The Missing Link In The Early Martian Water Cycle

Billions of years ago, water flowed on the surface of Mars. But scientists have an incomplete picture of how the Red Planet’s water cycle worked.

That could soon change after two graduate students at The University of Texas at Austin filled a large gap in knowledge about Mars’ water cycle — specifically, the part between surface water and groundwater.

Students Mohammad Afzal Shadab and Eric Hiatt developed a computer model that calculates how long it took for water on early Mars to percolate from the surface down to the aquifer, which is thought to have been about a mile underground. They found that it took anywhere from 50 to 200 years. On Earth, where the water table in most places is much closer to the surface, the same process typically takes just a few days.

The results were published in the journal Geophysical Research Letters.

The researchers also determined that the amount of water trickling between surface and aquifer could have been enough to cover Mars with at least 300 feet of water. This was potentially a significant portion of the planet’s total available water.

The research helps complete scientists’ understanding of the water cycle on early Mars, said Shadab, who earned his doctoral degree from UT Austin and is now a postdoctoral researcher at Princeton University. This new understanding will be useful in determining how much water was available to evaporate and fill lakes and oceans with rain, and ultimately, where the water ended up.

“We want to implement this into [an integrated model] of how the water and land evolved together over millions of years to the present state,” said Shadab, who was the study’s lead author. “That will bring us very close to answering what happened to the water on Mars.”

Infiltration on early Mars. (a) Difference in elevation of the topography and water table (m). The steady groundwater table has resulted from a banded recharge of 10 m/year in between −45 S to 45 N (black dashed line) with Arabia shoreline at −2,090 m elevation, including in Hellas and Argyre, all shown with solid black lines. (b) Infiltration time for homogeneous crust with uniform hydraulic conductivity of m/s and porosity of . (c) Infiltration time for heterogeneous crust with power-law decay in porosity and permeability with surface values and m/s. The power law exponents are given in the main text. Infiltration time refers to the time elapsed for surface water to reach to the groundwater table. White space on the map refers to water already present at or near surface, not considered in the present study.

Today, Mars is largely dry, at least at the surface. But 3 to 4 billion years ago — at around the time that life was getting started on Earth — oceans, lakes and rivers carved valleys through Mars’ mountains and craters and imprinted shorelines in the rocky surface.

Ultimately, Mars’ water took a different path than Earth’s. Most of it is now either locked in the crust or was lost to space along with Mars’ atmosphere. Understanding how much water was available near the surface could help scientists determine whether it was in the right places long enough to create the chemical ingredients needed for life.

The new findings add to an alternative picture of early Mars in which there was little water going back into the atmosphere through evaporation and raining down to refill oceans, lakes and rivers — as it would have on Earth — said coauthor Hiatt, who recently graduated with a doctoral degree from UT Jackson School of Geosciences.

“The way I think about early Mars is that any surface water you had — any oceans or large standing lakes — were very ephemeral,” he said. “Once water got into the ground on Mars, it was as good as gone. That water was never coming back out.”

The researchers said that the findings are not all bad news for potential life on Mars. If nothing else, the water seeping into the crust wasn’t being lost to space, Hiatt said. That knowledge could one day be important for future explorers looking for buried water resources to sustain a settlement on the Red Planet.

Shadab and Hiatt’s research was supported by a Blue Sky grant from the University of Texas Institute for Geophysics, a research unit of the Jackson School, and grants from UT Austin’s Center for Planetary Systems Habitability and NASA.

The work was conducted while Shadab was earning a doctoral degree from the Oden Institute for Computational Engineering and Sciences at UT Austin. Other coauthors include Rickbir Bahia and Eleni Bohacek from the European Space Agency (now at UK Space Agency), Vilmos Steinmann from the Eotvos Lorand University in Hungary, and Professor Marc Hesse from the Jackson School’s Department of Earth and Planetary Sciences at UT Austin.

Infiltration Dynamics on Early Mars: Geomorphic, Climatic, and Water Storage Implications, Computational simulation/modeling (open access)

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