The surface of Mars is barren and inhospitable, but perhaps it wasn't always that way. Billions of years ago, when life emerged on Earth, the climate of Mars could have been Earth-like as well, with a thicker atmosphere than today and oceans of liquid water. A study funded by NASA and international partners indicates this period could have lasted longer than originally thought.

"Our simulation revealed that three billion years ago, the climate in much of the northern hemisphere of Mars was very similar to present-day Earth, with a stable ocean," said Frederic Schmidt of the University Paris-Saclay, France, co-lead author of a paper on the research published in the Proceedings of the National Academy of Sciences Jan. 17. "Our result contradicts theories claiming that such a northern ocean could not be stable. It also increases the time period for an Earth-like climate on Mars."

The late Noachian period (from 4.1 billion to 3.5 billion years ago) is the period usually thought to be habitable on Mars, with significant rain near the equator, as demonstrated by the presence of valley networks - features formed by erosion from flowing water - at this age.

However, this clement period was not to last, and as the eons passed Mars gradually transitioned to its present-day climate, with an atmosphere too cold and thin to support liquid water, a necessary ingredient for life, on the surface. Scientists want to know the duration of the habitable period; the longer it was, the more time there would have been for any potential Martian life to form. The new work extends the potentially habitable period on Mars by about 500 million years, into the late Hesperian age.

"Discerning the climate of Mars approximately three billion years ago is challenging because the Martian surface features do not seem to fully support either a warm and wet or cold and dry climate during that time," said Michael Way, co-lead author of the paper at NASA's Goddard Institute for Space Studies, New York. "A warm and wet climate would have produced extensive erosion from flowing water, but few valley networks have been observed from this age. A too-cold climate would have kept any northern ocean frozen most of the time. A moderate cold climate would have transferred the water from the ocean to the land in the form of snow and ice. But this would prevent tsunami formation, for which there is some evidence."

The new simulation revealed that the Martian climate at this time could have been cold and wet instead. An ocean would have formed in the northern lowland basin where the atmosphere was denser and warmer. Water would evaporate from this ocean and return to the surface as rain or snow. In and near the ocean, it would be mainly rain, but in the southern highlands where the air was cold, it would be mostly snow. The snow would accumulate into extensive glaciers which would flow down to the lowland basin, returning the water to the ocean.

The model shows that the northern ocean could remain liquid even with global mean surface temperatures below the freezing point of water because ocean circulation can bring warm water from mid-latitudes to the pole, regionally warming the surface up to 4.5 Celsius (40 degrees Fahrenheit). Also, just as a dark asphalt parking lot is hotter than a white concrete sidewalk on a sunny day, liquid water is darker than snow and ice, allowing the ocean to absorb more heat from sunlight.

Mars' current atmosphere is mostly carbon dioxide and extremely thin, around one percent of Earth's atmospheric pressure at sea level, but there is evidence it was thicker in the past. The model predicted a stable northern ocean if Mars had an ancient atmosphere as thick as present-day Earth's, made primarily of carbon dioxide with 10 percent hydrogen (H2). Just as a heavy coat traps more heat than a light jacket, a thick atmosphere would have helped warm a young Mars by retaining more heat from sunlight. Also, hydrogen helps the atmosphere trap even more heat since it's an efficient greenhouse gas, and it could have been released by extensive volcanic eruptions or meteorite impacts on early Mars.

Mars' ancient climate was simulated using the ROCKE-3D Global Climate Model (GCM) developed at the NASA Goddard Institute for Space Studies. The team used the current Martian landscape and surface elevations, removed all present-day ice sheets, and included a small northern ocean whose boundaries were set to where the geological evidence points.

The simulation was one of the first fully coupled GCMs used for Mars. This means that the 3D atmospheric and oceanic components are calculated at the same time, making it more realistic. "Since incorporating the full 3D ocean circulation is computationally expensive and takes longer to complete, most Mars global climate models couple the 3D atmosphere to a simple, thin, single-layer ocean that has no horizontal or vertical heat transport unlike the full 3D ocean used in our study," said Way.

The cold and wet climate predicted by the team's model is consistent with geologic evidence on Mars from this period. Glaciers carve wide U-shaped valleys as they flow, and compatible valley structures are found in the southern highlands, where the model indicates glaciers formed because these areas had the coldest temperatures. Rain forms many small, V-shaped valley networks that resemble tree branches as water flows downhill and streams merge into larger rivers. Valley networks of this age only appear at low altitudes near the ancient shoreline, which would have had the warmer temperatures required to form rain, as the model predicts.

The team members plan to continue studying the Red Planet to see whether there is more evidence to support their model. They will examine more images of Martian surface features, such as the glacial valleys, from recent missions. Also, the Mars Ice Mapper, a proposed mission by NASA and international partners, will have an unprecedented radar sounder that will be able to study the structure of the shallow subsurface to look for evidence of an ancient ocean. Finally, China's Zhurong rover landed in the region where the ocean may have formed, and rocks examined by the mission could provide evidence that an ocean once existed there.

Research Report: "Circumpolar ocean stability on Mars 3 Gy ago"

Related Links
NASA's Goddard Institute for Space Studies,
Mars News and Information at MarsDaily.com
Lunar Dreams and more

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