Billions of years ago, Mars was a much different place than it is today. During the same period when life was first emerging on Earth, Mars had a thicker atmosphere, warmer surface temperatures, and flowing water on its surface. Evidence of this warmer, wetter past is preserved on the planet’s surface today in the form of river channels, lakebeds, alluvial fans, and sedimentary deposits. When this period began, and how long it lasted, remains the subject of much debate for scientists.
Knowing how long this period lasted helps establish how big the window of opportunity was for life on Mars. But according to new NASA-funded research from the Sellers Exoplanet Environments Collaboration (SEEC), Mars may have been wetter longer than previously expected. According to recently published results in the Proceedings of the National Academy of Sciences, Mars may have had a northern ocean as recent as three billion years ago.
The international SEEC team included researchers from the University Paris-Saclay, the Institut Universitaire de France, the Centre National de la Recherche Scientifique (CNRS), the Center for Climate Systems Research (CCRS) at Columbia University, Uppsala University, the NASA Goddard Institute for Space Studies (GISS), and the NASA Goddard Space Flight Center.
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Based on data provided by multiple robotic missions (landers, rovers, and orbiters), it is widely believed that the late Noachian Period (ca. 4.1 to 3.5 billion years ago) was the only geological period where Mars was habitable. Based on the valley networks observed near the equator (features that form from erosion due to flowing water), there was significant rain near the equator during this period.
This period did not last, as Mars lost its global magnetic field, and its atmosphere was slowly stripped away by solar wind. Over the ensuing eons, Mars gradually transitioned to the very cold, desiccated, and almost airless environment that we see there today. Knowing how long this period of habitability lasted is key to the current search for evidence of past life on Mars. The longer the window, the more likely it is that fossilized evidence of life can be found on Mars today.
In their study, the SEEC collaboration extended the potentially habitable period on Mars by about 500 million years into the late Hesperian Period (ca. 3.7 to 3 billion years ago). As co-author Frédéric Schmidt, a researcher with the University Paris-Saclay, explained in a NASA press release:
“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. 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 team used the ROCKE-3D Global Climate Model (GCM) developed at NASA GISS to simulate what the Martian environment was like during the Hesperian Period. They refined these simulations by factoring in the current Martian landscape, its surface elevations, and removing all the present-day ice sheets. Last, they accounted for a small ocean around the northern polar region whose boundaries were based on the current geological evidence for its existence.
Michael Way, the co-lead author of the paper at NASA GISS, explained in a recent NASA press release:
“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. 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.”
This simulation revealed that 3 billion years ago, an ocean would have formed in the Northern Lowlands, where the atmosphere was denser and warmer. In this region, water would evaporate and result in precipitation as rain or snow (depending on the latitude). It would mainly rain in or near the ocean, but in the colder Southern Highlands, it was mainly snow. The snow would accumulate to form large glaciers that would flow to the lowland basin, where they would melt and return water to the ocean.
This simulation was one of the first fully-coupled GCMs used for Mars, where 3D atmospheric and oceanic components are calculated simultaneously, making it that much more realistic. Consistent with previous findings, like those obtained by NASA’s Mars Atmosphere and Volatile Evolution Mission (MAVEN), this model shows that a northern ocean would have remained stable even if average global surface temperatures dropped below freezing.
Like Earth, ocean circulation could have transported warm water from mid-latitudes to the pole, warming the surface to 4.5 °C (40 °F). “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 multiple features on Mars from this period. These include U-shaped valley structures carved by slowly-moving glaciers and V-shaped valleys that form from streams as they flow downhill to merge with larger rivers. Whereas the former feature is found in the southern highlands – where the model indicates glaciers formed because these areas had the coldest temperatures – the latter appear at low altitudes near the ancient shoreline.
The SEEC team plans to continue studying Mars to see if there is more evidence to support their model. This will include incorporating evidence from recent surface and orbiter of Martian surface features, such as the glacial valleys in the north. There’s also the proposed Mars Ice Mapper, a collaborative effort between NASA and international partners that will be able to study the shallow Martian subsurface in unprecedented detail.
There’s also China’s Zhurong rover, which landed in the Northern Lowlands of Mars on May 22nd, 2021, and has examined the rocks and features there. The results of these missions could help answer the big questions about Mars’s ancient ocean, not the least of which are how long it existed there and if it retreated underground or was lost to space (or both). This data will also assist in the ongoing search for past (and maybe present) life!