In its distant past Mars had a denser atmosphere and warmer climate that allowed abundant liquid water to flow on the surface, creating rivers, lakes and possibly oceans
Observations from missions
Orbital images and surface geology show valley networks, deltas and sedimentary layers consistent with long‑lived liquid water on ancient Mars, evidence collected and interpreted by multiple missions and instruments.
Why it was warmer
Early Mars is believed to have had a thicker, greenhouse‑rich atmosphere that trapped heat and allowed surface temperatures to support liquid water. Researchers are exploring mechanisms such as episodic greenhouse warming, hydrogen‑rich atmospheres and carbon cycling to explain how Mars stayed warm despite a fainter young Sun.
Recent evidence and imagery
New analyses from rovers and orbital datasets continue to refine the picture of ancient Martian climate, identifying mineral signatures and landscape features that point to sustained aqueous environments and suggesting complex climate evolution rather than a single simple transition from wet to dry.
Implications for science
The loss of Mars’s atmosphere over billions of years transformed a warmer, wetter planet into the cold, arid world we see today, a process that informs models of planetary habitability, the search for past life and where to target future exploration for preserved biosignatures.
Takeaway
Geological and geochemical evidence shows ancient Mars was warmer and wetter, and understanding how the atmosphere was lost and climate changed is central to answering whether the planet was ever truly habitable for life as we know it.