If life ever existed on ancient Mars, it may not have found a way to survive on the surface, but several miles below it. A new study indicates that its subsurface was possibly the most habitable part of Mars in the past.
The research was recently published in the journal Science Advances.
Life needs certain essential ingredients, just like on the Earth. One of those is water. And NASA’s (National Aeronautics and Space Administration) succession of robotic missions has been “following the water on Mars for years to learn more about the history of the planet, including whether it has 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.
Young sun paradox
Four billion years ago, our sun was much fainter maybe about 30 percent 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, only about 43 percent of the concentrated sunlight received by Earth from the sun is received by Mars. This suggests that temperatures on ancient Mars would have struggled to climb above the melting point of water ice.
But evidence of hydrated minerals and ancient river beds and lakes can be seen in geological features on Mars. This evidence points to the fact that during the Noahian period, which is between 3.7 and 4.1 billion years ago, Mars actually had an abundance of liquid water.
The faint young sun paradox is this inconsistency between climate modeling and Mars’ geological record.
Water below the surface
Earth, Venus, Mercury and Mars which are the rocky planets in our solar system, contain elements that produce heat by radioactive decay. Uranium, potassium, and thorium are contained in these elements.
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.
To test their hypothesis of geothermal heating on Mars billions of years ago, the researchers used a number of datasets. This data included the thickness of ice deposits in the southern Martian highlands and estimates of the planet’s annual surface temperature and heat transfer 4 billion years ago from the inside to the surface.
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, a co-author of the research. “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 do not know what happened to make Mars so inhospitable, even though it once supported a warm humid climate, the red planet lost its magnetic field over time, stripped away most of its atmosphere, and suffered a decrease 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,” Mr. Ojha said. “So, the subsurface may represent the longest-lived habitable environment on Mars.”
Similar case with Earth
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 may be tucked beneath the surface where they have been protected from radiation.