Did it snow on Mars? New research suggests ancient Red Planet precipitation was a lot like Earth

Was Mars a friendly planet for water, even life, in its ancient past? Huge channel networks run across its surface. Rocks show signs of water immersion. Yet pictures sent back from NASA's Curiosity and Perseverance rover missions continue to show their tracks in the endless desert. Where did the water come from, how much of it was there, and why did it leave?

While the question of water is massively complex, a recent study in the Journal of Geophysical Research: Planets adds to the debate about how ancient water flowed – from the Red Planet's still icy poles, or in a water cycle that included precipitation, or in some combination. 

Precipitation, to be clear, includes any variant – not just water, but snow or freezing rain and other forms of water falling from the sky. Much like the Eastern Seaboard in winter, a "warm" Mars likely means slightly warmer than freezing, lead author Amanda Steckel said.

"Of course, we don't have [direct] access to Mars yet, so we tried to be really simple," Steckel told Salon. She did the study's work as a doctoral student at the University of Colorado Boulder, and has since moved to the California Institute of Technology as a postdoctoral scholar and research associate in planetary science. 

The study used landscape modeling techniques developed at CU Boulder. They placed a grid on a virtual surface and then ran models of a watery Martian climate to see how it evolved over time. To do so, they broke down the complexity of the climate into two scenarios, ideal for modeling and testing which might be stronger: ice caps pulling water downhill and forming the valley heads at a single elevation, or a precipitation-driven planet that created valley heads at many altitudes.

The authors compared their work with imagery of Mars in the equatorial southern highlands, which is heavily cratered but also full of valley networks. They particularly focused on the valley heads, which is the source of the water in each of the networks. 

And what they found suggests that some kind of water did fall on Mars, as the valley heads were situated at many elevations – a situation that is hard to explain with ice. And it also matches what is observed on Mars, where variations in valley heads range in altitude between thousands of feet.

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It currently does snow on Mars — just not really like it does here and far less frequently. As NASA's Jet Propulsion Laboratory puts it, "Cold as it is, don’t expect snow drifts worthy of the Rocky Mountains." Indeed, there are two kinds of snow on Mars: water ice and carbon dioxide, better known as dry ice, which has flakes that are actually cube-shaped. "Because Martian air is so thin and the temperatures so cold, water-ice snow sublimates, or becomes a gas, before it even touches the ground. Dry-ice snow actually does reach the ground," NASA's website states. That's pretty different from what we experience on Earth or what Mars may have had long ago.

That said, a lot more study will be required to understand the weird history of water on Mars. To be sure, Mars likely hosted some kind of water on the surface between 3.7 billion and 4.1 billion years ago, when Earth and the rest of the solar system were still young. But how exactly the water flowed is an open question. 

The young sun was likely burning a little cooler than it is today, raising questions about how much of its warmth reached the surface. The Martian atmosphere also might have been thicker, allowing water to flow more easily – that is, until solar pressure eroded the lighter molecules into space and thinned the planet's protective "envelope" into the thin amount we see today. That's because Mars, for all its charm, has a lower gravity than Earth and less ability to hold on to atmospheric compounds like carbon dioxide. 

Steckel emphasized that even accepting the idea that precipitation fell on Mars, it was unlikely to be the only way that water moved around on the surface. After all, recent studies of the Red Planet have not only suggested water in the icy poles, but underground. Smaller sources of water may also have come from ancient meteorite strikes.

"This is not a climate modeling study," she said of her work, adding her hope is that other climate scientists can use the data set to help inform future studies of the Red Planet. "There's a wide swath of possibilities" between the two water scenarios that her study identified, but to push forward on that, "I think that's where the climate modelers come in … when people try to replicate this dataset with climate modeling, that would be the natural next step."

Hansjörg Seybold, a physicist at ETZ not affiliated with the new study, said the methodology was sound but it was only one piece of understanding how liquid water shaped the Martian surface. Studies like this, he emphasized, are limited as they are based on a theoretical Red Planet surface and do not aim to exactly match what is seen in today's channel networks.

If Mars was "warm and wet," he continued, valley heads could be seen anywhere where rain accumulates. If the planet was instead a cold and wet location — fed by icy glaciers — the heads would be fed from a single elevation and would not create new branches downstream.

"In the end, we are left with one more hint telling us that we actually don't understand Mars' ancient climate and the processes forming its channel networks," he emphasized. "If either of the two cases is actually real remains elusive, and leaves the underlying question of how Mars could have sustained a hydrological cycle open."

Seybold said future studies should not only consider the valley networks, but also the geology of the area that we have picked up from Mars rover missions and from observations from orbiting satellites. Seybold also urged comparison with other planets; he led a study in Science Advances in 2018 that attempted to do just that. 

Seybold's study compared valley networks on Mars with valley networks on Earth to see if groundwater was important to forming Red Planet valley connections. They found that the Martian valleys' branching angles "are more similar to terrestrial valley networks incised by overland flow, than valley networks incised by re-emerging groundwater flow."

Understanding the history of weather on Mars helps us learn more about our own planet, as well as informing us of the possibilities of life on the Red Planet and its potential to (maybe) someday host humans.