Life, as we understand it on Earth, requires some basic ingredients. Water is one of those. And for years, NASA’s succession of robotic missions has been “following the water” on Mars to learn more about the planet’s history, including if it ever supported life.
While many scientists believe that Mars was warm and wet billions of years ago before it became the frozen desert it is today, others point to the faint young sun paradox.
Four billion years ago, our sun was much fainter — about 30% fainter. It has grown warmer and brighter over time. If that’s the case, then ancient Mars would have been too cold and dry for water or life on its surface.
Today, Mars only receives about 43% of the concentrated sunlight that Earth receives from the sun. This means that temperatures on ancient Mars would have struggled to rise above the melting point of water ice.
But geological features on Mars show evidence of hydrated minerals and ancient riverbeds and lakes. This evidence points to the fact that Mars likely had an abundance of liquid water during the Noachian era, or between 3.7 and 4.1 billion years ago.
This contradiction between climate modeling and the geologic record of Mars is the faint young sun paradox.
The rocky planets in our solar system — Earth, Venus, Mercury and Mars — contain elements that create heat through radioactive decay. These elements include uranium, potassium and thorium.
This kind of heating would be enough to melt the bottoms of thick ice sheets to create liquid water, even with a faint sun. On Earth, this type of heating, called geothermal heat, can be seen in subglacial lakes forming in parts of the West Antarctic ice sheet, the Canadian Arctic and Greenland.
The phenomenon of geothermal heat also explains liquid water on a freezing-cold Mars 4 billion years ago.
Understanding ancient Mars
The researchers used a variety of datasets to test their theory about geothermal heating on Mars billions of years ago. This data included the thickness of ice deposits in the Martian southern highlands and estimations of the planet’s annual surface temperature and the flow of heat from the interior to the surface 4 billion years ago.
Through modeling, the scientists found that subsurface melting of thick ice sheets would have led to an abundance of groundwater on Mars.
“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,” said lead study author Lujendra Ojha, an assistant professor in the department of Earth and planetary sciences in the School of Arts and Sciences at Rutgers University-New Brunswick, in a statement. “I and my co-authors propose that the faint young sun paradox may be reconciled, at least partly, if Mars had high geothermal heat in its past.”
Although they don’t know what happened to make Mars so inhospitable, even if it once supported a warm, wet climate, over time the red planet lost its magnetic field, had much of its atmosphere stripped away and experienced a drop in global temperature.
This means that in order for liquid water to have a stable presence on Mars, it would have to be beneath the surface.
“Regardless of the actual nature of the ancient martian climate, the subsurface would have been the most habitable region on Mars,” the authors wrote in the study.
As water penetrated deeper on Mars, researchers suggested, any extant life may have followed it miles below 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.”
This may have been the case on early Earth, too.
“Much of Earth’s microbial biomass resides within its crust, where water is readily available,” the authors wrote in the study. “Substantial biological diversity exists throughout the huge volume of subsurface habitable environments, which may reach (greater than 5 kilometers in) depth. Therefore, the subsurface could have been the most viable habitat for ancient simple life forms on early Earth and possibly Mars.”
Currently, the NASA Mars InSight lander is investigating the interior of Mars after landing in 2018. (InSight is short for Interior Exploration using Seismic Investigations, Geodesy and Heat Transport.) Data collected by the lander may help researchers learn more about how geothermal heating may have impacted Mars’ habitability billions of years ago, the researchers said.
And any evidence of potential past life on Mars, like biomarkers, may be tucked beneath the surface where they have been protected from radiation.