Ancient Mars Had Warm Weather and Rainstorms Before the Planet Turned Icy, Scientists Say

The climate on Mars between 3 and 4 billion years ago may have been warm enough to sustain significant rainstorms and flowing water before conditions turned cold and the planet froze over, according to research.

The existence of ancient valley networks and lake deposits on the red planet indicate that liquid water was once abundant on the surface. However, scientists have not been able to agree on whether the climate was warm and wet, or cold and icy

But now, findings presented at the Goldschmidt Geochemistry Conference in Barcelona on Monday by Briony Horgan have cast new light on Mars' ancient climate, indicating that the Red Planet had one or more long periods dominated by rainstorms and flowing water, before the surface water froze.

"We know that liquid water flowed on the surface of Mars between 3.5 to 4 billion years ago because we see clear evidence for ancient rivers and lakes across the oldest surfaces, including the ancient lake deposits that the Curiosity rover is investigating right now," Horgan told Newsweek. "However, climate models for this period have a hard time recreating a warm surface, in part because the Sun was fainter at this time—what's known as the 'faint young Sun paradox.'"

"So this has forced the Mars community to really carefully reconsider what we think we know about what the ancient martian climate and surface was really like," she said. "Could it have been mostly very cold and icy, with only rare periods of conditions where snow and ice melted to carve the rivers? Or were there long periods of warmer conditions where rain actually fell? This matters because the surface conditions would have had a very strong influence on where environments habitable to life occurred and how long they persisted—the Antarctic ice sheet is much less habitable than a warm lake at lower latitudes."

To understand more about the planet's ancient climate, Horgan and her colleagues compared data on patterns of Martian mineral deposits—collected by NASA spacecraft and rovers—with similar information on regions of Earth considered to be analogs of Mars.

"We're coming at this problem from a different perspective—what can the chemical record of ancient Mars tell us about climate? When rain falls or snow melts, that water interacts with rocks to create new minerals in soils and sediments, and those minerals should change depending on how much water is present, how fast it falls, and its temperature," Horgan said.

"But it turns out that there hasn't been much work done on what minerals form under cold and icy conditions in particular, and how they compare to minerals formed in warmer climates. So our first step was to go to places on Earth with different climates but that all contain volcanic rocks that are similar to Mars, and to investigate their weathering mineralogy," she said. "We could then compare the results from these sites to minerals detected across Mars using satellites and rovers."

The team's analysis led them to the conclusion that there was a general slow trend from warm to cold on Mars three to four billion years ago, with periods of thawing and freezing.

"We now know that minerals that form under cold climates, where water comes from melting of snow and ice, are different from those that form in warm climates," Horgan said. "In cold climates, weathering minerals form quickly and are more like silica gels than minerals, in that they tend to lack crystal structure. In warm climates, weathering forms abundant clay minerals, just like the clay you probably find in the soil in your backyard.

"This is important because it turns out that we actually see a lot of these poorly crystalline, silica-rich minerals in younger terrains on Mars, but we haven't yet found these minerals in the ancient surfaces," she said. "Instead, we have seen many ancient surfaces that contain clay minerals in what appear to be deeply weathered soils. So this suggests that the climate was warm for at least some period of time early on—millions of years or more—but eventually cooled off and became more dominated by snow and ice."

However, while these findings support the idea of this slow warming trend, climate models of ancient Mars struggle to produce scenarios where surface water could have remained unfrozen. This means scientists may have overlooked certain chemical, geological or other factors, which may have contributed to warming earlier in the planet's history.

"If we're correct, then it means that we're going to have to keep working with the climate models to try to understand what could have warmed ancient Mars—a lot of the current research is looking at whether or not really potent greenhouse gases could have warmed the planet," Horgan said. "We'll also get a chance to test out these theories with two really exciting new missions. Both the ESA ExoMars and NASA Mars 2020 rovers are launching next year, and both are going to visit landing sites dating from this most ancient period of martian history with evidence for lots of liquid water."

Furthermore, Horgan says that we don't know exactly how warm Mars might have been in its ancient past.

"Warm temperatures speed up chemical reactions, so it's possible that Mars may have been hot and rainy for a shorter period of time, or cooler and rainy for longer," she said. "It's also possible that the warm periods were short and spread apart. Mars might have otherwise still been pretty dry, but we know it was wet enough to form not only these minerals, but also huge river networks and lakes as big as seas. The jury is still out on whether or not Mars ever had an ancient ocean."

Nevertheless, warmer conditions increase the possibility that life developed on early Mars, the researchers say. Thus the latest results could have significant implications in the search for life there.

"We know that the building blocks of life on Earth developed very soon after the Earth's formation, and that flowing water is essential for life's development," Horgan said. "So evidence that we had early, flowing water on Mars, will increase the chances that simple life may have developed at around the same time as it did on Earth. We hope that the Mars 2020 mission will be able to look more closely at these minerals, and begin to answer exactly what conditions existed when Mars was still young."

Scott McLennan, a professor from Stony Brook University New York who was not involved in the research, praised Horgan's study.

"What is especially exciting about this work is that it used well-understood Earth-based geological processes from regions that are good analogs for Mars," he said. "The results not only make sense from the perspective of developing climate evolution models for Mars but also demonstrated a possible mechanism for forming the most interesting and perplexing and non-crystalline components that have been found in all of the samples analyzed so far by the Curiosity rover."

This article was updated to include additional comments from Briony Horgan.