Mars has a curious past. Rovers have shown unequivocal evidence that liquid water existed on its surface, for probably at least 100 years. But climate models haven’t come up with how exactly that happened with what we currently understand about what the Martian climate was like back then. A new paper, published in the journal AGU Advances by Eleanor Moreland, a graduate student at Rice University, and her co-authors, has a potential explanation for what might have happened - liquid lakes on the Red Planet would have hid under small, seasonal ice sheets similar to the way they do in Antarctica on Earth.
To understand why this finding is significant, it's best to understand the “Faint Young Sun Paradox” in Mars research. According to data collected by the Curiosity Rover in Gale Crater, there was absolutely liquid water pooled on the surface of Mars at some point in its history. There were clear signs of deltas and even river channels carved into the rock, and the best estimates for when this hydrological cycle was active was around 3.5 billion years ago.
However, 3.6 billion years ago, the Sun was 25% dimmer than it is currently - and we already know Mars is frozen even at the Sun’s current output power. So how could liquid water, which must, by definition, exist in environments above freezing, endure on the surface of Mars while it was possibly even colder than it is today.
Water discusses the possibility of water on Mars.There have been two main solutions to this paradox put forward in the literature. One is that there were “bursts” of warming on Mars that were caused by either active volcanoes or asteroid impacts, that allowed liquid water to run freely, but only until the energy from those events dissipated. The other is that Mars was always cold and icy, but that the liquid water existed under permanent ice sheets.
To settle this debate, Ms. Moreland and her co-authors developed a new computer model called the Lake Modeling on Mars for Atmospheric Reconstructions and Simulations (LakeM2ARS). This software package takes inputs like a potential lake’s location, its size, and the atmospheric composition, and predicts how long it could have held onto its liquid water. The results of the model were rather counterintuitive.
In warm scenarios, where the ambient temperature was above freezing for a significant amount of time, evaporation actually made the lakes dry out quickly. Whereas, in cold scenarios, the lake would have developed seasonal ice cover, which would have acted as a barrier to the evaporation. It would have still allowed liquid water to actively exist on the surface during seasonal warm periods though. “Warm” is relative though, with only a small part of the year existing at an average of above 0 ℃. Most of the rest of the year the average temperature would have been closer to -20 to -30 ℃.
We still haven't found existing liquid water on Mars - as Fraser explains.So how can researchers rule out permanent ice rather than the seasonal variety? If the atmosphere was too thin, that likely would have been the condition on Mars for millions of years. But, it also would have left distinct physical marks, such as dropstones and frost wedges, that weren’t present when Curiosity looked in Gale Crater. Seasonal ice provides a nice middle ground between the two extremes. It stopped the quick evaporation of the “warm and wet” climate model, while also explaining the absence of the features expected if the water was constantly frozen.
This study fits neatly into the ongoing discussion about the early climate on Mars, and what implications it might have had for the existence of life at some point on the planet. Luckily, we have another rover (Perseverance) wandering around different Martian terrain (Jezero Crater) that the LakeM2ARS software can be adapted to model that area, as well as other previously wet areas on Mars. As we continue our exploration of what is now a dry, arid world, the history of water will remain one of its most intriguing mysteries, even as we develop ever more sophisticated computer models to figure it out.
Learn More:
Rice University / EurekaAlert - Thin ice may have protected lake water on frozen Mars
E. L. Moreland et al. - Seasonal Ice Cover Could Allow Liquid Lakes to Persist in a Cold Mars Paleoclimate
UT - Mars' Gale Crater was Filled with Water for Much Longer Than Anyone Thought
UT - Curiosity has Reached an Ancient Debris Channel That Could Have Been Formed by Water