Mars today shows evidence of flowing water in its past from the presence of delta basins, canyons, and surface mineralogy. The remaining water on Mars today seems to be locked up in ice, but at some point in the early history of the solar system, Mars had flowing rivers, lakes, and even oceans.
The problem with this idea of a warm and wet early Mars is that the sun was fainter in the past, thereby providing even less energy than today to help thaw a frozen planet. One possibility is that early Mars had a much thicker greenhouse atmosphere than today, which could have provided enough additional warming to melt the ice. However, many climate models struggle to provide sufficient warming, even with a dense carbon dioxide atmosphere. Another option is that episodes of volcanic eruptions of meteor impacts could have temporarily warmed Mars long enough for water to flow and carve the fluvial features we see today. However, this type of episodic warming may not provide enough rainfall to carve the martian valleys observed today.
In a recent paper published in Earth and Planetary Science Letters, titled “Climate cycling on early Mars caused by the carbonate-silicate cycle,” my co-authors and I propose that climate cycling between warm and glacial states could have occurred on early Mars, driven by the carbonate-silicate cycle as we discuss in a previous study. For early Mars, the accumulation of greenhouse gases may have risen and fallen in episodic cycles, providing punctuated periods of warmth for carving the martian valleys. Our proposed hypothesis combines the notion of enhanced greenhouse warming with episodic warming, which can potentially be tested by future exploration of the martian surface.