recent research—published in December 2009 by Robert Grimm and Scott Painter in the journal Geophysical Research Letters—argues that the Martian subsurface must be largely dry today, he said.
That research modeled the ways ancient climate change on Mars moved water and carbon dioxide through the planet's subsurface. The scientists concluded that ice would have sublimated—turned directly from liquid to gas—and any groundwater would have subsequently evaporated at low latitudes.
"In this view," Andrews-Hanna said, "the only water on Mars today is the ice trapped in the permafrost near the polar regions and in a few other local deposits."