New evidence exists for hidden water reservoirs and rare magmas on ancient Mars

The study, led by Rice University's Cin-Ty Lee, demonstrates that the southern highlands' thick crust -- up to 80 kilometers in some areas -- was hot enough during the Noachian and early Hesperian periods (3-4 billion years ago) to undergo partial melting in the lower crust. This process, driven by radioactive heating, could have produced significant amounts of silicic magmas such as granites and supported subsurface aquifers beneath a frozen surface layer.

"Our findings indicate that Mars' crustal processes were far more dynamic than previously thought," said Lee, the Harry Carothers Wiess Professor of Geology and professor of Earth, environmental and planetary sciences. "Not only could thick crust in the southern highlands have generated granitic magmas without plate tectonics, but it also created the thermal conditions for stable groundwater aquifers -- reservoirs of liquid water -- on a planet we've often considered dry and frozen."

The research team -- including Rice professors Rajdeep Dasgupta and Kirsten Siebach, postdoctoral research associate Duncan Keller, graduate students Jackson Borchardt and Julin Zhang and Patrick McGovern of the Lunar and Planetary Institute -- employed advanced thermal modeling to reconstruct the thermal state of Mars' crust during the Noachian and early Hesperian periods. By considering factors such as crustal thickness, radioactive heat generation and mantle heat flow, the researchers simulated how heat affected the potential for crustal melting and groundwater stability.

Their models revealed that regions with crustal thicknesses exceeding 50 kilometers would have experienced widespread partial melting, producing felsic magmas either directly through dehydration melting or indirectly via fractional crystallization of intermediate magmas. Moreover, due to the elevated heat flow, the southern highlands' thick crust would have sustained significant groundwater aquifers extending several kilometers below the surface.

The study challenges the notion that granites are unique to Earth, demonstrating that Mars could also produce granitic magmas through radiogenic heating even without plate tectonics. These granites likely remain hidden beneath basaltic flows in the southern highlands, offering new insights into Martian geology. Additionally, the research highlights the possible formation of ancient groundwater systems in Mars' southern highlands, where high surface heat flux reduced the extent of permafrost and created stable subsurface aquifers. These reservoirs of water might have been periodically accessed by volcanic activity or impacts, resulting in episodic flooding events on the planet's surface.

The findings have significant implications for habitability as the presence of liquid water and the ability to generate granitic magmas, which often contain elements critical for life, suggest that Mars' southern highlands may have been more hospitable for life in the past than previously thought.

"Granites aren't just rocks; they're geological archives that tell us about a planet's thermal and chemical evolution," said Dasgupta, the Maurice Ewing Professor of Earth, Environmental and Planetary Sciences. "On Earth, granites are tied to tectonics and water recycling. The fact that we see evidence for similar magmas on Mars through deep crustal remelting underscores the planet's complexity and its potential for hosting life in the past."

The study highlights regions on Mars where future missions could focus on detecting granitic rocks or exploring ancient water reservoirs. Large craters and fractures in the southern highlands, for example, may provide glimpses into the planet's deep crust.

"Every insight into Mars' crustal processes brings us closer to answering some of the most profound questions in planetary science, including how Mars evolved and how it may have supported life," Siebach said. "Our research provides a roadmap for where to look and what to look for as we search for these answers."

This research was made possible by NASA grant 80NSSC18K0828 .