The evidence that water once existed on Mars is unmistakable: The planet is covered in valleys that appear carved by flowing water. For decades, these epic ravines and branching riverbeds beckoned planetary scientists to imagine a 3.5-billion-year-old world that was both warm and wet, covered with lakes and rivers possibly hosting an environment conducive to life.

But new research suggests Mars might not have been the balmy paradise scientists once envisioned. Now, the evidence seems to suggest that rivers may not have covered its surface, but instead flowed deep under large sheets of ice.

To come to this conclusion, the researchers performed a statistical analysis of the shapes and patterns of 66 networks of valleys on Mars, which are composed of over 10,000 individual valleys. They then compared these to similar features on Earth.

They found that 14 of the martian sites appeared to have characteristics reminiscent of above-ground rivers. But 31 seemed to be carved out by either glacial or subglacial meltwater, more like terrain found near former glaciers on Earth.

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Rivers on ancient Mars may have flowed beneath sheets of ice - Astronomy Magazine