Four billion years ago, our host star was far fainter and colder than it is today. And without the heat and energy of the young star, the planets around the Sun looked very different, too.
On Mars, temperatures would have been freezing, and yet early geological indicators suggest it had liquid water at one point during its early history. The cosmic mystery of how Mars was able to sustain liquid water despite the cold is known as the faint young Sun paradox.
New research may have the answer. In a study published Wednesday in the journal Science Advances, scientists suggest Mars may have had liquid water beneath the surface — and the ideal conditions for life.
A new hope — Scientists believe Mars was once a wet, warm world, and may even have hosted life forms at some point in its early history. But over time, Mars' atmosphere was stripped away because the planet lacks a magnetic field like Earth's, making it seemingly inhospitable.
Studies suggest Mars had abundant liquid water about 4.1 billion to 3.7 billion years ago. But temperatures on Mars today dip below -81 degrees Fahrenheit, and would have been much colder at a time when the Sun was not emitting as much heat during the planet's early history.
"Even if greenhouse gases like carbon dioxide and water vapor are pumped into the early Martian atmosphere in computer simulations, climate models still struggle to support a long-term warm and wet Mars," Lujendra Ojha, assistant professor in the Department of Earth and Planetary Science at Rutgers University, and lead author behind the new study, said in a statement.
To figure out how Mars was able to sustain liquid water during that time, the scientists behind the new study examined various Mars datasets to see if heat-producing elements like uranium, thorium, and potassium may have generated heat on Mars through radioactive decay. Much of these data come from NASA's Mars InSight Spacecraft, which landed on Mars in 2018. It has enabled scientists to reconstruct the geological history of the Red Planet in unprecedented detail.
If the elements were present on Mars, then the scientists theorized the bottom of the thick ice sheets beneath the Martian surface would have melted — even if the Sun was faint. Today, this process of geo-thermal heating results in subglacial lakes in areas of the West Antarctic ice sheet, Greenland, and the Canadian Arctic here on Earth.
From the data, the researchers found the conditions for geo-thermal heating would have existed on Mars four billion years ago. As a result, liquid water may have been present at extreme depths beneath Mars' surface. So if life did exist on the Red Planet, it would have had to evolved to live far beneath the surface.
"At such depths, life could have been sustained by hydrothermal — heating — activity and rock-water reactions," Ojha said. "So, the subsurface may represent the longest-lived habitable environment on Mars."
It is possible the subsurface continued to harbor Martian life long after the planet lost its protective atmosphere, the researchers suggest:
"If life ever originated on Mars, then it may have followed the groundwater table to progressively greater depths where stable liquid water could persist. In addition, the deep subsurface would have guarded early life from the Late Heavy Bombardment."
One of the tasks of Perseverance Rover, NASA's latest mission to Mars, is to look for signs of life on the surface of the planet. Perhaps in future missions, scientists will be able to dig a little deeper to look for relics of the Martian past.
Abstract: In explaining extensive evidence for past liquid water, the debate on whether Mars was primarily warm and wet or cold and arid 4 billion years (Ga) ago has continued for decades. The Sun’s luminosity was ~30% lower 4 Ga ago; thus, most martian climate models struggle to elevate the mean surface temperature past the melting point of water. Basal melting of ice sheets may help resolve that paradox. We modeled the thermophysical evolution of ice and estimate the geothermal heat flux required to produce meltwater on a cold, arid Mars. We then analyzed geophysical and geochemical data, showing that basal melting would have been feasible on Mars 4 Ga ago. If Mars were warm and wet 4 Ga ago, then the geothermal flux would have even sustained hydrothermal activity. Regardless of the actual nature of the ancient martian climate, the subsurface would have been the most habitable region on Mars.