More on Mars Climate Cycles

Fluvial features on Mars seem to indicate that liquid water once flowed on the surface, yet climate theorists remain divided among how the red planet was able to sustain warm enough conditions in the distant past when the sun was fainter. Popular ideas include a dense greenhouse atmosphere of carbon dioxide, hydrogen, and other gases permitted a lengthy period of warmth. Another option suggests that periodic impacts caused enough warming to carve the features in a shorter time.

My co-authors and I have argued in previous papers that climate cycles on early Mars could have been driven by oscillations in the carbonate-silicate cycle, which would have provided transient warming from the accumulation of greenhouse gases by volcanoes and subsequent loss by weathering. In a new paper, we respond to a critique of the limit cycle hypothesis in our “Reply to Shaw.”

We acknowledge that the biggest obstacle to any explanation for warming early Mars with carbon dioxide is their ultimate fate: are there carbonate rocks buried underneath the martian regolith? If not, where did all the carbon dioxide go? Even so, we maintain that the early Mars climate cycle hypothesis remains consistent with observable geologic evidence and could have played at least a partial role in providing warm conditions on early Mars.

Climate Cycles on Early Mars

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.

Was Early Mars Less Reflective?

Both Earth and Mars show geologic evidence of flowing liquid water in the distant past, nearly four billion years ago. The presence of liquid water on the surface of these planets is difficult to reconcile with the reduced luminosity of the sun at the time, so scientists have continued to search for possible explanations for the warm climates of early Earth and early Mars. A team of researchers recently suggested that early Earth may have had larger oceans and fewer clouds than today, which would have reflected away less incoming sunlight and might have allowed the planet to remain warm in spite of a fainter sun. Whether or not this solution will pan out for early Earth, it at least suggests the possibility of a similar mechanism on early Mars.

In a recent paper published in Astronomy & Astrophysics, on which I am co-author,
we examine the possibility that reduced reflectivity could have kept early Mars above freezing. We use a computer climate model to calculate the global average temperature at various values of ocean fraction and cloud coverage. We find that our model does indeed produce warm conditions for early Earth, but it fails to do the same for early Mars. In fact, our model can only produce warm conditions if early Mars were nearly entirely covered by oceans and also free of clouds, a result which is unlikely as well as inconsistent with geologic evidence. We conclude that some combination of climate and geochemical mechanisms, as yet unknown, may provide clues for understanding the stability of liquid water on early Mars.

If early Mars did harbor oceans, then the possibility remains that life could have developed. Examining the climates of both Earth and Mars in the past may therefore help in the quest to understand the origin of life. Future Mars exploration missions, as well as continued research on Earth, will slowly shed light on this mystery.

Cold Oceans on Early Mars

Billions of years ago, the planet Mars appears to have been covered by a liquid water ocean. Geologic evidence of riverbeds, deltas, canyons, and other features in the Martian landscape all suggest that a flowing liquid once meandered on the surface of the red planet. Even so, the fainter young sun at the time, combined with Mars’ orbital distance from the sun, suggests that even a wet early Mars was probably quite chilly.

In a recent paper published in Nature Geoscience, on which I am a co-author, we examine the idea that early Mars featured a cold glacial ocean on its northern hemisphere. This study combines some theoretical climate calculations (which was my contribution) along with a mineralogical analysis to reach this conclusion. In particular, the formation of minerals known as phyllosilicates would have been prevented in a cold ocean, which may explain the scarcity of phyllosilicates observed in the northern martian hemisphere today.

And if oceans did exist on Mars billions of years ago, then perhaps the processes of life also could have arisen in the early history of the red planet. Mars today appears barren and lifeless, but signs of past or present life could very well be lurking beneath the soil. Future Mars missions, and possibly human exploration, will eventually help to uncover this mystery.