Mars was once a wet world. The Red Planet’s geologic record shows evidence of flowing water on the surface — from river deltas to canyons carved by massive floods.

But a new study suggests that much of the rain that fell on the surface of ancient Mars, little of it inundated the planet’s southern highlands.

A graduate student at the University of Texas at Austin explored modeling groundwater recharge dynamics for aquifers using a variety of methods — from computer models to simple envelope calculations.

No matter the degree of complexity, the results converge on the same answer — an average of 0.3 mm of groundwater recharge per year. This means that wherever rainfall occurred in the model, only an average of .03 mm per year could have entered the aquifers and created the landforms that remain on the planet today.

For comparison, the annual rate of groundwater recharge for the Trinity and Edwards-Trinity Plateau aquifers that supply San Antonio is typically 2.5 to 50 millimeters per year, or what the researchers estimate is the recharge of water on Mars. About 80 to 1,600 times the rate.

Lead author Eric Hiatt, a doctoral student in the Jackson School of Geosciences, said there are several possible reasons for such a low rate of groundwater flow. When it rains, water can wash away much of the Martian landscape as runoff. Or maybe it didn’t rain much.

The findings could help scientists constrain the climatic conditions capable of producing rain on early Mars. They also suggest a very different water regime on the Red Planet than exists on Earth today.

“The fact that groundwater is not such a large process may mean that there are other things,” Hatt said. “This could increase the significance of the runoff, or it could mean that Mars didn’t get as much rain. [water] on the ground.”

The results were published in the journal Icarus. The paper is co-authored by Mohammad Afzal Shadab, a doctoral student at the Jackson School, and faculty members Sean Glick, Timothy Goodge, and Mark Hess.

The models used in the study work by simulating groundwater flow in a “steady-state” environment where the inflow and outflow of water into the aquifer is balanced. The scientists then changed the parameters affecting the flow – for example, where the rain falls or the average porosity of the rock – and observed how other variables would have to change to maintain a steady state. How reasonable are these charges?

While other researchers have simulated the flow of groundwater on Mars using similar techniques, this model is the first to incorporate the influence of oceans that have formed on the Martian surface for more than three billion years. Formerly present in the Hales, Argyre and Borealis basins.

The study also incorporates modern topographical data collected by satellites. Hyatt said the modern landscape still preserves one of the oldest and most influential topographical features on the planet — the Northern Hemisphere lowlands and the Southern Hemisphere highlands. Extreme difference in height between — called “” The Great Discontinuity.” This discontinuity preserves traces of past groundwater uplifts in which groundwater rose from aquifers to the surface. Researchers have used various model outputs to evaluate these past uplift events. Used geologic markers.

Across models, the researchers found an average rate of groundwater recharge of .03 mm per year, which closely matches what is known from the geologic record.

Research isn’t just about understanding the Red Planet’s past. It also has implications for future Mars exploration. Hiatt said understanding the flow of groundwater can help inform where to look for water today. Whether you’re looking for signs of ancient life, trying to sustain human explorers, or making rocket fuel to get back home to Earth, it’s important to know where water is most likely to be.

The research was funded by NASA, the University of Texas Institute for Geophysics, and the UT Center for Planetary Habitability.