How do we know Mars once had water?

No one watched an ancient Martian river. How can valley shape, rounded gravel, delta layers, mineral fingerprints, and buried ice build a historical case?

Walk from landform to rock texture to chemistry, then compare ancient liquid-water evidence with today's mainly frozen reservoirs.

Artistic scientific reconstruction transitioning from an ancient Martian river delta and crater lake to the same basin as a dry modern landscape. — Artistic scientific reconstruction: the wet and dry scenes synthesize several lines of evidence and are not a direct photograph of ancient Mars. Terrain, timing, and water extent are illustrative.

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