Astronomy
The Geology and History of Liquid Water on Mars
Quick fact
The youngest known lake on Mars may have existed around 110,000 years ago, possibly formed by melting snow from a past ice age—evidence that liquid water can appear even in the Amazonian period.
Why this is interesting
Mars looks like a bone-dry desert today, yet its planet-wide valleys, deltas, and lakebeds reveal that rivers and lakes once carved this world. What happened to all that water?
Read the full explanation
Understanding The Geology and History of Liquid Water on Mars
Imagine a planet that was once warm enough for flowing water: rain would gather into streams, carving valleys and carrying sediment downstream to form deltas where they entered lakes. When those lakes eventually dried up, they would leave behind layered sedimentary rocks, much like those in Earth's ancient lake basins. By studying satellite images of Martian deltas and mineral maps that show clays (altered volcanic ash) and sulfates (evaporite salts), scientists can deduce that water flowed across the surface, pooling, evaporating, and leaving chemical fingerprints. These observations let us reconstruct the sequence of wet and dry episodes in Martian history.
A deeper explanation
Two key mechanisms drive the history of liquid water on Mars. First, the early atmosphere was much denser, creating a greenhouse effect that kept surface temperatures above freezing, allowing rivers and lakes. However, as the planet lost its magnetic field, solar wind stripped away this thick atmosphere, leaving the current thin, cold one. Second, the low atmospheric pressure today (about 0.6% of Earth's) means pure liquid water is unstable—it boils away at low temperatures. Yet, briny water with dissolved salts can remain liquid at lower temperatures, and occasional warm-season seeps (recurring slope lineae) hint at transient flows. The geological evidence—eroded channels, deltaic deposits, and evaporite minerals—offers an observable timeline of when water was stable and how the planet dried out, shaping our search for ancient life and the feasibility of future human missions.