For decades, robotic missions have explored Mars to learn more about the planet's geological and ecological history. Next year the Perseverance rover will join the hunt and be the first mission to send samples back to Earth. The first crewed mission is expected to take place by 2030. All of these efforts are part of an ongoing effort to find evidence of past (and perhaps even present) life on Mars.
According to a new study from Rutgers University in New Brunswick, the most likely place where this evidence can be found is several miles below the surface. Here (so they argue), water still exists in liquid form, likely due to geothermal warming, which melts thick sheets of ice beneath the surface. This research could help resolve remaining questions like the solar weak young paradox.
The study, recently published in the journal Science Advances, was led by Lujendra Ojha, an assistant professor in the Department of Earth and Planetary Sciences at Rutgers University. He was joined by a team of engineers and planetary scientists from Dartmouth College (Hanover, NH), Louisiana State University (Baton Rouge, LA) and the Planetary Science Institute (PSI) in Tuscon, Arizona.
This artist's concept shows the early Martian environment (right) – presumably liquid water and a thicker atmosphere – compared to the cold, dry environment seen on Mars today (left). Photo credit: NASA's Goddard Space Flight Center
One of the most persistent questions about Mars was how to keep temperatures warm enough to have liquid water on its surface. During the Noachian era (about 4.1 to 3.7 billion years ago) the sun was much weaker and cooler than it is today. However, there are innumerable geological indicators on Mars that suggest the presence of rivers, lakes, and even an ocean in the northern lowlands, dated to this time.
This apparent contradiction between the geological record and the climate models is referred to by scientists as the “weak paradox of the young sun”. As Prof. Ojha stated in a Rutgers Today press release:
“Even if greenhouse gases such as carbon dioxide and water vapor are pumped into the early Martian atmosphere in computer simulations, climate models still have difficulties in supporting a long-term warm and humid Mars. Myself and my co-authors suggest that if Mars had high geothermal heat in its past, the weak paradox of the young sun can be at least partially reconciled. "
One possible mechanism is the heat generated by the radioactive decay of elements such as uranium, thorium, and potassium. On terrestrial planets like Earth, Mars, Venus, and Mercury, these elements generate enough heat inside to keep the mantle in a viscous state. They can also cause the undersides of thick ice sheets to melt, resulting in the formation of subglacial lakes.
Artist's impression of water beneath the surface of Mars. Photo credit: ESA / Medialab
It is believed that this phenomenon on Earth led to the formation of lakes under the ice cover of West Antarctica, Greenland, and the Canadian Arctic. It is likely that a similar melting could explain the presence of liquid water on Mars 4 billion years ago, at a time when surface temperatures would be frozen.
To verify this theory, the Rutgers team examined various Mars data sets to determine whether geothermal warming would have been possible in the Noachian era. What they found was that these conditions would have been commonplace on the Martian surface about 4 billion years ago. As the planet lost its magnetosphere and slowly saw its atmosphere receding, falling temperatures likely meant that liquid water was only stable at great depths.
So if life had ever originated on Mars, it would likely have followed the liquid water as it seeped underground. On these levels, said Ojha, the conditions would have been warm enough for life to continue:
“At such depths, life could have been sustained by hydrothermal (warming) activity and stone-water reactions. So the subsurface could represent the longest-lived habitable environment on Mars. "
These results support an ongoing theory among planetary scientists and astrobiologists currently in search of life on Mars. Since the orbiter Mariner 9 provided the first direct evidence of past water on the Red Planet – which has been confirmed several times since – scientists have begun to speculate where that water (and any life that supported it) might be found today.
In 2018, NASA interior exploration using seismic surveys, geodesy and heat transport (InSight) reached the surface and began studying the internal structure of Mars. The data collected could allow scientists to better understand the role geothermal warming played in the habitability of Mars during the Noachian era. It could also point the way to life on Mars today!
Further reading: Rutgers Today